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

Abstract

A picosecond pump and probe experiment has been applied to study the excited state dynamics of 7-azaindole–water 1 ∶ 2 and 1 ∶ 3 clusters [7AI(H₂O)_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(H₂O)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(H₂O)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.