Photosynthetic light-harvesting complexes capture sunlight and subsequently transfer the associated energy as electronic excitations to reaction centres where charge separation and charge transfer takes place. Experimental studies suggest that this energy transfer process is not always fully described by classical probability laws but instead can involve quantum-coherent evolution of electronic excitations. There is also evidence that suggests the possibility of coherent dynamics in the charge transfer step following energy transfer. Our group is interested in understanding the basic transfer mechanisms and implications of coherence during these primary steps in photosynthesis. To this aim we develop and apply theoretical frameworks to describe the processes of energy and charge transfer beyond the incoherent transfer limits of Förster (energy transfer) or Marcus (charge transfer) theories. In particular, we follow the approach of open quantum system, where the quantum system of interest is usually the electronic degrees of freedom (and possibly a few vibrational modes) and the bath represents all other degrees of freedom, mainly vibrations due to the protein and solvent environment of bio-molecules. Within this theoretical approach, we focus on investigating non-classical features of vibrational and electronic dynamics in biomolecules and their relation to the photosynthetic function.
- Electronic excitation dynamics in multichromophoric systems described via a polaron-representation master equation. Avinash Kolli, Ahsan Nazir and Alexandra Olaya-Castro. J. Chem. Phys. 135 154112 (2011) (http://jcp.aip.org/resource/1/jcpsa6/v135/i15/p154112_s1)
- Quantum state tuning of energy transfer in a correlated environment. Francesca Fassioli, Ahsan Nazir and Alexandra Olaya-Castro. J. Phys. Chem. Lett. 1 2139 (2010) (http://pubs.acs.org/doi/abs/10.1021/jz100717d)
This strand of research is focused on the possible ability of bio-molecules to exhibit and potentially utilize, non-classical phenomena to aid their performance. The presence of long-lived electronic (excitonic) coherences has been proposed as mechanism to improve the efficiency of excitation energy transfer in light harvesting systems. Our research aims to establish the possible relationship between functionality and the presence of quantum mechanical coherences. In this direction we are interested in answering several related questions. Among these, is what is the relevance of coherent dynamics under in vivo conditions of illumination (sunlight) as opposed to laboratory excitation conditions? We are also investigating how quantum features of energy transfer may alter as the spectroscopic properties of light-harvesting complexes acclimate to different light-illumination conditions. We are also studying the fundamental role that quantized vibrations strongly coupled to electronic degrees of freedom, typical of many biomolecules, play in determining their electronic dynamics. Broadening the investigation of non-classicality beyond the electronic degrees of freedom to include these vibrations may aid in establishing a relation between non-classicality and its functional role.
- Non-classicality of molecular vibrations activating electronic dynamics at room temperature, Edward J. O'Reilly and Alexandra Olaya-Castro. Nature Communications (2014) (http://www.nature.com/ncomms/2014/140109/ncomms4012/full/ncomms4012.html)
- The fundamental role of quantized vibrations in coherent light harvesting by cryptophyte algae, Avinash Kolli, Edward J. O'Reilly, Gregory D. Scholes and Alexandra Olaya-Castro. J. Chem. Phys. 137, 174109 (2012) (http://dx.doi.org/10.1063/1.4764100)
- F. Fassioli, A. Olaya-Castro, Distribution of entanglement in light-harvesting complexes and their quantum efficiency, New Journal of Physics, 12, 8, 085006 (2010) (http://iopscience.iop.org/1367-2630/12/8/085006)
The counting statistics of exciton and charge transfer can reveal detailed information about the underlying processes in a system. It can even give us information about whether quantum mechanical effects such as coherence are affecting the transport. This technique can be applied to energy transfer in light-harvesting antennae and charge separation in the photosynthetic reaction centre in order to unveil how these systems utilise quantum effects and whether they increase the efficiency of the transport processes. We are interested in developing experimental proposals that use counting statistics to unveil quantum features in transport dynamics without the interference of ultra-fast laser pulses, for example by studying the current through a reaction centre.
The interaction between electronic and vibrational degrees of freedom in biomolecular systems such as pigment-protein complexes has a rich structure as a function of energy. Here we investigate how such structure may have important implications for processes storage and extraction of energy.