title: Energetics and dynamics of the low-lying electronic states of constrained polyenes: implications for infinite polyenes.
abstract: Steady-state and ultrafast transient absorption spectra were obtained for a series of conformationally constrained, isomerically pure polyenes with 5-23 conjugated double bonds (N). These data and fluorescence spectra of the shorter polyenes reveal the N dependence of the energies of six (1)B(u)(+) and two (1)A(g)(-) excited states. The (1)B(u)(+) states converge to a common infinite polyene limit of 15,900 ± 100 cm(-1). The two excited (1)A(g)(-) states, however, exhibit a large (~9000 cm(-1)) energy difference in the infinite polyene limit, in contrast to the common value previously predicted by theory. EOM-CCSD ab initio and MNDO-PSDCI semiempirical MO theories account for the experimental transition energies and intensities. The complex, multistep dynamics of the 1(1)B(u)(+) → 2(1)A(g)(-) → 1(1)A(g)(-) excited state decay pathways as a function of N are compared with kinetic data from several natural and synthetic carotenoids. Distinctive transient absorption signals in the visible region, previously identified with S* states in carotenoids, also are observed for the longer polyenes. Analysis of the lifetimes of the 2(1)A(g)(-) states, using the energy gap law for nonradiative decay, reveals remarkable similarities in the N dependence of the 2(1)A(g)(-) decay kinetics of the carotenoid and polyene systems. These findings are important for understanding the mechanisms by which carotenoids carry out their roles as light-harvesting molecules and photoprotective agents in biological systems.
authors: Department of Chemistry, Bowdoin College, Brunswick, Maine 04011, USA. rchriste@bowdoin.edu
authors: Ronald L
authors: RL
authors: Christensen
authors: Miriam M
authors: MM
authors: Enriquez
authors: Nicole L
authors: NL
authors: Wagner
authors: Alexandra Y
authors: AY
authors: Peacock-Villada
authors: Corina
authors: C
authors: Scriban
authors: Richard R
authors: RR
authors: Schrock
authors: Tomáš
authors: T
authors: Polívka
authors: Harry A
authors: HA
authors: Frank
authors: Robert R
authors: RR
authors: Birge
mesh_terms: D002338:Carotenoids / Q000737:chemistry; D004583:Electrons; D008958:Models, Molecular; D011090:Polyenes / Q000737:chemistry*
chemical_list: D011090:Polyenes; D002338:Carotenoids