Ab initio based surface-hopping dynamics study on ultrafast internal conversion in cyclopropanone

Abstract

Cyclopropanone exhibits an intriguing phenomenon that the fluorescence from the S1 state disappears below 365 nm. This is ascribed to the ultrafast S1 → S0 internal conversion process via conical intersection, which deprives opportunity of the fluorescence emission. In this work, we have used ab initio based surface hopping dynamics method to study vibrational-mode-dependent S1 → S0 internal conversion of cyclopropanone. A new conical intersection between the S 1 and S0 states is determined by the state-averaged CASSCF/cc-pVDZ calculations, which is confirmed to play a critical role in the ultrafast S1 → S0 internal conversion by the nonadiabatic dynamics simulations. It is found that the internal conversion occurs more efficiently when the initial kinetic energies are distributed in the four vibrational modes related to the C=O group, especially in the C-O stretching and the O-C-C-C out-of-plane torsional modes. Meanwhile, the S 1 lifetime and the time scale of the S1 → S 0 internal conversion are estimated by the ab initio based dynamics simulations, which is consistent with the ultrafast S1 → S 0 internal conversion and provides further evidence that the ultrafast internal conversion is responsible for the fluorescence disappearance of cyclopropanone. © 2011 American Chemical Society.