Nonconjugated Substituents Modulate Excited-State Dynamics of Keto-Carotenoids

Nonconjugated groups strongly affect the spectroscopic and excited-state properties of keto-carotenoids. We studied crocetin derivatives with the same conjugated backbone but different nonconjugated groups. Crocetin and Na-crocetin, which lack gentiobiosyl groups, show essentially no response to solvent polarity, with S₁/ICT lifetimes of ∼124-130 ps in both EtOH and H₂O/D₂O. In contrast, crocin and crocin III, which contain two and one gentiobiosyl groups, respectively, display significant polarity-induced effects in H₂O/D₂O. Asymmetry of crocin III enhances these effects compared with the symmetric crocin. While in ethanol both molecules have the S₁/ICT lifetime of ∼130 ps, in H₂O it is shortened to 61 ps (crocin) and 11 ps (crocin III), and this change is associated with the presence of enhanced ICT bands in transient absorption spectra. Hydrogen bonding in polar solvents further strengthens these effects. These results show that nonconjugated groups, especially asymmetric ones, can tune the excited-state dynamics of symmetric keto-carotenoids.