Energetics and Dynamics of the Low-Lying Electronic States of Constrained Polyenes: Implications for Infinite Polyenes

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 ¹B_u⁺ and two ¹A_g^– excited states. The ¹B_u⁺ states converge to a common infinite polyene limit of 15 900 ± 100 cm^–1. The two excited ¹A_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 1¹B_u⁺ → 2¹A_g^– → 1¹A_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 2¹A_g^– states, using the energy gap law for nonradiative decay, reveals remarkable similarities in the N dependence of the 2¹A_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