Photon induced isomerization in the first excited state of the 7-azaindole-(H2O)3 cluster

Abstract

A picosecond pump and probe experiment has been applied to study the excited state dynamics of 7-azaindole-water 12 and 13 clusters [7AI(H 2O)2,3] in the gas phase. The vibrational-mode selective Excited-State-Triple-Proton Transfer (ESTPT) in 7AI(H2O)2 proposed from the frequency-resolved study has been confirmed by picosecond decays. The decay times for the vibronic states involving the ESTPT promoting mode σ(1) (850-1000 ps) are much shorter than those for the other vibronic states (2100-4600 ps). In the (1 + 1) REMPI spectrum of 7AI(H 2O)3 measured by nanosecond laser pulses, the vibronic bands with an energy higher than 200 cm-1 above the origin of the S1 state become very weak. In contrast, the vibronic bands in the same region emerge in the (1 + 1′) REMPI spectrum of 7AI(H 2O)3 with picosecond pulses. The decay times drastically decrease when increasing the vibrational energy above 200 cm-1. Ab initio calculations show that a second stable "cyclic-nonplanar isomer" exists in addition to a "bridged-planar isomer", and that an isomerization from a bridged-planar isomer to a cyclic-nonplanar isomer is most probably responsible for the short lifetimes of the vibronic states of 7AI(H2O)3. © the Owner Societies. 2011.