Ajeet Kumar Singh
@ictmumbai.edu.in
Ph.D. Scholar, Department of Chemistry
Institute of Chemical Technology Mumbai- IOC Odisha Campus Bhubaneswar
EDUCATION
Ph.D. Chemistry
RESEARCH, TEACHING, or OTHER INTERESTS
Chemistry, Physical and Theoretical Chemistry, Inorganic Chemistry, Electrochemistry
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
Ajeet Kumar Singh and Lisa Roy
Wiley
AbstractWater splitting is a potential pathway for hydrogen gas evolution and thereby realization of a carbon‐neutral sustainable energy scheme. However, oxidation of water to dioxygen is the major impediment in conversion of solar energy to fuel. Herein, density functional studies are conducted to explore the reactivity conduits of two molecular electro‐catalysts consisting of nickel and copper tetra‐anionic tetradentate amide ligand complexes of the type [(L1)MII]2−, where L1=o‐phenylenebis(oxamidate), and their substitutionally modified analogues. While nickel complexes demonstrate complex borderline chemistry between homogeneous and heterogeneous pathways, showing competition between water oxidation and molecular species degradation, copper complexes display robust and efficient molecular water oxidation behavior. Our analysis predict that this disparity is primarily due to the reversible O−O bond formation in nickel complexes, which provide the platform necessary for a direct attack of OH−/H+ on the metal and terminally accessible amidate groups of the 2e− oxidized anionic intermediate, [(L1⋅)NiIII(OH)]1−, respectively. This intermediate streamline ligand deactivation with a comparatively higher driving force for nickel complexes in acidic medium. Contrarily, the copper complexes display radical character on the hydroxyl ligand in the corresponding intermediate, [(L1⋅)CuII(OH⋅)]1−, that expedite O−O interaction, leading to predominant homogeneous water oxidation under all conditions.
Soumen Kuila, Ajeet Kumar Singh, Akash Shrivastava, Sukantha Dey, Tukai Singha, Lisa Roy, Biswarup Satpati, and Jayanta Nanda
American Chemical Society (ACS)
In this work, 1,8-naphthalimide (NMI)-conjugated three hybrid dipeptides constituted of a β-amino acid and an α-amino acid have been designed, synthesized, and purified. Here, in the design, the chirality of the α-amino acid was varied to study the effect of molecular chirality on the supramolecular assembly. Self-assembly and gelation of three NMI conjugates were studied in mixed solvent systems [water and dimethyl sulphoxide (DMSO)]. Interestingly, chiral NMI derivatives [NMI-βAla-lVal-OMe (NLV) and NMI-βAla-dVal-OMe (NDV)] formed self-supported gels, while the achiral NMI derivative [NMI-βAla-Aib-OMe, (NAA)] failed to form any kind of gel at 1 mM concentration and in a mixed solvent (70% water in DMSO medium). Self-assembly processes were thoroughly investigated using UV-vis spectroscopy, nuclear magnetic resonance (NMR), fluorescence, and circular dichroism (CD) spectroscopy. A J-type molecular assembly was observed in the mixed solvent system. The CD study indicated the formation of chiral assembled structures for NLV and NDV, which were mirror images of one another, and the self-assembled state by NAA was CD-silent. The nanoscale morphology of the three derivatives was studied using scanning electron microscopy (SEM). In the case of NLV and NDV, left- and right-handed fibrilar morphologies were observed, respectively. In contrast, a flake-like morphology was noticed for NAA. The DFT study indicated that the chirality of the α-amino acid influenced the orientation of π-π stacking interactions of naphthalimide units in the self-assembled structure that in turn regulated the helicity. This is a unique work where molecular chirality controls the nanoscale assembly as well as the macroscopic self-assembled state.
Payel Khanra, Ajeet Kumar Singh, Lisa Roy, and Anindita Das
American Chemical Society (ACS)
This study unravels the intricate kinetic and thermodynamic pathways involved in the supramolecular copolymerization of the two chiral dipolar naphthalene monoimide (NMI) building blocks (O-NMI and S-NMI), differing merely by a single heteroatom (oxygen vs sulfur). O-NMI exhibits distinct supramolecular polymerization features as compared to S-NMI in terms of its pathway complexity, hierarchical organization, and chiroptical properties. Two distinct self-assembly pathways in O-NMI occur due to the interplay between the competing dipolar interactions among the NMI chromophores and amide-amide hydrogen (H)-bonding that engenders distinct nanotapes and helical fibers, from its antiparallel and parallel stacking modes, respectively. In contrast, the propensity of S-NMI to form only a stable spherical assembly is ascribed to its much stronger amide-amide H-bonding, which outperforms other competing interactions. Under the thermodynamic route, an equimolar mixture of the two monomers generates a temporally controlled chiral statistical supramolecular copolymer that autocatalytically evolves from an initially formed metastable spherical heterostructure. In contrast, the sequence-controlled addition of the two monomers leads to the kinetically driven hetero-seeded block copolymerization. The ability to trap O-NMI in a metastable state allows its secondary nucleation from the surface of the thermodynamically stable S-NMI spherical "seed", which leads to the core-multiarmed "star" copolymer with reversibly and temporally controllable length of the growing O-NMI "arms" from the S-NMI "core". Unlike the one-dimensional self-assembly of O-NMI and its random co-assembly with S-NMI, which are both chiral, unprecedentedly, the preferred helical bias of the nucleating O-NMI fibers is completely inhibited by the absence of stereoregularity of the S-NMI "seed" in the "star" topology.
Saurajit Ghosh, Himanshi Bhambri, Ajeet Kumar Singh, Sanjay K. Mandal, Lisa Roy, and Partha Sarathi Addy
Royal Society of Chemistry (RSC)
A convenient two step synthetic protocol is reported to obtain a mechanochromic luminescent molecule with switchable optical property.
Ajeet Kumar Singh and Lisa Roy
American Chemical Society (ACS)
Increased demand for a carbon-neutral sustainable energy scheme augmented by climatic threats motivates the design and exploration of novel approaches that reserve intermittent solar energy in the form of chemical bonds in molecules and materials. In this context, inspired by biological processes, artificial photosynthesis has garnered significant attention as a promising solution to convert solar power into chemical fuels from abundantly found H2O. Among the two redox half-reactions in artificial photosynthesis, the four-electron oxidation of water according to 2H2O → O2 + 4H+ + 4e– comprises the major bottleneck and is a severe impediment toward sustainable energy production. As such, devising new catalytic platforms, with traditional concepts of molecular, materials and biological catalysis and capable of integrating the functional architectures of the natural oxygen-evolving complex in photosystem II would certainly be a value-addition toward this objective. In this review, we discuss the progress in construction of ideal water oxidation catalysts (WOCs), starting with the ingenuity of the biological design with earth-abundant transition metal ions, which then diverges into molecular, supramolecular and hybrid approaches, blurring any existing chemical or conceptual boundaries. We focus on the geometric, electronic, and mechanistic understanding of state-of-the-art homogeneous transition-metal containing molecular WOCs and summarize the limiting factors such as choice of ligands and predominance of environmentally unrewarding and expensive noble-metals, necessity of high-valency on metal, thermodynamic instability of intermediates, and reversibility of reactions that create challenges in construction of robust and efficient water oxidation catalyst. We highlight how judicious heterogenization of atom-efficient molecular WOCs in supramolecular and hybrid approaches put forth promising avenues to alleviate the existing problems in molecular catalysis, albeit retaining their fascinating intrinsic reactivities. Taken together, our overview is expected to provide guiding principles on opportunities, challenges, and crucial factors for designing novel water oxidation catalysts based on a synergy between conventional and contemporary methodologies that will incite the expansion of the domain of artificial photosynthesis.
Souvik Misra, Pijush Singh, Ajeet Kumar Singh, Lisa Roy, Soumen Kuila, Sukantha Dey, Ajit K. Mahapatra, and Jayanta Nanda
American Chemical Society (ACS)
Helical supramolecular architectures play important structural and functional roles in biological systems. The helicity of synthetic molecules can be tuned mainly by the chiral manipulation of the system. However, tuning of helicity by the achiral unit of the molecules is less studied. In this work, the helicity of naphthalimide-capped peptide-based gel nanofibers is tuned by the alteration of methylene units present in the achiral amino acid. The inversion of supramolecular helicity has been extensively studied by CD spectroscopy and morphological analysis. The density functional theory (DFT) study indicates that methylene spacers influence the orientation of π-π stacking interactions of naphthalimide units in the self-assembled structure that regulates the helicity. This work illustrates a new approach to tuning the supramolecular chirality of self-assembled biomaterials.
Aritra Rajak, Ajeet Kumar Singh, Lisa Roy, and Anindita Das
Wiley
AbstractMerocyanine dye assembly in nonpolar solvents is driven by electrostatic dipole‐dipole interactions, which make hierarchical structures of merocyanine less favorable, due to the compensation of its dipole moment in the discrete antiparallel dimer state. Herein, we describe the self‐assembly of a merocyanine dye (MC−OH) into higher aggregates in aqueous medium (with 10% dioxane) by synergistic effect of dipole‐dipole interactions and strong dispersion forces, which remains underexplored for its known molecular stabilization in polar solvents. Our results reveal that in the crystal packing, strong intermolecular hydrogen (H)‐bonding predominates over the dipole‐dipole interactions, which confines the dye into a head‐to‐head parallel π‐stacked assemblage. When intermolecular H‐bonding in water is curtailed, antiparallel dimers by dipolar interactions become predominant. Several of these antiparallel dimers laterally cluster through solvophobically‐induced π‐stacking and form stable nanodiscs, which exhibit efficient hydrophobic dye sequestering properties. At higher concentration, the nanodiscs are transformed into elongated nanotapes. The computational studies support the experimental findings and emphasize the competing nature of multiple noncovalent interactions in guiding the dye assembly under different conditions.
Akshoy Jamadar, Ajeet Kumar Singh, Lisa Roy, and Anindita Das
Royal Society of Chemistry (RSC)
Naphthalene monoimide derivatives produced distinct stimuli-responsive luminescent nanostructures through orthogonal dipole–dipole interactions and halogen bonding (XB) with diverse graftable XB donors and acceptors in organic solvents.