Tracking the early nonadiabatic events of ESIPT process in 2-acetylindan-1,3-dione by quantum wavepacket dynamics

Spectroscopy and dynamics associated with the three low-lying singlet excited electronic states of 2-acetylindan-1,3-dione (AID) are studied theoretically to interpret the early events of the excited-state intramolecular proton transfer process (ESIPT). Upon populating "bright"S-3, AID decays to "dark"lower electronic states (S-2 and S-1) on a timescale of about 100 fs. Nuclear densities associated with the O-H vibrations obtained from wavepacket calculations reveal their participation in the S-3 -> S-2/S-1 internal conversion. We show that AID requires very minimal distortions along these vibrations to access the conical intersections located near to the Franck-Condon (FC) point. Our findings demonstrate that the ultrafast nonadiabatic dynamics taking place within the FC zone make the assignment of ESIPT pathways and associated timescales in AID a challenging task.