Femtosecond Heterodyne Transient-Grating Studies of Nonradiative Decay of the S₂ (1¹B_u⁺) State of β-Carotene: Contributions from Dark Intermediates and Double-Quantum Coherences

Abstract

Femtosecond transient-grating spectroscopy with heterodyne detection was employed to characterize the nonradiative decay pathway in β-carotene from the S2 (1(1)Bu(+)) state to the S1 (2(1)Ag(-)) state in benzonitrile solution. The results indicate definitively that the S2 state populates an intermediate state, Sx, on an ultrafast time scale prior to nonradiative decay to the S1 state. Numerical simulations using the response function formalism and the multimode Brownian oscillator model were used to fit the absorption and dispersion components of the transient-grating signal with a common set of parameters for all of the relevant Feynman pathways, including double-quantum coherences. The requirement for inclusion of the Sx state in the nonradiative decay pathway is the observed fast rise time of the dispersion component, which is predominantly controlled by the decay of the stimulated emission signal from the optically prepared S2 state. The finding that the excited-state absorption spectrum from the Sx state is significantly red-shifted from that of S2 and S1 leads to a new assignment for the spectroscopic origin of the Sx state. Rather than assigning Sx to a discrete electronic state, such as the (1)Bu(-) state suggested in previous work, it is proposed that the Sx state corresponds to a transition-state-like structure on the S2 potential surface. In this hypothesis, the 12 fs time constant for the decay of the S2 state corresponds to a vibrational displacement of the C-C and C═C bond-length alternation coordinates of the conjugated polyene backbone from the optically prepared Franck-Condon structure to a potential energy barrier on the S2 surface that divides planar and torsionally displaced structures. The lifetime of the Sx state would be associated with a subsequent relaxation along torsional coordinates over a steep potential energy gradient toward a conical intersection with the S1 state. This hypothesis leads to the idea that twisted structures with intramolecular charge-transfer character along the S2 torsional gradient are active in excitation energy-transfer mechanisms to (bacterio)chlorophyll acceptors.

Torsional Dynamics and Intramolecular Charge Transfer in the S2 (11Bu+) Excited State of Peridinin: A Mechanism for Enhanced Mid-Visible Light Harvesting

“…The spectral time evolution in the sequential decay model is described by the evolutionassociated spectra (EAS). The nature of the time evolution of the EAS has been discussed in detail for β-carotene 3 and peridinin 1 in previous publications with support from a comprehensive set of numerical simulations using the response function formalism and the multimode Brownian oscillator model. Note that this work considered a range of alternative models and considered especially the possible assignment of the Sx state in peridinin to a vibrationally excited energy level of S1.…”

“…The experimental setup and techniques used for heterodyne transient grating spectroscopy were described in detail previously. 3 Briefly, a 250-kHz amplified Ti:sapphire laser (Coherent Mira Seed oscillator and RegA 9050 amplifier) was used to pump an optical parametric amplifier (OPA, Coherent OPA 9450).…”

“…As described in the previous work, 1,3 phasing of the heterodyne detected signal was performed using the method described by Xu et al 7 and Moran et al 5 with neat solvent in the same cuvette used for the peridinin samples and with kinematic replacement at the sample position.…”

A new energy transfer channel from carotenoids to chlorophylls in purple bacteria

“…Interestingly, the experimental 2D study resolves the diagonal peak due to the dark state. Furthermore, based on ultrafast-transient absorption and transient-grating experiments, some groups have suggested that the dark-state is due to a double-minima structure on the S 2 potential energy surface, not a distinct electronic state 43, 44 . This double-minima model hypothesizes that the lowest one of the S 2 potential surface minima exists at the conformation where the carotenoid is twisted from the all- trans isomer by ~90° with respect to one C=C bond of the conjugated polyene backbone.…”

Excited‐State Evolution of Keto‐Carotenoids after Excess Energy Excitation in the UV Region

“…[18] However, involvement of another dark intermediate state has also been invoked. The nature of this state, usually denoted as the S x state, is still unknown and either a separate electronic state of B u À symmetry [11,19,20] or a twisted S 2 state [21,22] have been proposed as the intermediate state facilitating the S 2 -S 1 relaxation.…”