Excited state conformational dynamics in carotenoids: Dark intermediates and excitation energy transfer

Abstract

A consideration of the excited state potential energy surfaces of carotenoids develops a new hypothesis for the nature of the conformational motions that follow optical preparation of the S2 (1(1)Bu(+)) state. After an initial displacement from the Franck-Condon geometry along bond length alternation coordinates, it is suggested that carotenoids pass over a transition-state barrier leading to twisted conformations. This hypothesis leads to assignments for several dark intermediate states encountered in femtosecond spectroscopic studies. The Sx state is assigned to the structure reached upon the onset of torsional motions near the transition state barrier that divides planar and twisted structures on the S2 state potential energy surface. The X state, detected recently in two-dimensional electronic spectra, corresponds to a twisted structure well past the barrier and approaching the S2 state torsional minimum. Lastly, the S(∗) state is assigned to a low lying S1 state structure with intramolecular charge transfer character (ICT) and a pyramidal conformation. It follows that the bent and twisted structures of carotenoids that are found in photosynthetic light-harvesting proteins yield excited-state structures that favor the development of an ICT character and optimized energy transfer yields to (bacterio)chlorophyll acceptors.

Understanding/unravelling carotenoid excited singlet states

“…However, the debate on the characterization of the S* state is still ongoing. Beck et al [102] reinterpreted the radiationless decay of carotenoids after photoexcitation up to the S 2 state by referring to a model derived from studies of polymethine cyanines [103]. They suggested that the S* state can be assigned to a low-lying S 1 state structure with intramolecular charge transfer character and a pyramidal conformation (figure 5).…”

Vibrationally Coherent Preparation of the Transition State for Photoisomerization of the Cyanine Dye Cy5 in Water

“…This point strongly suggests that the chromophore-binding site in a protein host can play a dominant role in controlling the quantum yield for photochemistry by selecting the ground-state conformation. 56 A similar suggestion, that the protein raises the quantum yield for photochemistry by "pretwisting" the chromophore in the ground state, was made by Kukura et al 32 on the basis of femtosecond stimulated Raman studies of the photochemistry of retinal PSB in rhodopsin. Additionally, the electrostatic environment of the binding site can control the minimum-energy path on the excited-state potential energy surface, the adiabaticity of the barrier crossing event, and the solvent friction that retards torsional motion.…”

Two wavelength‐dependent mechanisms of sensitisation of light‐induced quenching in the isolated light‐harvesting complex II

“…An alternative quenching sensitisation process has then to be sought. A possible explanation relies on the relatively high yield of trans‐cis isomerisation observed in model polyenes molecules upon population of carotenoids S 1 state , which would positively correlate with the higher quenching sensitisation yield detected when these states are excited directly. We stress that, in any case, these species are not the actual quenching sites.…”