THE PHOTOPHYSICS OF MONOMERIC BACTERIOCHLOROPHYLLS c AND d AND THEIR DERIVATIVES: PROPERTIES OF TRIPLET STATE AND SINGLET OXYGEN PHOTOGENERATION AND QUENCHING

Measurements of pigment triplet‐triplet absorption, pigment phosphorescence and photosensitized singlet oxygen luminescence were carried out on solutions containing monomeric bacteriochlorophylls (Bchl) c and d, isolated from green photosynthetic bacteria, and their magnesium‐free and farnesyl‐free analogs. The energies of the pigment triplet states fell in the range 1.29‐1.34 eV. The triplet lifetimes in aerobic solutions were200–250 ns; they increased to 280 pL 70 us after nitrogen purging in liquid solutions and to 0.7‐2.1 ms in a solid matrix at ambient or liquid nitrogen temperatures. Rate constants for quenching of the pigment triplet state by oxygen were (2.0‐2.5) × 109 M−1 s−1. which is close to 1/9 of the rate constant for diffusion‐controlled reactions. This quenching was accompanied by singlet oxygen formation. The quantum yields for the triplet state formation and singlet oxygen production were55–75% in air‐saturated solutions. Singlet oxygen quenching by ground‐state pigment molecules was observed. Quenching was the most efficient for magnesium‐containing pigments, kq= (0.31‐1.2) × 109 M−1 s−1. It is caused mainly by a physical process of singlet oxygen (1O2,) deactivation. Thus, Bchl c and d and their derivatives, as well as chlorophyll and Bchl a, combine a high efficiency of singlet oxygen production with the ability to protect photochemical and photobiological systems against damage by singlet oxygen.

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