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SPECTROSCOPIC CHARACTERICATION OF 6-CYANO-2-
NAPHTHOL FOR USE AS A CRYSTALLIZATION INITIATOR
IN AQUEOUS SOLUTIONS

presented by

Alayna Michelle Goetsch

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6/01 cJCIFiC/DaleDuopGS-DJS

 

SPECTROSCOPIC CHARACTERIZATION OF 6-CYANO-2-NAPHTHOL FOR
USE AS A CRYSTALLIZATION INITIATIOR IN AQUEOUS SOLUTIONS

By

Alayna Michelle Goetsch

A THESIS

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

MASTER OF SCIENCE

Department of Chemistry

2004

ABSTRACT

SPECTROSCOPTIC CHARACTERIZATION OF 6-CYANO-2-
NAPHTHOL FOR USE AS A CRYSTALLIZATION INITIATOR IN
AQUEOUS SOLUTIONS

By

Alayna Michelle Goetsch

Crystallization is a process that is compleit and difficult to control. Among
the experimental variables under our control are temperature, solution ionic
strength, pH, and precipitant concentration. In a pH dependent system, a weak
acid can be made to precipitate when the solution pH is set below the pKa. A
photoacid can be used as a (transient) source of protons in solution. When
excited by light, the photoacid deprotonates, altering the local solution pH. The
molecule 6-cyano-2-naphthol (GCN2) was used as a photoacid in this work in an
attempt to modulate the solubility of adipic acid, a bifunctional weak acid.
Spectroscopic characterization of 6CN2 was accomplished through the use of
UV-VIS absorption and fluorescence and Time Correlated Single Photon
Counting (T CSPC) measurements to evaluate the utility of SCN2 as a potential

crystallization initiator.

Copyright by
Alayna Michelle Goetsch
2004

This work is dedicated to my parents for all of their support through my education
and Mr. Hanson for teaching me the basics of chemistry through hands on
laboratory experiences.

ACKNOWLEDGMENTS

I would like to first of all acknowledge Professor Gary Blanchard for all of
his support and guidance during my graduate career. Without his enthusiasm
and support, I would not have been able to complete my research for this thesis.
Gary was an excellent teacher in helping me learn the laser system and how to
help troubleshoot any other problems that arose during the research, of which
there were many. I learned more about fixing instrumentation than I would ever
imagine I ever had to know during a research project and yet was still able to
complete work toward understanding the overall goal of my research project. I
would also like to acknowledge the National Science Foundation and the
Department of Energy through their grants for supporting the completion of this
research. The graduate office in the Department of Chemistry has also been
supporting me throughout my research career at Michigan State University. Dr.
Tom Carter was also beneficial in helping with keeping the laser system
operational and to supply appropriate software to do appropriate analysis and
develop programs to help research go smoother. I would also like to thank my
peers at and coworkers that helped me with ideas, support, and constructive

comments about my research to help me expand and develop as a scientist.

TABLE OF CONTENTS

Page
List of Tables .................................................................................. vll
List of Figures ................................................................... VII
List of Abbreviations .......................................................... XII
Chapter 1. Introduction ........................................................................... 1
1.1 Literature Cited ....................................................................... 6
Chapter 2. Dynamics of 6-cyano-2-naphthol in Aqueous 8
Solutions. An Examination of ProtonationlDeprotonation
Dynamics and Aggregate Formation. ..................................
2.1 Introduction ......................................................................................... 9
2.2 Experimental ........................................................................................ 10
2.3 Results and Discussion ...................................................................... 15
2.4 Conclusions ....................................................................................... 46
2.5 Literature Cited ................................................................................. 47
Chapter 3. Using the Photoacid 6CN2 as an Initiator for the
Crystallization of Adipic Acid. ..................................... 49
3.1 Introduction ....................................................................................... 50
3.2 Experimental ...................................................................................... 53
3.3 Results and Discussion ..................................................................... 54
3.4 Conclusions .................................................................................... 70
3.5 Literature Cited ............................................................................... 72
Chapter 4. Conclusions and Future Work 73
4.1 Conclusions ..................................................................................... . 75
4.2 Future Work ....................................................................................... 76
4.3 Literature Cited ................................................................................. 77

vi

Table 2.1

Table 2.2

Table 3.1

Table 3.2

LIST OF TABLES

Decay time constants and fractional contributions of the
protonated and deprotonated forms of GCN2. For both emission
at 370 nm from the protonated form and emission at 440 nm
from the deprotonated form, the data are of the form f(t) =
A1exp(-t/r1) + Azexp(-t/ 72). The physical significance of the
signs of A1 and A2 are discussed in the text

29

Reorientation time(s) for BCNZ as a function of pH and at
different excitation and emission wavelengths. In cases where 40
multiple components of the anisotropy decay are seen

Lifetime values of buffered H2AA/HAA‘ solutions with 1.0 x 10'5 66
M 60N2. Excitation at 300 nm.
Anisotropy values for buffered system with 6CN2 in solution.

Excitation 300 nm, emission collection 440 nm 68

vii

LIST OF FIGURES

Figure 1.1 Energy equilibrium diagram of the photoacid of its ground

and excited states. AH is merely an arbitrary photoacid. 4
Figure 1.2 Structures of some super photoacids Tolbert and coworkers 5
synthesized.18
Figure 2‘1 Schematic of the Time Correlated Single Photon Counter 12

Figure 2.2 The pH dependence of GCN2 steady state absorbance
spectrum. The spectra are shown in extinction units. All
solutions were 1.0x10‘5 M in water, with pH controlled by
concentration of HCI or NaOH. The protonated form is 17
characterized by with a...” = 8890 L mol" cm" at 300 nm,
and the deprotonated form has em, = 3410 L mol'1 cm'1 at
325 nm.

Figure 2'3 pH Dependence of GCN2 steady state emission spectra. All

6CN2 solutions were 1.0x10'5 M and were excited at 300 nm.
The emission band for protonated 6CN2 is centered at ca.

370 nm and the deprotonated form is centered near 440 nm.
Solution acid and base concentrations are (a) 2.0 M HCI, (b) 18
1.5 M HCI, (c) 1.0 M HCI, (d) 0.75 M HCI, (e) 0.50 M HCI, (f)

0.1 M HCI, (g) 0.01 M HCI, (h) 0.001 M HCI, (i) 2 M NaOH.

From these data we have determined pK; = 0.2

viii

Figure 2.4

Figure 2.5

Figure 2.6

Figure 2.7

Figure 2.8

Figure 2.9

Figure 2.10

Difference between excitation spectra of 60N2 taken at the
emission wavelengths of 440 nm and 500 nm, as a function
of pH. Spectra were normalized to the intensity of the 234
nm band and the 260 nm band also seen in the pH
dependence of the absorbance spectra seen in Figure 1.
The spectral feature centered at ca. 300 nm is the
absorption band of protonated GCN2. The band at 350 nm
is associated with a species that is neither the protonated or
deprotonated form of 6CN2. This new feature increases in
intensity with decreasing pH and is consistent with the
existence of an aggregated 6CN2 species (vide infra). All
solutions were 1.0x10'5M BCN2 and [HCI] ranged from 10'3
M to 2 M.

The known equilibria between the excited states and
ground state monomeric species

Solid lines: Steady state emission spectra of 60N2 excited
at 350 nm at selected HCI concentrations: (a) = 2 M HCI,
(b) = 0.6 M HCI, (c) = 10'3 M HCI. Dashed line: Emission
spectrum of a basic 6CN2 solution excited at 350 nm, (d) =
10'5 M NaOH

The new species and the possible pathways for
deprotonation

(a) Time resolved emission spectra of GCN2 in 0.1 M
NaOH. (b) Time resolved emission spectra of GCN2 in 2 M
HCI. For both spectral data sets the time after excitation
from top to bottom scans are 0, 20 , 30, 50, 70, 90, 110,
120, 140, 160, 180, 200, 220, 240, 260, 310, 360,410,460
and 510 ps.

(a) Fluorescence lifetime of 1x10'5M BCN2 in 2M HCI,
excited at 300 nm, emission collected at 370 nm. (b)
Fluorescence lifetime of 1x10'5M com in 1.0x10'5M NaOH.
excited at 300 nm, emission collected at 440 nm.

(a) Stern-Volmer plot for the protonated form of 6CN2 (o)
and the deprotonated form of 6CN2 (o) quenched by Cl'.
Lifetimes for the protonated form were acquired at 370 nm
and 440 nm for the deprotonated form. (b) Stern-Volmer
plot of quenching of the aggregate species by Cl-. Data
were acquired using 350 nm excitation and 500 nm
emission.

20

21

23

25

27

30

34

Figure 2.11

Figure 2.12

Figure 2.13

Figure 3.1

Figure 3.2

Figure 3.3

Figure 3.4

Figure 2.11 : Time Resolved spectra of 6CN2 in pH = 0.25

(0.6 M HCI), during the first 500 ps of the molecule’s lifetime.
Peak maximum at first is at 420nm and dies down. There is a
faint trace of peak at 500nm. The time after excitation from 36
top to bottom scans are 0, 20 , 30, 50, 70, 90, 110, 120, 140,
160, 180, 200, 220, 240, 260, 310, 360, 410, 460 and 510 ps.

(a) Fluorescence lifetime of 1x10'5M 60N2 in 0.6M HCI,
excited at 350 nm, emission collected at 420 nm. (b)
Fluorescence lifetime of 1x10’5M 6CN2 in 0.6M HCL,
excited at 350 nm, emission collected at 500 nm.

38

6CN2 equilibria relevant to this work. Top pane:
Protonation/deprotonation equilibria with ground state and
excited state Ka’s indicated. Absorption and emission

maxima are indicated. Bottom pane: Proposed 45
aggregation equilibria. We have implied the identity of the
aggregate species to be a dimer for reasons of physical and
chemical plausibility (see text).

Adipic acid (H2AA) and adipate anion (AA'Z’) and hydrogen
adipate anion 51
(HAA') equilibrium.19 The first pK. = 4.4 and the second pKa =
5.4.

Steady state spectra of samples for excitation at 300 nm,
solid line is spectrum taken prior to UV exposure and the
dotted line is the spectrum taken after the sample is exposed
to UV light. (a) Is H2AA and NazAA supersaturated and no
6CN2. b) Same concentration of H2AA and NazAA with also
1.0x10’ M 60N2 added.

56

Steady state spectra of 6CN2 in buffered H2AA/HAA'
solutions for excitation at 300 nm with spectra taken in time 5.,
intervals between 2 hrs to 12 hours. All spectra are identical.
Solution not exposed to UV light.

Steady state spectra of 60N2 in buffered H2AA/HAA' solution
with concentration ratio of H2AA/Na2AA 10:1 for excitation at
360 nm with spectra taken in time intervals between 2 hrs to 59
12 hours after exposure. (a) Solution kept in the dark (b)
Solution under UV exposure. Concentration of adipic acid to
sodium adipate is 10:1.

Figure 3.5

Figure 3.6

Figure 3.7

Figure 3.8

Figure 3.9

Steady state spectra of 6CN2 with H2AA/Na2AA concentration
ratio 30:1 and under UV exposure. The buffered H2AA/HAA'
and 6CN2 solution (a) 300 nm excitation (b) 360 nm
excitation.

61

Steady state spectra 6CN2 and H2AA with no NazAA added
in aqueous solution exposed to UV light. The buffered
H2AA/HAA' solution (a) for excitation at 300 nm (b) for
excitation at 360 nm.

63

Steady state spectra 6CN2 in buffered H2AA/HAN solution
for excitation at 350 nm. The solution was exposed to 355
nm UV light powered by a laser for 5 minutes and then
allowed to sit in the dark after exposure.

65

TCSPC data for excitation at 300nm, emission at 440nm.
Buffered H2AA/HAA' solution with concentration ratio of
H2AA and NagAA 30:1. The 6CN2 was in 1.0x10'5M
concentration.

67

Lifetime of H2AA solution with no NazAA and 10'5 M 6CN2 for

excitation at 300nm emission collection at 440 nm. 69

xi

LIST OF ABBREVIATIONS

TCSPC ............................................... Time Correlated Single Photon Counting
SCN2 ...................................................................................... 6-cyano-2-naphthol
H2AA .................................................................................................... Adipic acid
NazAA .......................................................................................... Sodium adipate
TAC ......................................................................... Tlme to amplitude converter
CFD ................................................................... Constant fraction discriminator
PMT ..................................................................................... Photomultipller tube
MCA ................................................................................... Multichannel analyzer
MCP ....................................................................................... Microchannel plate

xii

Chapter 1

INTRODUCTION

Many industries and pharmaceutical companies use crystallization to
purify chemicals. The range of materials purified by crystallization underscores
the importance of this fundamental chemical process. The details of
crystallization, such as growth kinetics and crystal size, depend on the system
properties, such as solute identity, solute concentration, solution ionic strength,
dielectric response, pH, and temperature. Crystallization, in many cases, is
influenced by the kinetics of crystal nucleation and growth, and in essentially all
cases, initiation of crystallization relies on an external event, such as shockwave
or the introduction of a nucleation impurity. The ultimate goal of our work is to
devise a means of initiating crystallization in a chemically controlled manner.
This thesis represents only the initial stages of this effort, focusing on the
properties of a photoacid.

Nucleation and crystal growth affect how large a crystal will be during the
crystallization process. If nucleation dominates, many small particles will result.
If crystal growth dominates, fewer larger crystals are obtained‘. Large crystals
are desirable from a chemical processing perspective. In order to produce large
crystals, the appropriate balance between nucleation and growth needs to be
achieved. Nucleation and growth are factors that are controlled by a variety of
system parameters with each system requiring optimization. Much effort has

been expended on investigating solution properties prior to crystallization, with an

-1-

eye toward understanding pro-crystallization organization.”8 Berglund et aP
started investigation of crystallization events with the use of a fluorescent probe.

Blanchard‘““

and coworkers have also used fluorescent probes and
spectroscopic techniques, such as Time Correlated Single Photon Counting
(TCSPC), to gain insight into crystallization precursors in glucose and adipic acid
solutions.

One solution property that can be used to control crystallization in
aqueous systems is pH. Carboxylic acid solubility depends on their charge.
Anionic, deprotonated species are typically more soluble in aqueous solution
than their protonated (neutral) forms. Reducing the pH of such solutions to the
point where pH<pKa of the weak acid can serve to initiate crystallization by
increasing the concentration of the less soluble, protonated form of the carboxylic
acid. The most well established means of controlling pH is either through the
addition of strong acids or bases to solution, or by the use of a buffer. Both of
these methods involve mixing and poorly controlled concentration gradients in
the crystallizing solution, thereby limiting the control that can be exercised over
crystallization. An alternative means to alter solution pH, albeit on a transient
basis, is to use organic photoacids in solution and with these species, control the
solution pH by means of light. The ultimate success of this approach is far from
being assured and rests on the kinetics of carboxylic acid deprotonation being
slow relative to the photon-induced proton production from the photoacid. The

first step in evaluating the feasibility of this approach is to understand the

protonation/deprotonation dynamics of the photoacid.

Photoacids are a class of molecules that, upon excitation, exhibit a
substantial decrease in pKa. This phenomenon typically yields an emission
spectrum for the deprotonated species that is shifted to the red of the protonated
form emission band. This red shift is commonly referred to as the Fbrster shift‘5'
16. One family of photoacids that has proven particularly useful is the
hydroxyarenes. Figure 1.1 shows a typical hydroxyarene energy level schematic
for the protonated and deprotonated forms. The equilibrium constant changes

for deprotonation upon photoexcitation. Both the pKa and pKa” can be

determined experimentally.

AH“ = A'* + H+

   

HF III-IIIIIIIIIIIIIIIIII-III

 

 

V

m:- ." 9“

AG

 

 

 

_thIIIIIIIIIIIIIIIIIIIIIIIII

pKa

= A'+H+

Figure 1.1: Energy equlllbrium diagram of the photoacid of its ground and exclted states.
AH is merely an arbitrary photoacid".

The difference value between the excited state equilibrium constant and
the ground state equilibrium constant varies within the structural identity of the
hydroxyarene. In 1994, Tolber't‘8'19 and coworkers synthesized a series of
photoacids with extremely large differences between pKa and pKa“, yielding some
pKa* values below zero. This family of molecules is referred to as super
photoacids,20 and all of the members reported to date are substituted

hydroxynaphthalenes (Figure 1.2).
CO °" °"
(2"

5-cyano-2-naphthol 6—cyano-2-naphthol

0

Z

Z

O

7-cyano-2-naphthol 8-cyano-2-naphthol

2

O

l I OH

5,8—cyano-2-naphthol

O

2

Figure 1.2: Structures of some super photoacids Tolbert and coworkers synthesized."

-4-

These 2-naphthols possess cyano group(s) on their distal ring. Different
locations of the cyano group(s) on the 2-naphthol ring structure results in
variations in pK; values. Tolbert has reported that these photoacids are strong

proton donors in a variety of solvents”23

and their protonation/deprotonation
equilibria can be monitored spectroscopically.2“'26 6-cyano-2-naphthol (6CN2)
has not been studied as extensively as some of the other substituted 2-
naphthols,”30 but SCNZ is available commercially and its spectroscopic and
photoacid properties are consistent with the other cyano-naphthols.

We will use 6CN2 as a proton donor in aqueous solutions of an organic
acid. We use a laser, already incorporated in our single photon counting system
to excite 60N2, producing protons in the light path. The spectral changes of

6CN2 on photoexcitation provide information on the kinetics of deprotonation and

reveal the conditions under which this chromophore may prove useful.

1.1 Literature Cited

1. Skoog, D.A.; Holler, F.J.; West, D. M.; Analytical Chemistry: An Introduction 7’"
Ed. Harcourt, Inc. Orlando, Florida 2000 182

2. Myerson, A.S.; Sorrel, L.S.; AlChE J. 1982, 28, 778

3. Sorrel, L.S.; Myerson, A.S.; AlChE J. 1982, 28, 772

4. Chang, Y.C.; Myerson, A.S.; AlChE J. 1985, 31, 980

5. Chang, Y.C.; Myerson, A.S.; AlChE J. 1986, 32, 1567

6. Chang, Y.C.; Myerson, A.S.; AlChE J. 1986, 32, 1747

7. Larson, M. A.; Garside, J.; Chem. Eng. Sci. 1986, 41, 1285

8. Larson, M. A.; Garside, J.; J. Crystal Growth. 1986, 76, 88

9. Chakraborty, R.; Berglund, K. A.; AlChE. Symp. Ser. 1991, 284, 113

10. Rasimas, J.P.; Berglund, K. A.; Blanchard, G. J.; J. Phys. Chem. 1996, 100,
7220 -

11. Rasimas, J.P.; Berglund, K. A.; Blanchard, G. J.; J. Phys. Chem. 1996, 100,
17034

12. Tulock, J. J.; Blanchard, G. J.; J. Phys. Chem. 81998, 102, 7148
13. Tulock, J. J.; Blanchard, G. J.; J. Phys. Chem. A 2000, 104, 8340
14. Kelepouris, L.; Blanchard, G. J.; J. Phys. Chem. A 2000, 104, 7261
15. F6rster, T.; Z. Elektrochem. 1950, 54, 311

16. Bell, R.P.; The Proton in Chemistry, 2”“ Edition. Cornell University Press,
New York. 1973

17. Caldin, E.; Gold, V. Proton Transfer Reactions. Chapman and Hall, New York
1975

18. Tolbert, L. M.; Haubrich, J.M.; J. Am. Chem. Soc. 1994, 116, 10593
19. Tolbert, L. M.; Haubrich, J.M.; J. Am. Chem. Soc. 1990, 112, 8163

20. Tolbert, L. M.; Solntsev, K. M.; Acc. Chem. Res. 2002, 35, 19

21.
22.

23.

24.

25.

26.

27.
28.
29.

30.

Solntsev, K. M..; Huppert, D.; J. Phys. Chem. A 1999, 103, 6984
Solntsev, K. M..; Huppert, D.; J. Am. Chem. Soc. 1998, 120, 7981

Solntsev, K. M..; Huppert, D.; Agmon, N.; Tolbert, L. M.; J. Am. Chem. Soc.
2000, 104, 46580

Knochenmuss, R.; Solntsev, K. M..; Tolbert, L. M.; J. Phys. Chem. A 2001,
105, 6393

Cohen, 8.; Segal, J.; Huppert, D.; J. Phys. Chem. A 2002, 106, 7462

Solntsev, K. M..; Tolbert, L. M.; Cohen, 8.; Huppert, D.; Yoshihito, H.;
Feldman Y.; J. Am. Chem. Soc. 2002, 124, 9046

Solntsev, K. M..; Agmon, N.; Chemical Physics Letters. 2000, 320, 262
Huppert, D.; J. Phys. Chem. A. 1997, 101, 4602
Agmon, N.; Rettig, W.; Groth, C.; J. Am. Chem. Soc. 2002, 124, 1089

Barroso, M.; Arnaut, L. G.; Fornosinho, S.J.; Journal of Photochemistry and
Photobiology. A: Chemistry 2002, 154, 13

Chapter 2

Dynamics of 6-Cyano-2-Naphthol in Aqueous Solutions. An
Examination of ProtonationlDeprotonation Dynamics and
Aggregate Formation.

Summary

We report on the behavior of 6-cyano-2-naphthol (6CN2) in aqueous solutions.
60N2 is a well known photoacid and we are interested in its ability to produce
local changes in solution pH upon excitation. We have measured the ground
state and excited state pKa values for 6CN2 and have studied the steady state
and transient optical properties of this chromophore as a function of pH. We use
transient spectral dynamics measurements to find the average deprotonation
time for 6CN2 as a function of solution pH and provide evidence for the existence
of a transient aggregate with a spectroscopic response suggesting it is a dimer.
With this information we are able to characterize the multiple equilibria

responsible for the spectroscopic behavior of 60N2.

2. 1 Introduction

Photoacids are a family of compounds that has attracted significant
attention for both practical and fundamental reasons. The substituted 2-
naphthols are one group of photoacids that has been studied extensively
because of the anomalously large difference in the acidity of their hydroxyl proton
in the ground and excited electronic states."‘5 2-Naphthols are known to exhibit
this effect based on the ability of the excited state anion to be resonance-
stabilized, giving rise to pKa* values more than 5 units lower than pK. values.3"°
Simple resonance structure predictions indicate that the placement of electron-
withdrawing substituents at certain positions on the distal ring of 2-naphthol will
result in an enhancement in the difference between pKa and pKa" values, and this
prediction has been verified with several positional substitutions and with several
different electron withdrawing groups.‘"'12 Of these so-called “super photoacid”
compounds, 6-cyano-2-naphthol (6CN2) has emerged as a relatively well
behaved member, and this compound is the focus of the work we report here.

A substantial reason for the interest in super photoacids lies in their ability
to protonate solvents, facilitating the study of photoinduced proton exchange
dynamics.2'3"'"7'9'‘2'15 We are interested in 60N2 because of its ability to alter the
local pH of a solution phase environment upon excitation. Such a local transient
decrease in pH could serve to mediate the solubility of certain species in solution
and 6CN2 could thus function as a “photo-trigger" for precipitation events,

depending on the proton association and dissociation kinetics of the putative

precipitating moiety. Prior to using 60N2 to photoinduce precipitation, however,
we need to understand its optical properties and protonation / deprotonation
dynamics. We report here on our measurements of the excited state pK, value
for this chromophore in aqueous solution and on the time-domain excited state
protonation / deprotonation behavior as a function of pH. Several reports in the
literature have hinted at the existence of substituted 2-naphthol aggregates in
solution,7'12 but we are unaware of any detailed information on such species. We
report here on our direct measurement of the linear optical properties of an
aggregated species. Our time-resolved reorientation and lifetime data provide
insight into the lifetime of this aggregate and suggest that excitation of the

aggregate gives rise to spontaneous dissociation.
2.2 Experimental

Chemicals. 6-Cyano—2-naphthol (60N2) was obtained from TCl America
and used as received. The solutions with varying pH of 60N2 were prepared
using deionized water obtained in-house. For all time-resolved measurements,
the concentration of 60N2 was 10‘5 M, with the pH of the solutions being
determined by the HCI or NaOH concentration. Electronic grade concentrated
HCI was purchased from CCI.

Steady state spectroscopy. Absorption measurements were made using a
Varian Cary model 300 double beam UV-visible absorption spectrometer. All
measurements were made with 1 nm resolution. Emission and excitation spectra

were recorded using a JY-Spex Fluorolog 3 emission spectrometer. For all

-10-

measurements, the excitation bandwidth was 2 nm and the emission bandwidth
was 2 nm.

Time-Correlated Single Photon Counting (TCSPC) Spectrometer. The
spectrometer used for the lifetime and dynamical measurements was similar to
one described in detail before and it is shown in Figure 2.116 The excitation light
pulses are produced by a synchronously pumped, cavity-dumped dye laser
(Coherent 702-2, 5 ps pulses, 4 MHz repetition rate). The dye laser is excited by
the second harmonic of the output of a cw mode-locked Nd:YAG laser (Coherent
Antares 768, 100 ps pulses, 76 MHz repetition rate). Samples were excited at
300 nm (Rhodamine 6G dye, Kodak, KDP Type I SHG) and 350 nm (LDS698
dye, Exciton, KDP Type I SHG).

The output of the dye laser in this configuration was ~ 90 mW average
power with pulses of 5 ps FWHM. The output of the dye laser was divided by a
beam splitter with 90% of the output being sent for excitation of the sample and
10% used as a reference pulse. This pulse sent into a fiber optic delay line
allowing for a temporal delay between the signal and the reference. The KDP
crystals used introduces astigmatism that is compensated by a piano-convex
cylindrical lens followed by filtering through a colored glass filter to remove longer
wavelength of light from the second harmonic light produced. The polarization of
the light is then rotated by a fused silica half-wave rhomb (CVl Laser
Corporation) to 0° to make the light vertically polarized. The polarized UV light is
focused into the sample with an 88mm focal length bi-convex fused silica lens

that results in an average power at the sample of s 1mW. For all experiments

-11-

emission is collected at 90° with respect to the incident light with a reflecting
microscope objective (Ealing x36/0.5) followed by a Glan-Taylor (G-T) prism for
selection of polarization. The polarization angles collected were at 0° and 90° for
anisotropy measurements and 54.7° for rather lifetime measurements. This G-T
prism was rotated by a motorized rotation stage (Newport Motion Controller
Model MM3000). The controller was operated using LabVlEW 7.0 software. The
selected polarization is then sent through a pseudo polarization scrambler and
focused into a subtractive double monochromator (CVI). The monochromator is
in communication to the computer and controlled by LabVlEW 7.0 software.

The detection electronics have been described in detail elsewhere", and
the significant features are reviewed here briefly. For detection, a Hamamatzu
(R3809U-50) cooled two plate microchannel plate photomultiplier tube (MCP-
PMT) is used and operated at a bias of - 3.10 W. The signal produced from the
detector is sent to one channel of a four channel constant fraction discriminator
(CFD, Tennelec T0455) for processing prior to its input to the time-to-amplitude
converter (TAC, TC864). We operate the TAC in “reverse mode”; the signal from
the MCP-PMT serves as the start signal and the stop signal originates from the
portion of the laser pulse that is split into the fiber optic delay line. This custom
made optical delay consists of varying lengths of fiber terminating in detection by
an avalanche photodiode detector (PD, RCA C30902E) whose signal is routed
into a second channel of the CFD. The output of the CFD reference signal is
electronically delayed (Tennelec TC412A) and sent to the TAC stop channel.

This configuration of the start and stop channels is termed “reverse mode” and

-12-

helps to eliminate pulse pile-up, and ensures operation of the TAC in a linear
conversion range. The valid conversions of the TAC output are displayed on an
oscilloscope (T ektronix 2230) and rate meter (Tennelec TC525) and sent to a
multichannel analyzer (MCA, Tennelec PCA-ll) for collection.

Emission was monitored using a 10 nm collection bandpass for each
wavelength of interest. The collection wavelength was stepped by computer
control and individual time-domain decays were collected to generate time-
resolved spectral profiles. Fluorescence was collected at 0°, 54.7°, and 90° with
respect to the vertically polarized excitation pulse. The instrument response
function for this system is typically 35 ps FWHM and transients measured range
from ~100 ps to ~6 ns. We did not deconvolute the instrument response function
from the experimental data. The shortest reorientation times we measure are on
the order of 50 ps, and because we can recover the entire anisotropy decay, loss
of the initial portion of the decay does not affect the accuracy of our

determinations.

-13-

pulse
diagnostics

  
 
  
 

2(1)

cavity dumped dye laser

a fiber optic
E switchable

   

I'll

560nm-900nm

     
 

filter

polarization
control

Figure 2.1: Schematic of the Time Correlated Single Photon Counter

A computer program written using National Instruments LabVlEW version
7.0° software was designed to automate the TCSPC system. For each
polarization collected an instrument response function was also acquired and
both data sets stored to the computer. This program created many ASCII data
files, which created a need manage the data more effectively. LabVlEW 7.0‘D
has a Report Generation analysis package that has commands to open Microsoft
Exce|°. A computer program was designed through LabVlEW 7.0‘‘3 to place all
magic angle/lifetime data in the appropriate Excel worksheets. In the Excel file
created, one worksheet was labeled “Responses” with the date attached to the

sheet and another worksheet was labeled “Lifetime” with the date attached to the

-14-

sheet. The Excel file was then saved with the same file name created in the
original program to run the TCSPC. Code was also develop to place the 0° and
90° responses and data files into a separate Excel program with similar names to
the worksheet, except the “Lifetime” sheet was named “Anisotropy”, and to add
on continuously if the program was started and stopped frequently. These sub-
programs were placed into the TCSPC data collection programs to operate and
collect the data for the instrument. Code was also developed to check before
data collection it the name provided already exists, if it did, the program stopped

and asked the user to proceed with a different file name and start over.
2.3 Results and Discussion

The purpose of this work is to establish an understanding of the
protonation and deprotonation dynamics of 6CN2. Ultimately, we would like to
mediate the solubility of compounds, such as adipic acid in aqueous solution, by
means of optically induced changes in solution pH. Any such change would be
transient, of course, and the ability to mediate solubility on timescales longer than
the excited state lifetime of a photoacid would hinge on the protonation and
deprotonation kinetics of the proton acceptor. As a first step in evaluating the
feasibility of any such approach to mediating solubility, we must understand the
behavior of the photo-induced proton donor. The excited state proton exchange
dynamics of cyano-2-naphthol derivatives have been examined extensively and
there have been several useful experimental and theoretical examinations of the
characteristic photoacidity of this family of compounds."2'514 Our experimental

data suggest that the time scale for proton transfer is on the order of tens to

-15-

hundreds of ps, depending in some instances on solution pH. In several studies,
aggregation phenomena have been invoked to explain long-lived features in the
transient optical response}12 but the putative aggregates remain poorly
characterized, especially in polar solvents such as water. We have also found
evidence for the aggregation of 60N2 at low pH, and our steady state and time-
resolved spectroscopic data provide some insight into its structural identity and
persistence time in solution. We propose a series of equilibria for 6CN2 in
aqueous solution to account for our findings. We have used steady state and
time-resolved spectroscopic measurements to obtain this information, and we
consider these measurements individually.

Steady state spectroscopic results. The absorption data shown in Figure
2.2 indicate that the optical absorption spectrum of 6CN2 in aqueous solution
does not vary substantially with pH over the range of -0.3 5 pH 5 3.0. The pK. of
6CN2 has been reported to be 8.4012 and we have not determined this quantity
experimentally because it does not lie within the pH range of the experiments we
perform here. We note that the absorption spectrum of 6CN2 does change when
the chromophore is deprotonated (Figure 2.2), but we do not access this pH

region in our experiments.

-16-

acidic solution
10'3 M to 2 M HCI

 

 

 
 
 

basic solution

 

 
  

. . ‘I~-4-'i"4 I~~~---7“1.__‘__J
225 250 275 300 325 350 375 400

wavelength (nm)

Figure 2.2: The pH dependence of 6CN2 steady state absorbance spectrum. The
spectra are shown In extinction units. All solutions were 1.0x10"5 M In water, with pH
controlled by concentration of HCI or NaOH. The protonated form Is characterized by
with a...” = 8890 L mol‘1 cm" at 300 nm, and the deprotonated form has em = 3410 L
mol‘1 cm'1 at 325 nm.

We have determined the pK.,* value for 6CN2 experimentally by means of
fluorescence titration18 because there is some uncertainty in the literature
regarding its value, but more importantly because it lies within the pH range
we access and it is thus important to have an accurate measure of this
quantity under our experimental conditions. Figure 2.3 shows the pH
dependence of the emission spectra of 6CN2. The band with a maximum
near 370 nm is associated with the protonated form of 60N2 and the 440 nm

band arises from the deprotonated form. This assignment is consistent with

-17-

the Forster model,2 which predicts a spectral red-shift of the emission band on
deprotonation. We have normalized the areas under these spectra and an
isoemissive point is seen at ~405 nm. From these data we find pK; = 0.25
for 6CN2 in water. Our value is in agreement with other reports which place
0.2 s pK; s 0.5. As has been noted elsewhere, the Fdrster shift can be used
to estimate the change in pK,3 of a molecule on excitation, and for 6CN2, this

model provides an underestimate of the pKa change.“15

normalized emission intensity

 

 

wavelength (nm)

Figure 2.3: pH Dependence of GCN2 steady state emission spectra. All 6CN2 solutions
were 1.0x10'5 M and were excited at 300 nm. The emission band for protonated 60N2
is centered at ca. 370 nm and the deprotonated form is centered near 440 nm. Solution
acid and base concentrations are (a) 2.0 M HCI, (b) 1.5 M HCI, (c) 1.0 M HCI, (d) 0.75 M
HCI, (e) 0.50 M HCI, (f) 0.1 M HCI, (g) 0.01 M HCI, (h) 0.001 M HCI, (I) 2 M NaOH. From
these data we have determined pK.‘ = 0.2

-13-

The absorption spectral data (Figure 2.2) indicate that the S +— So
absorption maximum of protonated 6CN2 is centered at 300 nm. Because the
pKa and pK; are so different, we excite exclusively protonated 60N2 in this
work, and monitor both the protonated and deprotonated excited species, and
the exchange of population between them. We will discuss these spectral
relaxation studies below. In addition to the dominant 300 nm absorption band,
we have identified a weak absorption feature centered near 350 nm, which is
present only at low pH. This 350 nm absorption band is associated with an
emission band in the region of 500 nm. In an effort to identify the origin of the
350 nm absorption band, we have acquired excitation spectra, monitoring
emission intensity at 430 nm and 500 nm. The 430 nm emission spectra are
associated with deprotonated 6CN2* and the 500 nm emission spectra are
associated with a different, previously unidentified species. By taking the
difference between the excitation spectra acquired for the two different emission
wavelengths, we can resolve the 350 nm absorption band. We show these
difference spectra in Figure 2.4, normalized for constant oscillator strength in this

spectral window. These data contain several interesting features. Near pH 3,

-19-

0.018

 

 

 

 

 

0.016 —
8
5 0.014 —
H ' decreasin H
7%. 0.012 - 31’
'U I-
.Q‘ 0.010 -
g .
§ 0.006 - l ,
E 0.004 l. . ,
O - ,
= 0.002 — A
01x» “Ni‘£".,|"~"‘: "if
-0.00Q 1 I

 

225 250 275 300 325 350 375 400

wavelength (nm)

Figure 2.4: Difference between excitation spectra of 6CN2 taken at the emission
wavelengths of 440 nm and 500 nm, as a function of pH. Spectra were normalized to
the intensity of the 234 nm band and the 260 nm band also seen in the pH dependence
of the absorbance spectra seen In Figure 1. The spectral feature centered at ca. 300
nm is the absorption band of protonated GCN2. The band at 350 nm is associated with
a species that Is neither the protonated or deprotonated form of 60N2. This new
feature Increases In intensity with decreasing pH and Is consistent with the existence
of an aggregated 6CN2 species (vide infra). All solutions were 1.0x10"M GCN2 and
[HCI] ranged from 10" M to 2 M.

the difference in intensity between the two excitation wavelengths is the result of
the different absorbance of 6CN2 at the two excitation wavelengths. At pH 3,
essentially the only species present is the protonated ground state — the 300 nm
excitation band is in excellent agreement with the absorption spectra shown in
Figure 2.2. As the pH is decreased, we observe a diminution of the 300 nm

excitation band and a corresponding increase in bands centered at 350 nm and

-20-

255 nm. It is clear from these difference spectra that there is a substantial
increase in the 350 nm band as the pH is lowered, indicating that the species
responsible for this subtle spectral feature we observe is associated with some
protonated form of 6CN2. A schematic of the equilibria between the ground state

and excited state forms of 60N2 is shown in Figure 2.5.

 

 

a
‘Q
a
‘0

km“

+----------------

 

+-----------

 

 

tr

 

Nl” k N2

Figure 2.5: The known equilibria between the excited states and ground state
monomeric species.

While it is tempting to interpret the difference spectra shown in Figure 2.4
in terms of an equilibrium between two species (e.g. two different protonated
forms) due to the form of these spectra, making such an interpretation is not
possible and not consistent with the spectra from which Figure 2.4 is derived.
The form of the difference spectra depends on the collection wavelengths used

and inferring equilibrium information from Figure 2.4 is not possible. The

-21-

excitation spectra used in the creation of Figure 2.4 show the dominant species
for all measurements to be protonated 6CN2.

We excite the sample at 350 nm and measure the emission spectral
profile associated with this new band. We observe that the emission band
associated with the 350 nm absorption feature has a maximum near 500 nm and
exhibits a pH dependence (Figure 2.6). For high pH solutions we obtain the
emission spectrum of the deprotonated form of the 6CN2 chromophore (Figure
2.6) and do not resolve any contribution from the 500 nm emission feature.
There is spectral overlap between the 300 nm absorption band of protonated
6CN2 and the 350 nm feature, resulting in a signal from both the monomeric
species and the new species appearing when the sample is excited at 350 nm.
Based on the spectral shift data and the pH-dependence for the 350 nm
absorption and 500 nm emission bands, we believe these features are those of

an aggregate of at least two 60N2 molecules.

-22-

normalized emission Intensity

 

 

 

wavelength (nm)

Figure 2.6: Solid lines: Steady state emission spectra of 6CN2 excited at 350 nm at
selected HCI concentrations: (a) = 2 M HCI, (b) = 0.6 M HCI, (c) = 10" M HCI. Dashed
line: Emission spectrum of a basic GCN2 solution excited at 350 nm, (d) = 10'5 M
NaOH.

If the 350 nm absorption band and 500 nm emission band are indeed
associated with an aggregate, the pH dependence of these bands should provide
some insight into possible structure(s) of the aggregate. We note that the
amount of aggregate present in solution is related to the solution proton
concentration. The form of the absorption and emission data are not consistent

with the 350 nm species being a doubly protonated form of 60N2. Rather, the

pH-dependence indicates the aggregate is formed from neutral

-23-

(monoprotonated) 60N2 molecules. The simplest aggregate would be a dimeric
species and we do not observe any precipitation of 6CN2 or other species in low
pH solutions. The existence of an aggregate is not unexpected based on the
dipolar nature of 60N2, with a face-to-face arrangement of opposing dipole
moments being one possible structure. This type of aggregate structure could
give rise to a modest lifetime for an aggregate, depending on the strength of
interaction, and our experimental data suggest this to be the case. Reasons for
the subtlety 0f the aggregate optical response may be that it is either short-lived
or that photoexcitation of the aggregate constituent(s) leads to dissociation. In
the latter picture, the deprotonation of one member of the aggregate would give
rise to an -O' functionality in close proximity to a nucleophilic -CEN moiety on
another 6CN2 molecule. Repulsive interactions for such a structure would force
the dimer constituents apart, leaving a deprotonated GCN2* and a protonated
ground state 6CN2. This explanation for the 350 and 500 nm spectral features is
speculative, and we can gain some additional insight into its behavior using time-
domain spectroscopy. Specifically, excitation of the 350 nm feature at low pH will
select for the putative aggregate, and its transient optical properties may provide
insight into its lifetime. The simplest aggregate would be a dimeric species and
we do not observe any precipitation of 6CN2 or other species in low pH solutions.
The existence of an aggregate is not unexpected based on the dipolar nature of
6CN2, with a face-to-face arrangement of opposing dipole moments being one
likely structure. There are two possible pathways that this particular dimer could

split apart and are shown in Figure 2.7.

-24-

 

Figure 2.7: The new species and the possible pathways for deprotonation

The first pathway suggests that upon excitation, there is no lifetime of the
dimer and it completely goes to an excited form of the deprotonated monomer
and a ground state monomer of 6CN2. However, the most probable pathway for
this type of aggregate structure would give rise to a modest lifetime for an
aggregate, and our experimental data suggest this to be the case. Reasons for
the subtlety of the aggregate optical response may be that it is either short-lived
or that photoexcitation of the aggregate constituent(s) leads to dissociation. In
this picture, the deprotonation of either member of the dimer would give rise to an
—O‘ functionality in close proximity to a nucleophilic -CsN moiety on another
6CN2 molecule. Repulsive interactions for such a structure would force the
dimer constituents apart, leaving a deprotonated 6CN2* and a protonated ground

state 6CN2. This explanation for the 360 and 500 nm spectral features is

-25-

speculative, and we can gain some additional insight into its behavior using time-
domain spectroscopy. Specifically, excitation of the 360 nm feature at low pH will
select for the putative aggregate, and its characteristic reorientation time
compared to that of monomeric 60N2 will place limits on the lifetime of the
aggregate.

Time Domain spectroscopic results. We have used time-correlated single
photon counting measurements to characterize the timescale of protonation and
deprotonation for 6CN2 and to understand the size of the species in solution.
For the former experiments we have acquired “3D” fluorescence spectra, where
both time- and frequency-resolved data are shown. Because the pK. of 6CN2 is
8.4, this molecule will exist exclusively in its protonated form in the ground state
at pH values less than 3. Upon excitation, because pKa" = 0.25, we expect
efficient deprotonation, leading to a decay of the 370 nm emission feature and a
corresponding build-up of the 440 nm emission band. We observe these trends
experimentally (Figure 2.8), and have acquired the SD emission spectra for 6CN2

at a series of pH values.

-26-

 

350 375 400 425 450 475 500
wavelength (nm)

 

l 4

350 375 400 425 450 475 500
wavelength (nm)

Figure 2.8 . (a) Time resolved emission spectra of 6CN2 In 0.1 M NaOH. (b) Time
resolved emission spectra of GCN2 In 2 M HCI. For both spectral data sets the time
after excitation from top to bottom scans are 0, 20 , 30, 50, 70, 90, 110, 120, 140, 160,
180, 200, 220, 240, 260, 310, 360, 410, 460 and 510 ps.

-27-

We report the 370 nm decay and associated 440 nm rise time at each of these
pHs in Table 1 and Figure 2.9. The data taken in 2 M HCI solution show a build-
up in intensity at early times at both 370 nm and 440 nm. Because the pH of the
2M solution (-0.3) is lower than the pK; of the excited chromophore, the majority
of the excited molecules will remain protonated and the build-up could, in
principle, be related to the time required for the solvent surrounding the
chromophore to accommodate to the excited state dipole moment or to the time
required by the chromophore to complete structural and vibrational relaxation
within the $1 manifold. The ~240 ps build-up time is far too slow to be accounted
for by solvent relaxation and it would be remarkable if population and structural
relaxation within the 6CN2 S, manifold required this much time. We do not have
sufficient data to elucidate the cause(s) of the build-up in both emission bands,
but it is possible that other protonated species play a role in the optical response
of 6CN2 at this low pH. The data for 0.6 M HCI and 0.001 M HCI yield the
expected result that following excitation there is a rapid decay of the protonated
species due to deprotonation, and a corresponding rise in the emission intensity
of the deprotonated form at 440 nm. For both 0.6 M and 0.001 M HCI solutions
the deprotonation time (fast 370 nm decay) is shorter than the build-up time for
the deprotonated emission intensity, suggesting the presence of either an
intermediate state or a measurably long (~50 — 100 ps) structural relaxation time
for the system. The latter explanation is not likely because we do not resolve any

spectral evolution aside from the expected bands in this time window.

-23-

Table 1.

Decay time constants and fractional contributions of the protonated and

deprotonated forms of GCN2. For both emission at 370 nm from the protonated form and
emission at 440 nm from the deprotonated form, the data are of the form f(t) = Arexp(-tln) +
Alexpotl 13). The physical gignlficance of the signs of A1 and A2 are discussed In the text.

 

Emission at 370 1 10 nm

Emission at 440 1 10 nm

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

300 nm exc.
A (normalized) ‘t (43s) A (normalized) T (ps)
-0.21 1 0.02 190 1 27
2 M HCI
0.79 1 0.02 1540 1 4
01010.02 1311 «02210.02 196113
0.6 M HCI
0.90 1 0.02 3847 1 10 0.78 1 0.02 3742 1 14
0.80 1 0.09 127 1 18 -0.36 1 0.01 198 1 7
0.001 M HCI
0.20 1 0.09 480 1 275 0.64 1 0.01 6071 1 32
350 nm ex c. Emission at 500 1 10 nm
A (normalized) 1 (ps)
-0.46 1 0.04 223 1 26
2 M HCI
0.54 1 0.04 2321 1 75
-0.59 1 0.05 247 1 35
0.6 M HCI
0.41 1 0.05 2984 1 357
-0.45 1 0.08 233 1 29
0.001 M HCI
0.55 1 0.08 4343 1 341

 

 

 

 

-29-

 

3500 '-
3000 '-
§ 2500 _-
8 2000 ~
1500 '-
1000 '-
500 '-

  

 

 

 

I I l l l I 1

0 1000 2000 3000 4000 5000
1800 mm (PS)

1600 _
1400 _

 

 

 

 

l'
02.13.12--- . . . . . . . .
0 1000 2000 3000 4000 5000
time (ps)

 

Figure 2. 9: (a) Fluorescence lifetime of 1x10“M 6CN2 In 2M HCI, excited at 300 nm,
emission collected at 370 nm. (b) Fluorescence lifetime of 1x10" M 6CN2 In 1 .0x10'5M
NaOH, excited at 300 nm, emission collected at 440 nm.

-30-

The lifetime data exhibit a relatively pH-independent time constant for the
evolution of the deprotonated form of 60N2, which is not expected. It has been
postulated that water dimers are adequate to accommodate the proton released
by the 6CN2 molecule upon excitation.2 Because we perform these experiments
in an aqueous medium, there is a substantial excess of water molecules in the
immediate vicinity of 6CN2, thereby placing the deprotonation reaction in a quasi-
zeroth order regime.

We consider next the long lifetime data for excitation at 300 nm and
collection of emission at 370 nm (protonated 6CN2) and 440 nm (deprotonated
6CN2). For emission at 370 nm, there is the expected pH dependence for the
2.0 M and 0.6 M HCI solutions (vide infra), with the long time emission behavior
of the 370 nm band of 6CN2 in 0.001 M HCI being anomalously short. We
ascribe this anomalous behavior to the fact that the pH of the solution is more
than 3 pH units above pK,,*, so the low efficiency of geminate recombination
plays a dominant role in determining the protonated 6CN2 lifetime. For emission
of the 440 nm band, we recover a pH dependence that is consonant with that
seen for the protonated form. The apparent pH dependence of the emission of
both the 370 and 440 nm bands is likely due not to [H‘], but to [Cl]. Chloride ion
is an efficient quencher and a Stern-Volmer plot of the emission rate constant as
a function of [CI'] yields a linear relationship and a quenching constant of Kq =
1.75 1 0.63 M'1 (Figure 2.10a).21 We note that the decay time constant for the
protonated and deprotonated forms of GCN2 is essentially the same for 0.6 M

HCI, which corresponds to a solution where pH = pKa‘. While it is not necessary,

-31-

and possibly not likely that Kq will be similar for the protonated and deprotonated
forms of 60N2, we note that phenomenologically, we observe Kq to be the same
for both species (Figure 2.10a), suggesting the relative unimportance of ionic
charge in this collisional quenching process.

The Stern-Volmer quenching analysis applied to the data assumes that
collisional (dynamic), quenching is occurring due to CI' in solution. The
quenching occurs as a result of a collisional interaction between the CI' and
excited 6CN2. The collisional quenching rate is limited by diffusion, the efficiency
of CI-6CN2* collisional relaxation, and changes in the temperatures and viscosity
of the solution, which can affect the quantum efficiency.22 In order to determine
the quenching constant of Cl' and 60N2 in the solution, the [CF] has to be high
enough so the probability of a Cl' - 6CN2 interaction is on the same order as the
probability of a radiative relaxation event for 60N2. The Stern-Volmer equation
is

(0°11 me) = 1 + K101 (1)
where 0% is the fluorescence quantum yield of 6CN2 without Cl’ present in
solution, (1).: is the fluorescence quantum yield with Cl' present, Kq is the Stern-
Volmer quenching constant, and [Q] is the concentration of the quencher O,
which in our case is Cl'. Upon rearrangement of this equation to solve for 01:", a
plot of the reciprocal of the observed fluorescence signal of 60N2 vs the [Cl]
yields a straight line as demonstrated in Figure 2.10, of which the Stern-Volmer
constant, Kq can be determined. The fluorescence quantum yield and the

fluorescence lifetimes are one in the same. Table 2.1 reports the fluorescence

-32-

lifetime values of 6CN2 in various concentrations of HCI. Upon higher HCI
concentrations, the lifetimes decrease confirming that Cl' is interacting with the
6CN2 and quenching the fluorescence of 6CN2 contributing to different lifetime
values.

We have also measured the lifetime of the 500 nm band after excitation at
350 nm. Our data show two important trends. The first is that the 500 nm
emission transients at all pHs are characterized by a ~230 ps build-up time. As
discussed above, the origin of this rise-time remains to be resolved
unambiguously, but for the aggregate such a transient response would be
consistent with the formation of an exciplex. The presence of the 350 nm
absorption band at low pH demonstrates the existence of ground state
aggregate, but excitation of this species may be associated with a structural
realignment to an emissive state. The second significant feature of these data is
the pH dependence of the long emission lifetime. These long time decays are
consistent with collisional quenching by Cl' and when the same Stern-Volmer
treatment is applied to these data, we recover a linear dependence with a Kq =
0.43 1 0.07 M'1 (Figure 2.10b). The fact that the Kq is smaller for the aggregate
than for either protonated or deprotonated 60N2 demonstrates that the
aggregate is structurally distinct from the monomeric forms. With this information
in hand, it is instructive to consider the motional dynamics of both the monomeric

and aggregate species.

-33-

 

 

1.0 1 . 1 . 1 . l 1 1 .
01) (15 11) L5 21)

[Cl']

4.5 -
- 1 b
4.0 -

35 e

31)-

rfl" (Hz)

25 -

 

2.0 1 . I . I . I r 1 1
01) 05 L0 L5 :10

[Cl']

 

Figure 2.10: (a) Stern-Volmer plot for the protonated form of GCN2 (o) and the
deprotonated form of 6CN2 (o) quenched by CT. Lifetimes for the protonated form
were acquired at 370 nm and 440 nm for the deprotonated form. (b) Stern-Volmer plot
of quenching of the aggregate species by Cl-. Data were acquired using 350 nm
excitation and 500 nm emission.

-34-

When probing the existence of the dimer at excitation 350 nm, very basic
solutions had similar results to excitation at 300 nm and this agrees with our
steady state spectra, since there should be no dimer present at pHs higher than
3. When looking at the “3D” spectral profile of 0.6 M HCI, Figure 2.11, one
notices there is a sharp peak around 420 nm with a rapid decay also with a
small, yet noticeable rising peak at 500 nm. This particular SD spectrum is noisy
compared to any obtained at excitation 300 nm and this is because the
fluorescence intensity probing at 350 nm for low pH solutions is extremely low.

There is a considerable amount of more noise present.

-35-

375 400 425 450 475 500 525 550 575
wavelength (nm)

Figure 2.11 : Time Resolved spectra of 6CN2 in pH = 0.25 (0.6 M HCI), during the first
500 ps of the molecule’s lifetime. Peak maximum at first Is at 420nm and dies down.
There Is a faint trace of peak at 500nm. The time after excitation from top to bottom
scans are 0, 20 , 30, 50, 70, 90, 110, 120, 140, 160, 180, 200, 220, 240, 260, 310, 360, 410,
460 and 510 ps.

To see the raw data, that was not altered, one can see a short lifetime of the
420 nm emission, Figure 2.12a, and a long component of a peak at 500 nm

emission, Figure 2.12b. When comparing all acid solutions of 6CN2 and the

-36-

steady state spectrum (Figure 2.6), it can be concluded that the 500 nm peak
shifts to shorter wavelengths upon raising the pH, and the 420 nm peak
increases in intensity and moves to longer wavelengths at the same time. This
suggests that the dimer is present but very short-lived, which is confirmed by our
time-domain data with the noise in the spectra. The 420 nm peak that is small at
very acidic concentrations but increases in intensity and broadness upon more
basic solutions is actually showing that there is a small amount of the excited

state deprotonated monomer present.

-37-

 

 

 

 

 

 

 

 

 

 

800-
i
6001 1
1% 4001 i
3
8
200- §§
0 ..... ,-
0 1000 2000 3000 4000 5000
time (ps)
4001
3001 i '
.g 2001
3
8 1
100-
0
0 1000 2000 3000 4000 5000
time (ps)

Figure 2.12 (a) Fluorescence lifetime of 1x10'5M GCN2 In 0.6M HCI, excited at 350 nm,
emission collected at 420 nm. (b) Fluorescence lifetime of 1x10"M 60N2 In 0.6M HCL,
excited at 350 nm, emission collected at 500 nm.

-33-

The lifetime data exhibit a relatively pH-independent protonation and
deprotonation time constant, which is surprising (Table 1). Several models have
been developed to account for proton transfer to the solvent and for many
photoacids it has been postulated that between two and four water molecules are
required to accommodate the proton released by the 6CN2 molecule upon
excitation. Robinson20 developed some of these theories, especially with 2-
napthols. These theories are especially applied to weak photoacids, in other
words photoacids that do not have a large pH jump. However, because GCN2 is
a super photoacid, the water dimers, if formed are effective proton acceptors
because solvent reorganization is less critical for proton to solvent transferz.
Because we perform these experiments in aqueous medium, it is likely that there
is a substantial excess of water molecules in the immediate vicinity of 6CN2,

thereby placing the deprotonation reaction in a quasi-zeroth order regime.

-39-

We have measured the reorientation times of 60N2 in several
environments, and present these data in Table 2. The reorientation information
is extracted from fluorescence intensity decays polarized parallel and
perpendicular to the vertical excitation polarization. These experimental intensity

data are used to generate the induced orientational anisotropy function, R(t),

R(t)=(1(t)uJam/(1(1)n+21(t)1) (2)
The function R(t) decays with up to five exponential components, but the most
common form is to observe a single exponential decay. Experimentally, two
decays are seen at most, and the resolution of multiple decay components is
based to a significant extent on the inherent symmetry of the reorienting
molecule and any restrictions the local environment places on the motion of the
chromophore. Owing to the low symmetry of 60N2 and the low viscosity of the
aqueous solutions we report on here, we would expect this chromophore to

exhibit a single exponential decay anisotropy.

Table 2. Reorlentation time(s) for 6CN2 as a function of pH and at different excitation and
emission wavelengths. In cases where multiple components of the anisotropy decay are
seen, we ascribe each decay component to an individual species.

 

 

 

 

 

 

 

 

 

 

 

300 nm exc. 370 nm emission 440 nm emission 500 nm emission

R(0) TOR (p8) R(O) Toe (98) mm tort (DS)
2 M HCI 0.401010 207121 0.181002 179125 0261008 167138
0.6M HCI 01810.04 224171 0131006 208129
1.0x10'5 M

0.19 1 0.05 123 1 23

NaOH
350 nm exc.
2 M HCI 0.31 1 0.04 159 1 35
0.6M HCI 02210.12 135158
1.0x10'5 M
NaOH

 

 

 

 

 

 

 

-40-

 

To interpret the reorientation times we use the modified Debye-Stokes-
Einstein (DSE) model,2‘““'23 where the time constant of the anisotropy decay is
related to the hydrodynamic volume (V) and shape (S) of the reorienting moiety

and the temperature (T) and viscosity (n) of the solvent medium.

M
r = 3
OR [(87.5 ( )

For 60N2 the hydrodynamic volume is 151 A3,“ we take the solution viscosity to
be 1 cP for all systems studied here, i: the solvent-solute friction coefficient,
which is 1 in water, k3 is the Boltzmann constant and T = 300 K. We assume the
solute shape factor to be unity (spherical solute) because the reorientation data
do not provide sufficient information to allow modeling this quantity accurately.
The DSE model assumes the solvent to be a continuum and is thus limited by its
inability to account for molecular interactions explicitly. Despite this limitation, the
modified DSE model is capable of predicting the reorientation behavior of polar
solutes in polar solvents with remarkable accuracy. Using Eq. 3, we estimate ton
= 50 ps for 6CN2 in water. We note that the DSE model significantly
underestimates the measured reorientation times for both protonated and
deprotonated 6CN2 in water. For excitation at 300 nm, we recover a
reorientation time constant of ca. 200 ps in all cases, with the time constant for
the basic form being the smallest (123 1 23 ps) by a substantial amount. Such
“super stick” behavior has been observed before for polar dyes in protic solvents,
but it is interesting to note here that both the neutral and anionic forms of 6CN2
are characterized by the same reorientation time to within the experimental

uncertainty, suggesting rapid proton exchange in this system. We note that

-41-

super photoacids are thought to exist within an extensive H-bonded network
formed between the solute and the solvent.2 If this is the case, the reorienting
moiety would be significantly larger than the bare 6CN2 molecule, consistent with
these data.

For excitation of 60N2 at 350 nm, only the 500 nm emission data yield
useful reorientation information, and this is expected based on the spectral
profiles of the monomer and aggregate species. We find that for 350 nm
excitation and 500 nm emission, the aggregated species reorients with the same
time constant as the monomer species, despite being characterized by a
substantially different Stern-Volmer quenching constant. The fact that the
aggregate exhibits a ca. 150 ps reorientation time suggests that either the
lifetime of the aggregate is on the order of the reorientation time, or the solvated
reorienting moiety is substantially the same in size as that of the solvated
monomer species. Because of the pH dependence of the long lifetime of the
excimer species it is not physically reasonable to conclude that the lifetime of the
aggregate is on the order of hundreds of ps. The fact that the reorientation time
of both the monomeric and aggregate species lies in the “super stick” regime
indicates that the reorienting moiety, is. the solute(s) and some amount of
solvent, is not substantially different in any of these cases. Based on the DSE
model, the monomer hydrodynamic volume of 150 A3 is consistent with a
reorientation time of 50 ps, as indicated above. The experimental reorientation
time of 150 - 200 ps observed experimentally suggests a reorienting moiety of

450 - 600 A3 in the limit of a “sticky” solvent-solute boundary condition. Since

-42-

the 450 — 600 A3 volume is substantially larger than either the monomer or a
putative dimeric species, we are not able to discern detailed information on the
aggregated species in solution, other than to say that its lifetime is on the order of
four ns or more based on the fluorescence lifetime data.

The steady state and time-resolved spectroscopic data on the excitation of
6CN2 at 300 nm indicate that it behaves primarily as a monomeric species in
solution. Our data for pK; are in agreement with other literature reports‘m'i'15
and our lifetime data show that protonation and deprotonation operates on the
~100 ps timescale, nominally independent of pH over a substantial range. When
6CN2 solutions at low pH are excited at 350 nm, we find evidence for a new
species in solution. The ca. 350 nm absorption band is associated with an
emission band at 500 nm, and the intensity of these features relative to those of
the 60N2 monomer increases with decreasing pH. These data collectively
reveal a complex dynamical system which we have schematized in Figure 2.13.
As is well established, the ground state and excited state protonation and
deprotonation dynamics are governed by substantially different equilibrium
constants, which are themselves the result of state-dependent changes in the it-
electron density distribution in the 6CN2 chromophore. These equilibria are well
characterized, but the equilibrium between monomeric and aggregated forms of
6CN2 is somewhat less well determined. Our data indicate that the aggregate is
present to a significant extent only in solutions with pH 3 pKa", and that the
lifetime of the aggregate is on the order of nanoseconds. The fact that we

observe the aggregate in the excitation spectrum indicates that the initial

-43-

complexation occurs in the ground state, consistent with the higher propensity of
ground state 6CN2 to retain its proton and thus remain neutral. We do not know
the extinction coefficient for the ca. 350 nm absorption band associated with the
aggregate, so it is not possible to readily estimate the value of the formation
constant for the aggregate. The fact that we monitored the presence of the
aggregate by emission spectroscopy accounts for the observation that the
aggregate is readily seen for pH 3 pKa*. Once the aggregate is excited, its
lifetime is at least as long as its fluorescence lifetime, several nanoseconds, and
the complex is susceptible to collisional quenching as seen by the Stern-Volmer
dependence of aggregate lifetime on [CT]. Reorientation data do not shed light
on the size of the complex directly, but all reorientation measurements indicate
that the reorienting moiety is substantially larger than the chromophore alone.
Thus, our frequency- and time-domain data on the aggregate provide limited
information on the structural details of the aggregate. Despite these limitations,
however, the clear demonstration of an aggregate of 6CN2 provides support for

an entity that has been hinted at in the literature for several years. 7'12

>i< _ >1:
OH K1156 O
C C

Nll’ : NI” :
A=300nm §A=370nm A=340nm §A=430nm
i i -
OH K1,...» 0
NI”C N”/C

u...___M..A-.—_—-_—__. .-_. -1, -- 7.5..» - - ._..1 .-.—-_ .... .ufl---— —-.—.—.—..~_-—--———

 

“__-_-_ ..- 1...- .._-__. v . _ - .1 . u.. .- _ - _. .7,kL_—_.___....-—___...._ ._

 

>l< )1
~00“ ”00°" ._ (6“ ‘6"
4c + ,0 H9 c s'm J
N ’ N ’ N’l
A=300nm §A=370nm A=350nm §A=500nm
i 1
OH cc
2 ’C HO ‘fiiumlgj
N” cc
N I
Figure 2.13: GCN2 equilibria relevant to this work. Top pane:

Protonationldeprotonatlon equilibria with ground state and excited state K.’s
Indicated. Absorption and emission maxima are indicated. Bottom pane: Proposed
aggregation equilibria. We have Implied the identity of the aggregate species to be a

dimer for reasons of physical and chemical plausibility (see text).

-45-

2.4 Conclusions

We have examined the time- and frequency-domain optical properties of
the photoacid 6CN2 in an effort to understand the dynamics of this chromophore
and its potential utility for effecting transient pH changes in solution. Our
determination of pKa" = 0.25 is in agreement with previous literature reports for
this compound and the time-domain spectra demonstrate the temporal evolution
of the protonated and deprotonated forms of the chromophore subsequent to
excitation. We account for the functional forms of the spectroscopic transients in
the context of protonation/deprotonation and collisional quenching by Cl' ions.
Excitation spectra reveal the presence of an aggregate characterized by an
absorption band centered at ca. 350 nm and an emission band centered near
500 nm. The emission of this aggregated species is quenched collisionally by Cl'
with a Stern-Volmer quenching constant different than that of either the neutral or
anionic 6CN2 monomer. Time-domain fluorescence depolarization
measurements show the aggregate species to reorient with the same diffusion
constant as the monomer to within the experimental uncertainty. The recovered
time constants for all species are longer than those expected based on the
modified Debye-Stokes-Einstein model, making reorientation measurements less
useful than anticipated. These data, taken collectively, demonstrate the
existence of a transient aggregate of 6CN2 in solution, with the likely dissociation
of the excited aggregate being determined by deprotonation of one of the

aggregate substituents.

~46-

 

25 Literature Cited

—L

10.

11.

12.

13.

14.

15.

16.

Tolbert, L. M.; Haubrich, J. E. J. Am. Chem. Soc., 1990, 112, 8163.
Tolbert, L. M.; Haubrich, J. E. J. Am. Chem. Soc., 1994, 116, 10593.

Huppert, D.; Tolbert, L. M.; Linares-Samaniego, S. J. Phys. Chem. A, 1997,
101, 4602.

Solntsev, K. M.; Huppert, D.; Tolbert, L. M.; Agmon, N. J. Am. Chem. Soc.,
1998, 120, 7981.

Solntsev, K. M.; Huppert, D.; Agmon, N. J. Phys. Chem. A, 1999, 103, 6984.
Cohen, 8.; Huppert, D. J. Phys. Chem. A, 2000, 104, 2663.

Solntsev, K. M.; Huppert, D.; Agmon, N.; Tolbert, L. M. J. Phys. Chem. A,
2000, 104, 4658.

Solntsev, K. M.; Agmon, N. Chem. Phys. Lett., 2000, 320,262.

Knochenmuss, R.; Solntsev, K. M.; Tolbert, L. M. J. Phys. Chem. A, 2001,
105, 6393.

Agmon, N.; Rettig, W.; Groth, C. J. Am. Chem. 800., 2002, 124, 1089.

Barroso, M.; Arnaut, L. G.; Formosinho, S. J. J. Photochem. Photobio. A,
2002, 154, 13.

Clower, C.; Solntsev, K. M.; Kowalik, J.; Tolbert, L. M.; Huppert, D. J. Phys.
Chem. A, 2002, 106, 3114.

Cohen, B.; Segal, J.; Huppert, D. J. Phys. Chem. A, 2002, 106, 7462.

Solntsev, K. M.; Tolbert, L. M.; Cohen, R; Huppert, D.; Hayashi, Y.;
Feldman, Y. J. Am. Chem. Soc., 2002, 124, 9046.

Tolbert, L. M.; Solntsev, K. M. Acc. Chem. Res, 2002, 35, 19.

DeWitt, L.; Blanchard, G. J.; LeGoff, E.; Benz, M. E.; Liao, J. H.; Kanatzidis,
M. G. J. Am. Chem. Soc, 1993, 115,12158.

-47-

17. Schulman, S. G. Acid Base Chemistry of Excited Singlet States. In Modern
Fluorescence Spectroscopy, Wehry, E. L., Ed.; Plenum Press: New York,
1976; PP 239.

18. lngle, J. D.; Crouch, S. R. SpectrochemicalAna/ysis", Prentice Hall: Upper
Saddle River, NJ, 1988.

19. Robinson, G.W. J. Phys. Chem, 1991, 95, 10386
20. Debye, P. Polar Molecules Chemical Catalog Co.: New York 1929.
21. Szabo, A. J. Chem. Phys, 1984, 81, 150.

22. lngle, JD. and Crouch, S.R. Spectrochemical Analysis Prentice Hall, New
Jersey 1988

23. Perrin, F. J. Phys. Radium, 1934 5, 497.
24. Edward, J. T. J. Chem. Ed, 1970, 47, 261.

-43-

Chapter 3

Using the Photoacid 6CN2 with a Weak Acid System

Summary

We report on the complexation between adipic acid and 6-cyano-2-naphthol
(SCN2) in aqueous solution. Adipic acid (H2AA) is a diprotic weak acid which has
a pH dependent solubility in an aqueous solution. The solution is buffered
slightly above pH=4.4, close to me for adipic acid. The goal is to poise the
system to contain enough hydrogen-adipate anions (HAA') that if protonated,
would produce a supersaturated H2AA concentration. The photoacid 6CN2 is
used to donate protons to this solution upon excitation. We investigate the
spectroscopic properties of 60N2 in this buffered system using steady state

spectroscopy and time correlated single photon counting (TCSPC).

-49-

3.1 Introduction

Many industries utilize crystallization as a method of production,
purification, or even to recover solid starting materials. Crystallization methods
can result in a product that is a large crystal or an amorphous solid with little
control over is the key factors that determine the size of the crystals."8 The larger
the particle the more techniques can be applied to determine the actual
molecular structure of the product and other properties of the crystal. The
mechanical, electrical, magnetic, and optical properties of a material can vary
according to the crystal size, habit, and packing of its constituent molecules. In
general, it is commonly considered the more crystalline a product, the higher its
purity. Many commercially mass produced products depend upon the pure
starting materials to produce useful product.‘ An example of a commercial
product whose production output is dependent upon the purity of its starting
materials is nylon 6, 69.

Nylon 6, 6 is a widely used polymer that is made from a reaction of adipic
acid and 1, 6-hexane diamine. The purity of nylon 6, 6 can be limited by the
purity of the adipic acid used. Adipic acid is rarely found in nature, and it is
synthetically produced in a variety of ways, with most of them starting with
reactants involving the oxidation of cyclohexane into cyclohexanol and
cyclohexanone‘mz. During this synthesis the products are then reacted with nitric
acid, resulting in the production N02 (g)

Adipic acid is a polyprotic acid that has two pKa’s, and the crystallization

event happens is below the first pKa. Investigations into the early events of pH

-50-

dependence of crystallization have been explored.“"18 Ideally the H2AA solution
is buffered to a pH near the pKa1 using HAA' ready to accept a proton and
precipitate the H2AA out of solution. Significant control over pH in a buffered
system is difficult, so the buffer constituent concentrations should be established
with an eye toward minimum buffer capacity. This condition may or may not be
consistent with poising the system near its solubility limit. Figure 3.1 shows the

equilibra adipic acid undergoes.

 

 

 

0
O
PKat
H0 —_* ”0 0 + H‘
OH
0 O
H2AA MA
0 0
PM
HO .0 . + H+
0 0
0 O
. M4
I'IAA

Figure 3.1: Adipic acid (H2AA) and adipate anion MA") and hydrogen adipate anion
(HAA') equilibrium." The first pK. = 4.4 and the second pK. = 5.4.

One means of controlling the pH of the solution is through the use of a

photoacid. In this case we use the photoacid 6CN2 to control the pH in the

-51-

adipic acid solution. The spectroscopic properties of 60N2 were reported in
Chapter 2 of this thesis. By knowing the spectroscopic properties of SCN2 we will
see how the properties change upon being in a buffered solution containing HAA'
and H2AA. The solutions used here will contain enough HAA' anions so that
upon protonation, a supersaturated concentration of H2AA can be created. The
photoexcitation of 6CN2 will ideally induce a local pH drop sufficient to induce
crystallization of H2AA if the concentrations of HAA', 60N2, and H2AA are set
appropriately. This chapter reports on our initial steps in evaluating the feasibility
of this means of crystallization.

A laser is used to excite the 6CN2 and in the path of the light, protons are
donated by the excited 6CN2 to its local environment. The disassociated proton
could associate with the solvent, water, or H2AA or it could recombine with the
6CN2 anion. We have demonstrated that there is an associative interaction
between the buffered adipic acid system and the photoacid, which we can
describe in the context of dipolar and hydrogen bonding interactions. We will also
investigate the steady state and time domain spectroscopic properties of 6CN2 in

the presence of adipic acid.

-52-

3.2 Experimental

Chemicals. 6CN2 was obtained from TCI America and adipic acid and
sodium adipate were obtained from Aldrich (both 99% purity). All chemicals were
used as received and no further purification was done. All solutions were made
with deionized water. Various concentrations of sodium adipate and adipic acid
were used and mixed together. The solution was constantly stirred and heated
until all solids were dissolved. Then the solution was slowly cooled to ensure
adipic acid would not precipitate. If any precipitate formed, the solution was
filtered and 6CN2 was added. The concentration of the 6CN2 was 1.0 x 10'5 M to
minimize incorporation of the photoacid serving as an impurity or nucleation sites
for the crystallization of H2AA. We note that this concentration of GCN2 is good
for spectroscopic purposes but may be too low to induce H2AA crystallization.

Steady state spectroscopy. Absorption measurements were made using a
Varian Cary model 300 double beam UV-visible absorption spectrometer. All
measurements were made with 1 nm resolution. Emission spectra were
recorded using a JY-Spex Fluorolog 3 emission spectrometer. For all emission
measurements, the excitation bandwidth was 2 nm and the emission bandwidth
was 2 nm. The excitation wavelengths were 300 nm and 360 nm.

Time-Correlated Single Photon Counting (TCSPC) Spectrometer. The
spectrometer we used for the lifetime and dynamical measurements is similar to
one that has been described in detail before20 and in Chapter 2. Fluorescence
was collected at polarizations of 0°, 54.7°, and 90° with respect to the vertically

polarized excitation pulse. The instrument response function for this system is

-53-

typically 35 ps FWHM and lifetimes measured range from ~100 ps to ~6 ns. We
did not deconvolute the instrument response function from the experimental data.
The shortest reorientation times we measure are on the order of 50 ps, and
because we can recover the entire anisotropy decay, loss of the initial portion of
the decay does not affect the accuracy of our determinations. The solution was
also stirred with a small magnetic stir bar inside the quartz cuvette to ensure that

there would be no photodegradation.

3.3 Results and Discussion

The purpose of this chapter is to explore the feasibility of 60N2 to be
crystallization initiator for H2AA. NazAA was added to a solution of H2AA and
HM’ was formed by disproportionation. NazAA is easily dissolved in aqueous
solutions to produce Na“ and AA2' ions. The NagAA is much more soluble than
H2AA, which has 1.1 wt% solubility in water. The HAA' is able to accept a proton
from an external proton donor such as super photoacid 6CN2. Upon protonation,
H2AA is formed and possibly starting a point to start a nucleation site and
facilitate crystallization growth. For samples that had H2AA and NazAA in a 30:1
molar ratio, [H2AA] = 0.157 M and [NazAA] = 5.17 x 10'3 M. For samples that had
a 10:1 ratio of H2AA/Na2AA, the concentrations were [H2AA] = 0.110 M and
[NazAA] = 9.64 x 10'3 M. Samples with no NazAA present had 0.242 M for H2AA.
All solutions considered for the different ratio amounts were made in 100 mL

solutions.

-54-

Steady state spectroscopy. Samples exposed to light were placed in a
quartz cuvette and were irradiated by ultraviolet lamp over a time span of 2 to 12
hours. Steady state emission spectra were excited at two different wavelengths,
300 nm and 360 nm, before and after ultraviolet exposure. All spectra were
normalized to have an integrated area equal to one to allow facile comparison
between samples. No fluorescence impurities were found in buffered H2AA/HAN
solutions without 6CN2 present. Figure 3.2a shows that if no 60N2 is present
before and after ultraviolet lamp exposure there are no major spectroscopic
changes in the buffered H2AA/HAA' system. The sharp peak found at 400 nm (for
300 nm excitation) is the second overtone of the Raman spectrum of water.
Figure 3.2a had no 6CN2 added to the system and is normalized however when
looking at the unnormalized spectrum, the fluorescence intensity of this solution
is negligible compared to the fluorescence intensity of the same solution with the
6CN2. It is important to note that when the two samples are analyzed and
normalized, it appears that there could be 6CN2 present in the system. A
possible cause for this is that the cuvette, even though it was washed thoroughly,
had minor traces of the fluorescing species present and probably sticking to the
side of the cuvette. 6CN2 therefore the only fluorescing species and it is a good
probe to use with the buffered solution. In a buffered H2AA/HAA' sample with
6CN2 present and the same exposure conditions results in a noticeable 440 nm
emission peak, corresponding to the deprotonated monomer anion of 6CN2. This
peak increases in intensity after constant exposure to the ultraviolet lamp (Figure

3.2b).

-55-

Normalized Relative Intensity

Normalized Relative Intensity

0.006 -
0.005 -
0.004-1
0.003 -
0.0021
0.001 I

0.000 -

, (a)

 

 

0.007 -
0.006:
0.005;
0.004 -
0.003;
0.002:
0.001 -

0.000 -

 

 

350 400 450 500 550 600
Wavelength (nm)

(b)

 

 

350 400 450

j

' l

500 ' 550 600
Wavelength (nm)

Figure 3.2: Steady state spectra of samples for excitation at 300 nm, solid line is spectrum
taken prior to UV exposure and the dotted line Is the spectrum taken after the sample Is
exposed to UV light. (a) Is H2AA and NazAA stipersaturated and no SCN2. (b) Same
concentration of H2AA and MM with also 1.0x10 M

6CN2 added.

-56-

Figure 3.3 is the emission spectrum of the buffered H2AA/HAA' solution
with 60N2 present resulting in no spectroscopic changes for excitation at 300nm
if not exposed to light, representing the stability of spectroscopic properties of the
60N2 fluorescing features. The 6CN2 in this solution has a pH that results in no
spectroscopic features of the protonated excited state monomer and only the
deprotonated anion is present. Another aliquot of the solution was exposed to the
ultraviolet lamp and resulted in no significant spectroscopic changes for

excitation at 300 nm.

 
 
 
  

 

z. °'°°6 ‘ — After 2 hrs
'7) —— After 3 hrs
5, 0.005 - —— After 4 hrs
E ——-— After 12 hrs
9 0.004 -
E
E 0.003 -
'0
o
-.-_"-‘ 0.002 -
to
E
o .-
2 0.001

0.000 -

I ' I ' I

 

I ' l ' l ' l '
300 350 400 450 500 550 600
Wavelength (nm)

Figure 3.3: Steady state spectra of GCN2 in buffered H2AA/HAN solutions for excitation at
300 nm with spectra taken In time Intervals between 2 hrs to 12 hours. All spectra are
identical. Solution not exposed to UV light.

-57-

Essentially no spectroscopic changes were seen for excitation of the
buffered H2AA/HAA‘ and 60N2 solutions for excitation at 300 nm. However,
spectroscopic changes in 60N2 features did occur with constant light exposure
for excitation at 360 nm. Samples were made and exposed to light under the
same conditions stated above. Figure 3.4a represents the sample unexposed to
ultraviolet light and resulted in no spectral changes with time, again confirming
the photostability of 6CN2. Figure 3.4b has a sample exposed to light between
the time span of 2 to 12 hours. There is the 440 nm emission peak, which is
spectral overlap from the deprotonated monomer anion with an additional feature
at 500 nm which decreased in intensity over constant light exposure. There is a
possibility more excited state anions are being created over time as a result of
the 440 nm peak increasing in intensity. The 500 nm peak maximum was found
to shift to shorter wavelengths with increasing pl-I resulting in a combination of
spectral overlap from the super photoacid anion monomer form and a
complexation, which could be an aggregate as discussed in Chapter 2, or a

complex between H2AA and 6CN2.

-53-

Normalized Relative Intensity

(a)

   
  

 

 

 
   
  

 

3‘ 0.004 - .
,5 —After 2 hrs in dark
8 ———After 3 hrs in dark
‘5 ———After 4 hrs in dark
3 0.003 - —After 12hrs in dark
.2
E
{E 0.002 -
8
.5
g 0.001 -
5
z 1
0.000 -
v I v I V I ' I ' l
350 400 450 500 550 600
Wavelength (nm)
b
0.006 - ( )
, — 2 hrs of UV exposure
11- — 3 hrs of UV exposure
0.005 - films —— 4 hrs of UV exposure
1 ' ' ‘1 — 5 hrs of UV exposure
0.004 d ' ' m 6 hrs of UV exposure
—— 7 hrs of UV exposure
8 hrs of UV exposure
0.003 - ———-— 12 hrs of UV exposure
1
0.002 -
0.001 -
0.000 -
T I l ' I ' I

 

400 I 460 ' 500 550 600
Wavelength (nm)

Figure 3.4: Steady state spectra of 6CN2 In buffered H2AA/HAN solution with
concentration ratio of H2AA/N82“ 10:1 for excitation at 360 nm with spectra taken In time
Intervals between 2 hrs to 12 hours after exposure. (a) Solution kept In the dark (b)
Solution under UV exposure. Concentration of adipic acid to sodium adipate Is 10:1.

-59-

Various concentration ratios of H2AA and NazAA for the buffered
H2AA/HAA' solution with 60N2 were investigated in steady state spectroscopy to
determine if 6CN2 changed in its spectroscopic properties. Regardless of the
concentration ratios of these two species, no spectral changes of 6CN2 for
excitation at 300 nm were found. However, various concentration ratios of
NazAA and H2AA affected the spectral features of 6CN2 for excitation at 360 nm.
The previous solutions analyzed were with a concentration ratio of H2AA and
NazAA of 10:1. Significant changes were found and investigated with a 30:1
concentration ratio of H2AA to NazAA respectively. Aliquots of the solutions kept
in the dark and analyzed under steady state conditions but are not shown since
they are time invariant. Figure 3.5a is the sample excited at 300 nm and (b) is an
emission spectrum of a sample excited at 360 nm for the 30:1 H2AA/Na2AA ratio.
By comparison of Figure 3.5b to Figure 3.4, the 500 nm emission peak is more
prominent for H2AA/Na2AA ratios that were 30:1, as opposed to 10:1. Again, for
excitation at 300nm, there are no spectral changes in 6CN2 fluorescence

spectrum with varying concentration ratios of H2AA/Na2AA.

-60-

0.007 -
0.006 -
0.005 -
0.004 -
0.003 -
0.002 —

0.001 -

Normalized Relative Intensity

0.000 -

 

(a)

— before UV exposure

— 2 hrs of UV exposure
- -_ 1 day of UV exposure
— 2 day of UV exposure

 
  
  

 

0.0045-
0.0040 -
0.0035-
0.0030-
0.0025-
0.0020 —

0.0015 -

Normalized Relative Intensity

0.0010 -
0.0005 -

0.0000

 

I ' I ' l ' I ' I ' fi'
350 400 450 500 550 600
Wavelength (nm)

(b)

—— before UV exposure
— 2 hrs of UV exposure
— 1 day of UV exposure
— 2 days of UV exposure

 
  
  

 

350

Wavelength (nm)

Figure 3.5: Steady state spectra of GCN2 with HzANNazAA concentration ratio 30:1 and
under UV exposure. The buffered H2AA/HM" and GCN2 solution (a) 300 nm excitation (b)

360 nm excitation.

-6l-

Figure 3.6 is an emission spectrum of a solution of only H2AA and 6CN2
exposed to ultraviolet light and monitored with steady state spectroscopy for
excitations at 300 nm and 360 nm. The solution was prepared as stated above to
ensure the system was saturated with HAA' molecules. Figure 3.6a represents
that there is no spectral change for excitation at 300 nm. Figure 3.6b is for
excitation at 360 nm. In comparison with the other concentration ratios, the
spectral feature at 500 nm is less pronounced with no NazAA present. In Chapter
2 it was discussed that there is a possible aggregate formation at low pH for
6CN2, and it resulted in spectral features at 500 nm emission for excitation at
360 nm. It was discussed that at 500 nm emission there is possible complex
formation and there is greater chance of a complex formation with the addition of
NazAA to be included to make the buffered H2AA/HAA' solution. By showing the
three different concentration ratios of H2AA/Na2AA, the optimal concentration of
the buffered system to monitor any changes is with the 30:1 concentration.
However, even though this ratio may be optimum to monitor spectral changes,
the optimal concentration to have the 6CN2 initiate the crystallization event upon
excitation still needs to be investigated. So far, no concentration ratios with 6CN2

present resulted in initiating crystallization of H2AA.

-62-

0.007 - (a)

 

 

 

 

 

2‘ 0.006 - —— before UV exposure

'17: ‘ —— 2 hrs of UV exposure

0:) 0.005 - —— 12 hrs of UV exposure

E

g 0.004 -

E 1

(“5’ 0.003 -

1:

at

g 0.002 -

E

o 0.001 -

2

0.000 -
I ' I ' l ' I ' l f l ' I ‘
300 350 400 450 500 550 600
Wavelength (nm)

0.005 " (a)
2‘ _ ,_ —— before UV exposure
'17) 0.004 - ,1iI'l , — 2 hrs of UV exposure

1‘ I

cc) .1 ,- —— 12 hrs of UV exposure
«s-o 4 I i - ‘
s y‘-
o 0.003 - . ‘ .
.2 1' "
a " 1
0) .1
cc 0.002 - l ,
'8 l 'V
N l r “I
= I II ‘I ,
g 0.001 - ,‘ ~1_ 1,
5 \M" {0
z .

0.000 -

' I ' l T l ' T ' I
350 400 450 500 550 600

Wavelength (nm)

Figure 3.6: Steady state spectra GCN2 and H2AA with no NazAA added In aqueous solution
exposed to UV light. The buffered H2AA/HAN solution (a) for excitation at 300 nm (b) for
excitation at 360 nm.

-63-

Another experiment was performed to investigate after 10 minute constant
exposure to 350 nm laser light beam path to see if the system was reversible
with the proton donation from the super photoacid (Figure 3.7). The spectral
feature at 500 nm diminishes after constant ultraviolet lamp exposure and the
440 nm emission peak increases in intensity. When the sample with all three
components in the solution is constantly exposed at 355 nm, the solution
fluoresces from green to blue in the path length of the light as well as tiny
particles were moving around in solution. A 250 mL buffered H2AA/HAA' solution
with 60N2 present was irradiated for excitation 355 nm by the laser for 30
minutes. There was no precipitation that resulted in the bottom of the flask
suggesting no definite determination that enough H2AA was added to precipitate
out of solution. After constant 355 nm exposure, the sample was removed and
allowed to sit in the dark. After 12 hours, a steady state spectrum was collected‘
for excitation 360 nm the 500 nm peak that had disappeared had returned and
the 440 nm peak had diminished in intensity. This 500 nm spectral feature did not
return to its original intensity prior to exposure. The solution was then stirred for 2
hours and another steady state spectrum was collected at 300 nm and yielded
the same spectral features as the one 2 hours prior. The 500 nm feature never
returns fully to its original bandshape prior to UV exposure, suggesting that the
system is partially irreversible. When the 6CN2 in the buffered H2AA/HAA’
solution is exposed to UV light, it causes proton donation to the adipate anion,
which based on the spectroscopic data is not fully reversible even on day long

time frames.

   
   

2, 1'6: — Before Exposure
g 1.41 — After 5 min. Exposure
o ‘ — After 5 min. Exposure
E 1'2: A; —1 day after
,3 1.0- " ——-—- 1 day and 2 hr of stirring
E .
(E (18: “\\\
E 0.6:
to _ .
g 0.4 4 \
z 0.2-
0.0:
-0.2

 

 

400 450 500 550 600
Wavelength (nm)

Figure 3.7: Steady state spectra 6CN2 In buffered H2AA/HAN solution for excitation at 350
nm. The solution was exposed to 355 nm UV light powered by a laser for 5 minutes and
then allowed to sit In the dark after exposure.

No visible precipitate was found, but that is not to say that very small
crystallites or rather aggregates are not forming. The Blanchard lab is not
equipped to detect such a species forming after excitation of 6CN2 in the
buffered H2AA/HAA' solution. One possibility that no precipitate was found could
be the concentrations of each species in solution were not correct to push the
adipic acid to nucleate and form a precipitation event to occur. If the solution did
not have enough HAA' accepting protons, then the probability for the H2AA

molecules that did form to come in contact with one another and form a

-65-

nucleation site is extremely small. The other factor discussed in affecting crystal
growth was particle size and if there are relatively few H2AA formed, the particle
size is probably small resulting in little chance for crystallization to occur. The
spectroscopic changes of 60N2 for excitation at 360nm show that there is a
permanent change signifying the possibility that a proton was donated to the
solution and possibly H2AA was formed.

Time correlated single photon counting (TCSPC). Time domain studies
were performed with SCN2 in aqueous solutions with a concentration ratio of
H2AA and NagAA to be 30:1 and also one with H2AA and 6CN2 in the solution.
When these mixtures were prepared and placed in the path of the light for the
TCSPC and it was excited at 300 nm the solution fluoresces a bright green.

Table 3.1 shows the lifetimes of buffered H2AA/HAN solutions with 10'5 M
GCN2 present. The longer lifetime values resemble those near a similar pH
SCN2 not in a buffered system, and reported in Chapter 2. Figure 3.8 shows the
raw data collected of a buffered sample with the adipic acid to sodium adipate
concentration ratio 30:1. The lifetimes fit a biexponential decay and the values
are similar to the values reported in Chapter 2, suggesting the spectroscopic time
domain features of 60N2 is not altered much in the presence of H2AA, HAA', and
AA'2 in the solution. The fluorescence lifetime is the same with NazAA present in

the solution and with it not present in the solution.

-66-

Table 3.1: Lifetime values of buttered H2AA/HAN solutions with 1.0 x 10“ M 6CN2.
Excitation at 300 nm.

 

 

 

 

 

Excitation 300nm A1 1:, A2 1:;

Emission 440nm

Buffered Solution 0.61 6049 i 55 -0.39 192 i 7.40
Solution, No NazAA 0.62 5905 i 39 -0.38 180 i 4.80

 

 

 

 

 

.0... WWW, with

 

Counts
TS‘
£-
:1:

200-

 

 

 

 

. , . T . . , .
o 1000 2000 3000 4000 5000
Time (ps)

Figure 3.8: TCSPC data for excitation at 300nm, emission at 440nm. Buffered H2AA/HAN
solution with concentration ratio of H2AA and NagAA 30:1. The GCNZ was in 1.0x10'5M
concentration.

~67-

 

Figure 3.9 has similar information as Figure 3.8, except there is no NazAA
present in the aqueous solution. initial inspection of the spectra, without looking
at the exponential fit results appears no significant spectral changes happened
for excitation at 300 nm and collection at 440 nm. Tables 3.1 reports the lifetime
information, which shows upon comparison to values found in Chapter 2,
resemble lifetime values for pH greater than 3. Table 3.2 is investigating if the
anisotropy of 6CN2 changes when other species are present in the system. The
reorientation information is extracted from fluorescence intensity decays
polarized parallel and perpendicular to the vertical excitation polarization. Table
3.2 shows the anisotropy values for these systems, and they correspond to what
happens for molecules in aqueous systems. in fact, when comparing to our
values in Chapter 2 and noting the pH, our data in a buffered system at excitation
300nm is in excellent agreement with our previous results. it only shows that at
this pH there is only a monomer form present of the solution. This is consistent
with our steady state spectra, that the concentration of H2AA and NazAA does

not affect the spectroscopic features of 6CN2 for excitation at 300 nm.

-68-

1400-
1200-
1000-

"MWW it
800 j z W? all

600-

Counts

4oo~

 

 

 

'200 I I I I l t I t I I I
0 1000 2000 3000 4000 5000

Time (ps)

Figure 3.9: Lifetime of H2AA solution with no N82“ and 10" M 6CN2 for excitation at
300nm emission collection at 440 nm.

Table 3.2: Anisotropy values for buffered system with 6CN2 in solution. Excitation 300 nm,
emission collection 440nm

 

 

 

 

 

 

 

Excite 300nm R(O) m
Emission 440nm
Buffered solution 0.160 i 0.021 89 i 13
Solution. no NazAA 0.1600 i 0.0098 72 i 5.40

 

No buffered H2AA/HAA' solutions with 6CN2 were analyzed with TCSPC
for excitation at 360nm because at this excitation, there fluorescence signal was
weak and therefore could not be detected from the detection for the TCSPC

system. The time domain spectra collected at this excitation for these buffered

-69-

 

solutions had a high signal to background ratio, making it difficult to analyze the

decay function of the molecule.

3.4 Conclusion

The buffered H2AA/HAA‘ solution does not fluoresce unless 6CN2 is
added. Over a period of 12 hours, 6CN2 in the buffered H2AA/HAA' solution
appears to remain stable if it is not exposed to constant ultraviolet light for either
excitation at 300 nm or 360 nm. However, upon constant ultraviolet exposure,
the 6CN2 in the buffered H2AA/HAA' solution does not result in significant
changes for excitation at 300 nm. In fact, it resembles the spectrum of 6CN2 in
an aqueous solution that is above pH=3. However, the concentration ratio of
H2AA/Na2AA present in the buffered solution does affect the spectroscopic
properties of 6CN2 in solution after ultraviolet exposure for excitation at 360nm.
In fact there is possibly a complexation between 60N2 and the adipate anion.
This complexation is probably with hydrogen bonding in nature. This complex is
500 nm emission and the complex probably breaks apart upon constant
ultraviolet light exposure. it was also found that using a laser to expose the
buffered H2AA/HAA' solution, the proton donation to the solution is partially
irreversible suggesting a proton is leaving the super photoacid 60N2 and
possibly going to the HAA' in the solution and forming H2AA. Even though H2AA
is possibly formed, the particle size is probably not large enough to help create a

nucleation site and initiate a crystallization event to happen.

-70-

It is not known how many protons are donated to the system per adipic
acid molecule that is formed. This would require much more in depth detailed
analysis. More studies with varying the concentration levels each species in the
buffered solution would need to be investigated to help encourage the

crystallization of adipic acid.

-71-

3.5 Literature Cited

1. Mullin, J. W. Crystallization; Butterworth-Heinemann: Oxford, England, 1993.
2. Myerson, A.S.; Sorrel, L.S.; AlChE J. 1982, 28, 778

3. Sorrel, L.S.; Myerson, A.S.; AlChE J. 1982, 28, 772

4. Chang, Y.C.; Myerson, A.S.; AlChE J. 1985, 31, 980

5. Chang, Y.C.; Myerson, A.S.; AlChE J. 1986, 32, 1567

6. Chang, Y.C.; Myerson, A.S.; AlChE J. 1986, 32, 1747

7. Larson, M. A.; Garside, J.; Chem. Eng. Sci. 1986, 41, 1285

8. Larson, M. A.; Garside, J.; J. Crystal Growth. 1986, 76, 88

9. Kohan, M.|. Nylon Plastics, J. Wiley & Sons: New York 1973

10. Luedeke, V., McKetta J. and Cunnigham, W. (Ed). Encyclopedia of Chemical
Processing and Design, 1977, 2, 128

11. Kirk-Othmer, 4th ed., 1, 466
12. Sittig, M. Dibasic acids and anhydrides, Noyes Development Corp: NJ 1966
13. Chakraborty, H.; Berglund, K. A.; AlChE. Symp. Ser. 1991, 284, 113

14. Rasimas, J.P.; Berglund, K. A.; Blanchard, G. J.; J. Phys. Chem. 1996, 100,
7220

15. Rasimas, J.P.; Berglund, K. A.; Blanchard, G. J.; J. Phys. Chem. 1996, 100,
17034

16. Tulock, J. J.; Blanchard, G. J.; J. Phys. Chem. B1998, 102, 7148
17. Tulock, J. J.; Blanchard, G. J.; J. Phys. Chem. A 2000, 104, 8340
18. Kelepouris, L.; Blanchard, G. J.; J. Phys. Chem. A 2000, 104, 7261

19. Skoog, D.A.; Holler, F.J.; West, D. M.; Analytical Chemistry: An Introduction
7‘h Ed. Harcourt, Inc. Orlando, Florida 2000 182

20. DeWitt, L.; Blanchard, G. J.; LeGoff, E.; Benz, M. E.; Liao, J. H.; Kanatzidis,
M. G. J. Am. Chem. 800.,1993, 115, 12158.

-72-

Chapter 4

Conclusions and Future Work

4.1 Conclusions

Inducing a crystallization event by optical means is a complicated task.
There are many factors to consider including solute concentration, solubility, type
of crystallization initiator, the initiation event, and how to monitor the
crystallization process. In order to try to control crystallization through an optically
induced event using a photoacid, the solubility of the solute chosen would have
be pH dependent. The initiator we have chosen was 6-cyano-2-naphthol, a
“super" photoacid that has a strong electron withdrawing group on the distal ring
of the naphthol. The buffered system chosen was a solution of 6CN2 with adipic
acid and sodium adipate so that the system could be driven into supersaturation
by photoexcitation of 6CN2.

The super photoacid’s protonation and deprotonation kinetics are not
simple and in order to use it successfully in a buffered system to induce a
crystallization event, the molecule itself needs to be characterized. The
protonation and deprotonation dynamics were studied for the 6-cyano-2-naphthol
at various pHs and the time-resolved lifetime and anisotropy data were also
obtained for 300 nm excitation and 360 nm excitation for different pH solutions. It
was found in the time domain data that for 300 nm excitation, very acidic
solutions kept the proton on 6CN2 within the first 500 ps of the molecule after

excitation. Basic solutions on the other hand, showed a protonated form at very

-73-

early times with a decrease in the protonated band to a corresponding rise in the
440 nm band within the first 500 ps. For the 350 nm excitation data, with the
TCSPC, it was found that acidic solutions had a peak corresponding to 500 nm
steady state spectra and a peak at 440 nm emission, which is the deprotonated
excited monomer anion. The 500 nm peak could be a complexation of 6CN2 as
well as the protonated/deprotonated species of 6CN2 in aqueous solution. We
have concluded that it is an aggregate of the protonated forms that exists in
acidic conditions, however, the lifetime of this molecule is extremely short.

For 360 nm excitation, the fluorescence intensity of 6CN2 is not as high as
exciting 6CN2 for 300 nm excitation. There is no evidence supporting the
existence of an aggregate species in basic solutions. The 6CN2 for excitation at
360 nm barely fluorescenced especially in acidic solutions, which makes it
difficult to obtain useful TCSPC measurements. With the buffered H2AA/HAN
and 6CN2 solutions, no spectral change occurs for excitation at 300nm and
constant UV exposure; since the pH of this solution is where the excited state
monomer of the anion of 6-cyano-2-naphthol is present. Spectral changes were
observed with the buffered solutions of H2AA/HAN and 6CN2 and constant UV
exposure. Concentration ratios were altered of the H2AA and Na2AA present in
the system to ensure the solution had HAA' present in the system. In the TCSPC
measurements, only 300 nm excitation was studied and it was found that no
major spectroscopic changes happened in the time domain in regards to the
lifetime of 60N2 in solution. The major spectroscopic changes observed of H2AA

and NazAA in solution were observed with steady state spectroscopy.

-74-

Theoretically the proton would come off of the super photoacid and attach
to the HAA' that is present in the buffered H2AA/HAA' forming H2AA. if enough
H2AA were formed, then possibly a nucleation site would form and crystallization
event would have been initiated. However, as of yet, a crystallization event has
not been observed. Therefore more studies need to be done on the
complexation events between adipic acid, sodium adipate, and the 6-cyano-2-

naphthol that are present in the aqueous solution. .
4.2 Future Work

More analysis needs to be done to understand the equilibrium between
the dimer formation and the monomer of the 6-cyano-2-naphthol. The other
equilibrium needed to be studied is from the excited state monomer back into a
possibly ground state protonated form and an excited state deprotonated form.
Once this information is found out then a very complete picture of the
complicated equilibrium studies can be obtained and applied and the 6-cyano-2-
naphthol can be used more effectively as a crystallization initiator in various
systems.

Adipic acid in a buffered solution was originally a good choice because its
crystallization events have been studied extensively by former Blanchard lab
members."5 It is quite possible that this weak acid is not a good choice; because
of the likely-hood that it could form an ester complex, since it is a polyprotic
carboxylic acid. In order to make this optical crystallization initiation event to
work, it might be more beneficial to study different weak acids that are more

sensitive to accepting protons in aqueous solution. it could also be that even

-75-

though the pKa to pK; jump is rather large for the photoacid, the pH regime we
were working in was too basic and so it would be beneficial to find a different
weak acid that works in a more acidic pH region and then go back to the adipic
acid system, once simpler systems are understood.

The 6-cyano-2-naphthol can also be used for biological systems that are
pH dependent. It can be used as a probe with systems since the protonated and
deprotonated dynamics are understood with this super photoacid. As long as the
pH is kept basic enough, then there will be no complexation of an aggregate

species present to be worrying about.

-76-

4.3 Literature Cited

1. Rasimas, J.P.; Berglund, K. A.; Blanchard, G. J.; J. Phys. Chem. 1996, 100,
7220

2. Rasimas, J.P.; Berglund, K. A.; Blanchard, G. J.; J. Phys. Chem. 1996, 100,
17034

3. Tulock, J. J.; Blanchard, G. J.; J. Phys. Chem. 81998, 102, 7148
4. Tulock, J. J.; Blanchard, G. J.; J. Phys. Chem. A 2000, 104, 8340

5. Kelepouris, L.; Blanchard, G. J.; J. Phys. Chem. A 2000, 104, 7261

-77-

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