SOLVENT AND TEMPERATURE EFFECTS IN THE ELECTRON-TRANSFER QUENCHING OFTRIPLET-STATES OF AROMATIC-HYDROCARBONS BY NITROBENZENES

The triplet quenching of anthracene and a,h-dibenzanthracene by four n itrobenzenes was studied by laser flash photolysis. Bimolecular quench ing rate constants were determined in acetonitrile and methanol. They follow the expected free energy correlation for an outer sphere electr on transfer process. The rate constants in methanol are generally high er than those in acetonitrile. The activation enthalpies and entropies were also measured in both solvents. The activation entropies are mor e negative in acetonitrile and the activation enthalpies are lower in acetonitrile with negative values being observed in some cases. Transi ent absorption spectra show that in the quenching process free radical ions are formed in methanol, but in acetonitrile free ions are almost absent. These results are compared with the current models for outer sphere electron transfer reactions. They cannot be explained if the so lvent is considered as a dielectric continuum.