Toward highly efficient NLO chromophores: Synthesis and properties of heterocycle-based electronically gradient dipolar NLO chromophores

To realize organic nonlinear optical (NLO) chromophores with optimized ground-state polarization and very large molecular optical nonlinearities, a novel series of heterocycle-based electronically gradient dipolar chromophores were designed and synthesized. These chromophores are featured by their same strong electron acceptor (i.e., 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran, TCF) and the same length of pi-conjugation, but different electron donors ( e. g., dialkylamine and dianisylamine), different (hetero) aromatics with varying electron densities (i.e., pyrrole, thiophene, and benzene) as the auxiliary donor, and electron-poor 1,3-heteroaromatic thiazole with different regiostructures (e.g., either electron-poor C2, matched'', or electron-rich C5, un-matched'', is connected to the acceptor) as the auxiliary acceptor, which allows for a systematic fine-tuning of the ground-state polarization. The gradient electronic structures and optical properties of these NLO chromophores were carefully characterized by H-1 NMR, CV, UV-vis, and Hyper-Rayleigh scattering experiments. All the NLO chromophores exhibited very large static molecular first hyperpolarizabilities (beta(0)) in the range of 450-960 x 10(-30) esu, which showed significant dependence on the gradient electronic structures. Upon using electron-rich heteroaromatic cycle as the auxiliary donor, matched'' thiazole as the auxiliary acceptor, and/or dianisylamine as the electron donor, substantially enhanced beta were obtained. Theoretical studies were carried out to understand the structure-property relationships, which showed that multiple states excitations contributed to the b values of this series of NLO chromophores. TGA investigations showed excellent thermal stability for most of the resulting NLO chromophores, with on-set temperatures for thermal decomposition higher than 250 degrees C. The very large beta(0) values coupled with the high thermal stability indicates good application potential of this series of NLO chromophores.