Search

A family of inherently chiral electroactive selectors based on the 2,2’‐biindole atropisomeric scaffold, of easy synthesis and modulable functional properties, is studied in cascade in two enantioselection contexts. They are at first investigated as probes in enantioselective HPLC, studying molecular structure and temperature effects, and achieving very efficient semipreparative enantioseparation. The enantiomers thus obtained, of remarkable chiroptical features (optical rotation as well as circular dichroism), are successfully applied as selectors in chiral voltammetry in different media for discrimination of the enantiomers of chiral electroactive probes, either by conversion into enantiopure electroactive electrode surfaces by electrooligomerization on glassy carbon substrate (the two monomers with shorter alkyl chains), or as chiral additive in achiral ionic liquid (the monomer with longest alkyl chains). Discrimination is conveniently and reproducibly achieved in terms of significant potential differences for the two enantiomers, specularly inverting either probe or selector configuration. In one case successful discrimination is also observed with the two probe enantiomers concurrently present, either as racemate or with enantiomeric excesses, neatly accounted for by the peak current ratios., A series of inherently chiral electroactive selectors based on the 2,2’‐biindole atropisomeric scaffold is studied in two enantiodiscrimination contests: as probes in enantioselective HPLC and as selectors in chiral voltammetry for successful discrimination of the enantiomers of chiral electroactive probes, either by conversion into enantiopure electroactive electrode surfaces or as chiral additive in achiral ionic liquid.

A series of 2,2’-biindole-based inherently chiral electroactive monomers are comparatively investigated with their 3,3’ analogues as an excellent study case of two equivalent redox centres interacting through a torsional barrier. The twin peak potential splitting observed in voltammetry for the first oxidation of the biheteroaromatic core accounts for the energy barrier height: the lower the barrier, the larger the peak potential splitting, with modulation by solvent and temperature. The height of the energy barrier is determining for the electrochemical and spectroscopic features of the monomers as well as for their configurational stability and applicability for enantioselection purposes. The 3,3’ monomers, featuring large twin peak splittings in CV, are “trópos” systems with a low torsional barrier, so they cannot exist as stable enantiomers at room temperature. Instead their 2,2’ isomers, with much smaller twin peak splittings, are “átropos” systems and can be separated by enantioselective HPLC into stable enantiomers, providing powerful “inherently chiral” selectors with outstanding enantioselection properties in chiral electroanalysis and electrochemistry as well as in chiroptical spectroscopy, with fascinating reciprocal correlations.

A voltammetric comparative investigation on the electrochemical activity of substituted bromothiophenes vs bromobenzenes highlights the combined effects of the aromatic or heteroaromatic ring substituents and the sulphur atom on both the intrinsic reactivity (accounted for by experiments on a glassy carbon electrode, assumed to have negligible specific interactions) and the reactivity in the presence of electrocatalytic effects, working on Au or Ag electrodes. The two series of compounds share similarities concerning the dissociative electron transfer mechanism for the reductive cleavage of the C-Br bond, including substituent effects. However, the presence of the sulphur atom in the heteroaromatic series significantly promotes the process both in non-catalytic conditions, on account of electronic effects, and on catalytic electrodes, performing as adsorption auxiliary. The effect is particularly remarkable on Au, partially compensating for the effect of the very negative surface charge, and with significant modulation from the S position with respect to the Br leaving group. The nitrile group might act as an additional adsorption auxiliary besides the S atom. In dibromobithiophene systems Au and Ag catalytic surfaces can also induce a remarkable modification in molecular conformation in order to optimize Br and S interactions with the catalytic surface for both conjugated thiophene rings.

Configurationally stable 5-aza[6]helicene (1) was envisaged as a promising scaffold for non-conventional ionic liquids (IL)s. It was prepared, purified, and separated into enantiomers by preparative HPLC on a chiral stationary phase. Enantiomerically pure quaternary salts of 1 with appropriate counterions were prepared and fully characterized. N-octyl-5-aza[6]helicenium bis triflimidate (2) was tested in very small quantities as a selector in achiral IL media to perform preliminary electrochemical enantiodifferentiation experiments on the antipodes of two different chiral probes. The new organic salt exhibited outstanding enantioselection performance with respect to these probes, thus opening the way to applications in the enantioselective electroanalysis of relevant bioactive molecules.

Deep Eutectic Solvents (DESs) offer advantages similar to ionic liquid (IL) ones, with easier and more sustainable synthesis; moreover, bio-based DESs often include chiral components, surprisingly underexploited. A proof of concept is now offered of the impressive potential of enantiopure chiral DESs as chiral media for enantioselective electroanalysis. Three chiral DESs, consisting of a molecular salt with bio-based chiral cation [NopolMIm]+ combined with three natural and/or low-cost partners (levulinic acid, glycerol and urea), are introduced and investigated as chiral voltammetry media. Significant potential differences are observed for the enantiomers of a model chiral probe, with a dramatic tuning depending on the achiral DES component, reaching an impressive ~0.5 V in the levulinic acid case (while less efficient appears [NopolMIm]+ as chiral additive in IL). With the same medium good enantiodiscrimination is also observed for aminoacid tryptophan, a quite different probe and of applicative interest. These findings can be considered as a remarkable step further in chiral electroanalysis as well as in the development of task-specific enantioselective media.

A voltammetric and scanning electrochemical microscopy (SECM) investigation was performed on an inherently chiral oligomer-coated gold electrode to establish its general properties (i.e., conductivity and topography), as well as its ability to discriminate chiral electroactive probe molecules. The electroactive monomer (S)-2,2&prime, bis(2,2&prime, bithiophene-5-yl)-3,3&prime, bibenzothiophene ((S)-BT2T4) was employed as reagent to electrodeposit, by cyclic voltammetry, the inherently chiral oligomer film of (S)-BT2T4 (oligo-(S)-BT2T4) onto the Au electrode surface (resulting in oligo-(S)-BT2T4-Au). SECM measurements, performed in either feedback or competition mode, using the redox mediators [Fe(CN)6]4&minus, and [Fe(CN)6]3&minus, in aqueous solutions, and ferrocene (Fc), (S)-FcEA, (R)-FcEA and rac-FcEA (FcEA is N,N-dimethyl-1-ferrocenylethylamine) in CH3CN solutions, indicated that the oligomer film, as produced, was uncharged. The use of [Fe(CN)6]3&minus, allowed establishing that the oligomer film behaved as a porous insulating membrane, presenting a rather rough surface. This was inferred from both the approach curves and linear and bidimensional SECM scans, which displayed negative feedback effects. The oligomer film acquired semiconducting or fully conducting properties when the Au electrode was biased at potential more positive than 0.6 V vs. Ag|AgCl|KCl. Under the latter conditions, the approach curves displayed positive feedback effects. SECM measurements, performed in competition mode, allowed verifying the discriminating ability of the oligo-(S)-BT2T4 film towards the (S)-FcEA and (R)-FcEA redox mediators, which confirmed the results obtained by cyclic voltammetry. SECM linear scans indicated that the enantiomeric discriminating ability of the oligo-(S)-BT2T4 was even across its entire surface.

Chiral electroanalysis could be regarded as the highest recognition degree in electrochemical sensing, implying the ability to discriminate between specular images of an electroactive molecule, particularly in terms of significant peak potential difference. A groundbreaking strategy was recently proposed, based on the use of "inherently chiral" molecular selectors, with chirality and key functional properties originating from the same structural element. Large differences in peak potentials have been observed for the enantiomers of different chiral molecules, also of applicative interest, using different selectors, all of them based on atropisomeric biheteroaromatic scaffolds of axial stereogenicity. However, helicene systems also provide inherently chiral building blocks with attractive features. In this paper the enantiodiscrimination performances of enantiopure inherently chiral films obtained by electrooxidation of a thiahelicene monomer with helicoidal stereogenicity are presented for the first time. The outstanding potentialities of this novel approach are evaluated towards chiral probes with different chemical nature and bulkiness, in comparison with a representative case of the so far exploited class of inherently chiral selectors with axial stereogenicity. It is also verified that the high enantiodiscrimination ability holds as well for electron spins, as for atropisomeric selectors.

Enantioselective electroanalysis, which aims to discriminate the enantiomers of electroactive chiral probes in terms of potential difference, is a very attractive goal. To achieve this, its implementation is being studied for various "inherently chiral" selectors, either at the electrode surface or in the medium, yielding outstanding performance. In this context, the new inherently chiral monomer Naph2T4 is introduced, based on a biaromatic atropisomeric core, which is advantageously obtainable in enantiopure form without HPLC separation steps by a synthetic route hinging on enantiopure 2,2&rsquo, dibromo-1,1&rsquo, binaphthalenes. The antipodes of the new inherently chiral monomer can be easily electrooligomerized, yielding inherently chiral electrode surfaces that perform well in both cyclic voltammetry (CV) enantiodiscrimination tests with pharmaceutically interesting molecules and in magnetoelectrochemistry experiments.

beta-Diketones are an important class of bidentate cyclometalating compounds, used in organometallic chemistry as ancillary ligands because of their wide commercial availability and easy synthesis. They are employed to finely tune the electronic, spectroscopic and physical properties of metal complexes. Heteroleptic iridium complexes often benefit from the use of -diketonate ligands, their properties being similar to those of the corresponding homoleptic tris-cyclometalated ones. Nevertheless, in some cases, their use results in a complete quenching of the phosphorescence. Aiming to understand the origin of this drawback, we designed a suitable class of heteroleptic complexes and studied their thermal stability (DSC/TGA). We explored the effect of the ancillary ligand in a series of Ir(iii) complexes bearing 2-phenylpyridine (ppy) as a cyclometalated ligand and acac (acetylacetonate), tta (2-thienoyltrifluoroacetonate), dtdk (1,3-di(thiophen-2-yl)propane-1,3-dionate) and BPhen (4,7-diphenyl-1,10-phenanthroline) as ancillary ligands. Through photochemical and electrochemical investigations, whose results agree with and support our density functional theory calculations, we demonstrate that -diketonate ligands with low triplet energy generate dark triplet excited states with negligible coupling to the ground state which indeed promote non-radiative relaxation through population of higher states.

In the present paper, the synthesis and characterization of 2,2′-bis{bi[2,2’-(3,4-ethylenedioxy)thiophen-5-yl]}-3,3′-bithianaphthene, nicknamed BT2E4, is reported. We show that electrooligomerization of BT2E4 leads to electroactive films which are evaluated by cyclic voltammetry (CV), UV/vis spectroelectrochemistry and CV coupled with in-situ conductance measurements. Direct comparisons to the properties of the oligomers obtained from the atropisomeric all-thiophene analogue BT2T4 show that the introduction of EDOT leads to a strong reduction of the band-gap, an overall red-shifted absorption spectrum and a rigidification of the structure. Finally, DFT and TD-DFT calculations were performed in parallel to investigate and to compare the electronic and molecular structures of neutral and charged monomer and oligomer species.

Two new inherently chiral oligothiophenes characterized by the atropisomeric 3,3'-bithianaphtene scaffold functionalized with fused ring bithiophene derivatives, namely 4H-cyclopenta[2,1-b3:4b']dithiophene (CPDT) and dithieno[3,3-b:2',3'-d]pyrrole (DTP), were synthesized. The racemates were fully characterized and resolved into antipodes by enantioselective HPLC. The enantiomers were analyzed through different chiroptical techniques: electronic circular dichroism (ECD) and vibrational circular dichroism (VCD) were employed to attribute the absolute configuration (AC). Comparison of experimental and calculated VCD spectra confirmed the DFT calculated conformational characteristics. The compound functionalized with two CPDT units was oxidized with FeCl3 , and ECD and CPL of the resulting material were measured. Circularly polarized luminescence (CPL) was measured to verify if inherently chiral oligothiophenes could be promising systems for chiral photonics applications.

A dialkyl-1,1′-bibenzimidazolium salt, consisting of an atropisomeric dication (i.e. featuring a stereogenic axis and thus “inherently chiral”) and an achiral counteranion, is employed as a chiral additive in three commercial ionic liquids, providing successful enantiodiscrimination in voltammetry experiments on screen-printed electrodes (SPEs) with the enantiomers of N,N′-dimethyl-1-ferrocenyl-ethylamine as model chiral probes. Significant differences in redox potentials are observed for the probe enantiomers despite the low concentration (0.01 M) of the chiral additive. The nature of the achiral ionic liquid in which the additive is employed significantly affects the peak potentials and potential differences, but does not alter the enantiomer sequence. Keywords: Chiral electrochemistry and electroanalysis, Ionic liquids, Chiral additives, Inherent chirality, Enantiorecognition

An increasing number of strategies and tools have been proposed to endow the electrochemical interphase with chirality, to achieve enantiodiscrimination in analytical and/or preparative applications. So far, chirality has mostly been implemented not only at the electrode surface side but also on the medium one. Recently, the attractiveness of the latter approach has remarkably increased on account of the increasing availability of advanced chiral molecular media with intrinsic attractive features for electrochemistry applications, such as chiral ionic liquids, chiral ionic liquid crystals, and chiral deep eutectic solvents. With respect to solid layer/fixed chiral networks, advanced chiral media can still offer a reasonably high degree of local structuring, while being less demanding concerning preparation and management protocols, as well as less sensitive to fouling/regeneration issues. Different ways to implement chirality in advanced molecular media, including cases of powerful ‘inherent chirality,’ will be presented and discussed, particularly focusing on recent applications in the electrochemical field.

The new configurationally stable, unsymmetrical 2,12-diaza[6]helicene was synthesized as a racemate and the enantiomers were separated in an enantiopure state by semi-preparative HPLC on chiral stationary phase. Under selected alkylation conditions it was possible to obtain both the enantiopure 2-N-mono- and di-N-ethyl quaternary iodides. Metathesis with bis(trifluoromethanesulfonyl)imide anion gave low-melting salts which were tested as inherently chiral additives to achiral ionic liquids for the electrochemical enantiodiscrimination of chiral organic probes in voltammetric experiments. Remarkable differences in the oxidation potentials of the enantiomers of two probes, a chiral ferrocenyl amine and an aminoacid, were achieved, the differences increase with increasing additive concentration and number of alkylated nitrogen atoms.

FeCl3 oxidation of the racemate of C2 symmetric, inherently chiral, sexithiophene monomer 1 (2,2′-bis(2,2′-bithiophene-5-yl)-3,3′-bithianaphthene) affords a mixture of cyclic oligomers, from the prevailing dimer to traces of the pentamer. The oligomers are constituted by mixtures of stereoisomers which are two for dimer 2, four for trimer 3 and six for tetramer 4. Cyclooligomers 2 and 3 could be separated by chromatography, while 4 was synthesized by ring closure of open chain dimer 2a, prepared in turn by controlled coupling of the anion of racemic 1. The optical properties of open-chain stereoisomer 2a and tetramer 4 have been compared with those of 2 and 3 respectively. The macrocyclic oligomers have been tested as donor materials in bulk heterojunction solar cell prototypes both as a crude mixture resulting from oxidation of 1 and as a single oligomer. Theoretical calculations support the photophysical properties of these new materials.

The racemate of an inherently chiral "spider-like" octathiophene monomer T83 , in which chirality is generated by torsion in its backbone, was synthesized. The racemate was resolved into configurationally stable antipodes by HPLC on a chiral stationary phase. Electrooxidation of the enantiomers resulted in materials displaying high enantiorecognition ability towards the antipodes of some chiral probes. Moreover, the T83 racemate demonstrated great aptitude to stimulate formation of 3D rigid architectures if used as a cross-linking monomer for molecular imprinting. This feature was exploited to devise a molecularly imprinted polymer-based chemosensor selective for a thymine-adenine oligonucleotide.

The new inherently chiral material shows outstanding chirality manifestations with chiral probes, as well as with circularly polarized light components and electron spins., Chiral oligothiophene monomers with C2 symmetry, based on 3,3′-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 2,2′-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 general-scope 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.

In spite of the increasing fundamental and practical interest of electrochemistry in ionic liquids (ILs), exploration of chiral ionic liquids (CILs) in view of enantioselective electrochemistry and electroanalysis is surprisingly overdue. In this study a family of chiral ionic liquids (CILs) based on natural chiral building blocks, of easy synthesis, is detailedly characterized in terms of thermal and electrochemical properties, achieving valuable information about structure−property relationships on account of the systematicity of available family terms. Moreover, they are submitted to a series of chiral electroanalysis tests. Cyclic voltammetry in bulk CILs or with CIL as additives in a bulk achiral ionic liquid IL, shows small but statistically significant potential differences for the enantiomers of two quite different chiral probes, an interesting result since enantiodiscrimination in terms of potential differences in chiral voltammetry (more desirable respect to current differences) has been only seldom obtained so far. The present first example of enantioselective voltammetry in CIL media with stereocenters as localized chirality sources also offers an important and so far missing confirmation of the intrinsically superior level of the inherent chirality strategy, recently resulting in larger potential differences with the same protocols implemented in "inherently chiral" ionic liquid media. Furthermore, an alternative transduction mode, based on electrochemical impedance spectroscopy EIS, is proposed to effectively highlight the enantiodiscrimination ability of CIL media in analytical experiments.

Combined experimental and theoretical study on the electronic characteristics of tetrathiahelicene (7-TH) alkyl and aryl phosphorus derivatives, including phosphanes, phosphine–borane complexes, phosphine oxides, and phosphine–selenides, is described. The collected data give useful information on the σ-donor ability of the phosphorus atoms, which mainly depends on the aryl or alkyl nature of the substituents at the phosphorus atom. Electrochemical investigations provide the relationships between the electronic and structural properties, and quantum chemical calculations show a clear dominance of the 7-TH scaffold on the electronic properties of all investigated molecules. Finally, different electronic and steric properties of 7-TH phosphanes strongly influenced their catalytic behavior, for example, as Lewis base organocatalysts in typical [3+2] annulation reactions.

A detailed electrochemical study has been carried out on 1,1’-bibenzo[d]imidazoles and the corresponding mono- and bibenzimidazolium salts, and compared to benzimidazole and benzimidazolium moiety models, and to 2,2’-bibenzo[d]imidazole constitutional isomers. The voltammetric experiments, supported by theoretical calculations and structural diffractometric characterizations, evidence how both electrochemical and chiral properties of bibenzimidazole systems depend on the torsional barrier between the two moieties, which determines at the same time the effective conjugation and communication between symmetrical redox centres, and the possibility of obtaining the molecule in two inherently chiral enantiomers that are configurationally stable at room temperature. In particular, the 1,1’-bibenzimidazole scaffold appears very promising for perspective development of inherently chiral substrates that may be used as additives, or supporting electrolytes, or ionic liquids. In fact, the high torsional angle granted by the 1,1’ connectivity results both in an energy barrier high enough to yield permanently stable enantiomers at room temperature, and in low effective conjugation between the two moieties, affording a very large operating window. The effect of single and double alkylation has also been considered. A preliminary enantiorecognition test, achieved performing the electrooligomerization of enantiopure 2,2’-bi[2-(5,2’-bithienyl)]3,3’-bithianaphthene in the presence of either the (R)- or the (S)- enantiomer of a 1,1’-bibenzimidazolium salt, has shown interesting results.

The novel [Re2(μ-H)(μ-OOC-CPDT)(CO)6(μ-3-Me-pydz)] complex (1; OOC-CPDT = 4H-cyclopenta[2,1-b:3,4-b']dithiophene-4-carboxylate, 3-Me-pydz = 3-methylpyridazine) has been prepared and characterized by single-crystal X-ray diffraction, density functional theory (DFT), and time-dependent DFT computations, UV-vis absorption and emission spectroscopy, and cyclic voltammetry (CV). The measured properties indicate the lack of electronic communication in the ground state between the CPDT and the rhenium diazine moieties. Oxidative electropolymerization of 1, achieved by repeated potential cycling (-0.4 to +1.0 V vs Fc(+)/Fc, in acetonitrile) with different supporting electrolytes, on different electrodes, afforded an electroactive and stable metallopolymer (poly-1). In situ measurements of the mass of the growing film (on a gold electrode, with an electrochemical quartz microbalance) confirmed the regularity of the polymerization process. The polymer exhibits two reversible oxidation peaks of the thiophene chain and a broad irreversible reduction peak (-1.4 V, quite close to that observed for the reduction of monomer 1), associated with a remarkably delayed sharp return peak, of comparable associated charge, appearing in close proximity (+0.3 V) to the first oxidation peak of the neutral polythiophene chain. This charge-trapping effect can be observed upon repeated cycles of p and n doping, and the negative charge is maintained even if the charged electrode is removed from the solution for many hours. Electrochemical impedance spectroscopy showed that the main CV oxidation peak corresponds to facile charge transfer, combined with very fast diffusion of both electrons and ions within the polymer. In summary, poly-1 provides a new example of a metallopolymer, in which the conductive properties of the π-conjugated system are added to the redox behavior of the pendant-isolated complexes.

We have synthesized a series of tetra(2-thienyl)ethylene (TTE) derivatives, selectively functionalized at the thiophene α positions, either by a coupling procedure or by direct electrophilic substitution of the TTE system. Optical spectroscopy revealed that TTE shows orange emission (470 nm) in a diluted 2-MeTHF matrix at 77 K and blue (420 nm) aggregation-induced emission (AIE) in the solid state. An exhaustive electrochemical study afforded a full rationalization of the electronic properties in terms of the HOMO and LUMO energy levels and gaps, as well as localization and features of the redox centers with respect to TTE functionalization.

Iridium complexes bearing cyclometalated (CN) ligands are the current emitters of choice for efficient phosphorescent organic light emitting diodes (OLEDs). Homoleptic iridium complexes Ir(CN)(3) and the analogous heteroleptic ones carrying a beta-diketonate ancillary ligand (CN)(2)Ir(OO) often exhibit similar photophysical properties and device performances; the choice among them usually depends both on the yield/ease of the respective synthetic preparations as well as on the device fabrication methods (i.e. vacuum-deposition or solution-process). In our recent study we found a significant spectral red shift on going from the homoleptic to the beta-diketonate Ir(III) derivatives. The NIR emitting complex Ir(iqbt)(2)dpm (lambda(max) = 710 nm) has almost 20 nm red shifted emission compared to the homologue Ir(iqbt) 3 making only the former a real NIR emitter. For comparison, we studied the Pt(iqbt) dpm complex as the suitable example to investigate metal ligand interactions. Noteworthily the Pt(iqbt) dpm emission perfectly overlaps that of the Ir(iqbt) 2dpm. In this paper we provide an in-depth investigation of these systems by electro-chemical and spectroscopic analyses and corroborate the results with DFT and TDDFT calculations to investigate whether the Pt(II) complex can be used as a model system to predict how far the emission can be pushed in a Ir(III) heteroleptic derivative bearing the same CN ligand.

Square planar platinum complexes are an important class of compounds used in (nano)technology, optoelectronics, medicinal chemistry and catalysis. The major research interests in cyclometalated Pt(ii) complexes focus on the selective modulation of their electronic properties and the control of the (cis/trans) geometry. For the first time, we unveil and demonstrate that cis-trans isomers of Pt(ii) complexes can be obtained in a derivative carrying the 1-phenyl-pirazolate (Hppz) and 2-hydroxy-1-naphtyl-(N-phenyl)imine ligands. The two isomers display significant differences in both optical and electronic properties. While luminescence is quenched in solution, they are brightly emissive in the PMMA matrix at room temperature and in the 2MeTHF rigid matrix at 77 K. The phosphorescent emission of the cis isomer, blue-shifted compared to that of the trans one, results from the significantly different trans influence of the ppz ligand. Theoretical investigation highlights the almost isoenergetic potential energy of the two isomers therefore explaining their formation and evidences a large geometry distortion of their triplet state, which should be responsible for the observed luminescence efficiency.

"Inherently chiral" thiophene-based electroactive oligomer films have recently been shown to exhibit outstanding chirality manifestations. One of the most exciting among them is an unprecedented enantioselection ability as electrode surfaces. In fact, in preliminary chiral voltammetry experiments, the new electrodes have been shown to both discriminate the enantiomers of chiral probes (either enantiopure or in a mixture, in terms of large differences in peak potentials) and quantify them (in terms of linear dynamic ranges in peak currents), without the need for preliminary separation steps. Such ability has now been tested on a series of chiral DOPA-related molecules, from phenolic amino acid tyrosine (together with its methyl ester) to catecholic amino acid DOPA (together with its methyl ester), to catecholamine epinephrine (adrenaline). The wide-range enantioselectivity of the new inherently chiral electrode surfaces is fully confirmed, as large peak potential differences are obtained for probe enantiomers of the whole series working in common aqueous buffers. Moreover, interesting modulating effects on enantiodiscrimination can be observed as a function of both molecular structure and pH. Graphical abstract Inherently chiral thiophene-based electrodes at work with pharmaceutically relevant probes.

In this contribution we provide an overview of the preparation and of the properties of different classes of luminescent dinuclear tricarbonyl rhenium(I) complexes of general formula [Re 2 (μ-X) 2 (CO) 6 (μ-1,2-diazine)] (X = halide, hydride, alkoxide, and thiolate). Their electrochemical and photophysical properties in solution are presented, as well as combined density functional (DFT) and time-dependent density functional (TDDFT) studies of their geometry, relative stability and electronic structure. The relationships between the structure and the emission properties of the complexes are discussed in detail, showing a modulation effect of the bridging ancillary ligands as well as of the diazine substituents on energy, lifetime and quantum yield of the emission. Photoluminescent quantum yields ( Φ ) up to 0.53 have been measured in fluid solution for the dichloro complexes containing diazines bearing alkyl groups in both the β positions, designed to optimize all the parameters responsible for the radiative decay. The photophysical properties in the solid state are also briefly presented, showing in some cases an increase of the emission intensity, discussed in terms of restriction of intramolecular roto-vibrational motions. These properties, together with their good processability and high stability, allowed the successful use of some of these complexes as a phosphorescent dopant in OLEDs. The good performances of OLED devices prepared both by solution- and vacuum-processing are reported. The preliminary work exploring the potential of these complexes as luminescent labels for bioimaging is also presented.

Push-pull Zn(II)-porphyrinates have recently shown attracting performances as light harvesting systems in dye-sensitized solar cells (DSSCs). To fully exploit their intrinsically high efficiency it is important to finely tune their HOMO and LUMO levels, which can be achieved by proper choice of the push and pull substituents. Of course such target-oriented molecular design requires the availability of reliable relationships between molecular structure and electronic properties; therefore we have carried out a combined electrochemical, spectroscopic and computational investigation on a wide, systematic range of Zn(II)-porphyrinates 5,15 meso substituted with phenylethynyl linkers, including a first symmetric series carrying on the opposite terminals the same substituent (-N(CH3)2,-OCH3,-COOCH3,-COOH,-NO2); and a second push-pull one, with the terminal positions carrying one donor and one acceptor group belonging to the series above. Moreover, two suitably modified porphyrins allowed evaluation of the effects of (i) the presence or absence of the phenyl group in the linker between the porphyrin core and the acceptor group, and (ii) the effect of perfluorination on the same phenyl group. A rationalization scheme is proposed encompassing the whole porphyrin set, affording inter alia interesting clues on the different localization of the redox centres and effective conjugation between the porphyrin core and the side chains as a function of the molecular design.

Tetrathia[7]helicene ([7]TH)-based complexes substituted at the thienyl ring ends by a ferrocenyl group (Fc) or by a (η5-cyclohexadienyl)Mn(CO)3 derivative have been prepared by Sonogashira coupling reactions starting from the mono- or diiodo [7]TH compounds. The molecular structure of one of the diferrocenyl [7]TH complexes was established by X-ray analysis. Electrochemical investigation on the Fc-[7]TH systems show that the Fc groups are significantly electron poorer with respect to Fc (ΔE° ≈ 0.15 V), due to the effective conjugation of the Fc redox moiety with the triple bond + helicene system, as also confirmed by spectroscopic data. Potential cycling around the second oxidation peak, assigned to the thiahelicene moiety, affords fast, regular growth of electrodeposited conducting films, provided that one terminal α-thiophene position be available for coupling; on the other hand, long alkyl chains hamper film formation. The conducting films feature a broad oxidation wave resulting from the merging of s...

To achieve rationalization criteria for target-oriented molecular design of Th–X–Th (Th = thiophene) semiconductor building blocks, we have carried out an extensive investigation on the effects of the X core (X = fluorene, carbazole or phenothiazine) on the electronic properties and polymerization ability of Th–X–Th monomers and on the electronic and structural properties of the corresponding periodic conducting polymers –(Th–X–Th) n –, obtained by electropolymerization and, for comparison's sake, by FeCl 3 -catalyzed polymerization and/or Suzuki coupling. The effects of molecule bending and of solubilising bulky alkyl substituents have also been considered. The systematic, exhaustive template sequence combined with a rigorous, multitechnique investigation protocol affords a unique data library and a complete set of reliable interpretative/predictive guidelines.

A series of 4-X-1-methylpyridinium cationic nonlinear optical (NLO) chromophores (X = (E)-CH=CHC(6)H(5); (E)-CH=CHC(6)H(4)-4'-C(CH(3))(3); (E)-CH=CHC(6)H(4)-4'-N(CH(3))(2); (E)-CH=CHC(6)H(4)-4'-N(C(4)H(9))(2); (E,E)-(CH=CH)(2)C(6)H(4)-4'-N(CH(3))(2)) with various organic (CF(3)SO(3)(-), p-CH(3)C(6)H(4)SO(3)(-)), inorganic (I(-), ClO(4)(-), SCN(-), [Hg(2)I(6)](2-)) and organometallic (cis-[Ir(CO)(2)I(2)](-)) counter anions are studied with the aim of investigating the role of ion pairing and of ionic dissociation or aggregation of ion pairs in controlling their second-order NLO response in anhydrous chloroform solution. The combined use of electronic absorption spectra, conductimetric measurements and pulsed field gradient spin echo (PGSE) NMR experiments show that the second-order NLO response, investigated by the electric-field-induced second harmonic generation (EFISH) technique, of the salts of the cationic NLO chromophores strongly depends upon the nature of the counter anion and concentration. The ion pairs are the major species at concentration around 10(-3) M, and their dipole moments were determined. Generally, below 5x10(-4) M, ion pairs start to dissociate into ions with parallel increase of the second-order NLO response, due to the increased concentration of purely cationic NLO chromophores with improved NLO response. At concentration higher than 10(-3) M, some multipolar aggregates, probably of H type, are formed, with parallel slight decrease of the second-order NLO response. Ion pairing is dependent upon the nature of the counter anion and on the electronic structure of the cationic NLO chromophore. It is very strong for the thiocyanate anion in particular and, albeit to a lesser extent, for the sulfonated anions. The latter show increased tendency to self-aggregate.

The anionic rhodium carbonyl clusters [Rh 7 (CO) 16 ] 3− and [Rh 14 (CO) 25 ] 4− can be easily prepared by a new simple and high yield one-pot synthesis starting from RhCl 3 · n H 2 O dissolved in ethylene glycol and involving two steps: (i) treatment of RhCl 3 · n H 2 O under 1 atm of CO at 50 °C to give [Rh(CO) 2 Cl 2 ] − ; (ii) addition of a base (CH 3 CO 2 Na or Na 2 CO 3 ) followed by reductive carbonylation under 1 atm of CO at an adequate temperature (50 °C for [Rh 7 (CO) 16 ] 3− ; 150 °C for [Rh 14 (CO) 25 ] 4− ). These new syntheses are more convenient than those previously reported, especially since such clusters are not accessible via silica surface-mediated reactions. This different behavior is due to the particular stabilization on the silica surface and under 1 atm of CO of an anionic carbonyl cluster, called A, which does not allow the formation of a higher nuclearity carbonyl cluster, called B, which was shown to be the key-intermediate in the synthesis of [Rh 14 (CO) 25 ] 4− working in ethylene glycol solution. Although it was not possible to isolate crystals of A and B suitable for X-ray structural determination, a combination of cyclovoltammetry, one of the few examples so far available of the use of this technique for anionic rhodium carbonyl clusters, infrared spectroscopy and elemental analyses suggest that A and B are probably the never reported [Rh 7 (CO) 14 ] − and [Rh 15 (CO) 28 ] 3− clusters, respectively. In particular the tentative formulation of the two clusters was carried out by a non-conventional method based on the existence of a linear correlation between carbonyl frequencies of the main band and the [(charge/Rh atoms)/CO number] ratio.

A series of neutral, dinuclear, luminescent rhenium(I) complexes suitable for phosphorescent organic light emitting devices (OLEDs) is reported. These compounds, of general formula [Re2(m-Cl)2(CO)6(m-1,2-diazine)], contain diazines bearing alkyl groups in one or in both the b positions. Their electrochemical and photophysical properties are presented, as well as a combined density functional and time-dependent density functional study of their geometry, relative stability and electronic structure. The complexes show intense green/yellow emissions in toluene solution and in the solid state and some of the complexes possess high emission quantum yields (f=0.18–0.22 for the derivatives with disubstituted diazines). In butyronitrile glass, at 77K, due to the charge transfer character of the lowest (emitting) excited state, strong blue shift of the emission is observed, accompanied by a strong increase in the lifetime values. The highest-performing emitting complex, containing cyclopentapyridazine as ligand, is tested in a polymer-based lightemitting device, with poly(9-vinylcarbazole) as matrix, as well as in a device obtained by vacuum sublimation of the complex in the 2,7bis(diphenylphosphine oxide)-9-(9-phenylcarbazol-3-yl)-9-phenylfluorene (PCF) matrix. This represents the first example of devices obtained with a rhenium complex which can be sublimed and is solution processable. Furthermore, the emission is the bluest ever reported for electrogenerated luminescence for rhenium complexes.

This works investigates the mechanistic features related to the reactions between Fe II Salen and several organic halides used as initiators in Atom Transfer Radical Polymerization. Within the time scale of cyclic voltammetry, a fast reaction is observed between electrogenerated Fe II Salen and either ethyl 2-bromoisobutyrate, carbon tetrachloride or 1-chloro-1-phenylethane. Since no catalytic current is observed in cyclic voltammetry in the presence of RX, no fast mediated reductive cleavage of RX into R + X − occured. An intermediate [R··X··FeSalen] complex was formed which was either relatively stable (ethyl 2-bromoisobutyrate and 1-chloro-1-phenylethane) or underwent further reduction (CCl 4 ). In the presence of styrene and at 110 °C, polymerization is observed and the polydispersity of the resulting polystyrene appears to depend on the Fe III Salen/Fe II Salen initial ratio. The role of both Fe III Salen and Fe II Salen in the regulation of radical concentration provided a controlled character to the polymerization.

The electrochemical reduction of a series of aromatic bromides including substituted bromobenzenes and polycyclic bromoarenes has been investigated in acetonitrile at silver and glassy carbon (GC) electrodes. Whereas GC behaves as a noncatalytic electrode, Ag exhibits remarkable electrocatalytic activities for the reduction of all compounds. The electrocatalytic effects are strongly influenced by the molecular structure of the aromatic bromide, decreasing with increasing electron-withdrawing ability of the substituents as well as with extension of the polycyclic aromatic system. Dissociative electron transfer (ET) to an organic halide RX may occur either in a single step (concerted mechanism) or in two distinct steps with the formation of an intermediate radical anion (stepwise mechanism). The mechanism of the reduction process was analyzed at both catalytic and noncatalytic electrodes. Electroreduction of all compounds at GC occurs according to a stepwise mechanism, whereas at Ag both mechanisms were obs...

The three complexes [Re2(mu-X1)(mu-X2)(CO)6(mu-ppd-kappaN3:kappaN4)] (X1, X2 ) H, 1; X1 ) H, X2 ) Cl, 2; X1, X2 ) Cl, 3; ppd) 2,5-diphenyl-1,3,4-oxadiazole) have been synthesized by different routes, involving the reaction of [Re4(mu3-H)4(CO)12]with ppd for 1, the reaction of 1 with HCl for 2, and the reaction of [ReCl(CO)5] with ppd for 3. The three complexes possess a different number of valence electrons, so the formal Re-Re bond order varies from 2 to 1 to 0 in complexes 1, 2, and 3, respectively. This is reflected in the Re-Re bond distance (277.9, 297.9, and 358.5 pm in the same series)and in the stability of the complexes in the coordinating solvent acetonitrile (t1/2 for ppd displacement 13.6, 4.5, and 3.7 h,for 1, 2, and 3, respectively). Both experimental and calculated structures indicates that coordination induces a distortion from planarity of the diphenyloxadiazole moiety due to the interaction of the equatorial carbonyls with the bridging ppd,which increases on going from 1 to 2 to 3 (dihedral angle between the oxadiazole and the phenyl rings 18.4 degrees, 23.3 degrees, and 45.0 degrees, respectively). The UV spectra show pi-pi* transitions of the oxadiazole ligand (which shift to higher energy on increasing the distortion from the planarity, from 252 to 267 nm) and metal-to-ligand charge transfer absorptions (from 300 to 362 nm). Upon irradiation between 340 and 380 nm, complex 2 only features a weak broad emission at 527 nm(phi)0.02%), whereas upon excitation at 300 nm, the emission typical of free ppd is observed, suggesting photodissociation.Cyclic voltammetry investigations in acetonitrile showed that the three complexes exhibit ligand-centered irreversible reduction peaks (from -1.83 to -1.93 V vs Fc+|Fc), shifted to more positive values with respect to free ppd (-2.50 V). The shift however is smaller than in the analogous derivatives containing 1,2-diazines, suggesting a smaller electron depletion of the heterocycle ligand upon coordination. The complexes also show a metal-centered, bi-electronic, irreversible oxidation peak (from 1.05 to 1.37 V vs Fc+/Fc). A combined density functional and time-dependent density functional (TD DFT)study allowed us to understand the factors affecting the stability of the three complexes and to rationalize their electrochemical and photophysical properties in terms of their electronic structure.

The primary and the secondary standards for the pH measurements in solvent media Z = (glycerol [G] + water [W]) at glycerol mass fractions wG up to 0.4 and at temperatures from −10 °C to +40 °C have been determined in compliance with the IUPAC criteria. To this purpose, Harned’s reversible cell {Pt|H2|Primary Buffer + KCl in Z|AgCl|Ag|Pt} and Baucke’s nonreversible cell {Pt|H2|Buffer at pHPS in Z|Salt Bridge in Z|Buffer at pHSS in Z|H2|Pt} were used, respectively. As a result, three primary standards (based on the “carbonate”, the “equimolal phosphate”, and the “phthalate” buffers) and three secondary standards (based on the “tetroxalate”, the “tartrate”, and the “unequimolal phosphate” buffers) are now available. Their internal consistency was ascertained through the linear pH dependence of the voltage of the cell {Pt|H2 |Buffer at pHPS [or pHSS] in Z|NH4Cl Bridge in Z|Reference Electrode}. These acquisitions enable the user to perform routine pHX measurements by the regular operational cell {Reference E...

The introduction of alkoxy chains in the molecular architecture of meso push-pull porphyrins is of paramount importance aiming at high performing dye-sensitized solar cells (DSSCs) based on these specific sensitizers. Recently, we have demonstrated that the same approach is fruitful even if it is applied to tetraarylporphyrins with an acceptor/anchoring substituent in the [small beta]-pyrrolic position. In particular, among the ortho-ortho, the ortho-para and the ortho-functionalization of the aryl rings with an octyloxy chain, we identified the latter as the most performing in the series, showing a good balance between the dye loading and the reduction of [small pi]-[small pi] aggregation. Herein, focusing our attention on the mono-ortho-functionalized molecular structure, we have investigated the effect of the alkoxy chain length and nature on the reduction of dye-to-dye aggregation as well as on the enhancement of light harvesting capabilities, finding an almost linear relationship between the device photon conversion efficiency (PCE) and the alkoxy chain length both in the presence and in the absence of a co-disaggregating agent.

Following the IUPAC criteria, four primary standards (pHPS) and three secondary standards (pHSS) have been determined at various temperatures using Harned's reversible cell and Baucke's nonreversible cell, respectively, the solvent media Z studied being N-methylacetamide and its mixture with water at 0.50 mass fraction. Their internal consistency was ascertained by the linear pH dependence of the emf of the cell (Pt|H2|pH standards in Z||NH4Cl bridge in Z|AgCl|Ag|Pt). Essential thermodynamic functions (standard emfs E° of cell and ionization constants of orthophthalic acid) hitherto unknown in the pertinent solvent Z had also to be determined from the emfs of the appropriate reversible cells. These pHPS and pHSS acquisitions enable the user to perform routine pHX measurements by the regular operational cell. A supplementary systematic search has been performed according to the method of Helmholtz transference cells both in N-methylacetamide and in its w = 0.50 mixture with water, to single out appropriate...

Novel poly(amidoamine)s (PAAs) containing disulfide linkages regularly arranged along the polymer chain, namely BP−CY and BAC−CY, were synthesized by stepwise polyaddition of l-cystine to 1,4-bis(acryloyl)piperazine (BP) and 2,2-bis(acrylamido)acetic acid (BAC), respectively. Even if l-cystine contains four acid hydrogens, no evidence of cross-linking was found. All products were characterized by 1H and 13C NMR spectroscopy, and their average molecular weight determined by size exclusion chromatography. The polymerization rates were investigated by means of 1H NMR spectroscopy. In both cases, the experimental data were consistent with pseudo-second-order kinetics. The calculated kinetic constants were kc,BP = 8.10 × 10-3 min-1 L mol-1 and kc,BAC = 1.41 × 10-3 min-1 L mol-1 for the polyaddition of l-cystine to BP and BAC, respectively. A potentiometric study was carried out of BP−CY and BAC−CY speciation as a function of pH, and the electrochemical activity of their disulfide bonds as a function of pH was ...

Carboxy-terminated polyvinylpyrrolidin-2-one (PVP) has been used as a new water-soluble and biocompatible polymeric support for a series of ferrocene labeled amino acid and peptide nucleic acid (PNA) monomer derivatives 4 – 7 . The organometallic polymer-conjugates thus obtained are new and potentially useful as water-soluble electrochemically active probes for biomolecules. In view of such application, their electrochemical activity has been evaluated and has proved very high notwithstanding the complexity and bulkiness of the molecule, affording detection limits down to 10 −8 M in the aqueous medium.

A systematic investigation was performed on the specific adsorption of bromide and iodide anions on controlled-surface polycrystalline silver electrodes, from three organic solvents (acetonitrile, propylene carbonate, and dimethylformamide), combining differential capacity and impedance experiments with a recently proposed “indirect” voltammetric method based on the monitoring of the negative shift of the reduction peak potential of a “probe” organic halide molecule induced by progressive additions of halide anions, resulting in increasing adsorption competition. Bromide and iodide ions are specifically adsorbed onto polycrystalline silver electrodes in the three organic solvents studied. The adsorption process, which is slow and it is characterised by a partial charge transfer and by a slow diffusion step, is modulated by the different coordination abilities of the solvents for the species involved in the adsorption. The three experimental methods provide consistent results. In particular, the strength of halide adsorption increases in the halide sequence Br−

The potential difference E of the amalgam cell {CsxHg1 − x|CsX (m)|AgX|Ag} (X = Cl, Br, I) has been measured as a function of the mole fraction xCs of Cs metal in amalgams and of the molality m of CsX in (methanol + water), (acetonitrile + water), and (1,4-dioxane + water) solvent mixtures containing up to 0.75 mass fraction of the organic component, at the temperature 298.15 K. The respective standard molal potential differences E m ∘ have been determined together with the relevant activity coefficients γ± as functions of the CsX molality. The found E m ∘ values show a parabolic decrease with increasing proportion of the organic component in the solvent mixture. Analysis of the relevant primary medium effects upon CsX shows that the CsX transfer from the standard state in water to the standard state in the (aqueous + organic) mixture is always unfavoured, and the acetonitrile is the least unfavoured co-solvent studied. Analysis of the primary medium effect upon CsI in terms of Feakins and French’s theory leads to a primary hydration number close to zero, which is consistent with the results of supplementary EXAFS experiments on Cs+ and I− in (acetonitrile + water) solvent mixtures.

We have designed and synthesised a new organometallic molecule containing three ferrocene groups for use as a highly sensitive electrochemical marker in biological assays. This trisferrocene derivative was conjugated to different PNA monomers, and the electrochemical activities of the conjugates were extensively investigated in organic solvents, in view of their potential diagnostic applications. The results showed that the introduction of a trisferrocene unit on the PNA monomer triples the current signal in comparison with the monoferrocene-labelled one. Despite their greater molecular molecular complexity, trisferrocene-conjugated PNA monomers are even more electrochemically active than the reference ferrocene. By using differential pulse voltammetry (DPV), the detection limit can reach 10-8 M in acetonitrile solution. These results are a good premise for the use of the trisferrocene unit as an effective electrochemical probe for biomolecules.

This work describes the synthesis and the electronic characterisation by electronic absorption spectroscopy, cyclic voltammetry and dipole moments of diphenyl porphyrins and their ZnII complexes substituted at the meso position by a pseudo-linear π-delocalised substituent carrying an electron-donor or an electron-withdrawing group. The second-order NLO response was investigated by the EFISH technique working with a nonresonant incident wavelength of 1.907 μm. This work confirms the ambivalent role of the polarisable porphyrin ring, which, already in the ground state, acts as a donor or acceptor depending on the nature (acceptor or donor) of the substituent in the meso position, as was pointed out in our previous work on tetraphenyl porphyrins substituted at the β-pyrrolic position. The second-order NLO response has been discussed by comparing the nature of the substituent (electron donor or acceptor) and the kind of substitution (β pyrrolic or meso). This comparison evidenced a significant increase of the electron-donor properties of NBu2 or NMe2 when they are connected to the meso position or to the β-pyrrolic position, probably because of an auxiliary donor effect of the electron-rich porphyrin ring. When the substituent is the electron-withdrawing NO2 group, the substituent position (meso or β pyrrolic) is influential, with the existance of a significant increase in the second order NLO response when substitution occurs at the meso position. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

[structure: see text] The role played by the electronic properties and the steric features of bis(oxazoline) ligands in the Cu(I)-catalyzed cyclopropanation of styrene effected with ethyl diazoacetate was investigated. Two pairs of new bis(oxazolines) displaying flexible and atropisomeric 3,3'-bithiophene backbones were synthesized and structurally and electronically characterized. For the first time, the electrochemical oxidative potential was used as a reliable index of the electronic density on the nitrogen atom of the chelating groups of new and, for comparative purposes, of already known bis(oxazolines). The Cu(I) complexes of the new ligands were prepared, and their enantioselection ability and catalytic efficiency were tested. This investigation suggests that steric factors and catalyst geometrical features are clearly more important than any consideration of the electronic properties of the chiral ligands.

A wide investigation on the electrochemical activity of four model organic bromides has been carried out in acetonitrile on nine cathodes of widely different affinity for halide anions (Pt, Zn, Hg, Sn, Bi, Pb, Au, Cu, Ag), and the electrocatalytic activities of the latter have been evaluated with respect to three possible inert reference cathode materials, i.e. glassy carbon, boron-doped diamond, and fluorinated boron-doped diamond. A general electrocatalytic activity scale for the process is proposed, with a discussion on its modulation by the configuration of the reacting molecule, and its connection with thermodynamic parameters accounting for halide adsorption.

For the amalgam cell {MxHg1 − x|MCl2 (m)|AgCl |Ag} (with M = Sr, Ba) the potential difference E has been measured as a function of the mole fraction xM of the metal M in amalgams and of the molality m of MCl2 in {methanol + water} and {ethanol + water} solvent mixtures Z = {A + W} with mass fractions of alcohol wA ⩽ 0.50 of alcohol, at temperature 298.15 K. The respective molal-scale standard potential differences E m ∘ have been determined together with the relevant activity coefficients γ± as functions of the MCl2 molality. The E m ∘ dependence on the alcohol mole fraction in the solvent mixture within the ranges explored turns out to be linear for both of these two metals M in the amalgams studied. Of course, also the molal-scale standard Gibbs free energy change ( Δ G W → Z ∘ ) m = 2 F ( W E m ∘ - Z E m ∘ ) , which constitutes the “primary medium effect” upon transferring MCl2 from pure water to the (alcohol + water) mixture, is linear in xA. In the same context, following Feakins and French’s treatment, which implies volume fraction statistics, the relevant primary medium effects upon MCl2 on the mol · dm−3 scale have been analysed in terms of the expected linear relation of ( Δ G W → Z ∘ ) c = 2 F ( W E c ∘ - Z E c ∘ ) against the logarithm of water volume fraction, leading to primary hydration numbers for SrCl2 and BaCl2, respectively, in acceptable agreement with Bockris’ data based on different methods.

To evidentiate the role of the nature of sulfonate ancillary ligands on the value of the quadratic hyperpolarizability of Zn(II) complexes with stilbazole-like ligands, the second-order nonlinear optical (NLO) properties of [ZnY 2 (4,4'-trans-NC 5 H 4 CH=CHC 6 H 4 NMe 2 ) 2 ] complexes (Y = CF 3 SO 3 , CH 3 SO 3 , or p-CH 3 C 6 H 4 SO 3 ) are investigated. By working at relatively high concentrations (>3 x 10 - 4 M), the positive effect of the triflate ligand remains unique while, with nonfluorinated sulfonate ligands, the second-order NLO response is comparable to that of the related complexes with acetate or trifluoroacetate as ancillary ligands. However, at dilutions higher than 10 - 4 M, all of the sulfonate complexes reach huge quadratic hyperpolarizabilities because of solvolysis with the formation of cationic species such as [ZnY(4,4'-trans-NC 5 H 4 CH=CHC 6 H 4 NMe 2 ) 2 ] + , characterized by a large second-order NLO response. This view is supported by careful conductivity measurements. The same behavior occurs if 4,4'-trans-NC 5 H 4 CH=CHC 6 H 4 . NMe 2 is substituted by 4,4'-trans,trans-NC 5 H 4 (CH=CH) 2 C 6 H 4 NMe 2 .

A new chiral ferrocene-labelled tyrosine PNA monomer 1 has been synthesised in good yield in both racemic and enantiomerically pure forms. It is suitable for insertion in various positions of PNA oligomers, a possibility that has been preliminarily demonstrated by synthesising the dimer 16. Moreover, in view of possible applications to nucleic acid detection, a preliminary voltammetric investigation on the electrochemical activity of monomer 1 and its synthetic precursors has been carried out in DMF. It appears that, despite the bulkiness of the PNA monomer backbone, its insertion on the ferrocene group only moderately lowers the latter’s diffusion coefficients and peak currents, thus affording voltammetric detection limits in the order of 10 −6 –10 −7 M.