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 [superscript 1]B[subscript u] [superscript +] and two [superscript 1]A[subscript g] [superscript –] excited states. The [superscript 1]B[subscript u] [superscript +] states converge to a common infinite polyene limit of 15 900 ± 100 cm [superscript –1]. The two excited [superscript 1]A[subscript g] [superscript –] 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[superscript 1]B[subscript u] [superscript +] → 2 [superscript 1]A[subscript g] [superscript –] → 1 [superscript 1]A[subscript g] [superscript –] 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 [superscript 1]A[subscript g] [superscript –] states, using the energy gap law for nonradiative decay, reveals remarkable similarities in the N dependence of the 2 [superscript 1]A[subscript g] [superscript –] 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.