Synthesis and properties of NLO chromophores with fine-tuned gradient electronic structures

A novel series of heterocycle-based NLO chromophores based on different combinations of auxiliary donor (i.e., benzene, thiophene and pyrrole) and auxiliary acceptor (i.e., thiazole with different regiochemistries) were designed and synthesized. Due to the different electron-rich and poor nature of the auxiliary donors and acceptors, respectively, the resulting NLO chromophores have systematically varied ground-state electronic structures, as evidenced by the ¹H NMR, CV and UV-vis investigations. The nonlinear optical properties of the resulting NLO chromophores were studied by UV-vis spectroscopy, Hyper-Rayleigh scattering (HRS), and semi-empirical computations. All the chromophores have very large molecular hyperpolarizabilities (β_{1000 nm}) in the range of 704-1500 × 10^-30 esu (or β₀, 318-768 × 10^-30 esu), which showed a great sensitivity to the gradient electronic structures. Upon increasing the electron density from benzene to thiophene and to pyrrole, substantial increases in β₀ were observed; significantly larger β₀ values were also observed for NLO chromophores based on "matched" thiazole (C2 is connected to the acceptor) than those based on "un-matched" thiazole (C5 is connected to the acceptor). TGA investigations showed good thermal stability for the resulting NLO chromophores. However, with the increase of electron density of the auxiliary donor, a decrease in thermal and photochemical stability was observed. It is interesting to note that NLO chromophores based on triarylamine as the donor and thiazole as the auxiliary acceptor exhibited not only high thermal stability but also very large β₀ values.