Singlet Oxygen Quenching Abilities of Carotenoids

Abstract

Abstract The bimolecular rate constants k q for quenching of singlet oxygen ( 1 Δ g state) by 26 different natural and novel synthetic carotenoids were determined at 37 °C in a mixture of chloroform and ethanol. The steady‐state technique used involves the generation of 1 O 2 by thermal decomposition of disodium 3,3′‐naphtalene‐1,4‐diyl‐dipropionate endoperoxide (NDPO 2 ) and the detection of its luminescence intensity at 1270 nm. Excitation energies (π,π*, 1 1 Ag → 1 1 B u ) and absorption maxima (430–590 nm) vary in the broadest range. Deeply coloured blue carotenoids are also included in the studies for the first time. An empirical correlation between the π,π* (1 1 A g → 1 1 B u ) excitation energy and carotenoid structure (effective chain length N eff ) was found: E (S) = 12642 cm −1 + 92027 cm −1 × 1/ N eff . The quenching ability of the investigated carotenoids depends on the excitation energy of their transition at long wavelengths in a characteristic way showing as limiting factors either the thermal Arrhenius activation or the diffusion‐controlled rate. This dependence and the suspected relationship between singlet E (S) and triplet E (T) energies, respectively, are discussed.