Quinomethides versus unsymmetric hybrids: two variations of non-radicaloid SEM-states in four-electron redox systems of bis-4H-imidazoles |
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Authors: | Martin Matschke Katrin Knop Rainer Beckert |
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Affiliation: | (1) Institute for Organic and Macromolecular Chemistry, Friedrich-Schiller-University Jena, Humboldt Street 10, 07743 Jena, Germany |
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Abstract: | A series of bis-4H-imidazoles 1a-h has been synthesized and characterized. Due to the integration of rigid substructures, the first fluorescent 4H-imidazole 1h which possesses the substructure of pyrrolo-pyrrol was obtained. Upon protonation of bis-4H-imidazoles, a bathochromic shift of 100 to almost 200 nm was observed originating from the transformation of merocyanine chromophores into cyanines. All 4H-imidazoles were reduced chemically yielding bridged 1H-imidazoles which immediately reoxidize when exposed to air. Stable quinomethide-like species 3a,b which represent the SEM form in this four-electron redox system are only observed during the reduction of bis-4H-imidazoles 1a,b. Influenced by larger bridging elements in derivatives 1c–1h, the electronic communication between terminal 4H-imidazoles is impaired. Therefore, detection and isolation of neither quinomethide-like nor biradikaloid SEM forms succeeded. Most likely a very fast electron-transfer-step leads to unsymmetric hybrids, consisting of one 1H- and one 4H-imidazole. The new synthesized derivative 1k is not only such a stable hybrid molecule, but also may be an indication for the existence of non-quinoid SEM-forms. |
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Keywords: | 4H-Imidazoles Quinomethides unsymmetric SEM-States Redox systems |
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