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A new synthetic route to mononitrated oligothiophenes is described, as well as the preparation of halogenated derivatives (Br, I) thereof. An unusual deep red colour is observed and explained, with the aid of DFT calculations, as arising from a significant quinoidal contribution to the molecular structure. The crystal structures of two compounds, H[([C.sub.4][H.sub.2]S).sub.3]N[O.sub.2] and Br([C.sub.4][H.sub.2]S)3N[O.sub.2], are presented. Both compounds have planar sheets held together by intermolecular short contacts (hydrogen bonds and, for the latter, N[O.sub.2] ... Br interactions); the sheets do not directly superimpose, so the effect of the π-stacking is not maximized. Solid-state fluorescence and extended-Huckel band-structure calculations are also presented for these materials. Key words: oligothiophene, organic semiconductor, crystal structure, band structure, nitro-substitution. On decrit une nouvelle methode generale de synthese des oligothiophenes mononitres ainsi que la preparation de leurs derives bromes et iodes. On a observe une couleur rouge fonce inhabituelle et, en se basant sur des calculs de la theorie de la fonctionnelle de la densite (TFD), on suggere qu'elle resulte d'une importante contribution quinonoidale a la structure moleculaire. On a determine les structures cristallines des deux composes H[([C.sub.4][H.sub.2]S).sub.3]N[O.sub.2] et Br[([C.sub.4][H.sub.2]S).sub.3]N[O.sub.2]. Les deux composes se presenten sous la forme de feuillets plans maintenus en place par des contacts intermoleculaires courts (liaisons hydrogenes et, dans le deuxieme cas, interactions N[O.sub.2] ... Br); les feuillets ne sont pas directement superposes et il en resulte que l'effet de superposition π n'est pas maximise. On rapporte aussi les resultats obtenus pour la fluorescence a l'etat solide de ces materiaux ainsi que les calculs etendus de Huckel de leur structure de bande. Mots-cles: oligothiophene, semiconducteur organique, structure cristalline, structure de bande, substitution par un groupe nitro. [Traduit par la Redaction], Introduction Organic materials are carbon-based small molecules or polymers with interesting electronic properties. Two of the most studied organic semiconductors are the oligothiophenes (1-3) and the acenes. (4) The promise [...]