Density Functional Theory Studies On The Optical Properties Of Organic And Organometallic Materials For Optoelectronics

This dissertation focuses on the application of density functional theory in the studies of molecular design and optical properties of the organic and organometallic optoelectronic materials and also the optimization and development of theoretical models employed. It is well-known that the functional optoelectronic materials, including small organic molecules, organometallic complexes, and self-assembly polymers, due to the excellent properties of photochromic, optical sensing, photoelectric conversion and nonlinear optical response, have attracted great attentions and been demonstrated broad potential applications in the photochemical field of novel display, transmission and storage of information, nonlinear optical switches and new energy."Structure determines performance", researchers also try to understand the relationship between the electronic structures and optical propertries. It can provide theoretical guidance for the development of optical materials with better performance.As the density functional theory (DFT) and its various functionals undergoing continuous development and the computer powder increasingly improved, DFT has become a promising calculation tool. However, it inevitably leads to an issue:are the practically applied theoretical models and methods reliable enough to calculate? This is also an important challenge and innovation point in this dissertation. We simulate the geometries and various optical properties of optoelectronic molecules and find good agreement with the experimental results (Chapter2and3). Based on this, we also systematically study the effect of various forms of density functionals on the calculation of the optical properties, and further raise the concept of optimal "tuning". The results show that the optimal "tuned" range separated functionals have great compatibility. It can achieve high accuracy on the calculations of various optical parameters of properties for most types of molecules (Chapter4-6). The conventional functionals (B3LYP and PBE) always lead to significant error because of the incorrect asymptotic behavior, significant delocalization error and lack of derivative discontinuity.The dissertation consists of seven chapters and the content is as follows.In Chpater1, a brief introduction to the theoretical and computational chemistry is firstly given, especially for the development of models of density functional theory (DFT). And then we review the advantages and shortcomings of DFT and introduce the concept of optimal "tuning". In addition, the related linear and nonlinear optical theories as well as the corresponding quantum calculations are also reviewed. The significance and advantages of organic and organometallic materials for optoelectronics are following introduced. Evently, we introduce the main work and innovation point of this dissertation.In Chapter2, the molecular geometries, electronic properties and second-order nonlinearities of a series of mono-and binuclear chromium carbazole complexes were studied by DFT and Time-Depedent DFT (TDDFT). The results show that the TDDFT can well-stimulate the strong metal to ligand charge transfer (’MLCT) spectra band, indicating significant charge-transfer interaction between the metal centers through the carbazole ligand bridge. The further theoretical study on the MLCT with different geometries will be helpful to develop nonlinear optical materials with large static first hyperpolarizabilities.Chapter3reports three novel organic photochromic spirooxazine derivatives, incorporating increasing numbers of thiophene units by vinyl bridges. We perform DFT and TDDFT calculations on the electronic absorption spectra and second-order nonlinear response. The results show that these compounds exhibit tunable photochromic properties that are sensitive to low-energy visible light. Theoretical calculations are performed to rationalize the "on and off switching nonlinear optical behaviors of compounds, showing the potential for applying the materials in nonlinear optical switches. This work also make up for the vacancy of the theoretical research on switching behavior of the second-order nonlinear response.Chapter4focuses on the numerical investigation of the extent of the delocalization error (DE) in Donor/Acceptor systems with π-conjugated bridges of various types and lengths. Further, the correlation of the magnitude of the DE with errors in calculated static first hyperpolarizabilities as well as in CT excitation energies is studied. Optimally tuned functionals incorporating range-separated exchange (RSE) can not only improve the accuracy of the calculations of charge-transfer excitation energies, but also greatly reduce the DE and produce reasonable magnitudes of static first hyperpolarizabilities.Chapter5investigates three different types of calculated energy gaps, including the optical gap△EO, fundamental gap△EF and orbital energy gap△ε), on the three traditional1D conjugated polymer (Polythiophene, PolyEDOT, and Polyfuran) by various functionals. Successful applications of optimal "tuned" RSE functional for the calculation of optical gaps, fundamental gaps, and electron attachment/detachment energies are demonstrated.Chapter6firstly raises a criterion to assess charge-transfer (CT) or CT-like character of electronic excitations for organic chromophores. A generalized form of the criterion amounts to taking the integral of the absolute value (modulus) of the electron density change upon excitation. This work has important significance on the selection of density functionals on the CT issues. The summary of theoretieal studies on the above organic and organometallic materials and the related outlook are presented in Chapter7.