The photoinduced excited-state double proton-transfer reaction of 7-azaindole has been examined in a variety of alcohol solvents. The influence of temperature and solvent deuteration has been investigated. A time-dependent Stokes shift of the initially excited normal species is observed, and this species is found to be the kinetic precursor to the tautomeric form. A substantial overlap of the normal and tautomer emissions is found, indicating that the tautomer emission must be monitored at wavelengths of 550 nm or greater to avoid contamination from the normal emission. The observed proton-transfer times in alcohols at room temperature are well correlated with the solvation parameter ET(30), suggesting that the rate of proton transfer is related to the strength of solvent-solute interactions. However, an unusual temperature-dependent isotope effect is also observed, in which the relative rates of proton transfer in normal and deuterated alcohols become closer as the temperature is lowered. These results are interpreted in terms of a two-step model for proton transfer, involving solvent rearrangement to an appropriate configuration for the reaction to occur, followed by rapid proton transfer.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry