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 π-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 ¹H NMR, CV, UV-vis, and Hyper-Rayleigh scattering experiments. All the NLO chromophores exhibited very large static molecular first hyperpolarizabilities (β₀) in the range of 450-960 × 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 β were obtained. Theoretical studies were carried out to understand the structure-property relationships, which showed that multiple states excitations contributed to the β 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 °C. The very large β₀ values coupled with the high thermal stability indicates good application potential of this series of NLO chromophores.