Theoretical Study On Nonlinear Optical Properties Of N-heterocyclic Radical

Recently, the study on nonlinear optical (NLO) materials of open-shell systems has received considerable attention both theoretically and experimentally. With the development of the molecular engineering theory and the perfection of the quantum chemistry computational technology, it provides a powerful tool to explore the relationship between the molecular electronic structures and properties. It offers the credible schemes and data for experiments, which further builds a foundation for the molecular design.The NLO properties of open-shell systems have been actively studied theoretically. It is possible that the highly efficient NLO material can be obtained. Nakano, M et al. have studied the NLO coefficients for a series of open-shell systems. Several studies have highlighted potential of open-shell systems and some of them have suggested the possibility of the multi-functional materials exhibiting both of magnetic and optical properties. In this study, the NLO properties of N-heterocyclic radical systems were calculated by using quantum chemical calculations combined the finite field (FF) approach. The research results suggest:(1) The geometrical structure and stability of neutralπ-conjugated C4H4N? with three spin states were investigated by using ab initio and density functional theory (DFT) methods. The results indicate that conjugation and stability of the neutral C4H4N? decreased subsequently with increasing its spin multiplicity. The linear and nonlinear optical coefficients for neutral C4H4N? were calculated by using the reliable UCCSD method. Compared with the UCCSD method, the polarizabilities (α) and hyperpolarizabilities (βandγ) obtained by other methods are a rather good approximation with that obtained by the UCCSD method. In addition, we obtained some methods that they can reduce calculation time and enhance the reliable results. Hence, the NLO activities can be modified by controlled the spin multiplicity.(2) By using quantum chemical DFT, the polarizabilities(α) and second hyperpolarizabilities (γ) of Pyyrole radical systems involving different linkers were studied. The singlet diradical systems with intermediate diradical characters (y) show a significant enhancement ofαandγvalues as compared to the nearly closed-shell systems. The BLA values decreased and the y values increased with increasing the similarπ-conjugation length. Theαandγvalues enhanced with increasing conjugation length. This suggests the possibility of controllingαandγof linked-type Pyrrole radical systems by changing the diradical character through adjusting the length and kind of conjugated linkers. These results demonstrate the possibility of spin multiplicity control of the NLO response further.