Enantioselective voltammetry on achiral electrodes: a comparison between inherently chiral additives based on different stereogenic elements

An attractive target in electroanalysis is the availability of chiral media affording enantioselection in terms of significant peak potential difference between the antipodes of chiral probes in voltammetry experiments on achiral electrodes. Previous literature attempts pointed to enantioselectivity increasing with the structural order of the chiral medium; on the other hand, outstanding enantioselection performances have been recently observed working on electrode surfaces consisting in "inherently chiral" oligomer films, that is, in which chirality does not originate from localized stereocenters, but is intrinsic of the whole main molecular backbone, featuring a tailored torsion [1-5]. Combining both strategies we have recently developed two inherently chiral ionic liquids ICILs, consisting of dialkylated bicollidinium salts, with an atropoisomeric bipyridinium cation featuring at least one octyl chain and bistrifilimide counteranions. They showed high enantioselectivity when tested even as low concentration additives in commercial achiral ionic liquid media [6]. Importantly, similar ability was also shown by other terms of the same family, having shorter alkyl chains and/or different counteranions, solid at room temperature but of easier synthesis. As a first temptative explanation we are considering the high supramolecular order of even simple ionic liquids at the interphase with a charged surface. A chiral additive could result in chiral reorganization of this peculiar interphase, as in the case of nematic-to-cholesteric transitions induced by chiral dopants in liquid crystals. [6] Such application of the chiral selector as additive in an achiral ionic liquid rather than as the bulk medium is indeed interesting and convenient, since only a small quantity of the enantiopure selector is needed and a low melting point is no more required, but only sufficient solubility in the achiral ionic liquid. This allowed us to include in our chiral voltammetry experiments a quite larger number of inherently chiral selectors based on different stereogenic elements, i.e., the bicollidine and bibenzimidazole atropoisomeric scaffolds and the tetrathielicene helicoidal scaffold. They all proved successful. Large potential differences were observed on achiral electrodes for the enantiomers of chiral probes even of very different structure and electroactivity, as in the former tests on inherently chiral electrodes, and possibly even in experiments with more than one probe, providing further evidence of the general validity of the "inherent chirality" strategy in the development of efficient chiral selectors in electroanalysis.