Professor, Institute for Molecular Science
Essentially, a quantum system never can be regarded as an isolated system. Quantum systems are always in contact with “the outside world.” Hence their quantum natures are sometimes sustained and sometimes destroyed. In condensed phase molecular systems, especially, quantum systems are affected by a huge number of dynamic degrees of freedom, such as solvent molecules, amino acid residues in proteins, and so forth. Balance between robustness and fragility of the quantum natures may dramatically alter behaviours of chemical and biophysical dynamics. On the experimental side, it has become possible to explore molecular processes on a timescale down to femtoseconds by means of ultrashort laser pulses. This progress in spectroscopy has opened up real-time observation of dynamic processes in complex molecular systems and has provided a strong impetus to theoretical studies of real-time quantum dissipative dynamics.
Investigation on the primary steps of photosynthesis is an example of such efforts. Photosynthetic energy conversion starts with the absorption of a photon of sunlight by one of the light-harvesting pigments, followed by the transfer of electronic excitation energy to the reaction centre. Ultrashort timescales of photosynthetic energy transfer require that all the relevant timescales in the problem be self-consistently included in any physical model that attempts to elucidate the mechanisms. Ordinarily, photosynthetic energy transfer is discussed in terms of the mutual relation between magnitudes of reorganization energy characterizing pigment-protein coupling and excitonic coupling between pigments. In our work, however, the main stress fell on the fact that the nature of the energy transfer is also dominated by the mutual relation between two timescales, the protein reorganization time, and the inverse of the excitonic coupling. Considerations about the finite timescale effects of protein-induced fluctuation-dissipation led to the rigorous theoretical framework describing photosynthetic energy transfer. As a consequence, we revealed that the photosynthetic energy transfer process is well-optimized in the parameter region corresponding to nature by utilizing a fine balance between the quantum mechanical delocalizing effect and the protein-induced localizing effect of the electronic excitations. A series of our papers was not only well accepted in the community of photosynthetic research, but also stimulated a burst of activity among multidisciplinary communities, such as condensed phase chemical physics and quantum physics.