The effect of molecular distortion in the rates of intersystem crossing S 1 T x S 0 processes

Abstract

It is shown that the interpretation of the experimental observations of Dewey and Hadley that led to the conclusion of the unexpectedly inefficient intersystem crossing S 1 ▪ T x process in 9,10-phenanthroline, and which prompted a new proposal by Azumi for the mechanism of intersystem crossing in N-heterocyclics is incorrect. The intersystem crossing process in aromatic hydrocarbons and the N-heterocyclics is briefly discussed and compared. The results indicate that the out-of-plane molecular distortions enhance the rate of intersystem crossing, S 1 → T x , mainly through electronic mixing and not the vibrational overlap integrals. The lack of phosphorescence emission for 9,10-phenanthroline in hydrocarbon solvents is attributed to a fast non-radiative T 1 ▪ S 0 channel resulting from the pseudo-Jahn-Teller effect.