title: Solvent-dependent activation of intermediate excited states in the energy relaxation pathways of spheroidene.
abstract: In carotenoids internal conversion between the allowed (S(2)) and forbidden (S(1)) excited states occurs on a sub-picosecond timescale; the involvement of an intermediate excited state(s) (S(x)) mediating the process is controversial. Here we use high time resolution (sub-20 fs) broadband (1.2-2.5 eV) pump-probe spectroscopy to study the solvent dependence of excited state dynamics of spheroidene, a naturally-occurring carotenoid with ten conjugated double bonds. In the high polarizability solvent, CS(2), we find no evidence of an intermediate state, and the traditional three-level (S(0), S(1), S(2)) model fully accounts for the S(2)→ S(1) process. On the other hand, in the low polarizability solvent, cyclohexane, we find that rapid (~30 fs) relaxation to an intermediate state, S(x), lying between S(1) and S(2) is required to account for the data. We interpret these results as due to a shift of the S(2) energy, which positions the state above or below the energy of S(x) in response to changes in solvent polarizability.
authors.affiliation: CNR-IFN, Dipartimento di Fisica, Politecnico di Milano, P.za L. da Vinci 32, Milan, Italy.
authors.forename: Margherita
authors.initials: M
authors.lastname: Maiuri
authors.forename: Dario
authors.initials: D
authors.lastname: Polli
authors.forename: Daniele
authors.initials: D
authors.lastname: Brida
authors.forename: Larry
authors.initials: L
authors.lastname: Lüer
authors.forename: Amy M
authors.initials: AM
authors.lastname: LaFountain
authors.forename: Marcel
authors.initials: M
authors.lastname: Fuciman
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Cogdell
authors.forename: Harry A
authors.initials: HA
authors.lastname: Frank
authors.forename: Giulio
authors.initials: G
authors.lastname: Cerullo
mesh_terms: D002246:Carbon Disulfide / Q000737:chemistry; D002338:Carotenoids / Q000737:chemistry*; D003510:Cyclohexanes / Q000737:chemistry; D012242:Rhodobacter sphaeroides / Q000737:chemistry; D012997:Solvents / Q000737:chemistry*; D013057:Spectrum Analysis; D013696:Temperature; D013997:Time Factors
chemical_list: D003510:Cyclohexanes; D012997:Solvents; C005637:spheroidene; D002338:Carotenoids; C506365:Cyclohexane; D002246:Carbon Disulfide