Self‐Standing Membranes Consisting of Inherently Chiral Electroactive Oligomers: Electrosynthesis, Characterization and Preliminary Tests in Potentiometric Setups

Abstract

Self‐standing chiral electroactive synthetic membranes are presented, prepared by oxidative electro‐oligomerization of a thiophene‐based “inherently chiral” electroactive monomer on indium tin oxide (ITO) or fluorine‐doped tin oxide (FTO) electrodes, followed by detachment of the electrodeposited thin films in aqueous solution. The membranes, possibly mesoporous, consist of a mixture of open and cyclic “inherently chiral” oligomers, that is, in which both chirality and electroactivity originate from the same source, and this is the main conjugated backbone featuring a tailored torsion. Such a combination can afford outstanding chirality manifestations. The electrosynthesis conditions significantly modulate the oligomer distribution. Racemate films are compared to enantiopure ones. Circular dichroism confirms that (R)‐ or (S)‐ enantiopure films are obtained, starting from the corresponding (R)‐ or (S)‐ enantiopure monomers. Reliable transmembrane potential readings are obtained in preliminary tests in ion‐selective electrode (ISE)‐like setups, consistent with those predicted considering the membrane features, offering a first step towards extension of the protocol to chiral experiments.

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

“…The so far best studied example of inherently chiral monomer is 2,2′-bis(2,2′-bithiophene-5-yl)-3,3′bibenzothiophene (BT2T4) (Scheme 1), featuring a 3,3′-bibenzothiophene atropisomeric core (with sterically hindered rotation between its two halves); its C2 symmetry results in propagation and amplification of its outstanding chirality properties in macro-and supramolecular structures [7]. Because these monomers are electroactive and have high electrooligomerization ability, they represent suitable starting reagents for the electrodeposition of inherently chiral oligomers onto electrode surfaces [6][7][8]. Chemically produced oligomers can also be dropcasted onto the electrode surface [9].…”

“…In this work, for the first time, we report a SECM study to characterize the enantiopure (S)-BT2T4 oligomer, electrodeposited onto the surface of a gold disk electrode, starting from the corresponding Because these monomers are electroactive and have high electrooligomerization ability, they represent suitable starting reagents for the electrodeposition of inherently chiral oligomers onto electrode surfaces [6][7][8]. Chemically produced oligomers can also be dropcasted onto the electrode surface [9].…”

Widening the Scope of “Inherently Chiral” Electrodes: Enantiodiscrimination of Chiral Electroactive Probes with Planar Stereogenicity

“…Oxidation of the P III into the respective P V sulfides was performed and open oligomers (bottom) constituting the electrodeposited chiral electrode surface. [35,45] by stirring of the parent compound with elemental sulfur. Substituents in 1'-position were introduced as described recently [82] .…”

“…Contrary to (S)-(À )-and (R)-(+)-N,N-dimethyl-1-ferrocenylethylamine, a standard chiral ferrocenyl probe with central stereogenicity, which undergoes first oxidation well before the film oxidation, and usually results in (quasi) canonical, reversible peaks on oligo-(BT 2 T 4 ) films, [35,36,45] the first oxidation process of the planar-stereogenicity probes is shifted to the onset of the film oxidation, on account of the highly electron attracting phosphane sulfide substituent, as discussed earlier. This can justify the irreversible, non-canonical first oxidation peak shape, quite different from the bare electrode case.…”

“…[35][36][37][38][39][40][41][42][43][44] A thoroughly investigated model example is provided by the 2,2'-bis[2-(5,2-bithienyl)]-3,3'-bi-thianaphthene (BT 2 T 4 ) electroactive monomer, inherently chiral on account of its atropisomeric bibenzothiophene core (i. e., with sterically hindered rotation between its two halves). By electrooxidation it enables fast, reproducible electrode modification with films constituted by a mixture of open and cyclic inherently chiral electroactive oligomers (Figure 1), [35,45] resulting in effective enantiodiscrimination of chiral electroactive probes of different nature. [35][36][37][38] Remarkably, the same inherent chirality strategy has proved successful with other inherently chiral electrodeposited films, either based on different atropisomeric cores, i. e. bithiophene [39] and biindole, [40] or on a helical element as tetrathia [7]helicene [41] (carbohelicene films [46] could be also tested).…”