关键词: 非线性光学, 羟醛缩合, 生色团, 密度泛函理论, 极化, 二次谐波法

A novel nonlinear optical (NLO) chromophore 2-[{2-[(4-diethylamino-styreneyl)-furyl-5]-vinyl}-3-cyano-5,5- dimethyl-5H-furan-2-ylidene]malononitrile (EFFC) containing furan conjugating bridge was designed with tricyanofuran as electron acceptors and dialkylamine as donors. The molecular has been synthesized via two steps of aldol condensation reactions. In the first step, 5-methylfurfural (1 equiv.) and drops of acetic acid in THF was added dropwise to a solution of 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) (1 equiv.) and pyridine. After the addition, the mixture was stirred for 24 h at room temperature. The reaction mixture was filtrated and the insoluble residue was washed several times with ethanol. Secondly, the intermediate (1 equiv.), 4-(diethylamino)benzaldehyde (1 equiv.) and ten drops of piperidine were dissolved in MeCN. The solvent was removed after heated to reflux for 24 h. The product EFFC was purified by a silica gel chromatograph column and characterized by FT-IR, ¹H NMR and elemental analysis. The thermal decomposition temperature (T_d) as high as 250℃ was determined by TGA testing. Furyl conjugating bridges lowered the T_d compared with the non-furyl corresponding structure DCDHF-2-V. Density functional theory (DFT) B3LYP/6-31G method was used to optimize the structures and calculate the static hyperpolarizability (β₀). The β₀ value of EFFC was calculated as large as 6.5×10^-28 esu. It was larger than the corresponding chromophores with dimethylamine donors. The chromophore was mixed into polysulfone (PSU) with the weight ratio of 18%. The mixture was dissolved in cyclohexanone and the films were spun onto ITO/glass substrates. The chromophores in the polymer matrix were aligned into a noncentrosymmetrical orientation by corona poling technique. The poling was performed in a wire-to-plane geometry under in-situ conditions. The discharging wire to plane distance was 1 cm. 12 kV of corona voltage was applied and kept for 15 min at 100℃. The film was cooled down to the room temperature at applied electric field maintaining a constant current. The second-order nonlinear coefficient r₃₃ value reached to 80 pm/V at 1064 nm was measured by second harmonic generation method.

Key words: nonlinear optics, aldol condensation, chromophores, density functional theory, polarization, second harmonic generation