Reaction and energy transfer dynamics in solutions and biological systems

We have been investigating excited state dynamics in condensed phases.

Reaction and energy transfer dynamics in solutions and biological systems
Quantitative Evaluation of Site Energies and Their Fluctuations of Pigments in the Fenna-Matthews-Olson Complex with an Efficient Method for Generating a Potential Energy Surface

We develop an efficient method to generate an accurate semi-global potential energy surface of a molecule in condensed phases with low computational cost. We apply the method to the analysis of the site energies and their fluctuations of bacteriochlorophyll (BChl) a pigments in the Fenna-Matthews-Olson (FMO) complex using the density functional properly describing the ground and excited states of BChl a in solutions in our previous work (J. Phys. Chem. B 2014, 118, 10906-10918). The errors of the potential energies calculated from the present and QM/MM methods are small: ~1 kcal/mol for both the ground and excited states. The calculated site energies are in good agreement with the experimentally fitted results. The calculated spectral density also agrees with the experimentally available data. The spectral densities of BChl 2 and BChl 5 are much larger than those of the other five sites. The present method is expected to provide new insights into the efficient excitation energy transfer in light-harvesting antennas.

Higashi & Saito, J.Chem.Theory Comput. (2016).
Theoretical Study on Excited States of Bacteriochlorophyll a in Solutions with Density Functional Assesment

The excited-state properties of bacteriochlorophyll (BChl) a in triethylamine, 1-propanol, and methanol are investigated with the time-dependent density functional theory by using the quantum mechanical and molecular mechanical reweighting free energy self-consistant field method. It is found that no prevalent density functionals can reproduce the experimental excited-state properties, i.e., the absorption and reorganization energies, of BChl a in the solutions. The parameter μ in the rangeseparated hybrid functional is therefore optimized to reproduce the differences of the absorption energies in the solutions. We examine the origin of the differences of the absorption energies in the solutions and find that sensitive balance between contributions of structural changes and solute−solvent interactions determines the differences. The accurate description of the excitation with the density functional with the adjusted parameter is therefore essential to the understanding of the excited-state properties of BChl a in proteins and also the mechanism of the photosynthetic systems.

Higashi, Kosugi, Hayashi, & Saito, J.Phys.Chem. B 118, 10906 (2014).
Excited state intramolecula proton transfer in solution
Direct Simulation of Excited-State Intramolecular Proton Transfer and Vibrational Coherence of 10-Hydroxybenzo[h]quinoline in Solution

We investigate an ultrafast excited-state intramolecular proton transfer (ESIPT) reaction and the subsequent coherent vibrational motion of 10-hydroxybenzo[h]quinoline in cyclohexane by the electronically embedded multi-configuration Shepard interpolation method, which enables us to generate the potential energy surface of the reaction effectively and thus carry out a direct excited-state dynamics simulation with low computational costs. The calculated time scale of the ESIPT and the frequencies and lifetimes of coherent motions are in good agreement with the experimental results. The present study reveals the coherent motions are caused by not only the proton transfer itself but also the backbone displacement induced by the ESIPT. We also discuss the effects of the solvent on the dynamics of the coherent vibrational modes.

Higashi & Saito, J.Phys.Chem.Lett. 2, 2366 (2011).