In Situ Electrochemical Investigations of Inherently Chiral 2,2′‐Biindole Architectures with Oligothiophene Terminals

Abstract

The synthesis and characterization of three new inherently chiral N,N′‐dipropyl‐3,3′‐diheteroaryl‐2,2′‐biindole monomers, nicknamed Ind2T4, Ind2T6 and Ind2Ph2T4, which differ in the number of thiophenes as terminals, are reported. In addition to a full monomer characterization, stable electroactive oligomeric films were obtained by electro‐oxidation upon cycling to potentials which activate the thiophene terminals. Cyclic voltammetry, UV‐Vis‐NIR spectroelectrochemistry and in situ conductance measurements show that oligomeric films of Ind2T6 present the best stability and electrochromic switching performance. Enantioselective tests with a chiral ferrocene amine clearly show the potential as chiral selectors for analytical and sensing purposes.

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

“…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.…”

“…A winning strategy to achieve chiral electrode surfaces of huge and wide-scope enantiodiscrimination ability consists in the electrodeposition of "inherently chiral" oligomer films from electroactive monomers [1] of helical or axial stereogenicity, i. e. based either on a helical backbone [2] or on an atropisomeric one, that is, consisting of two, usually identical, moieties with reciprocal high sterical hindrance. [3][4][5][6][7][8] In such monomers, both chirality and key functional properties, like electrochemical and optical activity, originate from the whole main molecular backbone, featuring a tailored torsion associated with an energy barrier (enantiomerization barrier) too high to be overcome at room temperature. [1] Thus such molecules can exist as stable (R)-and (S)-enantiomers, that can be separated and stored; they also fully retain their configuration when electrooligomerizing, yielding (R)-or (S)-enantiopure oligomer films.…”

“…[1] Thus such molecules can exist as stable (R)-and (S)-enantiomers, that can be separated and stored; they also fully retain their configuration when electrooligomerizing, yielding (R)-or (S)-enantiopure oligomer films. [3] Such electrode surfaces result in significant, often wide, potential differences for the enantiomers of a variety of chiral electroactive probes in voltammetry experiments, [3][4][5][6][7][8][9][10] which can be ascribed to energetically different conditions for the probe enantiomers at the enantiopure surface, achieved through the many coordination elements available (heteroatoms and π conjugated systems). [11][12][13][14] The axial stereogenicity approach, combining high three-dimensional character and oligomerization ability with convenient synthesis and wide structure modulability, has been so far more exploited than the helical one.…”

Cover Feature: Trópos and Átropos Biindole Chiral Electroactive Monomers: A Voltammetry and HPLC Comparative Insight (ChemElectroChem 6/2022)

“…A winning strategy to achieve chiral electrode surfaces of huge and wide-scope enantiodiscrimination ability consists in the electrodeposition of "inherently chiral" oligomer films from electroactive monomers [1] of helical or axial stereogenicity, i.e. based either on a helical backbone [2] or on an atropisomeric one, that is, consisting of two, usually identical, moieties with reciprocal high sterical hindrance [3][4][5][6][7][8]. In such monomers, both chirality and key functional properties, like electrochemical and optical activity, originate from the whole main molecular backbone, featuring a tailored torsion associated with an energy barrier (enantiomerization barrier) too high to be overcome at room temperature.…”

“…Thus such molecules can exist as stable (R)-and (S)-enantiomers, that can be separated and stored; they also fully retain their configuration when electrooligomerizing, yielding (R)-or (S)enantiopure oligomer films. [3] Such electrode surfaces result in significant, often wide, potential differences for the enantiomers of a variety of chiral electroactive probes in voltammetry experiments [3][4][5][6][7][8][9][10], which can be ascribed to energetically different conditions for the probe enantiomers at the enantiopure surface, achieved through the many coordination elements available (heteroatoms and  conjugated systems) [11][12][13][14]. The axial stereogenicity approach, combining high three-dimensional character and oligomerization ability with convenient synthesis and wide structure modulability, has been so far more exploited than the helical one.…”