Density Matrix Renormalization Group Theory For Electronic Processes In Complex Conjugated Systems

The organic conjugated systems can exhibit rich electronic processes and are the essential part of organic light-emitting devices,organic photovoltaic devices,and organic field-effect transistors.The microscopic processes that occur,including the charge trans-port,the excited state energy transfer and relaxation,the photoinduced exciton separation et al,determine their macroscopic properties.In order to better explain the experimental phenomenon,it is necessary to understand the microscopic mechanism from the perspec-tives of electronic structure and molecular quantum dynamics.The theoretical challenge lies in the fact that in these microscopic processes,there are not only electron-electron correlations,but also electron-vibration correlations,which are essentially a complex quantum many body problem.Density matrix renormalization group?DMRG?theory is a numerical method for dealing with strongly correlated electrons developed from the field of theoretical physics.In recent years,there has been considerable development for DMRG in the field of theoretical chemistry.In this thesis,based on the static DMRG method and the time-dependent TD-DMRG method,we develop new methodologies and apply them to the intramolecular singlet fission problem in polymer and spectrum of molecular aggregate.The results are:1.We studied the excited state electronic structure of a novel intramolecular singlet fission system——the donor-acceptor type copolymer PBTDO1.Through several many-body wavefunction analysis approaches,the dark 2A_gstate was confirmed as a¹TT state,which is an intermediate state participating in the singlet fission process.This finding answered the debate on the role of the dark 2A_gstate.2.We combined DMRG with the perturbation theory and developed the DMRG-is PT method,which could improve the computational efficiency with little sacrifice of the accuracy.Based on the Pariser-Parr-Pople model,we studied the electronic structures of the S₀and T₁states of the acene system.The calculation results showed that even for acene with a length of 32 units,there also will be no singlet-triplet crossover of the ground state.3.Under the matrix product states/operators?MPS/MPO?framework,a TD-DMRG method suitable for long-range interaction Hamiltonian was developed,which can per-form high-accuracy dynamic evolution of wave function?zero temperature?and density matrix?finite temperature?.Based on the Frenkel-Holstein exciton model,the TD-DMRG method was used to successfully calculate the absorption and emission spectra of the molecular aggregates composed of 2?18 molecules at both zero temperature and finite temperature.Compared with the ML-MCTDH and n-particle approximation method,TD-DMRG was proved to be an accurate and robust method.We have further developed TD-DMRG/TD-Hartree hybrid method.Through the combination of the high and low accuracy methods——the strongly coupled modes treated by DMRG,and the weakly cou-pled modes treated by Hartree,all vibration modes can be considered simultaneously.The calculation results comfirmed that this method can more accurately describe the dynamic behavior in the condensed phase,which is very suitable for the non-perturbative quantum dynamics problem of multi-molecules with multi-modes.