Singlet versus Triplet Reactivity in Photodegradation of C40 Carotenoids

Abstract

Direct observations of triplet excited states of astaxanthin (I), β-carotene (II), canthaxanthin (III), and zeaxanthin (IV) in toluene at 25 °C following unsensitized laser flash photolysis and using transient absorption spectroscopy have yielded the singlet−triplet intersystem crossing yields 3.7 × 10-3 (I), 5.4 × 10-4 (II), 9.7 × 10-3 (III), and 1.8 × 10-3 (IV), based on triplet−triplet extinction coefficients obtained in anthracene-sensitized experiments. A carbonyl rather than a hydroxy group distinguished the carotenoids from each other, as further evidenced by rate constants for oxygen quenching of triplet carotenoids, 2.1 × 108 (I), 1.1 × 108 (II), 2.5 × 108 (III), and 0.95 × 108 M-1 s-1 (IV), determined under similar conditions and by the fluorescence quantum yields which depend on excitation wavelength (355 versus 430 nm) more significantly for II (factor of 13) and IV (factor of 21) than for I and III (common factor of 2). For II and III, using continuous-wave photolysis, competing oxygen-dependent and oxygen-independent photodegradation was demonstrated, and for the latter, III was shown to degrade both in the singlet manifold with a photodegradation quantum yield Φ1 = 7 × 10-6 and in the triplet with Φ3 = 7 × 10-6 for 366 nm excitation, while II almost exclusively degraded in the singlet with Φ1 = 3.8 × 10-5. The nature of the excited states of III (and I) with some n, π* character yields (i) more facile transformation between excited states (higher intersystem crossing yield, less wavelength-dependent fluorescence) and (ii) less biradical character (less photolability) compared to excited states of II (and IV) with pure π, π* character. Keywords: Astaxanthin; β-carotene; canthaxanthin; zeaxanthin; singlet−triplet intersystem crossing yields; oxygen quenching; fluorescence quantum yield; photodegradation quantum yield