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Femtosecond Heterodyne Transient Grating Studies of Nonradiative Decay of the S (1B ) State of #-Carotene: Contributions from Dark Intermediates and Double Quantum Coherences 2
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Soumen Ghosh, Michael M. Bishop, Jerome D Roscioli, Jenny Jo Mueller, Nolan C Shepherd, Amy M. LaFountain, Harry A. Frank, and Warren F. Beck J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b09405 • Publication Date (Web): 27 Oct 2015 Downloaded from http://pubs.acs.org on October 28, 2015
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Femtosecond Heterodyne Transient Grating Studies of Nonradiative Decay of the S2 (11Bu+) State of β-Carotene: Contributions from Dark Intermediates and Double Quantum Coherences Soumen Ghosh,# Michael M. Bishop,#† Jerome D. Roscioli, Jenny Jo Mueller,‡ Nolan C. Shepherd,§ Amy M. LaFountain,¶ Harry A. Frank,¶ and Warren F. Beck* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322 USA Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060 USA
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†Current
address: Department of Physics, University of Connecticut, Storrs, Connecticut 06269-3060 USA
‡Current
address: Naval Medical Research Center, 503 Robert Grant Avenue, Silver Spring, MD 20910 USA.
§Current
address: Department of Chemistry, University of Chicago, Chicago, IL 60607 USA.
*Corresponding author. Email:
[email protected]. #These
authors made equal contributions to the work.
Abstract Femtosecond transient grating spectroscopy with heterodyne detection was employed to characterize the nonradiative decay pathway in β-carotene from the S2 (11Bu+) state to the S1 (21Ag−) state in benzonitrile solution. The results indicate definitively that the S2 state populates an intermediate state, Sx, on an ultrafast timescale 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
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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 1Bu− state suggested in previous work, it is proposed that the Sx state corresponds to a transition state 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 towards 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.
Introduction The light-harvesting function of carotenoids in photosynthesis1-4 is initiated by optical preparation of the second excited singlet state, S2 (11Bu+), which decays nonradiatively to yield the first excited singlet state, S1 (21Ag−). Both states can serve as excitation energy donors to the Q-band singlet states of (bacterio)chlorophyll ((B)Chl) molecules in lightharvesting complexes (Figure 1). The mechanism that enables the S1 state to function efficiently in energy transfer to (B)Chls is of considerable interest because only the S2 state can be directly populated by optical transitions; the S1 state is a dark state, lacking electric dipole-allowed one-photon transitions to or from the ground state, S0 (11Ag−). The bulk of the quantum yield of energy transfer is nevertheless carried by the S1 state, however, because the lifetime of the S2 state is very short, typically 150 fs. The rate of excitation 2
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energy transfer in light-harvesting proteins is optimized structurally by placing the carotenoid donor nearly in van der Waals contact with the (B)Chl acceptor. Additionally, it is likely that an intramolecular charge-transfer (ICT) character develops in carbonylcontaining carotenoids, such as peridinin and fucoxanthin, during the nonradiative decay process from the S2 state to the S1 state.
Figure 1. Pathways for nonradiative decay in carotenoids and for excitation energy transfer from carotenoids to (B)Chls in photosynthetic light-harvesting proteins after resonant one-photon excitation of the S2 (11Bu+) state. The one-photon transitions of most carotenoids effectively follow the selection rules for molecules belonging to the C2h point group; transitions between states of the same symmetry or pseudoparity are electricdipole forbidden.5,6 Nonradiative decay from the S2 and S1 (21Ag−) states is indicated with wavy arrows. The lifetimes for each state depend on the extent of π-electron conjugation of the carotenoid and on the nature of attached functional groups; the values indicated are approximately those of β-carotene. Energy transfer pathways from the carotenoid S2 and S1 states to the (B)Chl Qx and Qy states, are indicated with filled arrows. Whether the S2 state undergoes an ultrafast nonradiative decay process to populate one or more dark intermediates prior to populating the S1 state remains indeterminate despite extensive debate in the literature. Polívka and Sundström4 have reviewed the considerable body of evidence both for and against the existence of the state denoted Sx, which is detected in femtosecond pump–probe experiments in terms of an excited-state absorption (ESA) band in the near-IR in
