Final Report Summary - PHOTPROT (The Dynamic Protein Matrix in Photosynthesis: From Disorder to Life.)
Design Principles for Efficient Solar Energy Conversion
Based on our work on photosynthetic light-harvesting antennae and photosynthetic reaction centers we present four design principles with the aim of providing a guide for the design and construction of robust and efficient human-made energy conversion systems based on abundant materials [Romero et al., Nature 543, 355–365 (16 March 2017) doi:10.1038/nature22012].
1. Mixing of Excitons with Charge-Transfer States
To compete with back transfers the charge separation event (starting from a high-energy exciton state with subsequent energy localization within a low-energy CT state) must be as fast as possible. The quasi-classical energy hopping between non-mixed excitons and CT states is relatively slow, slow enough to allow back transfer to the antenna. In principle, a quantum superposition (or mixing) of the two states can create much faster transfer from exciton to CT.
2. Resonant vibrations
Exciton-CT mixing can be dramatically enhanced in the presence of an intra-molecular vibrational quantum with an energy close to the energy gap between exciton and CT states. The involvement of a resonant vibration promotes the exciton-CT interaction due to the mixing of the electronic zero-phonon origin of the exciton with a vibrational sublevel of the CT state, thus creating the non-classical penetration of the exciton into the CT potential, speeding up charge transfer
3. Multiple Charge-Separation Pathways controlled by the Smart Protein Matrix
The protein conformation-induced modulation of the energy gaps between states which creates and destroys exciton-CT mixing allows the switching between different CS pathways. In this manner, the possibility of multiple pathways increases the probability to have at least one effective channel of charge transfer within a disordered system providing functional flexibility and avoiding trapping of excitation in unproductive states. Note that the coherent exciton-CT mixing assisted by vibrations occurs within eigenstates (inner coherence), and therefore, it increases the rate of CS along a particular direction (corresponding to a particular pathway) not only upon coherent excitation, but also in natural sunlight.
4. Control of Exciton-Vibrational Coherence by the Smart Protein Matrix
The exciton-CT mixing controlled by the smart protein matrix promotes the fast and efficient formation of the CT state, however, this formation must be irreversible to avoid back transfer to the light-harvesting system. Therefore, both initial coherence and subsequent decoherence (or dephasing) are needed for efficient charge transfer. The protein matrix is characterized by a rich manifold of vibrational modes, it utilizes some of them to create the fast coherent population of the CT, whereas the remaining modes (acting as the bath) induce dephasing which stabilizes the primary CT photoproduct. In the bacterial RC a special mode was found to stabilize the photoproduct, i.e. the 30 cm-1 mode that has been assigned to the rotation of a water molecule located between the special pair (P) and the accessory BChl (BA). In addition, other modes could also contribute to the dephasing and reorganization within the CT potential. In the PSII RC, no specific modes responsible for the dephasing were found. Most probably the dephasing and dynamic localization within the primary CT state are a result of the simultaneous action of many phonon/vibrational modes. We propose that the coupling of specific vibrations to electronic states, both for coherence and decoherence, may be a general strategy nature has employed to selectively drive an electronic process and provide directionality
Based on our work on photosynthetic light-harvesting antennae and photosynthetic reaction centers we present four design principles with the aim of providing a guide for the design and construction of robust and efficient human-made energy conversion systems based on abundant materials [Romero et al., Nature 543, 355–365 (16 March 2017) doi:10.1038/nature22012].
1. Mixing of Excitons with Charge-Transfer States
To compete with back transfers the charge separation event (starting from a high-energy exciton state with subsequent energy localization within a low-energy CT state) must be as fast as possible. The quasi-classical energy hopping between non-mixed excitons and CT states is relatively slow, slow enough to allow back transfer to the antenna. In principle, a quantum superposition (or mixing) of the two states can create much faster transfer from exciton to CT.
2. Resonant vibrations
Exciton-CT mixing can be dramatically enhanced in the presence of an intra-molecular vibrational quantum with an energy close to the energy gap between exciton and CT states. The involvement of a resonant vibration promotes the exciton-CT interaction due to the mixing of the electronic zero-phonon origin of the exciton with a vibrational sublevel of the CT state, thus creating the non-classical penetration of the exciton into the CT potential, speeding up charge transfer
3. Multiple Charge-Separation Pathways controlled by the Smart Protein Matrix
The protein conformation-induced modulation of the energy gaps between states which creates and destroys exciton-CT mixing allows the switching between different CS pathways. In this manner, the possibility of multiple pathways increases the probability to have at least one effective channel of charge transfer within a disordered system providing functional flexibility and avoiding trapping of excitation in unproductive states. Note that the coherent exciton-CT mixing assisted by vibrations occurs within eigenstates (inner coherence), and therefore, it increases the rate of CS along a particular direction (corresponding to a particular pathway) not only upon coherent excitation, but also in natural sunlight.
4. Control of Exciton-Vibrational Coherence by the Smart Protein Matrix
The exciton-CT mixing controlled by the smart protein matrix promotes the fast and efficient formation of the CT state, however, this formation must be irreversible to avoid back transfer to the light-harvesting system. Therefore, both initial coherence and subsequent decoherence (or dephasing) are needed for efficient charge transfer. The protein matrix is characterized by a rich manifold of vibrational modes, it utilizes some of them to create the fast coherent population of the CT, whereas the remaining modes (acting as the bath) induce dephasing which stabilizes the primary CT photoproduct. In the bacterial RC a special mode was found to stabilize the photoproduct, i.e. the 30 cm-1 mode that has been assigned to the rotation of a water molecule located between the special pair (P) and the accessory BChl (BA). In addition, other modes could also contribute to the dephasing and reorganization within the CT potential. In the PSII RC, no specific modes responsible for the dephasing were found. Most probably the dephasing and dynamic localization within the primary CT state are a result of the simultaneous action of many phonon/vibrational modes. We propose that the coupling of specific vibrations to electronic states, both for coherence and decoherence, may be a general strategy nature has employed to selectively drive an electronic process and provide directionality