| Recent years have witnessed the tremendous developments in the electronic structure method and computer hardware, which made the quantum chemistry a powerful tool to calculate almost all kinds of molecule systems and play a very important role in almost all the branches of chemistry. More and more people are interested in the investigation of the excited state other than the ground state. In fact, the study of the excited state is the most difficult part and the front area in quantum chemistry. On the one hand, the Born-Oppenheimer approximation is broken to achieve high level calculations of the coupling of higher excited states, which include potential energy surface intersection. On the other hand, people are trying to calculate the excited states in the molecules of middle sizes using ab initio method. The present dissertation focuses not only on the potential energy surface intersections, the Spectroscopy and the reaction dynamics of three- or four-atoms molecular systems[C (1D) H2 and FN3], but also on the structures, the optical properties and the mechanism of charge transfer in the organic light emitting materials (A111, A212 and A313).The methylene radical CH2 is of interest to both chemists and astrophysicists. In the interstellar medium, CH2 plays an important role in the carbon chemistry. It is the direct chemical precursor of the widely observed CH radical as well as of more complex carbon-bearing molecules. In addition, CH2 is thought to be an important link in the photodissociation sequence of cometary methane. There are lots of investigations on the system and discrepancies are found between the theoretical calculations and the experimental results. A possible explanation for these discrepancies is that the ground state surface is not accurate enough in the long-range region. Other explanations are, however, possible. In particular, there are five singlet states, which correlate with C (1D) H2, and the PES intersections of different electronic states and nonadiabatic effects may play a role. The purpose of this work is to investigate the intersections of the singlet PESs in the C (1D) H2 system, which lie at relatively low energies and may play a role in the studies of the CH2 Spectroscopy and the C (1D) H2 reaction dynamics. Potential energy surface (PES) intersection seams of two or more electronic states from 11A' 21A', 31A', 11A " and 21 A " states in the C (1D) H2 reactive system, are investigated using the internally contracted multireference configuration interaction (MRCI) method and the correlation consistent polarized valence quadruple-zeta set augmented with diffuse functions (aug-cc-pVQZ). Intersection seams with energies less than 20 kcal/mol relative to the C (1D) +H2 asymptote are searched systematically, and finally several seam lines (at linear H-C-H, linear C-H-H and C2V geometries respectively) and a seam surface (at the Cs geometry) are discovered and determined. The minimum energy crossing points (MECP) on these seams are reported and the influences of the PES intersections on the CH2 Spectroscopy and the C (1D) +H2 reaction dynamics are discussed. In addition, geometries and energies of the 11A2 and 11 B2 states of methylene biradical CH2 are reported in detail for the first time.The reactions of the fluorine atom with the azide radical N3 are of great interest and may have potential applications in new lasers. For the F+N3 reaction, there are controversies in the previous studies of the conical intersection between the lowest singlet and triplet states. Herein, we explore the intersection between the singlet and triplet PESs of the F+N3 system with high-level ab initio methods and discuss the possible role of this intersection on the dynamics. The state-averaged coupled-perturbed multiconfiguration self-consistent field method (SA-CPMCSCF) and the the correlation consistent polarized valence double-zeta set (cc-pVDZ) are applied for the optimization of the MECP. Only one MECP is found. Its accurate geometry is determined and found to be in the Wans- form. This MECP appears after the singlet transition state (TS), and its possible role on the dynamics of the reactions is analyzed.Following the milestone report in 1987, organic light emitting diodes (OLEDs) are currently under intense investigation for application in next generation display technologies. To realize full color applications, high performance organic materials emitting the three elemental colors of red, green, and blue are required. The green and blue emitting materials of satisfactory properties are developed and feasible to utility. The situation for red emitters drops behind those of the other two colors in terms of color purity, efficiency, and lifetime. Recently, a serial of intramolecular charge transfer fluorescent materials, All, A21 and A31, are synthesized in our laboratory. They all belong to the unsymmetric D-π-A molecules and have the same electronic acceptor. The difference of the donor makes them different properties. We investigate the three molecules with high level methods. The ground-state structure and electronic properties of A31, are investigated using a hybrid density functional theory (DFT) approach, B3LYP, which corresponds to the combination of Becke's three parameter exchange functional (B3) with the Lee-Yang-Parr fit for the correlation functional (LYP), and the 6-31G* basis set. Eight geometrical isomers have been obtained. The theoretical infrared (IR) spectrum calculated by B3LYP/6-31G* level of theory is in very good agreement with the experimental measurement. The cation and anion are optimized to clarify the effects of the hole and electron injections and the energies needed by injections are estimated.The ground-state (So) and the lowest singlet excited-state (S1) of A31 are investigated. The S0 and S1 geometries are optimized at the ab initio Hartree-Fock (HF) and the singles configuration interaction (CIS) levels of theory, respectively. The 6-31G* basis set is used. The CIS and semi-empirical Zerner's Intermediate Neglect of Differential Overlap (ZINDO) methods provide the results for the absorption (S0→S1) and emission (S1→S0) transition energies. The Stokes shifts calculated at the CIS and ZINDO levels of theory are 19nm and 105nm, respectively. The absorption spectra in various solvents are calculated using the time-dependent (TD) density-functional theory (DFT) method in combination with the polarized continuum model (PCM), which are in very good agreement with the experimental measurements. The solvent effects are discussed.The structures and electronic properties of the ground-state of A11 and A21 are investigated using the hybrid DFT approach, B3LYP. The cation and anion are optimized to clarify the effects of the hole and electron injections and the energies needed by injections are estimated. The semi-empirical ZINDO method provides the result for the absorption (S0→S1) transition energies, which are compared with the available experimental measurements. The 6-31G* basis set is used all along. Comparisons are carried out among A11, A21 and A31.
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