J. Phys. Chem. 1991, 95, 10369-10373 of Tfl with ET(30) (Figure 7b). The greater motional freedom of the pendant group in alcohols versus a dimer partner could also be the source of the difference. This latter explanation is favored by the fact that the lifetimes do correlate reasonably with viscosity (Figure 6b) and show an activated temperature dependence suggestive of the importance of large-amplitude motion in alcohols. IV. Conclusions The results of the present study lead to a number of conclusions concerning the dynamics of excited-state proton transfer between 7AI and alcohol solvents. In addition to the proton-transfer reaction, time-dependent solvation of the initially excited normal species causes a time-dependent Stokes shift of the normal emission. The effect of this Stokes shift is apparent throughout most regions of the spectrum, complicating observation of the proton-transfer kinetics. The conflicting interpretations of the kinetics of 7AI/alcohol systems existing in the literature appear to have resulted from neglect of this previously unnoticed complication. Thus, after accounting for the improved signal-to-noise ratio afforded by the time-correlated single photon counting technique, the present results do not differ substantially from the primary data reported by others. It is merely the fact that different workers monitored emission at different wavelengths (often in regions where emission from the Stokes shifting normal band is significant) that has produced the varied interpretations reported for this reaction. The present results show that emission at wavelengths greater than 550 nm is essentially free from contamination by the normal emission and can be used to reliably measure the kinetics of the appearance of the tautomeric form. Results obtained in this way clearly show that the normal species is the direct kinetic precursor of the tautomer. In addition there may be some unresolvably fast tautomer formation, as has been previously suggested, but it accounts for a small percentage ( 0 the ESIPT rate is controlled by both the rate of barrier crossing and the rate of solvent reorganization. When AG* = 0 as in benzonitrile, 0.13:0.87 mole fraction acetonitri1e:benzene and ci.14:0.86 mole fraction acetonitrile:CCl,, the dynamics are controlled solely by the rate of solvent reorganization. Also, the agreement of the measured ESIPT rates in acetonitri1e:benzene ( a = 0.13) and acetonitrile:CCl, ( a = 0.14) solvent mixtures with the calculated T~ values suggest that the nonpolar solvents have little effect on the ESIPT dynamics other than to remove the short-range screening effects of the pure polar solvent. Acknowledgment. This work was supported by the National Science Foundation. Registry No. 3-Hydroxyflavone, 577-85-5.
