Donor-substituted cyanoethynylethenes: π-conjugation and band-gap tuning in strong charge-transfer chromophores

An extensive series of silyl-protected cyanoethynylethenes (CEEs) and N,N-dimethylanilino donor-substituted CEEs have been synthesized. More extended chromophores were constructed by selective silyl deprotection and subsequent oxidative acetylenic coupling. The strong electron-accepting nature of the CEEs was revealed by a combination of ¹³C NMR spectro-scopic and electrochemistry measurements. Donor-substituted CEEs display strong intramolecular charge-transfer (CT) character, resulting in intense, bathochromically shifted CT bands in the UV/Vis spectrum. Their structural diversity establishes them as suitable models for the study of π-conjugation and band gap tuning in strong charge-transfer chromophores. The extent of π-conjugation in the donor-substituted CEEs was investigated by a combination of ground-state techniques, such as X-ray crystallography, electrochemistry, B3 LYP calculations, and NMR spectroscopy. The comparison of these ground-state results with the features observed in the UV/Vis spectra reveals that - contrary to expectations - more extensive π - conjugation can lead to larger band gaps in molecules with strong donor and acceptor moieties.