Pradip K. Bhowmik

Room-Temperature Thermotropic Ionic Liquid Crystals: Viologen Bis(Triflimide) Salts

Several dicationic salts with bis(triflimide) as counterions—otherwise known as viologens—were prepared by metathesis reaction of the corresponding viologen dibromides (diiodides) with lithium triflimide in a polar solvent. They were characterized for their thermotropic liquid-crystalline properties with various experimental techniques. Two of them had low crystal-to-smectic phase transitions (Tm) at –4 and 28°C; the other two also had low Tm values, at 51 and 58°C. Each had a wide range of LC phase. Two had relatively low crystal-to-isotropic (Tm) transitions at 117 and 131°C. Each had excellent thermal stability in the temperature range of 327–396°C. They exhibited photoluminescence property in both nonpolar and polar solvents as well as in the solid state. Their emission spectra in the solid state exhibited hypsochromic shift when compared with those in solution of dimethoxyethane.

Synthesis and characterisation of thermotropic liquid-crystalline properties of azomethine dimers

A series of azomethine dimers were prepared by condensation reactions of benzaldehyde, biphenylcarboxaldehyde and 9-anthraldehyde with various aromatic diamines of varying flexibility in ethanol in the presence of tosic acid. Their chemical structures were determined by Fourier transform infrared and 1H and 13C nuclear magnetic resonance (NMR) spectroscopies, as well as elemental analysis. Their thermal properties were also examined by using a number of experimental techniques, including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), polarising optical microscopy (POM) and variable temperature X-ray diffraction (VTXRD). Azomethine dimer, prepared from benzaldehyde and 1,9-bis(4-aminophenoxy)nonane, exhibited a monotropic, nematic liquid-crystalline (LC) phase. The majority of the azomethine dimers containing biphenyl moieties exhibited enantiotropic, nematic LC phase on melting at relatively low temperatures, since they developed typical Schlieren, threaded or marbled textures in their LC phase. They also had accessible isotropisation temperatures well below their decomposition temperatures. Azomethine dimers containing anthracene moieties did not exhibit LC properties, but exhibited polymorphism as determined by POM and VTXRD in two cases. All of these azomethine dimers in the series had excellent thermal stability that was in the broad range of temperatures of 307–400°C depending on their degrees of aromaticity index.

The effect of stirring on the morphology of birnessite nanoparticles

The effect of mechanical stirring on the morphology of hexagonal layer-structure birnessite nanoparticles produced from decomposition of KMnO4 in dilute aqueous H2SO4 is investigated, with characterization by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), thermogravimetric analysis (TGA), and N2 adsorption (BET). Mechanical stirring during an initial stage of synthesis is shown to produce black birnessite containing nanofibers, whereas granular particulates of brown birnessite are produced without stirring. This is the first reduction synthesis of black birnessite nanoparticles with dendritic morphology without any use of organic reductant, and suggests that a particular morphology can arise from structural preferences of Mn in acidic conditions rather than particular organic reactants. These results enlighten the possibility of synthesizing nanoparticles with controlled size and morphology.

Solution, thermal and optical properties of new poly(pyridinium salt)s derived from bisquinoline diamines

Several novel poly(pyridinium salt)s with heterocyclic bisquinoline moieties having both tosylate and triflimide counterions were prepared by either ring-transmutation or metathesis reaction. Their chemical structures were established by using 1H and 13C NMR, FTIR and elemental analysis. Their number-average molecular weights (Mn) ranged from 53,000 to 79,000 and their polydispersities ranged from 1.30 to 3.24 as determined by gel permeation chromatography. They had excellent thermal stabilities in the temperature range of 336–447 °C and glass transition temperatures in the range of 119–174 °C. They had good solubilities in common organic solvents and in several cases their critical concentrations (*C) were exceeded to form lyotropic liquid-crystalline phases. Additionally, they emitted light in the blue and green region in both solution and solid state. Their quantum yields were also determined by using a spectrofluorometer.

A New Method of Synthesizing Black Birnessite Nanoparticles: From Brown to Black Birnessite with Nanostructures

A new method for preparing black birnessite nanoparticles is introduced. The initial synthesis process resembles the classical McKenzie method of preparing brown birnessite except for slower cooling and closing the system from the ambient air. Subsequent process, including wet-aging at for 48 hours, overnight freezing, and lyophilization, is shown to convert the brown birnessite into black birnessite with complex nanomorphology with folded sheets and spirals. Characterization of the product is performed by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), thermogravimetric analysis (TGA), and adsorption (BET) techniques. Wet-aging and lyophilization times are shown to affect the architecture of the product. XRD patterns show a single phase corresponding to a semicrystalline birnessite-based manganese oxide. TEM studies suggest its fibrous and petal-like structures. The HRTEM images at 5 and 10 nm length scales reveal the fibrils in folding sheets and also show filamentary breaks. The BET surface area of this nanomaterial was found to be 10.6 /g. The TGA measurement demonstrated that it possessed an excellent thermal stability up to . Layer-structured black birnessite nanomaterial containing sheets, spirals, and filamentary breaks can be produced at low temperature () from brown birnessite without the use of cross-linking reagents.

Dispersion of single-walled carbon nanotubes with poly(pyridinium salt)s

Dispersion of single-walled carbon nanotubes (SWNTs) with poly(pyridinium salt)s via non-covalent interactions in DMSO is demonstrated. The resulting composites display high quenching efficiency in the photoluminescent properties of this class of ionic polymers, and TEM study shows that SWNTs are wrapped by ionic polymers with thickness around 3 nm.

Design and synthesis of photoactive ionic amorphous molecular materials

Compounds bearing electrostatic charges have unique properties including outstanding ionic-conductivity when compared with neutral ones. On the other hand, amorphous molecular materials harness the advantages of small molecules with high purity, defined structures and monodispersity, while still being easily processable into thin films and fibers. Here, we report a series of bis(pyridinium salt)s that combine these properties to form ionic amorphous molecular materials. Their design allowed them to vitrify efficiently upon slow cooling. They exhibited glass-transition temperatures in the range of 81–191 °C and had excellent thermal stabilities of up to 461 °C. Fibers were easily drawn from their melts that were visible to the naked eye. Although they were completely conjugated and had large molecular sizes, they were soluble in many common organic solvents. Additionally, they fluoresced blue and green light in both solution and solid states. They could be useful for high-performance applications in opto- and microelectronic fields, among others.

Main chain, thermotropic, liquid crystalline, hydrogen‐bonded polymers of 4,4′‐bipyridyl with 4,4′‐dicarboxy‐α,ω‐diphenoxyalkanes

A series of main chain, thermotropic, liquid crystalline (LC) hydrogen‐bonded polymers based on 4,4′‐bipyridyl as a hydrogen bond acceptor and 4,4′‐dicarboxy‐α,ω‐diphenoxyalkanes as hydrogen bond donors were prepared by a slow evaporation technique from a pyridine solution and characterized for their thermotropic LC properties using a variety of experimental techniques. The homopolymer of 4,4′‐bipyridyl with 4,4′‐dicarboxy‐1,9‐diphenoxynonane exhibited relatively low T m at 205°C and low T i at 230°C, giving an LC phase range of 25°C. The other two homopolymers with 4,4′‐dicarboxy‐1,6‐diphenoxyhexane and 4,4′‐dicarboxy‐1,9‐diphenoxydecane exhibited relative low T m values, above which each of them formed high‐order smectic phases. With further heating at higher temperature they transformed into low‐order smectic phases that persisted up to their decomposition temperatures. Several copolymers also had relatively low T m values as well as low T i values and, therefore, had a broad LC phase range (22–77°C). All of the polymers including copolymers exhibited high‐order and low‐order smectic phases, since they developed usually mosaic and schlieren (or bâtonnets) textures. Generally, copolymerization increased the temperature range of the LC phases for these polymers. The thermal transitions of all of the polymers were well below their decomposition temperatures, which were in the ranges of 254–329°C.

Synthesis and characterization of ionic liquids: viologen bis{tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate} salts

Several viologen bis{tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate} salts were prepared by metathesis reaction of the corresponding viologen dibromides (diiodide) with sodium bis{tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate} salt in a polar organic solvent. They were characterized for their physical and thermal properties by experimental techniques including variable temperature X‐ray diffraction. All exhibited low T g and T m, and their T g/T m (K) ratios were in the range 0.66–0.79. Several exhibited polymorphism and formed isotropic ionic melts at <150°C. They were soluble in many common organic solvents such as ethers, alcohols, acetonitrile and methylene chloride. They exhibited photoluminescence in both 1,2‐dimethoxyethane and methanol, as well as in the solid state. In the solid state, the emission spectra exhibited hypsochromic shift when compared with those in solutions of 1,2‐dimethoxyethane and methanol. §. This article is dedicated to Professor Robert W. Lenz with best wishes on the occasion of his 80th birthday.

Synthesis and characterization of birnessite and cryptomelane nanostructures in presence of Hoffmeister anions

The effect of Hoffmeister anions Cl-, SO42-, and ClO4- on the structure and morphology of birnessite and cryptomelane-type manganese dioxide nanostructures, produced by the reduction reaction of KMnO4 and MnSO4 in aqueous acidic media, was studied. The syntheses were based on the decomposition of aqueous KMnO4 in presence of HCl for birnessite-type and acidified MnSO4 for cryptomelane-type manganese dioxide under soft hydrothermal conditions. They were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) techniques. XRD patterns show the formation of birnessite for the first synthesis and a mixture of cryptomelane and birnessite-types MnO2 for the second synthesis. XRD data revealed that the Hoffmeister anions have a significant effect on the nanostructures of birnessite. The sulphate ion-treated birnessite has the smallest crystals, whereas the chloride ion-treated birnessite has the largest crystals. Their TEM and HRTEM studies revealed a transformation from nanoplatelet morphology for chloride-treated samples to nanofibrous morphology for sulphate-treated birnessite. For the cryptomelane nanostructures, Hoffmeister anions also show a profound effect on their crystalline structures as determined by XRD analyses revealing a transformation of the cryptomelane phase to birnessite phase of MnO2. This transformation is also supported by TEM and HRTEM studies.