Quantum efficiency of the laser-excited singlet-oxygen-sensitised delayed fluorescence of the zinc complex of tetra(4-tert-butyl)phthalocyanine

An investigation was made of the ratios of the intensity $I_{df}$ of the singlet-oxygen($^1O_2$)-sensitised delayed fluorescence of the zinc complex of tetra(4-tert-butyl)phthalocyanine (ZnTBPc), with the maximum at $\lambda = 685$ nm, to the intensity $I_{1270}$ of the photosensitised phosphorescence of $^1O_2$ with the maximum at $\lambda = 1270$ nm in deuterated benzene when excited with $\lambda = 337$ nm nitrogen-laser pulses. Depending on the energy density of the laser radiation (0.25 – 0.7 mJ cm$^{-2}$) and on the concentration of ZnTBPc ($0.06$ – $3.4$ $\mu$M), the ratio of the zero-time intensities of the delayed fluorescence of ZnTBPc and of the singlet-oxygen phosphorescence $I_{df}^0/I_{1270}^0$ varied from $0.01$ to $0.2$ in air-saturated solutions of ZnTBPc. The intensity $I_{df}^0$ decreased fivefold as a result of saturation with oxygen of air-saturated solutions. The quantum efficiency of the delayed fluorescence was represented by the coefficient $\alpha =(I_{df}^0/I_{1270}^0)k_r/(\gamma_f[^1O_2]_0$[ZnTBPc]), where [$^1O_2]_0$ is the zero-time concentration of $^1O_2$ after a laser shot; $k_r$ is the rate constant of radiative deactivation of $^1O_2$ in the investigated solvent; $\gamma_f$ is the quantum yield of the ZnTBPc fluorescence. It was established that in the case of air-saturated solutions of ZnTBPc this coefficient was approximately 200 times less than for metal-free tetra(4-tert-butyl)phthalocyanine and its absolute value was $\sim 2 \times 10^{11}$ M$^{-2}$ s$^{-1}$.