PICOSECOND ABSORPTION STUDIES ON THE ROLE OF CHARGE-TRANSFER INTERACTIONS IN THE MECHANISM OF QUENCHING OF TRIPLET-STATES BY MOLECULAR-OXYGEN

The first picosecond pump-probe measurements from oxygenated hydrocarbon solutions exciting directly and exclusively into the charge transfer (CT) absorption bands is described. Upon excitation at 353 nm, which precludes population of the lowest excited singlet state of 1-ethylnaphthalene (EN), its triplet state is produced rapidly in < 10 ps in both oxygenated acetonitrile and cyclohexane solutions. The efficiency of this triplet state production exhibits a pronounced sensitivity to the solvent, being high in cyclohexane (almost-equal-to 80%) but considerably less in acetonitrile (almost-equal-to 40%). Measurements of the efficiency of singlet oxygen production following Cr excitation demonstrate that dynamic quenching of the triplet state is not the sole route to singlet oxygen. We suggest, that following absorption at 353 nm a proportion of the excited complexes, which are formed either by internal conversion from the initially excited CT state or by direct absorption to give a doubly excited complex state, dissociates to give both 3EN* and O2* (1DELTA(g)).