Decoding the excited-state dynamics of carbonyl-containing carotenoids: Insights from the Ind series.

Abstract

Carotenoids are naturally occurring pigments essential for both light-harvesting and photoprotection in photosynthetic processes. Among these, carbonyl-containing carotenoids exhibit distinctive excited state properties due to the presence of intramolecular charge transfer (ICT) in their excited states. In this study, we synthesized a novel family of carotenoid analogs with varying numbers of conjugated double bonds, denoted as the Ind series, and conducted femtosecond pump-probe spectroscopy on these molecules in both acetone and n-hexane. The objective was to elucidate how the excited-state dynamics depend on the conjugation length. The spectroscopic characterization of the Ind series revealed several unique features: the observation of stimulated emission from the ¹A_g^-/ICT state, the emergence of the ¹nπ^∗ state, triplet state formation mediated by the ¹nπ^∗ state, and an anomalous solvent dependence of the ¹A_g^-/ICT state lifetimes. The relationship between conjugation length and excited state dynamics, as well as the ICT character of the Ind series, are thoroughly discussed.