THERMOCHEMICAL PROFILES ON HYDROGEN-ATOM TRANSFER FROM TRIPLET NAPHTHOL AND PROTON-INDUCED ELECTRON-TRANSFER FROM TRIPLET METHOXYNAPHTHALENE TO BENZOPHENONE VIA TRIPLET EXCIPLEXES STUDIED BY LASER FLASH-PHOTOLYSIS

Temperature effects on hydrogen atom transfer (HT) and proton-induced electron transfer (p-ET) via triplet exciplexes in the l-naphthol-benzophenone (ROH-BP (or >CO)) and methoxynaphthalene-BP (ROMe-BP) systems in acetonitrile-H2O (ACN-H2O; 4:1 v/v) were studied by 355-nm laser flash photolysis. The HT rate in the ROH-BP system increased with an increase of temperature. The thermodynamic parameters for HT via the triplet exciplex (3)(ROH-->CO)* were obtained: the enthalpy change, Delta H-1 = -2.0 kcal.mol(-1), and the entropy change, Delta S-1 = -2.4 eu for formation of 3(ROH-->CO)*; the frequency factor, A(ex) = 7.7 x 10(9) s(-1) and activation energy, Delta E(ex) = 3.5 kcal.mol(-1), for intraexciplex HT. The negatively small values of Delta H-1 and Delta S-1 suggest that (3)(ROH-->CO)* has a loose sandwichlike structure. The corresponding parameters in the ROMe-BP system were obtained to be Delta H-1 = -2.2 kcal.mol(-1) and Delta S-1 = -2.8 eu for formation of the triplet exciplex (3)(ROMe-->CO)*. In the presence of protons in the ROH-BP system, proton-enhanced HT (p-HT, electron transfer followed by proton transfer) occurs efficiently due to the protonated triplet exciplex, (3)(ROH-->C+OH)*. The p-HT rate increased with an increase of temperature. In the ROMe-BP system with protons, formation of the protonated triplet exciplex 3(ROMe-->C+OH)* results in p-ET. The p-ET rate increased with an increase of temperature. The energy state diagrams for HT and p-ET are illustrated.