Ground-state reverse double proton transfer of 7-azaindole

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 unrec ognized finite rise kinetics for the long-lived transient species. Furtherm ore, the time-dependent spectral evolution indicates that the TSLIF spectru m obtained at the rise component is different from that of the decay compon ent, while both spectra an red shifted relative to the prompt tautomer emis sion. 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 (similar to 4%) of the excited 7AI(T) dimer, wh ich subsequently undergoes a slow reverse proton transfer via the formation of a 7AI(T)/7AI(N) hydrogen-bonded complex. This proposed mechanism ration alizes 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) w hile theoretical approaches,(38,39) in contrast, predict an energy differen ce of >20 kcal/mol.