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.