Theoretical Investigations On The Electronic Structures And Properties Of Several Heterocyclic Molecules

Nucleic acid is an important kind of biomacromolecules, it has decisive effect on the progress of organic evolution. Base, such as uracil, is the basic unit of nucleic acid composition, its characteristics are the nature of many properties and biological behavior of nucleic acids. For example, the ground-state and excited state proton-transfer tautomerism, as one of the important properties of base molecules, plays an significant role in the genetic process. Studies have shown that the capability of bases and their proton-transfer isomers of paring with other bases will determine the genetic variation progress. So theoretical studies on properties of bases may help people to understand the life progress on the molecular level. Halo derivatives of uracil are paid particular attention among all series of derivatives of nucleic pyrimidine bases on their medical applications. 5-Halouracils have been demonstrated as potential anti-tumor, anti-bacterial and anti-viral agents. The theoretical investigations on structures and characters of uracil have been reported a lot, but there are not so much studies on 5- and 6-halouracils, especially on 6-halouracils. Studies on proton-transfer reactions of ground states of the bases and their derivatives were also reported that many. Studies on proton-transfer reactions of excited states is the ground work for understanding the mechanisms of cancer induced by DNA photodamage and mechanisms of light repair after DNA photodamage. The further theoretical investigations on the properties and characters of halouracils will provide strong reference for pharmacy-mechanism understanding.The functional materials have special physical, chemical and biological effects, which have been widely applied in many high-tech fields in the last several decades because of their characteristics and function such as electricity, magnetism, light and heat etc. They are foundation and guide for the national economy and social development. The achievement in designing and developing functional materials not only has greatly promoted the revolution of scientific technology in the last century, but also will act as the foundation of the development of the advanced scientific technology in the future. The research, development and application of the functional materials is paid most attention to by every country, it becomes the heat and key point for investigation and development of the new materials in the world wide. The design of optical materials is one of the most important parts of the design of functional materials; scientists are making efforts in studying on synthesis and basic photoelectric properties of new kind of high functional optical materials. The life-time and monochromaticity of optical materials are closely related with properties of excited states and luminescence mechanism. We can discuss the information of electric structures of excited states on the theoretical level to define the microscopic luminescence mechanism.The electronic excited states of molecules have higher energy and unsteady characteristics, which can easily emit the energy to recur the steady ground state in a very short time. So it is difficult for experiment to obtain reliable information about the excited states and transition states of molecules. Theoretical chemists use various electronic structure theories to looking for the method that can be applied in the calculations of relatively large molecules without increasing excess calculation amount and accurately predict excited-state electronic structures. Up to now, the density functional theory (DFT), time-depended density functional theory (TD-DFT) and single excitation configuration interaction (CIS) have been widely used to treat the electronic excited states of large molecular systems.As the special rigid structure and molecular features of 3,6,9-bit easily being functional modified, carbazole is a kind of a good hole conduction molecule. The carbazole derivatives are often used for highly stabled hole transport materials in the field of electroluminescence. Such kind of compounds generally have a larger band gap and lower HOMO energy level, and such a large conjugated planar molecular structure is conducive to the transmission of current carrier. They also have a significant advantage of easily to be chemically modified. The environmental stability is very good owing to their lower HOMO energy level. The HOMO, LUMO energies and excited-state energies of carbazole main chain polymers can be adjusted by changing their main body connection pattern. Therefore, based on the previous theoretical and experimental work, we systematically theoretical studied five indolocarbazoles and fourteen bicarbazole isomers, the results are hoped to be theoretical support on materials design and synthesis in the future.There are five chapters in this paper. In Chapter 1, we made a brief introduction of the concept of electron affinity, a summary of optical materials, the development of theories and the theoretical and practical meaning of this study. In Chapter 2, an overview of theoretical methods including molecular orbital theory, density functional theory, correlation theories of excited stats and photochemistry, basic methods of structure optimization and solvent effect was given. We carried on the theoretical investigations on the electronic structures and properties of three kinds of heterocyclic molecules in the later three chapters. The main results are as follows:1, The molecular structures and electron affinities of 5- and 6-halouracils (XU, X=F, Cl, Br) were determined using five different density functional methods. The basis set used in this work is of double-ζplus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. The geometries are fully optimized with each density functional theory (DFT) method, and discussed. Three different types of the neutral-anion energy separations reported in this work are the adiabatic electron affinity (EA_(A)), the vertical electron affinity (EA_(v)), and the vertical detachment energy (IP_(V)). The differences in charge distribution between uracil and halogenated uracils are indicated, thus the ability to form the hydrogen bonds of halogenated uracils is discussed, too. The results show an excellent agreement between the results and those obtained by other calculations regarding the structural modifications and electron affinities of neutral and anion 5XU. 5-Bromouracil is expected to be used in radiotherapy as better radio-sensitizer owing to the more even charge distribution and the higher electron affinity than 5-fluoromouracil and 5-chlorouracil. This can explain by some means that AEA value of 5BrU is higher than that of 5FU and 5C1U. Although little direct information is available about the one-electron properties, in particular, the electron affinities, of these modified nucleobases, we fill a void in the available data for these systems, especially the 6-halouracils. We hope that our theoretical predictions will provide strong motivation for further experimental studies of these important halogenated nucleobases and their anions.2, Density functional theory (DFT) and configuration interaction with single excitations (CIS) method were used to optimize the ground state and excited state structures of five indolocarbazole molecules on the 6-31G(d,p) basis set level, respectively. Based on the geometry structures, the absorption and emission spectra were calculated with the time-dependent DFT (TD-DFT) method on the same basis set level. There are obvious differences in the emission spectra of these isomers. For instance, [3,2-b]indolocarbazole (5) bears larger oscillator strength in the emission spectrum but the lowest level of the transition energies compared with other isomers; the emission peak value of [3,2-a] indolocarbazole (4) is at highest energy level and the oscillator strengths of [2,3-b]indolocarbazole (2) are weakest. This is mainly because that the structures change in the excited state and molecular orbital (MO) energy levels of these molecules are different. We also evaluated the nonlinear optical response of this series of molecules. The calculated polarizabilities are at same level, but the static first hyperpolarizabilities (β₀) are discriminating. Theβ₀ of [2,3-b]indolocarbazole (2) is the biggest. The results provide a theoretical basis for the optical materials which using indolocarbazole as application unit.3, Density functional theory (DFT) and configuration interaction with single excitations (CIS) method were used to optimize the ground state and excited state structures of carbazole and 14 bicarbazole isomers on the 6-31G(d,p) basis set level, respectively. Based on the geometry structures, the absorption and emission wavelengths and charge transfer characters were calculated with the time-dependent DFT (TD-DFT) method on the same basis set level. In this paper we discussed the relationship between different molecular structures and properties, detailedly, the calculated data are in agreement with the reported corresponding experimental results. Based on the varied frontier molecular orbital energies, ionization potentials (IP), electron affinities (EA) and reorganization energies for bicarbazoles, the dissimilar applications are studied all round. It was found that the LUMO energy of [3,6']bicarbazole is the highest between the 14. The LUMO energies of [3,5'] bicarbazole, [4,5']bicarbazole and [2,6']bicarbazole are similar with [3,6']bicarbazole, which of the others are relatively low. There is the almost same trend for the HOMO energies but with less change. The studies can provide some help in designing and synthesizing more and greater molecular materials.