Ground-State Reverse Double Proton Transfer of 7-Azaindole

Abstract

Dynamics of the ground-state reverse proton transfer in 7-azaindole (7AI(N)) have been investigated by two-step laser-induced fluorescence (TSLIF) in various nonpolar solvents. Comprehensive analyses reveal a previously unrecognized finite rise kinetics for the long-lived transient species. Furthermore, the time-dependent spectral evolution indicates that the TSLIF spectrum obtained at the rise component is different from that of the decay component, while both spectra are red shifted relative to the prompt tautomer emission. The results lead us to propose that the transient species originates from the monomer of the 7AI proton-transfer tautomer (7AI(T)) produced by a minor dissociation channel (∼4%) of the excited 7AI(T) dimer, which subsequently undergoes a slow reverse proton transfer via the formation of a 7AI(T)/7AI(N) hydrogen-bonded complex. This proposed mechanism rationalizes the recent thermal lensing experiment which concluded that the 7AI(T) dimer is only 0.97 kcal/mol higher in energy than the 7AI(N) dimer,35 while theoretical approaches,38,39 in contrast, predict an energy difference of >20 kcal/mol.