Partha Bairi

Redox-Switchable Copper(I) Metallogel: A Metal–Organic Material for Selective and Naked-Eye Sensing of Picric Acid

Thiourea (TU), a commercially available laboratory chemical, has been discovered to introduce metallogelation when reacted with copper(II) chloride in aqueous medium. The chemistry involves the reduction of Cu(II) to Cu(I) with concomitant oxidation of thiourea to dithiobisformamidinium dichloride. The gel formation is triggered through metal-ligand complexation, i.e., Cu(I)-TU coordination and extensive hydrogen bonding interactions involving thiourea, the disulfide product, water, and chloride ions. Entangled network morphology of the gel selectively develops in water, maybe for its superior hydrogen-bonding ability, as accounted from Kamlet-Taft solvent parameters. Complete and systematic chemical analyses demonstrate the importance of both Cu(I) and chloride ions as the key ingredients in the metal-organic coordination gel framework. The gel is highly fluorescent. Again, exclusive presence of Cu(I) metal centers in the gel structure makes the gel redox-responsive and therefore it shows reversible gel-sol phase transition. However, the reversibility does not cause any morphological change in the gel phase. The gel practically exhibits its multiresponsive nature and therefore the influences of different probable interfering parameters (pH, selective metal ions and anions, selective complexing agents, etc.) have been studied mechanistically and the results might be promising for different applications. Finally, the gel material shows a highly selective visual response to a commonly used nitroexplosive, picric acid among a set of 19 congeners and the preferred selectivity has been mechanistically interpreted with density functional theory-based calculations.

Supramolecular assembly of melamine and its derivatives: nanostructures to functional materials

The last twenty years have witnessed increasing research activity in the area of supramolecular chemistry of 1 : 1 co-assembly of melamine (M)–cyanuric acid (CA), since the historic discovery of the M·CA aggregate in crystal form and its structural analysis by Wang and his coworkers in 1990. Its useful chemical structure and fascinating H-bonding interaction sites distinguish M and its analogous derivatives as scaffolding components in the field of supramolecular chemistry to develop desired nano-to-micro scaled architecture. To date, M-based supramolecular assemblies are known in diverse forms which include fascinating nano- to micromorphological structures, molecular guest boxes, small molecular gels, membrane, sensor and liquid crystal development, polymeric scaffolds etc. In this review, we have covered the development of M and its derivatives, encompassing both nano/micro-ordered structures and advanced functional materials.

Graphene Quantum Dots from a Facile Sono-Fenton Reaction and Its Hybrid with a Polythiophene Graft Copolymer toward Photovoltaic Application

A new and facile approach for synthesizing graphene quantum dots (GQDs) using sono-Fenton reaction in an aqueous dispersion of graphene oxide (GO) is reported. The transmission electron microscopy (TEM) micrographs of GQDs indicate its average diameter as ∼5.6 ± 1.4 nm having a lattice parameter of 0.24 nm. GQDs are used to fabricate composites (PG) with a water-soluble polymer, polythiophene-g-poly[(diethylene glycol methyl ether methacrylate)-co-poly(N,N-dimethylaminoethyl methacrylate)] [PT-g-P(MeO2MA-co-DMAEMA), P]. TEM micrographs indicate that both P and PG possess distinct core-shell morphology and the average particle size of P (0.16 ± 0.08 μm) increases in PG (0.95 ± 0.45 μm). Fourier transform infrared and X-ray photoelectron spectrometry spectra suggest an interaction between -OH and -COOH groups of GQDs and -NMe2 groups of P. A decrease of the intensity ratio of Raman D and G bands (ID/IG) is noticed during GQD and PG formation. In contrast to GO, GQDs do not exhibit any absorption peak for its smaller-sized sp(2) domain, and in PG, the π-π* absorption of polythiophene (430 nm) of P disappears. The photoluminescence (PL) peak of GQD shifts from 450 to 580 nm upon a change in excitation from 270 to 540 nm. PL emission of PG at 537 nm is quenched, and it shifts toward lower wavelength (∼430 nm) with increasing aging time for energy transfer from P to GQDs followed by up-converted emission of GQDs. Both P and PG exhibit semiconducting behavior, and PG produces an almost reproducible photocurrent. Dye-sensitized solar cells (DSSCs) fabricated with an indium-titanium oxide/PG/graphite device using the N719 dye exhibit a short-circuit current (Jsc) of 4.36 mA/cm(2), an open-circuit voltage (Voc) of 0.78 V, a fill factor of 0.52, and a power conversion efficiency (PCE, η) of 1.76%. Extending the use of GQDs to fabricate DSSCs with polypyrrole, both Voc and Jsc increase with increasing GQD concentration, showing a maximum PCE of 2.09%. The PG composite exhibits better cell viability than the components.

Hierarchically Structured Fullerene C70 Cube for Sensing Volatile Aromatic Solvent Vapors

We report the preparation of hierarchically structured fullerene C70 cubes (HFC) composed of mesoporous C70 nanorods with crystalline pore walls. Highly crystalline cubic shape C70 crystals (FC) were grown at a liquid-liquid interface formed between tert-butyl alcohol and C70 solution in mesitylene. HFCs were then prepared by washing with isopropanol of the FC at 25 °C. The growth directions and diameters of C70 nanorods could be controlled by varying washing conditions. HFCs perform as an excellent sensing system for vapor-phase aromatic solvents due to their easy diffusion through the mesoporous architecture and strong π-π interactions with the sp(2) carbon-rich pore walls. Moreover, HFCs offer an enhanced electrochemically active surface area resulting in an energy storage capacity 1 order of magnitude greater than pristine C70 and fullerene C70 cubes not containing mesoporous nanorods.

Bicomponent hydrogels of lumichrome and melamine: photoluminescence property and its dependency on pH and temperature.

Lumichrome (L) and melamine (M) produce thermoreversible hydrogels in LM31 and LM11 compositions, but LM13 composition does not produce hydrogel (the numbers indicate the respective molar ratio of the components). The formation of thermoreversible gels is confirmed from morphology, DSC, and rheological experiments where LM13 system does not meet the required characteristics of thermoreversible gels. FTIR spectra suggest that H-bonding between L and M produces the supramolecular complex, and (1)H NMR spectra suggest that pi-stacking of the complex produce fibrillar network structure entrapping a large amount of water producing the hydrogels. The nonplanar structure of LM13 complex probably causes difficulty in pi-stacking, prohibiting the gel formation. The UV-vis spectra show a blue shift of the pi-pi* transition band (354 nm) indicating H-aggregate formation but the pi-pi* band coupled with n-pi* transition (386 nm) shows a constant red shift by 7 nm, indicating independency of pi-stacking on the n-pi* transition in the different LM systems. The PL intensities of LM11 and LM31 gels become more quenched than the intensity of pure L due to formation of nonfluorescent complex (static quenching) in the gels. In the LM13 sol the degree of quenching is less than that of the gels because of absence of energy transfer through the junction points of gels. The increased lifetime values of LM gels compared to that of pure L is also indicative of H-aggregate formation. The PL intensity increases linearly with increase of temperature due to thinning of the fibers decreasing the exciton energy transfer. The emission peak shows a red shift with rise in temperature, indicating H- to J-aggregate transformation, and at the melting temperature it shows a sharp decrease. With both increase and decrease of pH from the neutral pH 7, the gels exhibit higher PL intensity because of sol formation.

Self-sustaining, fluorescent and semi-conducting co-assembled organogel of Fmoc protected phenylalanine with aromatic amines

N-Flourenylmethoxycarbonyl (Fmoc) phenylalanine (F) produces co-assembled organogel with 2-aminoanthracene (AA) and 2-aminonaphthalene (NA) at a 1 : 1 molar ratio of the components. The deep green co-assembled F-AA gel is rigid and can be cut into different shapes. At lower concentration, 0.2% (w/v), it shows a mixture of fibre and flake morphology, while at 1.5% (w/v) concentration only flake morphology is observed but the F-NA co-assembled gel produces tape morphology. The powder diffraction data of F-AA co-assembled gel indicate π–π stacking and lamellar packing which is supported by DFT calculation. The melting point of F gel is 15 °C higher over F-AA gel but the gel strength and stiffness of the F-AA co-assembled gel is 94 and 2.5 times higher than that of F gel. The F gel shows a smooth gel breaking point at 4 Pa but the F-AA co-assembled gel shows only a slippage at 160 Pa due to its high stiffness. The UV-vis spectra suggest the formation of H-aggregates and a charge transfer complex in the F-AA gel. The emission peak of AA shows a red shift in the F-AA co-assembled gel where both fluorescence intensity and peak position decrease with an increase in temperature. The F-AA xerogel shows semiconducting behaviour with a dc conductivity value 2.3 × 10−8 S cm−1 and the I–V characteristic curves indicate a semiconducting nature with a signature of negative differential resistance.

Improved mechanical and photophysical properties of chitosan incorporated folic acid gel possessing the characteristics of dye and metal ion absorption

The folic acid (F) gel, prepared in a 1:1 dimethyl sulfoxide (DMSO)–water mixture (v/v), at 0.2% (w/v) concentration has a nano-fibrous network morphology. FTIR spectra indicate the presence of intermolecular H-bonding interactions in the gel. The folic acid–chitosan (C) hybrid (FC) gel (prepared by mixing 0.1 ml of 1% (w/v) C solution with 2 ml 0.2% (w/v) F solution) appears as a yellow transparent semi-solid mass consisting of a nano-fibrous network with a higher density of branches. The fibrillar diameter of the FC hybrid gel (10.5 nm) is much lower than that of F gel (17.3 nm). The FTIR band at 3390 cm−1 of F gel shows a shift to 3435 cm−1 in the FC hybrid gel indicting a hydrogen bonding interaction between C and F. The fluorescence intensity of the FC hybrid gel is enhanced by 2.75 times than that of F gel. The shear viscosity of FC hybrid gel is 3 orders higher than that of the F gel and both gels exhibit shear thickening at a low shear rate (<10−3 s−1) but above that, shear-thinning occurs. In the creep phase, the FC hybrid gel exhibits strain recovery but the F gel does not. The complex modulus (G*) of both systems initially exhibit a sharp increase, followed by a slow increase with time (t) and dG*/dt varies with C concentration in the FC hybrid gel showing a maximum. The WAXS pattern of the FC hybrid xerogel does not exhibit any crystalline peak, suggesting that the additive (C) inhibits the crystallization of F. The dye absorption by the hybrid gel material is maximum for Eosin Yellow, (83.5%), however, Methyl Orange and Methylene Blue are absorbed to a lesser extent (∼40%). The FC hybrid gel absorbs 55% of Cu2+, 67.2% of Cr3+ and 49% of Co2+ from their respective solutions.

pH and anion sensitive silver(I) coordinated melamine hydrogel with dye absorbing properties: metastability at low melamine concentration

Ag(I)–melamine coordination polymer produces a metastable hydrogel at 1 : 1 molar ratio but 1 : 2 molar ratio produces a stable gel which has the helical fibrillar morphology, the ability of selective dye absorption and is highly sensitive to pH and addition of anions.

Supramolecular Differentiation for Construction of Anisotropic Fullerene Nanostructures by Time-Programmed Control of Interfacial Growth.

Supramolecular assembly can be used to construct a wide variety of ordered structures by exploiting the cumulative effects of multiple noncovalent interactions. However, the construction of anisotropic nanostructures remains subject to some limitations. Here, we demonstrate the preparation of anisotropic fullerene-based nanostructures by supramolecular differentiation, which is the programmed control of multiple assembly strategies. We have carefully combined interfacial assembly and local phase separation phenomena. Two fullerene derivatives, PhH and C12H, were together formed into self-assembled anisotropic nanostructures by using this approach. This technique is applicable for the construction of anisotropic nanostructures without requiring complex molecular design or complicated methodology.

Variation of physical and mechanical properties in the bicomponent hydrogels of melamine with positional isomers of hydroxybenzoic acid

The positional isomers of hydroxybenzoic acid have a significant influence on the physical and mechanical properties of their bicomponent hydrogels with melamine (M) produced at 1 : 1 molar composition. In the bicomponent hydrogels of salicylic acid (s), meta-hydroxy benzoic acid (m) and para-hdroxy benzoic acid (p), a gradation of morphology of the xerogels from tape to fibre occurs. WAXS study suggests different structures of the gels. FTIR spectra indicate that H-bonded complexes are produced in the gels. The aromatic protons in the gels become upfielded suggesting π–π stacking that follows the order Mp > Ms ≈ Mm. The UV-vis spectrum indicates better H-type aggregate formation in Mp than those in Ms and Mm systems. The photoluminescence (PL) intensity of the gels has increased by 200, 25 and 350 times those of pure acids for Ms, Mm and Mp gels, respectively. With the increase of temperature the PL intensity decreases and at 30 °C the highest PL intensity is observed at pH 6.8 but at pH 4 and 9.2 PL the intensity is drastically reduced due to breaking of the gel network. The melting point values suggest that the thermal stability of the gels follow the order Mp > Ms > Mm and both storage modulus (G′) and critical strain also follow the same order. In G′ vs. temperature plots, the Mp system exhibits a linear variation whereas the Ms and Mm systems exhibit peaks at 82 and 63 °C, respectively. This is attributed to the disaggregation of the gel macromolecule at high temperature forming a microgel and shear assisted aggregates.