Ionic Covalent Organic Framework as a Dual Functional Sensor for Temperature and Humidity.

Visual sensing of humidity and temperature by solids plays an important role in the everyday life and in industrial processes. Due to their hydrophobic nature, most covalent organic framework (COF) sensors often exhibit poor optical response when exposed to moisture. To overcome this challenge, the optical response is set out to improve, to moisture by incorporating H-bonding ionic functionalities into the COF network. A highly sensitive COF, consisting of guanidinium and diformylpyridine linkers (TG-DFP), capable of detecting changes in temperature and moisture content is fabricated. The hydrophilic nature of the framework enables enhanced water uptake, allowing the trapped water molecules to form a large number of hydrogen bonds. Despite the presence of non-emissive building blocks, the H-bonds restrict internal bond rotation within the COF, leading to reversible fluorescence and solid-state optical hydrochromism in response to relative humidity and temperature.

Tris(boranil) Columnar Liquid Crystalline Fluorophores: pseudo-Triphenylene Boron(III) Complexes with Peripheral N-B-O Linkages.

The borate complexes derived from salicylaldimine ligands, called boranils, possess a wide range of photophysical and electronic characteristics intrinsically. The unique combination of molecular rigidity, rendered by four-coordinate boron bridges, and extended π-conjugation enable them to serve as technically feasible fluorescent materials (dyes). The incorporation of liquid crystallinity in these boron(III) complexes, especially the columnar (Col) mesomorphism, which is overlooked hitherto, would provide a new dimension to these complexes. Herein, we report the first examples of tris(boranil) discotic liquid crystal (LC) dyes that have been readily synthesized by treating tris(N-salicylideneaniline)s, (TSAN)s, with BF3 .Et2 O in the presence of an acid quencher. These C3 -symmetric borate complexes self-assemble into the Col phase, existing over a wide thermal span including room temperature. The 2D periodic order of the Col phases shows dependence on the length of the peripheral tails. The photophysical measurements reveal the fluorescence emission in their two condensed states viz., solid and Col phase, and in solution. Their electrochemical, two-step oxidation process coupled with the aforesaid features upholds their significance in applied research.

Highly Soluble Ambipolar anti-Perylene-3,4 : 9,10-bis(benzimidazole)s Stabilize a Room-Temperature Columnar Hexagonal Phase.

A new series of highly soluble perylene anti-bis(4,5-dialkoxybenzimidazole)s bearing branched flexible chains stabilizing room temperature columnar hexagonal phase and with balanced ambipolar charge carrier mobility is reported for the first time. Only the anti isomer was successfully separated and characterized. These compounds have a high extinction coefficient, small optical band gap and wide absorption range, thus making them a promising class of ambipolar organic semiconductors capable of self-organizing.

Anisotropic sol-gel transition: the influence of sample thickness, pressure and strain.

We demonstrate an unprecedented influence of sample thickness (d) and pressure (P) on the gelation of a liquid crystalline nematic host. The temperature at which the anisotropic nematic sol transforms into a nematic gel is weakly dependent on 'd' down to its certain value, but surprisingly exhibits a precipitous drop below about 30 µm thickness. Temperature-dependent laser transmission exhibits characteristics of approach to a tricritical point, when d is varied. While the strain dependence of storage and loss moduli exhibit Payne effect/weak strain overshoot, the magnitudes of the moduli and their thermal variation present explicit dependence on d, both behaviours being well described by power-law expressions. Studies at elevated pressures also corroborate the observations of d-dependence with the nematic sol thermal range increasing with P, suggesting reduced favour for network formation. We strongly believe that these experiments pave a new pathway to realize the formation of gel fibres.

Topological defects due to twist-bend nematic drops mimicking colloidal particles in a nematic medium.

Colloids formed of solid/fluid particle dispersions in oriented nematic liquid crystals are known to be an ideal means of realizing fundamentally significant topological defect geometries. We find, experimentally, that twist-bend nematic (NTB) droplets formed in the N-NTB biphasic regime, either of pure compounds or mesogenic mixtures, completely mimic colloidal particles in their ability to generate a rich variety of defects. In the biphasic regime, the topological features of both liquid crystal colloids and chiral nematic droplets are revealed by (i) topological dipoles, quadrupoles and their patterned clusters formed in planar nematic liquid crystals orientationally perturbed by coexisting NTB drops, (ii) the transformation of hyperbolic hedgehogs into knotted Saturn rings encircling the NTB drops dispersed in a 90°-twisted nematic matrix and (iii) the Frank-Pryce defect texture evident in smaller (relative to sample thickness) NTB drops. In larger drops with fingerlike outgrowths, additional line defects appear; most of these are deemed to be pairs of disclinations to which are attached pairs of screw dislocations intervening in the growth process of the NTB droplets.

Effect of Atomic-Scale Differences on the Self-Assembly of Thiophene-based Polycatenars in Liquid Crystalline and Organogel States.

Two series of polycatenars are reported that contain a central thiophene moiety connected to two substituted oxadiazole or thiadiazole units. The number, position, and length of the peripheral chains connected to these molecules were varied. The oxadiazole-based polycatenars exhibited columnar phases with rectangular and hexagonal or oblique symmetry, whereas the thiadiazole-based polycatenars exhibited columnar phases with rectangular and/or hexagonal symmetry. All of the compounds exhibited bright emission in the solution and thin-film states. Two oxadiazole-based molecules and one thiadiazole-based molecule exhibited supergelation ability in hydrocarbon solvents, which is mainly supported by attractive π-π interactions. These gels showed aggregation-induced enhanced emission, which is of high technological importance for applications in solid-state emissive displays. X-ray diffraction studies of the xerogel fibers of oxadiazole-based polycatenars revealed a columnar rectangular organization, whereas a hexagonal columnar arrangement was observed for thiadiazole-based polycatenars. Rheological measurements carried out on the samples quantitatively confirmed the formation of gels and showed that these gels are mechanically robust. The impact of an atomic-scale difference (oxygen to sulfur, <2 % of the molecular weight) on the self-assembly and the macroscopic properties of those self-assembled structures are clearly visualized.

s-Triazine-Based Functional Discotic Liquid Crystals: Synthesis, Mesomorphism and Photoluminescence.

A new series of C3 -symmetric, π-conjugated molecules was designed, synthesized and characterized. The materials were derived from electron-accepting s-triazine, appended covalently to electron-donating styrylbenzene arms, and were readily prepared in excellent yield with high purity by means of three-fold condensation of triphosphonate with n-alkoxybenzaldehydes under Horner-Wadsworth-Emmons reaction conditions. Examination of the phase transitional properties by several complementary techniques evidenced self-assembly into a hexagonal columnar phase, occurring over wide and reasonable thermal ranges. The photophysical properties were studied both in solution and in the fluid/frozen columnar states by UV/Vis absorption and photoluminescence spectroscopy. The emission spectra obtained as a function of the temperature rule out the breaking-up of larger columns and a non-radiative, thermally activated process. A study carried out on thin films of the glassy columnar state, which accounts for conserved fluorescence, defect-free orientation, and freezing ionic species, with the help of atomic force microscopy (AFM) images, suggested a homogeneous granular morphology comprising fibrillar structures. Dissimilarities in the surface morphology and birefringence of thin films of the solid and frozen columnar states were clearly shown by Raman spectroscopy. An electrochemical investigation revealed a LUMO energy of -4.0 eV. Thus, the discotic motifs presented herein meet certain criteria of organic materials, which are essential for developing electronic devices.

Periodically clickable polyesters: study of intrachain self-segregation induced folding, crystallization, and mesophase formation.

A series of polyesters based on 2-propargyl-1,3-propanediol or 2,2-dipropargyl-1,3-propanediol or 2-allyl-2-propargyl-1,3-propanediol and 1,20-eicosanedioic acid were prepared by solution polycondensation using the corresponding diacid chloride; these polyesters were quantitatively "clicked" with a fluoroalkyl azide, namely CF3(CF2)7CH2CH2N3, to yield polyesters carrying long-chain alkylene segments in the backbone and either one or two perfluoroalkyl segments located at periodic intervals along the polymer chain. The immiscibility of the alkylene and fluoroalkyl segments causes the polymer chains to fold in a zigzag fashion to facilitate the segregation of these segments; the folded chains further organize in the solid state to form a lamellar structure with alternating domains of alkyl (HC) and fluoroalkyl (FC) segments. Evidence for the self-segregation is provided by DSC, SAXS, WAXS, and TEM studies; in two of the samples, the DSC thermograms showed two distinct endotherms associated with the melting of the individual domains, while the WAXS patterns confirm the existence of two separate peaks corresponding to the interchain distances within the crystalline lattices of the HC and FC domains. SAXS data, on the other hand, reveal the formation of an extended lamellar morphology with an interlamellar spacing that matches reasonably well with those estimated from TEM studies. Interestingly, a smectic-type liquid crystalline phase is observed at temperatures between the two melting transitions. These systems present a unique opportunity to develop interesting nanostructured polymeric materials with precise control over both the domain size and morphology; importantly, the domain sizes are far smaller than those typically observed in traditional block copolymers.

Supergelation via purely aromatic π-π driven self-assembly of pseudodiscotic oxadiazole mesogens.

A series of highly luminescent oxadiazole-based stilbene molecules (OXD4, OXD8, OXD10, and OXD12) exhibiting interesting enantiotropic liquid crystalline and gelation properties have been synthesized and characterized. The molecules possessing longer alkyl substituents, OXD10 and OXD12, possess a pseudodisc shape and are capable of behaving as supergelators in nonpolar solvents, forming self-standing gels with very high thermal and mechanical stability. Notably the self-assembly of these molecules, which do not possess any hydrogen-bonding motifs normally observed in most reported supergelators, is driven purely by π-stacking interactions of the constituent molecules. The d-spacing ratios estimated from XRD analysis of OXD derivatives possessing longer alkyl chains show that the molecules are arranged in a columnar fashion in the mesogens and the self-assembled nanofibers formed in the gelation process.

Competition between anisometric and aliphatic entities: an unusual phase sequence with the induction of a phase in an n-alkane-liquid crystal binary system.

In this work, we demonstrate two important features that arise out of introducing a liquid-crystalline (LC) compound into the rotator phase matrix and the consequent competition between the anisometric segments of the LC moieties and the aliphatic units. First, we show that the change in the structural character of the mixed medium depends on which of the entities forms the minority concentration: in the case of this being the alkane, the two components of the binary system are nanophase segregated, whereas if the LC molecules are present in a small concentration, then the layered structure merely gets roughened without any segregation. The second and more significant result of the calorimetric and X-ray experiments, at low LC concentrations, is the induction of a rotator phase that leads to unusual phase sequence not reported hitherto. Possible scenarios for the molecular arrangement are discussed. A Landau model is also presented that explains some of the observed features.

Tunable emissive lanthanidomesogen derived from a room-temperature liquid-crystalline Schiff-base ligand.

A novel photoluminescent room-temperature liquid-crystalline salicylaldimine Schiff base with a short alkoxy substituent and a series of lanthanide(III) complexes of the type [Ln(LH)3(NO3)3] (Ln = La, Pr, Sm, Gd, Tb, Dy; LH = (E)-5-(hexyloxy)-2-[{2-(2-hydroxyethylamino)ethylimino]methyl}phenol) have been synthesized and characterized by FTIR, (1)H and (13)C NMR, UV/Vis, and FAB-MS analyses. The ligand coordinates to the metal ions in its zwitterionic form. The thermal behavior of the compounds was investigated by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The ligand exhibits an enantiotropic hexagonal columnar (Col(h)) mesophase at room temperature and the complexes show an enantiotropic lamellar columnar (Col(L)) phase at around 120 °C with high thermal stability. Based on XRD results, different space-filling models have been proposed for the ligand and complexes to account for the columnar mesomorphism. The ligand exhibits intense blue emission both in solution and in the condensed state. The most intense emissions were observed for the samarium and terbium complexes, with the samarium complex glowing with a bright-orange light (ca. 560-644 nm) and the terbium complex emitting green light (ca. 490-622 nm) upon UV irradiation. DFT calculations performed by using the DMol3 program at the BLYP/DNP level of theory revealed a nine-coordinate structure for the lanthanide complexes.

Self-assembly of hekates-tris(N-salicylideneaniline)s into columnar structures: synthesis and characterization.

Two series of new, photoluminescent star-shaped discotic liquid crystals, recently termed as "hekates", derived from tris(N-salicylideneaniline)s (TSANs), were synthesized by the facile threefold condensation of 3,4-bis(alkoxy)phenyl 4-aminobenzoates/3,4,5-tris(alkoxy) phenyl 4-aminobenzoates with 1,3,5-triformylphloroglucinol and characterized. These two series of discotics with six and nine peripheral n-alkoxy tails were especially designed and accomplished to understand the relation between mesomorphic/photophysical properties and molecular structure. Proton NMR spectral analysis revealed their existence as an inseparable mixture of two keto-enamine tautomeric forms featuring C(3h) and C(s) rotational symmetries. A systematic study into the thermotropic liquid crystal behavior using polarizing optical microscopy, differential scanning calorimetry, and X-ray scattering confirmed the presence of columnar (Col) phase in vast majority of the TSANs prepared. The two-dimensional (2D) lattices of these fluid columnar phases were found to be characteristic of hexagonal Col (Col(h)), rectangular Col (Col(r)), or oblique Col (Col(ob)) phases depending on the number/length of the peripheral flexible chains. The stabilization of the Col(ob) phase, a less commonly found fluid columnar structure, and the first of its kind in TSAN systems, implies very intensive intermolecular (face-to-face) interactions among the TSAN cores within the column. The photophysical properties were investigated both in solution and the columnar states by UV-vis absorption and photoluminescence; markedly, the solution state emits light in the blue region. The light-emitting ability of the Col phase is particularly significant given the possibility that, in such cores, the protons and electrons interact with each other through the H-bonding environment.

Twist-bend nematic drops as colloidal particles: Structural features.

The mesogen CB7CB [1″,7″-bis(4-cyanobiphenyl-4'-yl)heptane], mixed with a small quantity of a long chain amphiphile, is examined for the structural features of twist-bend nematic (N_{TB}) drops acting as colloidal inclusions in the isotropic and nematic environments. In the isotropic phase, the drops nucleating in the radial (splay) geometry develop toward escaped radial, off-centered structures, involving both splay and bend distortions. With further growth, they transform into low-birefringence (near-homeotropic) objects, within which remarkably well-organized networks of parabolic focal conic defects evolve in time. In electrically reoriented near-homeotropic N_{TB} drops, the pseudolayers develop an undulatory boundary possibly attributable to saddle-splay elasticity. In the matrix of the planar nematic phase, N_{TB} droplets appearing as radial hedgehogs attain stability in the dipolar geometry, through their association with hyperbolic hedgehogs. With growth, on transformation of the hyperbolic defect into its topologically equivalent Saturn ring around the N_{TB} drop, the geometry turns quadrupolar. Significantly, dipoles are stable in smaller drops, while quadrupoles are stable in larger ones. The dipole-quadrupole transformation is reversible, but is hysteretic with respect to drop size. Importantly, this transformation is often mediated by nucleation of two loop disclinations, one appearing at a marginally lower temperature than the other. The existence of a metastable state with partial formation of a Saturn ring and persistence of the hyperbolic hedgehog raises a question relating to the conservation of topological charge. In twisted nematics, this state features in the formation of a giant unknot that binds all N_{TB} drops together.

First example of ambipolar naphthalene diimide exhibiting a room temperature columnar phase.

A self-assembled ambipolar organic semiconductor based on naphthalene diimide with low clearing temperature, solution processability, and high molar extinction coefficient, exhibiting a room temperature columnar hexagonal liquid crystalline phase is reported.

Chiral plasmonic liquid crystal gold nanoparticles: self-assembly into a circular dichroism responsive helical lamellar superstructure.

Owing to their proven and promising potential in various technological endeavors ranging from catalysis and sensing to invisibility cloaks made from metamaterials, chiral plasmonic superstructures resulting from the directed self-assembly of optically active metal nanoparticles (MNPs) have been pursued intensively in recent years. Several strategic efforts have emerged especially to accomplish advanced nanomaterials assembling into liquid crystalline (LC) helical structures, where MNPs are regularly packed in fluid/frozen arrays/layers or wires (columns). While the helical fluid columnar arrays (molecular wires) showing circular dichroism (CD) have been realized, the discovery of fluid chiral lamellar ordering, where the dielectric and conducting regimes are arranged alternatively, has hitherto remained highly elusive. Herein we report the first examples of monodisperse LC-gold NPs (LC-GNPs) self-assembling into a fluid/frozen lamellar structure exhibiting CD activity. Notably, these new, exceptional LC-GNPs have been realized by simple, hassle-free protocols that involve the room temperature addition of LC dimer-like arylamines to Au(iii), where the amines not only reduce Au(iii) to Au(0) but also bind strongly to the central GNP scaffold. Their molecular structure, mesomorphism, and ability to interact with circularly polarized light have been evidenced unambiguously and could play an important role in realizing metamaterials in the visible region.

Luminescent, liquid crystalline tris(N-salicylideneaniline)s: synthesis and characterization.

A new class of discotics derived from tris(N-salicylideneaniline)s have been synthesized and their thermal and photophysical properties are investigated. These systems with outer 1,3,4-oxadiazole wings exist in an inseparable mixture of two keto-enamine tautomeric forms with C(3h) and C(s) rotational symmetries, and self-assemble into fluid columnar phase over a wide thermal range as evidenced by several complementary studies. They possess emissive characteristics in both solution and columnar states; the blue light (lambda = 474 nm) emission has been evidenced for the former state.

Effect of the C-2 hydroxyl group on the mesomorphism of alkyl glycosides: synthesis and thermotropic behavior of alkyl 2-deoxy-D-arabino-hexopyranosides.

A homologous series of alkyl 2-deoxy-alpha-d-arabino-hexopyranosides and alkyl 2-deoxy-beta-d-arabino-hexopyranosides were synthesized, upon glycosylation of 1-alkanols (from C8 to C18 alkanols) with ethyl 2-deoxy-3,4,6-tri-O-acetyl-1-thio-d-arabino-hexopyranoside, followed by a deprotection. The thermotropic behavior of these new types of alkyl glycosides was investigated. It was observed that the beta-anomers of these alkyl glycosides, bearing nonyl to tetradecyl alkyl chain are mesomorphic, exhibiting monotropic smectic A phase. In contrast, the alpha-anomers are all non-mesomorphic. An effort to identify the liquid crystalline behavior of binary mixtures of the alpha- and beta-anomers was undertaken and it was found that mixtures containing equimolar amounts of the anomers exhibited mesomorphic behavior. A fine balance of the hydrophilic and hydrophobic components within the molecule is also found to be important for the alkyl 2-deoxy glycosides to form the mesophase.

Self-Assembling and Luminescent Properties of Chiral Bisoxadiazole Derivatives in Solution and Liquid-Crystalline Phases.

Herein, we report the synthesis, self-assembly, and electroluminescence characteristics of a new green-emitting, pseudodiscoid chiral molecule, OXDC, containing an electron-donating stilbene core and an electron-accepting oxadiazole substituent. The helical organization and specific interaction of the chiral pseudodiscoid molecule resulted in the formation of self-assembled nanofibers with a columnar superstructure. Macroscopic chirality was observed in both the liquid-crystalline phases and the self-assembled nanofibers of OXDC, a feature which was absent in the analogous achiral oxadiazole derivative reported earlier [ Sivadas , A. P. ; Supergelation via Purely Aromatic π-π Driven Self-Assembly of Pseudodiscotic Oxadiazole Mesogens . J. Am. Chem. Soc. 2014 , 136 , 5416 - 5423 ]. A high-performance organic light-emitting device was demonstrated using OXDC as the emitting material, with a luminous intensity of 10 115 cd m-2 at 5 V and chromaticity coordinates of (0.32, 0.51).

Ferroelectricity of a bent-core material with cholesteryl terminal chain.

We have studied the phase sequence and physical properties of an asymmetric bent-core material with a chiral rigid cholesteryl moiety, and a flexible achiral alkyl side chain at the other end. The combination of the achiral bent core with the chiral cholesteryl unit results in properties different from those of both the usual calamitic and bent-core materials. We find that the presence of the bent core unit induces a wide-temperature-range optically isotropic (probably BPIII) mesophase between the isotropic and cholesteric (chiral nematic) phases. Below the cholesteric phase a ferroelectric smectic-C* structure occurs in which the bent-core units do not seem to form a polar close packing.

Binary System Exhibiting the Nematic to Twist-Bend Nematic Transition: Behavior of Permittivity and Elastic Constants.

We describe measurements of the permittivity and Frank elastic constant in the nematic phase of a binary system displaying a transition between the nematic (N) and the recently discovered twist-bend nematic (NTB) phase. Among the salient features observed are (i) the existence of the NTB phase even when the system is loaded with a high concentration (∼64 mol %) of a rodlike component; (ii) a clear signature in permittivity of the N-NTB transition; and (iii) a lower value of the bend elastic constant compared to the splay over a large phase space, with the difference between the two becoming a maximum for an intermediate mixture. These studies further support the surprising idea that the elastic features associated with bent molecules can be further augmented by suitable rodlike additives.