Mechanism of singlet-oxygen induced delayed fluorescence of bacteriopheophytin in laser excitation

The relative intensity of photosensitized phosphorescence of singlet oxygen (¹O₂) at 1270 nm (L₁₂₇₀) and¹O₂-induced delayed fluorescence (L_df) of bacteriopheophytin a (BPh) (770 nm) in air-saturated solutions of BPh in hexafluorobenzene in excitation by 337-nm pulses of a nitrogen laso is investigated. It is established that L_df≪L₁₂₇₀. The ratio of the initial intensity of delayed fluorescence and phosphorescence of¹O₂(L_df)₀/(L₁₂₇₀)₀ changed from 0.02 to 0.30 as a function of the energies of laser pulses (2.5–5.0 mJ/cm¹) and the BPh concentration (6–18 μM). As the index of quantum efficiency of the delayed fluorescence, the authors used the coefficient where ¹O₂]₀ is the initial concentration of¹O₂ after the laser burst; Bpn] is the concentration of BPh; k_r is the constant of the rate of¹O₂ radiative deactivation in the solvent under study; γf is the quantum yield of BPh fluorescence. It is established that this coefficient is smaller by a factor of ∼2000 than in phthalocyanine, while its absolute value is ∼2·10¹⁰M⁻²sec⁻¹. The saturation of BPh solutions with oxygen at atmospheric pressure is shown to lead to a fivefold attenuation of the delayed fluorescence as compared to air-saturated solutions. The possibility of BPh triplet molecules being involved in the radiation of delayed fluorescence of the pigment is discussed. Reported at the VIIIth International Conference on Spectroscopy of Porphyrins and Their Analogs, Minsk, September 22–26, 1998. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 504–508, July–August, 1999.