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

Abstract

A family of inherently chiral electroactive selectors based on the 2,2'-biindole atropisomeric scaffold, of easy synthesis and modulable functional properties, is studied in cascade in two enantioselection contexts. They are at first investigated as probes in enantioselective HPLC, studying molecular structure and temperature effects, and achieving very efficient semipreparative enantioseparation. The enantiomers thus obtained, of remarkable chiroptical features (optical rotation as well as circular dichroism), are successfully applied as selectors in chiral voltammetry in different media for discrimination of the enantiomers of chiral electroactive probes, either by conversion into enantiopure electroactive electrode surfaces by electrooligomerization on glassy carbon substrate (the two monomers with shorter alkyl chains), or as chiral additive in achiral ionic liquid (the monomer with longest alkyl chains). Discrimination is conveniently and reproducibly achieved in terms of significant potential differences for the two enantiomers, specularly inverting either probe or selector configuration. In one case successful discrimination is also observed with the two probe enantiomers concurrently present, either as racemate or with enantiomeric excesses, neatly accounted for by the peak current ratios.

Trópos and Átropos Biindole Chiral Electroactive Monomers: A Voltammetry and HPLC Comparative Insight

“… different connectivity between biindole core moieties, i. e . 2,2’ [5,6] vs 3,3’ (with wings conversely attached in 3,3’ and in 2,2’ positions respectively, Figure 2 ), a feature which we realized has a huge impact on the potential difference for the twin peak system; and without or with a phenyl spacer [6] between the biindole core and the (bi)thiophene terminals, a feature affecting the core/terminal conjugation. …”

“…A phenyl group inserted between core and terminals (monomers 2 a , 2 b ) acts more as a spacer than as a linker, [6] resulting in a first oxidation twin peak system localized on the biindole core and shifted at significantly more positive potentials, since no more benefit can come from conjugation with adjacent thiophene rings (Figure 3, Table 1, [6] ). A further oxidation system localized on the bithiophene terminal follows, at a potential close to α‐bithiophene oxidation ( [45] normalized vs Fc + |Fc [46] ).…”

“…A further oxidation system localized on the bithiophene terminal follows, at a potential close to α‐bithiophene oxidation ( [45] normalized vs Fc + |Fc [46] ). (Figure 3, Table 1, [6] ). In other words, the phenyl spacer makes core and terminal oxidations nearly independent.…”

“…In particular, it has been so far implemented exploiting monomers based on four atropisomeric biheteroaromatic cores, i. e . 3,3’‐bibenzothiophene, [3,9,10] 3,3’‐bithiophene, [4] 2,2’‐biindole [5–7] and 1,1’‐binaphthyl [8] ones, in all cases combined with symmetrical oligothiophene‐based terminals (some examples are reported in Figure 1). Besides their above highly successful application for chiral electroanalysis, such monomers are also attractive model systems in the frame of intramolecular charge communication studies.…”

“…and without or with a phenyl spacer [6] between the biindole core and the (bi)thiophene terminals, a feature affecting the core/terminal conjugation.…”