Indu Bala

A porous, crystalline truxene-based covalent organic framework and its application in humidity sensing

Truxene is employed as a building block to successfully synthesize novel covalent organic frameworks (COFs). The condensation reaction between truxene (10,15-dihydro-5H-diindeno[1,2-a:1′,2′-c]fluorene, TX) and 1,4-phenylenediboronic acid (DBA) results in a crystalline COF with boron ester linkages (COF-TXDBA) and a surface area of 1526 m2 g−1, as confirmed by powder X-ray diffraction (PXRD) and Brunauer–Emmett–Teller (BET) surface area measurements. This is the first study where nanochannels generated by periodic COF planar layers are shown to ease the interactions of the boron ester linkages with the water molecules for efficient humidity sensing. The COF-TXDBA based % RH sensor exhibits a change of 3 orders of magnitude in impedance in the 11–98% RH range, with response and recovery times of 37 s and 42 s, respectively. The response transients measured by switching COF-TXDBA sensor back and forth in 4 loops of % RH range displays excellent reversibility of the sensor with a deviation of 2.3% in the switching process.

Heptazine: an Electron-Deficient Fluorescent Core for Discotic Liquid Crystals

Herein, room-temperature discotic liquid crystals based on heptazine, an electron deficient central core, are reported for the first time. Mesomorphic behaviors of the materials are also investigated. Supramolecular assembly of the mesophase derivatives were confirmed by X-ray scattering experiments. Heptazine-based solid thin films are strong blue light emitters, whereas in the solution state, they are weakly emissive or non-emissive. The band gap energy is found to be low in this class of compounds. Formation of room-temperature mesophases, low band-gap behavior, and strong blue-light emission in the solid state are promising attributes for optoelectronic applications of the materials.

Rod–disc oligomeric liquid crystal based on 4-cyanobiphenyl and truxene core

ABSTRACT A facile synthesis of a novel covalently linked disc–rod mesogen is reported consisting of a truxene-based core attached to which are six 4-cyanobiphenyl units via flexible alkyl spacers. The compound formed a stable Langmuir monolayer at the air–water interface. The atomic force microscope study on the Langmuir–Blodgett film of the molecule reveals a tilted orientation at air–solid interfaces. GRAPHICAL ABSTRACT

Phase behavior of a new class of anthraquinone-based discotic liquid crystals

Five novel columnar liquid crystalline compounds (4.1-4.5) consisting of a central anthraquinone core carrying four alkoxy chains (R = n-C6H13, n-C8H17, n-C10H21, n-C12H25, and 3,7-dimethyl octyl) with two diagonally opposite 1-ethynyl-4-pentylbenzene units were synthesized, and their phase transitions were investigated between changes in the molecular structure and their self-assembly into the columnar mesophases. Small and wide-angle X-ray scattering (SAXS/WAXS) studies were performed to deduce the exact nature of the mesophases, and their corresponding electron density maps were derived from the intensities of the peaks observed in the diffraction patterns. A comparison of compounds with different alkoxy chains indicated that the soft crystal columnar rectangular (Crcolrec) phase was stable at lower temperature for the shortest peripheral alkoxy chain (4.1; R = n-C6H13) and was found to exhibit the columnar hexagonal (Colh) phase and then the discotic nematic (ND) phase with increasing temperature. In contrast, increasing the peripheral chain length to n-C8H17 or the branched one (4.2 and 4.5) stabilized the Colh phase at lower temperature and showed the ND phase at higher temperature. Further increase in chain length (4.3 and 4.4; n-C10H21, n-C12H25) demonstrated the formation of the ND phase. Conductivity measurement in the Colh mesophase was found to be almost 10 times higher in magnitude than the corresponding Crcolrec phase. The HOMO-LUMO band gap of all the compounds was found to be in the range from 2.79 to 2.82 eV, which is quite less and comparable with the optical energy band gap.

Hydrogen-bond mediated columnar liquid crystalline assemblies of C3-symmetric heptazine derivatives at ambient temperature

A new class of hydrogen (H) bonded fluorescent liquid crystals (FLCs) based on a newly discovered s-heptazine fluorophore discotic component have been synthesized. The tendency of the s-heptazine core to form H-bonded LCs has been explored for the first time. Interestingly, the pure heptazine derivatives (non-mesomorphic) on complexation with tri-alkoxy benzoic acids exhibit enantiotropic columnar mesomorphism over a wide range of temperatures including room temperature. This indicates the strength of the resulting H-bonded complexes. The H-bonded supramolecular complexes were studied through FT-IR, temperature dependent FT-IR and NMR studies and H-D exchange studies, and their thermal behaviour was deduced through polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies. Because of the inherently fluorescent pure heptazine derivative, the resulting complexes exhibit fluorescent behaviour in the solution state as well as in the solid state.