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 (∼472
nm) emission bands. Detailed temperature-dependent studies showed
that the values of activation free energy (ΔG‡) were similar between N–H and N–D
isotopes. Therefore, water-catalyzed ESPT involves a stepwise mechanism
incorporating solvation equilibrium (Keq) 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 (∼450 nm). The distinct difference in ESPT properties
between water and ice makes azaindoles feasible for the investigation
of water–ice interface property.