Design of novel X-type second-order nonlinear optical chromophores with low ground state dipole moments and large first hyperpolarizabilities

Abstract

The novel low ground state dipole moment X-type second-order nonlinear optical (NLO) chromophores with dicyanovinyl group as acceptor and methylene-1,3-dithiole group as donor have been designed and theoretically investigated by employing AM1/FF approach. The electronic properties of the designed chromophores were investigated by ZINDO/S-CI method. The calculated results show that the ground state dipole moment ( μ g) of the designed chromophores are as low as 2 D and the zero-frequency first hyperpolarizabilities ( β 0) of them can reach as high as 50×10 −30 esu. These characteristics are well coincided with the reported thermally stable X-type chromophores and the one-dimensional low μ g chromophores. It is considered that the low ground state dipole moments may originate from their structural properties (including the ring-locking effect, steric hindrance factor and donor and acceptor properties). The large β 0 values are analyzed by utilizing two-level model. In addition, the merit of the low μ g of the NLO chromophores when they are incorporated into polymers has been also discussed. With the decrease of μ g, the NLO coefficient d 33 of the poled polymers have not been reduced due to the increase of the number density of chromophores in polymers. So the suitable low μ g chromophores are one of the solutions for the optical loss and enhancing the NLO coefficient of the poled polymer in the practical applications.