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dissertation entitled

Understanding Local Organization in Condensed Phases

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John L. Delacruz

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6/01 c;/CIRC/DateDue.p65-p. 15

 

UNDERSTANDING LOCAL ORGANIZATION IN CONDENSED PHASES
By

John Leonard DelaCruz

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSPHY
Department of Chemistry

2002

ABSTRACT
UNDERSTANDING LOCAL ORGANIZATION IN CONDENSED PHASES
By

John Leonard DelaCruz

We report on the reorientation dynamics of several Rhodamine chromophores in
condensed phases. The purpose of these measurements is to gain a better understanding
of local organization in condensed phases, which in turn will give a better understanding
of a diverse range of phenomena such as local heating, solvation and energy transfer
processes that occur on molecular length scales. We have employed a pump-probe
picosecond laser Spectrometer and a picosecond time-correlated single photon counting
laser system in these studies to examine the chromophores in polar protic and aprotic
condensed phases and anionic, cationic and neutral micellar systems. Specific
environments include the n-alcohols, N,N-dimethylformamide, dimethyl sulfoxide,
acetonitrile and the micellar systems of sodium dodecyl sulfate (SDS),
cetyltrimethylammonium bromide (CT AB), and poly(ethylene) glycol dodecyl ether 400
(Thesit®). From the reorientation measurements on these molecules we have determined
the effective rotor shape of the molecules in solution and the length scale upon which
local heating occurs. We have also gained insight into the relationship between structure
and local organization in condensed phases and the phenomenon of dielectric friction in
polar environments. Lastly, we have gained an understanding of how organization in
micellar environments occurs within the context of the ionic forces and the dispersive

forces that comprise the micellar environment.

ACKNOWLEDGMENTS

I would like to thank my advisor, Professor Gary J. Blanchard for his unswerving
dedication to my education and professional development, his wisdom when the
problems became tough, his ability to step in at the right time and his dogged
determination to stay away when warranted. Without his guidance, patience and humor, I
might not have made it this far.

I would like to thank my mother, Bonnie Drapeau, my step-father, Louis J.
Drapeau, and my father, Leonard U.Y. DelaCruz for being supportive of my endeavors,
no matter how crazy they sounded at the time and for believing in my ability even when I
didn’t.

Additionally, I would like to thank my group members in the Blanchard group,
past and present for the humor, dedication, persistence and excellence that they have
displayed. Good luck to everyone!

I’d like to specifically thank all of my close friends for the pleasure of their
company, the stress relief and their ability to see the lighter side of life when things got
tough.

Thank you to all of the professors, graduate Students and Staff members that I
have had the pleasure of knowing at Michigan State University during the performance of
my graduate work. Unfortunately you are too numerous to mention, but you will never
be forgotten.

Thanks to all the members of the softball team for great moments off and on the

field!

iii

TABLE OF CONTENTS

LIST OF TABLES ............................................................................................................. vi
LIST OF FIGURES .......................................................................................................... vii
Chapter 1. ............................................................................................................................ 1
Introduction ......................................................................................................................... 1
Literature Cited ................................................................................................................. 11
Chapter 2 ........................................................................................................................... 17
Introduction ....................................................................................................................... 17
Experimental ..................................................................................................................... 20
Chemicals and Sample Handling. ............................................................................. 20
Steady-sate spectroscopy. ......................................................................................... 20
Time-correlated single photon counting spectroscopy. ............................................ 20
Results and Discussion ..................................................................................................... 21
Reorientation of the probe molecule. ........................................................................ 22
Effective rotor shape. ................................................................................................ 25
Excitation-dependent reorientation dynamics ........................................................... 32
Conclusions ....................................................................................................................... 39
Literature Cited ................................................................................................................. 41
Chapter 3 ........................................................................................................................... 44
Introduction ....................................................................................................................... 44
Experimental Section ........................................................................................................ 47
Pump-Probe Laser System ........................................................................................ 47
Steady-Sate Spectroscopy. ........................................................................................ 48
Chemicals and Sample Handling. ............................................................................. 48
Results and Discussion ..................................................................................................... 49
Linear optical response. ............................................................................................ 49
Time resolved spectroscopy ...................................................................................... 50
Debye-Stokes-Einstein equation. .............................................................................. 56
Reorientation of R610, R640 and R610 lactone. ...................................................... 59
Reorientation of R700 and R800. ............................................................................. 61
Conclusions ....................................................................................................................... 63
Literature Cited ................................................................................................................. 68
Chapter 4 ........................................................................................................................... 72

iv

Introduction ....................................................................................................................... 72

Experimental Section ........................................................................................................ 75
Pump-probe laser system. ......................................................................................... 75
Time-correlated single photon counting system. ...................................................... 76
Steady-state spectroscopy. ........................................................................................ 76
Chemicals and Sample Handling. ............................................................................. 77

Results and Discussion ..................................................................................................... 77
Linear optical response. ............................................................................................ 78
Time resolved spectroscopy. ..................................................................................... 81
Wobbling-in-a—cone with translational diffusion and micelle rotation. .................... 93

Conclusion ........................................................................................................................ 95

Literature cited. ................................................................................................................. 97

Chapter 5 ......................................................................................................................... 100

Conclusions and Future Work ........................................................................................ 100

LIST OF TABLES

Table 2.1 - Experimental Reorientation Times and Changes in Solution Viscosity and
Temperature Associated with Internal Conversion of R640 ............................................. 36

Table 2.2 — Solvent Properties and Quantities Extracted from Transient Heating
Calculations. The quantity me1 is extracted from Ar and Vealc = Vsom + Vsolvm, where
these volumes are determined using Edward’s methods5 Some values (*) of C, are
interpolated from literature data For the other alcohols .................................................... 38

Table 3.1a. - Reorientation Times and Zero-Time Anisotropies for R610, R640 and R610
lactone in protic and aprotic solvents ................................................................................ 55

Table 3.1b — Reorientation Times and Zero-Time Anisotropies for R700 and R800 in
protic and aprotic solvents. ............................................................................................... 55

Table 3.2. — Calculated dielectric friction times for Rhodamine 610 (R610) and 640
(R640). The times reported are intended to be additive contributions to the reorientation
time calculated using the modified DSE model ................................................................. 59

Table 3.3. — Calculated molecular volumes and SO permanent dipole moments for the
rhodamines examined here. Volumes were calculated from Reference 57. Dipole
moments were calculated using Hyperchem® v. 6.0 with PM-3 parameterization. ........ 60

Table 3.4. — Selected properties for the solvents used. ..................................................... 61

Table 4.1 — Reorientation times and zero-time anisotropies for Rhodamine chromophores
in aqueous and micellar environments .............................................................................. 82

Table 4.2 — Various thermodynamic properties of Rhodamine chromophores in SDS and
CT AB micellar environments. .......................................................................................... 91

Table 4.3 — Wobbling constant (Dw) values for aqueous and Dw and 00 micellar
environments. .................................................................................................................... 94

vi

LIST OF FIGURES

Figure 1.1 -— Schematic of pump-probe laser spectrometer used in this work. ................... 6

Figure 1.2 - Schematic of the time-correlated single photon counting laser spectrometer
used in this work. ................................................................................................................ 7

Figure 2.1 -— Structure of the Rhodamine 640 molecule, drawn as a free cation. Cartesian
axes are indicated, with the n-plane defining the x-y plane .............................................. 23

Figure 2.2 — Reorientation for R640 in the n-alcohols: (I) indicates experimental data for
excitation of the $1 «— So transition. The calculated DSE result for the stick limit (f = l)
is shown as the solid line and the calculated DSE result where V = V801,,“ + Sme, is
indicated by the dashed line. ............................................................................................. 24

Figure 2.3 — Linear optical response of R640 in n-propanol. The spectra of R640 in other
alcohols are similar. Wavelengths of excitation are 280 and 580 nm respectively.
Absorption (solid line) and emission (dotted line) spectra are normalized for purposes of
presentation. ...................................................................................................................... 28

Figure 2.4 — (a) Experimental instrument response and 1"(t) and I 1(t) data for R640
excited at 580 nm (S. (— So) in n-propanol. (b) Experimental anisotropy function
generated from data presented in (a). The best fit anisotropy decay function for this data
set is R(t) = (0.29 :l: 0.01)exp(-t/(481 :l: 3 ps)) The residuals of the fit are shown in the
inset. .................................................................................................................................. 29

Figure 2.5 - (a) Experimental instrument response and I||(t) and I i(t) data for R640
excited at 290 nm (S; (— So) in n-propanol. (b) Experimental anisotropy function
generated from data presented in (a). The best fit anisotropy decay function for this data
set is R(t) = (-0.09 :l: 0.01)exp(-t/(383 1 12 ps)). The residuals of the fit are shown in the
inset. .................................................................................................................................. 31

Figure 2.6 - Reorientation time dependence on solvent viscosity for excitation of the S;

<— 80 transition (0) and the S; (— So transition (I). ........................................................ 33
Figure 3.1 — Structures of the five rhodamines studied here. .......................................... 46
Figure 3.2 - Absorption and emission spectra of (a) R610, (b) R640, in methanol. ........ 51
Figure 3.3 - Absorption and emission spectra of (a) R700, (b) R800, in n-propanol. ..... 52

Figure 3.4 - Absorption spectra of R610 lactone (solid line) in acetonitrile and ring-
opened R6101actone (dashed line) in methanol. .............................................................. 53

vii

Figure 3.5 — Calculated state energy levels for the rhoamines studied here. Calculations
were performed at the semi-empirical level with PM-3 parameterization. State energies
and oscillator strengths for SI) <— 80 transistions were calculated using configuration
interaction. ........................................................................................................................ 54

Figure 3.6 - Reorientation time constants of R610 (O) and R640 (O) as a function of
solvent viscosity. The calculated DSE stick limit line is shown for each chromophore,
with a dashed line for R610 and a dotted line for R610. .................................................. 65

Figure 3.7 — Reorientation time constants of ring-opened R610 lactone (o) and R610 (0)
as a function of solvent viscosity. The calculated DSE stick limit lines are shown for
each chromophore. For the ring-opened lactone, the calculated hydrodynamic volume
reflects an increase due to ester formation ........................................................................ 66

Figure 3.8 — Reorientation time constants of (a) R700 and (b) R800 as a function of
solvent viscosity. Aprotic solvents are shown with open (0) circles and protic solvents
are shown with open (0) squares. For each chromophore the calculated DSE stick and
slip limit lines are shown .................................................................................................. 67

Figure 4.1 - Rhodamines and surfactants used in this study. Top row, left to right:
tetramethylrhodamine methyl ester (R1), tetramethylrhodamine ethyl ester (R2). Second
row, left to right: tetraethylrhodamine hexyl ester (R6), tetraethylrhodarnine octadecyl
ester (R18). Third row: sodium dodecyl sulfate (SDS), Fourth row:
cetyltrimethylammonium bromide (CT AB), Bottom row: polyethylene glycol 400
dodecyl ether (Thesit®). .................................................................................................... 79

Figure 4.2 — Absorbance and fluorescence spectra of the rhodamine esters used in this
study. ................................................................................................................................. 80

Figure 4.3 — Time-resolved reorientation data for rhodamines in the aqueous phase.
Squares (I) are experimental data points, and the solid line represents the theoretical
reorientation calculated from the Debye-Stokes-Einstein equation .................................. 85

Figure 4.4 - Reorientation data for rhodamine aliphatic esters in aqueous SDS micelle
solution. Biexponential anisotropy decay functions produced a) normalized prefactors
for the contribution of the fast (0) and slow (I) components of the anisotropy decays. b)
Reorientation time constants plotted as a function of increasing ester chain length. ....... 87

Figure 4.5 - Reorientation data for rhodamine aliphatic esters in aqueous CT AB micelle
solution. Biexponential anisotropy decay functions produced a) normalized prefactors
for the contribution of the fast (O) and slow (I) components of the anisotropy decays. b)
Reorientation time constants plotted as a function of increasing ester chain length. ....... 88

viii

Figure 4.6 - Reorientation data for rhodamine aliphatic esters in aqueous Thesit® micelle
solution. Biexponential anisotropy decay functions produced a) normalized prefactors
for the contribution of the fast (0) and slow (I) components of the anisotropy decays. b)
Reorientation time constants plotted as a function of increasing ester chain length. ....... 89

Images in this dissertation are presented in color

ix

Chapter 1.

Introduction

Understanding local organization in the solution phase has been a subject of
considerable interest because of its effect on the process of solvation and the related
phenomena of chemical reactions and intermolecular energy transfer."9 Despite almost
three decades of nearly continuous investigation, the details of these solvent-solute
interactions, particularly in polar solvent systems, remain to be understood in detail.8'28

Condensed (liquid) phases are a widely used medium for chemical manipulation
and reaction and a further understanding of intermolecular interactions that occur in these
media would be of great value to the field of chemistry. Working in these phases
presents unique challenges that must be met, such as rapidly changing spatial
relationships between molecules and reaction time—scales that are often too rapid to be
accessed by techniques such as x-ray diffraction and NMR spectroscopy. In condensed
phases a combination of long-range forces, such as electronic and non.collisional energy
transfer, occur over relatively long length scales, and short-time duration collisional
phenomena, occurs and all of these processes contribute to the bulk behavior of the
system.”33 Through the use of time-domain spectroscopies it has become possible to
probe these interactions by measuring the molecular motion and energy relaxation
processes that occur on these length and time scales.

With the development of picosecond lasers, the measurement of these phenomena
has greatly expanded because many of the processes relevant to solvation occur on the

picosecond or longer timescale, and this is especially true in polar, strongly associative

systems.8'2‘7"23‘34'36 From time-domain spectroscopic data, information can be extracted
about how a molecule reorients in solution, and such results can then be analyzed within
the framework of a well-developed model to gain insight into interactions of the molecule
intuits its local environment.

Several different types of information are amenable to acquisition in the time-

38‘“ vibrational

domain, including fluorescence lifetime,” molecular reorientation,
relaxation4 and transient fluorescence spectral shift measurements.8'9'24 Of these
methods, molecular reorientation has proven to be among the most useful because of the
generality of the phenomenon and the well-developed theoretical framework for the
interpretation of the experimental data.

The starting point for interpreting molecular rotational motion measurements is

usually the modified Debye—Stokes-Einstein (DSE) equation.“43

0W
1' = —— 1.1
0” kBTS ( )

This expression has proven remarkably useful in providing a serniquantitative model for
rotational molecular motion in liquids. In this model, 77 is the solvent bulk viscosity, V is
the solute hydrodynamic volume and the terms f and S are frictional boundary condition
and solute molecular shape factors, respectively. This model assumes a continuum
solvent and it has been shown to model reorientation data well in the limit that the
individual solvent molecules are smaller than the solute.

A limitation to our understanding of motion in liquids is that we do not
understand the solvent-solute boundary condition at the molecular level. A critical point
in understanding the solvent-solute boundary condition is the notion of frictional

interactions and “local” viscosity versus bulk viscosity.6"“'45 It is understood that the

viscosity experienced by the reorienting molecule may be quite different than that of the
bulk liquid viscosity, but the exact transition point from local to bulk viscosity is often
poorly understood or ill defined.

It has been recognized that the concept of mechanical friction as the sole
contributor to mediating molecular reorientation, especially in polar and / or charged
systems may be outdated and should be amended to include the concept of dielectric
friction.29‘33"“5'53 In one model,32 dielectric friction can be best described by a dipole that
is contained within a spherical cavity that is immersed into a dielectric fluid with a
dielectric response that is frequency dependant. The dipole creates an electric field
within the solvent, that is coupled to the dipole. If this dipole then rotates at some
angular frequency, as in the case of rotational diffusion, an interaction will occur between
the solvent dielectric and the electric field created by the dipole. However, because the
solvent dielectric response is frequency dependant, a phase lag is generated between the
dipole moment of the solute and the solvent dielectric. This phase lag induces a torque
on the rotating dipole, thus leading to energy dissipation. This model was originally
developed without any consideration of the hydrodynamic effects that occur in solution,
but was later extended to incorporate these effects by Felderhof and others.54 Alavis5
extended this model by considering the dipole to be suspended among a distribution of
point charges in a cavity. This model predicts values of dielectric friction that are much
larger than those predicted by the point source model, and is a better model in that it
allows closer agreement between theory and experiment. However, both models suffer
from the same problem, which is that the conditions under which mechanical friction

becomes less significant than dielectric friction are poorly understood.

Another area that has remained ill-defined is the effect of solute structure on
intermolecular interactions and local organization. Exchanging one functional group for
another to improve reactivity or solubility is a well established practice in organic
chemistry, but our understanding of how and why these changes work often remains at an
intuitive level.2°'25'28’5‘H33

Micelles comprise another class of interesting, restricted motion environments
where local organization is present. Information has been collected on the size, shape
and formation of micelles.“67 It has been theorized that the forces that govern the
formation of micelles are ionic and / or dipolar forces that are dominant in the head group
region of the rrricelle and the hydrophobic dispersion forces that are dominant in the core
region of the micelle.”72 Clearly, these forces must exist in a balance or the formation of
the micelle could not occur, but how the ionic forces and the van der Waal’s forces
interact remains an active area of interest.71’73'84

The Rhodamine family chromophores used in this dissertation provide excellent
candidates to study local organization due to their ready solubility in a variety of polar
and amphiphilic media and their high photo-stability and fluorescence quantum yield.”85
They also provide a large variety of pendant functional groups from which the study of
structure and functionality on local organization was facilitated.

The use of laser spectrometers has become common in the pursuit of these
measurements and we present two such systems that have utilized in our work. The
picosecond pump-probe laser spectrometer used for most of the laser based

measurements has been described in detail elsewhere,86 and we present only a brief

outline of its characteristics here. Figure 1.1 shows a mode-locked CW Nd:YAG laser

(Coherent Antares 76-S) that produces 30 W of average power (1064 nm, 100 ps pulses,
76 MHz repetition rate). The output of this laser is frequency doubled to produce ~3 W
of average power at 532 nm, which is used to excite two cavity-dumped dye lasers
(Coherent 702-3). The output of each dye laser is ~40-110 mW average power at an 8
MHz repetition rate with a pulse that produces a 7 ps FWHM autocorrelation trace using
a three plate birefringent filter. The pump and probe wavelengths were chosen to access
the SI «— So transition of each chromophore to detect ground state population recovery.

The probe laser polarization was set alternately to 0‘ and 90’ relative to the pump laser

polarization depending upon the experiment being performed. The detection of the
transient signals was accomplished using a radio and audio frequency triple-modulation
scheme, with synchronous demodulation detection.”88 Each reported time constant is
the average of at least five individual determinations that are themselves the average of at
least ten to twelve time-scans. A computer controlled interface controls a translation
stage which, in turn, determines the arrival time of the probe pulse relative to that of the
pump pulse, with the arrival time being determined by the distance that the translation
stage travels. The time resolution of this system, ~10 ps is determined by the cross—
correlation between the pump and the probe laser pulse trains. Further information may

be found in chapters 2 and 3 which detail our experiments using this laser system.

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Figure 1.1 - Schematic of pump-probe laser spectrometer used in this work.

The time-correlated single-photon counting spectrometer used for the micellar
and state dependent studies has been described elsewhere,89 and we present only a brief
outline of its characteristics here. Figure 1.2 shows a mode-locked Nd:YAG laser
(Quantronix 416) producing 7W average power at 1064 nm with 100 ps pulses at an 80
MHz repetition rate. The fundamental light is frequency doubled to produce 532 nm

light which is used as an input to excite a synchronously pumped, cavity dumped dye

 
     
 
    
     

pulse
diagnostics

 
   

cav't dum edd elaser
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Figure 1.2 - Schematic of the time-correlated single photon counting laser spectrometer used in this
work.

laser (Coherent 701-2) operating with Pyromethine 567 (Exciton) laser dye. The output
of this laser is typically 80 mW average power with a 4 MHz repetition rate and 5 ps
pulses.

Detection of the transient signals in this system was accomplished using a
microchannel plate photomultiplier tube (Hamamatsu R3809U), where the light is
collected by a microscope objective and filtered through a subtractive double
monochromator (American Holographics DB-lO). Further information may be found in
chapters 2 and 4 of this dissertation which detail our experiments using this laser.

The study of dynamic interactions using rotational diffusion measurements is a
widely used method to probe the nature of solvent-solute interactions, and the theoretical
framework needed to understand these interactions is well documented. Application of
the theory to the experimental data can provide information such as solute rotational
diffusion constant, the dihedral angle between the excited and probed transition moments
and in some cases, the dielectric properties of the local environment. The average local
organization around the solute can then be inferred from the data and a picture can be
formed of the solute-solvent interaction.

Using linearly polarized light, an ensemble of molecules with transition dipole
moments oriented parallel to the polarization of the incident radiation can be excited and
induce a transient anisotropy in the system. The induced anisotropy will decay in time
due to the rotational dynamics of the system. The orientational anisotropy function can

be extracted from the raw data through equation 1.2.

I“(t) — 1l (t)

1.2
I”(t)+2li (t) ( )

 

R(t) =

I"(t) and I 1 (r) are the intensities of the response when the polarizations are

oriented either parallel or perpendicular to one another.

We have undertaken a series of studies in an attempt to better understand the
effects of local organization on the process of solvation by examining the phenomena of
local heating on short time scales, the effects of solute and solvent structure on local
organization and the balance of forces that are present in organized structures such as
rrricellar systems. We have used molecular reorientation, fluorescence lifetime and
steady-state fluorescence measurements in condensed phases, and report our finding in
this dissertation.

Chapter 2 of this dissertation outlines the steady state and time-resolved
spectroscopy of Rhodamine 640 in a series of n-alcohols, methanol through decanol.
From these studies we were able to form the framework from which the other studies
proceeded in examining local organization around these chromophores. Qualitative
assessments of the rotor shape, orientation of the ground and transition state dipole
moments and a qualitative understanding of how the local environment changes were
critical pieces of information for subsequent studies using this family of chromophores.

Chapter 3 of this dissertation details studies that were performed using a family of
Rhodamines to better understand the effects of structural freedom on solvation and local
organization. From these studies we have come to a better understanding of the effect of
solvent on reorientation behavior. We were also able to come to better understand some
of the forces that comprise these interactions.

Chapter 4 details a series of studies in micellar environments and an examination

of the balance of forces that are present in these environments. From these studies we

were able to make qualitative and quantitative assessments of the micellar environment
and how the organization present within these structures affects the reorientation
behavior of the Rhodamine chromophores.

Chapter 5 provides some final conclusions as well as some suggestions for future
work both with Rhodamines and in the condensed phase system that we have studied.
We draw some conclusion about the relationships between solute structure and local

organization in the solvent systems that we have studied.

10

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(37) Kahlow, M. A.; Kang, T. J .; Barbara, P. F. J. Phys. Chem. 1987, 91 , 6452.

(38) Madden, P.; Kivelson, D. J. Phys. Chem. 1982, 86, 4244.

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(42) Debye, P. Polar Molecules Chemical Catalog Go: New York 1929.

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13

(49) Jarzeba, W.; Walker, G. C.; Johnson, A. E.; Barbara, P. F. Chem. Phys. 1991, 152,
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Chem. 1986, 90, 5766.

14

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15

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16

Chapter 2

Reorientation Dynamics of Rhodarrrine 640 in Normal Alcohols. Measurement of the
Length and Time Scale of Transient Local Heating in Solution.

Introduction
Despite almost three decades of continuous investigation, the details of solvent-
solute interactions, particularly in polar solvent systems, remain to be understood in
detail. Most investigations of intermolecular interactions in solution have used a “probe”
molecule present at low concentration in neat or binary solvent systems. Typically, a
short pulse of light is used to establish some non-equilibrium condition in the ensemble
of probe molecules, with the object of the experiment being to monitor the return of this

population to equilibrium. Such studies have included fluorescence lifetime, molecular

22-34 3544

reorientation,"21 vibrational relaxation and fluorescence spectral shift
measurements. Of these methods, molecular reorientation has proven to be among the
most useful because of the combined generality of the effect and the well-developed
theoretical framework for the interpretation of the experimental data.45'50
The starting point for interpreting molecular rotational motion measurements is

usually the modified Debye-Stokes-Einstein equation.“47

7M
r = 2.1

In this model, 10R is the reorientation time constant (Chapter 1), t] is the solvent
bulk viscosity, V is the solute hydrodynamic volume, and the terms f and S are frictional

boundary condition and solute molecular shape factors, respectively. This model

assumes a continuum solvent and it has been shown to model reorientation data

17

quantitatively in the limit that the individual solvent molecules are smaller than the
solute. One present limitation to our understanding of motion in liquids is that we do not
understand the solvent-solute boundary condition at the molecular level. In the context of
the modified DSE model, discrepancies between experiment and theory are often
expressed in terms of the molecular-level breakdown of the notion of viscosity and in
terms of the frictional coefficient, f, an empirical parameter.

The frictional term, f, is equal to unity in the so-called “stick” limit, which is
taken as being representative of relatively strong solvent-solute interactions. For
interactions weaker than the stick limit, the “slip limit” is used.“49 In this model, the
strength of frictional interactions varies according to the shape of the molecule in such a
way that 0 < f < 1. This model describes the behavior of non-polar molecules reorienting
in nonpolar solvents well. In addition to frictional solvent-solute interactions, dielectric
friction, arising from the dielectric response of the probe molecule and the environment
can also contribute.5 1'53 For most systems, this contribution is small and in many cases
difficult to define precisely owing to the limited information available on the local
dielectric response of the probe molecule environment.

For polar solute molecules reorienting in polar solvents, it is common to recover
experimental reorientation times that are substantially longer than those predicted by Eq.
2.1. This regime, referred to as “super-stick”, remains ill defined but it is clear in such
systems that strong intermolecular forces are responsible for the reorienting moiety not
being simply the solute molecule, but rather the solute as well as some of the surrounding
solvent cage.5 An additional uncertainty in such measurements lies in the determination

of the solute shape factor, S. Perrin derived expressions for the term S to account for the

18

ellipsoidal shape of the solute rotor in solution.46 These expressions depend on the shape
of the solute and on the assumption of an effective rotor shape. The shape of the volume
swept out by the probe molecule is, in many cases, unclear because of ambiguity about
the details of the solute spectroscopic response and because of the limited information
typically available from the experimental data.

Another issue that is rarely considered in molecular reorientation measurements is
the local temperature of the solvent bath that is in closest proximity to the solute
molecule. In the limit that the solute exhibits no Stokes shift and is characterized by an
exclusively radiative decay, there is no reason to expect that the processes of excitation
and de-excitation will deposit energy into the solvent bath. For organic fluorophores,
where the fluorescence quantum yield is often on the order of 0.5 or less, and where
Stokes shifts are at least hundreds of cm", local heating must contribute to the
experimental data at some level.

The work presented here addresses several limitations of the understanding of
molecular motion in the liquid phase. The rotational diffusion behavior of the fluorescent
probe molecule Rhodarrrine 640 (R640, Fig. 2.1 top) has been studied in a series of
normal aliphatic alcohols, methanol through n-decanol. These experiments were
performed using a time-correlated single photon counting apparatus that is capable of
accessing both the S] (— So and the 82 <— So transitions of R640. Data from these
measurements reveal that R640 interacts strongly with multiple solvent molecules, giving
rise to a reorienting moiety that is substantially larger than the hydrodynamic volume of
the solute alone. By comparing the reorientation data taken for both excitation

conditions, an unambiguous determination has been made that this probe molecule

19

reorients as a prolate rotor in normal alcohols, that the Si (— So transition is polarized
along the chromophore long axis (x) and the S; (— So transition is polarized nominally
orthogonal to the S; (— So transition. Comparing the data taken for the two excitation
conditions, we find direct evidence for local heating that results from the nonradiative
dissipation of excitation energy from the 82 to the S1 state. The temperature rise was
determined to be ~5 - 10 K and the manifestation of this effect depends on the solvent.
The data shed new light on the timescale over which the thermal equilibration process
occurs.

Experimental

Chemicals. Rhodamine 640 perchlorate was purchased from Exciton Chemical
Company and used as received. All solvents were purchased from Aldrich Chemical
Company in their highest grade available and used without further purification.

Steady state spectroscopy. Absorption spectra of the R640 solutions were
measured with 1 nm resolution using a Cary 320 UV-visible absorption spectrometer.
Fluorescence spectra were measured using a SPEX Fluorolog 3 spectrometer, also with 1
nm resolution.

Time correlated single photon counting spectroscopy. Fluorescence lifetime and
rotational diffusion measurements were performed using a spectrometer that has been
reported previously.54 We provide a brief recap of its essential features here and a
schematic of this system is shown in Figure 1.2. The source laser is a mode-locked
Nd:YAG laser (Quantronix 416) producing 7 W average power at 1064 nm with 100 ps
pulses at a 80 MHz repetition rate. The second harmonic of the output of this laser (532

nm, ~700 mW) is used to excite a synchronously pumped, cavity dumped dye laser

20

(Coherent 702-2) operating with Rhodamine 6G dye (Kodak). The output of this laser is
typically 80 mW average power with a 4 MHz repetition rate and 5 ps pulses. For
excitation of the S] (-— So transition, the 580 nm output of the dye laser was used to excite
the sample directly. For excitation of the S; (— So transition, the output of the dye laser
was frequency-doubled using a Type I KDP crystal. Detection of the transient signals
was accomplished using a microchannel plate photomultiplier tube (Hamamatsu
R3809U), with fluorescence light collection through a reflecting microscope objective
and wavelength selection with a subtractive double monochromator (American
Holographics DB-lO). The electronics used for signal processing are a Tennelec 455
quad constant fraction discriminator, 864 time-to-amplitude converter and biased
amplifier. The reference channel was detected and delayed using an in-house built fiber
optic delay line. The experimental signal was collected using a multichannel analyzer
(PCA Multiport) and sent to a PC for processing. For this system, the instrument
response function is typically 30 — 35 ps fwhm.
Results and Discussion

The central foci of this work are understanding the reorientation dynamics of
R640 in the n-alcohols, methanol through n-decanol, and resolving the details of transient
heating in solution associated with nonradiative relaxation subsequent to optical
excitation. We consider three facets of the experimental data. The first is a comparison
of our results to the predictions of the modified DSE model. The second issue we
consider is the effective rotor shape swept out by the rotating probe molecule. This
information is often not available unambiguously for probe molecules of low symmetry,

such as R640, or for systems where the spectroscopy of the probe molecule is not

21

understood in detail. Finally, we consider the effect of initial excitation to different
electronic states on the reorientation dynamics of this probe molecule.

Reorientation of the probe molecule. As discussed in the Introduction, the
reorientation dynamics of probe molecules in solution are often treated using the
modified DSE equation. This model is effective for cases where individual solvent
molecules are small relative to the reorienting moiety, but in systems such as the ones we
consider here, this approximation is clearly not valid. Alcohols are strongly associative
liquids and the probe molecule, in its dissociated form, is a monocation. For these
experimental conditions, it is not uncommon to find that the experimental reorientation
dynamics are substantially slower than those predicted by the model.13 We find that to be
the case for the data we present in Fig. 2.2. To our knowledge, there has not been a
universally-accepted explanation for this experimental condition, although it is clear that
slower than expected reorientation arises from strong solvent-solute interactions. In
principle, according to Eq. 2.1, this phenomenon could be explained by an increase in
viscosity, hydrodynamic volume or frictional coefficient, or an unexpected decrease in
the value of S, the shape factor. From Perrin’s equations,“5 the structure of R640 is found
to have a shape factor of ~0.9 and the steric price that would have to be paid to make the
shape factor ~0.2 would be prohibitive. A slower than expected reorientation time, if
viewed in the context of a change in viscosity would suggest an increase in local
viscosity, which is unlikely. The terms V and f are the variables most capable of
accounting for the experimental data and, within the framework of the DSE model,

values of f greater than unity are required to bring Eq. 2.1 into agreement with our data.

22

 

 

 

i”
x OH
- /
Z x O
\
Z
/
+ N o N
160 ’-\ /./
A 140
E
3 .2.
3
g 100
O
80

 

 

60 70 80 90 100 110 120 130 140
dihedral angle

Figure 2.1 - Structure of the Rhodamine 640 molecule, drawn as a free cation. Cartesian axes are
indicated, with the n-plane defining the x-y plane.

23

i

35m ' III f

NNUJ
§§§
"l'
.
\
\
\
\
I-I-l

15m ' I /’

p—A

reorientation time (ps)
Ur
c. 8 §

 

 

0 2 4 6 8 10

solvent viscosity (cP)

Figure 2.2 - Reorientation for R640 in the n-alcohols: (I) indicates experimental data for excitation
of the S; .— S. transition. The calculated DSE result for the stick limit (f = l) is shown as the solid
line and the calculated DSE result where V = V...“ + SVW‘ is indicated by the dashed line.

The most straightforward means to treat these data is based on the ansatz that the
reorienting moiety observed experimentally is actually larger than that predicted by
simple calculations of the hydrodynamic volume of the probe molecule. If this is the
case, there are two possible ways to account for this finding. The first is to consider that
the volume of the solute is larger (by a factor of two or more) than is calculated using
Edward’s model.” Intuition suggests that any increase of the solute volume is
impractical, and the accuracy of molecular volumes calculated by Edward’s method is

well established. The second possible explanation for the data is that there is close

24

solvent association with the solute.”’"’ While it would not be appropriate to use the term
“attachment” because of the implication of covalent bonds, strong association between
the solute and some (average) number of solvent molecules, can produce reasonable
agreement with the experimental data. The agreement between experiment and the
model is best when the term V = V501,... + 5Vsolm.. The viscosity-dependence of the
reorientation time calculated using Eq. 2.1 with this value of V is in phenomenological
agreement with the experimental data. Even better agreement between the model and
experiment can be achieved if we choose only four solvent molecules for the longest
alcohols but, given the level of detail contained in the experimental data, we are not
justified in attempting such an empirical fit. Intuition suggests that there are four sites on
the solute where polar solvent-solute interactions will be facile (Fig. 2.1), but the
characteristic lifetime of such interactions is not known with certainty. Regardless of the
exact number of solvent molecules closely associated with the solute, our reorientation
data point to there being strongest interaction between the solute and a single layer of
solvent molecules in the immediate proximity of the solute.

Effective rotor shape. The rotational motion of a molecule in solution is rarely
consistent with the behavior expected of a spherical particle. For R640, the reorientation
dynamics are not well characterized by modeling the effective rotor shape as a sphere.
As noted above, we calculate a shape factor, S = 0.9, indicating a non-spherical probe
shape. This value may be somewhat surprising given the way most rhodarrrine molecules
are drawn. The reason for S being close to unity is that the phenyl ring substituent does
not lie in the same plane as that of the fused ring structure. Semi-empirical calculations

(Fig. 2.1, bottom) indicate a broad energetic minimum for this ring rotation angle

25

between ~70° and 125°, with substantial steric barriers (not shown) for structures
approaching planarity.

To elucidate the intrinsically anisotropic nature of the molecular motion, Chuang
and Eisenthal derived a series of equations that relate the spectroscopic and dynamical

properties of the solute molecules to the form of the experimental data.47

I t - I 1‘

R0) = ..<) .(i
1,.(1) + 21,0)

iquzm. eer-(D. + 0):) +%qzq.nr. exp(—(D, + 0):) + (2.2)
136(fl+a)exp(—(6D+2A)t)+-l%(,B—a)exp(-(6D-2A)t)

 

= %q.q,7.ry exr)(-(D. + D)t) +

I"(t) and I l(t) are the experimental signal intensities for fluorescence emitted at
polarizations parallel and perpendicular, respectively, to the polarization of the excitation
light. The terms q and y are the Cartesian components of the unit vector describing the
orientation of the excited and emitting transition dipole moments of the probe molecule,
respectively. The terms Dx, DJ, and D, are the Cartesian components of the rotational
diffusion constant and A is the average of these terms. The term 13 is related to the
orientation of the transition moments and or is related to both the spectroscopic and

motional properties of the system.

1
D=§(D,,+Dy +Dz)

 

A = J(03 + 0; + 0,2 — 0,0, - 0,0z — 0,0,)

1
fl=fifi+fifi+fifi-§ cm
a = (D. / AXqiri + (137’? - 261.3163 + 13 + q3)+

(Dy / mm??? + 61373 - 24373 + 73 + q?) +
(Dz / Aqul’f + 61573 - 261373 + 7.3 + q?)- (2D/ A)

26

Using these equations, expressions for the induced orientational anisotropy, R(t),
can be derived for either an oblate or a prolate ellipsoid under a variety of conditions
relating to the orientations of the transition dipole moments. For the chromophore 7:
system used to define the x-y plane.

Oblate: Dz > D, = Dy

Prolate: Dx > Dy = Dz

There are several possible forms of the anisotropy decay and we summarize those
relevant to this study below. For the S; (— So transition, where the excited and emitting

transition moments are nominally parallel to one another, and the transitions are polarized

along the x axis, we obtain
R(t) = T15 exp(—(2D. + 4D. )t) +;%exp(—6D.t) (oblate) (2.4)

and

R(t) =fiexp(—6th) (prolate) (2.5)
Conversely, if the transitions accessed are short-axis (y) polarized,
R(t) =45 exp(-(4Dx +2DZ )t) +1—% exp(—6D,t) (prolate) (2.6)

and

R(t) =-;%€XP(—(4Dx +2001) (oblate) (2.7)

For excitation of the S; <— 80 transition in R640, we observe that the experimental R(t)
function decays as a single exponential (Fig. 2.4). In principle, distinguishing between a
one- and two-component decay can be difficult, however our experience from

reorientation measurements on other systems has shown that determining the difference

27

between one and two decay components is straightforward, both in terms of the time

constants and prefactors.”56

Sl - So excitation

         
 

S2 - S0 excitation

/

 

linear response (normalized)

 

250 300 350 400 450 500 550 600 650 700

wavelength (nm)

Figure 2.3 - Linear optical response of R640 in n-propanol. The spectra of R640 in other alcohols

are similar. Wavelengths of excitation are 280 and 580 nm respectively. Absorption (solid line) and
emission (dotted line) spectra are normalized for purposes of presentation.

Based on Eq. 2.4-2.7 we see, for a two-component decay, the prefactor for the minority
constituent is 25% of the total. The residuals of the fits to the data demonstrate good
agreement with a single exponential decay functionality (Fig. 2.4b inset). For systems of
high symmetry, it is possible to assign the orientation of the transition moments but for

systems such as the rhodamines, such an assignment is not possible with the requisite

28

certainty. Absent this knowledge, we are left with the ambiguous situation of not

knowing if our molecule is characterized by a long-axis polarized transition and reorients

as a prolate rotor or has a short-axis polarized transition and reorients as an oblate rotor.

0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05

R(t)

I

    
 
   

parallel

perpendicular

 

\ response

 

 

 

 

0 A 500 A 1000 A 15004 20004 2500 A 3000
time (ps)
f 0.02 b

I ' I ' I fl l

V t V

0.01
0.00 . . "
-001 ‘

  

 

 

2000

 

 

 

 

p-

A

1000 ‘ 1500 ‘ 2000 ‘ 2500 A 3000
time (ps)

0 ‘500

Figure 2.4 - (a) Experimental instrument response and Iu(t) and 1;,(t) data for R640 excited at 580 nm
(S; (— S.) in n-propanol. (b) Experimental anisotropy function generated from data presented in (a).
The best fit anisotropy decay function for this data set is R(t) = (0.29 :l: 0.01)exp(-tl(481 :r: 3 138)) The
residuals of the fit are shown in the inset.

29

There has been discussion in the literature of the orientation of the transition moments for
certain rhodamines,4 but this discussion was based on theoretical grounds and we are not
aware of direct experimental verification.

We do have a means to resolve this issue and, at the same time, to provide
unambiguous assignment of the transition polarizations in R640. For R640, as with many
optical chromophores, the 82 <— 80 transition is polarized nominally perpendicular to the
S; (— So transition. We show the absorption and crrrission Spectra of R640 in Fig. 2.3.

For this condition, the expected form of R(t) for a prolate rotor is
— -2
R(t) — lo exp[-6th] (2.8)

and for an oblate rotor,

R(t)=m€XP(-6D.I)-r%ew(-(2Q +4090 (2.9)

By exciting the S; (— So transition in R640 and monitoring emission from the 8; state, we
obtain an experimental R(t) function that decays as a single exponential (Fig. 2.5). As
predicted from Eq. 2.8 and 2.9, we observe a negative zero-time anisotropy, a
characteristic signature of transitions polarized nominally perpendicular to one another.
The functionality of this anisotropy decay and that of the S; excitation experiment
demonstrates unambiguously that R640 in the alcohols reorients as a prolate rotor.
Because both excitation schemes sense the dynamics of R640 in the same state (S;), then
only Eq. 2.5 and 2.8 can be appropriate for our data. Specifically, our observation of a
single exponential decay for S; excitation (Fig. 2.4) shows that either Eq. 2.5 or Eq. 2.7 is
consistent with our data and that excitation of the S; (— So transition (Fig. 2.5) produces a

result that is consistent only with Eq. 2.8. These two pieces of information, taken

30

together, show that R640 reorients as a prolate rotor. Thus the S; (— So transition in

R640 must be long-axis (x) polarized and the 82 (— So transition is short axis (y)

 

 

  

 

 

 

 

 

 

 

 

polarized.
\ ndicular
8000- , Per!”
6000-
a
g 4000-
§\ response
2000- '
0,;...,,,_gll
0 500 1000 1500 2000 2500 3000
time (p8)
b
111.111.11.111
01X) 1,, “'1'. ; "’v‘i’r'i'ii” " 1|” '1" I H l"! i
9 H" 0.02-
E W 0.01-
'0.05" odd-Ellisjii.illi,lv
°-°°~ r; 1' 1'1"; 1101'” rm
/ -0.01r
010 0.020 . 10.00 - 20.00 3000
" 0 500 1000 1500 2000 2500 3000

time (ps)

Figure 2.5 - (a) Experimental instrument response and Iu(t) and I ;(t) data for R640 excited at 290 nm
(S; (— S.) in n-propanol. (b) Experimental anisotropy function generated from data presented in (a).
The best fit anisotropy decay function for this data set is R(t) = (-0.09 :r: 0.01)exp(-tl(383 :1: 12 136)).
The residuals of the fit are shown in the inset.

31

Excitation-dependent reorientation dynamics. A closer examination of the
reorientation data provides even greater insight into the energy dissipation dynamics
operating in this system. A comparison of Eq. 2.5 and 2.8 reveals that the Cartesian

component of the rotational diffusion constant sensed by both measurements is the same;

 

the experimental time constantZ' = . This is not a surprising result, given the fact

1
6D,
that we are observing motional relaxation from the S; state in both cases and it is well
established that relaxation from the S2 to the S; manifold is fast for complex organic
molecules. We present the solvent-dependent reorientation times for excitation of both
the S; and S2 states in Fig. 2.6. These data demonstrate an interesting effect. For small
solvents, such as methanol and ethanol, and for large solvents, such as n-nonanol and n-
decanol, we observe the same experimental reorientation times for excitation of either S;
or S2. For R640 in the intermediate solvents n-propanol through n-octanol, however, we
measure reorientation times for S; excitation that are faster than those for S; excitation.
This result is reproducible and appears to be inconsistent with the theory put forth in Eq.
2.2 — 2.9. Because we are measuring the dynamics of the same state of R640 for both
sets of data, the volume and shape of the probe as well as the solvent-solute boundary
condition must be the same for both sets of experiments. The only quantities that could
change as a result of the different excitation conditions are the temperature and the

viscosity of the environment in the immediate proximity of the probe molecule. We

believe that both of these quantities change and discuss our basis for this assertion below.

32

0 S1 excitation
I S2 excitation

 

 

m
PO 0 I
I

1 - e

. I

. i
O ' 1. l 1 L 1 l 1 l 1 l n
O 2 4 6 8 10
solvent viscosity (CP)

Figure 2.6 - Reorientation time dependence on solvent viscosity for excitation of the S; (— S.
transition (0) and the S; e— 8. transition (I).

The only difference between the two series of experiments lies in the excitation
wavelengths used (580 nm vs. 290 nm). Emission from the sample is identical for the
two excitation conditions and thus the difference between the two experiments is the
energy dissipated nonradiatively into the solvent bath. For excitation of the S; (— 8;;

transition, the Stokes shift is modest and the difference between excitation and

33

observation wavelengths is on the order of 30 nm. Excitation at 580 nm is close to the
origin for this band (Fig. 2.3). For excitation of the S2 (— So transition, the R640
molecule dissipates ~ 2.1 eV of energy by nonradiative relaxation from S2 to 8;, an
amount of energy sufficient to cause local heating. Such a phenomenon would give rise
to a reduction in the viscosity and through this change, the reorientation time constant of
the solute will decrease. Based on the magnitude of the decrease in reorientation time for
excitation of the S2 state, we can estimate the change in temperature using the following
strategy. From Eq. 2.1, we can relate the state-dependence of the reorientation time to
the transient change in viscosity. The bulk viscosity of liquids can be modeled
phenomenologically and from this model we can relate the change in viscosity inferred
from the reorientation data to the change in temperature sensed by the experimental
measurement. Using the phenomenological change in temperature and knowing the
amount of energy dissipated, we can estimate the effective distance over which
temperature gradient persists. We describe each step in this process below.

Relating the change in reorientation time to the bulk solvent viscosity is

accomplished through the modified DSE equation.

At =A[.’Z) if- z m (2.10)
0” T k,s kBTS

In this model, both the viscosity and the temperature are subject to change. We assert
that the dominant contribution to Aron is An and not AT. The reorientation time changes
at most ~15% while the temperature change consistent with this result is on the order of a
3% increase. Because the bulk viscosity depends sensitively on the temperature, it is At]

that is primarily responsible for ATOR.

34

The bulk viscosity of liquids is well understood and has been modeled accurately
in a number of different ways. The origin of the phenomenon of viscosity is the
interaction between molecules as they translate past one another in solution. Attractive
interactions between molecules impede their flow, and the energy required for this action
to occur is termed the viscous flow energy. Solution viscosity is typically treated in the
context of an activated process and, as such, it has a predictable temperature dependence.
The details of the temperature dependence are determined by the viscous flow energy and
the prefactor in an Arrhenius expression. Because this temperature dependence is
different for each liquid, the most common approach is to characterize this functionality
using a parameterized fit. One such expression of the relationship between t] and T is,57

T=fi—qB+C (2.11)
77

where A, B and C are parameters used to match the experimental viscosity temperature
dependence to the model. From Eq. 2.11, an empirical expression for the temperature

dependence of the viscosity can be derived.

2

£17. :___:_’_7____
(IT To A+nzB

(2.12)
where To is the ambient temperature of the bulk liquid, We use the published parameters

A and B to determine the quantity dn/dT. With this information and the quantity An (Eq.
2.10), we can estimate AT (Table 2.1). The quantity AT is the estimate of transient

heating that gives rise to the state-dependent reorientation dynamics we observe

experimentally.

35

Table 2.1 - Experimental Reorientation Times and Changes in Solution Viscosity and Temperature

Associated with Internal Conversion of R640.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Solvent TOR (Sr) TOR (32) ATOR (32 ' 31) ATI (GP) AT (K)
(98) (ps) (ps)

Methanol 134 :1: 3 146 2t 14 12 :1: 10 0.07 :l: 0.06 -8.59 :l: 7.36

Ethanol 254 :l: 5 209 :t 23 -45 :1: 17 -0.23 i 0.09 11.9 :1: 4.66
l-propanol 449 :1: 49 393 :l: 21 -56 :1: 38 -0.26 i 0.17 6.30 :l: 4.11
l-butanol 709 :1: 9 507 :t 61 -202 :l: 44 -0.84 :t 0.18 12.6 :t 2.69
l-pentanol 958 :1: 2 755 :1: 52 ~203 :l: 37 -0.77 :l: 0.14 7.50 d: 1.36
l-hexanol 1499 :l: 23 1223 :l: 26 -276 :i: 25 0.96 :1: 0.09 6.60 :1: 0.62
l-heptanol 1973 :l: 47 1635 :l: 23 -338 :l: 37 ~1.09 :l: 0.12 5.74 :l: 0.63
l-octanol 2280 :1: 54 1993 :1: 47 -287 :1: 51 -0.86 :1: 0.15 3.77 :t 0.66
l-nonanol 2580 :l: 102 2334 :1: 304 -246 :t 227 -0.70 :1: 0.65 2.71 :i: 2.51
l-decanol 3439 :1: 241 3212 :l: 358 -227 :1: 305 -0.59 :i: 0.80 2.11 :1: 2.86

 

At this point we consider the significance of the temperature jump we extract

 

from the experimental data. The excess energy deposited into the system is simply the
difference in energy between the excitation energies used to pump the S2 (— 8;; transition
(290 nm) and the S; (— So transition (580 nm). We calculate this energy difference to be
2.13 eV. This heating pulse occurs during the time required for the population to relax
from the S2 to S; states and this time is determined by the intramolecular relaxation time
or the duration of the excitation pulse, whichever is longer. Because of the
characteristically fast kinetics of internal conversion, we believe the time duration of the
excitation pulse (5 ps) is limiting in this case. The heat pulse associated with internal
conversion of R640 is related to the temperature gradient in the system according to Eq.

2.13.58

(2.13)

where K is the thermal conductivity of the bath, K = nCv, A is the “area” of the radiator,
which we take as the area of the chromophore 7: system, and dT/dr is the temperature

gradient in solution that is induced by dissipation of the excess excitation energy. We

36

take the heat pulse, q/t as the energy dissipated over the duration of the excitation pulse,
q/t as the energy dissipated over the duration of the excitation pulse, q/t = 2.13 eV/5 ps =
68 nW. The area of the radiator is that of the chromophore rt-system, and we
approximate the dimensions as 12 A x 14 A = 168 A2. The thermal conductivities of the
bulk solvents are determined by their viscosity and heat capacity, and both of these
quantities vary in a regular manner with solvent aliphatic chain length (Table 2.2). From
these data we calculate dT/dr and, using the phenomenological AT determined from the '
reorientation experiments, we can estimate Ar (Table 2.2). With the value of Ar, we
calculate the volume for the corresponding sphere and compare these results to the values
of the hydrodynamic volume of the reorienting moiety consistent with the experimental
data (Fig. 2.2, Table 2.2). What is immediately apparent from a comparison of the
thermal gradient and hydrodynamic volume data is that, for low and high viscosity
solvents, there is not a correspondence, while in the intermediate region, the agreement is
reasonable. Where there is a correspondence, we note the similarity of the two volumes.
It thus appears that the temperature gradient exists largely over a distance consistent with
a single layer of solvent molecules. Those molecules in closest proximity to the
chromophore will be the primary recipients of the excess energy. Solvent molecules
beyond the solvent cage in the immediate proximity of the probe molecule are coupled
strongly to the solvent bath and are thus characterized by the bulk temperature of the

system. The solvents that deviate from this model do so for reasons that we understand.

37

Table 2.2 - Solvent Properties and Quantities Extracted from Transient Heating Calculations. The

quantity Vw is extracted from Ar and Van. = V“... + Vm.” where these volumes are determined
using Edward’s method.55 Some values (‘) of C. are interpolated from literature data for the other

alcohols.

 

 

 

 

 

 

 

 

 

 

 

Solvent r; (cP) Cv (J/mol K) x (J/m s K) Ar (A) V.;...“ (A3) Vak (A3)
Methanol 0.576 73.2 1.32 -1.2 1 1.0 -- 624

Ethanol 1.032 104.0 2.28 2.8 1 1.1 92 709
l-propanol 1.796 135.6 3.92 2.6 1 1.7 74 794
1-butanol 2.377 168.8 5.20 6.9 1 1.5 1376 879
l-pentanol 3.160 200.7 6.88 5.4 1 1.0 660 964
l-hexanol 4.146 233.0 9.01 6.2 1 0.6 998 1049
l-heptanol 5.427 255.4" 11.3 6.8 1 0.8 1317 1134
l-octanol 6.878 276.2 13.9 5.5 1 1.0 697 1219
l-nonanol 8.685 3150* 18.0 5.3 1 4.8 623 1304
l-decanol 10.524 3448* 21.7 4.8 1 6.5 -- 1389

 

 

 

 

 

 

 

 

 

It is well established that the n-alcohols are characterized by long dielectric
relaxation times, ranging from <100 ps for methanol to ~ 2 ns for n-decanol.59 This
anomalous property of the alcohols is attributed to the extensive H-bonded network these
solvents form and it is this relaxation time that is related to solvent molecule exchange in
the solvent cage surrounding the solute. In order to detect the thermal relaxation event, it
is necessary to use a detection time window less than or on the order of the dielectric
relaxation time. For the solvents methanol, ethanol and l-propanol, we observe little or
no excitation energy dependence to the reorientation time. We assert that the reason for
this behavior is that exchange of solvent molecules in the solvent cage is fast relative to
the time constant for reorientation, the effective time window over which we sense the
transient temperature change. Thus, the solvent molecules heated initially by
nonradiative transfer are lost to the bath rapidly and the temperature of the average
solvent cage sensed by the reorienting moiety is close to that of the bulk solvent.

We also observe no excitation energy-dependence to the reorientation times for

the longest chain solvents, n-nonanol and n-decanol. We believe that this finding is due

38

to two factors. First, the thermal conductivity of the n-alcohols increases with solvent
aliphatic chain length because of the increased degrees of freedom in these molecules.
Thus the energy transfer to the bath mode is most efficient for these solvents. The second
factor is that the temperature we sense is averaged over the solute reorientation time. The
reorientation of R640 is slowest in these solvents, affording the greatest time for
dissipation of energy within the window of observation.

We note that we are not the first group to conclude that the transfer of
(vibrational) energy from solute to the immediate solvent environment is facile. Iwata
and Hamaguchi have found evidence from transient S; Raman spectra of stilbene in
selected solvents that the transfer of vibrational energy between the solute and the first
solvent sphere proceeds over a timescale that depends on solvent, but that is similar to the
reorientation time constant of stilbene in these systemsmz Subsequent solvent-solvent
energy transfer appears to proceed more slowly. Our findings are in excellent qualitative
agreement with that work.

Conclusions

We have studied the reorientation behavior of the polar probe molecule R640 in
the series of alcohols methanol through n-decanol. By measuring the reorientation
dynamics after exciting to two different excited electronic states, we have determined
unambiguously that this molecule reorients as a prolate rotor in these solvents. We have
also determined that the S; (— So transition is long-axis (x) polarized and the S2 (— 8;;
transition is short-axis (y) polarized for R640. Modeling these data in the context of the
modified DSE model indicates that the interactions between solvent and solute are

stronger than predicted by this model. The “super-stick” behavior seen for R640 in the n-

39

alcohols is consistent with that seen for other charged dye molecules in polar solvents and
we can model these data in the context of close solute association with four to five
solvent molecules. We take this finding to indicate that the reorienting moiety we
observe in solution is a solvent-solute complex where the lifetime of the solvent-solute
interactions are on the same order as the reorientation time.

Comparing the results of the reorientation measurements for the two different
excitation conditions reveals the effects of transient heating in such measurements. We
have found that the transient change in temperature associated with non-radiative
relaxation from the S2 to S; electronic states in R640 is on the order of 10 K. We observe
this transient temperature change with the relaxation time of the thermal gradient to be on
the same order as the reorientation time of the probe molecule and whenever this
condition is not obtained, the thermal gradient is not manifest in our experimental data.
This thermal gradient generated by excitation to the S2 state appears to be dissipated over

approximately one solvent shell.

40

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43

Chapter 3.

The Influence of Chromophore Structure on Intermolecular Interactions. A Study of

Selected Rhodamines in Polar Protic and Aprotic Solvents.

Introduction

The interactions between molecules determine both the microscopic and bulk
properties of essentially all solution phase systems. Achieving a detailed understanding
of these interactions has proven to be an elusive task because of their strength and their
characteristically short persistence time. The timescale of solvation phenomena in polar
liquids is considered to be on the order of the longitudinal relaxation time of the solvent,
and can range from ~ 100 fs for polar aprotic solvents to hundreds of picoseconds for
high viscosity solvents characterized by strong hydrogen bonding."2 The process of
measuring such phenomena usually requires the use of a chromophore that can be
accessed using short light pulses, and the properties of the chromophore can have a
substantial influence on the information obtained from the experiment. Owing to the
complexity of these systems, much information in the literature must be considered valid
for only a specific chromophore or, at best, a limited number of structurally similar
systems}1 I

The Rhodamines are a family of molecules that have been used extensively as
dyes for fabrics, biological stains, water markers, probes for studies of molecular-scale
processes in condensed phases and at interfaces, probes for host-guest interactions, and as
laser dyes.”22 The utility of this family of dye molecules stems from their

characteristically strong absorption at visible wavelengths, relatively high fluorescence

quantum yields and several-nanosecond fluorescence lifetimes. There are a substantial

44

number of rhodamines,12 with most of the structural variations being made with the intent
of controlling optical properties such as absorption energy or fluorescence lifetime.
While the rhodamines are versatile probe molecules, their structural features serve to
complicate the interpretation of experimental data in many cases. For example, several
rhodamines possess a pendant o-substituted phenyl ring, and both the substituents and
ring orientation relative to the chromophore plane can influence the motional and
spectroscopic properties of the molecule. The extent of structural freedom remains
largely undetermined for this ring, as does its role in mediating excited state relaxation
and motional properties.

We are interested in understanding the relationship between rhodamine structure,
steady state spectroscopy and solution phase dynamics. It is our ultimate intent to use
this family of molecules in interfacial and other sterically restricted environments, such
as lipid bilayers and micelles. A prerequisite for working in such environments is the
development of an understanding of the role of chromophore structure on the
experimental data we will acquire. For this reason we have undertaken a study of several
rhodamines in selected polar protic and aprotic solvents.

This study describes the steady state spectroscopy and orientational relaxation
dynamics of S substituted rhodarrrines (Figure 3.1). Our data, in conjunction with semi-
empirical calculations, indicate that, for several of these rhodamines, dipolar solvent-
solute interactions play at least as significant a role as hydrogen-bonding interactions in
determining reorientation dynamics, despite the significantly different timescales that are

characteristic for these interactions.

45

 

R610 lactone R700

 

Figure 3.1 - Structures of the five rhodamines studied here.

46

Experimental Section

Pump-Probe Laser System. The picosecond pump-probe laser spectrometer used
in these reorientation measurements has been described in detail previously,23 and we
present only a brief synopsis of its operation here. A mode-locked CW Nd:YAG laser
(Coherent Antares 76-S) produces 30 W of average power (1064 nm, 100 ps pulses, 76
MHz repetition rate). The output of this laser is frequency-doubled to produce ~3 W of
average power at 532 nm. The second harmonic light is used to excite two cavity-
dumped dye lasers (Coherent 702) synchronously, with the output of both lasers being
~70 mW average power at 8 MHz repetition rate, producing ~7 ps fwhm autocorrelation
trace using a three plate birefringent filter. The pump dye laser was operated using
Pyromethine 567 dye (Exciton) at 549 nm (for R610 and ring-opened R610 lactone
experiments) and 564 nm (for R640 experiments). For experiments on R700 and R800,
the pump laser was operated using LDS698 dye (Exciton) (645 nm for R700, 682 nm for
R800). The probe laser was operated with Rhodamine 6G dye (Kodak) at 564 nm (for
R610 and ring-opened R610 lactone experiments) and 580 nm (for R640 experiments).
Kiton Red dye (Exciton) was used in the probe laser for R700 and R800 chromophores
(615 nm for R700, 622 nm for R800). The pump wavelength and probe wavelengths

were chosen to access the S; <— So transition of each chromophore to detect ground state
population recovery. The probe laser polarization was set to 0' and 90‘ relative to the
pump laser polarization for individual scans of parallel and perpendicular polarization
used in acquiring orientational relaxation data. The time resolution of this system, ~10
ps, is determined by the cross-correlation of the pump and probe laser pulses. Detection

of the transient signals was accomplished using a radio- and audio-frequency triple-

47

modulation scheme, with synchronous demodulation detection.”26 The reorientation

time constants we report here are the average of 6 individual determinations, each
comprised of the average of 10 - 12 sets of I"(t) and I ;_(t) scans.

Steady-Sate Spectroscopy. The steady-state absorption spectra of the
chromophores used here were recorded with 1 nm resolution using a Cary 300 Bio UV-
visible Spectrophotometer. The spontaneous emission spectra for the same solutions were
obtained with 1 nm resolution using a SPEX Fluorolog-3 spectrometer. These data were
compared to semi-empirical computational results (Hyperchem® v. 6.0) and were used to
determine the appropriate pump and probe wavelengths for each chromophore I solvent
combination.

Chemicals and Sample Handling. The probe molecules Rhodamine 610 (R610),
Rhodamine 640 (R640), Rhodamine 700 (R700), and Rhodamine 800 (R800) were
obtained from Exciton Chemical Co. and used as received. Rhodamine 6101actone
(R6101actone) was obtained from Aldrich Chemical Co. and was used as received. All
solvents except water were obtained from Aldrich (99+% purity) and were used without
further purification. Distilled, deionized water was available in-house. The
concentration of all solutions used for laser measurements was 10 11M. Measurements of
R700 solutions were hampered by bleaching of the solution. This effect was mitigated by
replacement with fresh solution on at least a daily basis, or as warranted. All
measurements were taken using a flowcell with a 1 mm pathlength. Solution

temperatures were maintained at 298 1 0.5 K (Neslab EXIOO-DD).

48

Results and Discussion

We are interested in using Rhodamines as probes of their local environment in a
number of systems, such as bulk liquids, micelles, polymer matrices and interfacial
adlayers. To make use of these chromophores, we must first understand their optical
properties and the relationship(s) between chromophore structure and intermolecular
interactions. We consider first the structure-dependence of the linear optical response of
rhodamines, with an eye toward understanding experimental trends in the data. With that
understanding in place, we will consider the reorientation dynamics of several
rhodamines in polar protic and aprotic solvents. Our data point to the importance of
dipolar solvent-solute interactions, a result that is unusual in comparison to the behavior
of a variety of other chromophores.

Linear optical response. The normalized absorption and emission spectra of the
chromophores studied here are shown in Figures 3.2 — 3.4. The absorption and emission
spectra of R610 and R640 (Figure 3.2) are blue-shifted significantly from the spectra of
R700 and R800, (Figure 3.3) and the experimental trend is mirrored in the results of
semi-empirical calculations performed with a PM-3 parameterization (Figure 3.5).”30
As has been indicated by Drexhage,22 the o-benzoate ring at the 1 position of R610 and
R640 is effectively decoupled from the 3-ring chromophore system. For R800, we
understand the red-shift in the context of an effective increase in the extent of the rt-
system to include the -CEN group. R700 is red-shifted from R610 and R640 because the
fluorine 3p atomic orbitals on the -CF3 group at the 1 position can interact with the

chromophore rt-system.3"33 The rt-interaction with the F 3p atomic orbitals is not on the

same order as that for the -CEN rt orbitals of R800, and the spectral response of R700 is

49

not shifted as far to the red. The linear optical response of R610 lactone is qualitatively
different from that of the other rhodamines by virtue of the presence of the lactone ring.
”'38 Closure of the lactone ring at the 1 position breaks the conjugation of the
chromophore system, giving rise to an absorption spectrum that is blue Shifted by ~200
nm relative to R610 (Figure 3.4). In protic solvents, such as the primary n-alcohols used
in this study, hydrolytic ester formation proceeds efficiently with the resultant formation
of the rhodanrine chromophore, and a steady state optical response identical to R610
(Figure 3.3). This is an expected and well-documented result.”3942 With this
understanding of the steady state optical properties of the rhodamines, we turn to an
examination of the dynamical behavior of these chromophores.

Time resolved spectroscopy. Conversion of the experimental data (1"(t) and 12(0)

to the induced orientation anisotropy function, R(t), is accomplished using Eq. 3.1.

1.,(1) — 11(1)

1,,(1) + 21,0) (3'1)

R(t) =

 

The functionality of the R(t) decay provides important information on the dynamics of
the chromophores. In this work, we find that all of the chromophores exhibit single
exponential anisotropy decays and the S; (— So transitions accessed spectroscopically are
oriented along the rhodamine chromophore long axis. These pieces of information, taken
together, indicate that all of the rhodamine probe molecules we use reorient as prolate
rotors in the solvents studied. Because of the observed single exponential decay
functionality, we can interpret our data in the context of the modified DSE equation (Eq.

3.2).

50

 

450 500 550 600 650 700

linear response (normalized)

 

 

L a l a l 1 l a l 1 I

450 500 550 600 650 700

wavelength (nm)

Figure 3.2 - Absorption and emission spectra of (a) R610, (b) R640, in methanol.

51

 

 

l L J 1 l

1 1 1
500 550 600 650 700 750 800

linear response (normalized)

 

 

L l 1 l l 1 l

l 1 1 l 1
500 550 600 650 700 750 800

wavelength (nrn)

 

Figure 3.3 - Absorption and emission spectra of (a) R700, (b) R800, in n-propanol.

52

"'
""'
I---
'8'-"""
8",
"'
8'
'
’
I

 

J

 

 

 

 

 

 

A38: .253 8:90QO

350 400 450 500 550 600

250 300

wavelength (nm)

Figure 3.4 - Absorption spectra of R610 lactone (solid line) in acetonitrile and ring-opened R610

lactone (dashed line) in methanol.

53

i

i

energy relative to S0 (cm'l)

 

 

 

 

 

 

 

 

 

 

 

lactone methyl ester

1- T5

S.f=0.02
C “"1
. s3 f=0.25
I —Szfi-’0.0l
.- T3
: T2 s.f=0.04
_ S.f-}0.001 Ts
t 5_.sz=9-8,53 8113253 81250 T.
P T4 T —
+ T5
- Szf=0.07 S2f=0.07 s3 #0001
P
1 T; T _T§_
: 821211.006
. T3
: 13.12170 :S;f=1.73§r"""":
_ """”“ES;f=1.47:
' T
; T2 T2
1 T2
: Tl T;
P T
' R610 R610 R610 R640

 

T6 T

T5 T5
Saf=0-003 33 120.02

 

T
JA— —“—

 

ST 9002 321:0.002
T T

 

r ..
I
l
l

.Sif-l-49i lS;f=l.42

—————————
---------

 

 

T3 T2
T
T —
R700 R800

Figure 3.5 - Calculated state energy levels for the rhodamines studied here. Calculations were
performed at the semi-empirical level with PM-3 parameterization. State energies and oscillator
strengths for S. (— S. transitions were calculated using configuration interaction.

54

For all data, the reported zero-time anisotropies are derived from the regression of the
data for times greater than 15 ps after excitation, and the uncertainties reported for each
quantity are 95 % confidence intervals for six or more individual determinations. We
report the quantities R(0) and to); in Table 3.1 for the several rhodanrines and solvents

used.

Table 3.1a. Reorientation Times and Zero-Time Anisotropies for R610, R640 and R610 lactone in

 

 

 

 

 

 

 

 

 

 

 

 

 

rotic and a rotic solvents.
. . Ring-opened Rhodarrrine
Rhodamrne 610' Rhodamine 640‘ 610 lactone ., b
SOIVCI‘II R(O) 10L R(O) TQB R(O) TQB
Acetonitrile 0.261002 5713 0.251005 5013 - -
Methanol 0.261003 8716 0.241002 6812 0.291004 9515
DMF 0.261003 13815 0.281002 12512 - -
Water 0.231003 14116 0.241006 12113 0.21001 129110
Ethanol 0.291003 14314 0.251002 13516 0.221001 15419
n-Propanol 0.271001 22915 0.251002 227112 0.271001 310111
DMSO 0.321003 271115 0.241004 264118 - -

 

 

 

 

 

' The data are the best fit results of the data to the function R(t) = R(0)exp(-t/to;;). Time constants are in ps
and the uncertainties are standard deviations (116) for at least six determinations of each quantity.
b Some data for R610 lactone was not obtained. See text for explanation.

Table 3.1b - Reorientation Times and Zero-Time Anisotropies for R700 and R800 in protic and
aprotic solvents.

 

 

 

 

 

 

 

 

 

 

Rhodamine 7001 Rhodamine 800a
SOIVCDI R(O) T03; R(O) TOR—.—
Acetonitrile 0.201001 4211 0.221002 47 1 5
Methanol 0.171002 4814 0.221002 50 1 4
DMF 0.211001 9717 0.271001 101 :l: 7
Water 0.261002 110110 0.251002 122 :1: ll
Ethanol 0.23:1:001 8913 0.241004 94 :l: 6
n-Propanol 0.191002 15216 0.221004 156 :1: 7
DMSO 0.261003 213115 0.241002 217 :1: 10

 

 

 

 

 

 

' The data are the best fit results of the data to the function R(t) = R(0)exp(-tlto;;). Time constants are in ps
and the uncertainties are standard deviations (110) for at least six determinations of each quantity.

The intermolecular interactions of most interest to us are between polar organic
chromophores and associative solvents and systems that can exhibit spontaneous self-

assembly henomena.4348 For such studies, some spectroscopic property of the
P

55

chromophore is monitored as a function of a systematic change in the system, such as
viscosity or polarity.”53 In many cases, collecting information in the time domain is
most instructive, with orientational relaxation measurements being perhaps the most
information-rich for our purposes.

Debye-Stokes-Einstein equation. The starting point for the interpretation of
molecular reorientation measurements is usually the modified Debye-Stokes-Einstein

(DSE) equation.”56

 

’IVf
= 3.2
Ton kBTS ( )

This simple model has proven to be remarkably successful in providing at least a semi-
quantitative model for the rotational motion of molecules in solution. In this model, 7) is
the solvent bulk viscosity, V is the solute hydrodynamic volume,” and the terms f and S
are frictional boundary condition56 and molecular shape factors, respectively.” The
frictional boundary condition and molecular shape factors arise from modifications of the
original theory, which described the reorientation of spheroidal particles in a microwave
cavity. The DSE model assumes a continuum solvent and it has been shown to model
reorientation data quantitatively in the limit that the individual solvent molecules are
smaller than the solute.“3“‘5‘5 ”8'68 Discrepancies between the modified DSE model and
experimental data are often expressed in terms of the molecular-level breakdown of the
notion of viscosity or in terms of variations in the friction coefficient, f, that are related to
the interactions between solvent and solute molecules. The main limitation of the DSE
model in the prediction of solute reorientation lies with the molecular-scale definition of
the solute-solvent boundary condition. Indeed, this model treats the solvent as a

continuum and is thus incapable of accounting for specific chemical interactions. For

56

solvent-solute interactions with a substantial frictional component, the soocalled “stick

3 used,5'7'5 '53'66'69'72 and for cases where frictional interactions are expected to be

limit” i
less significant, e. g. reorientation of nonpolar solutes in nonpolar solvents, the “slip” limit
is used. In this limit, f can range in value from 0 to 1, with its exact value being
determined by the effective shape of the volume swept out by the reorienting molecule.
For some ionic solutes in polar solvents, the reorientation times measured experimentally
exceed those predicted by the modified DSE equation in the stick limit, and such systems
are termed “super-stick”. The central issue for such systems is determining the dominant
contribution(s) to the solvent-solute boundary condition, with dielectric friction73 and
hydrogen bondingw'“ being proposed to account for the experimental data.

The data we present in this paper are predicted reasonably well by stick limit
reorientation, save for one case. The ring-opened form of R610 lactone reorients more
slowly than R610 and we understand this behavior based on the formation of an alkyl
ester by reaction of the R610 lactone with the alcohol solvent (vide infra). We find that
the reorientation of rhodamines in polar aprotic solvents is consistent with stick or
slightly “super stick” limit behavior, while the reorientation of these same chromophores
in alcohols is slightly sub-stick. We examine these data in terms of solvent-solute
interactions and the internal structural freedom available to the chromophores, an effect
which can influence the effective rotor shape of the reorienting moiety and, under some
circumstances, can affect its apparent hydrodynamic volume.47

Among the factors that can contribute to stick or super-stick reorientation

behavior is dielectric friction.“‘°9'7°‘73‘76 Dielectric friction impedes rotational motion of

molecules in solution based on the strength of dipole-induced dipole interactions. The

S7

transient polarization of the solution arising spontaneously from solvent molecule
fluctuations will couple to the solute permanent dipole moment, with the strength of
coupling scaling as r‘. The interaction of the solvent fluctuations with the solute dipole
moment induces a torque on the rotating molecule that acts to slow its motion. This
contribution to the solvent-solute boundary condition is expected to operate in addition to
the frictional terms considered in the DSE treatment, and several methods have been used
to evaluate the contribution of dielectric friction to solute reorientation.65’73‘74'77 For the
rhodamines and solvents we use here, this effect is found to be a minor factor relative to
frictional interactions (see Table 3.2).

In protic solvents, both solvent-solvent and solvent-solute hydrogen bonding can
play a significant role in mediating solute motion. The characteristic persistence time of
these interactions in long-chain alkanols is similar to the reorientation times seen for
polar solutes in the size range of the rhodamines.60 Hydrogen bonding interactions are
known to play a significant role in determining solvent bulk viscosity and the Blanchard
group has reported previously on evidence for strong solvent-solute hydrogen bonding
interactions in alcohols and their effect on the reorientation dynamics observed for these
systems.“3‘6°‘6| In cases where H-bonding can play a role, the interaction is typically
between the solvent alcohol proton and a specific functionality on the solute, such as an
amine group. These interactions persist for sufficient time, on average, that the effect is
to increase the hydrodynamic volume of the reorienting moiety (solute plus attached
solvent), an effect that can be modeled effectively within the framework of the modified

DSE equation.

58

Table 3.2. Calculated dielectric friction times for Rhodamine 610 (R610) and 640 (R640). The times
reported are intended to be additive contributions to the reorientation time calculated using the
modified DSE model.

 

 

 

 

 

 

 

 

 

Solvent R610 (ps)”| R640 (ps)a
Acetonitrile 0.1 0.1
Methanol 0.9 0.8
DMF 0.4 0.3
Water 0. 1 0.0
Ethanol 2.2 2. 1
n-Propanol 10.9 10.2
DMSO 0.2 0.2

 

 

 

 

' Values are calculated from Nee and Zwanzig, reference 73. Some values necessary for completion of the
calculation are taken from Table 3.4 and references contained therein.

Reorientation of R61 0, R640 and R6101actone. We show the solvent viscosity-
dependence of R610 and R640 reorientation in Figure 3.6, along with the dependence
predicted from Eq. 3.2 (lines). The data show that, with minor exceptions, both R610 and
R640 reorient in the DSE stick limit, for both polar protic and aprotic solvents. Using the
finding that the rhodamines reorient as prolate rotors, we calculate a shape factor of S =
0.9 for both R610 and R640 using Perrin’s equations.”’56 The reorientation dynamics of
these two rhodamines are expected to be similar based on their hydrodynamic volumes,
418 A3 for R610 and 443 A3 for R640 (see Table 3.3).57 It is well established that polar
solutes such as rhodamines reorient more slowly in polar protic solvents than the DSE
model would predict while in polar aprotic solvents, they are modeled well by the
(modified) DSE equation.5°'58"““58 This trend can be understood to a significant extent
based on the characteristic persistence time of solvent-solute interactions. This
interaction time has been shown to correlate with the longitudinal relaxation time, 12L, of
the solvent, at least for polar aprotics."3'62 The longitudinal relaxation times for the

solvents used here (TL, Table 3.4) show that the dielectric relaxation times for the low

59

viscosity protic and aprotic solvents are very similar and all are fast on the timescale of

reorientation.

Table 3.3. Calculated molecular volumes and S0 permanent dipole moments for the rhodamines
examined here. Volumes were calculated from Reference 57. Dipole moments were calculated using
Hyperchem® v. 6.0 with PM-3 mrameterization.

 

 

 

 

 

 

 

Probe Volume (.313) [L (D)
R610 418 2.7
R640 443 2.6
R610 lactone 415 6.7
R700 375 5.7
R800 364 3.7

 

 

 

 

Thus, the results for this system are substantially in keeping with the DSE model and
there is little distinction between reorientation in polar protic and aprotic solvents because
the reorientation time constants for the two chromophores are much longer than the
longitudinal relaxation times for the solvents studied. Figure 3.4 shows the absorption
spectra of both the lactone and ring-opened forms of R610 lactone. R610 lactone
undergoes a ring-opening reaction in alcohols to form the alkyl ester of R610. This
reaction does not proceed in polar aprotic solvents and the reorientation data are available
only for the ring-opened R610 lactone only in alcohols. We show these data in Figure
3.7, along with the reorientation data for R610 for comparison. The reorientation of the
ring-opened R610 lactone and R610 are measurably different and we ascribe this
difference to the added volume of the alkyl ester. We note that the addition of the
aliphatic tail to the R610 chromophore yields reorientation behavior that is consistent
with the “super-stick” limit. We believe that this phenomenon is the result of relatively
strong interactions between the (alkyl) ester functionality and the solvent. This argument

is equivalent to the assertion that the interactions between the R610 aliphatic ester moiety

60

and the surrounding solvent is of the same order as solvent-solvent interactions that give

rise to the relatively high viscosity of the alkanols.

Table 3.4. Selected properties for the solvents used.

 

 

 

 

 

 

 

 

 

Solvent “:3?” ‘13:? pl (D) to' (ps) - n so IL” (ps)
Acetonitrile 0.369 47.1 3.92 3.9 1.34 37.5 0.2
Methanol 0.544 36.1 1.70 55.6 1.328 32.6 3.0
DMF 0.794 76.6 3.82 27.4 1.43 36.7 1.5
Water 0.89 20.6 1.85 8.2 1.333 78.5 0.2
Ethanol 1.074 53. l 1.69 105 1.361 24.3 8.0
n-Propanol 1.945 70.1 1.68 435 1.386 20.1 41.6
DMSO 1.987 76.8 3.96 20.6 1.478 4.7 9.6

 

 

 

 

 

 

 

 

' Values for “ID are from reference 45. Blanchard, G. J. J. Chem. Phys. 1991, 95, 6317—6325. and
references therein. Volumes are calculated from reference 50. Tao, T. Biopolymers 1969, 8, 609-
632. Viscosity values are from references 60. Blanchard, G. J. J. Phys. Chem. 1988, 92, 6303-
6307. and 78. Handbook of Chemistry and Physics; 82 ed.; Lide, D. R., Ed.; CRC Press: Boca
Raton, 2001. Solvent dipole moments (vapor phase) are taken from reference 78. Handbook of
Chemistry and Physics; 82 ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, 2001.
b Tl. = (Eu/801130. Em = n2

Reorientation of 700 and R800. The reorientation dynamics of R700 and R800
are shown in Figure 3.8 as a function of solvent viscosity, along with the DSE
predictions. Included with the data are two lines, the upper being the DSE predicted
reorientation in the “stick” limit and the lower the DSE predicted “slip” limit. These
values of the boundary conditions were obtained using the “stick” limit (f = l) or the
calculated “slip” limit (f ~ 0.1) for a prolate rotor with shape factors S = 0.81 (R700) and
0.84 (R800).56 The shape factor was calculated according to Perrin55 and the slip limit
frictional term was determined using the method of Hu and Zwanzig.56 The reorientation
data for these two molecules show a clear distinction between polar protic and aprotic
solvents.
In the solvents methanol, ethanol and propanol, both R700 and R800 exhibit slightly sub-

stick reorientation times while in water and the polar aprotic solvents, we observe stick-

lirnit behavior. The reorientation dynamics measured in the alcohols are not consistent

61

 

with slip-limit predictions, but there are two clearly discernible groups of data (Figure
3.8).The data for R610 and R640 reorientation differ qualitatively from that seen for
R700 and R800. For the first two chromophores, there is not a clear distinction between
their reorientation dynamics in polar protic and aprotic solvents, while there is clear
distinction for R700 and R800. It is tempting to ascribe this difference in behavior to
differences in the structures of these two groups of molecules, and, indeed, several
distinctions can be made based on structure. The first distinction is that the calculated
permanent ground state dipole moments of R610 and R640 are smaller than those for
R700 and R800 (Table 3.3). This finding is consistent with the experimental data in the
sense that, for R610 and R640, we observe little discernible difference between their
reorientation behavior in the alcohols and in polar aprotic solvents. For R700 and R800,
we observe stronger solvent-solute interactions for polar aprotic solvents than for the
alcohols or water. The larger permanent dipole moment of these two chromophores
serves to interact with solvents characterized by large permanent dipole moments (Table
3.4). Another structural feature that is worthy of examination is the shape of the
chromophores. R610 and R640, by virtue of their o-benzoate group at the 1 position, are
expected to sweep out volumes that are substantially more spherical than that seen for
R700 and R800. This effect is seen in the calculated shape factors for these two groups
of molecules; S ~ 0.9 for R610 and R640 and S ~ 0.8 for R700 and R800. In the stick
limit, such‘a distinction would not matter, but in the slip limit, the frictional term f would
be larger for R700 and R800 than for R610 and R640. We are not implying that the
rhodamines behave in the slip limit for the solvents we have studied here, but if there is

any slip-like component to frictional solvent-solute interactions, we assert that such

62

 

contributions would be more prominent in systems with a more anisotropic rotor shape,
hence the more pronounced solvent dependence for R700 and R800. In comparing the
reorientation dynamics of R610 and R640, we would expect R610 to be slightly faster
than R640 on the basis of the (small) difference in hydrodynamic volume. Instead, we
find that R610 exhibits reorientation times equal to or longer than those of R640 in all of
the solvents studied. This effect is especially pronounced in the solvents water, methanol
and ethanol (Table 3.1). We understand this behavior in terms of the motional freedom of
the terminal amines in R610 and their restricted motion in R640. Semi-empirical
calculations show that when free to rotate, the amino groups are capable of stabilizing a
partial positive charge, making them more amenable to strong solvent-solute interactions.
When both dipolar and structural contributions to the reorientation and linear response of
the rhodamines are considered, we can understand the behavior of these chromophores
within the framework of a single system.
Conclusions

We have examined the linear optical response and reorientation dynamics
of several rhodamines in a series of polar protic and aprotic solvents. The reorientation
data are all qualitatively consistent with the modified DSE model in the stick limit. At a
quantitative level, several trends emerge. The first is that dipolar solvent-solute
interactions are at least as important in determining reorientation dynamics as are H-
bonding interactions for the rhodanrines, and the distinction between polar protic and
aprotic solvents is most clear for rhodamines with small polar substituents at the 1
position. We also find that H-bonding interactions with the chromophore terminal amino

groups are more prominent in chromophores where the amino groups are free to rotate,

63

and we understand this phenomenon in the context of the ability of the free and locked
amino groups to stabilize charge. In the case of reactive systems, such as R6101actone,
only polar protic solvents are capable of participating in a lactone ring-opening reaction
to form an ester. Our reorientation data reflect the formation of the ester and, for the
esters, we observe stronger solvent-solute interactions than are seen for the native

chromophore, R610. The basis for this finding requires further investigation.

300 —

250 -

200 -

Ton (p8)

100 -

 

l a l a 1 a l n l a l

Viscosity (cP)

0 a n s
0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

Figure 3.6 - Reorientation time constants of R610 (0) and R640 (0) as a function of solvent viscosity.
The calculated DSE stick limit line is shown for each chromophore, with a dashed line for R610 and a

dotted line for R610.

65

350 r-
300 - 1
250 -

200 -

(p8)

5150-

T

100 -

 

50-

 

O l l l 1 L n l n l 1 l 1 l a l

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
Viscosity (cP)

 

Figure 3.7 - Reorientation time constants of ring-opened R610 lactone (o) and R610 (0) as a function
of solvent viscosity. The calculated DSE stick limit lines are shown for each chromophore. For the
ring-opened lactone, the calculated hydrodynamic volume reflects an increase due to ester formation.

66

250 r

 

 

 

 

 

200 -
150 F
é ..
100 -
Pg _
SO ' slip linrit
0 n ——irL i 1 l l 1 m l n l 1 l 1 l :41 n I
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
viscosity (cP)
250 - b
200 -
150 -
é .
100 -
P8 _
50 ' slip limit

 

 

 

O 1 +1 I s l l l n I s l l I s I l l n I
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
Viscosity (cP)

Figure 3.8 - Reorientation time constants of (a) R700 and (b) R800 as a function of solvent viscosity.
Aprotic solvents are shown with open (0) circles and protic solvents are shown with open (0) squares.
For each chromophore the calculated DSE stick and slip limit lines are shown..

67

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(55) Perrin, F. J. Phys. Radium 1934 5, 497.

(56) Hu, C.; Zwanzig, R. J. Chem. Phys. 1974, 60, 4354.

(57) Edward, J. T. J. Chem Ed. 1970, 47, 261.

(58) Fleming, G. R.; Morris, J. M.; Robinson, G. W. Chem. Phys. 1976, I 7, 91.

(59) Blanchard, G. J. J. Chem. Phys. 1987, 87, 6802.

(60) Blanchard, G. J. J. Phys. Chem. 1988, 92, 6303.

(61) Blanchard, G. J .; Cihal, C. A. J. Phys. Chem. 1988, 92, 5950.

(62) Homg, M.-L.; Gardecki, J. A.; Maroncelli, M. J. Phys. Chem. A 1997, 101, 1030.

(63) Balabai, N.; Kumikova, M. G.; Coalson, R. D.; Waldeck, D. H. J. Am. Chem. Soc.
1998, 120, 7944.

70

(64) Wiemers, K.; Kauffman, J. F. J. Phys. Chem. A 2000, 104, 451.

(65) Waldeck, D. H. The Role of Solute-Solvent Friction in Large-Amplitude Motions.
In Conformational Analysis of Molecules in Excited States; Waluk, J ., Ed.; Wiley-VCH,
2000; pp 113-176.

(66) von Jena, A.; Lessing, H. E. Chem. Phys. Lett. 1981, 78, 187.

(67) von Jena, A.; Lessing, H. E. Chem. Phys. 1979, 40, 245.

(68) von Jena, A.; Lessing, H. E. Ber. Bunsen-Ges. Phys. Chem. 1979, 83, 181.

(69) Madden, P.; Kivelson, D. J. Phys. Chem. 1982, 86, 4244.

(70) Philips, L. A.; Webb, S. P.; Clark, J. H. J. Chem. Phys. 1985, 83, 5810.

(71) Philips, L. A.; Webb, S. P.; Yeh, S. W.; Clark, J. H. J. Phys. Chem. 1985, 89, 17.
(72) Kahlow, M. A.; Kang, T. J .; Barbara, P. F. J. Phys. Chem. 1987, 91 , 6452.

(73) Nee, T.-W.; Zwanzig, R. J. Chem. Phys. 1970, 52, 6353.

(74) Alavi, D. S.; Waldeck, D. H. J. Chem. Phys. 1991, 94, 6196.

(75) Kivelson, D.; Spears, K. G. J. Phys. Chem. 1985, 89, 1999.

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(78) Handbook of Chemistry and Physics; 82 ed.; Lide, D. R., Ed.; CRC Press: Boca
Raton, 2001.

71

Chapter 4.

Dynamics of Rhodamine Esters in Micellar Media: Probing the Balance Between Ionic

and Dispersion Interactions

Introduction

Amphiphilic systems are important to a number of chemical processes ranging
from the existence of lipid bilayers to the use of detergents in industrial and home
applications. The single feature of such systems that allows such broad utility is their
ability to coexist with and function as an interface between polar and nonpolar phases.
This ability is determined by a balance between ionic and dipolar interactions with polar
media and dispersion interactions with nonpolar media. Micelles are a subset of
amphiphilic systems that manifest transient self-assembly behavior in certain
concentration regimes, and this family of solution-phase assemblies has been studied
extensively.

Micelles have attracted significant attention because of their ability to function as
encapsulating and/or biomimetic systems. Among the properties of micelles that have
been investigated are the effects of amphiphile concentration on size, shape and
permeability, and time-domain optical spectroscopy has emerged as a very useful tool for
such studies. Many dynamical studies of micelles have focused on associating a probe
molecule with the micellar environment and correlating the observed dynamics of the
probe molecule with that of the micelle as a function of pressure, viscosity and probe

solubility."'9

72

Despite the widely held understanding that the formation and existence of
micelles represents a balance between dispersion and ionic or dipolar intermolecular
interactions, a detailed understanding of this balance remains to be articulated. The
approach taken to understanding the behavior of micellar systems has been to use an
optical probe that exhibits limited solubility in both the rrricelle and the aqueous phases,
with the dynamical response of the system modeled in the context of the simultaneous
observation of the same chromophore in multiple environments. A motivation for
interpreting dynamical data within this framework has been the observation of
biexponential induced orientational anisotropy decays, with the assumption that the
chromophore in each environment is characterized by a single exponential anisotropy
decay. These multicomponent anisotropy decay data could be interpreted either as
evidence for lateral diffusion into and out of the micelles or as reorientation of
chromophores partitioned into two distinct environments. Save for systems designed to
force the chromophore into one environment or the other, chromophores will establish a
dynamic equilibrium between the two phases and a significant issue in understanding
such data is the time scale of dynamic exchange relative to the time scale of the
reorientation measurements.

For many systems, it makes sense to consider the average environment of the
chromophore. We expect that the environment of a micelle will vary in a regular manner
as a function of distance from the center of the micelle, going from a relatively dense
aliphatic environment near the center to a relatively diffuse region known as either the

20-22

Stern layer in charged micelles, or as the Palisade layer in neutral micelles, where the

head groups, counter-ions and solvent molecules coexist. Several groups have performed

73

studies using chromophores that were not water soluble, but were soluble in the aliphatic
portions of the micelle, and recovered the result that these chromophores “partitioned”
between the different regions within the micelle, a result consistent with the work of
several other groups.3““7'8'”"9'23 In systems where the probe chromophore is bound to
some specific location on an amphiphile, which presumably retains its ability to
participate in micelle formation, the chromophore is seen to exhibit a biexponential
anisotropy decay, and this finding has been modeled in the context of the probe
“wobbling” in a confined space. From these bodies of work, it is clear that the balance
between ionic/polar and dispersion interactions is such that micelles are highly dynamic
entities with the dynamics mediated by the details of the intermolecular interactions
within and around the micelles. We are interested in exploring the structural factors that
can influence this balance of forces. To elucidate these factors, we have performed
rotational diffusion measurements using four rhodanrine derivatives with varying
aliphatic ester chain lengths. These are tetramethylrhodamine-methyl ester (R1),
tetramethylrhodamine-ethyl ester (R2), tetraethylrhodamine-hexyl ester (R6) and
tetraethylrhodamine-octadecyl ester (R18) (Figure 4.1), in three different aqueous
micellar environments: cetyltrimethylamonium-bromide (CTAB), sodium dodecylsulfate
(SDS) and polyethylene glycol 400 dodecyl ether (ThesitQ) (Figure 4.1). The rhodamines
are a family of robust chromophores that have found wide use as optical probes. These
cationic dyes are water soluble and are thus good candidates for studies of nricellar
systems. Our data point to the expected partitioning between aqueous and micellar
phases, with the details of the partitioning depending sensitively on the identity of the

micelle head groups and the probe aliphatic chain length. We find that, for Thesit®, the

74

rhodamine hexyl ester partitions into the micellar phase with anomalous efficiency,
suggesting the importance of geometric issues related to the efficiency of probe-micelle
interactions.
Experimental Section

Pump-Probe Laser System. The picosecond pump-probe laser spectrometer used
to acquire reorientation data on the chromophores in water has been described in detail
previously,24 and we present only a brief synopsis of its operation here. A mode-locked
CW Nd:YAG laser (Coherent Antares 76-S) produces 30 W of average power (1064 nm,
100 ps pulses, 76 MHz repetition rate). The output of this laser is frequency-doubled to
produce ~3 W of average power at 532 nm, with the same pulse duration and repetition
rate. The second harmonic light is used to excite two cavity-dumped dye lasers
(Coherent 702-3) synchronously, with the output of both lasers being ~110 mW average
power at 8 MHz repetition rate, producing ~7 ps fwhm autocorrelation traces using a
three plate birefringent filter. The pump dye laser was operated using Pyromethine 567
dye (Exciton) at 556 nm and the probe laser was operated with Rhodamine 6G dye
(Kodak) at 580 nm. The pump and probe wavelengths were chosen to access the S1 <—
So transition of each chromophore to detect the stimulated emission from each probe.

The probe laser polarization was set alternately to 0' and 90‘ relative to the pump laser

polarization for individual time-domain scans of parallel (1“(t)) and perpendicular (11(0)

polarization used in constructing the orientational anisotropy function. The time
resolution of this system, ~10 ps, is determined by the cross-correlation of the pump and
probe laser pulses. Detection of the transient signals was accomplished using a radio-

and audio-frequency triple-modulation scheme, with synchronous demodulation

75

detection.”27 The reorientation time constants we report here are the average of 6
individual determinations, each comprised of the average of 10 - 12 sets of I“(t) and I 1(t)

scans.

Time-correlated single photon counting spectroscopy. Rotational diffusion
measurements of the probes in micellar systems were made using a spectrometer that has
been described previously,24 and we provide a synopsis of its essential features here. The
source laser is a mode-locked CW Nd:YAG laser (Quantronix 416) producing 7 W
average power at 1064 nm with 100 ps pulses at 80 MHz repetition rate. The second
harmonic of the output of this laser (532 nm, ~700 mW) is used to excite a synchronously
pumped, cavity dumped dye laser (Coherent 702-2) operating with Pyromethine 567 dye
(Exciton) at 556 nm. The output of this laser is typically 100 mW average power with a
4 MHz repetition rate and 5 ps pulses. Detection of the transient fluorescence signals was
accomplished using a microchannel plate photomultiplier tube (Hamamatsu R3809U),
with fluorescence light collection through a reflecting microscope objective and
wavelength selection with a subtractive double monochromator (American Holographics
DB-lO). The electronics used for signal processing are a Tennelec 455 quad constant
fraction discriminator and a Tennelec 864 time-to-amplitude converter and biased
amplifier. The reference channel was detected and delayed using an in-house built fiber
optic delay line. Signals are processed using a multichannel analyzer (PCA Multiport II)
and sent to a PC for acquisition. For this system, the instrument response function is
typically 30 — 35 ps fwhm.

Steady-Sate Spectroscopy. The steady-state absorption spectra of the

chromophores used here were recorded with 1 nm resolution using a Cary model 300

76

UV-visible spectrophotometer. The spontaneous emission spectra for the same solutions
were obtained with 1 nm resolution using a SPEX Fluorlog-3 spectrometer.

Chemicals and Sample Handling. The probe molecules tetramethylrhodamine-
methyl ester (R1), tetramethylrhodamine-ethyl ester (R2), tetraethylrhodamine-hexyl
ester (R6), and tetraethylrhodamine-octadecyl ester (R18) (Figure 4.1) were obtained
from Molecular Probes Inc. and used as received. Distilled, deionized water was
available in-house. The chromophore concentration of all solutions used for laser
measurements was 10 11M. Measurements of R18 solutions in neat water were hampered
by bleaching of the solution. This effect was mitigated by replacement with fresh
solution on at least a daily basis, or as warranted. All pump-probe measurements were
taken using a flowcell with a 1 mm pathlength, and TCSPC data were acquired using a
fluorescence cuvette with a 1 cm path length. Solution temperatures were maintained at
298 1 0.5 K (Neslab EX100-DD).

The surfactants sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide
(CT AB) and polyethylene glycol 400 dodecyl ether (Thesito) (Figure 4.1) were obtained
from Sigma-Aldrich, Inc.,_checked for fluorescent impurities, and used as received.

Results and Discussion
We are interested in using rhodamine alkyl esters as probes of micellar behavior.
Previous studies of rhodamines have provided a substantial understanding of their optical
properties and the relationship(s) between chromophore structure and intermolecular
interactions.”32 Using this knowledge, we consider how to interrogate the balance of
forces that give rise to nricellar structures in aqueous amphiphile solutions. We have

selected a series of rhodamine aliphatic esters with the idea that the ester aliphatic tails

77

will interact preferentially with the interior of the micelle and the cationic rhodamine
chromophore will interact primarily with the micelle head groups. For anionic SDS
micelles we expect attractive ionic interactions between chromophore and nricelle head
group and for cationic micelles we expect repulsive interactions. For this group of
systems, the extent to which the chromophores partition into the micelles should depend
sensitively on the chromophore aliphatic chain length. For neutral Thesit‘D micelles it is
not clear whether attractive or repulsive polar interactions will dominate. To evaluate
whether or not these expectations are realistic, we have examined the steady state
spectroscopy and reorientation behavior of the Rhodamine esters in water and aqueous
micellar media.

Linear optical response. The normalized absorption and emission spectra of the
chromophores studied here are shown in Figure 4.2. With the exception of R6, the
absorption and emission spectra of all the chromophores are not influenced by increasing
the length of the ester chain. R6 exhibits a slight red-shift of ~ 7 nm in its absorption and
emission spectra. Drexhage has studied the structure-spectroscopy relationships for
rhodamines,33 and his work has shown that the o-benzoate ring at the 1 position of these
chromophores (Figure l) is essentially decoupled from the three ring chromophore
system, precluding a direct electronic effect from being responsible for the spectral shift
we observe for R6. It is well established that substituents at the 4 and 12 positions do
influence the spectral profiles of the rhodanrines, and interactions between the ester
aliphatic chain and the protons at the 4 and/or 12 positions could, in principle, account for

our findings.

78

 

 

 

0\ //
IS.“~ Na+
SDS 0’ 0'
Br‘
CH3
CTAB Hsc’tm,

 

O‘EA Oh OH

Figure 4.1 - Rhodamines and surfactants used in this study. Top row, left to right:
tetramethylrhodamine methyl ester (R1), tetramethylrhodamine ethyl ester (R2). Second row, left to
right: tetraethylrhodamine hexyl ester (R6), tetraethylrhodamine octadecyl ester (R18). Third row:
sodium dodecyl sulfate (SDS), Fourth row: cetyltrimethylammonium bromide (CTAB), Bottom
row: polyethylene glycol 400 dodecyl ether (Thesit°).

Thesit®

79

1.0 -

52 0.8 —
C
3
o
O

'c 0.6 -
a)
.5
E

E 0.4 -
O
z

,

0.2 -

0.0 -

 

—R1
——Fl2

 

 

l l l I l
400 450 500 550 600 650 700

Wavelength (nm)

Figure 4.2 - Absorbance and fluorescence spectra of the rhodamine esters used in this study.

80

If such a sterically mediated interaction does account for our spectral data, it remains
unclear why the same spectral shifts are not seen for R18, and this is a matter that bears
further investigation. Semi—empirical calculations (HyperChem® v. 6.0) predict no
anomalous red-shift for long chain rhodanrine esters. Thin layer chromatography (TLC)
experiments show no unresolved impurities in our raw material and no other indications
of anomalies were apparent in our solution preparation procedures.

Time Resolved Spectroscopy. Experimental time—domain data (1"(t) and I J_(t)) are

combined according to Eq. 4.1 to produce the induced orientation anisotropy function,
R(t).

I" (r) - I,(:)
1H (t) + 21l (t)

 

R(t) = (4.1)

The functionality of the R(t) decay provides important information on the dynamics of
the chromophores. All of the rhodanrine esters exhibit single exponential anisotropy
decays in water and biexponential decays in micellar environments. Our previous work
has shown that the S. <— 80 transitions we access are oriented along the rhodamine
chromophore long axis. In water, the rhodamine esters behave as prolate rotors. For all
data, the reported zero-time anisotropies are derived from the regression of the data for
times greater than 15 ps after excitation, and the uncertainties reported for each quantity
are 95 % confidence intervals for six or more individual determinations. We report the
quantities R(O) and TOR for the rhodamine esters in aqueous and rrricellar environments in
Table 4.1.

The data we present in this paper for the reorientation of the rhodamine esters in

water (Figure 4.3) are predicted reasonably well by the well-established Debye-Stokes-

81

Einstein (DSE) model in the stick limit.34 We note that R18 reorients on a time scale that
is faster than the other rhodamine esters and is indeed faster than that predicted by the
modified DSE model. We understand this behavior based on the length of the alkyl chain
of the ester which interacts with the main portion of the chromophore in such a manner as
to screen the ionic charge of the chromophore, thus promoting more “slip” like behavior”
of the chromophore. Thus, even though the hydrodynamic volume”37 of the
chromophore increases, the frictional interaction decreases in such a way as to

compensate for the change in probe volume.

Table 4.1 - Reorientation times and zero-time anisotropies for Rhodamine chromophores in aqueous
and micellar environments.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Water“ sos" CTAB” Thea?"
111(0) 0.251003 0.051002 0.291001 0.321004

R1 11(ps) 132115 2431118 152113 16016
R;(0) — 0.271003 00210.01 00410.01
1:; (ps) - 1037148 14051399 15581298
R,(0) 0.221003 0.051002 0.341003 0.301001

R2 “(ps) 129115 3111111 16613 15119
112(0) - 0.261002 0.011001 0.041002
£21m - 10241120 18691494 15581207
111(0) 0.271004 0.031001 0.231002 0.131004

R6 mpg) 172132 97:14 200131 87134
111(0) - 0.281001 0.081002 0.231002
12(ps) - 1 142132 14811543 1361159
111(0) 0.221003 0.021002 0.171002 0.231001

R18 “(ps) 145121 190188 149118 200121
112(0) - 0.251001 0.141001 0.081002
12(95) - 1 101136 1275192 21891676

 

 

 

 

a. The data are the best fit results of the data to the function R(t) = R0(0)exp(-t/’tor). Time

 

constants are in ps and the uncertainties are standard deviations (110) for at least six
determinations of each quantity.

b. The values reported are the best fit results of the data to the function R(t) = Rl(0)exp(-t/tl) +
R2(0)exp(-t/12). Time constants are in ps and the uncertainties are standard deviations (110) for
at least six determinations of each quantity.

82

The reorientation data presented here for the rhodamine esters in micellar
environments are different for those of the reorientation of these same chromophores in
water, with biexponential anisotropy decays being seen for reorientation in micellar
solutions (Figures 4.4-4.6). This biexponential behavior is indicative of the intrinsic
complexity of these heterogeneous micellar systems. The simplest interpretation of the
data suggests that the chromophore reorients either in the water portion of the
environment or within the micelles. Examination of the experimental data shows this not
to be the case. The reorientation times of the short-time component do not match those
for reorientation in water, indicating that the simplest model is not applicable to the
treatment of these data. The next layer of complexity accounts for the motion of the
chromophore within the micelle as well as the motion of the micelle as a whole. In this
model, the chromophore is bound at or near the surface of the micelle, yielding a time

constant of 1,, that is related to restricted motion analogous to that of a hindered rotor and

rm for overall reorientation of the cone equal to micellar motion. The anisotropy function
for this model is shown in Eq. 4.2,

R(t) = 12(0)[s'2 + (1 — s'z)exp(—t/rw)]exp(—t(1It, +1/2'm)) (4.2)

where R(O) is the zero-time anisotropy, S. is an order parameter, sensitive to the
equilibrium orientational distribution width of the chromophore transition moment in its
restricted volume and Id is considered to be representative of the translational motion of
the chromophore about the surface of the micelle. In this model, there are three time
constants, but two are amenable to direct determination by virtue of the form of Eq. 4.2.
Previous work in the Blanchard group has shown that this expression does in fact model

the dynamics of bound and confined chromophores accurately.3843 Using Eq. 4.2, there

83

can be a qualitative correspondence made between the slow time constant arising from
motion related to the micelle and the fast time constant related to motion associated with
the probe in an environment where it is free to wobble, and we can estimate the
partitioning operative in these systems. In this picture, the chromophore aliphatic tail
interacts within the inner micelle, and the probe is tethered to the micelle. The length of
the tether controls the chromophore proximity to the micelle and thus its freedom of
motion. The distribution of the chromophore between a primarily aqueous and a
primarily micellar environment can be estimated through the normalized pre-exponential
factors (Rf and R2. for short- and long-time component pre-exponential factors) as

shown in Eq. 4.3.

Rf= R,
R, + R2
(4.3)
... R2
2 - R, + R2

This treatment assumes that the transition moment angle characteristic of a given
chromophore does not depend sensitively on its local environment and, for the systems
we consider here, this is likely to be an excellent approximation.“

The experimental data for the reorientation of the chromophores in anionic SDS micellar

solution (33 mM, 4x cmc) are shown in Figure 4.4, (Table 4.1) and show that R(t)

84

250-

175 r

150 -

125 .1

1(1)-

Time (ps)

75-

 

251lllllllll
2468101214161820

Rhodamine ester elmin length
(nurrber of carbons)

 

Figure 4.3 - Time-resolved reorientation data for rhodamines in the aqueous phase.
Squares (I) are experimental data points and the solid line represents the theoretical
reorientation calculated from the Debye-Stokes-Einstein equation.

85

decays biexponentially to within our ability to determine. For all of the rhodamine esters,
we recover the same time constants, to within the experimental uncertainty. The average
of the two decays, 210 1 82 ps and 1076 1 59 ps, is measurably different than the average
reorientation time seen for water (144 1 21 ps), despite the fact that the bulk viscosity of
the SDS solution is essentially the same as that of water. This finding suggests that there
is an interaction of the rhodamines with the micelle, which we would expect based on
charge association arguments alone, but which does not allow us to elucidate the nature
of the interaction in a direct fashion. The overall motion of SDS micelles is predicted to
be on the several nanosecond time scale,”"5'43 and the ~1100 ps time constant we recover
appears to be too fast to account for micelle motion. The chromophores do not yield time
constants consistent with being bound rigidly in the micelle, or free to rotate in an
aqueous environment. Since the slow time constant is not consistent with motion of the
free chromophore in bulk water, this leaves the quasi-translational motion of the
chromophore along the outer extent of the micelle as the only physical model consistent
with the recovered slow time constant.”'”‘16 We can use this model to estimate the
energetics of partitioning in these systems.

We assert that fast motion is indicative of the cationic rhodamine chromophore
head group existing in a region somewhat separated from the micelle, and that the slow
motion is reflective of the rhodamine chromophore that is tightly associated with the
micelle. Thus the normalized anisotropy prefactors R1. and R2. are proportional to the
concentrations of the loosely bound (C1) and tightly bound species (C.), respectively. If

there exists a dynamic equilibrium between these association conditions, we can use the

86

 

 

 

D 1.0 -
go 09 - I I
5 ”a 0.8 -' '
8 E 0.7 -
8- “ 06 -
8 fi '
_N 8 0.5 -
E .E 0.4 —
o V 0.3 1
Z 0.2 -. .
01 - I I I I I 2 I II I , I
1400 -
b
1200 -
6 l O 9
z; I”) " * i
E:
e 800;
[-1 400 _
300 — §
200 - i
100 I
0 [- I I I I I I I [III I I I
1234567161820
Rhodamine ester chain length
(number of carbons)

Figure 4.4 — Reorientation data for rhodamine aliphatic esters in aqueous SDS micelle
solution. Biexponential anisotropy decay functions produced a) normalized prefactors
for the contribution of the fast (0) and slow (I) components of the anisotropy decays.
b) Reorientation time constants plotted as a function of increasing ester chain length.

87

 

 

 

1.0 7, a
g, 0.9 '1‘
g A 0.8
g g 0.7 9
gfi 0.6 a
'8 —‘= 0.5 ’
1; 8 0.4 '
E 0.3
o 0.2 I
Z
0.1 l
00 I I ! l I I I I II I I
II
2400 -
2000 - l i b
1600 - L 1 Q
g 1200 - 1 l 2
E 8004’
E" 400 -
300 -
200 - i
100 I ! ! I I I I I I’ll I !
0 1 2 3 4 5 6 16 18
Rhodamine ester chain length
(number of carbons)

Figure 4.5 - Reorientation data for rhodamine aliphatic esters in aqueous CTAB
micelle solution. Biexponential anisotropy decay functions produced a) normalized
prefactors for the contribution of the fast (0) and slow (I) components of the
anisotropy decays. b) Reorientation time constants plotted as a function of increasing
ester chain length.

88

0.9— Q a;

1%

Normalized percentage
(in each phase)
0
U!
l

 

0.0 "l I I I I J I I II I I
II

2800 -
2400 -
2000 -
1600 -
1200 -

800.;

d

400-

1—13 ‘1
119 1
F—G—i

Time (ps)

200- I I I

O I I I I I I I I II I I
ll

012345671618

Rhodamine ester chain length
(number of carbons)

 

Figure 4.6 — Reorientation data for rhodamine aliphatic esters in aqueous Thesitm
micelle solution. Biexponential anisotropy decay functions produced a) normalized
prefactors for the contribution of the fast (0) and slow (I) components of the anisotropy
decays. b) Reorientation time constants plotted as a function of increasing ester chain

length.

89

concentration ratios to obtain the equilibrium constant, which is related to the free energy

of interaction between the micelle and chromophore.

 

 

Clt—K—‘q—a Cr

[C] R;
K =-—'--z , 4.4
AG =-RT1nK,q

The free energy we extract from these data can be decomposed into ionic and dispersion

contributions.

AG = AGW + AG

1.1.1.... (4.5)
We can estimate the dispersion contribution as follows. For highly ordered aliphatic
chains, such as alkanethiol monolayers, the energy of interaction between chains is taken
to be ~ -125 kJ/mol-CH2.4°‘“ We believe this value to be too high for micellar systems
owing to the substantially lower extent of aliphatic chain ordering in these systems. We
estimate the strength of interaction to be -0.5 kJ/mol-CH; unit, and calculate the ionic
contribution to the interaction energy as a function of rhodamine alkyl ester chain length.
We present these calculated results for SDS and CTAB micelles in Table 4.2. The
partitioning data reveal that the interaction with the headgroup region is significant, and
the dependence of the data on rhodamine alkyl ester chain length reveals the important
role of the double layer around the perimeter of the micelle. We will return to a detailed

discussion of these results after considering the reorientation data for the esters in each

micellar environment.

90

Table 4.2 - Various thermodynamic properties of Rhodamine chromophores in SDS and CTAB

 

 

 

 

 

 

 

 

 

micellar environments.
SDS CTAB
(Id/mow (Ulmol)' (U/mol)' (kJ/mol)' (kl/moi)" (UlmoD'
R: 5.25 -4.14 -0.5 -3.64 0.06 +6.86 -0.5 +7.36
R: 5.25 -4.14 -1.0 -3.14 0.03 +8.67 -l.0 +9.67
Re 9.0 -5.48 -3.0 -2.48 0.35 +2.61 -3.0 +5.61
Ru 12.5 -6.30 -9.0 +2.70 0.82 +0.5 -9.0 +9.50

 

 

 

 

 

 

 

 

a Values for AGW are calculated from Eq. 4.4. Values for AG,“ are calculated from Eq. 4.5.

The reorientation dynamics of the rhodamine esters in cationic CT AB micellar solution
(3.0 mM, 3x cmc) are shown in Figure 4.5. Again, there seems to be no clear dependence
of the reorientation time (Table 4.1) on increasing ester chain length and, although the
short time constant that is recovered is closer in absolute value to that for bulk water, it is
clear from the biexponential decay dynamics that there is a significant interaction
between the CTAB micelles and the chromophores. As with reorientation in SDS
micelles, the recovered time constants do not correspond to independent motion of the
chromophores within the micelle (too fast), or the motion of the micelle itself (too slow).
When the data on chromophore distribution are examined, a clear picture emerges
of increasing chromophore interaction with CTAB as a function of increasing ester chain
length (Figure 4.53). Much like the SDS micelles, the interaction is primarily with the
headgroup region for the methyl and ethyl esters, and then increases substantially for the
hexyl and octadecyl esters. This effect is dominated by the increase in dispersion
interactions between the micelle aliphatic region and the chromophore ester alkyl chains.
The effects of ionic interactions are compensated substantially by the presence of the
double layer at the micelle perimeter. From the results for ionic interaction energies

reported in Table 4.2, it can be seen that the strength of ionic interactions remain

91

relatively constant and repulsive with increasing probe alkyl chain. Specifically, we
recover an ionic interaction energy term for probe-SDS systems that becomes
increasingly unfavorable with increasing probe alkyl chain length, and an ionic
interaction remains approximately constant for interactions between CT AB and the
rhodamine probes. While intuition would suggest strongly attractive ionic interactions
between the probe and SDS, with the overall interaction increasing in strength as the
probe alkyl ester length increased, this is not what is seen experimentally. We find
approximately constant total interaction energy for the probes with SDS, with ionic
interactions becoming increasingly unfavorable with increasing alkyl chain length. For
these systems, the dominant ionic interactions are between the probe cationic moiety and
the double layer at the perimeter of the micelle. For SDS, the counterions are Na+ and H+
and these ions will interact repulsively with the cationic rhodamine. As the probe alkyl
ester chain length is increased, the probe is drawn more efficiently into the micelle and
repulsive interactions between the micelle double layer and the probe head group become
more pronounced. For CTAB, the opposite result is obtained. The ionic interactions
between the rhodamine probe and the CTAB counter ions in the proximity of the micelle
head groups remain essentially constant (~8.5 kJ/mol) as a function of rhodamine alkyl
ester chain length. For CT AB, the ionic interactions are essentially constant and appear
to be mediated by the presence of the counterions at the micelle surface. We note that,
despite the importance of ionic interactions being between the micelle counter ions and
the cationic probe, the magnitude of the interactions for both systems are similar, and

modest.

92

For the reorientation of the rhodamine esters in the neutral micellar Thesit®
system (1.0 mM, 10x cmc), it is not clear how the balance of forces will operate, i.e.
whether polar and dispersion interactions will act in a cooperative or opposing manner.
The recovered time constants (Table 4.1) reveal that interaction seems to increase slightly
with increasing ester chain length. However, an inspection of the individual time
constants for rhodamine reveals that the interaction between the chromophores and the
micellar system is anomalous for the hexyl ester. There is a decrease in the time
constants measured for R6 and this behavior is unique to the R6I'I‘hesitQ system. This
trend is also seen in the chromophore partitioning data (Figure 4.5). Starting with the
ethyl ester, there is an increase in the chromophore partitioning into the micelle to the
point where the chromophore is interacting with the core more strongly than with the
headgroup. This trend is reversed for R18, counter to any predictions that can be made
based solely on energetic trends.

Wobbling-in-a-Cone with Translational Difi’usion and Micelle Rotation. With the
partitioning issues understood, we consider next the time constants of the anisotropy
decays we measure. We expect that the reorientation time constant for a micelle is
several nanoseconds, and we do not observe any such long-time anisotropy decay in our
data. The possible reasons for this absence are either a lack of sensitivity to a long-time
component, which is not reasonable given the sensitivity and the time resolution of our
TCSPC system, or that the persistence time of the interactions between chromophore and
micelle is shorter than the reorientation time of the micelle. This explanation is
consistent with the modest interactions between the chromophore and the micelle (vide

infra). We extract the wobbling diffusion constant Dw using the experimental quantities

93

1., and S. using Eq. 4.2 and 4.6-4.7. The quantities 00, the confining cone angle and Dw

in water Dw = (61:0,) " are reported in Table 4.3.

Dw = {2',,(1-S'2)}"[—cos2 60(1+cos60)2{ln[(l+cost90)/2]
+(l-cos60)/ 2} {2(1-cos60)}'l +(1—cos60) (4.6)
X(6+8COS 60 -cos2 6,, -120083 60 —7cos4 00)/24]

S ' = 0.5 cos 6,,(1 + cos 6,) (4.7)

Table 4.3 - Wobbling constant (D!) values for aqueous and D. and 0. micellar environments.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R1 R2 R6 R18

Water Dw (x109 Hz) 1.26 1.30 0.97 1.15
SDS Dw (x109 Hz) 0.66 0.46 2.15 1.03
00 58.7 58.2 64.3 67.0

Dw (x109 Hz) 0.07 0.03 0.23 0.58

CTAB 00 12.0 8.0 25.2 35.3
4, DW (x109 Hz) 0.12 0.14 1.57 0.24
Tm“ 00 16.0 16.5 45.2 25.2

 

 

The values of the order parameter, S. vary from 0.84 to 0.94 for reorientation of
the rhodamine esters in the micelles. The order parameter is a measure of the time-
averaged orientational distribution of the chromophores and can take on values from zero
for a completely random distribution of chromophores to one for a rigid crystalline
system. Our derived values for S’ indicate that, on average, the micellar systems are well
ordered with the order increasing from the neutral to cationic, to anionic micelles. These
data are related to the values recovered for the cone angles associated with the wobbling
chromophores. For SDS micelles, the cone angle decreases with increasing ester chain
length, indicating more confinement of the chromophore, consistent with our calculations
of the ionic interaction energy. For CT AB micelles, the cone angle becomes larger with

increasing chromophore aliphatic chain length, again in keeping with the energetic

94

estimates. We view the interaction of the chromophore in the micelle as an activated
process, so its residence time and thus the measured cone angle will scale with the overall
energy of interaction. For the Thesit® micelles, substantial disorder exists, as is seen by
the larger cone angles present for the very short chain esters and the rapid increase in
diffusion coefficient with increasing probe alkyl chain length. While the diffusion
coefficients for the chromophores in water decrease with ester size, this is an expected
result and is due to increasing correlation times in the bulk water. The esters are free to
reorient about their tethering bonds as indicated by the quantities of wobbling diffusion
coefficient, Dw, which also indicate that there is substantial confinement in these systems.
The value of DW in micelles is 6-9 times slower than that for the free chromophores in
water, but it is important to keep in mind that the free volume accessible to the
chromophores in the micelle is substantially less than that of the same chromophores in
water.
Conclusion

We have studied the steady state spectroscopy and reorientation dynamics of
selected rhodamine esters in water and in the aqueous micellar systems, SDS, CI‘AB and
Thesit®. The charges of the rhodamines and the lengths of the ester chains dictate that
there is an interaction with each of these micellar systems and we have used this
interaction to probe the balance of forces operating between micelles and amphiphilic
probes. The reorientation dynamics of the chromophores are consistent with translational
motion of the tightly bound chromophore along the surface of the micelle and less
restricted motion of the chromophore when it is tethered less tightly to the micelle. Our

data indicate that there is indeed a balance of forces between the ionic moieties in the

95

headgroup region and the dispersion forces that characterize the inner core region. For
the ionic micelles, the role of the double layer formed by counter-ions is substantial in
mediating repulsive or attractive ionic interactions. For Thesit®, the absence of an ionic
double layer and the presence of substantial disorder in the mantle region gives rise to
behavior that is qualitatively consistent with that seen in the ionic micelles, but differing
in detail. It is clear from these data that, for ionic micelles, the presence of counter ions
to form a double layer near the micelle head groups serves to mediate ionic interactions.
Because of this ionic compensation effect, the role of dispersion interactions assume
substantial importance in determining the dynamics of micellar media. An important
issue will be to consider the extent to which this balance of forces is altered in a two-

dimensional amphiphilic system.
Acknowledgment

We are grateful to the National Science Foundation for their support of this work through

Grant 0090864.

96

 

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Chapter 5

Conclusions and Future Work

In this dissertation we have studied the reorientation dynamics of Rhodamine
chromophores in polar protic and aprotic, associative condensed phases and in micellar
systems. We have examined the dynamics of these chromophores in n-alcohols,
methanol to decanol. the polar aprotic solvents acetonitrile, DMF and DMSO, aqueous
environments and finally the cationic, anionic and neutral micellar environments of SDS,
CT AB and Thesit®. These studies have been conducted to better understand local
organization in restricted, condensed phases and to gain a better understanding of the
interactions that occur in polar, strongly associative systems.

We now better understand the role of local heating, which changes the viscosity
of the liquid. We have studied this effect (Chapter 2) in polar protic environments and
understand how this effect can be attributed to the excess excitation energy given to the
molecule in terms of a local heating affect. A better understanding of this phenomenon
could be achieved by extending this work to other environments and other chromophores.
Future work would include investigating the reorientation dynamics of different
chromophores and different solvents to understand if the local heating effect is localized
to the rhodamine chromophores that we studied, or is a more generalized effect that is
observable in a broad range of environments.

Chapter 3 detailed experiments involving the use of solute structure and solvent
functionality to probe intermolecular interactions that occur on molecular length scales.
We understand these interactions in the context of the environments that we have studied,

but recognize that this work can be extended to include other environments in order to

100

better probe the interactions that we have studied. Specifically, extensions of this work
are required to include a broader range of aprotic environments, environments that
sample specific regions of the viscosity range and different structural environments in
order to probe the structure-solvation relationship. We understand the hydrogen bonding
interactions that occur between the nitrogen, oxygen and hydrogen atoms in condensed
phases in the context of these measurements, but a need to extend this work to other types
of functional groups exists. Only through extending these measurements to other
functional groups can we complete a picture of structure and local organization in
condensed phases.

Chapter 4 of this dissertation detailed work involving the interactions of
Rhodamine esters and micellar systems. We examined the interaction of four different
esters in cationic, anionic and neutral systems in order to gain a better understanding of
the balance of forces that are present in these systems. We understand the interactions of
the cationic Rhodamine esters with the anionic SDS micelles and to an extent with the
cationic CTAB micelles, where the longer ester chains balance the repulsive forces that
are present between the chromophores and the headgroups of the micelle. We have a
limited understanding of the interactions that occur in the neutral micellar system,
Thesit®, because of the nature of the interactions that we have observed. We understand
that this system is the most disordered of the three systems that we have chosen to study
and believe that this disorder contributes to the observed interactions such that the
chromophores have greater degrees of freedom than in either the cationic or anionic
systems. Several different length scales of ester (propyl, butyl, heptyl or octyl esters, for

instance) could be employed to probe this effect further and to better understand the

101

interactions that occur not only in the neutral system, but also the cationic and anionic
systems.

The information gathered in the pursuit of this dissertation has provided a
contribution toward the understanding of local organization in condensed phases and

allowed the notions of local heating, hydrogen bonding, dielectric friction and solute-

 

'- \

solvent interactions to be probed in the context of the reorientation dynamics of

Rhodamine chromophores.

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