New Push-Pull Chromophores Featuring TCAQ (11,11,12,12-Tetracyano- 9,10-anthraquinodimethane) and Other Dicyanovinyl Acceptors

Stable, highly colored push-pull chromophores with NMe2 donor and C=C(CN)2 acceptor moieties, featuring intense intramolecular charge-transfer (CT) bands in the UV/Vis spectra, are reported. In an attempt to prepare the quinoid push-pull systems 2, chromophores 10 and 11, with a central cyclohexene spacer, were obtained and characterized by X-ray analysis. A series of donor-substituted TCAQ (11,11,12,12-tetracyano-9,10-anthraquinodimethane) derivatives were synthesized, using the Knoevenagel condensation between appropriately functionalized anthraquinones and malononitrile, mediated by the Lehnert reagent (TiCl4/pyridine), as the key step. HCl addition to triple bonds was observed when this transformation was applied to alkynylated anthraquinones. Electrochemical studies by cyclic voltammetry (CV) and rotating-disk voltammetry (RDV) showed that introduction of donor substituents into the TCAQ core of 25, 26, and 31 shifts the first reduction potential to more negative values, while chromophores bearing guanidine moieties (27, 28) displayed a specific and complex redox behavior. Both electrochemical and UV/Vis data provide good evidence that D–A conjugation is more efficient through olefinic (in 10) than through acetylenic (in 37) spacers. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)