THE IN VITRO PHOTOCHEMISTRY OF BIOLOGICAL MOLECULES-III. ABSORPTION SPECTRA, LIFETIMES AND RATES OF OXYGEN QUENCHING OF THE TRIPLET STATES OF β-CAROTENE, RETINAL AND RELATED POLYENES

Abstract— The triplet-triplet absorption spectra of six polyenes have been characterised using flash photolysis, in the presence of anthracene as sensitizer, and pulse radiolysis, in the absence of a sensitizer. The polyenes include several which contain carbonyl groups whose triplet states, unlike retinal , could not be detected unsensitized by flash photolysis. The triplet lifetimes appear to be a function of the number of conjugated double bonds, and vary between 7 and 14 μ sec. In general, the longer the polyene, the shorter the lifetime. An empirical linear relation was found between the frequencies of the polyene triplet-triplet absorption maxima, and the frequencies of the corresponding ground singlet-singlet maxima. The rate constants for quenching by oxygen of nine polyene triplet states were determined to lie in the range 2–7 × 10⁹ M ^-1 sec^-1. The possible mechanisms for oxygen quenching of triplet states are discussed and analogies between the results for oxygen quenching of polyenes and of polyacenes are drawn. The rate constant for oxygen quenching of all- trans -β-carotene triplet was the same in benzene and hexane.