Photosynthesis powers life on our planet. The basic photosynthetic architecture consists of antenna complexes that harvest solar energy and reaction centres that convert the energy into stable separated charge. In oxygenic photosynthesis, the initial charge separation occurs in the photosystem II reaction centre, the only known natural enzyme that uses solar energy to split water. Both energy transfer and charge separation in photosynthesis are rapid events with high quantum efficiencies. In recent nonlinear spectroscopic experiments, long-lived coherences have been observed in photosynthetic antenna complexes, and theoretical work suggests that they reflect underlying electronic-vibrational resonances, which may play a functional role in enhancing energy transfer. Here, we report the observation of coherent dynamics persisting on a picosecond timescale at 77 K in the photosystem II reaction centre using two-dimensional electronic spectroscopy. Supporting simulations suggest that the coherences are of a mixed electronic-vibrational (vibronic) nature and may enhance the rate of charge separation in oxygenic photosynthesis.

Two-dimensional spectroscopy has recently revealed the oscillatory behavior of the excitation dynamics of molecular systems. However, in the majority of cases there is considerable debate over what is actually being observed: excitonic or vibrational wavepacket motion or evidence of quantum transport. In this letter we present a method for distinguishing between vibrational and excitonic wavepacket motion, based on the phase and amplitude relationships of oscillations of distinct peaks as revealed through a fundamental analysis of the two-dimensional spectra of two representative systems.Two-dimensional photon-echo (2DPE) spectroscopy is a powerful tool capable of resolving quantum correlations on the femtosecond timescale 1-3 . They appear as beats of specific peaks in the 2DPE spectrum for a number of molecular systems 3,4 . However, the underlying processes are often ambiguous. At first, the beats were attributed to the wave-like quantum transport with quantum coherences being responsible for an ultra-efficient excitation transfer 3-6 . The same process was associated with the opposite phase beats in the spectral regions which are symmetric with respect to the diagonal line 7 .In molecules and their aggregates, electronic transitions are coupled to various intra-and intermolecular vibrational modes. Vibrational energies of these are of the order of 100 -3000 cm −1 , while the magnitudes of the resonant couplings, J, in excitonic aggregates (e.g. in photosynthetic pigment-protein complexes or in J-aggregates) are in the same range. Thus, vibronic and excitonic systems show considerable spectroscopic similarities, and presence of electronic and/or vibrational beats in the 2DPE spectrum is expected. Indeed, similar spectral beats originating entirely from a high-energy vibrational wavepacket motion have been observed 8,9 . The possibility of distinguishing the electronic and vibrational origin of the beats from a 2DPE spectrum has been emphasized in a recent letter 10 . However, the reported conclusions have not been supported by theoretical arguments, and thus are questionable. Therefore, the highly relevant question of how vibrations interfere with electronic coherences in 2DPE spectrum is still an open one. A theoretical study of the origin of spectral beats, their phase relationships in the rephasing and non-rephasing components of the 2DPE spectrum is presented in this article.We address this problem by considering two generic model systems which exhibit distinct internal coherent dynamics. The simplest model of an isolated molecular electronic excitation is the vibronic system represented a) Electronic mail: darius.abramavicius@ff.vu.lt|e 2 . . . 1234 |g |e 1 |e 2 2 3 | f -ω 0 a b FIG. 1. Energy level structure of the displaced oscillator (a) and electronic dimer (b) and corresponding linear absorption spectra.by two electronic states, |g and |e , which are coupled to a one-dimensional nuclear coordinate q. We denote the model by a displaced oscillator (DO) system (Fig. 1a). Taking =1, the vibronic potentia...

Coherent dynamics of coupled molecules are effectively characterized by the two-dimensional (2D) electronic coherent spectroscopy. Depending on the coupling between electronic and vibrational states, oscillating signals of purely electronic, purely vibrational or mixed character can be observed with the help of oscillation maps, constructed from the time-resolved 2D spectra. The amplitude of the beatings caused by the electronic coherence is heavily affected by the energetic disorder and consequently the electronic coherences are quickly dephased. Beatings with the vibrational character weakly depend on the disorder, assuring their long-time survival. We show that detailed modeling of 2D spectroscopy signals of molecular aggregates provides direct information on the origin of the coherent beatings.

A general theory of electronic excitations in aggregates of molecules coupled to intramolecular vibrations and the harmonic environment is developed for simulation of the third-order nonlinear spectroscopy signals. It is applied in studies of the time-resolved two-dimensional coherent spectra of four characteristic model systems: weakly/strongly vibronically coupled molecular dimers interacting with high/low frequency intramolecular vibrations. The results allow us to (i) classify and define the typical spectroscopic features of vibronically coupled molecules, (ii) separate the cases, when the long-lived quantum coherences due to vibrational lifetime borrowing should be expected, (iii) define when the complete exciton-vibrational mixing occurs, and (iv) when separation of excitonic and vibrational coherences is possible.

Quantum beats in nonlinear spectroscopy of molecular aggregates are often attributed to electronic phenomena of excitonic systems, while nuclear degrees of freedom are commonly included into models as overdamped oscillations of bath constituents responsible for dephasing. However, molecular systems are coupled to various high-frequency molecular vibrations, which can cause the spectral beats hardly distinguishable from those created by purely electronic coherences. Models containing damped, undamped, and overdamped vibrational modes coupled to an electronic molecular transition are discussed in this paper in context of linear absorption and two-dimensional electronic spectroscopy. Analysis of different types of bath models demonstrates how do vibrations map onto two-dimensional spectra and how the damping strength of the coherent vibrational modes can be resolved from spectroscopic signals.

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Light-harvesting in fucoxanthin-chlorophyll protein (FCP) of diatoms is performed by a cluster of chromophores: chlorophylls a (Chl a), chlorophylls c 2 (Chl c 2 ), and carotenoids fucoxanthins. It is well-known that energy captured by fucoxanthin is transferred to Chl a on a subpicosecond time scale. However, the energy flow channel connecting Chl c 2 and Chl a remained elusive. In this study, the energy transfer between Chl c 2 and Chl a molecules in the FCP complex from the diatom algae C. meneghiniana at room temperature is investigated using pump−probe and coherent two-dimensional electronic spectroscopy. Measured dynamics of the absorption band associated with the Q y transition of the Chl c 2 reveals an ultrafast energy transfer pathway to Chl a. This conclusion is supported by the theoretical simulations based on the effective oscillator model. SECTION: Energy Conversion and Storage; Energy and Charge Transport

The role of quantum coherence in photochemical functions of molecular systems such as photosynthetic complexes is a broadly debated topic. Coexistence and intermixing of electronic and vibrational coherences has been proposed to be responsible for the observed long-lived coherences and high energy transfer efficiency. However, clear experimental evidence of coherences with different origins operating at the same time has been elusive. In this work, multidimensional spectra obtained from a six-porphyrin nanoring system are analyzed in detail with support from theoretical modeling. We uncover a great diversity of separable electronic, vibrational, and mixed coherences and show their cooperation in shaping the spectroscopic response. The results permit direct assignment of electronic and vibronic states and characterization of the excitation dynamics. The clear disentanglement of coherences in molecules with extended π-conjugation opens up new avenues for exploring coherent phenomena and understanding their importance for the function of complex systems.