Quantum Chemical Simulation Of The Q_y Absorption Spectrum Of Zn Chlorin Aggregates For Artificial Photosynthesis

The chlorosomes in green photosynthetic bacteria are the largest light-harvesting antenna in the nature,which are composed of more than 200,000 of bacteriochlorophyll c,d,and e molecules.It can capture light energy in low light conditions,and transfers the excitation ener-gy to the photosynthetic reaction center through baseplate protein to carry out photosynthetic,so that the green photosynthetic bacteria may be found in the conditions of low light levels.Inspired by the study of chlorosomes in the nature,Tamiaki et al.synthesized the Zn-centered chlorin(Znchl)by replacing the Mg atom in the center of chlorophyll-a with Zn so as to simu-late the bacterial chlorophyll c(Bchl c),the main component of chlorosomes in the nature,and its optical properties were systematically studied.However,the underlying mechanisms are not clear in experiments.In this study,a theoretical model of the monomer as well as the possible aggregates were constructed based on Znchl.The Qy absorption spectra of Znchl monomer and its aggregates were simulated by using the time-dependent density functional theory combined with the clas-sical exciton theory.The effects of intermolecular coupling and solvent environment on molec-ular absorption spectra were explored as well,which provide a basis and reference for further exploration of the energy transfer of artificial light-harvesting antennas.This study includes the following aspects:1.Znchl monomer model.The density functional theory(DFT)and the time dependent density functional theory(TDDFT)were used to optimize the structure of the monomer model and calculate the excitation energy,respectively.By combining the implicit sol-vent model and the explicit solvent model with the optimized monomer structure model,the experimentally detected Qy absorption spectrum in solvent was well simulated.The importance of including the dispersion correction in the optimization of weakly interact-ing systems was verified,and the existence of THF ligand could significantly affect the structure of Znchl and finally its first excitation energy.This monomer model provided a basis for the construction of the structure of the Znchl aggregations and the simulation of absorption spectra.2.Aggregates of Znchl.The layered,spiral tubular,and tubular supermolecular aggregate structures of Znchl were constructed on the basis of the single molecule model of Znch-1.The classical exciton theory and TDDFT method were used to investigate the effects of different aggregate structures on the Qy absorption spectra,and the site energies and coupling strengths in the aggregates were also compared theoretically.The results in-dicated that it is very important to consider the implicit solvent effect in the calculation of the site energy,and the variation of the spectra of Znchl aggregates with the size of aggregates is related to the aggregation mode.The simulated Qy absorption spectrum of the layered aggregate is consistent with the experimental one,which provides a basis for the subsequent study of aggregate energy transfer in the future.Our results indicate that the J-aggregates along the z-direction due to intermolecular coordination bonds are the dominant factor in extending the Qy band of Znchl into the near infrared region.