Pyrylium- and Pyridinium-Based Ionic Liquids as Friction Modifiers for Greases

The use of ionic liquids (ILs) as lubricants or additives has been studied extensively over the past few decades. However, the ILs considered for lubricant applications have been part of a limited structural class of phosphonium- or imidazolium-type compounds. Here, new pyrylium- and pyridinium-based ILs bearing long alkyl chains were prepared and evaluated as friction- and wear-reducing additives in naphthenic greases. The physical properties of the synthetic ILs and additized naphthenic grease were measured. The tribological performance of the greases was measured by using standard benchtop tests. The addition of ILs was detrimental to wear, causing an increase in the amount of material removed by sliding relative to the base greases in most cases. In contrast, the friction performance improved under nearly all conditions tested due to the IL additives. The compatibility of the synthetic ILs with the naphthenic greases and its potential influence upon miscibility and tribological performance are tentatively proposed to be a result of the molecular structure.

Facile access to pyridinium-based bent aromatic amphiphiles: nonionic surface modification of nanocarbons in water

Efficient water-solubilization of nanocarbons is desirable for both their biological and material applications, but so far has mainly relied on covalent modifications or amphiphiles featuring ionic side-chains. Here, we report a facile 2-4-step synthesis of pyridinium-based, bent aromatic amphiphiles with modular nonionic side-chains (i.e., CH3 and CH2CH2(OCH2CH2)2-Y (Y = OCH3, OH, and imidazole)). The new amphiphiles quantitatively self-assemble into ≈2 nm-sized aromatic micelles in water independent of the side-chain. Importantly, efficient water-solubilization and nonionic surface modification of various nanocarbons (e.g., fullerene C60, carbon nanotubes, and graphene nanoplatelets) are achieved through noncovalent encircling with the bent amphiphiles. The resultant imidazole-modified nanocarbons display a pH-responsive surface charge, as evidenced by NMR and zeta-potential measurements. In addition, solubilization of a nitrogen-doped nanocarbon (i.e., graphitic carbon nitride) in the form of 10-30 nm-sized stacks is also demonstrated using the present amphiphiles.

Synthesis, biological evaluation, and computational studies of N-benzyl pyridinium-curcumin derivatives as potent AChE inhibitors with antioxidant activity

A library of N-benzylpyridinium-based compounds, 7a-j and 8a-j, was designed and synthesised as potential acetylcholinesterase) AChE (inhibitors. An in vitro assay for the synthesised compounds showed that most compounds had significant AChE inhibitory activities at the nanomolar and submicromolar levels. The benzyl (8a) and fluoro (8b) derivatives were the most active, with IC50 values ≤56 nM. Compound 7f, which had a benzyl moiety, showed the highest potency among all the target compounds, with an IC50 value of 7.5 ± 0.19 nM against AChE, which was higher than that of the activities of tacrine (IC50 = 30 ± 0.2 nM) and donepezil (IC50 = 14 ± 0.12 nM). Compounds with vanillin moieties exhibited antioxidant activity. Among the tested compounds, four derivatives (7f, 7 g, 8f, and 8 g) exhibited superior AChE inhibitory activity, with Ki values of 6-16 nM, which were potent in the same range as the approved drug, donepezil. These compounds showed moderate antioxidant activities, as indicated by the results of the ABTS assay.

Modified Viologen- and Carbonylpyridinium-Based Electrodes for Organic Batteries

Efficient electrochemical energy storage has been identified as one of the most pressing needs for a sustainable energy economy. Inorganic battery materials have traditionally been the center of attention, with the current state-of-the-art device being the lithium-ion battery. Recent pursuits have led to organic materials for their beneficial chemistry and properties, but suitable materials for organic batteries are still few and far between. This Spotlight on Applications highlights two intriguing pyridinium-based organic materials, modified viologens and carbonylpyridiniums, that have both been successfully employed in electrode materials for solid-state Li-ion-type organic batteries (LOBs). We first provide an overview of the inherent electronic properties of each building block and how they can effectively be modified while maintaining or enhancing their desirable electrochemical properties for practical applications. We then describe a range of different material designs for a battery context and their application in various organic device settings, with some examples showing competitive performance with traditional Li-ion batteries.

Bright NIR-Emitting Styryl Pyridinium Dyes with Large Stokes' Shift for Sensing Applications

Two NIR-emitting donor-π-acceptor (D-π-A) type regioisomeric styryl pyridinium dyes (1a-1b) were synthesized and studied for their photophysical performance and environment sensitivity. The two regioisomers, 1a and 1b, exhibited interesting photophysical properties including, longer wavelength excitation (λex ≈ 530-560 nm), bright near-infrared emission (λem ≈ 690-720 nm), high-fluorescence quantum yields (ϕfl ≈ 0.24-0.72) large Stokes' shift (∆λ ≈ 150-240 nm) and high-environmental sensitivity. Probe's photophysical properties were studied in different environmental conditions such as polarity, viscosity, temperature, and concentration. Probes (1a-1b) exhibited noticeable changes in absorbance, emission and Stokes' shift while responding to the changes in physical environment. Probe 1b exhibited a significant bathochromic shift in optical spectra (∆λ ≈ 20-40 nm) compared to its isomer 1a, due to the regio-effect. Probes (1a-1b) exhibited an excellent ability to visualize bacteria (Bacillus megaterium, Escherichia coli), and yeast (Saccharomyces cerevisiae) via fluorescence microscopy.

Photoredox Catalysis for the Synthesis of N-CF2H Compounds Using 1-((N-(difluoromethyl)-4-methylphenyl)-sulfonamido)pyridin-1-ium Trifluoromethanesulfonate

: N-(difluoromethyl)amino (-NCF2H) compounds are of great interest given their unique and underexplored physiochemical properties. The lack of structural diversity in NCF2H compounds is likely due in part to the shortage of protocols for efficient installation. Presented herein is a new shelf-stable pyridinium reagent that enables the direct installation of the N-(difluoromethyl)sulfonamide moiety [N(Ts)CF2H)] onto (hetero)arenes and alkenes for the diversification of aryl and alkyl NCF2H compounds. The described protocol utilizes blue light photoredox catalysis and displays broad functional group tolerance with excellent chemoselectivity. Additional transformations and applicability towards a photoredox continuous flow protocol are also demonstrated.

Recent Progress in Heterocycle Synthesis: Cyclization Reaction with Pyridinium and Quinolinium 1,4-Zwitterions

Heteroarene 1, n-zwitterions are powerful and versatile building blocks in the construction of heterocycles and have received increasing attention in recent years. In particular, pyridinium and quinolinium 1,4-zwitterions have been widely studied and used in a variety of cyclization reactions due to their air stability, ease of use, and high efficiency. Sulfur- and nitrogen-based pyridinium and quinolinium 1,4-zwitterions, types of emerging heteroatom-containing synthons, have attracted much attention from chemists. These 1,4-zwitterions, which contain multiple reaction sites, have been successfully used in the synthesis of three- to eight-membered cyclic compounds over the last decade. In this review, we present the exciting progress made in the field of cyclization reactions of sulfur- and nitrogen-based pyridinium and quinolinium 1,4-zwitterions. Moreover, the mechanistic insights, the transition states, some synthetic applications, and the challenges and opportunities are also discussed. We hope to provide an overview for synthetic chemists who are interested in the heterocycle synthesis from cyclization reaction with pyridinium and quinolinium 1,4-zwitterions pyridinium and quinolinium 1,4-zwitterions.

Facile One-Pot Synthesis of Polysubstituted Pyridinium Salts by Annulation of Enamines with Alkynes

An efficient and facile synthesis of highly substituted pyridinium salts through the annulation of enamines with alkynes is reported herein. A Ag₂ CO₃ /HNTf₂ synergistically acting catalyst system was developed and used in a condensation reaction between carbonyl substrates and propargylamine to afford structurally diverse pyridinium salts. A mechanistic investigation shows that this one-pot transformation proceeded via selective 6-endo-dig cyclization of the in situ generated propargylenamine and protonolysis of the resulting vinyl-silver intermediate. The reaction conditions are mild, and the substrate scope is broad. During the cyclization, an unusual inversion of the normal reactivity of α,β-unsaturated carbonyl systems was observed.

Pyridinium Salts of Dehydrated Lanthanide Polychlorides

The reaction of lanthanide (Ln) chloride hydrates ([Ln(H₂O)_n(Cl)₃]) with pyridine (py) yielded a set of dehydrated pyridinium (py-H) Ln-polychloride salts. These species were crystallographically characterized as [[py-H-py][py-H]₂[LnCl₆]] (Ln-6; Ln = La, Ce, Pr, Nd, Sm, Eu, Gd) or [[py-H]₂[LnCl₅(py)]] ((Ln-5; Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu). The Ln-6 metal centers adopt an octahedral (OC-6) geometry, binding six Cl ligands. The -3 charge is off-set by two py-H moieties and a di-pyridinium (py-H-py) ion. For the Ln-5 species, an OC-6 anion is formed by the Ln cation binding a single py and five Cl ligands. The remaining -2 charge is offset by two py-H⁺ cations that H-bond to the anion. Significant H-bonding occurs between the various cation/anion moieties inducing the molecular stability. The change in structure from the Ln-6 to Ln-5 is believed to be due to the Ln-contraction producing a smaller unit cell, which prevents formation of the py-H-py⁺ cation, leading to the loss of the H-bonding-induced stability. Based on this, it was determined that the Ln-5 structures only exist when the lattice energy is small. While dehydrated polychloride salts can be produced by simply mixing in pyridine, the final structures adopted result from a delicate balance of cation size, Coulombic charge, and stabilizing H-bonding.

Regiodivergent Asymmetric Pyridinium Additions: Mechanistic Insight and Synthetic Applications

A practical protocol for the first regiodivergent asymmetric addition of aryl and alkenyl organometallic reagents to substituted N-alkyl pyridinium heterocycles is described. The couplings proceed with high regiochemical and stereochemical selectivities, and provide access to chiral 1,2,3- and 1,3,4-trisubstituted dihydropyridine products, controlled by judicious choice of nitrogen activating agent. To this end, a correlation was found between the parameterized size of the activating group and the C2/C4 regioselectivity in the couplings. The utility of the described chemistry was demonstrated in two concise asymmetric syntheses of (+)-N-methylaspidospermidine and (-)-paroxetine.

Synthesis of pyridinium-based ionic liquids and their application to improve Candida rugosa lipase stability in a methanol-water solvent system

This paper studied the effect of pyridinium-based ionic liquids as cosolvents in a methanol-water solvent system on the hydrolytic activity of Candida rugosa lipase. These ionic liquids were successfully synthesized using imidazolium-based ionic liquid synthesizing methods with a certain adjustment. The hydrolytic activity of Candida rugosa lipase was analyzed using 4-nitrophenol acetate (pNPA) and 4-nitrophenol palmitate (pNPP) as substrates. The addition of ionic liquids had no significant effect on the hydrolytic activity of lipase in a water solvent, and it had a greater effect in methanol. The addition of [C6Py] Br ionic liquid as a methanol cosolvent (methanol: ionic liquid, 10:5) could increase the hydrolytic activity of lipase. The use of ionic liquid as a cosolvent could increase the hydrolytic activity of lipase by about 15.61% while using pNPP as a substrate in the methanol system. A molecular dynamics study for the interaction between lipase and ionic liquids supported the experimental results. The ionic liquid using bromide as an anion provided more stability on lipase conformation. It tends to form the short-range interaction between the lipase and bromide anion.

Synthesis, Characterization, Biological Evaluation, and In Silico Studies of Imidazolium-, Pyridinium-, and Ammonium-Based Ionic Liquids Containing n-Butyl Side Chains

Ionic liquids (ILs) have emerged as active pharmaceutical ingredients because of their excellent antibacterial and biological activities. Herein, we used the green-chemistry-synthesis procedure, also known as the metathesis method, to develop three series of ionic liquids using 1-methyl-3-butyl imidazolium, butyl pyridinium, and diethyldibutylammonium as cations, and bromide (Br⁻), methanesulfonate (CH₃SO₃⁻), bis(trifluoromethanesulfonyl)imide (NTf₂⁻), dichloroacetate (CHCl₂CO₂⁻), tetrafluoroborate (BF₄⁻), and hydrogen sulfate (HSO₄⁻) as anions. Spectroscopic methods were used to validate the structures of the lab-synthesized ILs. We performed an agar well diffusion assay by using pathogenic bacteria that cause various infections (Escherichia coli; Enterobacter aerogenes; Klebsiella pneumoniae; Proteus vulgaris; Pseudomonas aeruginosa; Streptococcus pneumoniae; Streptococcus pyogenes) to scrutinize the in vitro antibacterial activity of the ILs. It was established that the nature and unique combination of the cations and anions were responsible for the antibacterial activity of the ILs. Among the tested ionic liquids, the imidazolium cation and NTf₂⁻ and HSO₄⁻ anions exhibited the highest antibacterial activity. The antibacterial potential was further investigated by in silico studies, and it was observed that bis(trifluoromethanesulfonyl)imide (NTf₂⁻) containing imidazolium and pyridinium ionic liquids showed the maximum inhibition against the targeted bacterial strains and could be utilized in antibiotics. These antibacterial activities float the ILs as a promising alternative to the existing antibiotics and antiseptics.

A Comparison of the Photophysical, Electrochemical and Cytotoxic Properties of meso-(2-, 3- and 4-Pyridyl)-BODIPYs and Their Derivatives

Boron dipyrromethene (BODIPY) dyes bearing a pyridyl moiety have been used as metal ion sensors, pH sensors, fluorescence probes, and as sensitizers for phototherapy. A comparative study of the properties of the three structural isomers of meso-pyridyl-BODIPYs, their 2,6-dichloro derivatives, and their corresponding methylated cationic pyridinium-BODIPYs was conducted using spectroscopic and electrochemical methods, X-ray analyses, and TD-DFT calculations. Among the neutral derivatives, the 3Py and 4Py isomers showed the highest relative fluorescence quantum yields in organic solvents, which were further enhanced 2-4-fold via the introduction of two chlorines at the 2,6-positions. Among the cationic derivatives, the 2catPy showed the highest relative fluorescence quantum yield in organic solvents, which was further enhanced by the use of a bulky counter anion (PF₆^-). In water, the quantum yields were greatly reduced for all three isomers but were shown to be enhanced upon introduction of 2,6-dichloro groups. Our results indicate that 2,6-dichloro-meso-(2- and 3-pyridinium)-BODIPYs are the most promising for sensing applications. Furthermore, all pyridinium BODIPYs are highly water-soluble and display low cytotoxicity towards human HEp2 cells.

Solvent-Free Strategy for Direct Access to Versatile Quaternary Ammonium Salts with Complete Atom Economy

A solvent-free method for the synthesis of quaternary ammonium salts (QAS) by iodoquaternization of alkenes with N-heteroarenes was reported. Its advantages lie in energy-saving and clean production by using iodine as the oxidant and manual grinding the starting materials, together with the complete atom economy and low process mass intensity (PMI) value. Demonstrated by 50 examples, the generated QAS was proved to be able to produce valuable chemicals, such as biological protease inhibitors, anti-cancer agents, and organic fluorescent materials.

Improved Effect of Metal Coordination on Molecular Oxygen Activation for Selective Aerobic Photooxidation

A pyridinium-based complex with environment-friendly and earth-abundant Zn^II ion was synthesized and explored as a green catalyst applied in activating molecular oxygen for the simple and efficient photooxidation of alcohols into aldehydes under additive-free and mild conditions. The metal coordination was conducive to promoting the electron transfer efficiency and introducing the heavy-atom effect for the increased generation of ⋅O₂ ^- and ¹ O₂ . Accordingly, improved photocatalytic performance of this complex compared to its precursor, no matter activity or selectivity, was obtained, facilitating the transformation of alcohols into aldehydes in a sustainable way.

IndiFluors: A New Full-Visible Color-Tunable Donor-Acceptor-Donor (D1-A-D2) Fluorophore Family for Ratiometric pH Imaging during Mitophagy

Full-visible color-tunable new fluorophores are essential in bioimaging research. However, it is significantly challenging to design fluorophores with the desired optical and biological properties owing to their structural complexity. We report a unified design of an interesting molecular framework, IndiFluors, based on the principle of a donor-acceptor-donor (D1-A-D2) system. The IndiFluors comprise pyrylium, pyridinium, and pyridine derivatives, which exhibit full-visible emission color (375-700 nm) by varying donor and acceptor strengths of the core scaffolds. With a minimal change of structure, the bright fluorophores (Φ: 0.96) can be tuned to become nonfluorescent (Φ: 0.01), which is well explained by time-dependent density functional theory (TD-DFT/PCM) by oscillator strengths in the S1 state. Within IndiFluors, pyridinium offers several advantages, including a large Stokes shift (∼154 nm) and excellent stability, compared to pentacyclic pyrylium fluorophores. Especially, the designed probe, PM-Mito-OH, demonstrated specific colocalization in mitochondria and a monitored ratiometric pH change during mitochondrial damage, autolysosomes, and the mitophagy process. Hence, IndiFluors and the derived probe show great potential for cellular pH imaging in live cells while exhibiting minimal cytotoxicity.

On the Supra-LUMO Interaction: Case Study of a Sudden Change of Electronic Structure as a Functional Emergence

The synergistic functioning of redox-active components that emerges from prototypical 2,2'-di(N-methylpyrid-4-ylium)-1,1'-biphenyl is described. Interestingly, even if a trans conformation of the native assembly is expected, due to electrostatic repulsion between cationic pyridinium units, we demonstrate that cis conformation is equally energy-stabilized on account of a peculiar LUMO (SupLUMO) that develops through space, encompassing the two pyridiniums in a single, made-in-one-piece, electronic entity (superelectrophoric behavior). This SupLUMO emergence, with the cis species as superelectrophore embodiment, originates in a sudden change of electronic structure. This finding is substantiated by insights from solid state (single-crystal X-ray diffraction) and solution (NOE NMR and UV-vis-NIR spectroelectrochemistry) studies, combined with electronic structure computations. Electrochemistry shows that electron transfers are so strongly correlated that two-electron reduction manifests itself as a single-step process with a large potential inversion consistent with inner creation of a carbon-carbon bond (digital simulation). Besides, absence of reductive formation of dimers is a further indication of a preferential intramolecular reactivity determined by the SupLUMO interaction (cis isomer pre-organization). The redox-gated covalent bond, serving as electron reservoir, was studied via atropisomerism of the reduction product (VT NMR study). The overall picture derived from this in-depth study of 2,2'-di(N-methylpyrid-4-ylium)-1,1'-biphenyl proves that trans and cis species are worth considered as intrinsically sharply different, that is, as doubly-electrophoric and singly-superelectrophoric switchable assemblies, beyond conformational isomerism. Most importantly, the through-space-mediated SupLUMO may come in complement of other weak interactions encountered in Supramolecular Chemistry as a tool for the design of electroactive architectures.

Radical Stabilization of a Tripyridinium-Triazine Molecule Enables Reversible Storage of Multiple Electrons

A novel organic molecule, 2,4,6-tris[1-(trimethylamonium)propyl-4-pyridiniumyl]-1,3,5-triazine hexachloride, was developed as a reversible six-electron storage electrolyte for use in an aqueous redox flow battery (ARFB). Physicochemical characterization reveals that the molecule evolves from a radical to a biradical and finally to a quinoid structure upon accepting four electrons. Both the diffusion coefficient and the rate constant were sufficiently high to run a flow battery with low concentration and kinetics polarization losses. In a demonstration unit, the assembled flow battery affords a high specific capacity of 33.0 Ah L^-1 and a peak power density of 273 mW cm^-2 . This work highlights the rational design of electroactive organics that can manipulate multi-electron transfer in a reversible way, which will pave the way to development of energy-dense, manageable and low-cost ARFBs.

Stand-Alone CdS Nanocrystals for Photocatalytic CO2 Reduction with High Efficiency and Selectivity

The development of a cost-effective photocatalyst is highly anticipated to achieve efficient photocatalytic CO₂ reduction with superior selectivity, which is still facing the lack of valid settlements. Herein, 4-mercaptopyridine (PD) as the building block of a capping ligand is tightly decorated on the surface of CdS nanocrystals (CdS-PD) using a facile ligand-exchange strategy, to exploit a cost-effective photocatalyst for photocatalytic CO₂ reduction without any cocatalysts. The conjugated structure of PD can facilitate the delocalization of photogenerated electrons in CdS nanocrystals, bringing forth an improved charge separation efficiency. More importantly, N-protonated PD can enable the easy formation of a six-membered ring intermediate with CO₂ assisted by water, which can serve as the efficient active site to achieve photocatalytic CO₂ reduction. In the absence of a cocatalyst, stand-alone CdS-PD nanocrystals exhibit an excellent CO yield of 20.35 mmol g^-1 h^-1 concomitant with a high selectivity of 95.3% for the CO₂-to-CO conversion under visible light, which are remarkably superior than those of CdS nanocrystals possessing traditional alkyl-chain and other conjugated capping ligands.

Theoretical Analysis of the Heterocyclic [4+2] Cycloaddition Between Pyridinium Ion and Enol Ether

Dearomative heterocyclic [4+2] cycloaddition between the N-(2,4-dinitrophenyl)pyridinium ion of nicotinamide and an enol ether was analyzed by Density Functional Theory (DFT) calculations. The calculation revealed that the reaction undergoes stepwise bond formation rather than occurring in a concerted manner. The experimental products were found to be both kinetically and thermodynamically favored. The calculated transition states and intermediate suggested that the high diastereoselectivity is derived from the electrostatic interaction between the 2-nitro group of the pyridinium ion and the hydrogen of the enol ether.

Antibacterial and Antibiofilm Formation Activities of Pyridinium-Based Cationic Pillar[5]arene Against Pseudomonas aeruginosa

An omnipresent pathogenic bacterium, Pseudomonas aeruginosa (P. aeruginosa PAO1), is easy to contaminate environmental water or foods, causing daily food spoilage and infections. The biofilm-forming ability and bacterial resistance of P. aeruginosa PAO1 make it difficult to be eradicated by traditional bacteriostatic agents. In this work, we designed and synthesized a pyridinium-based pillar[5]arene (PP5), while trimethylammonium-based pillar[5]arene (TP5) was used as a control compound. After clear characterization, the antibacterial and antibiofilm activities as well as the microbial resistance of TP5 and PP5 against P. aeruginosa PAO1 were extensively examined. It was revealed that PP5 exhibited good inhibition activity with the minimum inhibitory concentration (MIC) of 0.051 mmol/L, while no significant antibacterial and biofilm formation activity for TP5 against P. aeruginosa PAO1 was observed. More importantly, PP5 had negligible antimicrobial resistance even after 18th passages. A transmission electron microscope (TEM) showed that PP5 could physically disrupt the cell membranes, causing the leakage of internal constituents, which is possibly ascribed to the synergistic penetrability and π-π interactions of strain, thus greatly reduced the development of bacterial resistance. Overall, the presented studies indicated that pyridinium moieties could facilitate the cationic pillar[5]arene to generate surprising antibacterial and antibiofilm formation ability against P. aeruginosa PAO1.

2- and 2,7-Substituted para-N-Methylpyridinium Pyrenes: Syntheses, Molecular and Electronic Structures, Photophysical, Electrochemical, and Spectroelectrochemical Properties and Binding to Double-Stranded (ds) DNA

Two N-methylpyridinium compounds and analogous N-protonated salts of 2- and 2,7-substituted 4-pyridyl-pyrene compounds were synthesised and their crystal structures, photophysical properties both in solution and in the solid state, electrochemical and spectroelectrochemical properties were studied. Upon methylation or protonation, the emission maxima are significantly bathochromically shifted compared to the neutral compounds, although the absorption maxima remain almost unchanged. As a result, the cationic compounds show very large apparent Stokes shifts of up to 7200 cm-1 . The N-methylpyridinium compounds have a single reduction at ca. -1.5 V vs. Fc/Fc+ in MeCN. While the reduction process was reversible for the 2,7-disubstituted compound, it was irreversible for the mono-substituted one. Experimental findings are complemented by DFT and TD-DFT calculations. Furthermore, the N-methylpyridinium compounds show strong interactions with calf thymus (ct)-DNA, presumably by intercalation, which paves the way for further applications of these multi-functional compounds as potential DNA-bioactive agents.

Phenylene-Bridged Bispyridinium with High Capacity and Stability for Aqueous Flow Batteries

A rotating phenyl ring is introduced between the two pyridinium rings, namely, 1,1'-bis[3-(trimethylamonium)propyl]-4,4'-(1,4-phenylene)bispyridinium tetrachloride ((APBPy)Cl₄ ), to form a switchable conjugation. In this design, the conjugation is switched "off" in the oxidized state and the two pyridinium rings behave independently during the redox process, yielding a concomitant transfer of two electrons at the same potential and, thus, simplifying the battery management. The conjugation is switched "on" in the reduced state and the charge can be effectively delocalized, lowering the Lewis basicity and improving its chemical stability. By pairing 0.50 m (APBPy)Cl₄ with a 2,2,6,6-tetramethylpiperidin-1-yl oxyl derivative as the positive electrolyte, a flow battery delivers a high standard cell voltage of 1.730 V and a high specific capacity of 20.0 Ah L^-1 . The battery also shows an exceptionally high energy efficiency of 80.8% and a superior cycling stability at 80 mA cm^-2 . This strategy proves itself a great success in engineering viologen as a two-electron storage mediator with high capacity and stability.

Structure-Activity Relationship Modeling and Experimental Validation of the Imidazolium and Pyridinium Based Ionic Liquids as Potential Antibacterials of MDR Acinetobacter Baumannii and Staphylococcus Aureus

Online Chemical Modeling Environment (OCHEM) was used for QSAR analysis of a set of ionic liquids (ILs) tested against multi-drug resistant (MDR) clinical isolate Acinetobacter baumannii and Staphylococcus aureus strains. The predictive accuracy of regression models has coefficient of determination q² = 0.66 - 0.79 with cross-validation and independent test sets. The models were used to screen a virtual chemical library of ILs, which was designed with targeted activity against MDR Acinetobacter baumannii and Staphylococcus aureus strains. Seven most promising ILs were selected, synthesized, and tested. Three ILs showed high activity against both these MDR clinical isolates.

Functional Single-Walled Carbon Nanotubes for Anion Sensing

We report an anion-sensing platform wherein conductance changes are triggered by chemical interactions between selectors and anions. The selector design incorporates both a cationic moiety (i.e., pyridinium) and a thiourea-based dual-hydrogen-bond donor. Anion binding by a model selector (2) was studied using ¹H NMR and UV-vis titrations, which reveal a binding strength toward acetate ions (AcO^-) followed by Cl^- > Br^- > NO₃^-. These studies reveal that selector 2 is deprotonated upon addition of AcO^-, whereas it undergoes hydrogen bonding associated with Cl^-, Br^-, and NO₃^-. The cationic pyridinium moiety improves anion binding affinity by lowering the pK_a value of selector 2 and enhancing the hydrogen-bond donor capability as confirmed by spectroscopic titrations and DFT calculations. The selector is covalently attached to poly(4-vinylpyridine) (P4VP), which wraps single-walled carbon nanotubes (SWCNTs) (i.e., P4VP-2-SWCNT) to transduce an electrical signal. As a result, continuous anion sensing was achieved with high sensitivity represented by a normalized resistance change of 101.9 ± 10.3% toward 16.7 mM AcO^-, whereas negligible sensitivity was observed toward Cl^-, Br^-, and NO₃^-. The sensitivity transition was attributed to the internal charge transfer of 2 by deprotonation of the thiourea proton upon addition of AcO^-.

Pyridinium-Functionalized Ionic Metal-Organic Frameworks Designed as Bifunctional Catalysts for CO2 Fixation into Cyclic Carbonates

Ionic metal-organic frameworks (MOFs) offer a new platform to design and construct complete heterogeneous bifunctional catalytic systems for the chemical fixation of CO₂ with epoxides. Herein, we developed a series of bifunctional pyridinium ionic MOF heterogeneous catalysts (66Pym-RXs and 67BPym-MeI) by the postsynthetic N-alkylation of noncoordinated pyridine sites in porous MOFs. The synergetic catalytic effect of acidic sites with nucleophilic anions in the ionic MOF significantly enhanced the catalytic activity toward the cycloaddition of CO₂ with epoxides to produce cyclic carbonates under cocatalyst-free and solvent-free mild conditions. The catalytic activity of ionic MOFs is easily tuned by the introduction of different alkyl groups into pyridinium cations and halide ions. The 66Pym-iPrI catalyst displayed the highest catalytic performance in terms of the turnover number value for the synthesis of cyclic carbonates. The proposed alternative method provides the means of developing functional N-heterocyclic groups for the new design of bifunctional ionic MOFs as potential heterogeneous catalysts for CO₂ fixation applications.

A new photoluminescent coordination polymer constructed with an N-donor ligand having extended coordination capabilities derived from quinoline and pyridine

Employment of the organic 2-(pyridin-4-yl)quinoline-4-carboxylic acid ligand with extended coordination capabilities leads to the formation of the one-dimensional copper(II) coordination polymer catena-poly[[diaquacopper(II)]-bis[μ-2-(pyridin-4-yl)quinoline-4-carboxylato]-κ²N²:O;κ²O:N], {[Cu(C₁₅H₉N₂O₂)₂(H₂O)₂]·2H₂O}_n, under hydrothermal conditions. The ligand, isolated as its hydrochloride salt, namely, 4-(4-carboxyquinolin-2-yl)pyridinium chloride monohydrate, C₁₅H₁₁N₂O₂⁺·Cl^-·H₂O, reveals a pseudosymmetry element (translation a/2) in its crystal structure. The additional pyridyl N atom, in comparison with the previously reported analogues with an arene ring instead of the pyridyl ring in the present ligand molecule, promotes the formation of a one-dimensional coordination polymer, rather than discrete molecules. This polymer shows photoluminescent properties with bathochromic/hypsochromic shifts of the ligand absorption bands, leading to a single band at 479 nm. The Cu^II ions are involved in weak antiferromagnetic interactions within dimeric units, as evidenced by SQUID magnetometry.

A Simple and Robust Functionalization of Graphene for Advanced Energy Devices

Efficient and selective methods for graphene functionalization are needed because they allow tuning of the graphene surface and electronic properties. To date, graphene has been functionalized using ionic bonds, π-π interactions, and covalent bonds. Graphene derivatives based on these methods have been used in various applications, but a new functionalization strategy that improves the properties of graphene is still needed. Herein, a new concept for graphene functionalization using halogenated graphene has been developed, in which brominated graphene is successfully functionalized by heteroatom-containing molecules to form onium bonds, such as pyridinium or ammonium. The counterion bromide is replaced with other anions, such as sulfate, by treating with sulfuric acid while retaining the molecules, which demonstrates the durable properties of onium bonding. To emphasize the advantages of this strategy for graphene functionalization, the performance for energy-related applications, such as biofuel cells, supercapacitors, and Li-ion batteries, is evaluated after introducing redox-active moieties onto graphene through onium bonding. This new graphene functionalization concept will provide a new approach to the design of tailor-made materials with targeted functions.

Tetra-aqua-[3-oxo-1,3-bis-(pyridinium-2-yl)propan-1-olato]nickel(II) tribromide dihydrate

The crystal structure of the title compound, [Ni(C13H11N2O2)(H2O)4]Br3·2H2O, contains an octa-hedral NiII atom coordinated to the enol form of 1,3-di-pyridyl-propane-1,3-dione (dppo) and four water mol-ecules. Both pyridyl rings on the ligand are protonated, forming pyridinium rings and creating an overall ligand charge of +1. The protonated nitro-gen-containing rings are involved in hydrogen-bonding inter-actions with neighoring bromide anions. There are many additional hydrogen-bonding inter-actions involving coordinated water mol-ecules on the NiII atom, bromide anions and hydration water mol-ecules.

Crystal structure of pyridinium tetra-iso-thio-cyanato-dipyridine-chromium(III) pyridine monosolvate

In the crystal structure of the title compound, (C5H6N)[Cr(NCS)4(C5H5N)2]·C5H5N, the CrIII ions are octa-hedrally coordinated by four N-bonding thio-cyanate anions and two pyridine ligands into discrete negatively charged complexes, with the CrIII ion, as well as the two pyridine ligands, located on crystallographic mirror planes. The mean planes of the two pyridine ligands are rotated with respect to each other by 90°. Charge balance is achieved by one protonated pyridine mol-ecule that is hydrogen bonded to one additional pyridine solvent mol-ecule, with both located on crystallographic mirror planes and again rotated by exactly 90°. The pyridinium H atom was refined as disordered between both pyridine N atoms in a 70:30 ratio, leading to a linear N-H⋯N hydrogen bond. In the crystal, discrete complexes are linked by weak C-H⋯S hydrogen bonds into chains that are connected by additional C-H⋯S hydrogen bonding via the pyridinium cations and solvent mol-ecules into layers and finally into a three-dimensional network.

Anion Influence on the Packing of 1,3-Bis(4-Ethynyl-3-Iodopyridinium)-Benzene Halogen Bond Receptors

Rigid and directional arylethynyl scaffolds have been widely successful across diverse areas of chemistry. Utilizing this platform, we present three new structures of a dicationic 1,3-bis(4-ethynyl-3-iodopyridinium)-benzene halogen bonding receptor with tetrafluoroborate, nitrate, and hydrogen sulfate. Structural analysis focuses on receptor conformation, anion shape, solvation, and long range packing of these systems. Coupled with our previously reported structures, we conclude that anions can be classified as building units within this family of halogen bonding receptors. Two kinds of antiparallel dimers are observed for these dicationic receptors. An off-centered species is most frequent, present among geometrically diverse anions, and assorted receptor conformations. In contrast, the centered antiparallel dimers are observed with receptors adopting a bidentate conformation in the solid-state. While anions support the solid-state formation of dimers, the molecular geometry and characteristics (planarity, rigidity, and directionality) of arylethynyl systems increases the likelihood of dimer formation by limiting efficient packing arrangements. The significantly larger cation may have considerable influence on the solid-state packing, as similar cationic arylethynyl systems also display these dimers, suggesting.

Effect of pH on the construction of CdII coordination polymers involving the 1,1'-[1,4-phenylenebis(methylene)]bis(3,5-dicarboxylatopyridinium) ligand

Changing the pH value of a reaction system can result in polymers with very different compositions and architectures. Two new coordination polymers based on 1,1'-[1,4-phenylenebis(methylene)]bis(3,5-dicarboxylatopyridinium) (L^2-), namely catena-poly[[[tetraaquacadmium(II)]-μ₂-1,1'-[1,4-phenylenebis(methylene)]bis(3,5-dicarboxylatopyridinium)] 1.66-hydrate], {[Cd(C₂₂H₁₄N₂O₈)(H₂O)₄]·1.66H₂O}_n, (I), and poly[{μ₆-1,1'-[1,4-phenylenebis(methylene)]bis(3,5-dicarboxylatopyridinium)}cadmium(II)], [Cd(C₂₂H₁₄N₂O₈)]_n, (II), have been prepared in the presence of NaOH or HNO₃ and structurally characterized by single-crystal X-ray diffraction. In polymer (I), each Cd^II ion is coordinated by two halves of independent L^2- ligands, forming a one-dimensional chain structure. In the crystal, these chains are further connected through O-H...O hydrogen bonds, leading to a three-dimensional hydrogen-bonded network. In polymer (II), each hexadentate L^2- ligand coordinates to six Cd^II ions, resulting in a three-dimensional network structure, in which all of the Cd^II ions and L^2- ligands are equivalent, respectively. The IR spectra, thermogravimetric analyses and fluorescence properties of both reported compounds were investigated.

Supramolecular interaction-induced assemblies of polyanions and 2-aminopyridinium in two polyoxometalate-based hybrids

Organic-inorganic hybrids consisting of organic cations and polyanions are promising functional materials due to their various compositions and structures. An important aspect of these materials is the interactions between the organic and inorganic components, which not only produce the final structures, but also influence the properties. Here, we investigated the interactions between organic cations and polyanions using protonated 2-aminopyridinium (Hap) as the cation, and successfully obtained two polyoxometalate-based hybrids, namely (C₅H₇N₂)₄[Mo₈O₂₆], (I), and (C₅H₇N₂)₂[NiMo₆O₁₆(OH)₂{CH₃C(CH₂O)₃}₂]·4H₂O, (II). In the crystal structure of (I), every Hap cation links with two polyanions by donating one or two N-H...O hydrogen bonds, and every polyanion is surrounded by eight Hap cations via terminal or bridging O atoms. Conversely, in compound (II), every Hap cation only links with one polyanion decorated by a triol ligand; this organic-inorganic component further assembles via uncoordinated water molecules. In the extended structures, Hap plays a key role, not only providing a counter charge, but also acting as `glue' linking polyanions in the role of hydrogen-bond donors. In both compounds, as the nodes of the supramolecular network, the polyanions exhibit an ordered two-dimensional arrangement due to strong hydrogen bonds and electrostatic interactions between the organic and inorganic parts. The electrochemistry of compound (I) shows that redox sourcing from polyanions is a surface-controlled process. Conversely, the magnetic behaviour of compound (II) indicates dominant antiferromagnetic properties.

New Pyridinium Type Poly(Ionic Liquids) as Membranes for CO₂ Separation

New pyridinium based PILs have been prepared by modification of their precursors based on high molecular weight aromatic polyethers bearing main chain pyridine units. The proposed methodology involves the conversion of the precursors to their ionic analogues via N-methylation reaction, followed by anion exchange methathesis reaction to result in PILs with the desirable anions (tetrafluoroborate and bis(trifluoromethylsulfonyl)imide). These PILs show excellent thermal stability, excellent mechanical properties, and most importantly can form very thin, free standing films with minimum thickness of 3 μm. As expected, the PIL containing the TFSI⁻ anion showed improved CO₂ and CH₄ permeabilities compared to its analogue containing the BF₄⁻. PIL-IL composites membranes have also been prepared using the same PIL and different percentages of pyridinium based IL where it was shown that the membrane with the highest IL weight percentage (45 wt %) showed the highest CO₂ permeability (11.8 Barrer) and a high CO₂/CH₄ ideal selectivity of 35 at room temperature.

A rare positively charged nicotinic acid di-sulfide: 2,2'-di-thio-dinicotinic acid hydro-chloride monohydrate

The title compound {systematic name: 3-carb-oxy-2-[2-(3-carb-oxy-pyridin-2-yl)disulfan-1-yl)]pyridin-1-ium chloride monohydrate}, C12H9N2O4S2+·Cl-·H2O, crystallizes in the triclinic space group P . A pair of 2-mercaptonicotinic acid moieties is connected by a 2,2'-di-sulfide bond with a dihedral angle of 78.79 (3)°. One of the N atom is protonated, as are both carboxyl-ate groups, resulting in an overall +1 charge on the dimer. The structure comprises a zigzagging layer of the dimerized di-thio-dinicotinic acid rings, with charge-balancing chloride ions and water mol-ecules between the layers. Hydrogen bonding between the chloride and water sites with the dimer appears to hold the structure together. Nearest neighbor nicotinic acid rings are offset when viewed down the a axis, suggesting no added stability from ring stacking. The asymmetric unit corresponds to the empirical formula of the compound, and it packs with two formula units per unit cell.

Sensitive and Specific Guest Recognition through Pyridinium-Modification in Spindle-Like Coordination Containers

An elaborately designed pyridinium-functionalized octanuclear zinc(II) coordination container 1-Zn was prepared through the self-assembly of Zn²⁺ , p-tert-butylsulfonylcalix[4]arene, and pyridinium-functionalized angular flexible dicarboxylate linker (H₂ BrL1). The structure was determined by a single-crystal X-ray diffractometer. 1-Zn displays highly sensitive and specific recognition to 2-picolylamine as revealed by drastic blueshifts of the absorption and emission spectra, ascribed to the decrease of intramolecular charge transfer (ICT) character of the container and the occurrence of intermolecular charge transfer between the host and guest molecules. The intramolecular charge transfer plays a key role in the modulation of the electronic properties and is tunable through endo-encapsulation of specific guest molecules.

(E)-2,6-Di-bromo-4-{2-[1-(1H,1H,2H,2H-perfluoro-oct-yl)pyridinium-4-yl]ethen-yl}phenolate methanol disolvate, a fluoro-ponytailed solvatochromic dye

The title compound, C₂₁H₁₂Br₂F₁₃NO·2CH₃OH, was obtained by condensation of 4-methyl-1-(1H,1H,2H,2H-perfluoro-oct-yl)pyridinium iodide and 3,5-di-bromo-4-hy-droxy-benzaldehyde, followed by deprotonation. It crystallizes as a methanol disolvate and exhibits short O-H⋯O hydrogen bonds and a disordered perfluoro-alkyl chain [occupancy ratio 0.538 (7):0.462 (7)]. Significant π-π stacking inter-actions are observed between the benzene and pyridine rings of neighbouring mol-ecules along the b-axis direction.

Interfacially Engineered Pyridinium Pseudogemini Surfactants as Versatile and Efficient Supramolecular Delivery Systems for DNA, siRNA, and mRNA

This article presents the synthesis, self-assembly, and biological activity as transfection agents for pDNA, siRNA, and mRNA of novel pyridinium pseudogemini surfactants, interfacially engineered from the most efficient gemini surfactants and lipids generated in our amphiphile research program. Formulation of novel amphiphiles in water revealed supramolecular properties very similar to those of gemini surfactants, despite their lipidlike charge/mass ratio. This dual character was found also to enhance endosomal escape and significantly increase the transfection efficiency. We were also successful in identifying the parameters governing the efficient delivery of pDNA, siRNA, and mRNA, drawing valuable structure-activity and structure-property relationships for each nucleic acid type, and establishing DNA/siRNA/mRNA comparisons. Several supramolecular complexes identified in this study proved to be extremely efficient nucleic acid delivery systems, displaying excellent serum stability and tissue penetration in three-dimensional organoids.

N-Benzylnicotinamide and N-benzyl-1,4-dihydronicotinamide: useful models for NAD+ and NADH

3-Aminocarbonyl-1-benzylpyridinium bromide (N-benzylnicotinamide, BNA), C₁₃H₁₃N₂O⁺·Br^-, (I), and 1-benzyl-1,4-dihydropyridine-3-carboxamide (N-benzyl-1,4-dihydronicotinamide, rBNA), C₁₃H₁₄N₂O, (II), are valuable model compounds used to study the enzymatic cofactors NAD(P)⁺ and NAD(P)H. BNA was crystallized successfully and its structure determined for the first time, while a low-temperature high-resolution structure of rBNA was obtained. Together, these structures provide the most detailed view of the reactive portions of NAD(P)⁺ and NAD(P)H. The amide group in BNA is rotated 8.4 (4)° out of the plane of the pyridine ring, while the two rings display a dihedral angle of 70.48 (17)°. In the rBNA structure, the dihydropyridine ring is essentially planar, indicating significant delocalization of the formal double bonds, and the amide group is coplanar with the ring [dihedral angle = 4.35 (9)°]. This rBNA conformation may lower the transition-state energy of an ene reaction between a substrate double bond and the dihydropyridine ring. The transition state would involve one atom of the double bond binding to the carbon ortho to both the ring N atom and the amide substituent of the dihydropyridine ring, while the other end of the double bond accepts an H atom from the methylene group para to the N atom.

Dibenzothiophene-S,S-dioxide and Bispyridinium-Based Cationic Polyfluorene Derivative as an Efficient Cathode Modifier for Polymer Solar Cells

A novel n-type conjugated polymer containing dibenzothiophene-S,S-dioxide (FSO), bispyridinium, and fluorene scaffolds in the backbone (PFSOPyCl) was synthesized and used in the cathode interfacial layers (CILs) of conventional polymer solar cells (PSCs). The high electron affinities and large planar structures of the FSO and bispyridinium units endowed this polymer with good energy level alignments with [6,6]-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM) and metal cathode, and excellent electron transport and extraction properties. Polymer solar cells (PSCs) based on the poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT):PC₇₁BM system with PFSOPyCl CIL exhibited simultaneous enhancement in open-circuit voltage (V_oc), short-circuit current density (J_sc), and fill factor (FF), while the power conversion efficiency increased from 5.47% to 6.79%, relative to the bare Al device. Besides, PSC based on the poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b']dithio-phene-2,6-diyl-alt-ethylhexyl-3-fluorothithieno [3,4-b]thiophene-2-carboxylate-4,6-diyl] (PTB7):PC₇₁BM system achieved a PCE of 8.43% when using PFSOPyCl as CIL. Hence, PFSOPyCl is a promising candidate CIL for PSCs.

Crystal structure of 4,4'-[(1,3,5,7-tetra-oxo-1,3,3a,4,4a,5,7,7a,8,8a-deca-hydro-4,8-etheno-pyrrolo-[3,4-f]iso-indole-2,6-di-yl)bis-(methyl-ene)]bis-(pyridin-1-ium) dinitrate

In the title salt, C24H22N4O4 (2+)·2NO3 (-), the cation is U-shaped with the two iso-indole dione rings inclined to one another by 60.41 (13)°, while the two outer pyridine rings are inclined to one another by 2.77 (12)°. The dihedral angles between the pyridine ring and the adjacent iso-indole dione ring are 71.82 (12) and 86.44 (13)°. In the crystal, each nitrate anion is linked to a protonated pyridine ring by N-H⋯O hydrogen bonds. These units are linked by a series of C-H⋯O hydrogen bonds, forming a three-dimensional structure.

Crystal structure of 2-cyano-1-methyl-pyridinium perchlorate

The asymmetric unit of the title salt, C7H7N2 (+)·ClO4 (-), comprises two independent formula units. The solid-state structure comprises corrugated layers of cations and of anions, approximately parallel to (010). The supra-molecular layers are stabilized and connected by C-H⋯O hydrogen bonding to consolidate a three-dimensional architecture. A close pyridin-ium-perchlorate N⋯O contact [2.867 (5) Å] is noted. The crystal was refined as an inversion twin.

X-ray Photoelectron Spectroscopy of Pyridinium-Based Ionic Liquids: Comparison to Imidazolium- and Pyrrolidinium-Based Analogues

We investigate eight 1-alkylpyridinium-based ionic liquids of the form [Cn Py][A] by using X-ray photoelectron spectroscopy (XPS). The electronic environment of each element of the ionic liquids is analyzed. In particular, a reliable fitting model is developed for the C 1s region that applies to each of the ionic liquids. This model allows the accurate charge correction of binding energies and the determination of reliable and reproducible binding energies for each ionic liquid. Shake-up/off phenomena are determinedfor both C 1s and N 1s spectra. The electronic interaction between cations and anions is investigated for both simple ionic liquids and an example of an ionic-liquid mixture; the effect of the anion on the electronic environment of the cation is also explored. Throughout the study, a detailed comparison is made between [C8 Py][A] and analogues including 1-octyl-1-methylpyrrolidinium- ([C8 C1 Pyrr][A]), and 1-octyl-3-methylimidazolium- ([C8 C1 Im][A]) based samples, where X is common to all ionic liquids.

Crystal structure and thermal behaviour of pyridinium styphnate

In the crystal structure of the title mol-ecular salt, C5H6N(+)·C6H2N3O8 (-) (systematic name: pyridinium 3-hy-droxy-2,4,6-tri-nitro-phenolate), the pyridin-ium cation and the 3-hy-droxy-2,4,6-tri-nitro-phenolate anion are linked through bifurcated N-H⋯(O,O) hydrogen bonds, forming an R 1 (2)(6) ring motif. The nitro group para with respect to phenolate ion forms an intra-molecular hydrogen bond with the adjacent phenolic -OH group, which results in an S(6) ring motif. The nitro group flanked by the phenolate ion and the phenolic -OH group deviates noticeably from the benzene ring, subtending a dihedral angle of 89.2 (4)°. The other two nitro groups deviate only slightly from the plane of the benzene ring, making dihedral angles of 2.8 (4) and 3.4 (3)°. In the crystal, the 3-hy-droxy-2,4,6-tri-nitro-phenolate anions are linked through O-H⋯O hydrogen bonds, forming chains along [100]. These anionic chains, to which the cations are attached, are linked via C-H⋯O hydrogen bonds, forming a three-dimensional structure. Impact friction sensitivity tests and TGA/DTA studies on the title mol-ecular salt imply that it is an insensitive high-energy-density material.

Crystal structure of (E)-3-{[2-(2,4-di-chloro-benzyl-idene)hydrazin-1-yl]carbon-yl}pyridinium chloride trihydrate

In the title hydrated salt, C₁₃H₁₀Cl₂N₃O⁺·Cl⁻·3H₂O, the organic cation exhibits a dihedral angle of 8.26 (14)° between the mean planes of the pyridinium and benzene rings, and dihedral angles of 8.70 (15) and 15.93 (5)° between the mean planes of the hydrazide group and the benzene and pyridinium rings, respectively. In the crystal, N—H⋯O, N—H⋯Cl, C—H⋯O, C—H⋯Cl, O—H⋯O, O—H⋯N and O—H⋯Cl hydrogen bonds link the complex cations, chloride anions and solvent water mol­ecules into a three-dimensional network.

Crystal structure of 1-[(1-methyl-5-nitro-1H-imidazol-2-yl)meth-yl]pyridinium iodide

In the title salt, C10H11N4O2 (+)·I(-), the asymmetric unit consists of a pyridinium cation bearning a (1-methyl-5-nitro-1H-imidazol-2-yl)methyl group at the N position and an iodide anion. The imidazole ring is quasiplanar, with a maxiumum deviation of 0.0032 (16) Å, and forms a dihedral angle of 67.39 (6)° with the plane of the pyridinium ring. The crystal packing can be described as alternating zigzag layers of cations parallel to the (001) plane, which are sandwiched by the iodide ions. The structure features two types of hydrogen bonds (C-H⋯O and C-H⋯I), viz. cation-anion and cation-cation, which lead to the form ation of a three-dimensional network.

Crystal structure of 4-amino-1-(4-methyl-benz-yl)pyridinium bromide

The title mol­ecular salt, C₁₃H₁₅N₂⁺·Br⁻, crystallized with two independent ion pairs (A and B) in the asymmetric unit. In the cations, the planes of the pyridine and benzene rings are inclined to one another by 79.32 (8) and 82.30 (10)° in ion pairs A and B, respectively. In the crystal, the anions and cations are connected by N—H⋯Br hydrogen bonds, forming a centrosymmetric tetra­mer-like unit enclosing an R₈⁴(16) ring motif. These units are linked via C—H⋯Br hydrogen bonds, forming a three-dimensional network.

Crystal structure of bis-[1-(naphthalen-1-ylmeth-yl)pyridinium] bis-(2,2-di-cyano-ethene-1,1-di-thiol-ato-κ(2) S,S')nickelate(II)

In the ion-pair complex, bis­[1-(naphthalen-1-ylmeth­yl)pyridinium] bis­(2,2-di­cyano­ethene-1,1-di­thiol­ate-κ²S,S′)nickelate(II), C—H⋯N and C—H⋯Ni hydrogen bonds as well as π–π inter­actions between the ions result in the formation of a three-dimensional network.

A new ion-pair complex, (C₁₆H₁₄N)₂[Ni(C₄N₂S₂)₂] or (1-NaMePy)₂[Ni(imnt)₂], where 1-NaMePy is 1-(4-naphthyl­methyl­ene)pyridinium and imnt is 2,2-di­cyano­ethene-1,1-di­thiol­ate, was obtained by the direct reaction of NiCl₂, K₂imnt and (1-NaMePy)⁺Br⁻ in H₂O. The asymmetric unit contains a [1-NaMePy]⁺ cation and one half of an Ni(imnt)₂²⁻ anion. The Ni^II ion lies on an inversion centre and adopts a square-planar configuration with Ni—S bond lengths of 2.200 (1) and 2.216 (1) Å. In the [1-NaMePy]⁺ cation, the naphthyl ringsystem and the pyridinium ring make a dihedral angle of 90.0 (2)°. In the crystal, C—H⋯N and C—H⋯Ni hydrogen bonds, as well as π–π inter­actions between the chelate ring and the pyridinium ring [centroid–centroid distance = 3.675 (2) Å] link the ions into a three-dimensional network.

Modulation of pyridinium cationic lipid-DNA complex properties by pyridinium gemini surfactants and its impact on lipoplex transfection properties

The study presents the effects of blending a cationic gemini surfactant into cationic lipid bilayers and its impact on the plasmid DNA compaction and delivery process. Using nanoDSC, dynamic light scattering, zeta potential, and electrophoretic mobility measurements, together with transfection (2D- and 3D-) and viability assays, we identified the main physicochemical parameters of the lipid bilayers, liposomes, and lipoplexes that are affected by the gemini surfactant addition. We also correlated the cationic bilayer composition with the dynamics of the DNA compaction process and with transfection efficiency, cytotoxicity, and the internalization mechanism of the resultant nucleic acid complexes. We found that the blending of gemini surfactant into the cationic bilayers fluidized the supramolecular assemblies, reduced the amount of positive charge required to fully compact the plasmid DNA and, in certain cases, changed the internalization mechanism of the lipoplexes. The transfection efficiency of select ternary lipoplexes derived from cationic gemini surfactants and lipids was several times superior to the transfection efficiency of corresponding binary lipoplexes, also surpassing standard transfection systems. The overall impact of gemini surfactants into the formation and dynamic of cationic bilayers was found to depend heavily on the presence of colipids, their nature, and amount present in lipoplexes. The study confirmed the possibility of combining the specific properties of pyridinium gemini surfactants and cationic lipids synergistically to obtain efficient synthetic transfection systems with negligible cytotoxicity useful for therapeutic gene delivery.

Efficient organic dye-sensitized solar cells: molecular engineering of donor-acceptor-acceptor cationic dyes

Three metal-free donor-acceptor-acceptor sensitizers with ionized pyridine and a reference dye were synthesized, and a detailed investigation of the relationship between the dye structure and the photophysical and photoelectrochemical properties and the performance of dye-sensitized solar cells (DSSCs) is described. The ionization of pyridine results in a red shift of the absorption spectrum in comparison to that of the reference dye. This is mainly attributable to the ionization of pyridine increasing the electron-withdrawing ability of the total acceptor part. Incorporation of the strong electron-withdrawing units of pyridinium and cyano acrylic acid gives rise to optimized energy levels, resulting in a large response range of wavelengths. When attached to TiO2 film, the conduction band of TiO2 is negatively shifted to a different extent depending on the dye. This is attributed to the electron recombination rate between the TiO2 film and the electrolyte being efficiently suppressed by the introduction of long alkyl chains and thiophene units. DSSCs assembled using these dyes show efficiencies as high as 8.8 %.