Auxiliary donor for tetrahydroquinoline-containing nonlinear optical chromophores: enhanced electro-optical activity and thermal stability

A series of chromophores FTC, L1 and L2 have been synthesized based on three different types of electron donors, including diethylaminophenyl, tetrahydroquinolinyl and N-(4-dimethylaminophenyl) tetrahydroquinolinyl groups respectively, with the same thiophene bridges and strong tricyanovinyldihydrofuran (TCF) acceptors. In particular, the donor part of the chromophore L2 was modified with an additional donor N-(4-dimethylaminophenyl) substituent, resulting in enhanced thermal stability and electro-optic activity. Cyclic voltammetry measurements showed that chromophore L2 had a smaller energy gap than chromophore L1 due to the additional donor. Moreover, density functional theory calculations suggested that the molecular quadratic hyperpolarizability (mu beta) value of chromophore L2 is 29% and 44% larger than that of chromophores L1 and FTC, respectively. The doped film containing the chromophore L2 showed an r(33) value of 100 pm V-1 at the concentration of 25 wt% which is much higher than the EO activity of the chromophore L1 (57 pm V-1) and two times higher than that of the traditional chromophore FTC (39 pm V-1).