Synthesis and properties of novel second-order NLO chromophores containing pyrrole as an auxiliary electron donor

Abstract

A novel series of second-order NLO chromophores containing pyrrole as an auxiliary electron donor was synthesized via Knoevenagel reactions between 5-aminated N-methylpyrrole-2-carbaldehydes and different electron-accepting groups, i.e., malononitrile, picolinium tosylate and 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF). Their corresponding NLO chromophores containing thiophene in the place of pyrrole were also prepared for comparison. The resulting NLO chromophores showed good solubility in common organic solvents such as CHCl₃, THF and DMF, except for TTCF containing thiophene and TCF, which is soluble in polar aprotic solvents but poorly soluble in less polar solvents. NMR studies of these chromophores showed that, in comparison with thiophene rings in the same type of NLO chromophores, pyrrole rings had higher electron density, as evidenced by the up-field chemical shifts of pyrrole protons. TGA investigations showed good thermal stability of these chromophores in nitrogen with the onset weight loss temperatures in the range of 203 to 296 °C. Positive solvatochromism of 10–44 nm from dioxane to chloroform were found for these chromophores, and moderate to very large molecular static hyperpolarizabilities (β₀) of 57–1490 × 10⁻³⁰ esu were revealed by hyper-Rayleigh scattering measurements. For chemical bonding to polymer chains, hydroxyl-containing NLO chromophores were also prepared and characterized for their linear and nonlinear optical properties.