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This is to certify that the thesis entitled OPruan AND Lumwesceoce PROPeRTuES or 9-AMTH&mc ESTeRS ; (5mm! M was 5de Sa‘mm‘mbbml KHHDER A. AL—Hassam has been accepted towards fulfillment of the requirements for ELSE... ___degree in My I . .1141 ..1 SI/ ‘14 I .41 Major professor © 1978 KHADER AHMAD AL-HASSAN ALL RIGHTS RESERVED OPTICAL AND LUMINESCENCE PROPERTIES OF 9-ANTHROIC ESTERS (Excited State Geometric and Solvent Relaxations) By Khader A. Al-Hassan A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Chemistry 1978 ABSTRACT OPTICAL AND LUMINESCENCE PROPERTIES OF 9-ANTHROIC ESTERS (Excited State Geometric and Solvent Relaxations) By Khader A. Al-Hassan The excited statescfi‘9-anthroic esters undergo geo- metric relaxation to different equilibrium positions de- pending on the size of the ester group, viscosity, struc- ture and polarity of the solvent matrix. However, one should not view geometric and solvent-cage relaxations as being completely independent since the equilibrium configuration of the solvent cage itself depends on the equilibrium configuration of the carboxyl group. The fluorescence spectra of 9-methyl anthroate (9MA) and 9—tertbutyl anthroate (9TBA) have been studied in various media in order to investigate the effect of vis- cosity and polarity of the medium. Fluorescence quantum yields and lifetimes were also obtained. As the viscosity of the solvent is increased, solvent relaxation is frozen at first followed by geometric relaxation. The structure of the solvent and its packing around the solute molecules Khader A. Al—Hassan at high viscosity is crucial in locking the ester group in its ground state equilibrium configuration. Relaxation of (9MA) in the excited state is not completely prevented in a "rigid" hydrocarbon glass at 77 K while (9TBA) under the same condition does not relax to its equilibrium posi- tion in the excited state. This is supported by the shapes of the decay curves of these esters at 77 K. In (9TBA) the branched tertbutyl group requires a relatively large "free volume" for relaxation. The fluorescence spectra of 9-anthroyl palmitic acid (A816) which is used as a "fluidity probe" in membranes have been also studied under various conditions. To my mother and in memory of my father 11 ACKNOWLEDGMENTS I would like to express my deep appreciation to my advisor, Dr. M. Ashraf El-Bayoumi, for his guidance, en- couragement and friendship during the course of this in- vestigation. I would also like to express my gratitude to the members of my committee, Dr. James F. Harrison, Dr. Lynn R. Sousa and Dr. Andrew Timnick. I would like to express my deep appreciation to my friend, David Carr, for his expert assistance with time resolved spectrofluorimeter and other devices in this laboratory. Special thanks are extended to Yarmouk University in Jordan for their support. iii TABLE OF CONTENTS Chapter Page LIST OF TABLES. . vii LIST OF FIGURES . . . . . . . . . . . . . . . . .viii CHAPTER I - ELECTRONIC SPECTRA AND GEOMETRY OF ORGANIC MOLECULES 1 CHAPTER II - OPTICAL AND LUMINESCENCE PROPERTIES OF FLEXIBLE MOLECULES. ll A. Introduction. 11 B. Three Dimensional Potential Energy Diagram . . . . . . . . . . . . . . 13 C. Steric Effects on Electronic Spectra of Molecules that May Undergo Intra- molecular Twisting About an Essential Single Bond . 16 D. Excited State Intramolecular Tor- sional Relaxation.Viscosity, Tem- perature, and Medium Effect on the Fluorescence Characteristics of a Sterically Crowded Molecule. . . 24 CHAPTER III - CORRELATION OF ELECTRONIC SPECTRA OF FLEXIBLE MOLECULES WITH CHANGES OF TWIST ANGLE OF TWO INTER- ACTING MOIETIES . . . . . . . . . . 30 A. Qualitative Prediction of Geometric Changes From Spectra. . . . . . . . . . . 30 B. Excited State Geometric Relaxation as a Useful Probe of "Microscopic" Viscosity of Liquids and Membranes. . 31 C. Empirical Correlation of Absorption and Fluorescence Characteristics of Aromatic Molecules. . . . . . . . 32 (1) Flexible Molecules and Geometry Changes due to Excitation . . . . . . . . . . . . . 36 iv Chapter Page (ii) Effect of Rendering a Flexible Molecule Rigid. . . . . . . . . . . 51 CHAPTER IV - OPTICAL AND LUMINESCENCE PROPERTIES OF 9-ANTHROIC ESTERS (Excited State Geometric and Solvent Relaxations). . . 56 A. Introduction. . . . . . . . . . . . . . . 56 B. Ground State Geometrycfl‘9-Anthroic Esters. . . . . . . . . . . . . . . . . . 60 0 Fluorescence Spectra and Excited State Geometric Relaxation in 9- Anthroic Esters . . . . . . . . . . . . . 6A D. Solvent Relaxation in the Excited State of 9-Anthroic Esters. . . . . . . . 81 E. Excited State Relaxation of 9- Anthroyl Palmitic Acid (A816) . . . . . . 88 CHAPTER V - EXPERIMENTAL. . . . . . . . . . . . . 92 A. Materials . . . . . . . . . . . . . . . . 92 I. Purification of Solvents. . . . . . . 92 l. 3-Methyl Pentane (3MP). . . . . . 92 2. Isopentane (2-methylbutane) . . . 92 3. Diethyl Ether . . . . . . . . . . 93 A. Ethanol . . . . . . . . . . . . . 93 II. Purification of Chemical Compounds StUdied. o o o o o o o o o o o o o 0 9’4 1. 9-Anthroic acid (9-anthracene Carboxylic Acid). . . . . . . . . 9A 2. 9-Methyl Anthroate (Methyl Ester of 9-antroic acid) (9MA). . . . . 9A 3. 9-Tertbutylanthroate (tert- butyl ester of 9-anthroic ACid) (9TBA)9 o o o o o o o . o o 9“ Chapter A. ASl6 (9-anthroyl—palmitic Acid) . . . . . . . . . . B. Spectral Measurements l. 2. 3. A. 5. Absorption Spectra. Emission Spectra. Quantum Yields. Temperature Variation System. Fluorescence Decay Curves BIBLIOGRAPHY. vi Page 9A 95 95 95 95 96 96 100 Table II III IV LIST OF TABLES Optical and luminescence charac- teristics of some anthracene derivatives Abs. band maxima of 9- anthracene derivatives. . . . Viscosities of 3MP-IP mixtures at 77.5 °K. . . . . . . . . Lifetimes and quantum yields of 9MA and 9TBA in different media vii Page NO 65 7A 77 Figure LIST OF FIGURES Page Franck-Condon potential energy curves of diatomic molecules and their typical absorption spectra. (a) Similar equilibrium separation; (b) Equilibrium separation in the excited state is slightly larger than that of the ground state; (c) equilibrium separation is much greater in the excited state. . . . . . . . . . . . . . . . . 5 Room temperature absorption and fluorescence spectra of anthracene in hydrocarbon solvent (b), po- tential energy curves representing the ground state (SO), first and second excited singlet states (S1 and S2 are shown (a) together with radiationless transitions (wavy lines) and radiative processes (solid line) . . . . . . . . . . . . . 8 Potential energy cross sections drawn along two coordinates QV and Qr for the ground and excited viii Figure Page states. The diagram illustrates vertical excitation (1), vibra- tional excitation (2), emission before relaxation along Qr occurs (3), relaxation along or (u), emis- sion from an intermediate configura- tion (5), and emission from the equilibrium excited state configura- tion (6) . . . . . . . . . . . . . . . .15 Potential energy curves for a system RS in the ground and excited states as a function of 6 and the probability distribution functions. (a) unhindered system RS and (b) the corresponding probability distribution function; (c) slightly hindered system RS and (d) the corresponding distribution func- tion; (e) moderately hindered system RS and (f) the corresponding distribu- tion function. . . . . . . . . . . . . .20 Absorption (A) and fluorescence (F) spectra of TPB (trans-1,1,“,A-tetra- phenyl butadiene) at 300 K (---) and at 77 K (--) . . . . . . . . . . . . . . 25 ix Figure Page Effect of temperature on 3% the fluorescence energy maximum of TPMB in three different media. Arrows indicate the temperature at which the viscosity of the medium is 10M cp . . . . . . . . . . . . . . . . . . 27 (a) Absorption and fluorescence spectra of 9,10—Bis(phenylethynyl)- anthracene and (b) the correspond- ing potential energy curves of the ground and excited states as a function of 6. . . . . . . . . . . . . 39 (a) Absorption and fluorescence spectra of 9-vinyl anthracene and (b) the corresponding potential energy curves of the ground and the excited states as a function of e. (---) steric repulsion energy curve; ( ----- -) n- interaction energy curve; (-——) result- ant potential energy curves. . . . . . A3 (a) Room temperature absorption (A) and fluorescence (F) spectra of 9-pheny1 anthracene in hydrocarbon solvent, and (b) the corresponding potential energy curves . . . . . . . . . . . . . . . . A6 Figure 10 ll l2 13 Page (a) Room temperature absorption and fluorescence spectra of 9-anthroic acid and (b) the corresponding po- tential energy curves of the ground and excited states as a function of e. (---) steric repulsion energy curve; (-----) n-interaction energy curve; (-——) resultant potential energy curves. . . . . . . . . . . . . A9 (a) Room temperature absorption and fluorescence spectra of 2-phenyl INDOLE and (b) the corresponding ground and excited state potential energy curves (the solid curves ———0 as a function of e. The dashed curves (--—) correspond to the ground and excited state potential energy curves of Figure (12) as a function of e. . . 53 Room temperature absorption (A) and fluorescence (F) spectra of l-methyl-3,2-methylene-2-phenyl INDOLE in hydrocarbon solvent. . . . . 5A Room temperature absorption spectra of (7x10'5M) 9-tertbutyl anthroate in solvents of different polarities. xi Figure 14 l5 l6 l7 18 Page (---) 3MP; (—) Ether; ( ----- ) Ethanol. . . . . . . . . . . . . . . . 61 Room temperature absorption spectra of (7x 10'5M) in 3MP of (———) 9-tertbutyl anthroate; (---) 9-phenylanthracene; (...) 9-vinylanthracene. . . . . . . . . . . 63 Room temperature absorption (-——0 and fluorescence spectra of 9TBA (---) and (9MA) (...) in 3MP . . . . . 66 Room temperature absorption spec- trum of (7x10-5M) (9TBA) and 77 K fluorescence spectra of (9TBA) ( ----- ) and (9MA) (---) in 3MP . . . . 68 Room temperature absorption spectrum of (7xlO'5M) 9MA and its fluorescence spectra at room temperature (...) and at 77 K (--—) in 3MP. . . . . . . . . . . 69 Room temperature absorption spectrum of (7x10'5M)(9TBA) and its fluorescence spectra at room temperature (...) and at 77 K (---) in 3MP . . . . . . . . . . . . . 70 xii Figure 19 2O 21 22 23 2” Room temperature fluorescence spectrum of (9TBA) (...) and 77 K fluorescence spectra in 3MP MP/IP mixtures: (-——) 0:1; ( ----- ) A:l; (---) 1:0 Room temperature fluorescence spectrum of (9MA) (...) and 77 K fluorescence spectra in 3 MP/IP mixtures (-——) 0:1; (---) 1:3; ( ----- ) 8:1. Fluorescence spectra of (9TBA) in 3MP at different temperatures ("'") R-T03 ( """ ) 77 OK, (—) and (...) intermediate tempera- tures. . . . Fluorescence spectra of (9TBA) in triacetine at different tem— peratures. (-—-) R.T.; ( ----- ) —2l4 °c; (...)—1:0 °c; (...) -57 °C; (—----'-) -118 °C. The decay curves of 9MA (a) and 9TBA (b) in 3MP at 77 K. Log intensity vs. nsec of the decay curves corresponding to (a) and (b) in Figure 23 . xiii Page 71 72 75 76 79 80 Figure 25 26 27 28 29 Room temperature fluorescence spectra of (9TBA) in solvents of different polarities ( ----- ) 3MP; (...) Ether; (---) octanol; (———) ethanol. Room temperature fluorescence spectra of (9MA) in solvents of different polarities (———) 3MP; (---) Ether; ( ----- ) Oc- tanol; (...) Ethanol Room temperature and 77 K fluorescence spectra of (9TBA) in 3MP, ether and ethanol. ----- 3MP R.T.; ... 3MP 77 K; --- Ethanol R.T.; ——— Ethanol or Ether 77 K. Room temperature and 77 K fluores- cence spectra of (9MA) in IP, ether and ethanol. (...) ethanol R.T.; (———) Ethanol 77 K; (----..-) Ether 77 K; ( ----- ) IP R.T.; (...)IP77K. . . . .. Fluorescence spectra of (9MA) in triacetine at different tem- peratures ( ----- ) R.T.; (...) -29 °C; (—) -uo.5 °c; xiv Page 82 83 8A 86 Figure 30 31 32 (---) -93 °C; (-------) 77 K . Fluorescence spectra of (A816) in paraffin oil at different temperatures. (———) R.T.; -..-..— -u1 °C; —-- -80 °C; (...) 77 K . Fluorescence spectra of (A816) in triacetine at different tem- peratures. -——-R.T.; --- -17 °C; - 27 °C; ----- -3A.5 °C; —.o—oo- -u2.5 °C; -000—000- — 52 °C; I-oooo—ooo Diagram of the time resolved spectroscopy instrument. XV .- —102 °C Page 87 9O 97 CHAPTER 1 ELECTRONIC SPECTRA AND GEOMETRY OF ORGANIC MOLECULES Different electronic states have different geometries due to electron density redistribution. Thus absorption and emission contain valuable information regarding the change in geometry due to electronic excitation. Ab- sorption spectra result from light induced transitions between the ground state and the excited electronic states. At low temperature the first absorption band represents transitions from the lowest vibrational level of the ground state and the various vibrational levels of the lowest excited singlet state. The role of absorption is given by the following expression: 8H3 [af(t)]2 = 3;? 20(vif)t where p(vif) is the radiation density, M(X) = eir1 is the dipole moment operator, W1 is the initial state (ground state) and wf is the final state (excited state). The probability of a given transition is proportional to |M(X)|2 where |M(x)|2 = 2 <2) In order to deal with this matrix element, one uses the Born Oppenheimer approximation, i.e., electron motion is fast compared to the sluggish nuclei, so one imagines the motion of the electrons in the field of the static nuclei and plots the electronic energy as a function of nuclear coordinate to get a potential energy curve. To put this mathematically, one factorizes the wavefunction into an electronic wavefunction, 6, and a nuclear wavefunction, ¢, w(X,Q) = 6(X,Q) ° ¢(Q) (3) where w(X,Q) is the total wavefunction. Substitution of (3) in the transition moment matrix element (2) gives M(X) = ff6*(X,Q)¢*(Q)|M(X)|6(X,Q)¢(Q)dXdQ = [6*(X,Qe)|M(X)|e(X,Qe)de¢*(Q)¢(Q)dQ (A) where the first integral is the electronic transition moment, Q appears as a parameter, and the second integral represents the overlap integrals between the zero vibra- tional wavefunction of the ground state with various vibra- tional wave functions of the excited state. Such overlap integrals are the Franck-Condon factors that determine the relative intensities of vibronic levels in a given ab- sorption band. Another way of stating the Frank-Condon principle is to say that, the most intense transition in a given absorption band is the vertical transition, mean— ing a transition that does not involve a change in the nuclear coordinate. This vertical transition can be the O + O, O + l, O + 2 depending on the relative positions of the potential curves of the two states. In Figure 1,(2) various band shapes are shown that can result depend- ing on the relative change of nuclear coordinates due to excitation. Thus in Figure la where no change in nuclear coordinate occurs, the O + O is the most intense transition. In Figure lb where there is a displacement of the potential energy in the excited state to a larger value of nuclear coordinate, the O + 2 is the most intense transition. In Figure 1c where large displacement of the upper curve to a large value of nuclear coordinate, the O + O is very weak and a continuum begin after 0 + 5. Likewise, the relative intensities of the vibronic bands in the fluorescence spectra is determined by the Franck- Condon factors; in this case the overlap integrals between the zero vibrational wavefunction of the excited singlet state and the various vibrational wavefunctions of the ground state. Thus the fluorescence spectrum and the first absorption band exhibit a mirror image relationship; the structures of such mirror symmetry depends on the extent of displacement of the potential energy curves in the states involved in the transitions and change in their Figure l. Franck-Condom potential energy curves of diatomic molecules and their typical absorp- tion spectra. (a) Similar equilibrium separation; (b) Equilibrium separation in the excited state is slightly larger than that of the ground state; (c) Equilibrium separation is much greater in the excited state. Lm'rgy 1 Ixffi (a) Encrgy t '1" fl 0)) Energy I Ix" —' (e) n—oO v ("n") —o- "—04 l l "-5 ”—00 n... I l . " V (Cln- ') “ Continuum of absorption (0 - 6. 0 -o 7. CK.) shapes that will alter vibrational frequencies as well as the Franck-Condon factors. One must recall that in polyatomic molecules the only emitting level of the singlet manifold is the lowest excited singlet state (Kasha's Rule) (Figure 2a). In the case of anthracene which is an aromatic planar molecule, rigid in both the ground state and excited state, no significant change in the equilibrium configuration as a result of excitation and Av = v0+o (absorption) - v0+0 (fluorescence) is negligible as shown in Figure 2b.(3) In Chapter II,I will review the optical and lumines- cence properties of flexible molecules where intramolecular twisting occurs about an essential single bond connecting two parts of the n-chromophore. A summary of the effects of viscosity, temperature and medium polarity are the fluorescence characteristics of a sterically crowded molecule, i.e., trans-1,1,A,A-tetraphenylmethyl butadiene (TPMB) will be given together with a three-dimensional potential energy diagram that helps to distinguish the role of the twist angle coordinate (widths of individual vibronic band and Stokes shift) and the role of vibrational coordinate that is prominent in the emission and absorption spectrum (vibrational coordinate that gives the vibrational structure of the absorption and emission bands). The goal of this work is to discuss the type of quali- tative information regarding the change in geometry as a Figure 2. Room temperature absorption and fluorescence spectra of anthracene in hydrocarbon solvent (b), potential energy curves representing the ground state (SO), first and second excited singlet states (31 and 82 are shown (a) together with radiationless transitions (wavy line) and radiative processes (solid line). INTENSITY s t lj')'tl ”.tjl r4(_l Energy (a) HRVELENGTH (R) 5000 «030 «600 ' «00 «200 «:00 39;. 3600 I.mt'—“_'LT-J‘ ‘7'] “ L T ‘ Efi J + J ‘Y— ‘ ‘ *V 4‘ 2‘90 v 13290 fifl YSOPC fl 1 MYMt-Yfif'lt 1 0-90 » scum Crammer I . 9°00 ; l 000 0.00 ‘- ‘ I . 0000 * [KCIYRHOn “VILINSYH 2537“ | I .— l 3 .7 . ‘ 2000 ° ° mm— mm 0.35 A 1 ‘5 H 2 x I 0.60 H 4 6000 g r . u u. U o 0.50 t .. 5000 U ’ h i b— OJIO L 1 1060 L.) I z I ,_ X a.” > . 3003 L“ ' I . c: ..J o 0.20 . F A . 2000 I “I’D » 4 IC' 0.” ' L A ' ___L ‘ AL 4* A n A L __&___—A A A _x A 20000 21000 22000 23000 3000 25000 26000 30000 31000 32000 33000 30000 13000 27000200003000 HRVE NUMBER (CH‘U (b) result of excitation that one can derive from absorption and fluorescence spectra. We will deal with geometric changes where rotation of a part of the molecule may occur relative to the rest of the molecule and effects of angle of twist e on the absorption and fluorescence spectra. The characteristics of absorption that we will focus upon in Chapter III are the degree of resolution of the vibra- tional structure, the shape of the band (which vibronic level is the most intense),shift of absorption and fluores- cence bands and change in the molar extinction coefficients relative to the parent chromophore. The characteristic of fluorescence that we will examine are the degree of resolution of the vibrational structure, the shape of fluorescent band, i.e., intensity distribution among vib- ronic levels of fluorescence, Av = v0_0 abs - VO-O Fl.’ lifetimes (IF) and the quantum yields 0F, To account for the qualitative features of absorption and fluorescence we will consider the shapes of potential energy curves in the ground and lowest excited singlet states. Such potential energy curves will be constructed by considering the changes in n-interaction as a function of angle of twist 6 and steric repulsion that may occur between atoms that may come in close contact in certain configurations. We will also consider changes in equilib- rium configuration with respect to 6 (angle of twist) due to electronic excitation. Changes in equilibrium con— figuration may lead to geometric relaxation in the excited 10 state that will cause the fluorescence to shift to longer wavelengths and to be weak and diffuse. The other goal of this work will be discussed in Chapter IV. More specifically, we will examine the optical and luminescence properties of 9—anthroic esters under various conditions. The effect of polarity, viscosity and struc- ture of the medium will be studied. The relaxation of the ester group during the lifetime of the excited state will be examined. CHAPTER II OPTICAL AND LUMINESCENCE PROPERTIES OF FLEXIBLE MOLECULES A. Introduction Flexible molecules that undergo intramolecular tor- sional vibration around a bond linking two interacting moieties of the chromophore are at best weakly fluores- cent. The low frequency oscillations lead to efficient radiationless deactivation of the excited molecule in fluid media. Upon "freezing" these modes or limiting their amplitudes, the molecule becomes strongly fluorescent. This may be accomplished by using media of high Viscosities or by rendering the molecule rigid via a chemical bond. In addition to the fluorescent enhancement effect, flexible molecules may undergo a change in their equilib- rium geometry configuration upon excitation. In these cases a large Stokes shift of fluorescence maximum is observed in fluid media. The fluorescence emission may originate from the equilibrium excited state, Franck- Condon state, or an intermediate geometric configuration depending on the relative magnitudes of the rate constants of the rotational relaxation process kr and fluorescence kf. If kr is viscosity dependent one may expect the 11 l2 fluorescence maximum to be a sensitive function of the viscosity provided that kf << kr' As the medium becomes more viscous the fluorescence maximum shifts progressively to higher energies and a blue shift of the fluorescence maximum is observed in rigid media. The vibrational structure that appear in the spectrum is related to a vibrational coordinate QV which is distinct from the twisting mode coordinate Q6. The latter determines the shape of individual vibronic bands and hence the degree of resolution of the vibrational structureixithe absorp- tion and emission bands. Trans-1,1,u,u-tetraphenyl methyl butadiene (TPMB) offers an example of intramolecular and twisting on the optical and luminescence properties of a flexible molecule. In this laboratory, the effects of viscosity, temperature and medium on the fluorescence properties of TPMB have been studied.(u) A summary of the results are given at the end of this chapter. (TPMB) offers a nice example where a competition occurs between the rate constant of fluorescence and that of intramolecular geometric relaxation. The result of such competition depends on the viscosity of the medium. Thus the fluores- cence shifts to higher energy, undergoes a dramatic en- hancement and becomes relatively resolved for TPB when the medium becomes rigid. 13 B. Three Dimensional Potential Energy Diagram Following electronic excitation, molecules in a con- 12 densed phase undergo a rapid (>10' 8) vibrational relaxa- tion leading to the emitting level of the excited state. In addition to vibrational relaxation, an excited molecular system may undergo various other relaxation processes that occur on the nanosecond time scale. These processes include (”-6) intramolecular (10,11) intramolecular twisting relaxation, excimer formationf7'9) solvent-cage relaxation and proton transfer.(l2-15) Dramatic changes in the luminescence properties often occur as a result of these relaxation processes, e.g., large Stokes shift, appear- ance of a new fluorescence band, significant fluores- cence intensity changes, and/or fluorescence lifetimes different from those calculated from absorption intensities. In addition, unique medium and temperature effects as well as large viscosity effects on the fluorescence spectra may be observed. In order to account for the fluorescence spectra ob- served in relaxing molecular system, it is necessary to draw a potential energy surface along at least two co- ordinates. The vibrational coordinate, QV, represents a specific vibration which is coupled with the electronic transition; it is assumed for simplicity that only one vibrational mode is coupled. The relaxation coordinate, Q , may represent an angle of twist e, the distance between I’ 1A a proton and a specific atom in a molecule, a coordinate specifying the separation and orientation of two chromo- phores involved in excimer formation, or generalized solute- solvent cage relaxation. Figure (3) shows how vertical excitation (Process 1) of the equilibrium ground state leads to the Franck-Condon state S' 0 configuration Sr,v’ r,v in which both QV and Qr did not change. This state under- goes rapid vibrational relaxation (Process 2) leading to a state, S' , where Qr did not change but QV has the value r,V' of the equilibrium excited state configuration. Fluores- cence may originate from Sé,,v, where both Qr and QV as— sume their values in the equilibrium excited state con- figuration, from state 8;,v, or from an intermediate con- figuration depending on the relative magnitudes of the rate constants of the relaxation process kr and fluores- cence kf; processes 6, 3 and 5 in Figure (3) represent those situations respectively. The vibrational spacing, usually of several hundred wave numbers, which appearsin the absorp- tion and fluorescence spectra is illustrated by the po- tential energy cross section drawn along QV which represents the intramolecular vibrational mode coupled with the elec- tronic transition. Relaxation along Qr leads to a large Stokes shift, the magnitude of which depends on the dif- ference between the values of Qr in the equilibrium ground state and equilibrium excited state configurations. Energy levels that characterize potential energy cross sections l5 Figure 3. Potential energy cross sections drawn along two coordinates Qv and Qr for the ground and excited states. The diagram illustrates ver- tical excitation (1), vibrational relaxation (2), emission before relaxation along Qr occurs (3), relaxation along Qr (A), emission from an intermediate configuration (5), and emission from the equilibrium excited state configuration (6). 16 along Qr correspond to low frequency modes such as those associated with excimer liberation, torsional motion, proton oscillation with respect to a proton acceptor, as well as solvent—solute oscillations. Coupling with these low frequency modes in addition to relaxation along Qr in a fluid medium lead to an extensive broadening of each vibronic band and the disappearance of vibrational struc- ture corresponding to QV' In a rigid medium, relaxation along Qr is prevented giving rise to an emission originat- ing from Sé,v,, in addition the low frequency modes will be frozen leading to a resolution of vibrational structure corresponding to QV' Emission in rigid media occurs at higher energies relative to emission in fluid media. C. Steric Effects on Electronic Spectra of Molecules that May Undergo Intramolecular Twisting About an Essential Single Bond Many of the studies dealing with steric effects on ultraviolet absorption spectra have been concerned with chromophores connected by essentially single bonds. Intro- duction of substituents around the single bond results in more or less interference and steric inhibition of coplan- arity of the chromophores. The reduced interaction of the chromophores in the nonplanar compound is responsible for the deviations in the absorption spectrum of the hindered compound from that expected for the planar model. 17 The extent of interaction between orbitals on carbon atoms r and 3 may be measured by the overlap integral defined as S = f¢r ¢s dr which can have an absolute value between zero and unity. If atoms r and s are the atoms at which two chromophores R and S are connected, as, e.g., atoms 1 and 1' of biphenyl the overlap integral of the P1T orbitals on r and s varies Biphenyl from a maximum, sgs, at 0° twist [i.e., when the chromo- phores (the two rings) are coplanar] to a value of zero when the angle of twist, e, is 90°. In particular the overlap integral, S is proportional to cose: rs’ S6 = S° 0056 r8 rs The resonance integral, 8 is proportional to the overlap 106) I'S’ integra ~ 0 3 rs Brs cose 18 The change in energy of the system (RS) relative to (R+S), as, due to interaction is related to the resonance integral, 8. Using perturbation theory to express the resonance energy as a function of the angle of twist, one can find the change as, in the total energy 5, as an infi- nite series in 38 Be = a as + a (as)2 + a (as)3 + l 2 3 First order perturbation theory neglect all terms but the first. Thus, for small angles of twist, B is propor- tional to cose, and so is the total energy. However, as soon as 6 is appreciable, the second term can't be ignored, and the simple cosine dependence breaks down. Using the second order perturbation theory gives a 2 00828 dependence(l7) and e z cos a is a fair approximation. A cos2 6 function can be used to draw a potential energy curve in system RS where the shape of the potentials in the ground and excited states depend on the degree of sterci crowding.(18) (l) The potential energy of an unhindered system RS in the ground and excited states as a function of e is shown in Figure (Ha). Both potential energy curves have a minimum near 6 = 0, (where potential energy curve will behave as Prscose), but the curve of the excited state, because of Figure A. 19 Potential energy curves for a system RS in the ground and excited states as a function of 0 and the probability distribution func- tions. U‘fl) ( ( AAA/x ”SW 000 ) ) ) . ) ) ) unhindered system RS and the corresponding probability distribu— tion function slightly hindered system RS and the corresponding distribution function moderately hindered system RS and the corresponding distribution function. 20 O 0: 0% l‘lll".|l'|llllllll| “ 050 a; a; (b) (a) 0 . \ .-.: ...Iyl.-.'nl ~~-' 0'60 0 as. to I o, a 09' 0 of G > t 2 0< 15 < 0.5 05 (d) (0) l ’ \ -001 of. o. 0 0‘9. 03 . (f) (e) 21 the larger value of Pr is steeper than that of the ground s’ state. Consideration of Figure (Ab) shows the quantum mechani- cal distributions of molecules in the lowest vibrational level among different values of the angle 0. The dashed lines in Figure (Ab) represent the classical limits of 6 in the lowest level of the excited state, and by mixing classical and quantum mechanical argument one arrives at the conclusion that molecules represented by part of the area under the distribution function which is shaded cannot undergo a 0 + 0 transition. The values 6G<, 0E< and 6G>, 6E > are the minimum and maximum angles, respectively, of twist 0, classically per- mitted in a given vibrational level (e.g., the lowest) of the ground and the excited states. (2) The potential energy Figure (Ac) and distribution Figure (Ad) in slightly hindered RS. 6' E' 9 and 0 e e represent the equilibrium value of 0 in the ground and excited state, where the prime identify the slightly hindered molecule. It should be again remembered that it is assumed that the bond order of the essential single bond in RS increases on excitation, and the result- ing greater resistance to twist lead to 62' > 65'. Since 0080 is a slowly changing function of 6 near 6 = O, and since the steric repulsion energy also is small 22 t 1 where 5G, EE and eG , 6E are the energies at the potential minima in the ground and excited states of the unhindered and slightly hindered RS respectively. (3) The potential energy Figure (He) and distribution Figure (Hf) functions in moderately hindered RS: The potential-energy curve for the ground state must have a fairly high and reasonably steep maximum at G" 6 = 0°. The equilibrium value of Se (double prime iden- tifies the moderately hindered molecule) for 9 occurs at considerably larger values of 6 than in the slightly hindered case. The potential-energy curve asymptotically approaches that of the unhindered case (broken line), but only at relatively large values of e - unless the hinder- ing groups are so bulky that even the completely twisted conformation is hindered, so that the chromophores are isolated. Just as in the slightly hindered case, the equilibrium value of e is increased more in the ground than G" E" in the excited state, 6e > 6e , and both quantities are considerably larger than they were in the slightly hindered G" G! E" E! molecule, ee e and 9e > 6e . But in the moderately > 6 hindered case, the distortion in 6 is considerable, and hence c030 is less than 1 by an appreciable amount. This information, together with the knowledge that the bond 23 order in the excited state (pE) is greater than in the E G ground state (pG), p > p , leads to the inequality Further, the higher bond order in the excited state indi- cates a smaller bond distance, and consequently E5" is slightly larger than E3" where ES represents the steric repulsion energy. Figure (Hf) illustrates the distribution function in which the shaded area represents the fraction of molecules unavailable for excitation to the lowest vibrational level of the excited state is seen to be further increased. Also, the transition moment decreases with increasing 6, so that the total (integrated intensity), or the oscillator strength, may be expected to decrease. The above considerations explain the effects observed when the system RS of two chromophores R and S is subjected to steric interference by introducing groups which prevent the ground state of RS from assuming coplanarity. In the case of slight hindrance, a Franck-Condon effect, manifested by a hypochromic* effect relative to the unhindered model, is expected; where moderate hindrance occurs, both a hypsochro- ** mic and hypochromic effect should be observed. Naturally, * Hypochromic effect: A decrease in the intensity of a par- ticular absorption band. **Hypsochromic shift: A displacement of a particular ab- sorption band under discussion toward shorter wavelength. 2“ there is no sharp demarcation between slight and moderate hindrance, and each case must be considered. Several examples of the affects discussed can be examined on mono and poly—ortho—substituted biphenyls. D. Excited State Intramolecular Torsional Relaxation Viscosity,_Temperature, and Medium Effect on the Fluores- cence Characteristics of a Sterically Crowded Molecule Trans-1,1,“,A-tetraphenyl-2-methylbutadiene (TPMB) undergoes an intramolecular twisting relaxation after excitation. The extent of this relaxation depends on the viscosity of the medium. The fluorescence energy maximum exhibits a blue shift and the fluorescence intensity is greatly enhanced as the medium becomes rigid. Absorption and fluorescence spectra of TPB (trans-1,1,4,”- tetraphenylbutadiene) in isopentane-methylcyclohexane sol- vent mixture at two different temperatures illustrate this phenomenon.(u) In Figure (5) a large blue shift of the fluorescence maximum is observed (1650 cm’l) in going from a fluid medium at 300 K to a rigid glass at 77 K. Both the fluorescence and absorption spectra show a 1300 cm-1 vibrational structure at 77 K which disappears at room temperature. In Figure (3), QV may represent this 1300 cm-1 vibrational mode While Qr represents 9, the angle of twist about an essential single C—C bond, probably that of 25 J 500 450 400 350 300 >. (nm) Figure 5. Absorption (A) and fluorescence (F) spectra of TPB (trans-1,1,A,U tetraphenyl butadiene) at 300 K (---) and at 77 K (-——). 26 butadiene. In a fluid medium, k the rate constant 6’ corresponding to intramolecular twisting relaxation (Process A in Figure 3), is larger than kf and fluores- cence originates from the equilibrium configuration, Sé',V', k6 is small compared to kf and fluorescence originates from Sé v' as represented by Process 3. It is obvious 3 l as represented by Process 6. In a rigid medium that in order to account for the 1300 cm' vibrational structure one must consider the potential energy cross section drawn along that specific vibrational mode represented by QV. The absorption band maximum of TPMB lies 2300 cm-1 at higher energy compared to TPB. This shift is due to the steric effect of the methyl group which leads the ground state configuration of TPMB to be less planar. In a fluid medium at room temperature TPMB exhibits a weak yellow emission in hydrocarbon solvent. A remarkable intensity enhancement (about 2 order of magnitudes) and a large blue shift of 3700 cm'1 occurs as the sample forms a rigid glass at 77 K. Figure (6) shows the general increase of 3% upon lowering the temperature in three different solvents. The rapid increase of 3% occurs in a narrow temperature range which differs widely from one medium to the other. In contrast, these changes of 3% occur at nearly the same viscosity. The arrows in the figure mark a viscosity of 10” Cp- 27 .90 OH we Esfloos one no haam00ma> on» cofinz pm madampodEop on» opmomocfi msonp< .mfimoe economm«o omen» :H mZmB no Esefixme mwhoco mocmomopozflm on» 9 co manpwpanop mo vacuum .m opsmfim v..._. 00m Ohm CON On— GDP 4 . _ A _ 0— . sir—W2 . l on o ....n. a02<¢0¢mlc l _.N ..Ver fly 3 02:85,: w. o I ILNN fl .JmN 28 The apparent increase of V? in the temperature range 220 K to room temperature in hydrocarbon solvent and in l-propanol is not due to a viscosity change. The small viscosity change which occurs in this temperature range does not cause any change in 3% as shown by experiments where the viscosity is changed at room temperature. This effect has been interpreted as a temperature-induced selective quenching where emission from the equilibrium configuration Sé',v" is selectively quenched by populating the upper torsional modes which undergo more efficient internal conversion S1 80' Emission originating from inter- mediate relaxed configurations occurs at shorter wave- length and will dominate the spectrum as the temperature is increased from 220 to 300 K, giving rise to the ob- served blue shift. Selective red quenching has been ob- (3) served in other cases and may be common in relaxing molecular systems. Experimental results also show a large increase of the oscillator strength and a red shift of the TPMB first absorption band as the temperature is lowered. It has been interpreted in terms of depopulation of TPMB ground-state molecules which have several quanta of high amplitude-low frequency torsional modes and an electric dipole transition moment that is dependent on e, the angle of twist. At low temperature there will be more TPMB molecules with planar configurations which have larger 29 conjugation, higher oscillator strengths, and smaller excitation energies. CHAPTER III CORRELATION OF ELECTRONIC SPECTRA OF FLEXIBLE MOLECULES WITH CHANGES OF TWIST ANGLE OF TWO INTERACTING MOIETIES A. Qualitative Prediction of Geometric Changes From Spectra Different electronic states may have different equilib- rium configurations depending on the extent of electronic density redistribution that may occur as a result of excita- tion and on whether the molecule is rigid or flexible. Changes in geometry as a result of excitation will reflect itself in the absorption and emission spectra. Thus flex- ible molecules that twist around a bond linking two inter- acting moieties of the molecule usually exhibit diffuse weak fluorescence. By rendering the molecule rigid via a chemical bond, the fluorescence spectrum becomes sharp and the quantum yield is enhanced. The goal of this chapter is to discuss fluorescence and absorption charac- teristics of aromatic molecules and examine the quali- tative information that may be derived, concerning the conformation of the chromophore in the ground and the first excited states. It would be useful to obtain emperical correlations between nuclear topology and electronic spectra. In particular we will discuss 30 31 various cases that may arise in aromatic molecules where one part of the chromophore can rotate with respect to the rest of the molecule about an essential single bond. B. Excited State Geometric Relaxation as a Useful Probe of "Microscopic" Viscosity of Liquids and Membranes Sterically crowded molecules that undergo intramolecular twisting relaxation after excitation exhibit luminescence characteristics that can be used to probe microfluidity. The fluorescence characteristics that may be used are: emission maximum 3%, relative fluorescence yield 0F, polari- zation P, the rate of fluorescence decay, kf, as well as the rate of twisting relaxation, kr’ obtained from time resolved spectra. Simple measurement of U? will give a good indication of the fluidity of the medium. Relative measurements of the fluidity using this technique may be more meaningful than absolute measurement. One must point out that geometric relaxation processes reflect packing properties of the medium which depend on the molecular structure rather than the macroscopic viscosity of the medium. Thus 3? may be different in two hydrocarbon solvents having the same macroscopic viscosity (n). The rotating moiety ex- perience different "free volume" in the two media inspect of equal bulk viscosity. The problem here is that of defining microviscosity as compared to macroviscosity. 32 Microviscosity may include a factor reflecting the struc- ture of the molecule of the medium; where as macroviscosity as measured by the gravity flow method and by the pres- d(19) reflects isotropic and continuous sure extrusion metho properties. Care must therefore be exercized in utilizing complex geometrically relaxing molecules in probe micro- fluidity. It is necessary to study quantitatively the fluorescence properties of the molecule in different media and examine the temperature and viscosity effects in de- tails as well as possible dielectric effects before apply- ing the molecular system to probe studies. C. Empirical Correlation of Absorption and Fluorescence Characteristics of Aromatic Molecules The fluorescence and absorption characteristics of a large number of aromatic compounds in solution have been (20) made an attempt to correlate measured.(3) Berlman the fluorescence and absorption spectra of aromatic mole- cules to the topology of the chromophore in the ground state as well as in the excited state. A summary of (20) Berlman's work is now given: 1. A planar system in the ground state has a more 33 structured absorption spectrum and larger value of 8 than the comparatively nonplanar system. 2. A planar chromophore in the excited state has a more structured fluorescence spectrum and larger quantum yield than the comparatively nonplanar mole- cular system. 3. If the topology of the chromophore changes upon excitation from a nonplanar to a more planar con- figuration, then the absorption spectrum is more dif- fuse and broader than the fluorescence spectrum. On the other hand, when the conformational change upon excitation from a planar (or nonpolanar) to a less planar configuration, the absorption spectrum is more structured and narrower than the fluorescence spec- trum. A. When a phenyl substituent makes an angle about 50° or larger with the main chromophore in the ground state, the angle becomes larger in the excited state. When the angle in the ground state is less than 50°, it becomes smaller in the excited state. Thus, anthracene being a rigid planar molecule and thereforetflueabsorption and fluorescence spectra are well resolved, the mirror image relationship is maintained (20) and Stokes shift is small. Berlman's interpretation of the fluorescence spectra of 9,10-diphenylanthracene 3A rests upon the observation that T: calculated from absorp- tion intensity is smaller than r3 obtained experimentally from decay and quantum yield measurements. From this he concluded that the angle of the phenyl group with respect to the anthracene plane is larger than 57° (the ground state value),similar conclusion was recorded by Berlman regarding l,l-diphenyl ethylene. As will be seen later in this chapter the proper emphasis should be on the relative positions and shapes of the potential energy curves drawn with respect to e, the angle of twist of the phenyl group with respect to the rest of the molecule. One must point (20) analysis neglected the effect of out that Berlman's relaxation in the excited state which causes diffusness and a shift of the fluorescence spectra. It is not clear why l,l-diphenyl ethylene becomes less planar in the ex- cited state while biphenyl becomes more planar. In order to answer such questions the nodal properties of the highest filled and lowest vacant molecular orbitals must be con- sidered to determine whether fl-interactions are enhanced or not in the excited state across the bond about which (21) (22) twisting occurs. In biphenyl and l,2-diphenyl ethylene (transestilbene)tflman-interaction between the two phenyl groups in biphenyl and between the phenyl and ethylene groups in trans-stilbene is enhanced as a result of excitation to the lowest singlet state in both cases. Thus, the n-interaction curve with respect to 0 is 35 expected to be steeper in the lowest singlet state. Berlman(20) did not explain why some molecules become more or less planar in the excited states, i.e., what inter- action should be considered to predict topological change upon electronic excitation? Many molecules which become more planar in the excited state compared to the non— planar ground state, yet their fluorescence spectra are more diffuse. This is opposite to the generalizations derived by Berlman(20). Reviewing various absorption and fluorescence spectra(3) in cases where rotation is possible lead us to believe that very useful qualitative information regarding changes in geometry as a result of excitation can be derived. In order to account for the qualitative features of absorp- tion and fluorescence one needs to consider: 1. Shapes of potential energy curves in the ground and lowest excited singlet states.' This depends on the extent of fl-interaction between the interacting moieties of the chromophore. In the cases which we will deal, it is assumed that n-interaction in the excited state is more than that of the ground state and that steric repulsions may occur between atoms in close contact. 2. Changes in equilibrium configuration with respect to 9 (angle of twist) due to excitation. Such changes may lead to geometric relaxation in the excited state 36 if the rate constant of relaxation is larger or com— parable to fluorescence rate constant. This will cause the fluorescence spectra to be diffuse, weak and shifted to longer wavelength in fluid media. The weakness of the emission results from more ef- ficient radiationless transition (internal conver- sion)to the ground state. (1) Flexible Molecules and Geometry Changes due to Excitation Now we will examine various cases of flexible mole- cules and attempt to correlate for their absorption and fluorescence characteristics with geometry changes. Case a: No steric repulsion between the interacting O C II GOG moieties; example ner- 9,10-Bis(phenyl-ethynyl)anthracene 37 Absorption and fluorescence spectra are shown in Figure (7a)(3). It should be noticed that the first absorption band is relatively diffuse compared to the structured fluorescence band, both taken at room tempera- ture. The vibrational spacing observed in the absorption spectrum is less than the vibrational spacing observed in fluorescence. Table (I) gives (300-301) for the absorp- tion and fluorescence. The mirror-image relationship is not as good as the anthracene case (Figure 2b, Chapter I), [(vOO)A—(vOO)F] is negligible, showing no change in geo- metrical configuration as a result of excitation. In order to interpret the above spectral information in terms of the potential energy diagrams, one uses the function E e Prscos2e, Chapter II, (6 is the angle of twist), to represent the n-interaction energy of the ground and the excited states. Since the repulsion energy is zero and under the assumption of more n-interaction in the excited state we get a steeper potential energy curve for the excited state compared with that for the ground state, the minimum of both potentials is at e = 0° as shown in Figure (7b). One can conclude that the diffuseness of the absorption spectrum is due to a relatively shallow ground state potential compared to the steeper excited state potential energy curves. The fluorescence spectrum is well resolved consistent with this,[(vOO)A-(VOO)F] is negligible because the equilibrium configurations of 38 Figure 7. (a) Absorption and fluorescence spectra of 9,10-Bis(phenylethynyl)anthracene and (b) the corresponding potential energy curves of the ground and excited states as a func- tion of 6. INTENSITY F L UORL SCENCI. ewneansanswuasan sun sun 1mm: «an «an mwo 4 man nan . ran an» '.m A #1? l A 1‘ A A JrL 1 FA A'A A 171 1% jA If A r 4 Y1 % J .- IO-IIS Mun-Emma) MIMI! 0-90 suvrm CYCLOIM (P .1 °"° mums mvstrucm man i c 0'70 mm: mm 1.0 000 ‘ moo i d c mso A mac 4 030 020 ' duo 4 m A l I A A A A A A A A A A 15am ream 12am 10am lawn and) :nan 2am0 zawo 2ww0 zuwo 2nw0 ammo zuwo HAVE NUMBER (CH'U 39 HRVELENGTH (9) g It) § § HOLRR EXIINCTION COEFFICIENT Enemy (a) 40 Table 2. Optical and luminescence characteristics of some anthracene derivatives. Extent of (3 -; )cm-1 4 00 0‘1 V V Ster-c ( 00)A’( 00)? Compound Repulsicn Absorption Fluorescence Diffuseness Parent Compound lAOO lACO zero At? No «1000 1250 Negligible A>F (NLCO) Small 1300 1000 Small F>A (WlOOO) Medium 1350 1150 Small FlA (x800) Large leo 950 Large F>>A (3000) Al both potential energy curves are coincident. One should note from Table (1) that the vibrational spacing in ab- sorption is about A00 cm"l smaller compared to anthracene. In fact such vibrational spacing decreases as the degree of w-interaction of the substituent in the 9 or 10 posi- tions increases. The vibrational spacing in fluorescence is 150 cm"1 smaller than that of anthracene. Such spacing reflect ground-state vibrations; in the ground state one expects n-interaction to be smaller compared to the ex- cited state. Case b: Small and moderate steric repulsion (1) CH C «a 9-Vinyl anthracene The vinyl group at position number 9 of the anthra- cene chromophore lies at 13° in a different plane(23) than that of the ring because of little repulsion with the peri hydrogens at positions 1 and 8. The absorption and fluorescence spectra of this mole- cule, Figure (8a) shows(3) a relatively diffuse fluores- cence spectrum compared to the structured absorption Figure 8. A2 (a) Absorption and fluorescence spectra of 9-vinyl anthracene and (b) the corresponding potential energy curves of the ground and the excited states as a function of 0. (---) steric repulsion energy curve; (-.-.—) n-interaction energy curve; (-——) resultant potential energy curves. FLUORESCENCE A3 HRVELENCTH In) 5500 S300 5100 0000 0700 1500 0300 «:00 3900 3700 3500 3300 3:00 A l.m>‘ A L" 1 :7 L471. 111 L1) L' I i J v ‘5 I." L_'_L YL fi‘ 'L V 0.90. scum maroon: . 1mm 0"“ utmuc mmmm 30500 7 «no 0 or aannvnuo ans ‘ 050. . awn 050. .4 ’4” 0.00, d ”W 0.w. J 2000 d2c. 010, . Ian Otto —L—+ J A A 4 _;l A A _A A J A A A 0 10000 moo 20000 21000 22000 23000 24000 25000 20000 27000 20000 20000 30000 31000 32000 33000 3:000 E NUMBER ICM'U Energy HRV (a) It) HOLRR EXIINCIION COEFFICIENI AA spectrum. The vibrational spacing observed in the absorp- 1 smaller than that of an- tion spectrum is only 100 cm- thracene reflecting small n-interaction between the vinyl group and anthracene. However, the vibrational spacing observed in fluorescence is significantly reduced (A00 cm"l less) compared with that of anthracene. [(vOO)A— (V00)F] is small, the mirror—image relationship is poor. Some relaxation appears to occur prior to fluorescence at room temperature. In order to correlate this spectral information to the shape of the potential energy curves of the ground and the excited states one assumes that the steric repul- sion energy is the same in both ground and excited states represented by the dashed curves in Figure (8b), and that smaller n-interaction occurs in the ground state compared with n-interaction in the excited state. The resultant potential energy curves are the solid curves of Figure (8b), which shows that the excited state potential energy curve has a minimum at a smaller angle compared with that of the excited state. Diffuseness of the fluorescence spectrum is probably due to the relaxation of the vinyl group that occurs in the excited state. The smaller vibrational spacing observed in the absorption spectrum of 9-vinyl anthracene compared with that of anthracene reflects the n-interaction of the vinyl group in the ground state. In the excited state the vinyl group relaxes to A5 its equilibrium configuration, in such configuration the n-interaction with the ring is more significant. The potential energy curve with respect to QV for the relaxed molecule is not identical to that of the equilibrium ground state configuration but corresponds to a relatively more planar molecule. Thus the spacing shown in fluorescence is smaller than that in absorption. O O30 9-Phenyl anthracene (2“) with the The phenyl group lies at an angle of 57° anthracene chromophore indicating greater repulsion of the phenyl hydrogens with the peri hydrogens at position 1 and 8 compared to the vinyl group. Figure (9a)(3) shows resolved absorption spectrum for 9-phenyl anthracene but the fluorescence spectrum is relatively less resolved. The fluorescence spectrum appears more resolved than in the case of 9-vinyl anthracene. Figure (9b) shows the potential energy curves for a case where n-interaction is small in the excited state and is negligible in the ground state giving rise to a relatively broader potential energy curve for the excited state. The phenyl group is more out of plane compared to the vinyl group (57° vs 13°) in the ground state. The larger A6 HRVELENCTH IR) 5200 5000 7 0000 1 0030 J «00 4 0200 11000 3000 3690 . 3000 3200 3000 . A A l A A A 1 A ._L hm ‘T‘ fi—i V v v Y r r v r 1' v r uncut mmuctur 0-90 _ scum manual: 1 10000 0.00, “(guns '1erth 36509 9 6' ”It! YIELD 0.319 O o 0. O INTENSITY 9 9 6 S g 3 110100 EXTINCTION COEFFICIENT ((1 —_T _._—__r_.——1————_r ? __s m > 5 fl IIORF. SCE NCf p .0 a s 0.00 _ AL A A 4 A A 0 19200 20200 21200 22200 23200 231200 25200 27300 20200 29200 30200 31200 322“ 13200 31200 352W “FIVE NUMBER (CH "I (a) Energy I Figure 9. (a) Room temperature absorption (A) and fluores- cence (F) spectra of 9-phenyl anthracene in hydrocarbon solvent and (b) the corresponding potential energy curves of the ground and the excited states as a function of 0. (---) steric repulsion energy curve; (-.-.-) n-interaction energy curve; (-—-) resultant potential energy curves. A7 vibrational spacing observed in the absorption spectrum of 9-phenyl anthracene reflect this. In the excited state some relaxation occurs and the phenyl group is more planar compared with the ground state; however, it is less planar than the vinyl group in the excited state. The vibrational spacing observed in fluorescence reflects this. Case c: Large steric effect: 9-Anthroic acid Because of the large steric repulsion between the lone- pair electrons of the oxygen atoms of the carboxyl group with the peri hydrogens at positions 1 and 8, the carboxyl group lies in a perpendicular plane to the anthracene chromophore.(25’26) Figure (10a) shows the absorption spectrum of 9- anthroic acid. The spectrum is well resolved and essen- tially identical to that of anthracene. The fluorescence spectrum is broad and shifted to longer wavelength.(27) Figure 10. A8 (a) Room temperature absorption and fluores- cence spectra of 9-anthroic acid and (b) the corresponding potential energy curves of the ground and excited states as a function of 0. (—--) steric repulsion energy curve; ( ----- ) n-interaction energy curve; (———0 resultant potential energy curves. A9 2.32:. 350332.... 0.3 0.2 '- O.1 — 3 3:33 4 (a) 90 (b) 50 The mirror-image relationship is poor and [(VOO)A_(vOO)F] is very large indicating a large change in geometry as a result of excitation. To interpret its spectra we assume that the n—interaction in the excited state brings the carboxyl group at a smaller angle than that of the ground state. The resultant curves are shown in Figure (10b). Relaxation to the equilibrium configuration in the excited state before emission occurs in a fluid medium giving rise to broad fluorescence spectrum where the vibra- tional structure essentially disappears. The vibrational spacing in absorption spectrum is identical to that of anthracene, this is consistent with an essentially perpen- dicular carboxyl group in the ground state. The very small vibrational spacing observed in the fluorescence spectrum in hydrocarbon solvent and the large Stokes shift is consistent with an emission from a configuration where the carboxyl group is more planar with the anthracene ring. The spectra of 9-anthroic acid esters has been examined in more detail as a function of viscosity and medium polarity, the results are discussed in Chapter IV. 51 (ii) Effect of Rendering a Flexible Molecule Rigid (I) (II) 2-Phenyl Indole 2,Phenyl—3,2'- methylene Indole The spectra of I and II are shown in Figures (11a) (3) and (12) respectively. The diffuse absorption spectrum of (I) indicates a shallow ground state potential energy curve while the structured fluorescence spectrum indicates a relatively steep excited state potential energy curve. The enhanced n-interaction in the excited state could be responsible for this. Linking the two interacting chromo- phores of (I) by a covalent bond will make it rigid. A comparison of the spectra of (I) and (II) shows that ren- dering (I) rigid via a covalent bond makes the absorption spectrum more resolved particularly the 0-0 vibronic band. While the fluorescence spectrum did not change signifi- cantly because in the excited state more n-interaction Figure 11. 52 (a) Room temperature absorption and fluores- cence spectra of 2-phenyl INDOLE and (b) the corresponding ground and excited state po- tential energy curves (the solid curves (-——) as a function of 0. The dashed curves (---) correspond to the ground and excited state potential energy curves of Figure (12) as a function of 0. 0100 1.00 ‘ 24mm "out 0J0 awn (10100014 0.00 0.70 0.00 0.50 P P 9 8 8 6 fl UOHI SCIENCE INTENSI II P (ICIIIUG MLIKM 3|” ”7w ”(L0 0.00 > '05“) '1100 VIN 3000 3700 L A A A g A A A A A T f V f r f 53 HRVELENGTH ”so 0.00 _ 21000 2200023000260020000200002700020000 Energy zinc new 1160:: 32000 “NE NUMBER (01") (a) A A A i ”an Home EXTINCTION COEFFICIENT HI 5A znv mad wHCOdmmomsom Amv monondm ow Hlamnwww1w.m.amnswpmsmlm1odmsww quorm H: smadoomwuoz mow( \-\ - l - \\ l I / -.\\ -,.\ 2‘ i II I \'-\\ °. \ .g) i, I \.. \ \ 93 .-I / \\ \ £5 I I / -.\ '. \\ : I \\\ '. . 8 'I/ / -. \\ '- \ C o a; I, 5. I \k \ \\ 3; [=1 . / \.\ \ ‘9; J ./ \.\\ \ fl: .' / .\ \ \ .‘lf "\\:‘ ‘\\ I .31]; ‘\~‘:::=- '/ l I I I I Ii I 380 420 460 500 . Wavelength (nm) Figure 29. Fluorescence spectra of (9MA) in triacetine at different )temperatures ( ----- ) R. T. (. .) -29°C; ( 0 5°C; (----) -93 °C; (- - -) 77 K. 88 triacetine glass but does not relax in ether glass. This is in spite of the fact that ether glass has smaller macro- scopic viscosity than ethanol glass pointing to the im- portance of the specific packing of the solvent molecules around the anthroic esters. E. Excited State Relaxation of 9-Anthroyl Palmitic Acid (A816) The excited state configuration of 9-anthroic esters have different geometries depending on the size of the ester group, temperature and solvent matrix. In the previous pages, it was shown that (9MA) does relax to the equilibrium excited state configuration even in a rigid glass of hydrocarbon while under the same condition (9TBA) does not relax. The tert-butyl ester group is interlocked in the rigid solvent matrix but the methyl ester group can undergo some relaxation in the same matrix element. It should be noted that rigid parafin oil at -80 °C does not prevent excited state relaxation of anthroic esters. The structure of the solvent matrix around the anthroic ester molecule is such that there is enough free volume for the ester group to relax during the excited state lifetime. The fluorescence spectrum of 9-anthroyl palmitic acid (A816) in fluid medium and rigid glasses of paraffin oil and triacetine has been measured and are shown in Figures (30) and (31) respectively. Geometric relaxation occurs 89 ... . ' // ?‘\.\\': / I \§~.\ : / \ '. > 3 / '° \\- \ 35-) .' I f r. -. I: .' I ' V. \ 93 : / ,/ \ '- ‘E ~° I / \s \ m .' . ' § 5 II / \'-\\-\ o - -, g I], / \\ \. g8 37'}, .?§\. 2\\. E .'// .>.\ .\-. 5,, ‘h..\ 4/ .. I I I l J l J 380 I 420 7 460 500 Wavelength (nm) Figure 30. Fluorescence spectra of (A816) in paraffin oil at different emperatures. ( ) R.T-3 ...-00. ”'41 °C; ---- ‘80 °C; (.00.) 77 K0 9O Fluorescence Intensity Figure 31. l l 420 460 500 Wavelength (nm) Fluorescence spectra of (A816) in triacetine T. at different temperatures. ; -——- -17 °C; III. -27 °C; - "' - -3u. .5. :C; —Ia—o¢— -l-I2.5 °C; "OOI-oao— —52 °;C .- 91 in paraffin oil at room temperature, however below -“0 °C such relaxation appears to be limited and an emission spectrum similar to (9MA) in rigid 3MP at 77 K is observed. The long aliphatic chain of A816 did not prevent some relaxation of the ester group even in a rigid hydrocarbon. Molecular model show that relaxation that renders the carboxyl group near coplanar with the anthracene group requires minor conformational changes mainly rotation about C-C bonds. In the (9TBA),the branched tertbutyl group requires a relatively large free volume for relaxa- tion, moreover a rigid medium looks it in the ground state configuration. In triacetine solvent as well as geometric relaxation of A816 occurs depending on the viscosity of triacetine. CHAPTER V EXPERIMENTAL A. Materials I. Purification of Solvents l. 3-Methy1 Pentane (3MP) - The Phillips pure grade 3-methyl pentane was mixed with nitric acid and sulfuric acid, then stirred for two days. Then it was separated and washed with base and water. After drying with sodium sulfate for one night, the solution was refluxed over sodium wires for two days and distilled. The vapor passed through a four foot vacuum Jacketed column and condensed at a speed of two drops per minute. The purity was checked by obtaining the absorption spectrum. 2. Isopentane (2-methylbutane) - Phillips pure grade isopentane was purified and purified by repeated recrystal- lization from ethanol. “. A816 (granthroyltpalmitic acid) - A816 (Prob- ing Chemical Company) was used without further purifica- tion. 95 B. Spectral Measurements 1. Absorption Spectra All reported absorption spectra were run on a Cary 17 Spectro-photometer. 2. Emission spectra Fluorescence spectra were recorded on a multicomponent system used in this lab consisting of a 500 W xenon light source, 500 mm Bausch and Lamb excitation monochromator, McPherson Model 235 Emission Monochromator and EMI Model 9558QB phototube. Noise reduction and amplification of the PMT signal is achieved by using a Princeton Applied Research Model HR-8-Lock In Amplifier and appropriate chopping apparatus. 3. Quantum Yields Fluorescence quantum yield (¢F) determinations were made using the following equation where r and u stand for reference and unknown, respectively 2 n _ _E .3 .Ji ¢u ’ ¢r x n2 x X F I" :2 "S n: is the solvent refractive index, A: is the solution absorbance at the exciting wavelength 96 (>0.02, 1 cm cells); and F: is the area under the emission spectrum obtained by the cut and weigh technique. The quantum yield reference was 9-methy1 anthroate in ethanol<27> which has a quantum yield of 0.18. Our samples were not degassed. “. Temperature Variation System A quartz dewar with a flat quartz excitation window and a glass narrow tube for the samples were used. The temperature of the sample was controlled by boiling liquid nitrogen using a power resistor and allowing the N2 gas to flow into the sample dewar. The temperature of the sample was monitored through a thermocouple (copper, constantan) dipped inside the solution in the sample tube immediately above the point of excitation. The lowest stable temperature which can be reached by such a system is around -200 °C. 5. Fluorescence Decay Curves The decay curves and the lifetimes measurements were taken on an instrument similar to that developed and described by Ware.(u3) It is shown in Figure (32). This consists of a gated nanosecond pulse lamp, Model 501A of Photochemical Research Associates, Ontario, run 97 DUVP Emission Phototube Monochromater /Polarizers Sample Flasi 1P28 bJ Holder Dia hototub i:i§y Line DelaylLine Timing Filter 100 MHz Amplifier Discriminator [ Delay Constant Fractio - Discriminator _ I I Multichannel Dispersion [Oscilloscope - Analyzer Amplifier J Time to Amplitude Converter [(stop) l Teletype ] Acoustic ] Coupler HP 7200A CDC 6500 Plotter Computer Figure 32. Diagram of the time resolved spectroscopy instrument. 98 at one half atmosphere pressure of deuterium (D2) gas and “.2 kV giving a flash frequency of approximately 30 kHz and a pulse width at half-height of about 2-3 nsec. The flash is detected by an RCA 1P28 phototube. The output of the phototube is delayed 12“ nsec by Cable and passed through an ORTEC Model “36 one hundred megaherz discriminator and is then used to gate the 'start' of ORTEC Model “57 Time to Pulse height convertor. Corning Glass Filters and/or Wratten Diffraction Gratings were used to select radiation of appropriate frequency for excita- tion. The sample cell was an Aminco-Bowman with a tem- perature Jacket to allow connection to a constant tem- perature control unit. Emission is viewed at 90° to the excitation. A Bausch-Lamb Monochromator served as the emission monochromator. An Amperex 56DUVP103 photo- multiplier tube served as the detector of fluorescence emission. The output pulse is passed through a 12“ nsec delay Cable and into an ORTEC Model “5“ Timing Filter Amplifier. The filtered signal is then passed through an ORTEC Delay Unit Model “25 giving the pulse a constant 1“ nsec delay before passing through an ORTEC Model “63 Constant Fraction Discriminator. The signal then gates the 'stop' input of the time to pulse height converter. 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