NONRADIATIVE DEACTIVATION AND TRIPLET-STATES IN DONOR ARYL ACCEPTOR COMPOUNDS (DIALKYLAMINOBENZONITRILES)

Intersystem crossing rates and triplet state properties of TICT (twist ed intramolecular charge transfer) model compounds (dimethylaminobenzo nitrile - DMABN - and its methyl substituted derivatives, 3-cyanoindol ine, 3-cyanobenzquinuclidine - CBQ - and benzonitrile) were studied by various experimental and theoretical methods (emission spectroscopy, transient absorption techniques, EPR spectroscopy and semiempirical qu antum chemical calculations) with the aim to obtain a quantitative dea ctivation pattern of this class of compounds in solvents of different polarity. The lowest triplet state, populated after excitation, is gen erally a non-charge-transfer state, i.e. the 3A1 of planar DMABN, irre spective of solvent polarity and compound flexibility, with the only e xception of the rigidly perpendicular CBQ, which relaxes to the 3A1 st ate of the benzonitrile moiety. In non-polar solvents intersystem cros sing increases with increasing charge transfer character of the excite d 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 des cribed as a thermal charge recombination process in the normal Marcus region. With increasing solvent polarity, the decreasing energy gap be tween 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 se cond dominant deactivation pathway of 1TICT states; it shows a signifi cant deuterium isotope effect.