J. Phys. Chem. 1986,90, 5089-5093 a. Intra-ET efficiency is strongly dependent on molecular geometry for both singlet-singlet and triplet-triplet routes. b. Intersystem crossing efficiency in cyclic a-diketones depends on the particular molecular geometry and is always smaller than that for biacetyl. c. The interrelation between S-S and T-T intra-ET can be established from observations based on quantum yield ratios. In
5089
particular, we note the competition between these two transfer processes, resulting in less efficient T-T intra-ET for bichromophoric molecules for which S-S intra-ET is efficient. Registry No. I (n = 4), 87179-28-0; I (n = 5), 87179-29-1; I (n = 6), 87179-30-4; 11 (n = 3). 92346-37-7; 11 (n = 4), 92346-38-8; 111 ( n = 2), 103711-88-2; 111 (n = 3), 92346-39-9; I11 (n = 4), 92346-40-2.
Effect of Solvent on Excited-State Intramolecular Proton Transfer in Benzotriazole Photostabilizers K. P. Ghiggino,* A. D. Scully, Department of Physical Chemistry, The University of Melbourne, Parkville, Australia, 3052
and I. H. Leaver C.S.I.R.O. Division of Protein Chemistry, 343 Royal Parade, Parkville, Australia, 3052 (Received: February 20, 1986)
The absorption and fluorescence characteristics of 2-(2’-hydroxy-5’-methylphenyl)benzotriazole and its sulfonated analogue sodium 2-(2’-hydroxy-5’-methylphenyl)benzotriazolesulfonate have been studied in a range of solvents. The ground-state conformationscontributing to absorption in these solutions have been resolved and characterized. Excited-state intramolecular proton transfer (ESIPT) is found to be strongly dependent on the hydrogen-bonding properties of the solvent. ESIPT is effectively prevented in those molecules which are hydrogen bonded to aprotic solvents whereas in protic hydrogen-bonding solvents at room temperature the results suggest that ESIPT may occur via an encounter complex involving solvent molecules. Rate constants for radiative and nonradiative excited-state decay processes are reported.
Introduction The exceptional photostability of many compounds with intramolecular hydrogen bonds has been attributed to a highly efficient radiationless decay process from an excited-state proton-transferred form of the molecules.’-8 The phenomenon of excited-state intramolecular proton transfer (ESIPT) has been observed in salicylic acid derivatives, and for hydroxyphenylsubstituted benzothiazoles, benzoxazoles, and benzotriazoles.* In addition, ESIFT has been implicated in the photochromic behavior of certain anils,* and picosecond fluorescence measurements for ~alicylidenaniline~ indicate a rate for tautomeric proton transfer of 3 2 x 10” s-1. The photophysics of the photostabilizer 2-(2’-hydroxy-5‘methylpheny1)benzotriazole (TIN) have been studied by several workers using both steady-~tate’.~.~ and time-resolvede7 spectroscopic measurements. In nonpolar solvents TIN is believed to exist mainly in an intramolecularly hydrogen-bonded form and the photophysical scheme shown in Figure l a has been proposed.’~~ Originally the initial excited state (I) was believed to be non(1) Otterstedt, J. A. J. Chem. Phys. 1973, 58, 5716-5725. (2) Klopffer, W. Adu. Photochem. 1977, 10, 311-358. (3) Werner, T. J. Phys. Chem. 1979,83, 320-325. (4) Huston, A. L.; Scott, G. W.; Gupta, A. J. Chem. Phys. 1982, 76, 4978-4985. ( 5 ) Flom,S. R.; Barbara, P. F. Chem. Phys. Lett. 1983, 94, 488-493. (6) Wotssner, G.; Goeller, G.; Kollat, P.;Stezowski, J. J.; Klein, U. K.A.; Kramer, H. E. A. J . Phys. Chem. 1984,88, 5544-5550. (7) Woessner, G.; Goeller, G.; Rieker, J.; Hoier, H.; Stezowski, J. J.; Daltrozzo, E.; Neureiter, M.; Dramer, H. E. A. J. Phys. Chem. 1985, 89, 3629-3636. ( 8 ) Bocian, D. F.; Huston, A. L.; Scott, G. W. J. Chem. Phys. 1983, 79, 5802-5807. ( 9 ) Barbara, P. F.; Rentzepis, P. M.; BNS, L. E. J. Am. Chem. SOC.1980, 102, 2786-2791.
fluorescent’ but later measurements using a high-sensitivity spectrofluorimeter revealed the presence of a very weak emission with a maximum near 400 nm: In more recent work the origin of this weak emission has been attributed to the presence of trace amounts of moisture in the sample.s,6 Fluorescence from the proton-transferred form of the molecule (11) has also been observed (A, 630 rim) in low temperature g l a s s e ~ , l in * ~mixed ~~~~ A rate constant of >lo1’ crystals,8 and in pure crystalline s-l has been estimated for the excited-state intramolecular proton-transfer In polar, hydrogen-bonding solvents such as ethanol, a short wavelength fluorescence with a maximum near 400 nm has been proposed to originate from those molecules which are intermolecularly hydrogen bonded to the solvent and which do not undergo ESIPT (Figure lb).3-8 This paper reports the results of a study on the room temperature absorption and emission characteristics of T I N and its sulfonated analogue, sodium 2-(2’-hydroxy-Y-methylphenyl)benzotriazolesulfonate (TINS), in a range of solvents. A consistent photophysical model is proposed. The results indicate that the mechanism for excited-state relaxation in these molecules is strongly dependent on the hydrogen-bonding properties of the solvent.
-
Experimental Section The synthesis and spectral properties of the sulfonated benzotriazole, sodium 2-(2’-hydroxy-5’-methylphenyl)benzotriazolesulfonate (TINS) are described elsewhere.1° A sample of the unsulfonated benzotriazole, 2-(2’-hydroxy-5’-methylpheny1)benzotriazole (TIN), was generously provided by Dr J. F. K. Wilshire, CSIRO Division of Protein Chemistry. The (10) Leaver, I. H.; Waters, P. J.; Evans, N. A . J . Polym. Sci.: Polym. Chem. Ed. 1979, 17, 1531-1541.
0022-3654/86/2090-5089%01.50/00 1986 American Chemical Societv I
,
5090
The Journal of Physical Chemistry, Vol. 90, No. 21. 1986
Ghiggino et al.
* I
a
A H 0,
b
Figure 1. (a) Photophysical scheme for TIN in nonpolar solvents (“TIN(intra)“). (b) Suggested structure of “TIN(inter)”
solvents methanol, deuterated methanol (methanol-d), ethanol, tetrahydrofuran (THF), dimethyl sulfoxide (Me2SO), dimethylformamide (DMF), diethyl ether, cyclohexane, and ethyl acetate were the best available grade. Where necessary, the solvents were carefully purified, dried, and stored over molecular sieves in the dark. Following purification, the solvents exhibited negligible absorption or fluorescence in the spectral region studied. Solutions were prepared to have concentrations in the range 10-5-10-4 mol dm-3. Fluorescence spectra were recorded on a Perkin-Elmer MPF-44A spectrofluorimeter and corrected for the wavelength sensitivity of the detection system as described elsewhere.” Digitized absorption spectra were recorded on an Hitachi 150-20 spectrophotometer/data processor system. Quantum yields (A,, = 300 nm) were determined relative to that of quinine bisulfate in 0.5 M H2S04(I& = 0.546)12 with appropriate corrections for the refractive indices of the solvents.” Fluorescence decay profiles were obtained on a picosecond laser/streak camera system described in detail e1~ewhere.l~The fourth harmonic of a single pulse from a mode-locked Nd3+/ phosphate glass laser was used for excitation (&,,, = 265 nm) and the fluorescence above 400 nm observed through a polarizer set at 54.7O to the excitation polarization to remove any distortions in the decay due to rotational relaxation of the m~lecules.’~ The digitized fluorescence decay profile provided by the streak camera/optical multichannel analyzer combination was fitted to a single exponential function using iterative nonlinear least-squares reconvolution methods.” For each sample at least three separate lifetime determinations were made. Results and Discussion Absorption Spectra. It has been previously reported that the absorption spectra of (2-hydroxypheny1)benzotriazoles show two well-resolved m a ~ i m a (Figure ~ ~ ~ 2). ~ ~ The ~ ’ longer ~ wavelength ( 1 1 ) Ghiggino, K. P.; Skilton, P. F.;Thistlethwaite, P. J. J. Photochem. 1985,31, 111-119. (12) Melhuish, W. H. J . Phys. Chem. 1961,65, 229-235. (13) Fleming, G. R.;Morns, J. M.; Robinson, G. W. Aust. J. Chem. 1977,
30,’2331-2352: (14) Fleming, G. R.;Morris, J. M.; Robinson, G . W. Chem. Phys. 1976, 17, 91-100. (15) Heller, H. J. Eur. P d y m . J . 1969, 105-132.
0
280
320 WAVELENGTH
360
400
nrn
figure 2. Absorption spectra of TIN in (A) THF, (B) DMF, (C) MezSO and TINS in (A) water, (B) methanol, (C) THF. EH values for THF, DMF, and Me2S0 are 24.0, 29.1, and 28.9 kJ/mol, respectively.*’
band (near 340 nm) has been assigned to intramolecularly hydrogen-bonded molecules in which the benzotriazole and phenol ring systems lie in the same plane. The shorter wavelength band (near 300 nm) has been attributed to a nonplanar conformation in which conjugation between the two ring systems is substantially reduced. In hydrocarbon solvents the absorption spectrum of T I N corresponds to the intramolecularly hydrogen-bonded species, “TIN(intra)” (Figure l ) , as no interference with the internal hydrogen bond should occur in these solvents. It has been found that the absorption spectrum of TIN is independent of viscosity and t e m p e r a t ~ r esuggesting ,~ that in hydrocarbon solvents only TIN(intra) is present. The crystalline form of TIN is a good model for TIN(intra), and x-ray crystal structure determination shows that the intramolecularly hydrogen-bonded tautomer is almost perfectly planarS6 In solvents with large values of EH (the solvent-phenol hydrogen bond strength*’) the intermolecularly hydrogen-bonded form of T I N “TIN(inter)” (Figure lb) might be expected to be present to an appreciable e ~ t e n t . ~ *Again ~ * ~ two J ~ absorption bands are usually observed at approximately the same wavelengths as those in hydrocarbon solvents. However, it can be seen in Figure 2 that the relative intensity of the longer wavelength absorption band is strongly solvent dependent and tends to decrease as the EH value of the solvent increases. This strongly suggests that the absorption spectrum of TIN in these solvents is a result of the superimposition of absorption by two distinct ground-state species: TIN(intra) and TIN(inter). Further evidence for the existence of two different conformers comes from the fluorescence excitation spectra (see the following section). An unusual solvent dependence of the absorption spectrum is observed in the case of the sulfonated benzotriazole derivative,
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5091
Proton Transfer in Benzotriazole Photostabilizers TABLE I: Fluorescence Characteristics of TIN solvent % “interno $F x 103
methanol methanol-d ethanol
19 19 8
0.33 0.49 0.51
Me2SOb
66
9.9
&a*
x IO3
7F, Ps 20 f 1 30 f 4 53 f 1 400
1.7 f 0.2
2.6 f 0.3 6fl 15*2
(kf/ 10’)
(knr/ 109)s-I
s-I
9fl 9f2
50 f 3
33 f 4 19 f 1
l l f 2 3.8 f 0.6
2.5 f 0.2
“Contribution to absorbance by ’inter” form at 300 nm for methanol, methanol-d, and ethanol and 313 nm for Me2S0. bReference 7. TABLE Ik Fluorescence Characteristics of TINS % “inter” solvent (300 nm) 6F x 103
methanol methanol-d ethanol THF
76 12
91 97
8.7 13.4 29 176
L~ x 103
TF, Ps
(kf/ lo7) s-l
12f 1 19 f 2 32 f 3 181 f 18
64 f 7 93 h 6 128 f 6 751 f 39
19 f 2 20 f 3 25 f 4 24 f 4
T I N S (Figure 2). In THF and ethanol (not shown) the longer wavelength absorption band is almost completely suppressed, indicating that the majority of T I N S molecules adopt the nonplanar conformation. However, in methanol and water the band near 340 nm becomes quite intense. This apparent increase in the ground-state concentration of “TINS(intra)” in the presence of small hydroxylic solvent molecules may possibly be due to a specific solvation effect which favors the more planar conformation. On the basis of these observations, the absorption spectra of T I N and T I N S were resolved into their separate “inter” and “intra” components by using the mathematical least-squares method of principal component analysis (PCOMP).I6I8 In this technique no assumptions are made as to the shapes of the curve profiles. A unique solution is found if the two components have zero intensity at different wavelengths within the absorption envelope. Should this condition not prevail, then a narrow band of solutions containing the possible spectra for each component is obtained. In this case the midpoint of the band may be taken as a reasonable estimate of the true solution. The narrowness of such bands depends on the selection of sample mixtures which contain a high proportion of one or other component. Absorption spectra were recorded for at least five different solvents, each spectrum containing a minimum of 131 data points. The component spectra thus obtained accounted for at least 99.87% of the variance of The root-mean-square deviation of the the original calculated composite curves from the original data was no greater than 1%. The resolved absorption spectra of T I N and T I N S produced by PCOMP analysis are shown in Figure 3. The resolved absorption spectra for T I N and TINS are quite similar, and they agree very well with previously recorded excitation spectra of the -400- and -630-nm fluorescence emission bands of TIN.’ The component spectra can thus be assigned to the “inter” and ”intra” forms (Figure 3, A and B, respectively). The PCOMP analysis also provides an estimate of the contribution that each component makes to the total absorption spectrum in each solvent (Tables I and 11). Fluorescence Emission and Excitation Spectra. N o emission could be detected from solutions of T I N in cyclohexane at room temperature. However, the room temperature fluorescence spectra of TIN and TINS have been recorded in a variety of other solvents and quantum yields are reported in Tables I and 11. Quantum yields based on the total amount of light absorbed by the solute (&) were also corrected for the light absorbed by the nonfluorescent intramolecularly hydrogen-bonded species in each solvent (&,rr). This correction was obtained from the results of the PCOMP analysis to estimate the contribution to the total absorbance by the ”inter- and ”intra” conformers at the excitation wavelength in each solvent. In all solvents the emission maximum for T I N and TINS occurs around 400 nm. The relatively small Stokes shifts indicate that (16) Lawton, W. H.; Sylvestre, E. A. Technometrics 1971, 13, 617-633. (17) Warner, I. M.; Christian, G. D.; Davidson, E. R.; Callis, J. B. Anal.
280
(kn,/ 109)SKI
15 f 2 11 f 1
7.6 f 0.4 1.1 f 0.1
320
360
f w
80 4
m K
0 Y)
m
60
40
20
0 WAVELENGTH
nm
Figure 3. Resolved absorption spectra of the “inter” (A) and “intra” (B) forms of TIN and TINS, obtained from a PCOMP analysis. Solvents in which absorption spectra were recorded and used for the PCOMP analysis were as follows: for TIN methanol, methanol-d, ethanol, THF, DMF, Me2S0,N-methylformamide;for TINS water, methanol, meth-
anol-d, ethanol, THF. the emitting species can be assigned to a non-proton-transferred form of the molecules since the proton-transferred form emits fluorescence with a very large Stokes ~ h i f t . ~The - ~ fluorescent species in each of these solvents is therefore one in which the internal hydrogen bond has been disrupted. Since the hydrogen bonding between the two ring system provides a stabilization energy of 30 kJ/m01,’~species lacking intramolecular hydrogen bonding will exist in solvents which are strong proton acceptors. ESIPT will not occur in such “strongly solvated” species due to the absence of a direct pathway for proton transfer between
-
Chem. 1977, 49, 564-573.
(18) Aartsma, T. J.; Gouterman, M.; Jochum, C.; Dwiram, A. L.; Pepich, B. V.; Williams, L. D. J . Am. Chem. SOC. 1982, 104, 6278-6283.
(19) Vinogradov, S.N.; Linnell, R. M. Hydrogen Bonding, Van Nostrand: Princeton, 1971.
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The Journal of Physical Chemistry, Vol. 90, No. 21, 19‘86
Ghiggino et al.
A
320
360
400 WAVELENGTH
440
480
520
nm
Figure 4. Fluorescence emission spectra of TIN in (A) diethyl ether, (B) THF, and (C) DMF. ET(30)solvent polarity parameters for solvents are 145, 156, and 183 kJ/mol, respectively.21
al;+ Figure 5. Proposed encounter complex for ESIPT in protic hydrogenbonding solvents.
“donor” and “acceptor’! groups. An interesting feature of the emission spectra of TIN in aprotic hydrogen-bonding solvents is the red shift in the wavelength of maximum emission with increasing solvent polarity (Figure 4). This shift can be attributed to solvent relaxation around the h,a* state, which has an appreciably higher dipole moment than the ground state. Unlike aprotic solvents, protic hydrogen-bonding solvents such as alcohols can act as proton donors as well as proton acceptors. Because of the very large hydrogen-bonding capabilities of alcohols, solutions of the benzotriazole derivatives in these solvents would be expected to display quantum yields comparable to those obtained in aprotic hydrogen-bonding solvents. However, the quantum yields in hydroxylic solvents were markedly reduced compared with those measured in aprotic hydrogen-bonding solvents. This result suggests that an efficient nonradiative process is operating in hydroxylic solvents and which is absent in aprotic solvents. One possible explanation for this effect is the formation of an encounter complex involving an eight-membered chelate ring incorporating the benzotriazole molecule and a solvent molecule (Figure 5 ) . In such a complex, proton transfer may still occur via the hydrogen-bonded solvent bridge. A similar mechanism has previously been proposed for proton transfer in methanol solutions of 3-hydro~yflavone~~ and solvent-mediated proton transfer in 7-azaindole/alcohol complexes has also recently been suggested.22 However, in other molecules such as salicylidenaniline,g solvent effects on the rate of intramolecular proton transfer have not been detected although the solvent has been shown to influence the ground-state tautomeric equilibria2 and the relaxation processes of the excited proton-transferred species.g For the case of (2-hydroxyphenyl)benzotriazolederivatives, complexation with a protic solvent molecule would be assisted by the increase in electron density (from 5.28 in the ground state to 5.46 in the first excited singlet state) of the triazo nitrogen (20) Woolfe, G. J.; Thistlethwaite, P. J. J . Am. Chem. SOC.1981, 103, 69 16-6923. (21) Sakurovs, R.;Ghiggino, K. P. J . Phorochem. 1983, 22, 373-377. (22) McMorrow. D.;Aartsma, T. J. Chem. Phys. Lett. 1986, 125, 581-585.
0.?i
2. Q
4.0 TIME
6. e
ns
Figure 6. Fluorescence decay profiles for TIN in (A) methanol and (B) THF: points, experimental data; solid lines, fitted curves with lifetimes 20 ps (methanol) and 1.2 ns (THF).
involved in hydrogen bonding, and the corresponding decrease in electron density (from 6.29 to 6.22) of the phenolic oxygen (assuming a coplanar conformation).* Consequently, complexation with a protic solvent molecule would appear favorable. A similar solvent dependence of the fluorescence quantum yields is observed for TINS. This derivative becomes quite fluorescent in aprotic solvents (I$,,,, 0.18 in THF). There is an increase in quantum yield in deuterated methanol, compared with methanol, for both TIN and TINS. This result provides further support for the involvement of solvent molecules in the proton-transfer process, since in deuterated solvents the proton transfer through the solvent encounter complex would be expected to be retarded. The room temperature fluorescence excitation spectra of T I N and TINS in aprotic and protic hydrogen bonding solvents have their maximum near 300 nm, and they show a close correspondence to the resolved spectra of the “inter” form in each case. This indicates that the ground-state conformation leading to fluorescence in these solvents consists of a tautomer intermolecularly hydrogen bonded to the solvent. Fluorescence Decay Measurements. No evidence of multiexponential decay behavior was observed in the room temperature fluorescence decay of T I N and TINS. The decay curves for TIN in methanol and T H F are shown in Figure 6. Values for the decay times obtained for T I N and TINS in a range of solvents are reported in tables I and 11, together with radiative and nonradiative rate constants. These were calculated by using the corrected fluorescence quantum yields obtained in the previous section. It has previously been reported that the lifetime of T I N in ethanol at room temperature is substantially longer than that observed in nonpolar solvent^.^ The lifetime obtained here for T I N in ethanol is in good agreement with these previous measurements. However, the similarity of the lifetimes of TIN in methanol (Table I) to those measured earlier in nonpolar solvents4 suggests that the very low quantum yield of “normal” Stokes shifted emission obtained in the latter study was most likely the
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Proton Transfer in Benzotriazole Photostabilizers A
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5093
k,, values calculated for T I N and TINS in deuterated methanol (Tables I and 11) are consistent with the participation of the solvent in the major deactivation process, although it should be noted that the deuterated solvent may affect the rates of other nonradiative processes in addition to proton transfer.23 The significantly lower values of k,, in aprotic hydrogen-bonding solvents suggest that ESIPT does not occur in those molecules that are intermolecularly hydrogen bonded to the solvent. A longer fluorescence lifetime ( 1.2 ns) was observed for T I N in T H F and ethyl acetate both of which are less polar than Me,SO, suggesting a polarity dependence of the nonradiative decay pathway of the “inter” form. Intersystem crossing (isc) is known to occur at room temperature in aprotic solvents (& = 0.15).’ One possibility is that the increasing energy of the SI state observed on lowering the solvent polarity (Figure 4) affects the efficiency of isc. The energy gap between the S1 and TI states is of the order of 50 kJ/mo13 and changes in the energy of the SI state relative to the TI (or T,) excited state in more polar aprotic solvents may promote isc, thus increasing the value of knr. In view of the comparatively small effect of solvent on the radiative rates for each derivative, the same excited-state species is probably responsible for fluorescence in each case, i.e. the non-proton-transferred tautomer. The calculated radiative decay rates for T I N are in good agreement with values obtained by Woessner et al.’when corrected for absorption by the nonfluorescent “intra” conformer. N
0
C
Figure 7. Proposed photophysical model for the singlet states of TIN and TINS in (A) nonpolar solvents, (B) protic hydrogen-bonding solvents, (C) aprotic hydrogen-bonding solvents.
result of trace amounts of an alcohol (or possibly water) complexing with T I N molecules. N o emission from T I N in nonpolar solvents was observed in the present work in accord with recent st~dies.~,~ If proton transfer via a solvent bridge occurs in hydroxylic solvents, then the calculated nonradiative decay rates of the solute in these solvents should reflect the time required for solvent and molecular rearrangement into the necessary configuration and for proton transfer to occur. The rate of ESIPT for benzotriazole molecules containing a preexisting intramolecular hydrogen bond (Figure l a , I) has been estimated as >lo” s - I . ~ , ~ The lower nonradiative decay rates in alcohol solvents can thus be attributed to molecular reorientation of the solvent molecules and also most likely to rotation of the hydroxy group out of the plane of the molecule in order to accomodate the solvent bridge. A reaction scheme which describes the photophysical properties of T I N and TINS in protic and aprotic solvents as well as in nonpolar solvents is shown in Figure 7. The mechanism involving participation of a protic hydrogenbonding solvent molecule in ESIPT is supported by the marked increase in the lifetimes when the solvent is deuterated. The lower
Conclusion The absorption spectra of TIN and TINS in a range of solvents have been resolved into two components representing the “intra” and “inter” forms. At room temperature no emission is observed from the “intra” form whereas “normal” Stokes shifted fluorescence near 400 nm is observed from the “inter” form. The “intern form cannot undergo ESIPT in aprotic hydrogen-bonding solvents as the intramolecular hydrogen bond is disrupted by hydrogen bonding to the solvent molecules. Much lower quantum yields and shorter fluorescence lifetimes are observed in hydroxylic solvents than in aprotic solvents. It is suggested that ESIPT may proceed in hydroxylic solvents via an encounter complex involving a solvent molecule thus providing a fast nonradiative decay pathway (Figure 7B). Consequently, the nonradiative decay rates calculated in hydroxylic solvents ( > l o l o s-l) are an order of magnitude larger than in aprotic hydrogen-bonding solvents and represent the time required for molecular/solvent reorientation and subsequent proton transfer. This compares with a rate for ESIPT of > l o L 1s-l in nonpolar solvent^,^^^ in which the excited-state originates from “intra” ground-state species which have a direct, preexisting proton-transfer pathway. The nonradiative decay rates in aprotic hydrogen-bonding solvents are in good agreement with values obtained by Woessner et al.,’ corrected for absorption by “intra” species. The results obtained for T I N and TINS in this work emphasize the important role the solvent plays in determining both the ground-state species present in solution and the excited-state relaxation mechanisms of the solute molecules. The conclusion that the solvent may be directly involved in the proton-transfer process in hydroxylic solvents is in accord with recent data reported for 3-hydroxyflavone in methanolz0 and 7-azaindole/alcohol complexes.zz However, the solvent has been shown not to influence the rate of ESIPT in salicylidenaniline’ suggesting that such solvent interactions are quite molecule specific and might be influenced by the excited-state charge distribution in the solute and steric factors which favor solute/solvent complexation. Acknowledgment. This project is supported by a University of Melbourne/CSIRO collaborative research grant. Registry No. TIN, 2440-22-4; TINS, 86677-59-0.
(23) Woolfe, G. J.; Melzig, M.; Schneider, S.;Dorr, F. Chem. Phys. 1983, 77, 213-221.
