Electrochemically active chiral molecular materials: the "inherent chirality" approach

[Please note that this contribution has been presented by Dr. Arnaboldi in place of Patrizia Mussini; for this reason Dr. Arnaboldi is indicated above as "corresponding" although in the printed abstract it is not so] The introduction of chirality in molecular polyconjugated electroconductive materials is an attracting goal for a wide variety of purposes, among which, in the electrochemical field, the ability to discriminate between antipodes, required in sensors designed for the detection of chiral analytes, and the preparation of chiral electrode surfaces for asymmetric redox reactions. So far chirality in organic semiconductors has mostly been introduced by attaching chiral pendants to the electroactive conjugated backbone through suitable linkers; however, this approach usually leads to poor chirality manifestations. We are therefore developing electroactive chiral polyheterocycles, where chirality is not external to the electroactive backbone, but inherent to it, resulting from a tailored torsion produced by the periodical presence of atropisomeric, conjugatively active biheteroaromatic scaffolds. The high interconversion energy barrier results in the enantiomers being easily separable and fully stable upon storage, while their intrinsic 3D character, effective conjugation, and C2 simmetry endows them with high and highly regioregular electrooligomerization ability. Chirality is fully transferred to the resulting films: CD spectroelectrochemistry confirms that the electrode surfaces electrodeposited from enantiomers are perfectly specular and highly chiral. Moreover, the surface chirality can be finely modulated by the amount of injected charge (reducing the torsion angle to achieve better p conjugation), which suggested us the image of a breathing system. A whole palette of members of this innovative class have already been designed, synthesized, and characterized, both as monomers and as electrodeposited oligomer films, changing the biheteroaromatic core, the conjugated side chains, and also inserting further stereogenic elements. The electrochemical properties of the new compound family will be described and discussed as a function of the molecular structure, as monomers (both as racemates and pure enantiomers), as well as electrodeposited chiral conducting surfaces of promising enantiorecognition ability.