The Calculated Study On Nonlinear Optical Properties Of Nitrogen-oxide Radical

Materials with large nonlinear optical (NLO) properties are promising candidates for applications to telecommunications, computer, solar-energy conversion and light-treatment, and so on. So the research field of NLO has been comprehensively and rapidly developed. The synthesis of materials with large NLO properties is very currently popular topics.The development of the molecular engineering theory and the perfection of the quantum chemistry computional technology offer the strong means for experiment. Theoretical studies provide the relationship between NLO properties and molecular structure, it is a useful tool for molecular designingIn recent years, the NLO properties of open-shell systems have attached considerable attention., open-shell systems may have higher NLO activity than that of closed-shell. It is possible that the open-shell systems could become highly efficient NLO materials. The NLO properties of open-shell systems have been studied theoretically. Such as Nakano, M et al, they have studied the NLO properties for a series of open-shell systems. The results showed that the open-shell systems have better NLO activity than the closed-shell systems. Scientists hope that the multi-functional materials will be exploited in future.The NLO properties of nitrogen-oxide radicals have been studied by using quantum chemical calculations, the relationship between molecular structure and NLO properties have been analyzed. The research results suggest:(1) Spin-coupling properties of imidazole-oxide radicals and their isomers were investigated by using the DFT UB3LYP/6-31(d) method. NLO coefficients were studied at the same level by using FF approach. The calculated results showed that the ground state spin accord with spin coupling rules. Their polarizability decreased with increasing spin multiplicity. Due to molecular symmetry effects, the first hyperpolarizability changed when molecular symmetry changed. The second hyperpolarizability increased with increasing spin multiplicity.(2) Seven radicals were designed according to di-acridine nitrogen-oxide radical which had been synthesized. DFT UB3LYP/6-31G(d) method was employed to optimize molecular structure, the stabilities and NLO responses of these radicals are different with various spin states. The results showed that the ground state of di-acridine nitrigen-oxide radicals and derivatives are stable in triplet state. Introduction of donor and acceptor increased the polarizability and the second hyperpolarizability. Moreover, the polarizability and the second hyperpolarizability increased obviously with increasing the strength of donor and acceptor. Substituents had important influence on the first hyperpolarizability in singlet state. The singlet radicals had the larger NLO coefficients than the triplet radicals.(3) On the basis of optimized molecular structure by using UMP2/6-31g(d,p) method, we employed the DFT UB3LYP/6-31G(d,p) approach to calculate the polarizability and the second hyperpolarizability of the benzo-dithiazolyl dimer radical. The interplanar distance was enlonged and shorten by step of 0.5?, we use the five point to calculate and discuss. The singlet radical is slightly more stable than the triplet radical according to UMP2 calculations. The polarizability of the vertical-stacking radical enhanced with increasing the interplanar distance in singlet and triplet state, the second hyperpolarizability increased with increasing the interplanar distance in triplet state. The polarizability of the singlet shifted-stacking radical decreased with increasing the interplanar distance. The absolute value of the second hyperpolarizability increased with increasing the interplanar distance for the triplet shifted-stacking radical.