Characterization of Inherently Chiral Electrosynthesized Oligomeric Films by Voltammetry and Scanning Electrochemical Microscopy (SECM)

Abstract

A voltammetric and scanning electrochemical microscopy (SECM) investigation was performed on an inherently chiral oligomer-coated gold electrode to establish its general properties (i.e., conductivity and topography), as well as its ability to discriminate chiral electroactive probe molecules. The electroactive monomer (S)-2,2′-bis(2,2′-bithiophene-5-yl)-3,3′-bibenzothiophene ((S)-BT2T4) was employed as reagent to electrodeposit, by cyclic voltammetry, the inherently chiral oligomer film of (S)-BT2T4 (oligo-(S)-BT2T4) onto the Au electrode surface (resulting in oligo-(S)-BT2T4-Au). SECM measurements, performed in either feedback or competition mode, using the redox mediators [Fe(CN)6]4− and [Fe(CN)6]3− in aqueous solutions, and ferrocene (Fc), (S)-FcEA, (R)-FcEA and rac-FcEA (FcEA is N,N-dimethyl-1-ferrocenylethylamine) in CH3CN solutions, indicated that the oligomer film, as produced, was uncharged. The use of [Fe(CN)6]3− allowed establishing that the oligomer film behaved as a porous insulating membrane, presenting a rather rough surface. This was inferred from both the approach curves and linear and bidimensional SECM scans, which displayed negative feedback effects. The oligomer film acquired semiconducting or fully conducting properties when the Au electrode was biased at potential more positive than 0.6 V vs. Ag|AgCl|KCl. Under the latter conditions, the approach curves displayed positive feedback effects. SECM measurements, performed in competition mode, allowed verifying the discriminating ability of the oligo-(S)-BT2T4 film towards the (S)-FcEA and (R)-FcEA redox mediators, which confirmed the results obtained by cyclic voltammetry. SECM linear scans indicated that the enantiomeric discriminating ability of the oligo-(S)-BT2T4 was even across its entire surface.

Modulating the Enantiodiscrimination Features of Inherently Chiral Selectors by Molecular Design: A HPLC and Voltammetry Study Case with Atropisomeric 2,2’‐Biindole‐Based Monomers and Oligomer Films

“…Concerning currents, they again exhibit a linear dynamic range for the probe enantiomer concentrations, but in this case they appear comparable or even higher respect to those recorded on bare electrode. This feature looks consistent with the whole film having become conductive and thus potentially acting as electrode [33] …”

“…In the light of this, the smaller peak potential difference in the oligo-2 case should point to less effective coordination and/or smaller differentiation in coordination ability, linked to the perturbing presence of the long aliphatic chains, more likely than to the chiral film being possibly thinner (Figure SI.4). Concerning peak currents, they are notably lower than on the bare electrode; this looks consistent with the neutral state of the oligomer film, implying that the probe can only react diffusing inside the film to the GC electrode surface; [33] this could be easier in the case of oligo-2, possibly resulting in the same conditions in a thinner film, as already mentioned, which could justify the higher currents with respect to the oligo-1 case.…”

“…This feature looks consistent with the whole film having become conductive and thus potentially acting as electrode. [33] …”

“…This feature looks consistent with the whole film having become conductive and thus potentially acting as electrode. [33] Selector implemented as inherently chiral additive in ionic liquid: The enantiodiscrimination ability of monomer 3, unsuit- able for electrooligomerization, was tested employing it as chiral additive in achiral ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide BMIMTf 2 , according to our alternative recently developed enantiodiscrimination strategy which exploits the high and modulable local order of ionic liquid media at the electrochemical interphase. [8,13] In this case CV tests were performed with (R)-Fc and (S)-Fc probes in very thin solution layers on screen printed graphite working electrodes, resulting in a potential difference for the two enantiomers even larger (about 230 mV) than that obtained for the same probes on oligo-1 films in CH 2 Cl 2 + 0.1 M TBAPF 6 (Figure 12).…”