ABSTRACT. Excited-state relaxation dynamics and energy-transfer processes in the chlorophyll a (Chl a) manifold of the light-harvesting complex II (LHCII) were examined at physiological temperature using femtosecond two-dimensional electronic spectroscopy (2DES). The experiments were done under conditions free from singlet-singlet annihilation and anisotropic decay. Energy transfer between the different domains of the Chl a manifold was found to proceed on timescales from hundreds of femtoseconds to five picoseconds, before reaching equilibration.No component slower than 10 ps was observed in the spectral equilibration dynamics. We clearly observe the bidirectional (uphill and downhill) energy transfer of the equilibration process between excited states. This bidirectional energy flow, although implicit in the modelling and simulation of the EET processes, has not been observed in any prior transient absorption studies. Furthermore, we identified the spectral forms associated with the different energy transfer lifetimes in the equilibration process. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 All photosynthetic organisms rely on light-harvesting antenna systems to maximize the flux of excitation energy at the photochemical reaction centers. Efficient photochemistry relies on the fact that the excitation energy transfer (EET) through the exciton network occurs on a much shorter timescale than the excitation lifetime. Light-harvesting complex II, the most abundant pigmentprotein complex in the biosphere, 1,2 is the major light-harvesting antenna complex in plants and green algae. The relatively rigid protein structure of LHCII 3,4 ensures fine-tuned localization and environment of the noncovalently bound pigment molecules -chlorophyll (Chl) a and b and xanthophylls. The EET dynamics in LHCII has been studied extensively over the past decades using various time-resolved spectroscopic techniques and modelling approaches. Yet, despite the wealth of literature data, there is no complete consensus on the dynamics and pathways of EET, particularly in the equilibration process in the manifold of Chl a exciton states. Upon photoexcitation, due to the narrow spread in energy of the manifold of Chl a exciton states, significant uphill and downhill EET will occur concurrently, leading eventually to a dynamic thermal equilibrium. A wide range of lifetimes for this equilibration process have been reported for LHCII monomers and trimers. At temperatures of 77 K or lower, excitation on the blue side of the Qy band of Chl a is followed by downhill EET with time constants from hundreds of femtoseconds to about 20 ps. 5-8At ...

Note:The paper is part of the JCP Special Topic on Ultrafast Molecular Sciences by Femtosecond Photons and Electrons.

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We measured two-dimensional electronic spectra of light-harvesting complex II (LHCII) at various temperatures (77, 110, 150, 230, and 295 K) under conditions free from singlet−singlet annihilation. We elucidated the temperature-dependent excitation energy transfer dynamics in the Chl a manifold of LHCII. Global analysis revealed that the dynamics can be summarized in distinct time scales from 200 fs up to 15 ps. While the fastest dynamics with a decay time of ∼0.2−0.3 ps are relatively temperature-independent, the lifetimes and relative contributions of slower components showed considerable temperature dependence. The slowest time scale of equilibration with the lowest-energy Chl a increased from ∼5 ps at 295 K to ∼15 ps at 77 K. The final excited state is independent of initial excitation at 230 K and above, whereas static energy disorder is apparent at lower temperatures. A clear temperature dependence of uphill energy transfer processes was also discerned, which is consistent with the detailed-balance condition.

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