Enhanced Nonlinear Optical Activity of Molecules Containing Two D−π−A Chromophores Locked Parallel to Each Other

A series of H-shaped second-order nonlinear optical (NLO) chromophores with two parallel and nonconjugated D−π−A units have been synthesized, in which a 9,10-dihydroanthracene is employed as molecular backbone. The measurement results of hyper-Rayleigh scattering (HRS) and UV−vis spectra show that the first molecular hyperpolarizability (β) values of H-shaped chromophores are remarkably increased compared with the corresponding mono-D−π−A unit reference compounds (the enhancements per azo D−π−A unit from 1.2 to 3.0 are observed), without causing a large shift of the absorption band to longer wavelength. Therefore, it gives an available way of solving the trade-off between nonlinearity and transparency in designing NLO chromophores. Furthermore, some main-chain fluoro-containing polyimides embedded with the H-shaped chromophores have also been prepared. The elemental investigation results show that the polyimides exhibit a higher macroscopic nonlinear optical coefficient ( d 33 = 35.0 to ∼70.2 pm/V), good thermal stability ( T g 198 to ∼210 °C, T d 240 to ∼245 °C), and optical transparency (λ max < 400 nm).