Modulation of Intersystem Crossing by Chemical Composition and Solvent Effects: Benzophenone, Anthrone and Fluorenone

Abstract

Combined density functional theory and multireference configuration interaction methods including spin-orbit coupling and spin-vibronic interactions have been used to elucidate the photophysical pathways of benzophenone (BP), anthrone (A) and fluorenone (FL). Our results reveal that the slower S1(1nπ*) T1(3ππ*) passage of FL in the gas phase, as compared to BP and A, originates from the different electronic structures of the T1(3ππ*) states in these compounds and is not related to the planarity of the nuclear arrangement. Temperature effects on the rate constants of intersystem-crossing (ISC), reverse ISC (rISC) and fluorescence have been investigated for FL in solvents of different polarity. Experimentally observed trends for rate constants are well reproduced. A nearly temperature- and solvent-independent, slower down-hill ISC component is attributed to the El-Sayed-forbidden S1(1ππ*) T1(3ππ*) transition in these environments. The faster El-Sayed-allowed S1(1ππ*) T2(3nπ*) ISC requires thermal activation in tetrahydrofuran and acetonitrile solution so that fluorescence may compete against triplet formation. In cyclohexane solution, S1(1ππ*) and T2(3nπ*) are nearly degenerate according to our calculations. Hence S1 T2 ISC and S1 T2 rISC have similar rate constants. If T2 T1 internal conversion could be slowed down by deuteration or perfluorination, for example, FL-derivatives might show delayed fluorescence.