Synthesis of novel nonlinear optical chromophores: achieving enhanced electro-optic activity and thermal stability by introducing rigid steric hindrance groups into the julolidine donor

Abstract

A series of chromophores z1–z4 have been synthesized based on julolidine donors modified by four different rigid steric hindrance groups including benzene, 2,3,4,5,6-pentafluorobenzene, 3,5-bis(benzyloxy)benzene and 3,5-bis(2,3,4,5,6-pentafluorobenzoate)benzene, respectively, with the same thiophene bridges and strong tricyanovinyldihydrofuran (TCF) acceptors and with high yields. Density functional theory (DFT) was used to calculate the HOMO–LUMO energy gaps and first-order hyperpolarizability (β) of these chromophores. Besides, to determine the redox properties of these chromophores, cyclic voltammetry (CV) experiments were performed. All these four chromophores showed superb thermal stabilities with high thermal decomposition temperatures above 265 °C. Most importantly, the introduction of rigid steric hindrance groups can effectively reduce dipole–dipole interactions to translate their relatively β values into bulk high EO activities. By doping chromophores z1–z4 with a high loading of 25 wt% in APC, EO coefficients (r₃₃) of up to 99, 104, 97 and 89 pm V⁻¹ at 1310 nm can be achieved, respectively. The normalized r₃₃ value was increased to 6.84 × 10⁻¹⁸ pm cc per (V molecules) for z4 possessing the largest steric hindrance group which indicates the weakest dipole–dipole interaction and highest polarization efficiency. The high r₃₃ value, good thermal stability and high yield suggest the potential use of the new chromophores in nonlinear optical areas.