[Mechanism of photosensitized luminescence of singlet oxygen dimols in air-saturated pigment solutions]

Abstract

Luminescence of singlet oxygen dimols (1O2)2 with the main spectral maximum at 703-706 nm and much weaker bands at 640 and 770-780 nm was studied using mechanical phosphoroscopes in aerobic solutions of the nonfluorescent photosensitizer phenalenone in CCl4 and C6F6 at relatively low radiant power. The spectrum of this luminescence resembles that of dimol luminescence, which we had detected previously in solutions of porphyrins and other compounds. It was shown that, in phenalenone solutions, the mechanism of dimol luminescence involves reaction of two 1O2 molecules and one ground-state pigment molecule. It is most likely that light is emitted by the dimol-pigment contact complexes formed as a result of collisions of 1O2 with metastable, probably triplet intermediates that result from reactions of 1O2 with pigment molecules. It is proposed that this luminescence mechanism is of general importance for many organic and biologically important systems where singlet oxygen is generated. However, a comparison with the literature data suggests that the luminescence of this type can be detected at relatively low rates of 1O2 generation. At high singlet oxygen generation rates, dimol luminescence with the main maximum at 635-637 nm dominates, which is likely caused by direct collisions of 1O2 molecules.