Theoretical Studies On Electronic Structures And Spectroscopic Properties Of Organic Molecule Systems Containing N And S

Organic optoelectronic functional materials have many advantages compared to traditional inorganic materials, such as greater flexibility, the tunable band and color, easy processing, and lower cost and so on. Therefore to replace the inorganic optoelectronic materials by organic materials in the field will become a development trend. Now, organic light emitting devices (OLED) and organic photovoltaic devices (OPVC) are the main applications to organic optoelectronic materials. In theory, electroluminescence and photovoltaic devices work in reverse process with each other. However, both the light absorption and emission properties of the materials should be considered in the two processes.Now the experimental chemists have paid much attention on the synthesis of new kinds of optoelectronic materials with good performance. As the key components of OLED, the luminescent system should emit suitable color with high efficiency. In this aspect, the red, green, and blue light emitting materials has been the focus of the study since the reasonable construction of the red, green, and blue (RGB) light emitting materials can achieve the full-color image display. As for organic photovoltaic devices, to improve the energy conversion efficiency of the devices and make them to be applied in practice as soon as possible are the main task of current research. Therefore, the photovoltaic material should exhibit large scale absorptions to the solar energy with high intensity. Experimental chemists in this area have always been trying to synthesize this kind of material. In addition it also can improve the device performance by dye-sensitization and building heterojunction, and both of the projects need to choose the materials with better properties. Therefore, the oxazole, anthracene, thiophene, fluorene, and other organic compounds, including small molecules, oligomers, and the polymer system, are the main frames of new materials that the experimental chemists have selected in recent years.In the meantime, the theoretical chemists are engaged in the investigation on the optoelectronic materials by means of the theoretical calculation. The electronic structures, the mechanism of electronic absorption and emission as well as the carrier transport etc. of the materials can be revealed through the theoretical calculation. The theoretical findings can help to design the new kinds of materials with better properties.In this thesis, we chose pyridine, thiophene and other small molecules containing the N and S atoms as the main framework moderated with the special functional group in theory to form the now kinds of optoelectronic material. Through the adjustment to the molecular structures of the theoretical models, the electronic structures can be affected and then the optical and electrical properties of the system can be regulated. We adopted density functional theory (DFT, Density functional theory), single-excitation configuration interaction (CIS, Single-excitation configuration interaction) and Time-dependent density functional theory (TD-DFT) to calculate the geometries and electronic structures of the systems in the ground and excited state respectively, together with the absorption and emission spectra properties. The main research results are as follows:1. The electronic structures and spectroscopic properties of a series of donor-π-acceptor (D-π-A) organic compounds based on [1,3]dithiolan-2-ylidene-actaldehyde, which acts as the acceptor, and four different donors, i.e. phenothiazine (1), benzenamine (2), triphenylamine (3), and carbazole (4), bridged by a C-C double bond were investigated theoretically. The quantum calculation revealed the geometry structures of the four compounds in the ground and excited state at the B3LYP/6-311G* and CIS/6-311G* level respectively. Under the TD-DFT level and considering the solvent effect with the PCM model, the absorption and photoluminescence properties of the compounds in toluene, THF, CH2Cl2, and DMSO solvent were explored based on the optimized geometries in the ground and excited states, respectively. The calculation results showed that the lowest-energy absorptions and photoluminescence of these D-π-A molecules are due to the intramolecular charge transfer (ICT) transition. The dipole moments of 1-4 change dramatically from the ground to the excited states and the variation of 1 and 3 is much remarkable compared with 2 and 4. Furthermore, the donor becomes more positive while the bridge and acceptor fragments turn more negative from the ground to excited state and this is in line with the typical characteristic of the donor-acceptor system. Not only the spectroscopic properties (absorption and emission) but also the dipole moments as well as the charge distribution of the systems are influenced by the solvent polarity, and the influence in the excited state is remarkable.2. Four new small organic molecules involving N and S atoms were designed theotetically by taking two thiophene rings as the main fragment combined with a 2,3-Dimethyl-thieno[3,4-b]pyrazine (1), 2,1,3-benzothiadiazole (2), 2,3-Dimethyl-quinoxaline (3) and pyridine (4), in which the two thiophenes are located on the head and tail and the heterocycles lie as the center. The geometry structures of the systems in the groud and excited state were optimized at the B3LYP/6-311G* and CIS/6-311g* level, respectively. The optimization results showed that the compounds 1-3 have the C2 symmetry. Compound 1 exhibits quasi-planar conformation both in the ground and excited state, while the two thiophene ring and the middle heterocyclic part showed twist angles of ca. 160.0°in 2 and 3 and 148.4°in 4 in the ground state. However, the geometries of 2-4 were tuned to quasi-plane configuration in the excited state due to the rearrangement of the electrons. Based on the optimized geometry structures, the absorptions and emissions were calculated with the TD-B3LYP/6-311g* method. The absorption results revealed that the lowest-energy absorptions of 1-4 originate from the electron transition from the thiophene to the middle heterocycle units upon excitation. The calculated lowest-energy intense absorptions are blue-shifted from 1 to 4 and the overlapped spectra of them covered the area from 200-600 nm. Compounds 1-4 emit fluorescence in the visible region. Specially, 1 and 3 emits in the red and green color, respectively. The calculations with the SCRF method showed that the solvent has hardly influence to the spectroscopic properties of the system. These theoretical findings can provide the guidance for designing the new kinds of optoelectronic materials.