1,721,114 research outputs found
"Inherently Chiral" electrodes: tools for chiral voltammetry and enantiomeric excess evaluation
No full textThe development of artificial "intelligent" electrodes, capable to discriminate and quantify the enantiomers of chiral analytes, particularly of biological and pharmaceutical interest, is a quite attractive issue in electroanalysis. Obviously, selectivity towards specular molecules can only be achieved on enantiopure chiral electrodes. For this aim, many approaches have been proposed in the last years. However, even the most successful attempts at chiral discimination almost invariably resulted in the detection of a difference in current intensity between the signals of the two antipodes of a chiral probe, without differentiation of their redox potentials; the chiral enantioselective layer is in many instances not of general use, but tailored for a given probe; many preparation procedures are very sophisticated and/or the active films fragile. A winning solution comes from a new class, which we have recently presented1‐3 and patented4, of "inherently chiral" molecular semiconductors, whose stereogenic element is a tailored torsion in the electroactive conductive backbone. The coincidence of the element granting both electroactivity and chirality with the entire molecular backbone results in extraordinary chirality manifestations (such as circularly polarized luminescence), that can be finely and reversibly tuned by the electric potential. Above all, enantiopure electrode surfaces can be easily prepared e.g. by fast electrooligomerization, mostly consisting of cyclic oligomers, highly electroactive and chiral, idealizing conducting polymers without ends and of high complexing ability; they are able to discriminate enantiomers of chiral molecules in terms of large peak potential differences (80‐200 mV and more), with linear dynamic ranges for peak currents, thus affording enantiomeric
ratio evaluation. The same spectacular enantioselectivity is obtained on chemically different surfaces of the same structural concept, which demonstrates the general validity of our proposed strategy. A simple reconditioning protocol affords performing more experiments on a single electrode. The new electrodes have been tested with very good results on chiral probes even very different and of applicative interest3 (Dopa and methyl‐Dopa [see our parallel presentation] ofloxacin, norepinephrine, tyrosine, naproxen, catechines, ascorbic acid...), on different supports, including commercial screen printed ones, and in different media (aqueous and nonaqueous ones, as well as small ionic liquid drops on SPEs).
This work was supported by Fondazione Cariplo (Grant no. 2011‐0417)
References
1. F. Sannicolò, S. Arnaboldi, T. Benincori, V. Bonometti, R. Cirilli, L. Dunsch, W. Kutner, G. Longhi, P.R. Mussini, M.
Panigati, M. Pierino, S. Rizzo, Angew. Chem. Int. Ed. 2014, 53, 2623.
2. F. Sannicolò, P.R. Mussini, T. Benincori, R. Cirilli, S. Abbate, S. Arnaboldi, S. Casolo, E. Castiglioni, G. Longhi, R.
Martinazzo, M. Panigati, M. Pappini, E. Quartapelle Procopio, S. Rizzo, Chem. Eur. J. 2014, 20, 15296..
3. S. Arnaboldi, T. Benincori, R. Cirilli, W. Kutner, M. Magni, P.R. Mussini, K. Noworyta, F. Sannicolò, Chemical
Science, 2015, 6, 2041.
4. F. Sannicolò, P.R. Mussini, S. Arnaboldi, T. Benincori, R. Cirilli et al., Patent appl. MI2014A000948 (2014
Spin selectivity properties displayed by chiral electrode surfaces under an external magnetic field
No full textThe combination of spintronics with magneto- electrochemistry, particularly involving truly chiral molecular spin selectors, was promoted by the discovery of the Chiral Induced Spin Selectivity (CISS) effect by Ron Naaman and coworkers, observing spin polarization in photo-ejected electrons transmitted through a thin layer of enantiopure material adsorbed on gold, acting as an electron spin filter [1].
In this frame we present an innovative set-up which includes i) a non-ferromagnetic electrode (ITO) modified with a thin electroactive chiral film (the spin filter), ii) achiral redox couples dissolved in aqueous or organic solutions and iii) an external permanent magnet which was placed near the chiral film, considering as spin filters four different types of chiral selectors (with different stereogenic elements, i.e. helix, stereogenic axis and chiral pendant). [2,3,4]
A spectacular unforeseen effect was observed by means of cyclovoltammetry (CV), in fact CV peaks recorded in the presence of achiral redox couples reveal an impressive potential shift by flipping the magnet orientation (north vs south). This effect was also specular by changing the spin injector configuration.
The importance of these studies includes possible applications in the field of spintronics, electronics, chemical sensoristic and so on and provides a striking evidence of the spin selectivity properties of chiral thin films.
References:
[1] O. B Dor, S. Yochelis, S. P. Mathew, R. Naaman, Y. Paltiel, Nat. Commun. 4 (2013), 3256.
[2] T. Benincori, S. Arnaboldi, M. Magni, S. Grecchi, R. Cirilli, C. Fontanesi, P. R. Mussini, Chem. Sci., DOI: 10.1039/C8SC04126A (2019).
[3] S. Arnaboldi, S. Cauteruccio, S. Grecchi, T. Benincori, M. Marcaccio, A. Orbelli Biroli, G. Longhi, E. Licandro P. R. Mussini, Chem. Sci., 10 (2019) 1539-1548.
[4] 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., DOI: 10.1039/C8SC04862B (2019)
Spin Filter Properties of Chiral Thin Film Highlighted by an External Magnetic Field
No full textThe study of the mutual influence between chirality, electron spin and magnetism is a well-documented field of research, in fact implications span from pure fundamental research, to chemical applications of both analytical and synthetic character, to multidisciplinary purposes.1,2 In this context the interrelated disciplines of magnetoelectrochemistry, spintronics and Spin Dependent Electrochemistry (SDE) play a crucial role. In particular, the combination of spintronics with magneto- electrochemistry, involving truly chiral molecular spin selectors, was promoted by the discovery of the Chiral Induced Spin Selectivity (CISS) effect by Ron Naaman and coworkers, observing spin polarization in photo-ejected electrons transmitted through a thin layer of enantiopure material adsorbed on gold, acting as an electron spin filter3.
Our proposed strategy is a variation of the SDE protocol, in fact the innovative set-up involves i) a non-ferromagnetic electrode (ITO as working electrode) covered by thin electroactive chiral films as electron source, ii) achiral redox couples dissolved in aqueous or organic solutions and iii) an external permanent magnet which was placed perpendicular to the electrode surface, considering as spin filters four different types of chiral selectors (with different stereogenic elements, i.e. helix, stereogenic axis and chiral pendant). 4,5,6
A spectacular unforeseen effect was observed performing cyclic voltammetry (CV) experiments under applied magnetic field, in fact the CV peaks of achiral, chemically reversible redox couples undergo impressive potential shifts by flipping the magnet orientation (north vs south), with specular results changing the film configuration.
The importance of these studies includes possible applications in the field of spintronics, electronics, chemical sensoristic and so on and provides a striking evidence of the spin selectivity properties of chiral thin films.
References:
1 G. A. Printz, Science, 1998, 282, 1660-1663.
2 P. L. Popa, N. T. Kemp, H. Majjad, G. Dalmas, V. Faramarzi, C. Andreas, R. Hertel, B. Doudin, Proc. Natl. Acad. Sci., 2014, 111, 10433-10437
3 O. B Dor, S. Yochelis, S. P. Mathew, R. Naaman, Y. Paltiel, Nat. Commun., 2013, 4, 3256-3262.
4 T. Benincori, S. Arnaboldi, M. Magni, S. Grecchi, R. Cirilli, C. Fontanesi, P. R. Mussini, Chem. Sci., 2019, 10, 2750-2757.
5 S. Arnaboldi, S. Cauteruccio, S. Grecchi, T. Benincori, M. Marcaccio, A. Orbelli Biroli, G. Longhi, E. Licandro P. R. Mussini, Chem. Sci., 2019, 10, 1539-1548.
6 . 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
Electrochemistry of, and electrochemistry in, chiral and inherently chiral ionic liquid media
No full textIonic liquids ILs are attractive media for electrochemistry and electroanalysis, since, in addition to other useful properties, they provide both solvent and supporting electrolyte; moreover, they feature an extremely well defined structure at the interphase with a charged electrode, resembling somehow a bulk liquid crystal, extending for many layers, also standing in the presence of water traces, and possibly tunable by in-situ present additives. [1-3] For this reason chiral ionic liquids (CILs), of which a rich palette is already available, are surprisingly still nearly unexplored by electrochemists, in spite of appearing quite attractive, since they could transmit chiral information more effectively than chiral organic solvents or chiral supporting electrolytes. [1]
In this context, we have recently started a detailed investigation of (a) bio-based chiral ionic liquids and (b) inherently chiral ionic liquids (ICILs), investigating both their physico-chemical and electrochemical features and their performance as media for chiral electroanalysis experiments. Our bio-based CILs featu- re cations with a building block of natural origin, including one or more localized stereocentres, from which their chirality arises (as in most so far available CILs) [4]. Instead, in our ICILs chirality is intrinsic of the whole biheteroaromatic cation, which features a stereogenic axis related to a torsion between two equal heteroaromatic moieties, with a related energy barrier too high to be overcome at room temperature, so that the ICIL can be obtained in two stable enantiopure antipodes. In inherently chiral molecules and materials both chirality and functional properties originate from the same structural element, which can coincide with the main molecular backbone; thus their chirality manifestations can be very powerful, as recently evidenced by some of us in electroanalytical, chiroptical and magneto-electrochemistry experiments with electrodes modified by inherently chiral oligomer thin films.
Actually, while interesting but (at least so far) small are the chirality effects observed working in our bio- based CILs with localized stereocentres [4], large peak potential differences have been observed for the enantiomers of very different chiral probes in CV experiments in an enantiopure bulk ICIL. Very conve- niently, impressive enantiodiscrimination is observed even using ICILs (or other related inherently chiral molecular salts, solid at room T, but of easier synthesis) as low-concentration chiral additives in common achiral ionic liquids (ILs) [5,6]. Furthermore, similar impressive performances have also been observed dissolving in an achiral IL a thiahelicene-based additive, an inherently chiral oligothiophene, but un- charged and based on a different stereogenic element, i.e. a helical scaffold. Among possible explanations we are considering the possibility of a chiral reorganization effect by the inherently chiral additives in the locally highly ordered IL structure at the interphase with the charged electrode, similarly to the well- known transitions from nematic to cholesteric order induced by chiral additives in bulk liquid crystals.
The support of Fondazione Cariplo/Regione Lombardia "Avviso congiunto per l’incremento del- l’attrattività del sistema di ricerca lombardo e della competitività dei ricercatori candidati su strumenti ERC - edizione 2016” (Project 2016-0923) is gratefully acknowledged.
[1] S. Arnaboldi, M. Magni, P.R. Mussini, Curr. Opin. Electrochem. 8 (2018) 60-72.
[2] K. Ma, R. Jarosova, G.M. Swain, G.J. Blanchard, Langmuir 32 (2016) 9507-9512.
[3] A.Lahiri Cui, T. Carstens, N. Borisenko, G. Pulletikurthi, C. Kuhl, F. Endres, J. Phys. Chem. C 120 (2016) 9341-9349.
[4] M. Longhi, S. Arnaboldi, E. Husanu, S. Grecchi, I.F. Buzzi, R. Cirilli, S. Rizzo, C. Chiappe, P.R. Mussini, L. Guazzelli, Electrochim. Acta 298 (2019) 194-209.
[5] S. Rizzo, S. Arnaboldi, V. Mihali, R. Cirilli, A. Forni, A. Gennaro, A.A. Isse, M. Pierini, P.R. Mussini, F. Sannicolò, Angew. Chem. Int. Ed. 56 (2017) 2079-2082.
[6] S. Rizzo, S. Arnaboldi, R. Cirilli, A. Gennaro, A.A. Isse, F. Sannicolò, P.R. Mussini, Electrochem. Comm. 89 (2018) 57-61
Enantiomeric Separation of Amino-alcohols on Dinitrobenzoyl-diaminocyclohexane Chiral Stationary Phases
No full textA silica-bonded chiral stationary phase, containing the N,N'-3,5-dinitrobenzoyl derivative of 1R,2R-diaminocyclohexane, was used to separate the enantiomers of some amino alcohols with beta-blocking activity after their conversion to oxazolidin-2-ones. The influence of mobile phase composition (mixtures of hexane with dichloromethane, chloroform, dioxane and isopropanol) on the enantioselectivity and efficiency of the column was evaluated. Furthermore, a tandem arrangement of the chiral column and its racemic version was used to resolve all the stereoisomers of one amino alcohol containing two stereogenic centres
Chiral electrochemistry : introducing the inherent chirality Concept in ionic liquids
Full text linkThe typical design of chiral electroactive materials involves attaching chiral pendants to an electroactive polyconjugated backbone and generally results in modest chirality manifestations.
In this frame we have proposed our innovative approach based on the concept of "inherent chirality" implying a stereogenic element not external to, but coinciding with, the whole electroactive backbone, derived from a tailored torsion originating in the molecule by the insertion of atropisomeric scaffolds. Therefore both chirality and electroactivity are strictly connected, deriving from the same element, which constitutes the main part of the molecule. Such property is fully transmitted from inherently chiral monomers to the oligomers obtained by their electrooxidation. Thus, the inherent chirality concept has turned out to afford molecular materials endowed with powerful chirality manifestations plus an unprecedented pool of other outstanding properties [1,2,3].
Besides extending such exciting study, we are now trying to apply the same "inherent chirality" approach to the development of inherently chiral ionic liquids (ICILs), hopefully endowed with high enantioselectivity. The new molecules are based on cations including different atropisomeric bis-benzimidazolium and bis-collidinium groups acting as the stereogenic element, responsible for both the molecular chirality and the IL properties of the material, modulated by number, position and length of alkyl chain substituents.
An overview will be given of the first structures obtained and of their electrochemical properties compared with those of the corresponding scaffolds.
Results emerging from the comparison of the cyclic voltammetry patterns of each scaffold family are here summarized.
1,1’-bis-benzimidazoles [4] display a racemization barrier of ~40 kcal mol-1.
First reduction and first oxidation peaks are close to the boundaries of the available potential windows, consistent with very low effective conjugation deriving from the high energy barrier. The effect of alkyl chain length is very slight.
2,2’-bis-benzimidazoles exhibit the narrowest potential window of all scaffold families: both first reduction and first oxidation peaks are significantly shifted to less extreme potentials, pointing to higher effective conjugation between the two equal scaffold moieties, which is in accordance with the much lower computed racemization barriers. Actually the scaffolds are in this case nearly planar, and their reduction CV pattern, featuring two reversible monoelectronic peaks, is consistent with two reciprocally interacting equivalent redox centers.
It is worthwhile noticing that bridging alkyl substituents result in a different reduction mechanism with respect to simple alkyl chains and appear to destabilize the first ET product, which reacts much more rapidly. Moreover, the 3-term bridge results in a significantly lower Egap with respect to the 4-term one, consistently with its lower calculated racemization barrier.
3,3’-bis-collidines have the highest computed racemization barrier (42.5 kcal mol-1) and yield the largest potential window in CV experiments (Egap ~4.8 eV). First reduction and oxidation are in this case barely perceivable as background shoulders, coherently with very low effective conjugation.
These inherently chiral scaffolds can be employed, even without salification, as chirality inductors in stereoselective processes, either dissolved in achiral commercial ionic liquid media or, together with a supporting electrolyte, in traditional solvents.
When mono- or di-alkylation occurs the quaternarization of the imidazole and pyridine rings results in a huge positive shift of the first reduction potentials.
The 1,1’-bis-benzimidazolium di- (or mono-) alkyl salt family [4] shows a very high racemization barrier; thus enantiomers can be separated by preparative HPLC, stored and characterized.
Unfortunately such salts are solid or wax-like at room temperature, since so far double alkylation has succeeded only when either R or R’ chains were short.
However, they could be used as chiral supporting electrolytes or even, at lower concentration, as chirality inductors (see below).
The 2,2’-bis-benzimidazolium mono-alkyl salt family are waxes or oils at room temperature. Unfortunately, however, in this case the low barrier and easy rotation between the two equal halves, which results in high effective conjugation, also makes it impossibile to separate stable enantiomers.
The 3,3’-bis-collidinium di- (or mono-) alkyl salt family is the most promising one displaying the highest racemization barrier in the three families as the corresponding scaffold; moreover, alkylation with long chains has been successfully achieved, finally affording, with proper counteranion choice, low melting points.
Products are synthesized from inexpensive starting reagents; the scaffold enantiomers can be separated by fractional crystallization of diastereomeric salts with o,o-dibenzoyltartaric acids, and also the mono- and dialkylated salts can be isolated by a convenient protocol; thus low-cost synthesis is possible, avoiding the necessity of preparative chiral HPLC. In view of testing the new ICILs as media for electrooligomerizations a first proof was carried out using a 3,3’-bis-collidinium salt as supporting electrolyte evidencing the powerful influence of the anion choice in the oligomerization process. The anion choice is also very important for obtaining the desired melting points.
Finally, very promising appear the preliminary enantioselectivity tests (figure below) using 1,1'-bibenzimidazolium enantiopure salts as additives, even in small amount, in the electrooligomerization of our inherently chiral monomer [4].
We are now looking forward to testing as supporting electrolytes and/or ionic liquids the newly achieved low-melting 3,3'-bis-collidinium salts, which look the most promising family, being low-cost, having the highest racemization barriers, the largest potential windows, as well as reaching low melting points.
Figure. Electrooligomerization tests of racemic (left) or enantiopure (right) inherently chiral monomer (parent BT2T4) in the absence (red) or in the presence of 2:1 enantiopure (R)- (blue) or (S)- (green)1,1’-bis-benzimidazolium based salt.
Acknowledgements
With the contribution of Fondazione Cariplo, grant no. 2011-1851 (inherently chiral ionic liquids)
References
[1] 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
Potential-driven chirality manifestations and impressive enantioselectivity by inherently chiral electroactive organic films
Angew. Chem. Int. Ed., 53 (2014), p. 2623
[2] F. Sannicolò, P.R. Mussini, T. Benincori, R. Cirilli, S. Abbate, S. Arnaboldi, S. Casolo, E. Castiglioni, G. Longhi, R. Martinazzo, M. Panigati, M. Pappini, E. Quartapelle Procopio, S. Rizzo
Inherently chiral macrocyclic oligothiophenes: easily accessible electrosensitive cavities with outstanding enantioselection performances
Chem. Eur. J., 20 (2014), p. 15298
[3] S. Arnaboldi, T. Benincori, R. Cirilli, W. Kutner, M. Magni, P.R. Mussini, K. Noworyta, F. Sannicolò
Inherently chiral electrodes: the tool for chiral voltammetry
Chem. Sci., 6 (2015), p. 1706
[4] S. Arnaboldi, R. Cirilli, A. Forni, A. Gennaro, A. A. Isse, V. Mihali, P. R. Mussini, M. Pierini, S. Rizzo, F. Sannicolò
Electrochemistry and Chirality in bis-Benzimidazole Systems
Electrochim. Acta, (2015), doi:10.1016/j.electacta.2015.03.17
Unconventional Electrochemical Approaches for the Direct Readout of Chiral Information
No full textAmong the different strategies to endow conducting polymers with chiral features, the concept of inherent chirality is extremely attractive and efficient for several reasons1. Inherent chirality is an intrinsic propriety of the whole molecule in which the stereogenic element responsible for chirality coincides with the functional group responsible for the specific properties of the molecular material. The racemization barrier of these molecules is high enough to allow the separation into stable enantiomers by chiral HPLC. Their electro-oligomerization results in chiral electrode surfaces able to discriminate the antipodes of chiral species by differences in peak potential recorded by voltammetry2. This is a true advantage from an analytical point of view, because it is possible to achieve the enantioselective discrimination in racemic mixtures of the analyte, in contrast to most of the approaches presented in literature for which the recognition is only based on differences in current intensity. In this context the high enantioselectivity of such chiral surfaces was used to develop i) a new approach based on bipolar electrochemistry for the absolute on-off recognition of enantiomers of a chiral probe3, and ii) new autonomous enzymes based swimmers able to convert chiral information present at the molecular level into macroscopic enantioselective trajectories4.
1 S. Arnaboldi, M. Magni, P. R. Mussini, Curr. Opin. Electrochem. 2018, 8, 60−72.
2 S. Arnaboldi, T. Benincori, R. Cirilli, W. Kutner, M. Magni, P. R. Mussini, K. Noworyta, F. Sannicolo,̀ Chem. Sci. 2015, 6, 1706− 1711.
3 S. Arnaboldi, B. Gupta, T. Benincori, G. Bonetti, R. Cirilli, A. Kuhn, Anal. Chem. 2020, 92, 10042−10047.
4 S. Arnaboldi, G. Salinas, A. Karajic, P. Garrigue, T. Benincori, R. Cirilli, S. Bichon, S. Gounel, N. Mano, A. Kuhn, Nat. Chem. 2021, in press
Influence of the nature of alkyl substituents on the high-performance liquid chromatography enantioseparation and retention of new atropisomeric 1,1’-bibenzimidazole derivatives on amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phase
No full textSix new atropisomeric heteroarenes were synthesized by connecting two 2-alkylbenzimidazole fragments via N-N junction. They differ by the substituent nature (methyl, ethyl, propyl, butyl, pentyl and hexyl) of the aliphatic function. The novel atropisomeric compounds were used as chiral probes to study the chromatographic behavior of the amylose tris(3,5-dimethylphenyl carbamate) (Chiralpak AD-3) chiral stationary phase (CSP) under normal phase mode. The pivotal role of the length and flexibility of the 2,2'-alkyl groups on retention, enantioselectivity and enantiomer elution order was demonstrated by enantioselective HPLC analysis. Additional information on the chiral recognition mechanism was obtained from the evaluation of the correlated thermodynamic data
Synthesis of novel chiral bisthiophene-based phosphine oxides as Lewis bases in organocatalytic stereoselective reactions
No full textNovel enantiomerically pure tetramethylbithiophene diphosphine oxides (tetraMe-BITIOPO) featuring differently substituted aromatic rings at the phosphorous atoms were synthesized, fully characterized and isolated in enantiomerically pure form. The new Lewis bases were tested as organocatalysts in two different reactions involving trichlorosilyl compounds. The introduction of electron donating substituents on the aromatic rings connected to the phosphine oxide groups positively affected the chemical and stereochemical catalytic efficiency of these ligands. The new catalysts were able to promote the allylation of aldehydes with allyltrichlorosilane in up to 76% yield and up to 86% enantiomeric excess (ee), and the direct aldol reaction to afford β-hydroxy ketones in high diastereoselectivity (up to 88:12 anti:syn ratio) and up to 72% ee
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