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

Absorption spectroscopy and voltammetry, of known analogies and connections, share even more fascinating similarities and connections at a higher complexity level, when "upgrading" them with the ability to discriminate between enantiomers by chiral selector implementation. In both techniques either "molecular" selectors or "electromagnetic" ones (L- versus Rcircularly polarized light components for spectroscopy, ccversus b-spin electrons for voltammetry) can be considered; moreover, external magnetic field application can replace a truly chiral actor. A tentative schematization is provided. Analogies and connections also concern molecular features of the enantiodiscrimination actors. In both techniques outstanding performances are obtained with inherently chiral molecules, in which a conjugated backbone with tailored torsion is source of chirality as well as spectroscopic and electrochemical activity, in an attractive three-fold interconnection. Their outstanding effects can be justified by a combination of chemical and electromagnetic properties (excellent potential molecular spin filters), a fascinating challenge for future developments

Chiral Recognition: A Spin-Driven Process in Chiral Oligothiophene. A Chiral-Induced Spin Selectivity (CISS) Effect Manifestation

In this paper it is experimentally demonstrated that the electron-spin/molecular-handedness interaction plays a fundamental role in the chiral recognition process. This conclusion is inferred comparing current versus potential (I-V) curves recorded using chiral electrode surfaces, which are obtained via chemisorption of an enantiopure thiophene derivative: 3,3 & PRIME;-bibenzothiophene core functionalized with 2,2 & PRIME;-bithiophene wings (BT2T4). The chiral recognition capability of these chiral-electrodes is probed via cyclic voltammetry measurements, where, Ag nanoparticles (AgNPs) capped with enantiopure BT2T4 (BT2T4@AgNP) are used as the chiral redox probe. Then, the interface handedness is explored by recording spin-polarized I-V curves in spin-dependent electrochemistry (SDE) and magnetic-conductive atomic force microscopy (mc-AFM) experiments. The quality of the interfaces is thoroughly cross-checked using X-ray photoemission spectroscopy, Raman, electrodesorption measurements, which further substantiate the metal(electrode)-sulfur(thiophene) central role in the chemisorption process. Spin-polarization values of about 15% and 30% are obtained in the case of SDE and mc-AFM experiments, respectively.It is demonstrated that probing the handedness of a chiral system (here a chiral-electrode-surface/solution interface) by using a spin-polarized current, allows for chiral recognition. This conclusion is inferred by tight comparison with cyclic voltammetry results, where the handedness of the "chiral-electrode-surface/solution interface" is recognized by using an enantiopure chiral redox couple.imag

Metal-Free Alpha Trifluoromethylselenolation of Carbonyl Derivatives under Batch and Flow Conditions

Trifluoromethylselenolated carbonyl compounds represent an emerging class with potential applications in several fields; however, a widespread use of such compound is hampered by the very limited number of strategies for their preparation. In this study we developed a method for the preparation of alpha-SeCF3 substituted carbonyl derivatives using an in situ generated electrophilic ClSeCF3 species. We also implemented an in-flow protocol to improve the safety features of the process

"Solid state charge trapping": Examples of polymer systems showing memory effect

The paper reports on a characteristic property of electroactive materials bearing an electron-rich and an electron-poor moiety, known as charge trapping. As examples of materials that exhibit this phenomenon, films of poly(4,4"-dipentoxy-4\u27-(2,2\u27-dicyano)ethenyl-2,2\u27:5\u27,2"-terthiophene), poly(2,3-dihexylthieno[3,4-b]pyrazine) and a blend between a fulleropyrrolidine derivative and poly(3-hexylthiophene) were investigated by cyclic voltammetry, spectroelectrochemistry and electrochemical quartz crystal microbalance. In the cyclic voltammetry, the reduction processes show the reverse oxidation potential about 1 V higher than the expected value, indicating a strong stabilization of the corresponding anion species. The mechanism leading to the stabilisation of the anions is discussed and the results indicate that the investigated materials exhibit a remarkable and quite stable memory effect

Panoramic Overview on the Enantioselection Performance of Inherently Chiral Surfaces: a Comparison between Systems with Different Atropisomeric Cores and Stereogenic Elements

Enantiorecognition is a key issue in advanced analytical chemistry, particularly concerning the biological and pharmaceutical field. Enantiomeric molecules, being mirror-image structures, have identical physico-chemical scalar properties, but opposite pseudo-scalar ones. When interacting with a racemic probe, chiral molecules are able to recognize the enantiomers through diasteromeric interactions. Similarly, chiral electrodes are required for enantioselective electroanalysis, and the development of "intelligent" electrodes capable of discriminating enantiomers, in particular molecules of biological and pharmaceutical importance, remains as one of the major challenges in electroanalysis. We have recently proposed the first synthetic inherently chiral electrode surfaces able to neatly discriminate as separate peaks (in terms of potential values) the antipodes of model chiral probes, also drugs, both as enantiopure and racemate. [1-2] We have also verified the general validity of the inherently chiral concept, which does not depend from the chemical nature of the atropisomeric scaffold, testing chiral surfaces electrooligomerized from starting monomers with different molecular design (i.e. bithiophene, bibenzothiophene, biindole and paracyclophane cores) and different stereogenic elements (stereogenic axis vs helix vs plane). In order to fully elucidate the enantioselection capability of all of these heteroaromatic systems we propose a detailed comparison (an example in Figure) of our inherently chiral surfaces with different atropisomeric core vs thiahelicene-based films vs \u201ctwo floor\u201d paracyclophanic oligomers. Figure. Enantioselection properties of inherently chiral oligomers with bibenzothiophene and biindole units towards L- and D-DOPA probes. The support of Fondazione Cariplo/Regione Lombardia "Avviso congiunto per l\u2019incremento dell\u2019attrattivit\ue0 del sistema di ricerca lombardo e della competitivit\ue0 dei ricercatori candidati su strumenti ERC - edizione 2016\u201d (Project 2016-0923) is gratefully acknowledged. References: [1] F. Sannicol\uf2, 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. [2] S. Arnaboldi, T. Benincori, R. Cirilli, W. Kutner, M. Magni, P. R. Mussini, K. Noworyta, F. Sannicol\uf2, Chem. Sci., 6 (2015) 1706-171

Highly enantioselective “inherently chiral” electroactive materials based on a 2,2' -biindole atropisomeric scaffold

Chiral oligothiophene monomers with C2 symmetry, based on 3,30 -bithiophene atropisomeric cores with high racemization barriers, have recently been shown to provide excellent chiral starting materials with high electroactivity for the easy preparation of enantiopure electroactive films endowed with powerful chirality manifestations. We now introduce an inherently chiral monomer based on a 2,20 -biindole core, as the prototype of a new inherently chiral monomer family, whose properties could be modulable through functionalization of the pyrrolic N atoms. By fast, regular electrooligomerization the new monomer yields inherently chiral films with high, reversible electroactivity and, above all, impressive enantioselectivity towards very different chiral probes, some of pharmaceutical interest, as generalscope electrode surfaces. Such results, while opening the way to a new, attractive inherently chiral selector class, nicely confirm the general validity of the inherent chirality strategy for chiral electrochemistry. Furthermore, the enantioselectivity of the new selectors not only holds with electroactive chiral probes, but also with circularly polarized light components as well as electron spins, resulting in good chiroptical and spin filter performances, which suggests fascinating correlations between the three contexts

Electroactive Inherently Chiral Surfaces at Work: Clues Toward the Elucidation of the Enantioselection Mechanism

Chirality is a concept strictly related to life and to its evolution. Capability to discriminate antipodes and/or produce enantiopure chiral chemicals through cheap and efficient protocols is a crucial task for our modern civilization. So identification of increasingly effective and robust chiral selectors is a challenging task also for the electrochemical community [1,2]. In this frame our research group is working on the so called \u201cinherently chiral functional molecular materials\u201d, ICFMMs; the idea is simple: make the stereogenic element responsible for chirality coincident with the functional group responsible for the material specific property (Figure, left). This approach has constituted an actual breakthrough in chiral electrochemistry, resulting in the preparation of efficient chiral electroactive surfaces [3,4,5] (and chiral additives/media, too [6]) invariably characterized by outstanding enantiodiscrimination ability in quite different working conditions and with chemically different chiral electroactive analytes. Notwithstanding plenty of proofs pointing to a general validity of the ICFMMs concept, a clear rationalization of the enantiodiscrimination mechanism still lacks. To fill the gap a deeper knowledge of the behavior of our electrodeposited chiral films is mandatory. As a first step some of the most important experimental parameters governing the growth of the conductive coatings have been changed, one by one, to evaluate their impact on the morphological, optical and electronic properties of the final deposit. Results of the multi-technique characterization will be discussed, including profilometry, electrochemical impedance spectroscopy (Figure, right) and spectroelectrochemistry data, all aimed to collect clues useful to rationalize the way in which ICFMMs work. The support of Fondazione Cariplo/Regione Lombardia (Project 2016-0923) and SmartMatLab are gratefully acknowledged. References: [1] S. Arnaboldi, M. Magni, P. Mussini, Curr. Opin. Electrochem., 2018, 8, 60. [2] S. Arnaboldi, S. Grecchi, M. Magni, P. Mussini, Curr. Opin. Electrochem., 2018, 7, 188. [3] F. Sannicol\uf2, P.R. Mussini, T. Benincori, R. Martinazzo, S. Arnaboldi, G. Appoloni, M. Panigati, E. Quartapelle Procopio, V. Marino, R. Cirilli, S. Casolo, W. Kutner, K. Noworyta, A. Pietrzyk-Le, Z. Iskierko, K. Bartold, Chem. Eur. J., 2016, 22, 10839. [4] S. Arnaboldi, P.R. Mussini, M. Magni, F. Sannicol\uf2, T. Benincori, R. Cirilli, K. Noworyta, W. Kutner, Chem. Sci., 2015, 6, 1706. [5] F. Sannicol\uf2, 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., 2014, 53, 2623. [6] S. Rizzo S. Arnaboldi, V. Mihali, R. Cirilli, A. Forni, A. Gennaro, A.A. Isse, M. Pierini, P.R. Mussini, F. Sannicol\uf2, Angew. Chem. Int. Ed., 2017, 56, 2079

Artificial enantiopure inherently chiral membranes: enantiodiscrimination trough a new “ion-selective like” setup

High-efficiency resolution technology is fundamental for scaling-up separation of enantiomerically pure substances. Membrane technology fulfils this requisite, in fact it is characterized by i) high efficiency, ii) simplicity and iii) convenience for up- and/or down-scaling. Membrane-based chiral resolution can be achieved using either enantioselective or non-enantioselective membranes. Enantioselective membranes can be used for chiral separation of enantiomers because they contain chiral recognition sites. In this frame we have discovered that the electrooligomerization, in acetonitrile as solvent, for 108 deposition cycles, on an ITO electrode support, of our \u201cinherently chiral\u201d benchmark monomer, leads to self-standing racemic or enantiopure membranes. These ones were obtained by simply peeling off the solid deposit from the ITO immersed in water after the electrodeposition in acetonitrile. We have then characterized inherently chiral membranes by a multivariate technique approach (e.g. electrochemical impedance spectroscopy, scanning electron microscopy, BET for surface area and pore size distribution, and atomic force microscopy) comparing the racemic vs enantiopure deposit properties. Considering i) the outstanding enantioselection ability achieved with our both inherently chiral electrode surfaces and media [1-2] and ii) the perfectly specular CD spectra displayed by the two membrane enantiomers, we have decided to implement enantiopure inherently chiral membranes in a \u201cion-selective like\u201d set-up in order to study their enantiorecognition capability (as depicted in Figure on the right). First of all we have verified the potential difference was read correctly through the membrane to allow correct determinations of transmembrane potentials. After that we have tested enantiopure membranes in the presence of chiral charged species (in all configurations for both membranes and internal/external electrode solutions) for determining their enantioselective capability. Preliminary results are very promising and encourage us to perform the scaling up of the membrane electrosynthesis to be used for industrial scopes and to extend the study to other probe useful in the analytical and pharmaceutical field. References: [1] S. Arnaboldi, M. Magni, P. R. Mussini, Curr. Op. in Electrochemistry 8 (2018) 60-72. [2] S. Arnaboldi, S. Grecchi, M. Magni, P.R. Mussini, Curr. Op. in Electrochemistry 7 (2018) 188-199

Going beyond the Surface: a Glance inside Smart Conducting Molecular Surfaces through a Multitechnique Approach

Conducting organic polymers, COPs, are smart materials that merge some of the most interesting properties of common polymers (e.g. flexibility, processability, etc.) with high electrical conductivity of metals. Research in this field is currently attracting increasing attention, since these innovative materials are very promising for a great variety of applications, from energetics to electronics and sensoristics, even from an industrial point of view. Chirality makes COPs even smarter materials, opening the way to enantioselective electroanalysis/electrosynthesis. In particular the \u201cinherent chirality\u201d concept proposed by our groups some years ago actually represented a breakthrough, significantly improving all other literature approaches so far proposed, making possible deposition of conducting homochiral oligomeric films acting as effective, efficient and robust enantioselectors toward a great variety of chiral analytes, in different media.. The further natural step is the comprehension of the actual working mechanism of these intelligent surfaces. To reach such intriguing target a deep and multivariate characterization is mandatory, to reveal as much properties as possible that could be finally combined to depict a complete portrait of these conducting inherently chiral films. In this short presentation we will glance at these smart chiral conducting molecular surfaces, following an ideal tour from outside (i.e. surface appearance) to their inner parts (i.e. optical and electronic features). The support of Fondazione Cariplo/Regione Lombardia (Project 2016-0923) and SmartMatLab are gratefully acknowledged

Enantiorecognition performances of "inherently chiral" film electrodes: a successful first example with planar-chirality probes

Enantiorecognition performances of "inherently chiral" film electrodes: a successful first example with planar-chirality probes Patrizia Mussini,a Serena Arnaboldi,aGiorgio Tomboni,a Mirko Magni?,a Francesco Sannicol\uf2a Heinrich Lang,b Marcus Korbb,Tiziana Benincoric aUniversit\ue0 degli Studi di Milano, Dipartimento di Chimica,Via Golgi 19,20133 Milano, bTechnische Universit\ue4t Chemnitz, Stra fe der Nationen 62, 09111 Chemnitz, Germany cUniv. degli Studi dell\u2019Insubria, Dip. di Scienza e Alta Tecnologia, Via Valleggio 11, 22100 Como, Italy [email protected] Enantiorecognition in voltammetry is a quite attractive target, implying a superior selectivity degree, which necessarily requires the electron transfer to take place in a chiral interphase environment, exploiting a chiral electrode surfaces or a chiral medium. Many approaches have been proposed, but unfortunately however most of them suffer from some drawback, like complex preparation, high cost and/or lack of robustness...., but above all, in most cases, they result in current differences between the two enantiomer of chiral probes, while a difference in peak potentials would be the desirable feature for enantiorecognition purposes.[1,2] In this context, we have recently reported the outstanding performances of "inherently chiral" electrodes prepared by fast and reproducible electrodeposition of a thin film of thiophene-based oligomers from "inherently chiral" monomers like BT2T4 (right) [3-6]. "Inherent chirality" implies that chirality and key functional properties originate from the same structural element; in the cited monomer cases, this is obtained by a tailored torsion in the conjugated electroactive system, with an energy barrier too high to be overcome at room T, so that the monomer exists in two stable enantiomers. Upon electrooligomerization of the (R)- or (S)- enantiomer, electroactive oligomer films are obtained, including linear and cyclic terms of different dimensions, and fully retaining the monomer configuration. Testing such electrode surfaces in chiral CV experiments with chiral probes, neat differences in peak potentials are observed for the enantiomers of chiral probes even of significantly different structure, specular upon inverting the film or probe configuration. The property appears to be of general character, testing a first small series of probes of different structures, even of pharmaceutical interest [3-6] (and also working on achiral electrodes in inherently chiral ionic-liquid based media[7,8]). Of course, it is important to widen the range of investigated cases. One issue concerns testing probes having other stereogenic elements than stereogenic centres (which is the most current occurence, as in our first tested cases), considering for instance axial chirality, helical chirality, and planar chirality. In this presentation we will focus on the latter case, for which convenient, electrochemically reversible model probes are provided by disubstituted ferrocene scaffolds like the one shown on the left (with R1\uf0b9 R2). Successful and reproducible chiral voltammetry tests with a first couple of such planar chirality examples on oligo BT2T4 films nicely confirm the general character of the successful inherent chirality electrode strategy. The current support of Fondazione Cariplo/Regione Lombardia "Avviso congiunto per l\u2019incremento dell\u2019attrattivit\ue0 del sistema di ricerca lombardo e della competitivit\ue0 dei ricercatori candidati su strumenti ERC - edizione 2016\u201d (Project 2016-0923) to our inherently chiral research is gratefully acknowledged- References: 1. Curr.Op. 2018, 7, 188-199 2 Curr. Op. 2018, 8, 60-72 3. Angew. Chem. Int. Ed. 2014, 53, 2623. 4. Chem. Eur. J. 2014, 20, 15298. 5. Chem. Sci. 2015, 6,1706. 6. Chem. Eur.2016 , 22,10839. 7. Anal. Bioanal. Chem. 2016, 408, 7243. 8. Angew Chem.2017 9. Electrochem. Comm. 2018, 89, 57-6