Theoretical Study on the Substituent Effect on the Excited‐State Proton Transfer in the 7‐Azaindole‐Water Derivatives

The first excited‐state proton transfer (ESPT ) in 7AI ‐H₂O complex and its derivatives, in which the hydrogen atom at the C₂ position in pyrrole ring was replaced by halogen atom X (X = F, Cl, Br), were studied at the TD ‐M06‐2X/6‐31 + G(d, p) level. The double proton transfer took place in a concerted but asynchronous protolysis pathway. The vibrational‐mode selectivity of excited‐state double proton transfer in the model system was confirmed. The specific vibrational‐mode could shorten the reaction path and accelerate the reaction rate. The substituent effects on the excited‐state proton transfer process were discussed. When the H atom at C₂ position in 7AI ‐H₂O complex was replaced by halogen atom, some geometrical parameters changed obviously, the barrier height of ESDPT reduced, and the asynchronicity of proton transfer enlarged. The above changes correlated with the Pauling electronegativity of halogen atom.