TRIPLET STATES OF CAROTENOIDS FROM PHOTOSYNTHETIC BACTERIA STUDIED BY NANOSECOND ULTRAVIOLET AND ELECTRON PULSE IRRADIATION

Abstract— Absorptions of the triplet excited states of five carotenoids (15,15'‐ds phytoene, all‐ trans phytoene, C‐carotene, spheroidene and spirilloxanthin), extracted from the photosynthetic bacteria Rhodopseudomonas spheroides and Rhodospirillum rubrum, have been detected in solution using pulse radiolysis and laser flash photolysis. Triplet lifetimes, extinction coefficients, lowest energy levels and quantum efficiencies of formation have been determined. Comparison of the carotenoid triplet energy levels with that of O 2 ('Δ g ) suggests that spirilloxanthin, spheroidene and possibly alsoζ‐carotene, would be expected to protect against photodynamic action caused by O 2 ('Δ g ), but not cis or trans phytoene. The S → T intersystem crossing efficiences of all five polyenes were found to be low, being a few per cent or less. In their protective role these triplet states can only therefore be effectively reached via energy transfer from another triplet, except in the case of O 2 ('Δ g ). The low crossover efficiencies also mean that light absorbed by such carotenoids in their possible role as accessory pigments would not be wasted in crossing over to the triplet state.