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.

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.

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.

Noise in supercontinuum generated using PM and non-PM tellurite glass all-normal dispersion fibers.

Intensity fluctuations in supercontinuum generation are studied in polarization-maintaining (PM) and non-PM all-normal dispersion tellurite photonic crystal fibers. Dispersive Fourier transformation is used to resolve the shot-to-shot spectra generated using 225-fs pump pulses at 1.55 µm, with experimental results well reproduced by vector and scalar numerical simulations. By comparing the relative intensity noise for the PM and non-PM cases, supported by simulations, we demonstrate the advantage of the polarization-maintaining property of the PM fibers in preserving low-noise dynamics. We associate the low-noise in the PM case with the suppression of polarization modulation instability.

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.

Ultra-low-noise supercontinuum generation with a flat near-zero normal dispersion fiber.

A pure silica photonic crystal fiber with a group velocity dispersion (ß2) of 4 ps2/km at 1.55 µm and less than 7 ps2/km from 1.32 µm to the zero dispersion wavelength (ZDW) 1.80 µm was designed and fabricated. The dispersion of the fiber was measured experimentally and found to agree with the fiber design, which also provides low loss below 1.83 µm due to eight outer rings with increased hole diameters. The fiber was pumped with a 1.55 µm, 125 fs laser and, at the maximum in-coupled peak power (P0) of 9 kW, a 1.34-1.82 µm low-noise spectrum with a relative intensity noise below 2.2% was measured. The numerical modeling agreed very well with the experiments and showed that P0 could be increased to 26 kW before noise from solitons above the ZDW started to influence the spectrum by pushing high-noise dispersive waves through the spectrum.

Viologen-Based Conjugated Covalent Organic Networks via Zincke Reaction.

Morphology influences the functionality of covalent organic networks and determines potential applications. Here, we report for the first time the use of Zincke reaction to fabricate, under either solvothermal or microwave conditions, a viologen-linked covalent organic network in the form of hollow particles or nanosheets. The synthesized materials are stable in acidic, neutral, and basic aqueous solutions. Under basic conditions, the neutral network assumes radical cationic character without decomposing or changing structure. Solvent polarity and heating method determine product morphology. Depending upon solvent polarity, the resulting polymeric network forms either uniform self-templated hollow spheres (HS) or hollow tubes (HT). The spheres develop via an inside-out Ostwald ripening mechanism. Interestingly, microwave conditions and certain solvent polarities result in the formation of a robust covalent organic gel framework (COGF) that is organized in nanosheets stacked several layers thick. In the gel phase, the nanosheets are crystalline and form honeycomb lattices. The use of the Zincke reaction has previously been limited to the synthesis of small viologen molecules and conjugated viologen oligomers. Its application here expands the repertoire of tools for the fabrication of covalent organic networks (which are usually prepared by dynamic covalent chemistry) and for the synthesis of viologen-based materials. All three materials-HT, HS, and COGF-serve as efficient adsorbents of iodine due to the presence of the cationic viologen linker and, in the cases of HT and HS, permanent porosity.

Influence of chirality on the thermal and electric properties of the columnar mesophase exhibited by homomeric dipeptides.

We present the first investigation of the influence of chirality on the thermal and electric properties in a biologically important homomeric dipeptide that exhibits a hexagonal columnar liquid crystal mesophase. The peptide employed has two chiral centres, and thus the two possible enantiopures are the (R,R) and (S,S) forms having opposite chirality. The measurements reported the span of the binary phase space between these two enantiopures. Any point in the binary diagram is identified by the enantiomeric excess Xee (the excess content of the R,R enantiopure over its S,S counterpart). We observe that the magnitude of Xee plays a pivotal role in governing the properties as evidenced by X-ray diffraction (XRD), electric polarization (Ps), dielectric relaxation spectroscopy (DRS) measurements, and the isotropic-columnar transition temperature. For example, XRD shows that while other features pointing to a hexagonal columnar phase remain the same, additional short-range ordering, indicating correlated discs within the column, is present for the enantiopures (Xee = ±1) but not for the racemate (Xee = 0). Similarly, an electric-field driven switching whose profile suggests the phase structure to be antiferroelectric is seen over the entire binary space, but the magnitude is dependent on Xee; interestingly the polarization direction is axial, i.e., along the column axis. DRS studies display two dielectric modes over a limited temperature range and one mode (mode 2) connected with the antiferroelectric nature of the columnar structure covering the entire mesophase. The relaxation frequency and the thermal behaviour of mode 2 are strongly influenced by Xee. The most attractive effect of chirality is its influence on the polar order, a measure of which is the magnitude of the axial polarization. This result can be taken to be a direct evidence of the manifestation of molecular recognition and the delicate interplay between chiral perturbations and the magnitude of the polar order, a feature attractive from the viewpoint of devices based on, e.g., remnant polarization-a currently hot topic. To add further dimension to the work, the DRS measurements are also extended to elevated pressures.

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.

Self-assembly of luminescent N-annulated perylene tetraesters into fluid columnar phases.

A new class of N-annulated perylene tetraesters and their N-alkylated derivatives has been synthesized. N-Annulated perylene tetraesters stabilize a hexagonal columnar phase over a broad temperature range. The hexagonal columnar phase exhibited by these compounds shows good homeotropic alignment with few defects. Annulation in the bay region of the perylene tetraesters enhanced the width of the mesophase compared with the parent tetraesters. N-Alkylation of these compounds perturbed the self-assembly behaviour and the resulting compounds were non-mesomorphic. A bright green luminescence was visible under long wavelength UV light. These properties suggest that these materials may have promising applications in organic electronics.

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.

IR regulation through preferential placement of h-BN nanosheets in a polymer network liquid crystal.

Recently, there has been a great deal of interest in devices which effectively shield near-infrared light with an additional feature of external field tunability, particularly for energy-saving applications. This article demonstrates an approach for fabricating a highly efficient near-infrared regulating device based on a polymer network liquid crystal reinforced with nanosheets of hexagonal-boron nitride (BN). The device achieves ∼84% IR scattering capability over a wavelength range of 800-2300 nm, and can also be regulated by an electric field. Interestingly, the observed high IR regulation is despite individual components of the composite being IR transparent, in stark contrast to earlier attempted incorporation of IR-absorbing/scattering particles. Detailed experimental characterization methods including FESEM corroborated with EDS and Raman spectroscopy suggest that the preferential positioning of the BN nanosheets, a consequence of the photo-polymerization process, is responsible for the observed feature. The IR reflectivity/back scattering that is doubled upon incorporation of the nanosheets results in an enhanced convective/radiative heat barrier capability, as evidenced by thermal imaging and significant (2 °C) reduction in ambient temperature upon one-Sun illumination. Numerical simulation results are also found to be in good agreement with the observed enhanced reflectance values for the BN-incorporated case. The presence of BN augments the mechanical rigidity of the system by a factor of 6.8 without compromising on the device operating voltage. The protocol employed is quite general and thus advantageous with far-reaching applications in passive cooling of buildings and structures, in thermal camouflaging, and in overall energy management.

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.

Effect of gender and geographic location on the expression of primary hyperparathyroidism.

BACKGROUND: The effect of gender on the expression of primary hyperparathyroidism (PHPT) is not well characterized. AIM: We therefore evaluated two Caucasian populations (US and Italian) of men and women with PHPT, matched for age and body mass index (BMI), in a cross-sectional retrospective observational study. METHODS: We studied 74 US (23 men) and 126 Italian (42 men) patients evaluating main biochemical indices of the disease and bone mineral density (BMD) at the spine and proximal femur. RESULTS: Mean serum calcium levels were higher both in Italian men compared to women (11.7 ± 1.22 mg/dl and 11.1 ± 0.83, p<0.01) and in Italian compared to US patients (11.3 ± 1.01 and 10.8 ± 0.58, p<0.001), with similar results for the serum ionized calcium. Mean serum PTH levels were not different either between the genders or between the countries. After controlling for BMI, the mean BMD at both the femoral neck and total hip in females US patients was significantly higher compared with Italian female patients. CONCLUSION: Despite similar levels of circulating PTH, Italian patients have more pronounced effects of the disease as assessed by serum calcium and a more significant cortical involvement in women as assessed by BMD.

Twist-bend nematic drops as colloidal particles: Electric instabilities.

The mesogen 1,''7''-bis(4-cyanobiphenyl-4'-yl)heptane (CB7CB), doped with a small quantity of an amphiphilic compound, is examined in its biphasic state in which twist-bend nematic (N_{TB}) drops are dispersed in the isotropic fluid. Various flexoelectric and electrokinetic responses of small drops in their escaped-radial-like (ER) geometry, and also of larger ones with parabolic focal conic defects, are discussed. A pair of confocal parabolas with their axes along the applied low-frequency electric field undergo periodic dimensional changes so as to contribute flexoelectrically to free-energy reduction. In an ER droplet, the same result is achieved by periodic relocations of the hedgehog core. Sine-wave fields of low frequency and high voltage excite patterned states near zero-voltage crossings and homeotropic alignment at peak voltages. ER drops also exhibit electrohydrodynamic effects; in relatively weak fields, they undergo translatory motion with a velocity that is a quadratic in the field strength; the drift, which occurs over a very wide frequency range, extending from dc to MHz region, is enabled by radial symmetry breaking that their off-centered geometry entails; and the drift direction reverses across a critical frequency. In high fields, vortical flows occurring within an ER N_{TB} drop become discernible. The hydrodynamic effects are discussed based on the Taylor-Melcher leaky dielectric model.