Self-assembled viologens on HOPG: solid-state NMR and AFM unravel the location of the anions

The controlled growth of self-assembled networks on surfaces based on viologen salts is a major scientific challenge due to their unique electronic properties. The combination of solid-state NMR spectroscopy and atomic force microscopy at ambient conditions can unravel the fine organization of the supramolecular network on a graphitic surface by positioning the counter-ions relative to the viologen cation.

Redox of Viologen for Powering and Coloring

Viologen is among the most attractive and easiest-to-use organic redox active group in many functional molecular assemblies. It plays crucial roles as an electron transfer mediator in the artificial photo-energy conversion systems and electron-transfer protein assemblies and as a building block of supramolecules. Its features include electrochemically reversible redox activity and stability. Strong blue color and tendency to dimerization of the one-electron reduced form, viologen mono-radical mono-cation, are remarkable. In this Account, we describe the use of viologen to give a powered movement of small molecules and motion of millimetre-sized macroscopic soft-matters and the use of viologen ionic liquid as electrochromic materials. Attractivities of the use of viologen units for powering and coloring are demonstrated and discussed. In particular, we highlight driving of mechanical movements by π-π stacking dimerization, incorporation in a hydrogel to attain highly deformable material, induction of 2D phase transformation, and sharp color change of very thin ionic liquid layer in a compartment-less electrochromic display.

Influence of the ion size on the stability of the smectic phase of ionic liquid crystals

The thermotropic phase behavior of ionic liquids and ionic liquid crystals based on novel N-alkyl-3-methylpyridinium halides, trihalides and dichloroiodates was experimentally studied by polarized optical spectroscopy (POM) and differential scanning calorimetry (DSC) as well as by molecular dynamics (MD) simulation. In the experiments, the existence and thermal range of stability of the smectic phase of these ionic liquid crystals are found to strongly depend on the volume ratio between the cation and anion, that is their relative size. Only compounds with a relatively large volume ratio of the cation to anion, i.e., those with longer cationic alkyl chains and monoatomic halide anions, have a stable smectic A phase. Both melting points and clearing points increase with such a ratio. The MD simulation results qualitatively agree very well with the experimental data and provide molecular details which can explain the experimentally observed phenomena: the stronger van der Waals interactions from the longer alkyl chains and the stronger electrostatic interactions from the smaller anions with a higher charge density increase the stability of both the crystal phase and the smectic phase; this also prevents the ionic layers from easily mixing with the hydrophobic regions, a mechanism that ultimately leads to a nanosegregated isotropic liquid phase.

Mono- and Di-Quaternized 4,4'-Bipyridine Derivatives as Key Building Blocks for Medium- and Environment-Responsive Compounds and Materials

Mono- and di-quaternized 4,4'-bipyridine derivatives constitute a family of heterocyclic compounds, which in recent years have been employed in numerous applications. These applications correspond to various disciplines of research and technology. In their majority, two key features of these 4,4'-bipyridine-based derivatives are exploited: their redox activity and their electrochromic aptitude. Contemporary materials and compounds encompassing these skeletons as building blocks are often characterized as multifunctional, as their presence often gives rise to interesting phenomena, e.g., various types of chromism. This research trend is acknowledged, and, in this review article, recent examples of multifunctional chromic materials/compounds of this class are presented. Emphasis is placed on solvent-/medium- and environment-responsive 4,4'-bipyridine derivatives. Two important classes of 4,4'-bipyridine-based products with solvatochromic and/or environment-responsive character are reviewed: viologens (i.e., N,N'-disubstituted derivatives) and monoquats (i.e., monosubstituted 4,4'-bipyridine derivatives). The multifunctional nature of these derivatives is analyzed and structure-property relations are discussed in connection to the role of these derivatives in various novel applications.

Computational Study of the Substituent Effects for the Spectroscopic Properties of Thiazolo[5,4-d]thiazole Derivatives

Inspired by the structure and optical properties of N,N'-dialkylated/dibenzylated 2,5-bis(4-pyridinium)thiazolo[5,4-d]thiazole, we proposed a series of disubstituted thiazolo[5,4-d]thiazole derivatives as promising materials for multifunctional optoelectronic, electron transfer sensing, and other photochemical applications. Density functional theory study of the electronic structures and transition properties of those newly proposed molecules indicates that the electron-donating and electron-withdrawing groups introduced to the peripheral pyridyl ligands extend the distributions of molecular frontier orbitals, increase the electron density in thiazolo[5,4-d]thiazolea, and therefore lead to remarkable red-shifts of their absorption and emission peaks.

A coarse-grained model of ionic liquid crystals: the effect of stoichiometry on the stability of the ionic nematic phase

We have investigated, by means of molecular dynamics simulations, the phase behaviour of mixtures of charged ellipsoidal Gay-Berne (GB) particles and spherical Lennard-Jones (LJ) particles, as a coarse-grained model of ionic liquid crystals (ILCs). The anisotropic GB particles represent cations usually found in ILCs, for example, pyridinium or bipyridinium salts, while the spherical LJ particles are taken as a model of anions like common halides, hexafluorophosphate and tetrafluoroborate. Here we have focused our attention on the effect of the stoichiometry of the system (that is, the GB : LJ ratio n : m in the salt formula [GB]n[LJ]m) on the stability and thermal range of the ionic liquid crystal phases formed, with special attention to the ionic nematic phase. To isolate the stoichiometry effect, a comparison of four different systems with GB : LJ ratios of 1 : 3, 1 : 2, 1 : 1 and 2 : 1 is made by keeping the packing fraction and the charge of the minor component fixed. Our results suggest a way to improve the stability of the ionic nematic phase by enhancing the anisotropic van der Waals interaction compared to the Coulomb interaction, and by increasing the proportion of anisotropic particles in the mixture.

Nonfunctionalized Cation of an Ionic Liquid as a Ligand in the Synthesis of a New Coordination Compound and Assessment of Its Biological Activity

Literature evidences reveal the affinity of ionic liquids for biomembranes that they are readily absorbed into the cell, resulting in a variety of biological effects, including broad antibacterial potential and anticancer activity. Recent research directions considered the ions of this class of compounds as a new choice of ligands in the synthesis of transition metal complexes for various applications. Based on this, the present work reports the synthesis, structural characterization, and in vitro antibacterial activities of a tetrahedral hexacationic Co(II) complex formed by coordinating with the cation of an ionic liquid, N-butyl-4,4-bipyridinium bis(trifluoromethylsulfonyl)amide ([C₄Bip][Tf₂N]). It has been demonstrated by the isolation and characterization of tetrakis-(N-butyl-4,4′-bipyridinium)cobalt(II)dichloride-tetrakis-(bis(trifluoromethylsulfonyl)amide, ([(C₄Bip)₄Co]Cl₂(Tf₂N)₄). The ligand and complex are characterized spectroscopically (¹H, ¹³C, and ¹⁹F NMR, ESI MS, ICP OES), and by CHNS elemental analysis, halide estimation, and conductivity studies. The antibacterial activities of the compounds against two bacteria, Klebsiella pneumoniae (K. pneumoniae) and Staphylococcus aureus (S. aureus), are screened using the agar well-diffusion method and were compared with a reference (gentamicin). The metal complex demonstrated better inhibition than the ionic liquid and the reference.

A combined LX-NMR and molecular dynamics investigation of the bulk and local structure of ionic liquid crystals

The unique power of NMR spectroscopy in anisotropic media (LX-NMR) as a tool to obtain local and bulk structural information, combined with the effectiveness of molecular dynamics simulations at the atomistic level, shows very attractive potentialities for the study of interesting, even though still poorly understood, materials such as Ionic Liquid Crystals (ILCs). In this work, we focused our attention, in particular, on the orientational ordering of two mesophases: 1-dodecyl-3-methylimidazolium chloride, [C12C1im]Cl, and 1-dodecyl-3-methylimidazolium tetrafluoroborate, [C12C1im][BF4]. Both ILCs were studied by a 2H NMR direct investigation of the molecules forming the phases, suitably deuterated, and by 1H NMR spectroscopy, using the small rigid probe-solutes 1,4-dichlorobenzene (DCB), dissolved in [C12C1im][BF4] and [C12C1im]Cl, and 1,4-dibromobenzene (DBB) dissolved in [C12C1im][BF4], to probe the local, internal structure and organization of the mesophases. The experimental results were then compared with the predictions, by atomistic MD simulations, of the structure of the smectic phase of the two salts, at two selected temperatures, containing a single DCB molecule as a probe. The MD simulations show that the DCB solute is distributed only within the hydrophobic layers of the ILC. Orientational order parameters of the imidazolium cations and of the DCB molecule were obtained and compared with the experiments, showing a general good agreement and allowing a deeper understanding of the microscopic structure of the systems.

Thermotropic Liquid-Crystalline Properties of Viologens Containing 4-n-alkylbenzenesulfonates

A series of viologens containing 4-n-alkylbenzenesulfonates were synthesized by the metathesis reaction of 4-n-alkylbenzenesulfonic acids or sodium 4-n-alkylbezenesulfonates with the respective viologen dibromide in alcohols. Their chemical structures were characterized by Fourier Transform Infrared, 1H and 13C Nuclear Magnetic Resonance spectra and elemental analysis. Their thermotropic liquid-crystalline (LC) properties were examined by differential scanning calorimetry and polarizing optical microscopy. They formed LC phases above their melting transitions and showed isotropic transitions. As expected, all the viologen salts had excellent stabilities in the temperature range of 278–295 °C as determined by thermogravimetric analysis.

Crystal melting and glass formation in copper thiocyanate based coordination polymers

The melting point of Cu⁺ coordination polymer crystals is controlled by ligands, and the reversible crystal-to-glass state is observed.

Synthesis of N-Tetradecyl-1,10-phenathrolinium-Based New Salts for Biological Applications

New organic salts were synthesized by quaternizing 1,10-phenanthroline using 1-bromotetradecane. The first step yielded an organic salt of formula [C₂₆H₃₇N₂]Br. Anion exchange reaction using Li[(CF₃SO₂)₂N] resulted in a more stable salt of formula [C₂₆H₃₇N₂][(CF₃SO₂)₂N]. The organic salts were investigated by spectrometry (¹H, ¹³C, ¹⁹F NMR, X-ray photoelectron spectroscopy (XPS), UV-Vis, and matrix-assisted laser desorption/ionization mass spectroscopy (MALDI MS), CHNSBr elemental analysis, and thermal analysis (TGA and DSC). The thermal characterization showed the melting and decomposition points of [C₂₆H₃₇N₂][(CF₃SO₂)₂N] to be 48°C and 290°C, respectively, which indicates it is an ionic liquid with large liquidus range. The biological activities of the salts were investigated against two Gram-positive (Staphylococcus aureus and Streptococcus pyogenes) and two Gram-negative (Escherichia coli and Klebsiella pneumoniae) bacteria, and they are found to be active against all of them. They were compared with [Cu(1,10-phenanthroline)₂Cl]Cl. They are found more active against the Gram-negative bacteria. The salts demonstrated minimum inhibitory concentration as low as 50 µg/L. These results suggest the synthesized salts can be considered as a better alternative to certain transition metal complex drugs. This minimizes the concern of introducing metal ions into the organism.

Reversible 2D Supramolecular Organic Frameworks encompassing Viologen Cation Radicals and CB[8]

Reversible 2D supramolecular organic frameworks encompassing branched viologen architectures and cucurbit[8]uril (CB[8]) were constructed and investigated. UV-vis investigation clearly indicates the formation and intermolecular dimerization of monocation radicals and their encapsulation into the hydrophobic CB[8] cavity which is further complemented by EPR (electron paramagnetic resonance) spectroscopy. Particle size measurements by dynamic light scattering method showed particle sizes in the range of several µm indicating larger aggregates. Zeta potential measurements suggested the instability of these particles and their tendency to form aggregates. TEM (transmission electron microscope) analysis further revealed the formation of supramolecular polymer (monocation radical with cucurbit[8]uril) whose diameter were in the range of several µm as indicated by DLS measurements; however the oxidized form, i.e., the viologen dication with cucurbit[8]uril showed dotted spots in the range of sub nanometer level. The internal periodicities of the supramolecular polymers were analyzed by SAXs (small angle X-ray scattering) measurements. Additionally, we have demonstrated that these supramolecular organic frameworks can be depolymerized by oxidation in air and again can be polymerized (intermolecular radical dimerization) by reduction under inert atmosphere demonstrating that these systems will be of broad interest.

Design of Viologen-Based Liquid Crystals Exhibiting Bicontinuous Cubic Phases and Their Redox-Active Behavior

We have succeeded in developing viologen-based liquid-crystalline materials forming bicontinuous cubic phases. They are composed of amphiphilic zwitterions with a viologen ionic-head-group and sulfonyl-imide-type acids. In the bicontinuous cubic liquid-crystalline assemblies, the ionic-head groups of the amphiphiles align along a gyroid minimal surface, forming a 3D continuous viologen layer. The ionic state of the viologen-moieties can be tuned from a dication state (V²⁺) to a radical mono cation state (V^1+•) by UV irradiation and/or electric field. This redox reaction proceeds in bulk, accompanying the change of their color from colorless to purple. Interestingly, they preserve the 3D molecular assembled structures beyond the redox reaction, which has been confirmed by polarizing optical microscopy and X-ray diffraction measurements.

Hierarchical Growth of Supramolecular Structures Driven by Pimerization of Tetrahedrally Arranged Bipyridinium Units

A shape-persistent molecule, featuring four bipyridinium units, has been synthesized that upon reduction undergoes intermolecular pimerization because of the rigid architecture of the molecule. The pimerization process has been investigated by a variety of techniques, such as absorption measurements, EPR spectroscopy, as well as gamma and pulse radiolysis, and compared with the behavior of a model compound. Computational studies have also been performed to support the experimental data. The most interesting feature of the tetramer is that pimerization occurs only above a threshold concentration of monoreduced species, on the contrary to the model compound. Furthermore, there is an increase of the apparent pimerization constant by increasing the concentration of reduced bipyridinium units. These results have been interpreted by the fact that pimerization is favored in the tetrahedrally shaped molecule because of a cooperative mechanism. Each multiply reduced molecule can indeed undergo multiple intermolecular interactions that enhance the stabilization of the system, also leading to hierarchical supramolecular growth. The resulting supramolecular system formed by such intermolecular pimerization should exhibit a diamond-like structure, as suggested by a simplified modeling approach. The intermolecular nature of the pimerization process occurring in the tetramer has been demonstrated by measuring the corresponding bimolecular rate constant by pulsed radiolysis experiments.

Doped ionic liquid crystals as effective weakly alignment media for polar solutes

The ionic liquid crystal 1-dodecyl-3-methylimidazolium tetrafluoroborate slightly doped with water is presented as a promising NMR alignment medium for the measurement of residual dipolar couplings for polar molecules dissolved therein.

Key Developments in Ionic Liquid Crystals

Ionic liquid crystals are materials that combine the classes of liquid crystals and ionic liquids. The first one is based on the multi-billion-dollar flat panel display industry, whilst the latter quickly developed in the past decades into a family of highly-tunable non-volatile solvents. The combination yields materials with a unique set of properties, but also with many challenges ahead. In this review, we provide an overview of the key concepts in ionic liquid crystals, particularly from a molecular perspective. What are the important molecular parameters that determine the phase behavior? How should they be introduced into the molecules? Finally, which other tools does one have to realize specific properties in the material?

Ionic Liquid Crystals: Versatile Materials

This Review covers the recent developments (2005-2015) in the design, synthesis, characterization, and application of thermotropic ionic liquid crystals. It was designed to give a comprehensive overview of the "state-of-the-art" in the field. The discussion is focused on low molar mass and dendrimeric thermotropic ionic mesogens, as well as selected metal-containing compounds (metallomesogens), but some references to polymeric and/or lyotropic ionic liquid crystals and particularly to ionic liquids will also be provided. Although zwitterionic and mesoionic mesogens are also treated to some extent, emphasis will be directed toward liquid-crystalline materials consisting of organic cations and organic/inorganic anions that are not covalently bound but interact via electrostatic and other noncovalent interactions.

Switchable electrochromic devices based on disubstituted bipyridinium derivatives

Gel switchable electrochromic devices based on disubstituted bipyridinium derivatives have been developed.

Viologen cyclophanes: redox controlled host–guest interactions

Bis(viologen) cyclophanes with alkyl linkers demonstrate redox-dependent host–guest properties, including the formation of a crystalline mixed valence species.

Viologen-based ionic liquid crystals: induction of a smectic A phase by dimerisation

The stability of thermotropic ionic liquid crystals is essentially due to micro-phase segregation between the ionic heads and the long alkyl chains. Here we show, using newly synthesized viologen dimers, that the structure of the central core is another key parameter to play with in order to tune the mesomorphic behaviour.

Photoresponsive supramolecular complexes as efficient DNA regulator

Two supramolecular complexes of trans-1⊂CB[8] and trans-2⊂CB[8] were successfully achieved by the controlled selective complexation process of cucurbit[8]uril (CB[8]) with hetero-guest pair containing azobenzene and bispyridinium moieties in aqueous solution, exhibiting the reversibly light-driven movements of CB[8] upon the photocontrollable isomerization of azophenyl axle components. Significantly, the obtained bistable supramolecular complexes and their corresponding [2]pseudorotaxanes could act as a promising concentrator and cleavage agent to regulate the binding behaviors with DNA molecules.

Synthesis of guanidinium-sulfonimide ion pairs: towards novel ionic liquid crystals

The recently introduced concept of ionic liquid crystals (ILCs) with complementary ion pairs, consisting of both, mesogenic cation and anion, was extended from guanidinium sulfonates to guanidinium sulfonimides. In this preliminary study, the synthesis and mesomorphic properties of selected derivatives were described, which provide the first example of an ILC with the sulfonimide anion directly attached to the mesogenic unit.