The study on regulating the conjugate position of NLO chromophore for reducing dipole moment and enhancing electro-optic activities of organic materials
In order to improve the first-order hyperpolarizability (β) of the chromophore and transform it into high macroscopic electro-optic activity, a series of novel second-order nonlinear optical chromophores with different push-pull electron groups introduced on the thiophene π-conjugate bridge for tuning the shape and dipole moment (µ) of chromophores have been designed and synthesized. These chromophores are based on the same thiophene π-conjugated bridge, where the donor (N, N-diethylaniline) and acceptor (2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene) malononitrile or Malononitrile) linked to position 2 and 3 of thiophene, respectively, affording to boomerang-like shape instead of rod-like shape. Besides, electron-poor group Br (bromine atom) or electron-rich group DEA (N, N-diethylaniline) as auxiliary acceptor or donor linked at the position 5 of thiophene. And all chromophores showed good thermal stability, and their decomposition temperatures are all above 200 °C as the DSC and TGA analysis. Through UV-vis analysis and DFT calculation, it has been concluded that chromophores with additional electron-rich group as auxiliary donor display better intermolecular charge-transfer (ICT) absorption and the lower HOMO-LUMO energy gaps (∆E). Furthermore, the boomerang-like chromophore with same push-pull structure shows a smaller dipole moment (μ) and β value than the traditional FTC. The poling results of guest-host EO polymers FTC/APC FTC-H/APC, FTC-Br/APC and FTC-DEA/APC with the same number density afford r33 values of 17 pm V-1, 11 pm V-1, 10 pm V-1 and 25 pm V-1, respectively. Although the β value of FTC-DEA is smaller than FTC, the r33 value of FTC-DEA (25 pm V-1) is 47% greater than FTC (17 pm V-1) under the same number density. Hence, the results above indicated that regulating the conjugate position of chromophores and can efficiently decrease the dipole-moment of the chromophores so that weaken dipole-dipole interactions, thus enhancing the macroscopic electro-optical activity in poled polymers. These results indicate the potential application of this novel chromophores in electro-optical devices.