title: Unusual relaxation pathway from the two-photon excited first singlet state of carotenoids.
abstract: Transient infrared and visible absorption measurements along with density functional theory (DFT) calculations on carotenoids 8'-apo-beta-caroten-8'-al (I) and 7',7'-dicyano-7'-apo-beta-carotene (II) were used to explore the nature of a long-lived species observed in transient infrared absorption measurements following two-photon excitation (Pang et al. J. Phys. Chem. B 2009, 113, 13806). The long-lived species of I has a very strong infrared absorption around 1510 cm(-1) and a visible transient absorption band centered at 760 nm. The long-lived species appears on two different time scales of approximately 16 and 140-270 ps. The longer rise component is absent in nonpolar solvents. DFT calculations using the B3LYP functional and the 6-31G(d) basis set were used to investigate the ground-state potential-energy surface of I and II including its conformational isomers, a pi-diradical "kinked" structure, and cation and neutral radicals. From the simulated infrared spectra of all the structures considered, we found a close match in the cation radical spectrum to the experimental infrared spectrum of the long-lived species. However, the visible absorption band does not match that of the monomeric cation radical. On the basis of our experimental and theoretical results, we propose a charge-transfer complex between a carotenoid and a solvent molecule for the origin of the long-lived species formed from the direct two-photon excitation of the S(1) state.
authors.affiliation: Department of Chemistry, University of California, Berkeley, and Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720-1460, USA.
authors.forename: Yoonsoo
authors.initials: Y
authors.lastname: Pang
authors.forename: Garth A
authors.initials: GA
authors.lastname: Jones
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Prantil
authors.forename: Graham R
authors.initials: GR
authors.lastname: Fleming
mesh_terms: D002338:Carotenoids / Q000737:chemistry*; D003198:Computer Simulation; D013055:Spectrophotometry, Infrared / Q000379:methods; D013499:Surface Properties; D013816:Thermodynamics; D019207:beta Carotene / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: C547006:7',7'-dicyano-7'-apo-beta-carotene; C110652:beta-apo-8'-carotenal; D019207:beta Carotene; D002338:Carotenoids