Two-dimensional spectroscopy of a molecular dimer unveils the effects of vibronic coupling on exciton coherences (original) (raw)
Related papers
Scientific Reports, 2013
A vibronic-exciton model is applied to investigate the recently proposed mechanism of enhancement of coherent oscillations due to mixing of electronic and nuclear degrees of freedom. We study a dimer system to elucidate the role of resonance coupling, site energies, vibrational frequency and energy disorder in the enhancement of vibronic-exciton and ground-state vibrational coherences, and to identify regimes where this enhancement is significant. For a heterodimer representing two coupled bachteriochloropylls of the FMO complex, long-lived vibronic coherences are found to be generated only when the frequency of the mode is in the vicinity of the electronic energy difference. Although the vibronic-exciton coherences exhibit a larger initial amplitude compared to the ground-state vibrational coherences, we conclude that, due to the dephasing of the former, both type of coherences have a similar magnitude at longer population time.
Vibronic resonances sustain excited state coherence in light harvesting proteins at room temperature
Until recently it was believed that photosynthesis, a fundamental process for life on earth, could be fully understood with semi-classical models. However, puzzling quantum phenomena have been observed in several photosynthetic pigment-protein complexes, prompting questions regarding the nature and role of these effects. Recent attention has focused on discrete vibrational modes that are resonant or quasi-resonant with excitonic energy splittings and strongly coupled to these excitonic states. Here we report a series of experiments that unambiguously identify excited state coherent superpositions that dephase on the timescale of the excited state lifetime. Low energy (56 cm -1 ) oscillations on the signal intensity provide direct experimental evidence for the role of vibrational modes resonant with excitonic splittings in sustaining coherences involving different excited excitonic states at physiological temperature.
Nature Physics, 2013
Recent observations of oscillatory features in the optical response of photosynthetic complexes have revealed evidence for surprisingly long-lasting electronic coherences which can coexist with energy transport. These observations have ignited multidisciplinary interest in the role of quantum effects in biological systems, including the fundamental question of how electronic coherence can survive in biological surroundings. Here we show that the non-trivial spectral structures of protein fluctuations can generate non-equilibrium processes that lead to the spontaneous creation and sustenance of electronic coherence, even at physiological temperatures. Developing new advanced simulation tools to treat these effects, we provide a firm microscopic basis to successfully reproduce the experimentally observed coherence times in the Fenna-Matthews-Olson complex, and illustrate how detailed quantum modelling and simulation can shed further light on a wide range of other non-equilibrium processes which may be important in different photosynthetic systems.
The journal of physical chemistry letters, 2015
Until recently it was believed that photosynthesis, a fundamental process for life on earth, could be fully understood with semi-classical models. However, puzzling quantum phenomena have been observed in several photosynthetic pigment-protein complexes, prompting questions regarding the nature and role of these effects. Recent attention has focused on discrete vibrational modes that are resonant or quasi-resonant with excitonic energy splittings and strongly coupled to these excitonic states. Here we unambiguously identify excited state coherent superpositions in photosynthetic light-harvesting complexes using a new experimental approach. Decoherence on the timescale of the excited state lifetime allows low energy (56 cm-1) oscillations on the signal intensity to be observed. In conjunction with an appropriate model, these oscillations provide clear and direct experimental evidence that the persistent coherences observed originate from quantum superpositions among vibronic excited ...
Electronic coherence transfer in photosynthetic complexes and its signatures in optical spectroscopy
Spectroscopy-an International Journal, 2008
Effects of electronic coherence transfer after photoexcitation of excitonic complexes and their manifestation in optical spectroscopy are discussed. A general excitonic model Hamiltonian is considered in detail to elucidate the origin of energy relaxation in excitonic complexes. We suggest that the second-order quantum master equation for the reduced density matrix of electronic degrees of freedom provides the most suitable theoretical framework for the study of coherence transfer in photosynthetic bacteriochlorophyll complexes. Temperature dependence of the absorption band maximum of a simple excitonic dimer is interpreted in terms of coherence transfer between two excited states. The role of reorganization energy of the transitions in the magnitude of the effect is discussed. A large reorganization energy difference between the two states is found to induce significant band shift. The predictions of the theory are compared to experimental measurements of the bacterial reaction center absorption spectra of Rhodobacter sphaeroides. As an example of a time-dependent spectroscopic method sensitive to coherences and possibly to their transfer, we present recent two-dimensional photon echo measurements of energy relaxation in the so-called Fenna-Matthews-Olson complex of Chlorobium tepidum, where distinct oscillatory patters predicted to be signatures of electronic coherence have been observed.
Coherent Vibronic Coupling in Light-Harvesting Complexes from Photosynthetic Marine Algae
The Journal of Physical Chemistry Letters, 2012
Observations of long-lived coherences in photosynthetic lightharvesting complexes utilize short pulses with broad spectral bandwidths to coherently excite multiple transitions and coherent superpositions. In order to identify the role that such quantum effects might play in efficient energy transfer, however, an alternative approach is required. We have developed a technique for two-color photon echo spectroscopy to selectively excite the pathway of interest and measure its evolution in the absence of any other excitation. We use this technique to excite a coherence pathway in phycocyanin-645 from cryptophyte algae and measure the dynamics of this coherence. A decoherence time of 500 fs was measured, and clear signatures for strong coupling between the electronic states and phonon modes were observed, allowing coherent coupling between otherwise nonresonant transitions. This provides detailed experimental evidence of the long-lived coherences and the nature of the quantum mechanical interactions between electronic states and phonon modes in phycocyanin-645 from cryptophyte marine algae. SECTION: Kinetics, Spectroscopy T he role of quantum effects in photosynthesis has been a subject of great speculation since the first observations of long-lived coherences in light-harvesting complexes. 1−4 The initial theoretical models showed that quantum coherence, by itself, actually reduces the efficiency of energy transfer, but by including some dephasing, the combination of quantum tunnelling and noise leads to highly efficient energy transfer. 5−11 Part of the reason for this enhanced transfer efficiency is that the dephasing caused by a Markovian bath of phonon modes leads to spectral broadening, which brings otherwise nonresonant transitions into resonance. Subsequent studies looked at the effect of spatial correlations of the phonon interactions 12 and non-Markovian dynamics of the system− bath interactions. 13−15 The conclusions of these studies point to a regime where the separation of the system and bath is not clear, and it is necessary to include both electronic states and vibrational modes within the description of the system. The form of the interactions between vibrational and electronic states and assumptions made about the phonon spectral density have been shown to significantly alter the calculated dynamics. Experimental details of the interactions between electronic states and the vibrational/phonon modes of the chromophores and surrounding protein matrix remain lacking. Similarly, beyond the observation of long-lived coherent coupling between exciton states, any experimental evidence for the role of quantum effects in photosynthetic energy transfer remains elusive.
Exciton dynamics in photosynthetic complexes: excitation by coherent and incoherent light
New Journal of Physics, 2010
In this paper we consider dynamics of a molecular system subjected to external pumping by a light source. Within a completely quantum mechanical treatment, we derive a general formula, which enables to asses effects of different light properties on the photo-induced dynamics of a molecular system. We show that once the properties of light are known in terms of certain two-point correlation function, the only information needed to reconstruct the system dynamics is the reduced evolution superoperator. The later quantity is in principle accessible through ultrafast non-linear spectroscopy. Considering a direct excitation of a small molecular antenna by incoherent light we find that excitation of coherences is possible due to overlap of homogeneous line shapes associated with different excitonic states. In Markov and secular approximations, the amount of coherence is significant only under fast relaxation, and both the populations and coherences between exciton states become static at long time. We also study the case when the excitation of a photosynthetic complex is mediated by a mesoscopic system. We find that such case can be treated by the same formalism with a special correlation function characterizing ultrafast fluctuations of the mesoscopic system. We discuss bacterial chlorosom as an example of such a mesoscopic mediator and propose that the properties of energy transferring chromophore-protein complexes might be specially tuned for the fluctuation properties of their associated antennae.
Dynamic coherence in excitonic molecular complexes under various excitation conditions
Chemical Physics, 2014
We investigate the relevance of dynamic quantum coherence in the energy transfer efficiency of molecular aggregates. We contrast the dynamics after excitation of a quantum mechanical system with that of a classical system. We demonstrate how a classical description of an ensemble average can be satisfactorily interpreted either as a single system driven by a continuous force or as an ensemble of systems each driven by an impulsive force. We derive the time evolution of the density matrix for an open quantum system excited by light or by a neighboring antenna. We argue that unlike in the classical case, the quantum description does not allow for a formal decomposition of the dynamics into sudden jumps in the quantum mechanical state. Rather, there is a natural finite time-scale associated with the excitation process. We propose a simple experiment how to test the influence of this time scale on the yield of photosynthesis. Because photosynthesis is intrinsically an average process, the efficiency of photosynthesis can be assessed from the quantum mechanical expectation value calculated from the second-order response theory, which has the same validity as the perturbative description of ultrafast experiments. We demonstrate using typical parameters of the currently most studied photosynthetic antenna, the Fenna-Matthews-Olson (FMO) complex, and a typical energy transfer rate from the chlorosome baseplate, that dynamic coherences are averaged out in the complex despite excitation proceeding through a coherent superposition of its eigenstates. The dynamic coherence averages out even when the FMO model is completely free of all dissipation and dephasing. We conclude that under natural excitation conditions, coherent dynamics cannot be responsible for the remarkable efficiency of the photosynthesis, even when considering the dynamics at a single molecular level.
Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems
Nature, 2007
Photosynthetic complexes are exquisitely tuned to capture solar light efficiently, and then transmit the excitation energy to reaction centres, where long term energy storage is initiated. The energy transfer mechanism is often described by semiclassical models that invoke 'hopping' of excited-state populations along discrete energy levels 1,2 . Two-dimensional Fourier transform electronic spectroscopy 3-5 has mapped 6 these energy levels and their coupling in the Fenna-Matthews-Olson (FMO) bacteriochlorophyll complex, which is found in green sulphur bacteria and acts as an energy 'wire' connecting a large peripheral light-harvesting antenna, the chlorosome, to the reaction centre 7-9 . The spectroscopic data clearly document the dependence of the dominant energy transport pathways on the spatial properties of the excited-state wavefunctions of the whole bacteriochlorophyll complex 6,10 . But the intricate dynamics of quantum coherence, which has no classical analogue, was largely neglected in the analyses-even though electronic energy transfer involving oscillatory populations of donors and acceptors was first discussed more than 70 years ago 11 , and electronic quantum beats arising from quantum coherence in photosynthetic complexes have been predicted 12,13 and indirectly observed 14 . Here we extend previous twodimensional electronic spectroscopy investigations of the FMO bacteriochlorophyll complex, and obtain direct evidence for remarkably long-lived electronic quantum coherence playing an important part in energy transfer processes within this system. The quantum coherence manifests itself in characteristic, directly observable quantum beating signals among the excitons within the Chlorobium tepidum FMO complex at 77 K. This wavelike characteristic of the energy transfer within the photosynthetic complex can explain its extreme efficiency, in that it allows the complexes to sample vast areas of phase space to find the most efficient path.