Photosynthetic light harvesting in excess light is regulated by a process known as feedback deexcitation. Femtosecond transient absorption measurements on thylakoid membranes show selective formation of a carotenoid radical cation upon excitation of chlorophyll under conditions of maximum, steady-state feedback deexcitation. Studies on transgenic Arabidopsis thaliana plants confirmed that this carotenoid radical cation formation is correlated with feedback deexcitation and requires the presence of zeaxanthin, the specific carotenoid synthesized during high light exposure. These results indicate that energy transfer from chlorophyll molecules to a chlorophyllzeaxanthin heterodimer, which then undergoes charge separation, is the mechanism for excess energy dissipation during feedback deexcitation.

The crucial step in the conversion of solar to chemical energy in Photosynthesis takes place in the reaction center where the absorbed excitation energy is converted into a stable charge separated state by ultrafast electron transfer events. However, the fundamental mechanism responsible for the near unity quantum efficiency of this process is unknown. Here we elucidate the role of coherence in determining the efficiency of charge separation in the plant photosystem II reaction centre (PSII RC) by comprehensively combining experiment (two-dimensional electronic spectroscopy) and theory (Redfield theory). We reveal the presence of electronic coherence between excitons as well as between exciton and charge transfer states which we argue to be maintained by vibrational modes. Furthermore, we present evidence for the strong correlation between the degree of electronic coherence and efficient and ultrafast charge separation. We propose that this coherent mechanism will inspire the development of new energy technologies.

Abstract:Singlet fission is the spin-allowed conversion of a spin-singlet exciton into a pair of spintriplet excitons residing on neighbouring molecules. To rationalise this phenomenon, a multiexcitonic spin-zero triplet-pair state has been hypothesised as an intermediate in singlet fission. However, the nature of the intermediate states and the underlying mechanism of ultrafast fission have not been elucidated experimentally. Here, we study a series of pentacene derivatives using ultrafast 2D electronic spectroscopy and unravel the origin of the states involved in fission. Our data reveal the crucial role of vibrational degrees of freedom coupled to electronic excitations that facilitate the mixing of multiexcitonic states with singlet excitons.The resulting manifold of vibronic states drives sub-100-fs fission with unity efficiency. Our results provide a framework for understanding singlet fission and show how the formation of vibronic manifolds with a high density of states facilitates fast and efficient electronic processes in molecular systems. 2" "Singlet fission (SF) is an exciton multiplication process in organic semiconductors that allows one photogenerated spin-singlet excited state to be converted to two spin-triplet excitons.1 !The two generated spin-triplet excitons are initially correlated to form an overall spin-singlet state, making SF a spin-allowed process in contrast to intersystem crossing that involves a spin flip. For systems where the energy of the lowest lying singlet exciton (S) is close to double the energy of the triplet state (T), such as pentacene and its derivatives, SF can occur on a sub-100fs timescale with every singlet being converted to two triplets.2 SF has attracted great attention lately as it enables photovoltaic devices to overcome thermalisation losses by generating two electron-hole pairs per high-energy photon absorbed, potentially allowing single-junction devices that could beat the Shockley-Queisser limit on power conversion efficiency 3 . The first steps towards this goal have been taken with the demonstration of organic solar cells based on pentacene, that show external quantum efficiencies above 129%, the highest for any solar technology to date. 4,5 Despite advances in the experimental characterization of SF in several molecular systems 6-13 as well as extensive theoretical work, 1, 14-22 the fundamental mechanism of ultrafast SF remains unclear. In the kinetic model proposed by Merrifield and co-workers 23 ! the process can be represented as S!TT!T+TWhere: S is the lowest singlet excited singlet state, T is the molecular triplet state and T+T is a pair of fully independent T states. TT corresponds to a doubly excited pair of spin-correlated triplets, forming an overall spin singlet. The TT state, often referred to as the multiexciton state, is considered a dark state that cannot be optically populated from the ground state g, but serves as an intermediate to the formation of free independent triplets T+T.Current theoretical models for SF focus on characterising t...

Effects of introducing a carbonyl group into the conjugation system of carotenoids were studied for four naturally occurring carotenoids: peridinin, fucoxanthin, siphonaxanthin and spheroidenone. The conjugated carbonyl group affects energetics and dynamics of all these carotenoids in a similar way, although the magnitude of the changes depends strongly on the carotenoid structure. Firstly, presence of a carbonyl group considerably narrows the S 1 /ICT-S 2 gap, and this effect does not depend on polarity. The S 1 /ICT energies of carotenoids were measured by recording S 1 /ICT-S 2 spectral profiles in the near-infrared region and the resulting energies were 16100 cm À1 for peridinin, 16520 cm À1 for fucoxanthin and 16610 cm À1 for siphonaxanthin. Narrowing of the S 1 /ICT-S 2 gap has important consequences for functionality of these carotenoids in light-harvesting systems of oceanic organisms, since while the S 2 state is red-shifted to capture green light, the S 1 /ICT state is still high enough to transfer energy to chlorophyll. The S 1 /ICT energy of spheroidenone was determined to be 13000 cm À1 . Secondly the carbonyl group introduces some polarity-dependent effects: (1) polarity-induced change of the S 1 /ICT lifetime. When changing from nonpolar to polar solvent, the S 1 /ICT lifetime is changed from 160 to 8.5 ps for peridinin, from 60 to 30 ps for fucoxanthin, from 60 to 20 ps for fucoxanthin, while for the longer carotenoid spheroidenone the S 1 /ICT lifetime remains 6 ps regardless of solvent polarity. This effect is explained in terms of stabilization of charge-transfer character of both ground and excited states. (2) stabilization of the charge-transfer character in polar solvents is also demonstrated by appearance of new polarity-induced bands in the transient absorption spectra. (3) polarity-induced changes of the ground state are manifested by asymmetric broadening of the absorption spectrum accompanied by a loss of vibrational structure.