The Excited-State Proton Transfer in 3-Cyano-7-Azaindole: From Aqueous Solution to Ice.

We investigated the excited-state proton transfer (ESPT) reaction for 3-cyano-7-azaindole (3CAI) in aqueous solution and in ice. 3CAI undergoes water-catalyzed ESPT in the aqueous solution, giving normal (355 nm) and proton transfer tautomer (similar to 472 nm) emission bands. Detailed temperature dependent studies showed that the values of activation free energy (triangle G(double dagger)) were similar between N-H and N-D isotopes. Therefore, water-catalyzed ESPT involves a stepwise mechanism incorporating solvation equilibrium (K-eq) to form a 1:1 (molar ratio) water:3CAI cyclic hydrogen-bonded complex as an intermediate, followed by perhaps proton tunneling reaction. In sharp contrast, 3CAI in ice undergoes entirely different photophysical properties, in which 3CAI self-organizes to form a double-hydrogen-bonded dimers at the grain boundary of the polycrystalline. Upon excitation, the dimer proceeds with a fast excited-state double proton transfer reaction, giving rise to solely a tautomer emission (similar to 450 nm). The distinct difference in ESPT properties between water and ice makes azaindoles feasible for the investigation of water-ice interface property.