Excitation-wavelength dependence of the femtosecond fluorescence dynamics of 7-azaindole dimer: further evidence for the concerted double proton transfer in solution

Excitation-wavelength dependence of the ultraviolet fluorescence dynamics of 7-azaindole dimer was examined in solution by femtosecond up-conversion method. It was found that the fluorescence decay of the dimer excited state showed a significant wavelength dependence. It changes from a bi-exponential decay to a single-exponential decay, when we scanned the excitation wavelength from 280 toward 313 nm (the red-edge of the dimer absorption). This result demonstrates that the proton-transfer dynamics itself exhibits a single exponential behavior. The obtained fluorescence data deny the appearance of the intermediate species and strongly support the concerted mechanism of the double proton transfer reaction.