Excited-State Double-Proton Transfer on 3-Methyl-7-azaindole in a Single Crystal: Deuterium Isotope/Tunneling Effect

Unlike 7-azaindole consisting of the tetrameric configuration, 3-methyl-7-azaindole (3MAI) exists solely as intact double hydrogen-bonded dimeric forms in a single crystal. Both steady-state and time-resolved measurements down to 8.0 K reveal remarkable deuterium isotope effects on the rate of excited-state double proton transfer (ESDPT) in the N(1)-deuterated 3MAI (3MAI- d ) single crystal. The rates of ESDPT for the 3MAI- d dimer resolved at <150 K are mainly governed by the proton tunneling mechanism. At <12 K, the nearly temperature-independent ESDPT dynamics lead us to qualitatively deduce a barrier height of ∼1.73 kcal/mol for the 3MAI- d dimer. The results provide an ideal model to investigate the intrinsic ESDPT dynamics for 7-azaindole analogues in which the structural information is well documented.