Influence of solvents on the quantum efficiency of laser-excited delayed fluorescence of phthalocyanine sensitised by singlet oxygen

An investigation was made of the intensity ratios of the phosphorescence of singlet oxygen $^1$O$_2$ at $\lambda$ = 1270 nm ($L_{1270}$) and of the $^1$O$_2$-sensitised delayed fluorescence ($L_{df}$) of tetra-(4-tert-butyl)phthalocyanine (Pc) in hexafluorobenzene, benzene, and acetone were also studied. Tetraphenylporphine and Pc were used as photosensitisers of O$_2$ formation. Excitation was provided by a pulsed nitrogen laser emitting at $\lambda$ = 337 nm. The ratio $L_{df}^0 / L^0_{1270}$ of the initial intensities of the delayed fluorescence of Pc and of the $^1$O$_2$ phosphorescence varied from 14 to 70, depending on the energy of the laser pulses (1 – 2 mJ cm$^{-2}$), on the Pc concentration (0.25 – 1.2 $\mu$ M), and on the nature of the photosensitiser and its optical density (0.1 – 0.2) in the excitation region. A criterion of the quantum efficiency of the delayed fluorescence was defined as the coefficient $\alpha$ = ($L_{df}^0/L_{1270}^0 k_r/\gamma_f$) [$^1$O$_2$]$_0$[Pc], where [$^1$O$_2$]$_0$ is the initial concentration of $^1$O$_2$ after a laser pulse; [Pc] is the concentration of Pc; $k_r$ is the rate constant of radiative deactivation of $^1$O$_2$ in the investigated solvent; $\gamma_f$ is the quantum yield of the Pc fluorescence. Within the limits of the experimental error, the coefficient $\alpha$ was the same for all the investigated media and amounted approximately to 4 $\times$ 10$^{13}$ M$^{-2}$ s$^{-1}$. An analysis of the results indicated that the quantum efficiency of the delayed fluorescence of Pc excited in aprotic media remained high irrespective of the presence and number of hydrogen atoms in the solvent molecule and also independent of the solvent permittivity in the range 2.3(benzene) – 21.5(acetone).