Unique ultrafast excited states dynamics of artificial short-polyene carotenoid analog 2-(all-trans-β-ionylideneetinylidene)-indan-1,3-dione

Abstract

Carotenoids are naturally occurring pigments and essential for light-harvesting and photoprotection in photosynthesis of plants, algae, and bacteria. Carbonyl-containing carotenoids exhibit unique excited state properties due to the occurrence of intramolecular charge transfer (ICT) in the excited state. In the present study, we performed femtosecond pump-probe spectroscopy on the short-chain polyene compound 2-(all-trans-β-ionylideneetinylidene)-indan-1,3-dione with three conjugated double bonds and two carbonyl groups, denoted as C15Ind in acetone and in n-hexane. The spectroscopic properties of C15Ind both in polar and nonpolar solvents suggests an ICT character based on the observations of the stimulated emission due to the coupled 1Ag-/ICT state. In acetone, an additional excited state was found to be located below the 1Ag-/ICT state. The ultrafast triplet generation via the 1nπ* state was also observed both in acetone and in n-hexane, while the amplitude of the triplet transient absorption of C15Ind was rather small compared with those in retinal and Retinyl-1 reported previously. Thus, an ICT character of carbonyl-containing carotenoids may prevent a triplet generation upon excitation into the optically allowed 1Bu+ (S2) state.