Bappaditya Roy

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.

Time sensitive, temperature and pH responsive photoluminescence behaviour of a melamine containing bicomponent hydrogel

Melamine (M) produces supramolecular complexes MQ31, MQ11 and MQ13 with 6,7-dimethoxy-2,4[1H,3H]-quinazolinedione (Q) (numbers indicate respective molar ratios of components) and they produce thermoreversible hydrogels [≥0.1% (w/v)] at 30 °C. Optical microscopy shows fibrillar network morphology and on irradiation with 300 nm light, the fibrils emit blue light. DSC results indicate a thermoreversible first order phase transition and the storage modulus (G′) is invariant with frequency supporting the formation of thermoreversible hydrogel. The magnitude of G′ follows the order MQ11 > MQ13 > MQ31 and a probable cause is attributed to number of H-bonds, which follows the same order in the nanofibers. FTIR-spectra suggest H-bonding interaction between components and WAXS results indicate a different crystalline structure of the complexes. The UV-vis spectra of the MQ systems shows red shift of both π–π* and n–π* bands and a maximum red shift is observed for the MQ13 system. The PL intensity decreases with increase in M concentration and lifetime data indicates PL quenching due to the formation of less fluorescent complexes. The PL property of the gel depends on aging time. A gradual development of a new emission peak at the expanse of an initial peak suggests the formation of an initial metastable state, which transforms into the stable state after 24 h of aging. The gel is stable in the pH range 6–9, above or below which the gel breaks down showing a significant PL-quenching and a blue shift of the emission peak. With increase of temperature, the emission peak intensity increases at first up to 45 °C, followed by an abrupt decrease at ≥55 °C. Possible reasons are attributed to the gradual disassembly of the complexes converting thinner fibers and finally melting.

Cyclization-induced turn-on fluorescence system applicable to dicarboxylate sensing

A novel tetraphenylethene-based fluorescence (FL) chemosensor exhibits nonlinear turn-on FL switching though cooperative binding of L-tartarate, where its convergent binding to form cyclic substructures is responsible for the FL increase. This binding scheme achieves selective detection of dicarboxylates over monocarboxylates, thus is potentially applicable to the preliminary screening for metabolic disorders.

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.

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.

Translation of Dicarboxylate Structural Information to Fluorometric Optical Signals through Self‐Assembly of Guanidinium‐Tethered Oligophenylenevinylene

Although self-assembly has realized the spontaneous formation of nanoarchitectures, the nanoscopic expression of chemical structural information at the molecular level can alternatively be regarded as a tool to translate molecular structural information with high precision. We have found that a newly developed guanidinium-tethered oligophenylenevinylene exhibits characteristic fluorescence (FL) responses toward L- and meso-tartarate, wherein the different self-assembly modes, termed J- or H-type aggregation, are directed according to the molecular information encoded as the chemical structure. This morphological difference originates from the geometric anti versus gauche conformational difference between L- and meso-tartarate. A similar morphological difference can be reproduced with the geometric C=C bond difference between fumarate and maleate. In the present system, the dicarboxylate structural information is embodied in the inherent threshold concentration of the FL response, the signal-to-noise ratio, and the maximum FL wavelength. These results indicate that self-assembly is meticulous enough to sense subtle differences in molecular information and thus demonstrate the potential ability of self-assembly for the expression of a FL sensory system.

Metastability in a bi-component hydrogel of thymine and 6-methyl-1,3,5-triazine-2,4-diamine: ultrasound induced vs. thermo gelation

A new metastable bi-component hydrogel of thymine and 6-methyl-1,3,5-triazine-2,4-diamine, that can also cage a number of organic solvents, has different stabilities depending on its synthesis; the ultrasound induced gels transform more quickly into a crystalline precipitate than the thermo gels and the structural, thermal and photoluminescence properties indicate that the Ostwald ripening is the probable cause of the metastability.