Inherently chiral molecular materials with 2,2'- and 3,3'-bisindole atropoisomeric cores: interactions between equivalent redox sites, configurational stability and enantioselection ability

The peculiarity of inherently chiral molecular materials is that the same element endows the molecule with both its key functional property and with chirality, coinciding with the main molecular backbone featuring a tailored torsion; this results in outstanding chirality manifestations. Recently we have presented "inherently chiral" enantiopure electrodes resulting in large potential differences for the enantiomers of chiral probes in voltammetry experiments; they were prepared by electrooligomerization of monomers having atropoisomeric bibenzothiophene or bithiophene cores. [1-5]. Concurrently, we have also developed a large family of inherently chiral monomers having 2,2'- or 3,3'-bisindole atropoisomeric cores. Since indole is electron richer than thiophene, the first two oxidations are shifted at significantly less positive potentials, and localized on the two interacting moieties of the bisindole core rather than on the terminal thiophene wings, and therefore are chemically reversible (oligomerization can be achieved cycling around the third oxidation peak). Moreover, indoles can be N-alkylated, affording modulation of important properties such as solubility and therefore processability. We will present a detailed electrochemical study on the monomer redox properties and oligomerization ability as a function of the molecular structure in this compound family. A quite original and attractive feature concerns the interaction between the two equivalent redox centers in the biindole cores, (which can be estimated from the potential difference between the corresponding oxidation peaks), since it can be shown to account for the atropoisomeric energy barrier (depending on the 2,2' or 3,3' connectivity and on the N-alkyl substituents), and to be also nicely modulated by temperature and solvent polarity. Thus electrochemistry can provide information on the torsional energy barrier and on the enantiomer stability, confirmed by other approaches. Finally, enantioselectivity tests on films obtained by electrooligomerization of the more configurationally stable 2,2'-oligomers yield large potential differences for the antipodes of very different chiral probes, also of pharmaceutical interest. The support of Fondazione Cariplo/Regione Lombardia "Avviso congiunto per l’incremento dell’attrattività del sistema di ricerca lombardo e della competitività dei ricercatori candidati su strumenti ERC - edizione 2016” (Project 2016-0923) is gratefully acknowledged.