Enantiomer discrimination in absorption spectroscopy and in voltammetry: highlighting fascinating similarities and connections

Enantiomer discrimination in absorption spectroscopy and in voltammetry: highlighting fascinating similarities and connections Patrizia Romana Mussini1, Serena Arnaboldi1, Mirko Magni1,2, Sara Grecchi1 , Giovanna Longhi3, Tiziana Benincori4 1 Università degli Studi di Milano, Dipartimento di Chimica 2 present address: Università degli Studi di Milano, Dipartimento di Scienze e Politiche Ambientali; 3Università degli Studi di Brescia, Dipartimento di Medicina Molecolare e Translazionale 4Università degli Studi dell’Insubria, Dipartimento di Scienza e Alta Tecnologia е-mail: patrizia.mussini@unimi.it Electronic absorption spectroscopy, involving intramolecular electron transitions triggered by light, and voltammetry, involving electrode-to/from-molecule electron transfers triggered by the electrode potential, have well known analogies and connections, and are usually exploited in synergy for the investigation of electronic properties of advanced molecules and materials. In our recent investigations 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, when adding chirality to the involved actors, i.e. considering chiral molecules interacting with the left-handed and right-handed helicoidal components of polarized light (in circular dichroism CD spectroscopy) as well as chiral molecules undergoing electron transfer at a chiral electrode surface (in enantioselective voltammetry). [1] In particular, the following points are indeed worthy of interest: (i) High performance achieved with inherently chiral molecules: “inherently chiral” molecules of helical or axial stereogenicity usually result in high enantiodiscrimination performances both in chiroptical spectroscopy, in terms of neat differences in absorption of the above left-handed and right-handed 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, reciprocally interacting redox centres in CV, in both cases resulting in loss of degeneration of energy levels. This implies an absorption wavelength difference with “Davydov splitting” in the CD pattern [3] as well as a twin peak system in the CV pattern [5,6]; (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 chiral electrode surfaces for achiral molecules in a magnetic field [7]; both phenomena are modulated by the magnetic field intensity and orientation. [1] P.R. Mussini, S. Arnaboldi, M. Magni, S. Grecchi, G. Longhi, T. Benincori, Curr. Opin. Electrochem. 37 (2023) 101128. [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. 8 (2018) 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. 10 (2019) 2750-2757. [7] 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. 10 (2019) 2708-2717.