Nonradiative deactivation and triplet states in donor-aryl-acceptor compounds (dialkylaminobenzonitriles)

Abstract

Intersystem crossing rates and triplet state properties of TICT (twisted intramolecular charge transfer) model compounds (dimethylaminobenzonitrile — DMABN — and its methyl substituted derivatives, 3-cyanoindoline, 3-cyanobenzquinuclidine — CBQ — and benzonitrile) were studied by various experimental and theoretical methods (emission spectroscopy, transient absorption techniques, EPR spectroscopy and semiempirical quantum chemical calculations) with the aim to obtain a quantitative deactivation pattern of this class of compounds in solvents of different polarity. The lowest triplet state, populated after excitation, is generally, a non-charge-transfer state, i.e. the 3A 1 of planar DMABN, irrespective of solvent polarity and compound flexibility, with the only exception of the rigidly perpendicular CBQ, which relaxes to the 3A 1 state of the benzonitrile moiety. In non-polar solvents intersystem crossing increases with increasing charge transfer character of the excited singlet; in ethanol, the intersystem crossing rate is about the same for all compounds studied, with the exception of benzonitrile, and in general significantly lower than in a non-polar environment. From the mechanistic point of view, intersystem crossing from 1TICT can be described as a thermal charge recombination process in the normal Marcus region. With increasing solvent polarity, the decreasing energy gap between these two states then entails a decreasing intersystem crossing rate and an increasing 1TICT lifetime, as experimentally observed. In hydroxylic solvents, internal conversion to the ground state is the second dominant deactivation pathway of 1TICT states; it shows a significant deuterium isotope effect.