Electronic Structure Studies Of Conjugated Polymers

Conjugated polymer as a new type of(photo)electronic materials,is characterized with both optical and electronic semiconductor properties.Different polymers can be used as the electron donor(D)or acceptor(A),then the constructed D-A pair will complete the photoelectric conversion process in a polymer solar cell(PSC).In recent years,some progresses have been achieved in experiment,through the molecular design and device structure optimization,the power conversion efficiency(PCE)of PSCs have been achieved to 11%or more.However,due to some limitations of experiment,such as the difficulties in synthesis and characterization of polymers,the photoelectric conversion mechanism in PSCs is not clear,which seriously affects the further rapid increasing PCE of PSC.On the orther hand,the theoretical calculations have been developed for a longer time,and the application on small organic crystal have been relatively mature.Furthermore,new quantum chemical method should be developed to reveal the relationship between the geometrical structure and the electronic structure of the complicated polymers,as well as the mechanism of the photoelectric conversion(such as coherent or non-coherent energy transfer),which can provide a theoretical reference for increasing PCE of the PSCs.How to provide electronic structure information of such complex large(macro)molecules from basic quantum chemical calculations is still one of the most important scientific frontier problems.The current theoretical problem is how to quantitatively describe the electronic structure of polymers?And how to quantitatively construct the exciton model used to study the energy transfer process in multi-chromophore systems?This is the main problem solved in this thesis.In this thesis,the first part of the work is the construction of excitonic diabatic basis and the calculation of intramolecular excitonic coupling by a new fragmentation-based quantum chemical method.In the second part,the differences in the electronic structure of one-dimensional and two-dimensional polymers consisted of three common organic structural units are systematically studied by density functional theory.In the third part,two important mechanochemical reactions of conjugated oligomer and metal-ligand complexes under external force stretching have been studied by density functional theory.In the last part,the mechanism of ultra-high electrochemical capacity of nitrogen-rich porous graphene as lithium battery anode material has been studied by means of quantum chemical calculations combined with genetic algorithm.Our results are summarized as follows:(1)A Fragmentation-Based Approach for Evaluating the Intra-Chain Excitonic Couplings in Conjugated PolymersIn this chapter,we propose the fragmentation-based approach for the first time by constructing the diabatic basis of the model PPV chain through "bottom-up" method.We construct the method without relying on the experimental parameters and the whole system calculation,which can be expected to evaluate excitonic coupling on complicated large(macro)molecules.In addition,the contributions of the lowest four excited states and charge transfer excited states of each chromophore to the low-excited state of the whole PPV chain have been analyzed in detail.It finally shows that our scheme can be used to obtain the results compared to that of the quantum chemical calculations in the weak coupled system.In the strong coupled system,we need to consider the higher excited states of each chromophore to get more accurate excitonic coupling.(2)Electronic structure properties of two-dimensional ?conjugated polymersIn order to understand how ? conjugation extends in 2D material and quantitativly calculate and analyze the wave function information,firstly,three common building blocks(biphenyl,phthalocyanine,and styrene)are selected in this chapter to construct 1D and the corresponding 2D system through covalent bond.The HOMO-LUMO energy gap(HLG),frontier molecular orbital(FMO),Hirshfeld charge distribution,and reorganization energy are analyzed systematically after the geometry optimization with constrained plane.The structure-property relationships of the neutral and charged systems are systematically studied.The results show that:(1)2D porous systems have HLGs that decreases rapidly with system size,but differ from the ideal graphene characterized with zero HLG,and the FMOs are more delocalized.(2)The charge polarization effect of 2D systems are stronger than that of corresponding 1D systems,and it can be potentially good candidates for novel non-linear optical materials.(3)2D systems with big size can be potentially good bipolar semiconductors.(3)Mechanochemical reactions of conjugated oligomers Through theoretical calculations,combined with atomic force microscopy(AFM),we have studied two important mechanochemical reactions from the single molecular level to understand the mechanism of cis-trans isomerization and mechanical response of catechol-Fe3+.The main conclusions are as follows:1.Alkene chain isomerization is usually difficult and only found in a small number of highly conjugated systems.The Alkene chain we used in this study do not promote cis-to-trans isomerization by thermal activation,however,mechanical forces can be used to trigger this reaction.This indicates that the mechanical force has the potential to trigger a reaction that is banned in the thermal activation process.Through the theoretical simulations of the external force stretching process,we have carefully analyzed the change of activation energies and activation lengths.The results show that the model molecule can undergo a cis-to-trans isomerization process through a diradical transition state,but the opposite pathway is forbidden.2.Catechol-Fe3+ shows unique mechanical properties when applying external force on its gel state,but the mechanism is unknown experimentally.Uslly,the catechol-Fe3+ can form two complexes,namely,bis-and tris-complex in hydrogel.Through the theoretical simulations on the two compelxes,the results show that:the two complexes have different stretched displacement and force intensity when bond breaking,which lead the unique mechanical properties so that they can be used as ideal sacrificial bonds in high strength and toughness materials.Theoretical calculations can provide an understanding of the mechanical response of catechol-Fe3+ coordination bonds at molecular level,and the references for the rational design and regulation of the mechanical properties of synthetic materials in the future.(4)Mechanism of N-enriched Hierarchically Porous Carbon for Ultrahigh Electrochemical CapacityFirst,we designed a theoretical adsorption/desorption model of lithium clusters on the nitrogen-doped graphene.Then,using the ab initio molecular dynamics(AIMD)and the density functional theory(DFT)and genetic algorithm(GA),we systematically studied the lithium clusters in nitrogen-doped graphene with different inner-hole sizes.According to the change of the hole size for lithium adsorption,the number of adsorbed lithiumatoms(NLi)and the number of nitrogen atoms(NN)doped in the polymer is NLi/NN=12 in the vicinity of the hole size measured by experiment.Calculated lithium capacity is about 2297 mAhg'1 by the Faraday's law,which well explains the measured ultra-high electrochemical capacity(2722 mAhg-1)of the lithium battery.