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

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• 作者：Ronald L. Christensen ; Miriam M. Enriquez ; Nicole L. Wagner ; Alexandra Y. Peacock-Villada ; Corina Scriban ; Richard R. Schrock ; Tom谩拧 Pol铆vka ; Harry A. Frank ; Robert R. Birge
• 刊名：The Journal of Physical Chemistry A
• 出版年：2013
• 出版时间：February 21, 2013
• 年：2013
• 卷：117
• 期：7
• 页码：1449-1465
• 全文大小：802K
• 年卷期：v.117,no.7(February 21, 2013)
• ISSN：1520-5215

文摘

Steady-state and ultrafast transient absorption spectra were obtained for a series of conformationally constrained, isomerically pure polyenes with 5鈥?3 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^{鈥?/sup> excited states. The 1B_u+ states converge to a common infinite polyene limit of 15鈥?00 卤 100 cm鈥?. The two excited 1A_g鈥?/sup> states, however, exhibit a large (9000 cm鈥?) 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+ 鈫?21A_g鈥?/sup> 鈫?11A_g鈥?/sup> 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鈥?/sup> states, using the energy gap law for nonradiative decay, reveals remarkable similarities in the N dependence of the 21A_g鈥?/sup> 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.}