Donor-Substituted Cyanoethynylethenes: π-Conjugation and Band-Gap Tuning in Strong Charge-Transfer Chromophores

• Published in: Chemistry : a European journal Vol. 11; no. 11; pp. 3325 - 3341
• Main Authors: Moonen, Nicolle N. P., Pomerantz, William C., Gist, Robin, Boudon, Corinne, Gisselbrecht, Jean-Paul, Kawai, Tsuyoshi, Kishioka, Atsushi, Gross, Maurice, Irie, Masahiro, Diederich, François
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 20.05.2005; WILEY‐VCH Verlag; Wiley
• Subjects: alkynes; charge transfer; Chemistry; Chemistry, Multidisciplinary; conjugation; electrochemistry; Physical Sciences; Science & Technology; two-photon absorption; ELECTROCHEMICAL PROPERTIES; electrochemistry; PHOTODYNAMIC THERAPY; TETRAETHYNYLETHENES; STABILIZATION; alkynes; COMPLEXES; NONLINEAR ABSORPTION; conjugation; charge transfer; MOLECULES; two-photon absorption; SYSTEMS; SOLVATOCHROMIC SHIFTS; ELECTRONIC-STRUCTURE
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200500082

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 spectroscopic 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. Tuning the band gap! An extensive series of silyl‐protected cyanoethynylethenes (CEEs) and N,N‐dimethylanilino (DMA) donor‐substituted CEEs has been synthesized (an example of which is shown here). The efficiency of the conjugation pathways in the donor‐substituted CEEs is investigated in great detail. A set of rules for π‐conjugation and band‐gap tuning in strong charge‐transfer chromophores is established.