Pathik Sahoo

Gel Sculpture: Moldable, Load‐Bearing and Self‐Healing Non‐Polymeric Supramolecular Gel Derived from a Simple Organic Salt

An easy access to a library of simple organic salts derived from tert-butoxycarbonyl (Boc)-protected L-amino acids and two secondary amines (dicyclohexyl- and dibenzyl amine) are synthesized following a supramolecular synthon rationale to generate a new series of low molecular weight gelators (LMWGs). Out of the 12 salts that we prepared, the nitrobenzene gel of dicyclohexylammonium Boc-glycinate (GLY.1) displayed remarkable load-bearing, moldable and self-healing properties. These remarkable properties displayed by GLY.1 and the inability to display such properties by its dibenzylammonium counterpart (GLY.2) were explained using microscopic and rheological data. Single crystal structures of eight salts displayed the presence of a 1D hydrogen-bonded network (HBN) that is believed to be important in gelation. Powder X-ray diffraction in combination with the single crystal X-ray structure of GLY.1 clearly established the presence of a 1D hydrogen-bonded network in the xerogel of the nitrobenzene gel of GLY.1. The fact that such remarkable properties arising from an easily accessible (salt formation) small molecule are due to supramolecular (non-covalent) interactions is quite intriguing and such easily synthesizable materials may be useful in stress-bearing and other applications.

Supramolecular Synthons in Designing Low Molecular Mass Gelling Agents: l-Amino Acid Methyl Ester Cinnamate Salts and their Anti-Solvent-Induced Instant Gelation

Easy access to a class of chiral gelators has been achieved by exploiting primary ammonium monocarboxylate (PAM), a supramolecular synthon. A combinatorial library comprising of 16 salts, derived from 5 L-amino acid methyl esters and 4 cinnamic acid derivatives, has been prepared and scanned for gelation. Remarkably, 14 out of 16 salts prepared (87.5 % of the salts) show moderate to good gelation abilities with various solvents, including commercial fuels, such as petrol. Anti-solvent induced instant gelation at room temperature has been achieved in all the gelator salts, indicating that the gelation process is indeed an aborted crystallization phenomenon. Rheology, optical and scanning electron microscopy, small angle neutron scattering, and X-ray powder diffraction have been used to characterize the gels. A structure-property correlation has been attempted, based on these data, in addition to the single-crystal structures of 5 gelator salts. Analysis of the FT-IR and (1)H NMR spectroscopy data reveals that some of these salts can be used as supramolecular containers for the slow release of certain pest sex pheromones. The present study clearly demonstrates the merit of crystal engineering and the supramolecular synthon approach in designing new materials with multiple properties.

Naked-eye discrimination of methanol from ethanol using composite film of oxoporphyrinogen and layered double hydroxide.

Methanol is a highly toxic substance, but it is unfortunately very difficult to differentiate from other alcohols (especially ethanol) without performing chemical analyses. Here we report that a composite film prepared from oxoporphyrinogen (OxP) and a layered double hydroxide (LDH) undergoes a visible color change (from magenta to purple) when exposed to methanol, a change that does not occur upon exposure to ethanol. Interestingly, methanol-induced color variation of the OxP-LDH composite film is retained even after removal of methanol under reduced pressure, a condition that does not occur in the case of conventional solvatochromic dyes. The original state of the OxP-LDH composite film could be recovered by rinsing it with tetrahydrofuran (THF), enabling repeated usage of the composite film. The mechanism of color variation, based on solid-state (13)C-CP/MAS NMR and solution-state (13)C NMR studies, is proposed to be anion transfer from LDH to OxP triggered by methanol exposure.

Combinatorial library of primaryalkylammonium dicarboxylate gelators: a supramolecular synthon approach.

Following the supramolecular synthon approach, a combinatorial library comprising 35 organic salts derived from 7 dicarboxylic acids (malonic-, succinic-, adipic-, L-tartaric-, maleic-, phthalic-, and isophthalicacid) and 5 primaryalkyl amines Me-(CH2)n-NH2 (n = 11-15) was prepared and scanned for gelation. About 66% of the salts in the combinatorial library were found to show moderate to good gelling ability in various polar and nonpolar solvents including commercial fuels such as petrol. The majority of the salts having a rigid, unsaturated anionic backbone (maleate, phthalate, and isophthalate) did not show gelation; only the corresponding hexadecylammonium salts showed gelation. Some of the representative gels were characterized by rheology, small-angle neutron scattering (SANS), optical microscopy (OM), and scanning electron microscopy (SEM). Single-crystal structures of two gelator and two nongelator salts were also discussed in the context of supramolecular synthon and structure-property correlation.

Dynamic Breathing of CO2 by Hydrotalcite

The carbon cycle of carbonate solids (e.g., limestone) involves weathering and metamorphic events, which usually occur over millions of years. Here we show that carbonate anion intercalated layered double hydroxide (LDH), a class of hydrotalcite, undergoes an ultrarapid carbon cycle with uptake of atmospheric CO2 under ambient conditions. The use of (13)C-labeling enabled monitoring by IR spectroscopy of the dynamic exchange between initially intercalated (13)C-labeled carbonate anions and carbonate anions derived from atmospheric CO2. Exchange is promoted by conditions of low humidity with a half-life of exchange of ~24 h. Since hydrotalcite-like clay minerals exist in Nature, our finding implies that the global carbon cycle involving exchange between lithosphere and atmosphere is much more dynamic than previously thought.

Rapid Exchange between Atmospheric CO2 and Carbonate Anion Intercalated within Magnesium Rich Layered Double Hydroxide

The carbon cycle, by which carbon atoms circulate between atmosphere, oceans, lithosphere, and the biosphere of Earth, is a current hot research topic. The carbon cycle occurring in the lithosphere (e.g., sedimentary carbonates) is based on weathering and metamorphic events so that its processes are considered to occur on the geological time scale (i.e., over millions of years). In contrast, we have recently reported that carbonate anions intercalated within a hydrotalcite (Mg0.75Al0.25(OH)2(CO3)0.125·yH2O), a class of a layered double hydroxide (LDH), are dynamically exchanging on time scale of hours with atmospheric CO2 under ambient conditions. (Ishihara et al., J. Am. Chem. Soc. 2013, 135, 18040-18043). The use of (13)C-labeling enabled monitoring by infrared spectroscopy of the dynamic exchange between the initially intercalated (13)C-labeled carbonate anions and carbonate anions derived from atmospheric CO2. In this article, we report the significant influence of Mg/Al ratio of LDH on the carbonate anion exchange dynamics. Of three LDHs of various Mg/Al ratios of 2, 3, or 4, magnesium-rich LDH (i.e., Mg/Al ratio = 4) underwent extremely rapid exchange of carbonate anions, and most of the initially intercalated carbonate anions were replaced with carbonate anions derived from atmospheric CO2 within 30 min. Detailed investigations by using infrared spectroscopy, scanning electron microscopy, powder X-ray diffraction, elemental analysis, adsorption, thermogravimetric analysis, and solid-state NMR revealed that magnesium rich LDH has chemical and structural features that promote the exchange of carbonate anions. Our results indicate that the unique interactions between LDH and CO2 can be optimized simply by varying the chemical composition of LDH, implying that LDH is a promising material for CO2 storage and/or separation.

Ferrocene based organometallic gelators: a supramolecular synthon approach

The supramolecular synthon namely primary ammonium dicarboxylate (PAD) synthon has been exploited to generate a new series of salt based low molecular weight gelators (LMWGs) derived from ferrocenedicarboxylic acid (FDCA) and primary amines Me–(CH2)n–NH2 (n = 3–15). While most of the salts are capable of forming gels with DMSO and DMF, nearly all of them show a tendency to form gels with at least one of the solvents studied. Structure property correlation based on single crystal and powder X-ray diffraction data in combination with optical-, scanning-, and transmission-electron microscopy reveals that the supramolecular synthon approach adopted herein for designing LMWGs is indeed useful and allows one to get an easy access to a new series of organometallic gelators.

Reverse thermal gelation of aromatic solvents by a series of easily accessible organic salt based gelators

Exploiting crystal engineering and supramolecular synthon concepts, a series of new gelator salts based on primary ammonium dicarboxylate (PAD) salts of azobenzene-4,4′-dicarboxylic acid and primary alkyl amines have been synthesized and characterized by various physico-chemical techniques. Most of the salts were shown to form gels with various solvents. Interestingly, most of the gelator salts possessed rarely observed reverse-thermal gelation (gelation with the rise of temperature) ability of aromatic solvents which may be relevant in developing thermo-responsive materials.

Multinuclear solid-state NMR spectroscopy of a paramagnetic layered double hydroxide

NMR spectroscopy is rarely utilized when analytes include paramagnetic ions. We show that multinuclear solid-state NMR is actually potentially useful for investigating nanostructures of a layered double hydroxide (LDH) containing paramagnetic Ni2+ cations. In particular, 13C NMR spectra of interlayer carbonate anions are well resolved by using conventional MAS/DD.

Efficient photocatalytic hydrogen evolution under visible light by ternary composite CdS@NU-1000/RGO

Herein, hydrogen production from an aqueous medium over a Zr(IV) metal–organic framework (NU-1000) under visible-light irradiation is presented. Photodeposition of CdS nanoparticles onto NU-1000 gives CdS@NU-1000. A ternary composite that hybridizes CdS with reduced graphene oxide (RGO) (different proportions) and NU-1000 was prepared (CdS@NU-1000/RGO). CdS@NU-1000 and CdS@NU-1000/1%RGO exhibit 9.35 and 12.1 times higher photocatalytic activities than commercial CdS under visible light, respectively. More importantly, in sharp contrast with the obvious deterioration in photoactivity of CdS, CdS@NU-1000/1%RGO displayed significantly enhanced photostability. This study clearly demonstrates the advantage of MOF supported CdS nanoparticles for enhancing the photocatalytic H2 evolution activity with good stability.