Syntheses, structures, and spectroscopic properties of push-pull heteroquinoid compounds

A variety of push-pull heteroquinoid compounds (4-10) bearing an electron-donating 1,3-dithiol-2-ylidene group and an electron-accepting dicyanomethylene group were prepared by the coupling reactions of the corresponding dicyanomethyl heteroaromatics with 2-methylthio-1,3-dithiolium methyl sulfate according to the Gompper method. All these compounds are highly colored and amphoteric. The electronic absorption spectra have revealed that their deep colors arise from a strong intramolecular charge-transfer absorption band in the visible region. The infrared nitrile vibrational spectra have indicated that these molecules exist as a resonance hybrid of the quinoid and zwitterionic forms, accordingly, assuming considerably polarizable characters. The amphoteric and polarizable characters of the simple heteroquinoid compounds (4-7) are nearly independent of the kind of the constituent chalcogen atom and of the introduction of the octylthio or hexyl groups into the frameworks. On the other hand, those of the extensive heteroquinoid compounds (8-10) are largely enhanced because of increasing gain in aromaticity of the heterocyclic rings in the zwitterionic form.