Effect of π-electron conjugation length on the solvent-dependent S1 lifetime of peridinin

Peridinin exhibits an anomalous solvent dependence of its S1 excited state lifetime attributed to the presence of an intramolecular charge transfer (ICT) state. The nature of this state has yet to be elucidated. Ultrafast time-resolved optical spectroscopy has been performed on a synthetic analog, C35-peridinin, having one less conjugated double bond than peridinin. The data reveal the lifetime decreases from 1.5 ns in n-hexane to 9.2 ps in methanol, an order of magnitude larger than peridinin. This is the strongest solvent dependence on the lifetime of an S1 state of a carotenoid yet reported. The data support the view that the S1 and ICT states are strongly coupled.