Development of an Exciton Model of Light-Harvesting Antenna LH2 Considering Charge Transfer States and Their Fluctuations

The light-harvesting complex 2 (LH2) of the purple bacteria absorbs sunlight and transfers energy to the RC with high efficiency. To achieve a microscopic understanding of the energy transfer process, theoretical analysis of the excited states of the bacteriochlorophyll <i>a</i> pigment aggregates is effective. In particular, not only the locally excited (LE) states of pigments but also the charge transfer (CT) states between pigments play a significant role in the excited states of the B850 ring, where the pigments are in close proximity to each other. We developed a new method to incorporate the fluctuations of such CT states into the exciton model of LH2, based on quantum chemical calculations and molecular dynamics (MD) simulations, with the aid of a machine learning model. The excitation energies to LE and CT states and the electronic couplings between these states were calculated from the MD trajectory with a low computational cost to obtain a time-dependent exciton Hamiltonian. The calculated absorption spectra of LH2 using the exciton Hamiltonian revealed that fluctuations of the CT states play an important role in the broadening of the B850 absorption band.