Excited state properties of peridinin: Observation of a solvent dependenceof the lowest excited singlet state lifetime and spectral behavior unique among carotenoids

The spectroscopic properties and dynamic behavior of peridinin in several d ifferent solvents were studied by steady-state absorption, fluorescence, an d transient optical spectroscopy. The lifetime of the lowest excited single t state of peridinin is found to be strongly dependent on solvent polarity and ranges from 7 ps in the strongly polar solvent trifluoroethanol to 172 ps in the nonpolar solvents cyclohexane and benzene. The lifetimes show no obvious correlation with solvent polarizability, and hydrogen bonding of th e solvent molecules to peridinin is not an important factor in determining the dynamic behavior of the lowest excited singlet state. The wavelengths o f emission maxima, the quantum yields of fluorescence, and the transient ab sorption spectra are also affected by the solvent environment. A model cons istent with the data and supported by preliminary semiempirical calculation s invokes the presence of a charge transfer state in the excited state mani fold of peridinin to account for the observations. The charge transfer stat e most probably results from the presence of the lactone ring in the pi-ele ctron conjugation of peridinin analogous to previous findings on aminocouma rins and related compounds. The behavior of peridinin reported here is high ly unusual for carotenoids, which generally show little dependence of the s pectral properties and lifetimes of the lowest excited singlet state on the solvent environment.