On the short and long phosphorescence lifetimes of aromatic carbonyls

This work uses theoretical and computational methods to investigate the relationship between phosphorescence lifetime and the electronic character of the lowest triplet state of aromatic carbonyls. It shows that phosphorescence is due to a direct spin-orbit coupling (SOC) mechanism modulated by permanent dipoles when the T1 minimum is 3np*. If the minimum is a totally symmetric 3pp*, phosphorescence is due to an indirect SOC mechanism involving transition dipole moments with other excited states. The magnitude difference between permanent and transition dipoles leads to 3np* phosphoresce to be 100 times faster than 3pp*. A vertical approximation and the nuclear ensemble approach (NEA) were tested on benzaldehyde and three derivatives in the gas phase chosen to have both 3np* and 3pp* phosphorescence. Both simulation methods deliver good results for 3n* systems. Nevertheless, vertical simulations fail for 3pp* due to the overwhelming importance of vibronic couplings.