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.

Dynamics of the photo-thermo-mechanical actuations in NIR-dye doped liquid crystal polymer networks.

We describe photo-thermo-mechanical actuation and its dynamics in thin films of a liquid crystal networks (LCN) under near infrared (NIR) illumination through experiments and simulations. Splay aligned films of different thicknesses (25 µm to 100 µm) were obtained by crosslinking a mixture of mono-functional and bi-functional liquid crystal monomers. The NIR-driven thermo-mechanical actuation was achieved by adding an NIR dye to the monomer mixture. The absorption of incoming radiation by the dye molecules raises the local temperature of the film causing an order-disorder (nematic-isotropic) transition, thereby resulting in a macroscopic shape change. We have investigated the effect of film thickness, NIR laser power and dye concentration on the tip displacement of the films in a cantilever configuration. The experimental findings and finite element simulation results are in reasonably good quantitative agreement. Despite using lower NIR powers than typically employed, the films show high actuation and large displacements. After achieving saturation in actuation, the films exhibit a flutter behavior which is discussed in light of the observed overshoot in the tip displacement for certain intensities and thicknesses. Finally, using a solar simulator, we also show the visible light response of the film.

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.

Columnar Self-Assembly of Electron-Deficient Dendronized Bay-Annulated Perylene Bisimides.

Three new heteroatom bay-annulated perylene bisimides (PBIs) have been synthesized by microwave-assisted synthesis in excellent yield. N-annulated and S-annulated perylene bisimides exhibited columnar hexagonal phase, whereas Se-annulated perylene bisimide exhibited low temperature columnar oblique phase in addition to the high temperature columnar hexagonal phase. The cup shaped bay-annulated PBIs pack into columns with enhanced intermolecular interactions. In comparison to PBI, these molecules exhibited lower melting and clearing temperature, with good solubility. A small red shift in the absorption was seen in the case of N-annulated PBI, whereas S- and Se-annulated PBIs exhibited blue-shifted absorption spectra. Bay-annulation increased the HOMO and LUMO levels of the N-annulated perylene bisimide, whereas a slight increase in the LUMO level and a decrease in the HOMO levels were observed in the case of S- and Se-annulated perylene bisimides, in comparison to the simple perylene bisimide. The band gaps of PBI and PBI-N were almost same, whereas an increase in the band gaps were observed in the case of S- and Se-annulated PBIs. The tendency to freeze in the ordered glassy columnar phase for PBI-N and PBI-S will help to overcome the charge traps due to crystallization, which are detrimental to one-dimensional charge carrier mobility. These solution processable electron deficient columnar semiconductors possessing good thermal stability may form an easily accessible promising class of n-type materials.

Confinement-driven radical change in a sequence of rotator phases: a study on n-octacosane.

Rotator-phase forming n-alkanes have been studied extensively in both their bulk state and in nanoconfinement. While some alkanes maintain their bulk-state rotator phases in nanoconfinement albeit with increased disorder, there are others exhibiting new rotator phases upon confinement. We present here a temperature dependent X-ray diffraction (XRD) and differential scanning calorimetric (DSC) study on n-octacosane (C28H58), which almost completely loses its bulk state RIV phase and undergoes complete disappearance of its RIII phase. In their place, when confined in cylindrical anodized alumina nanopores, a phase highly resembling the hexatic mesophase is formed at a higher temperature and the RI rotator phase at a lower temperature. The effects of finite size, interfacial interactions with the host matrix and alkyl chain flexibility are used to provide an explanation for such unexpected behaviour.

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.

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.

Influence of polarization-tilt coupling on the ferroelectric properties of smectic gels.

We have studied composites of a ferroelectric liquid crystal mixture with a simple organic gelating agent, employing structural, thermal, electrical and mechanical probes, investigating the influence of the coupling between the polarization and the tilt angle on the ferroelectric properties of smectic gels. The calorimetric data, presenting clear signatures of the gelation occurring in the smectic A (SmA) phase or the isotropic phase, depending on the concentration of the gelator, help in constructing a rich diagram in the temperature-gelator concentration phase space. The atomic force microscopy imaging brings out the interesting feature of the transfer of chirality from the ferroelectric liquid crystal (FLC) to the gel strands, as exemplified by the creation of nanorope structures which have attracted much attention in recent times. The influence of gelation on the magnitude of the tilt angle appears to be dependent on the probe employed: there is no change in the values obtained by X-ray diffraction, which looks at the projection of the entire molecular length onto the layer normal. In contrast, the value from the electro-optic method, wherein the molecular-core is responsible for the results, diminishes with gelator concentration. The latter feature is copied by the magnitude of the polarization also. Dielectric spectroscopy shows that gelation weakly influences the soft mode in the SmA phase. However, the Goldstone mode behaviour is strongly dependent on the gelator concentration, with the appearance of two modes in the smectic C* (SmC*) phase of higher gel concentrations. With information obtained upon application of DC bias, the origin of the two relaxations is discussed. These data are analyzed in terms of the predictions of the Landau model proposed for the ordinary (non-gel) SmA-SmC* transition showing that the gel network enhances the linear polarization-tilt coupling over the biquadratic one. Upon gelation the system becomes mechanically strong with a large increase in the elastic moduli.

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.

Anchoring transition induced by gelation in a liquid crystal system.

We report a novel type of anchoring transition (ANT) in a liquid crystal driven by the physical gelation of the system. The ANT manifests as anomaly in the thermal behaviour of the dielectric permittivity. Data from X-ray studies suggest that ANT is caused by the ability of the gel fibres to compete with the substrate-driven orientation conditions. It is further found that the molecular reorientation is possible only in cases where the gel is weak, the difference between weak and strong gels being established by rheological measurements.

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.

Dimer-parity-dependent odd-even effects in photoinduced transitions to cholesteric and twist grain boundary smectic-C

We describe investigations on the influence of the flexible spacer parity and length of the guest photoactive liquid-crystalline dimers in guest-host mixtures exhibiting photoinduced transitions involving isotropic (I), cholesteric (N^{*}), and twist grain boundary smectic-C^{*} (TGBC^{*}) phases. Despite a small concentration (3 wt. %) of the guest molecules, the transition temperatures and their photodriven shift (δT) show a strong odd-even parity (of the dimer) dependent effect, with the even-parity systems having a larger value than their odd-parity counterparts; δT is larger for the N^{*}-TGBC^{*} transition than for the I-N^{*} one. The photocalorimetric measurements corroborate these features in addition to showing that, in comparison with the absence-of-ultraviolet (UV) case, the transition enthalpy (ΔH) of the I-N^{*} transition in the UV-on case is diminished by 33 and 12% for the mixtures with even- and odd-parity dimers, respectively. The duration for relaxation from the isothermal photodriven transition also exhibits general features of an odd-even influence. Molecular dynamics simulations demonstrate the presence of significant conformational heterogeneity and associated shift in the conformational space on photostimulation of the guest molecules. The change in the effective shape and nematic order parameter is more pronounced in the even-parity system.

Triazole-modified triphenylene derivative: self-assembly and sensing applications.

Triphenylene-based discotic liquid crystal 3 bearing 1,2,3-triazole groups has been synthesized using "click" chemistry. Discotic mesogen 3 has good thermal stability, and incorporation of triazole groups results in stabilization of columnar mesophases down to room temperature and formation of organogels in cyclohexane and mixed solvents such as hexane and dichloromethane (4:1 v/v). Characterization of the organogel of 3 in cyclohexane revealed a porous network. However, presence of Cd(2+) ions in solution obstructed the self-assembly of this derivative due to preferred interactions between Cd(2+) ions and triphenylene units over π-π interactions among triphenylene groups. Further, strong emission of derivative 3 in its nonaggregated form makes it a promising fluorescence sensory material for nitroaromatic compounds.

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.

Electric-field-dictated phase diagram and accelerated dynamics of a reentrant nematic liquid crystal under photostimulation.

In a system consisting of photoactive molecules that exhibit light-driven isomerization transformations, actinic light can diminish or enhance ordering to the extent that transitions from the equilibrium to a more disordered phase can be brought about isothermally. This feature enables light to be used as a thermodynamiclike parameter to investigate phase behavior and adds another dimension to the studies owing to the nonequilibrium character of the isothermal transitions. We have carried out experiments which exploit the combination of two recent findings, viz., an electric field can accelerate the return to the nematic liquid crystalline phase from a photodriven isotropic phase; and in a reentrant mesogen, the photoinduced phase can be more ordered. To photostimulate the nonequilibrium transitions a low power uv radiation (0.1 mW cm(-2)) has been used. Unique temperature-electric-field phase diagrams of a liquid crystal exhibiting isotropic-nematic-smectic- A -reentrant nematic sequence, mapped using light transmission as probe reveal that the electric field influences all the transitions, but the effect is maximum on the equilibrium reentrant nematic to the photoinduced smectic- A transition. Temporal measurements have been performed under nonequilibrium conditions to study the dynamics of both the photochemical and the back relaxation processes across the different transitions. The electric field is indeed observed to accelerate the thermal back relaxation in each case, and especially the recovery of the reentrant phase is hastened by three orders of magnitude in time. We explore possible causes for the acceleration and present a finding which can be associated with one of the predictions of density-functional calculations for isomerization of azobenzenes.

Mechanochemical Synthesis and Temperature-Dependent Optical Properties of Thermochromic (Ag1-x Cux )2 HgI4.

Thermochromic materials are generally synthesized via high-temperature melting reaction or solution-based synthesis. Herein, all-inorganic thermochromic compounds of (Ag1-x Cux )2 HgI4 were synthesized by solvent-free simple and scalable mechanochemical grinding at room temperature. Temperature-dependent electronic absorption spectroscopy along with DSC analysis confirmed the thermochromic events within these materials, and the phase transition temperature varied with solid solution compositions. The photoluminescence (PL) spectra is red-shifted with the increase in the Cu content in (Ag1-x Cux )2 HgI4 (x=0-1).

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.

Monodispersive linear supermolecules stabilizing unusual fluid layered phases.

The first examples of monodisperse liquid crystalline pentamers, synthesized by covalently linking five mesogenic segments through four flexible spacers, exhibiting mono-/partially bilayered phases with unusual textures, as established by optical, calorimetric, and X-ray diffraction studies, are reported. Importantly, the two types of multifunctional pentamers, namely, unsymmetric and C2 symmetric are soluble in organic solvents and exhibit analogous electrochemical features.