Spectral Densities Of Molecular Crystal Of Tetracene Calculated By Trajectory-based Approaches And Harmonic Oscillator Model

To describe the photosynthesis sysytem,organic light-emitting and photovoltaic mate-rials and many other systems,one has to treat the molecular aggregates or organic molecules crystals or films.For those systems,it is impossible to use molecular quantum mechanics or quantum chemical method to describe them due to their considerable sizes.To describe the photophysical properties of molecular aggregates and the light-induced electron transfer and energy transfer processes in the materials,the effective model Hamiltonian was usually used.When the intermolecular distance is large or the intermolecular interaction is weak,one can use the Frenkel exciton model.When the intermolecular distance is close,the inter-molecular electron exchange effect can not be neglected.Model Hamiltonian should include the delocalized intermolecular excitation,such as Anderson-News Hamiltonian used to de-scribe the one-dimensional accumulations.Constructing an effective model Hamiltonian requires many microscopic physical quantities such as exciton energy,exciton-exciton cou-pling intensity,exciton-phonon coupling,exciton-environment coupling,and so on,which were usually obtained by fitting the experimental spectrum.This fitting method is very much dependent on the experimental measurement.Besides,one can obtain different set-s of parameters by fitting to one single experiment.The non-uniqueness and uncertainty keep us away from an understanding of the microscopic mechanism.Furthermore,for some new materials,there exist no experimental data,the fitting method can not be used to pre-dict the experimental results.Therefore,in order to achieve the in-depth understanding to the experimental phenomenon and micro-process and to give a substantial theoretical basis for multi-scale model,it is essential to develop and apply the effective quantum chemical methods to calculate these micro-physical quantities.Electron-phonon coupling represents the interaction between electrons and nuclear motions,which is an important characteristic quantity to describe organic materials.Usually the strength of electron-phonon coupling is represented by the function of spectral density,and spectral density is closely related to re-organization energy.The reorganization energy in organic materials is closely related to the organic molecular structural change induced by the internal and external interaction,which affects the transport properties of organic molecular crystals.In this work,the tetracene molecular crystal was used as the research object,and the spectral density of diagonal and nondiagonal electron-phonon coupling were studied theoretically.The paper is divided into two parts:The first part introduces the basic concepts and theoretical methods.The first chapter mainly introduces the history and properties of organic molecular crystal;the second chapter introduces the theoretical basis of quantum chemistry involved in theoretical calculations,including Hartree-Fock theory,DFT,TDDFT,Molecular Dynamics and QM/MM.The second part mainly presents the calculated spectral densities of exciton-phonon(e-p)interaction for molecular crystal of tetracene by the trajectory-based(TB)approaches and the shifted harmonic oscillator(SHO)model.The influence of force fields and quan-tum chemistry calculation methods on the spectral density calculated by TB approaches is checked.The shifted harmonic oscillator(SHO)model performed the geometries optimiza-tion and vibrational frequencies of tetracene molecule embedded in its crystal,and obtained the reorganization energy and spectral densities by fitting.Compared with the benchmark results from the SHO model based on QM/MM or QM/MMpol calculations,the TB approaches always overestimate the e-p coupling of in-tramolecular high-frequency regime,and they predict the strongest e-p coupling locates at above 1500 cm-1 while the SHO model predicts it locates at around 1400 cm-1.Neither the classical force fields nor ab initio MD simulation can accurately describe the high-frequency e-p coupling.Furthermore,the spectral densities of the excitonic coupling fluctuation for three different dimers are presented.It is found that they continuously distribute in the range of 0?150 cm-1 and are 2-3 order of magnitude smaller than the maxima of the e-p coupling strength.The tilted dimers in tetracene crystal have larger e-p coupling than the parallel dimers.