The Theoretical Study Of The Electronic Structure And Spectral Properties Of Functional Materials

This thesis covers my major research work during the period of my PhD study.We investigate the structure-property relationship of the interesting functional materi-als such as, exible organicπ-conjugated polymers, light-harvesting antenna systemand the dye-sensitized solar cells by using the Density Functional Theory (DFT)/theTime-Dependent Density Functional Theory (TDDFT) with the reasonable Exchange-Correlation functionals and other theoretical models. Their electronic, optical spectralproperties and other interesting characters have been investigated theoretically and sys-tematically.These study objects, which have larger size than single molecules, usually pos-sess supermolecular structures and are multi-scale complex. They have been well or-ganized in structure arrangement to realize one or various functions. The task of thetheoretical simulation is to describe the essential phenomena and processes occurredin these molecular aggregates. We can study the nature of self-organization which isdetermined by the molecular interaction, the connection between microcosmic config-uration and macroscopical properties, and correlative rules of catalysis and transfer.The multi-scale theoretical approaches have to be used unavoidably in investigatingthese multi-scale complexes.DFT possesses excellent performance on the description of geometrical structuresand structure-property relationship of single molecules. It requires low computer re-sources and can gives highly accurate results. As a result, we utilize first-principlemolecular quantum mechanics methods to describe the small molecules exactly. Forexample, we adopt TDDFT and TDHF theory to calculate the electronic absorptionspectra of oligouorenes(OFs)and then combine TDDFT with correlation func-tion method to calculate their vibrationally-resolved optical absorption and emissionspectra. For molecular aggregates, especially when the complex contain the big-sizechromophores, both the wavefunction-based Ab initio quantum chemistry methods andDFT are expensive. The alternative choices are to adopt the parameterized effectivemodel Hamiltonian. For example, in this thesis we have employed Frenkel excitonmodel to simulate the electronic absorption spectra and circular dichroism spectra ofLH2 complex. The microcosmic parameters of effective Hamiltonian are accuratelygained by first-principles DFT. The main contents of this thesis are summarized up in the following.1)We present systematic, theoretical investigations on structure-property corre-lations in polyuorenes (PFs) derived mainly from the chain morphology, oligomerlength, and chemical substitutent. Both the vertical absorptions and the vibrationalcontributions to electronic absorption and uorescence spectra have been calculated. Itis found that the vibronic (electronic and vibrational) structures of PFs are morphology-dependent.β-phase oligouorenes and ladder-type poly(p-phenylene) (LPPP) oligomersshow a red shift compared to the spectra ofα(FL)n. The asymmetry of the absorp-tion and uorescence spectra inα(FL)n and the uorenone (FLO) defect oligou-orenesα(FL)nm(FLO)m is significantly more pronounced than that inβ-phaseand LPPP oligomers. By properly taking into account the anharmonic torsion poten-tials resulting from the strong electronic and nuclear coupling in the oligouorenes, wehave reasonably reproduced the experimentally observed spectroscopic features.2)For LH2 complex, we combined the Frenkel exciton model and the long-rangecorrected (LRC) DFT to study the exction coupling, energy transfer and optical spec-tra theoretically. The LRC DFT can describe the low-lying Qy and Qx excitations ofBChl accurately. The study of the typical dimer of BChls indicate that the Frenkelmodel dominant the energy transfer manner among LH2. The experimentally observedelectronic absorption spectra and circular dichroism spectra of LH2 are reasonably re-produced.3)The optical spectra of dyes and the complex of dye-TiO2 have been system-atically investigated by using TDDFT. Significant charge transfer characteristics hasbeen visualized in dipole-allowed excited state of Ru-pyridine complex. By usingLRC-PBE (ω= 0.30 a0 1) functional, we have achieved a satisfied consistent withthe experimentally-measured spectra. Through appropriate analysis, we found that theHTSs adsorption, which two carboxylic groups residing on the same bipyridine ligand,is more beneficial to electron injecting than HMSs adsorption, which two carboxylicgroups coming from different bipyridine ligand.