Enantioselective Voltammetry & Chiroptical Spectroscopy: Exploring Intriguing Analogies and Connections

UV-Vis absorption spectroscopy, involving intramolecular electron transitions triggered by light, and voltammetry, involving electrode-to/from-molecule electron transfers triggered by the electrode po¬ten¬tial, have well known analogies and connections, and are usually exploited in synergy for the in¬ve¬sti¬gation of electronic properties of advanced molecules and materials. In our recent investi-gations of “inherently chiral” electroactive molecules of axial stereogenicity to be exploited as chiral selectors in electroanalysis and electrochemistry, we realized that, fascinatingly, the two techniques also share many connections and analogies at a superior complexity level, adding chirality to the involved ac¬tors, i.e. considering chiral molecules interacting with the left-handed and right-handed helicoidal com¬ponents of polarized light, (in circular dichroism CD spectroscopy) as well as chiral molecules undergoing electron transfer at a chiral electrode surface (in enantioselective vol-tammetry). [1] In particular we will discuss with appropriate examples the following shared features: (i) Top enantioselection with inherently chiral molecules: “inherently chiral” molecules of helical or axial stereogenicity result in top enantiodiscrimination performance both in chiroptical spectroscopy, in terms of neat differences in absorption of the above L- and D- polarized light components, [2,3] and in enantioselective voltammetry, in terms of neat differences in the electron transfer potentials for (R)- or (S)-molecular probes; [4] (ii) Loss of energy level degeneration for interacting chromophores/redox sites: chiral electroactive molecules of axial stereogenicity, consisting of two equal moieties, behave as equivalent, reciprocally interacting chromophores in CD, and as equivalent, reci¬pro-cally interacting redox centres in CV, in both cases resulting in loss of degeneration of energy levels. This implies absorption wavelength difference and consequent “Davydov splitting” in the CD pattern, [3] as well as a twin peak system in the CV pattern; [1,5] (iii) Pseudochiral manifestations with achiral molecular probes+magnetic fields: CD spectra can also be obtained from polarized light absorption by achiral molecules in a magnetic field, and peak potential differences have been observed on a chiral electrode for achiral molecules in a magnetic field [6]; both phenomena are modulated by the magnetic field intensity and orientation. Support to our chiral electroanalysis research line by Fondazione Cariplo/Regione Lombardia as well as by Università degli Studi di Milano is gratefully acknowledged. [1] S. Arnaboldi, T. Benincori, A. Penoni, L. Vaghi, R. Cirilli, S. Abbate, G. Longhi, G. Mazzeo, S. Grecchi, M. Panigati, P. R. Mussini, Chem. Sci. 2019, 10, 2708-2717. [2] N. Berova, L. Di Bari, G. Pescitelli, Chem. Soc. Rev.,36, 2007, 914-931. [3] J. T. Vázquez, Tetrahedron: Asymmetry, 28, 2017, 1199-1211. [4] S. Arnaboldi, M. Magni, P. R. Mussini, Curr. Opin. Electrochem. 2018, 8, 60-72. [5] F. Sannicolò, S. Arnaboldi, T. Benincori, V. Bonometti, R. Cirilli, L. Dunsch, W. Kutner, G. Longhi, P. R. Mussini, M. Panigati, M. Pierini, S. Rizzo, Angew. Chem. Int. Ed., 53, 2014, 2623-2627. [6] T. Benincori, S. Arnaboldi, M. Magni, S. Grecchi, R. Cirilli, C. Fontanesi, P. R. Mussini, Chem. Sci. 2019, 10, 2750-2757.