Deactivation of O₂(¹Δg) Singlet Oxygen by Carotenoids:  Internal Conversion of Excited Encounter Complexes

Literature data of rate constants k Δ Q of electronic energy transfer (EET) from O 2 ( 1 Δ g ) singlet oxygen to 28 carotenoids have been analyzed by means of a semiempirical model derived for the deactivation of triplet excited (T 1 ) molecules by O 2 ( 3 Σ g - ) triplet ground-state oxygen. The analysis demonstrates that EET from O 2 ( 1 Δ g ) to carotenoids occurs not via the electron exchange mechanism according to Dexter but principally via formation of 1 ( 1 Δ·S 0 ) encounter complexes that deactivate irreversibly by internal conversion to 1 (T 1 · 3 Σ) encounter complexes leading to formation of the T 1 excited carotenoid. The dependence of log( k Δ Q ) on the excess energy of the EET process is quantitatively described. The results indicate that EET processes to and from O 2 could generally occur via IC of excited complexes.