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

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 sigma(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 S-1 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).