Reaction of the triplet state of retinol with oxygen

Abstract

The yield of singlet oxygen from the reaction of oxygen with the triplet state of retinol is 0.25 ± 0.05. The low yield can be explained by the existence of three competing pathways for the reaction of the triplet state of retinol with oxygen: (a) energy transfer: ³T^*+³O₂→¹T +¹O₂^*, (b) charge transfer: ³T^*+³O₂→¹T +³O₂, (c) spin exchange with a triplet state in a twisted conformation: ³P^*+³O₂→βT +(1 –β)C +³O₂.

The rates of isomerization of all-trans retinol sensitized by triphenylene were measured in the presence and absence of oxygen. From the ratio of these rates and the yield of singlet oxygen it is concluded that the disappearance of triplet state via pathway (c) is much slower than the combined rates via pathways (a) and (b). Also, the rate of path (b) is 2.7 times faster than path (a). Theoretical considerations show that path (b) is probably mediated by charge-transfer interactions between the all-trans triplet state of retinol and oxygen.