On the short and long phosphorescence lifetimes of aromatic carbonyls

Abstract

This work applies theoretical and computational methods to investigate the relationship between phosphorescence lifetime and the electronic character of the lowest triplet state of aromatic carbonyls. A formal analysis of the spin-perturbed wave functions shows that phosphorescence is due to a direct spin–orbit coupling mechanism modulated by permanent dipoles when the T 1 minimum is 3 nπ*. If the minimum is a totally symmetric 3 ππ*, phosphorescence is due to an indirect spin–orbit coupling mechanism involving transition dipole moments with other excited states. The magnitude difference between permanent and transition dipoles leads to a much faster 3 nπ* phosphoresce than 3 ππ*. These predictions were verified with phosphorescence lifetime simulations of benzaldehyde and its three derivatives in the gas phase employing a vertical approximation and the nuclear ensemble approaches. Both predict 3 nπ* emission within a few tens of milliseconds. While the vertical approach indicates a 3 ππ* emission within a few seconds, vibronic corrections bring this value down to about 200 ms.