title: Femtosecond transient infrared and stimulated Raman spectroscopy shed light on the relaxation mechanisms of photo-excited peridinin.
abstract: By means of one- and two-dimensional transient infrared spectroscopy and femtosecond stimulated Raman spectroscopy, we investigated the excited state dynamics of peridinin, a carbonyl carotenoid occurring in natural light harvesting complexes. The presence of singly and doubly excited states, as well as of an intramolecular charge transfer (ICT) state, makes the behavior of carbonyl carotenoids in the excited state very complex. In this work, we investigated by time resolved spectroscopy the relaxation of photo-excited peridinin in solvents of different polarities and as a function of the excitation wavelength. Our experimental results show that a characteristic pattern of one- and two-dimensional infrared bands in the C=C stretching region allows monitoring the relaxation pathway. In polar solvents, moderate distortions of the molecular geometry cause a variation of the single/double carbon bond character, so that the partially ionic ICT state is largely stabilized by the solvent reorganization. After vertical photoexcitation at 400 nm of the S2 state, the off-equilibrium population moves to the S1 state with ca. 175 fs time constant; from there, in less than 5 ps, the non-Franck Condon ICT state is reached, and finally, the ground state is recovered in 70 ps. That the relevant excited state dynamics takes place far from the Franck Condon region is demonstrated by its noticeable dependence on the excitation wavelength.
authors.affiliation: LENS (European Laboratory for Non-Linear Spectroscopy), via N. Carrara 1, 50019 Sesto Fiorentino, Italy.
authors.forename: Mariangela
authors.initials: M
authors.lastname: Di Donato
authors.affiliation: LENS (European Laboratory for Non-Linear Spectroscopy), via N. Carrara 1, 50019 Sesto Fiorentino, Italy.
authors.forename: Elena
authors.initials: E
authors.lastname: Ragnoni
authors.affiliation: LENS (European Laboratory for Non-Linear Spectroscopy), via N. Carrara 1, 50019 Sesto Fiorentino, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Lapini
authors.affiliation: LENS (European Laboratory for Non-Linear Spectroscopy), via N. Carrara 1, 50019 Sesto Fiorentino, Italy.
authors.forename: Paolo
authors.initials: P
authors.lastname: Foggi
authors.affiliation: Department of Biology, Faculty of Science and Engineering, Macquarie University, Sidney 2109, Australia.
authors.forename: Roger G
authors.initials: RG
authors.lastname: Hiller
authors.affiliation: LENS (European Laboratory for Non-Linear Spectroscopy), via N. Carrara 1, 50019 Sesto Fiorentino, Italy.
authors.forename: Roberto
authors.initials: R
authors.lastname: Righini
mesh_terms: D002338:Carotenoids / Q000737:chemistry*; D008027:Light; D011789:Quantum Theory; D017550:Spectroscopy, Fourier Transform Infrared; D013059:Spectrum Analysis, Raman; D013997:Time Factors
chemical_list: C016040:peridinin; D002338:Carotenoids