Fine Tuning of the Electronic Properties of Linear π-Conjugated Oligomers by Covalent Bridging

• Published in: Chemistry : a European journal 2006-01, Vol.12 (4), p.1244-1255
• Main Authors: Blanchard, Philippe, Verlhac, Patrick, Michaux, Laurent, Frère, Pierre, Roncali, Jean
• Format: Article
• Language: English
• Subjects: conjugation; electronic properties; oligomers; structure-property relationships; thiophene chemistry
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200500853

A series of oligothienylenevinylenes, π‐conjugated oligomers rigidified by ethylene bridges attached at different sites of the conjugated backbone, have been constructed by multistep synthetic methodologies. Electronic absorption spectra show that the rigidification of the conjugated system produces a bathochromic shift of the absorption maximum and a narrowing of the HOMO–LUMO energy gap, as compared to the spectra of an open‐chain reference compound. The cyclic voltammograms of all oligomers show that these compounds can be reversibly oxidized into their cation radicals and dications and that rigidification produces a large negative shift of the first oxidation potential, which is indicative of a considerable increase of the HOMO level. Electrochemical data confirm that covalent bridging strongly affects the HOMO and LUMO levels and these data demonstrate that the sites of fixation of the bridges on the π‐conjugated backbone exert a determining effect on the relative stability of the cation radical and dication. Examination of these various results in the light of theoretical calculations shows that in addition to a local control of bond length alternation, and hence of the HOMO–LUMO gap, the fixation of covalent bridges at selected positions of the π‐conjugated system limits the deformation of the π‐conjugated structure upon oxidation to the charged states. Bridge to improved properties: Rigidified oligothienylenevinylenes, such as those depicted with optimized π‐electron delocalization, have been synthesized by covalent bridging of ethylene linkages. Cyclic voltammetry, UV/Vis spectroscopy, and theoretical results highlight the influence of the position of the covalent bridge on the electronic properties of the title molecules.