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Optimizing the electrodeposition protocol of enantioselective inherently chiral electrode surfaces: a multi-technique investigation
No full textWe have recently introduced "inherently chiral" enantiopure electrode surfaces of outstanding chirality manifestations, including circularly polarized luminescence, reversibly potential-driven circular dichroism, and large potential differences for the enantiomers of chiral probes in voltammetry experiments performed on such surfaces. [1-3]
The outstandingly powerful “inherent chirality” concept implies a molecular structure where the stereogenic element does not consist in an isolated stereocentre or an external chirality source, but originates from a tailored torsion in the whole main backbone endowing the molecule with its key functional property (here electroactivity).
A key issue is now to investigate the enantioselection mechanism and to optimize the experimental protocols for the deposition of our inherently chiral surfaces. For both aims it is important to study the thickness and regularity of the chiral oligomer films as a function of the experimental conditions. We started a systematic profilometry study correlated to electrochemical impedance spectroscopy measurements of the oligomer films obtained by carefully controlled electrodeposition, varying one by one different experimental parameters. The study is also important to properly compare enantioselection by films prepared from different inherently chiral monomers, including e.g. bisindole and tetrathiahelicene ones.
[1] S. Arnaboldi, S. Grecchi, M. Magni, P. Mussini, Electroactive chiral oligo- and polymer layers for electrochemical enantiorecognition, Current Opinion in Electrochemistry, 7 (2018) 188-199.
[2] S. Arnaboldi, M. Magni, P. R. Mussini, Enantioselective selectors for chiral electrochemistry and electroanalysis: Stereogenic elements and enantioselection performance, Current Opinion in Electrochemistry, 8 (2018) 60-72.
[3] S. Arnaboldi, T. Benincori, R. Cirilli, S. Grecchi, L. Santagostini, F. Sannicolò, P.R. Mussini, “Inherently chiral” thiophene-based electrodes at work: a screening of enantioselection ability toward a series of pharmaceutically relevant phenolic or catecholic amino acids, amino esters, and amine, Analytical and Bioanalytical Chemistry, 408 (2016) 7243-7254
Electroactive Inherently Chiral Surfaces at Work: Clues Toward the Elucidation of the Enantioselection Mechanism
Full text linkChirality 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 “inherently chiral functional molecular materials”, 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. Sannicolò, 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. Sannicolò, T. Benincori, R. Cirilli, K. Noworyta, W. Kutner, Chem. Sci., 2015, 6, 1706.
[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., 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. Sannicolò, Angew. Chem. Int. Ed., 2017, 56, 2079
Highly Enantioselective “Inherently Chiral" Film Electrodes at Work
Full text linkThe usual approaches to chiral electroactive molecular materials, relying on attaching chiral pendants to an electroactive polyconjugated backbone, generally result in modest chirality manifestations; other approaches, such as chiral templating agents, chiral counteranions, etc. actually imply the chirality source to be external to the electroactive material.
Very recently, our research group has unveiled by electrochemical experiments the unprecedented enantioselectivity properties of molecular films based on a new family of "inherently chiral" polyheterocycles, where chirality is not external to the electroactive backbone, but inherent to it, resulting from a tailored torsion produced by the periodical presence of atropoisomeric, conjugatively active biheteroaromatic scaffolds (e.g. 3,3'-bithiophene [as in the monomers (R)- and (S)-BT2T4 represented in Figure 1], 2,2'-biindole and 3,3' bithiophene). [1]
The electroactive films obtained by electrooligomerization mostly consist of cyclic electroactive oligomers, constituted by several fully conjugated thiophene units.
These ringlets, which can also be obtained by chemical oligomerization, are endowed with an outstanding pool of attractive properties both as racemates and as enantiopure antipodes. [1,2] The neat peak separation for two enantiomers of electroactive chiral probes (Figure 2) together with the linearity of the peak currents with enantiomer concentration affords estimation of the enantiomeric excess when both enantiomers are present [3].
The new electrodes have been tested with very good results towards several chiral probes, quite different in molecular structure and of applicative interest, on different supports, and in different media. Moreover, we have verified that the same spectacular enantioselectivity is obtained on surfaces prepared starting from monomers designed according to the same structural concept but chemically different, which demonstrates the general validity of the new strategy.
Albeit preliminary, such unprecedented outstanding results open the way to chiral voltammetry.
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 and S. Rizzo, Angewandte Chemie 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 and S. Rizzo, Chemistry-A European Journal, 2014, 10, 15261
[3] S. Arnaboldi, P. Mussini, M. Magni, F. Sannicolò, T. Benincori, R. Cirilli, K. Noworyta and W. Kutner, Chemical Science, 2015, 6, 170
Enantioselective voltammetry on achiral electrodes
Full text linkAn attractive target in electroanalysis is the availability of chiral media affording enantioselection in terms of significant peak potential difference between the antipodes of chiral probes in voltammetry experiments on achiral electrodes.
Previous literature attempts pointed to enantioselectivity increasing with the structural order of the chiral medium; on the other hand, outstanding enantioselection performance has been recently observed working on electrode surfaces consisting in "inherently chiral" oligomer films [1-2].
Combining both strategies, we have recently developed two inherently chiral ionic liquids, ICILs, consisting of dialkylated bicollidinium salts, with an atropoisomeric bipyridinium cation featuring at least one octyl chain and bistrifilimide counteranions. They showed high enantioselectivity when tested even as low concentration additives in commercial achiral ionic liquid media [3] and also as chiral bulk media.
Importantly, similar ability was also shown by other terms of the same family,
having shorter alkyl chains and/or different counteranions, solid at room temperature but of easier synthesis. As a first tentative explanation we are considering the high supramolecular order of even simple ionic liquids at the interphase with a charged surface. A chiral additive could result in chiral reorganization of this peculiar interphase, as in the case of nematic-to-cholesteric transitions induced by chiral dopants in liquid crystals.
This allowed us to include in our chiral voltammetry experiments a quite larger number of inherently chiral selectors based on different stereogenic elements, i.e., the bicollidine and bibenzimidazole atropoisomeric scaffolds and the tetrathielicene helicoidal scaffold. They all proved successful.
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.
[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, Angew. Chem. Int. Ed. 53 (2014) 2623
[2] S. Arnaboldi, P. Mussini, M. Magni, F. Sannicolò, T. Benincori, R. Cirilli, K. Noworyta, W. Kutner, Chem. Sci. 6 (2015) 1706
[3] 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) 207
Electrochemical impendance spectroscopy: a useful tool to study conducting polymers and the electroreductive cleavage of carbon-halogen bond
No full textThe electrochemical impedance spectroscopy (EIS) is a powerful and polyhedral
technique that can find application in many fields of scientific research; actually it can
be exploited for the study of practically any kind of electron transfer process.
Considering the EIS potentialities we employed the technique in two quite different
fields: the characterization of conducting organic polymers and the mechanistic study
of the electroreductive cleavage of carbon-halogen (C-X) bonds in non-catalytic and
catalytic conditions. We will present the main results of the thesis project.
The work focused on the conducting polymers was structured as follows: i) a
preliminary study focused on poly(4H-cyclopenta[3,2-b]dithiophene), poly-CPDT,
chosen as a conducting polymer model which made us able to understand the
behavior of these materials in different experimental conditions (see figure); ii) the
study of a very promising class of inherently chiral conducting polymers with a
particular attention to electrodeposited oligomeric films of 2,2’-bis(2,2’-bithiophen-5-
yl)-3,3’-bi-1-benzothiophene, applying EIS (in combination with cyclic voltammetry,
CV) in order to prove the enantiorecognition capability of its enantiopure films. A
further investigation was aimed to test EIS as an auxiliary and/or complementary
technique to CV in mechanistic electron transfer studies, applying it, for the first time,
to the investigation of the electroreductive cleavage of C-X bond in a series of aromatic
and heteroaromatic organic bromides, on non-catalytic GC, moderately catalytic Au,
and highly catalytic Ag electrode. EIS very nicely and neatly accounted for the
“orientation effect” of bromo-thiophenes on gold electrode surfaces by the anchoring
sulphur atom, affording to monitor the variation of the charge transfer resistance for
the C-Br bond cleavage on two ad-hoc chosen constitutional isomers (see figure)
rH-metric controls and primary standardization in aqueous-organic media
No full textThe rationale for the extension of the rH-metric standardization and measurements to aqueous-organic solvent mixtures is here introduced, and examples of establishment of the ranges of conventional rH-metric scales as well as primary standards rH(S) in some typical aqueous-organic solvent mixtures are described
Ionization constant of o-phthalic acid and standard pH values of potassium hydrogen phthalate buffer solutions in (glycerol+water) solvent mixtures at normal and subzero temperatures
No full textDetermination of primary standards for pH measurements in glycerol + water solvent mixtures has been carried out based on reversible emf measurements of the cell Pt|H2|KHPh (mPS) + KCl (mCl)|AgCl|Ag|Pt where KHPh denotes the potassium hydrogen phthalate buffer solution of molality mPS = 0.05 mol-kg-1, at glycerol mass fractions wG = 0.2 and 0.4, within the temperature range -10 to 40°C. A multilinear regression procedure as a function of electrolyte molality, glycerol mass fraction wG, and temperature T has been applied for the data processing leading to the values of primary standards pHPS. These can be represented by the following regression equation pHPS = (4.007037±0.001113) + (3.55844±0.01776)XG +(0.39622±0.01410)Z + (4.3084±0.3377)Z2 - (50.66±10.53)XG Z2 + (457.10±78.48)XG2 Z2 where Z = (T - 298.15)/298.15. Parallel values of the first ionization constant of o-phthalic acid (H2Ph; benzene- 1,2-dicarboxylic acid, the parent acid of KHPh), which are essential for the above calculations, have been determined from reversible emf measurements of the cell Pt|H2|H2Ph (m1) + KHPh (m2) + KCl (m3)|AgCl|Ag|Pt over the range of solvent composition and temperatures mentioned above
Thermodynamics of the cell Pt|H2(p)|HCl(m)|AgCl|Ag and primary medium effects upon HCl in (glycerol + water) solvent
No full textThe electromotive force (e.m.f.)E of the cellPt|H2(p)|HCl(m) in Z|AgCl|Ag in {glycerol (G) + water (W)} solvents, Z = (G + W), up to glycerol mass fraction wG = 0.7 has been measured within the temperature range from 273.15 K to 313.15 K at HCl molalities up to 0.1mol · kg − 1. On this basis, the standard molar e.m.f. Em ring operator values pertaining to such solvent mixtures have been obtained, and have been combined with sparse literature data for optimization. At glycerol mass fractions up towG ≈ 0.5, at constant temperature,Em ring operator shows a linear dependence on the glycerol mole fractionxG and, in parallel, Ec ring operator (on the amount-of-substance concentration scale) shows linear dependence on the glycerol volume fractionphiG . The primary medium effect upon HCl, defined as the difference (Ec ring operator )W − (Ec ring operator )Z, has been considered as a function of the water volume fraction phiWin terms of Feakins and French’s theory: this would lead to a primary hydration number n(hydr) = 2.4 for HCl, in good agreement with previous results obtained with solvents other than (glycerol + water). In this connection, some basic methodological aspects are discussed. Ancillary values of the densities ρZof the relevant solvent mixtures, which were hitherto unavailable and are necessary for the data processing leading toEm ring operator , have also been measured
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