J . Phys. Chem. 1993,97, 2618-2622
2618
Reversal of Excitation Behavior of Proton-Transfer vs. Charge-Transfer by Dielectric Perturbation of Electronic Manifolds Pi-Tai Chou,' Marty L. Martinez, and John H. Clements Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 Received: October 23, 1992; In Final Form: December 16, 1992
Reversal of excitation behavior of proton-transfer vs. charge-transfer by dielectric perturbation is reported for 4'-(diethylamino)-3-hydroxyflavone (I). At room temperature I exhibits a dominant proton transfer emission and a nonnegligible normal emission with maxima at 555 and 440 nm, respectively in alkane solvents, in contrast to 3-hydroxyflavone in which only tautomer emission is observed. The result may be rationalized by a mechanism incorporating fast proton tunneling between normal and tautomer forms which are in equilibrium in the excited singlet state. The normal emission maximum exhibits a drastic solvent dependence with a red shift of >3000 cm-' from n-heptane to methanol. In ethanol a unique normal emission with an unusually high yield (a 0.52 f 0.02) is observed. It is proposed that the SIstate of I may be ascribed to a zwitterionic configuration
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induced by the charge transfer from the 4'-diethylamino group to the carbonyl oxygen, for which the energy is even lower than that of the tautomer in strong polar, protic solvents, precluding the proton-transfer reaction.
Introduction Excited-state intramolecular proton transfer (ESIPT) has received considerable attention both experimentally and theoretically.14 The ESIPT process generally involves transfer of a hydroxyl (or amino) proton to an acceptor such as a carbonyl oxygen or a nitrogen atom in the excited state, resulting in a large Stokes shifted tautomer emission. This unusual property has led to several recent practical application^.^-^ As a prominent example 3-hydroxyflavone (3HF) has been widely used as a radiation hard-scintillator counter5.* based on its large Stokes' shifted (- 1000 cm-I) tautomer emission and relatively high quantum yield. Unfortunately, its photochemical reactivity9 limits its practical long-term application. Thus, one focus of the search for an ideal radiation-hard scintillator has been the design of derivatives of 3HF which can overcome these drawbacks. We proposed that the addition of auxiliary substituents such as a 4'-dialkylamino group on the &phenyl ring of 3HF (see Figure 1) may greatly change the electron density distribution due to its strong electron donating property. Thus, the photophysical and photochemical properties may besignificantly altered with respect to that of 3HF, resulting in an ideal candidate for radiation-hard scintillators. In this report the photophysicsof the 3HFderivative, 4'-(diethylamino)-3-hydroxyflavone (I, Figure 1) have been investigated.I0 The non-negligible normal emission in alkane solvents at room temperature, in contrast to 3HF in which a unique tautomer emission is observed, may be rationalized by an excited-stateequilibrium between the normal and tautomer species associated with a rapid proton tunneling mechanism. The drastical solvent polarity dependent maximum of the normal emission is discussed based on the charge transfer property of the diethylamino substituent (electron donor) coupled with carbonyl oxygen (electron acceptor). In strong polar, protic solvents such as ethanol the charge transfer state is proposed to be lower in energy than the excited singlet tautomer state. Thus, ESIPT is energetically unfavorable, resulting in a dominant charge transfer emission. The observation of the reversal of the excitation behavior of proton-transfer vs. charge transfer by dielectric perturbation is unique among the class of flavonols.
x AIb
la
-0
H
IC
Figure 1. Structure of a. the normal species and b. the tautomer species of I, c. the proposed structure for the charge-transfer state.
The product was purified by repeated recrystallization from n-heptane. The purity of the product was checked by NMR, GC and the fluorescence excitation spectrum. 4'-(Diethylamino)3methoxyflavone (11) was synthesized by the reaction of I with dimethyl sulfate under a nitrogen atmosphere. The product was purified by column chromatography (eluent 4: 1 v/v hexanes/ ethyl acetate) and repeated recrystallization from n-heptane. All solvents were of spectrograde quality and purified as previously described.12 The solutions were either saturated with 760 torr oxygen or degassed by four freeze-pump-thaw cycles under vigorous stirring conditions on the vacuum line. Measurements. Steady-state absorption and emission spectra were recorded by an H P (Model 8452A) spectrophotometer and Shimadzu (Model RF5000U) fluorometer, respectively. Details of the setup of the transient absorption, two-step laser-induced fluorescence and low temperature measurements have been described elsewhere.12-'4 Quantum yields were measured using quinine sulfate/ 1.O N H$304 as a reference, assuming a yield of 0.564 with 360 nm excitation.'s
Experimental Section Materials. I was synthesized by the reaction of o-hydroxyacetophenone and 4-(diethylamino)benzaaldehyde by the standard Claisen-Schmidt condensation followed by oxidation with H2O2.II 0022-3654/93/2097-261 8S04.00/0
Results Steady-State Measurements. The UV absorption of I in n-heptane'is shown in Figure 2a. The SO- S I ( m * )absorption 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2619
Dielectric Perturbation of Electronic Manifolds 1.01
L
11.0
1
fi.
0
W.\VFl.tB(;TtI 300
400
500
600
Wavlength nm Figure 2. Absorption and emission spectra of I a h n-heptane and b. in ethanol (emission is denoted by the prime sign). c. The emission spectrum of a expanded 30 times in the 400-500 nm region.
1
+o
1.0
400
500
Wavelength
600
nm
Figure 3. Absorption and emission spectra of I1 a. in n-heptane and b. in ethanol (emission is denoted by the prime sign).
exhibits an onset at -440 nm with a band maximum a t 400 nm. The absorption maximum of I shows a red shift with increasing solvent polarity. In addition, the sharp-rise of the absorption profile becomes broadened when solvated in polar solvents as opposed to alkane solvents. In ethanol, the absorption (Figure 2b) is structureless with a maximum which is only - 5 nm redshifted with respect to thpt in n-heptane. However, the onset of the absorption is unusually long extending to -500 nm. The concentration independent absorption spectra in various solvents applied indicate that there is no dimer or oligomer formation for I in therangeof 1W-lWM. Theroomtemperatureluminescence of I in n-heptane (Figure 2a') is characterized by a predominant yellow-orange emission maximized at 555 nm with a quantum efficiency @fof0.25 f 0.02. In addition, a normal Stokes shifted emission maximum at 440 nm is observed with an emission intensity which is -2 orders of magnitude weaker than that of the 555 nm band (Figure 2c). The excitation spectra monitored at both440and 555 nmareidentical with theabsorptionspectrum. A similar ratio of 555 nm/440 nm emission intensities was observed in various alkane solvents, such as 3-methylpentane, n-pentane, n-hexane, methylcyclohexane and dodecane. On the other hand, 11, which is considered as a non-proton-transfer model with an electronic configuration similar to I exhibits a normal Stokes shifted fluorescence maximum at -420 nm in n-heptane (Figure 3). No long wavelength emission in the 500-600 nm region was observed. In conclusion, the 555 nm emission band observed in I is consistent with a tautomerization (proton transfer) occurring in the lowest excited singlet state following electronic excitation. The energy gap between absorption and tautomer emission maxima is calculated to be -6,400 cm-1. This is believed to be one of the smallest Stokes shifts among this class of 3HF derivatives. Accordingly, the 440 nm emission of I observed in
UiI
Figure 4. Room temperature emission spectra of I in (-) ether, dioxane, (- -) CHCI3, CH2C12, (- -) CH3CN. It is noted that all spectra measured are uncorrected. The ratio of longwavelength/ shortwavelength emission intensities may not be correct.
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(-9)
(e-)
n-heptane, similar to that of 420 nm for 11, is ascribed to normal emission (vide infra). In weakly polar, aprotic solvents the ratio of normal/tautomer emission intensities for I increases as the solvent polarity increases. The maximum of the tautomer emission is slightly red-shifted with increasing solvent polarity. However, the red shift of the normal emission with respect to the solvent polarity is drastic (Figure 4). In protic solvents such as ethanol a unique, strong emission maximum at 510 nm was observed. This 510 nm emission, 1400 cm-l blue shifted with respect to the tautomer emission measured in n-heptane, does not fit the polarity dependent tautomer emission maxima, which all show a slight red shift in polar, aproticsolvents with respect to that in hydrocarbon solvents. Furthermore, in spite of the 3600 cm-l gap between absorption and emission maxima the 0-0 onsets closely overlap in ethanol (Figure 2). These observations lead us to conclude that the 510 nm emission does not originate from the excited tautomer species. This argument can be supported by the observation of the emission maximum at 505 nm for I1 in ethanol (Figure 3), for which no ESIPT takes place. Since alkoxy1 anion formation for I1 is not possible the possibility that the 510 nm emission for I results from adiabatic proton dissociation to the surrounding solvent in the excited state is also eliminated. In contrast to the weak normal emission (@ < 0.01) of 3HF in ethanol, the fluorescence quantum yield for I in ethanol is usually high with a value of 0.52 f 0.02. Laser action was observed for I at 560 nm in alkane solvents and 510 nm in ethanol. Consequently, for I an unusually wide range of lasing frequencies can be achieved from 535 nm to 575 nm, depending on the chosen solvents. A strong laser action was also observed for I1 in alcohol solvents. A detailed study of the lasing properties of I and I1 will be published elsewhere.16 In comparison to the 440 and 420 nm emission maxima for the normal forms of I and I1 in n-heptane, the 510 nm and 505 nm emissions in ethanol are -3,000 cm-' and -2,800 cm-I redshifted, respectively. This drastic solvent dependence is clearly shown in the continuous spectral changes observed when various amounts of ethanol are added to solutions of I and I1 in n-heptane. As shown in Figure 5 a significant red-shift of the normal emission was observed with the added ethanol concentration varying from 0-4.0 M, while the maximum of the tautomer emission shows negligible spectral shift. In addition, the ratio of normal/tautomer emission bands for I increases during the titration. Similar redshifted emission maximum as a function of ethanol concentration was observed for 11. Nano-microsecond Time-ResolvedStudies. Figure 6 shows the photoinduced transient absorption spectra of I in degassed n-heptane at various delay times. The spectra are characterized by broad absorption maxima at -430 nm and 520 nm. The intensities of these two absorption bands, within theexperimental error, show the same time dependence, indicating that they originate from the same transient species. Two important
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2620 The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 a
n
500
450
550
600
nm Figure 5. Room temperature emission spectra of I in n-heptane with varying ethanol concentrations: a. no ethanol b. 0.04 M c. 0.06 M d. 0.08 M e. 1.2 M f. 2.0 M g. 4.0 M. Wavelength
500
460
L1!4\’FLFNCTH
580
540 nm
Figure 6. Time-dependent transient absorption spectra of I in degassed n-heptane (1.7 X 10.’ M) at various delay times: a . 200 ns; b. 6 p s ; c. 20 ps; d. 100 gs. All spectra are measured at room temperature. The pump pulse energy is 3.0 mJ cm-?/pulse. Data are taken by an average of 60 shots for a. and b. and an average of 100 shots for c. and d. Due to the strong SO Si absorption at < 420 nm the transient absorption maximum at 430 nm may be even shifted toward higher energy.
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observations for the decay dynamics of the transient absorption can be pointed out: (1) The decay is excitation energy dependent and behaves primarily by second-order kinetics at the highest excitation energy applied. (2) The time-profiles of these two transient absorption bands are extremely sensitive to the presence of oxygen and the pseudo-first-order quenching rate constant for oxygen is calculated to be -2.0 f 0.2 X lo9 SKI. The results can be well explained by assigning the long-lived transient species to a triplet state. Since the triplet-triplet (TI T,) absorption of I1 exhibits a maximum at -460 nm which is different from that of I observed in n-heptane we conclude that the triplet-triplet absorption of I originates from the tautomer triplet state (TtI). It is noted that although in the two-step laser-induced fluorescence experiment the tautomer emission was observed when the probe laser was tuned to the transient absorption maximum of 430 nm, the tautomer emission was also observed when the probe laser was tuned to > 600 nm in which no S’O S’l absorption takes place. Therefore, our previously proposed mechanism for 3HF and 7-hydroxy-l-indanone, which incorporates TtI T’, absorption followed by T’2 SI intersystem crossing, resulting in the tautomer emissionl3,14 is also suitable for I.
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Discussion A detailed understanding of the photophysics of 3HF in various solvents has been achieved due to the efforts of many research groups. In alkane solvents the rate of ESIPT is extremely rapid, even in inert gas matrices at cryogenic temperatures, indicating that the ESIPT barrier for 3HF in alkane solvents and inert gas
Chou et al. matrices is either negligible or small with the ESIPT dynamics dominated by proton tunneling.”-I9 On the other hand, several laboratories have reported that in protic solvents the rate of proton transfer consists of both slow and fast component~.I~J@2~ The slow proton transfer component, k,~,,,depends on the hydrogenbonding ability of the solvent and follows Arrhenius-type behavior. Barbara et al.*O concluded that in alcohol solvents the proton transfer process is not a function of the solvent relaxation dynamics, but rather is more sensitive to energetic factors of the solute/solvent interactions. The hydrogen bonding effect of protic solventson the proton transfer of 3 H F has been recently extended by Harris et al.17based on femtosecond dynamics. Their results for the slow component of the ESIPT in alcohol solvents are consistent with those of previous reports.2G24 In addition, the fast component is attributed to the rate of excited-state proton transfer of a 1:l 3HF/monohydroxyl alcohol complex in which the wavepacket moves along the reaction coordinate even faster than the intact five-membered ring intramolecularly hydrogen bonded 3HF due to the higher frequency 0 - H vibration. hast of ESIPT is determined to be in the range of 10’3s-l - lOI4 s-I, depending on thestrengthof the 3HF-solvent hydrogen b0nding.l’ Although Harris et al.3 work failed to consider the spectral evolution associated with vibrational relaxation the result unambiguously concludes that in alcohol solvents the deactivation of the electronically excited 3HF normal species is dominated by the overall rate of proton transfer (including solvent dynamics and intrinsic proton transfer), resulting in a short lifetime and low quantum yield for the normal emission in alcohol solvents. In comparison to the known photophysics of 3HF the results for I provide interesting new features in proton transfer dynamics among flavonols. First, the appreciable normal emission in alkane solvents is unlike 3HF for which only tautomer emission was observed at room temperature. Since the excitation spectra are identical when monitored at normal (440 nm) and tautomer ( 5 5 5 nm) emission regions, the possibility that the short wavelength emission is caused by an impurity is small. In addition, the intensity of the normal emission was unchanged after further solvent purification attempts. Thus, perturbation due to trace polar solvent impurities at room temperature also seems unlikely. The observation of an intrinsic normal emission in alkane solvents leads to three postulated proton transfer mechanisms: (1) It may indicate that the proton transferreactionisassociated with a significantly large activation barrier crossing in the sense of transition-state theory. (2) It may indicate a slow rate of proton transfer due to nontransition-state effects. These include either the dielectric relaxation of solvent configuration or a modulation of the slow isomerization rate due to coupling of the large amplitude motion of the solute to the configurational dynamics of the solvent. (3) It may imply that there exists an equilibrium between normal and tautomer excited singlet states. For this case the rateof forward and reverse ESIFTcan beextremely rapid, possibly dominated by proton tunneling. Although our instrument response limits dynamic studies in t h e excited state, a mechanism may be deduced through steadystate measurements in combination with numerous dynamic results for 3HF.I7-*4 Since n-alkanes are non-polar and generally treated as “inert solvents” the proposed slow dielectric relaxation of the solvent preceding ESIPTcan be eliminated. In additioqdue to thestrong electron donating property of the diethylamino group in the para (4’) position theC2-CI’(seeFigure1) bondshould bemoredouble bond like for I than 3HF, resulting in a coplanar structure for the y-pyrone and phenyl rings. Thus, ESIPT may take place with negligible coupling with phenyl torsional motion. Consequently, the possibility of the proposed mechanism (2) in which the proton-transfer activation energy is a function of large amplitude motion, such as the torsional motion of the phenyl
Dielectric Perturbation of Electronic Manifolds
X
The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2621
2
3 Y
3~
xp
5
7 4.
5.
8 7
8
4~
5~
60
80
7
Figure 7. Frequencies at normal (closed circles) and tautomer (open circles) emission maxima of I in various polar solvents with respect to that measured in n-heptane. ‘X” symbol represents the frequencies a t normal emission maxima of I1 in various polar solvents with respect to that measured in n-heptane. Solvents used in this experiment are 1. n-heptane, 2. diethyl ether, 3. dioxane, 4. ethyl acetate, 5 . CHC13, 6 . CHzCll 7 . acetone 8. acetonitrile.
group is discounted. In fact, studies of 3HF and (8-ring)methylsubstituted 3HF by Barbara and co-workers showed that phenyl torsion is not important to the ESIPT dynamic^.^^.^^ Theoretically, mechanism (1) can be verified by examining the temperaturedependent dynamics of ESIPT or the intensity ratio of tautomer/ normal emissions. However, one should note that in a temperature-dependent study of 3HF the dynamics of ESIPT are strongly perturbed by trace polar solvent i m p ~ r i t y . 2 ~Similar 3 ~ ~ to 3HF, the emission spectrum of I unfortunately is also highly sensitive to trace solvent impurities in the temperature dependent study. The spectrum of I in a 77 K methylcyclohexane glass exhibited a dominant green emission when the methylcyclohexane was not purified. This green emission maximum at 500 nm is identical with the emission of I in a 77 K ethanol glass, indicating that the 500 nm emission of I observed in a 77 K methylcyclohexane glass originates from the I/(protic solvent impurity) hydrogen bondedcomplex. After extensive purification of methylcyclohexane according to reported method~l2.2~,*~ the spectrum shows a dominant yellow-orange tautomer emission maximum at 560 nm. Although our best purified solvents still exhibited 10% of the normal emission, the result clearly demonstrates that under
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--
the solvent impurity perturbation-free condition the tautomer emission is expected to be dominant and thus cannot be explained by mechanism (1) in which the ratioof normal/tautomeremission intensities should increase as the temperature decreases. Accordingly, mechanism (3) is favorable with an equilibrium between normal and tautomer species in the excited singlet state, where the energy level of the tautomer state is lower than that of the normal state. Since the Stokes shift (the frequency difference between absorption and tautomer emission maxima) for the tautomer emission of I is only -6,000 cm-I, based on the AM1 calculation that the energy difference between normal and tautomer forms is (24.2 f 5.0 kcal/mol) in the ground state, the energy levels of SI (normal species) and S’I (tautomer species) are expected to be very close. This property is different from 3HF for which a highly unsymmetric potential surface along the reaction coordinate, i.e. large exothermic ESIPT, is generally accepted, resulting in a negligible degree of reverse proton transfer during the lifetime of the excited state. It is noted that close energy levels between normal and tautomer excited singlet states have been recently reported for several other classes of ESIPT molec~les.2~J8 The increase in the ratio of normal/tautomer emission intensities observed in polar solvents is intriguing. For instance, a 2: 1 ratio of normal/tautomer emission intensities was observed in CH2C12. Since the proton accepting ability of CHzClz is weak, the hydrogen bonding between I and the solvent is negligible. Thus, the intramolecular hydrogen bond in I is expected to remain intact. This is supported by the dominant tautomer emission observed for 3HF in CHzC12. In order to achieve a detailed understanding of the solvent effect, a series of solvents was used to permit a study of environmental perturbation of the competing electronic process. Figure 7 shows the solvent polarity dependence of the normal and tautomer peak frequencies for I and I1 as a function of solvent polarity ( E ~ ( 3 0 ) ) . *It~ is expected that the intramolecular proton transfer fluorescence for I should be slightly affected by the solvent polarity index, consistent with the result shown in Figure 7. On the other hand, the frequency shift of the normal fluorescence peak is drastically dependent on the solvent polarity index. A Av change from 1200 cm-l in CHzClz to 3000 cm-i in acetonitrile is observed. Similar results were obtained for the normal emission of 11. Since the excitation spectra monitored at tautomer and normal emissions are identical in various solvents studied, attributing the dual emission to two different conformers which equilibrates in the ground state is not possible. We thus propose that for I and I1 the SI state responsible for the normal emission has charge transfer properties induced by the electron donor, the para-diethylamino substituent coupling with the electron acceptor, the carbonyl oxygen (Figure IC).This assignment can be supported by an experiment in which I was dissolved in CHC13 saturated with HC1 gas. The absorption maximum was significantly blue shifted to 350 with a spectral
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S;
E
emission charge transfer emission
so so
--
so in
nonpolar solvents
in polar. aprotic solvents
in polar. proric solvenis
Figure 8. The proposed ESIPT/charge transfer mechanisms incorporating solvent dielectric perturbation.
2622 The Journal of Physical Chemistry, Vol. 97, No. 11, 1993
profile identical with that of 3HF. The tautomer emission was also analogous to that of 3HF with a emission maximum at 528 nm. The result can be rationalized by the protonation of the diethylamino substituent, prohibiting the charge transfer reaction. Although without ultrafast time-resolved studies the relaxation dynamics cannot be resolved at this stage, a mechanism incorporating the reversal of excitation behavior of proton-transfer vs. charge-transfer by dielectric perturbation of electronic manifolds is tentatively proposed in Figure 8. In nonpolar, alkane solvents thecharge-transfer SI state, due to thelackof solvent stabilization, is higher in energy than the proton-transfer S’Istate. According to Figure 7 it is reasonable to assume that the normal (charge transfer) state is more sensitive to solvent polarization than the tautomer state. Thus, the relatively small energy difference for I with respect to that of 3HF in alkane solvents may lead to a near degeneracy between SI and S’,states in weak polar media. In relatively strong polar solvents the reversal of the relative energies of the SIand S’Istates may even take place. Thus, dual fluorescences were observed corresponding to proton transfer and charge transfer emissions. Thedeductionof the kinetics involving forward and reverse proton transfer competing with solvational relaxation and nonradiative emissions of SIand S’]states may be complicated and required further timeresolved study. In protic solvents, such as in ethanol, the unrelaxed charge transfer (SI) state may even be energetically much lower than the SI’state, prohibiting the excited-state proton transfer. Our results also suggest that in polar solvents significant (possibly with ultrafast dynamics) time-dependent Stokes shifts should be observed for the charge transfer emission. In contrast, nearly time-independent Stokes shifts are expected for the tautomer emission. It is noted that our attempt is to qualitatively derive a mechanistic picture to explain our results. Detailed studies of proton transfer versus charge transfer dynamics in various solvents require pico-femtosecond time-resolved studies with the assistance of a theoretical approach based on the calculation of the molecular dynamics. Due to the unusually high fluorescence yield for both tautomer (proton transfer) and normal (charge transfer) emissions, the proton/charge transfer dynamics of I are believed to be of interest to a broad spectrum of researchers.
References and Notes ( I ) Kasha, M. J . Chem. Soc., Faraday, Trans. 2 1986, 82, 2379.
Chou et al. 29.
(2) Barbara, P. F.; Walsh. P. K.; Brus, L. E. J . Phys. Chem. 1989, 93,
(3) Special Issue (Spectroscopy and Dynamics of Elementary Proton Transfer in PolyatomicSystems, ed. by P. F. Barbara and H. D. Trommsdorff), Chem. Phys. 1989, 136, 153-360. (4) Special Issue(M. Kasha Festschirift),J. Phys. Chem. 1991,95,l022& 10524. ( 5 ) Renschler, C. L.; Harrah, L. A. Nucl. Inst. Methods Phys. Res., A235, Sept., U S .Patent, 1985, 636. (6) Costela, A.; Munoz, J. M.; Douhal, A,; Figuera, J. M.; Acuna, A. Appl. Phys. 1989, 49, 545. (7) Chou, P. T.; Studer, S. L.;Martinez, M. L. Appl. Spec. 1991, 45, 5 13. laser (8) Radial. Phys. Chem. (special issue), in press. (9) (a) Studer, S.L.; Brewer, W. E.;Martinez, M. L.; Chou, P. T. J. Am. Chem. SOC.1989, 7643. (b) Chou, P. T.; Studer, S. L.; Martinez, M. L.; Orton, E.; Young, M. Photochemistry and Photobiology 1991, 581. (IO) Itoh, M.; Fujiwara, Y.; Sumitani, M.; Yoshihara, K. J . Phys. Chem. 1986, 90, 5672. In this paper the tautomer emission of the analogue of I, 3-hydroxy-2-(4’-dimethylaminophenyl)-4H1-benzopyran-4-one, in 3-methylpentane has been reported. ( I I ) Smith, M. A.; Neumann, R. M.; Webb, R. A. J . Heterocyclic Chemistrv 1968. 5. 425. (12) Brewer,’W. E.; Studer, S. L.; Standiford, M.; Chou, P. T. J. Phys. Chem. 1989, 93, 6088. (13) Martinez,M.L.;Studer,S.L.;Chou,P.T.J.Am.Chem.Soc.1990, 112, 2427. (14) Chou, P. T.; Martinez, M. L.; Studer, S. L. J. Phys. Chem. 1991,95, 10306. (15) Demas, J. N.; Crosby, G. A. J . Phys. Chem. 1971, 75, 991. (16) Chou, P. T.; Martinez, M. L.;Clements, J. H. Chem. Phys. Lett, in press. (17) Schwartz, B. J.; Peteanu, L. A.; Harris, C. B. J . Phys. Chem. 1992, 96, 3591. (18) Dick, B.; Ernsting, N. P. J . Phys. Chem. 1987, 91, 4261. (19) Brucker, G. A.; Kelley, D. F. J. Phys. Chem. 1987, 91, 2856. (20) Strandjord, A. J. G.;Courtney, S. H.;Friedrich, D. M.; Barbara, P. F. J . Phys. Chem. 1983, 87, 1125. (21) Strandjord, A. J. G.; Barbara, P. F. Chem. Phys. Chem. 1983, 98, 21. (22) Strandjord, A. J. G.; Barbara, P. F. J . Phys. Chem. 1985,89,2355. (23) Strandjord, A. J. G.; Smith, D. E.; Barbara, P. F. J . Phys. Chem. 1985,89, 2362. (24) Brucker, G . A.; Swinney, T. C.; Kelley, D. F. J . Phys. Chem. 1991, 95, 3 190. (25) McMorrow, D.; Kasha, M. J . Phys. Chem. 1984,193, 2235. (26) McMorrow, D.; Kasha, M. Proc. Natl. Aead. Sci. 1984, 81, 3375. (27) Smith, T. P.;Zaklika, K. A.; Thakur, K.; Barbara, P. F. J . Am. Chem.Soc. 1991,IJ3,4035. Smith,T. P.;Zaklika, K. A.;Thakur,K.; Walker, G. C.; Tominaga, K.; Barbara, P. F. J . Phys. Chem. 1991, 95, 10465. (28) Mordzinski, A.; Grabowska, A.; Kuhnle, W.; Krowczynski, A. Chem. Phys. Lett. 1983, 101, 291. Mordzinski, A.; Grabowska, A.; Teuchner, K. Chem. Phys. Lett. 1984, 1 1 1 , 384. (29) Christian, R. Ed. Solvent and solvent effects in organic chemistry VCH publishers, chap. 7 pp. 365.
