SINGLET OXYGEN DIMOL-SENSITIZED LUMINESCENCE FROM THERMALLY GENERATED SINGLET OXYGEN

Delayed fluorescence (700 nm) of tetra-tert-butylphthalocyanine (Pc) can be sensitized by singlet oxygen generated by thermolysis of 1,4-dimethylnaphthalene endoperoxide. The initial intensity is proportional to the square of the initial concentration of the endoperoxide, and the apparent rate constants of its decay are twice those for singlet oxygen monomol emission. The apparent activation energies for the reaction of the endoperoxide from Arrhenius plots of the first-order rate constants for luminescence decay at 1270 and 700 nm were DELTAH(double dagger) = 21.1 +/- 1.1 kcal/mol, DELTAS(double dagger) = -7 +/- 3 eu and DELTAH(double dagger) = 21.2 +/- 1.1 kcal/mol, DELTAS(double dagger) = -6 +/- 3 eu, respectively, in reasonable agreement with the literature (Turro, N.J.; Chow, M.F.; Rigaudy, J. J. Am. Chem. Soc. 1981, 103, 7218-7224). However, values of DELTAH(double dagger) calculated from the luminescence intensities were 22.7 +/- 4.4 and 50.3 +/- 1.4 kcal/mol, respectively; the apparent activation enthalpy at 700 nm is roughly twice that at 1270 nm. These results are in accord with a mechanism in which singlet oxygen is in reversible equilibrium with the dimol, which transfers energy to produce the phthalocyanine excited singlet state, which fluoresces.