On the mechanism of photosensitized luminescence of singlet oxygen dimols in air-saturated pigment solutions

Luminescence of singlet oxygen dimols ( 1 O 2 ) 2 was studied in aerobic solutions of a nonfluorescent photosensitizer phenalenone in CCl 4 and C 6 F 6 using a setup with a mechanical phosphoroscope and relatively low rates of photosensitizer excitation. The luminescence spectrum was found to resemble those reported in our previous papers dealing with dimol luminescence in solutions of porphyrins and other organic dyes. The main maximum was located at 703–706 nm, and two much weaker bands at 640 and 770–780 nm. These data suggest that dimol luminescence arises owing to interaction of two 1 O 2 molecules and one ground-state pigment molecule. Light is emitted by the dimol-pigment contact complexes, which are formed as a result of 1 O 2 collisions with metastable, probably triplet, intermediates appearing in 1 O 2 reaction with pigment molecules. It is proposed that this mechanism of dimol luminescence might be of general importance for photochemical, chemical, and biological systems where singlet oxygen is generated. However, the luminescence of this type dominates at relatively low rates of 1 O 2 generation. According to the literature data, at high 1 O 2 generation rates the prevalent type of dimol luminescence has the main maximum at 635–637 nm and is caused by direct collisions of two 1 O 2 molecules.