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Item β-substituted donor-acceptor porphyrins: Synthesis, energy transfer and electrochemical redox properties(Elsevier, 2019-02) Grover, Nitikaβ-substituted D-A porphyrins have been synthesized via Suzuki coupling reaction and characterized by various spectroscopic techniques. Two different electron donating moieties i.e. 4-(carbazol-9-ylmethyl)phenyl and 4-(phenothiazin-10-ylmethyl)phenyl were appended at antipodal β-positions of electron acceptor moiety i.e. H2TPP(NO2). The absorption and emission data suggested an interaction between the donor and acceptor subunits. An efficient energy transfer (78–88%) from β-substituents to macrocyclic core was supported by steady state and time resolved fluorescence measurements. Density functional theory (DFT) and time dependent-density functional theory (TD-DFT) studies support the existence of intramolecular energy and charge transfer from β-donor substituents to macrocyclic core in the dyads. Förster energy transfer studies revealed that the nature of energy transfer was mainly through bond (χTB = 0.71–0.83) in nature. The basic nature of porphyrin core was elucidated by very high protonation constants (logβ2 = 9.59–10.45). The first and second oxidation potentials of 1, 2 and their Zn(II) complexes (1a and 2a) were cathodically shifted as compared to H2TPP(NO2) and ZnTPP(NO2) due to effective electron donating nature of β-substituents which led to lower HOMO-LUMO gap.Item Incremental Introduction of Organocatalytic Activity into Conformationally Engineered Porphyrins(Wiley, 2019-01) Grover, NitikaTo study the correlation of macrocycle nonplanarity and catalytic activity of free base porphyrins in detail, a series of six tetraphenylporphyrins with graded degree of β-ethyl substitution (“H2EtxTPPs” 1–6; x = 0, 2, 4, 6, 8) was applied in organocatalyzed reactions. The macrocycles display incrementally increasing nonplanarity due to repulsive peri-interactions. This creates an out-of-plane vector and better accessibility of the core amine and imine groups as the number of alkyl substituents increases. Following such a molecular engineering approach, the inner core system could be used to activate small molecules as a result of significant saddle distortion. The potential organocatalyst “H2EtxTPPs” were used in benchmark sulfa-Michael reactions and we found a distinct relationship between nonplanarity and conversion. These observations were attributed to the combined effect of enhanced basicity and increased H-bonding potential that could facilitate bifunctional organocatalysis. Ultimately, density functional theory (DFT) calculations were performed on 1–6 to monitor some electronic properties of the title compounds.Item Adsorption of hydrogen on single-walled carbon nanotubes with defects(Elsevier, 2015-10) Ghosh, SarbaniWe present molecular dynamics (MD) simulations and density functional theory (DFT) calculations of hydrogen adsorption on single-walled carbon nanotubes (SWCNT) with various kinds of defects. The nature of defects, which is characterized here by the number of carbon atoms present in a ring on the surface of nanotube, plays a significant role in determining the hydrogen adsorption capacity of the SWCNT. Nanotubes containing the Stone–Wales defect with 5 and 8-member rings were found to have the largest hydrogen adsorption ability that increases further with the number of rings with such defects. Whereas, the presence of defects with 5, 3-5-8-member rings and the Stone–Wales defect with 5 and 7-member rings decreases the adsorption ability of the defective SWCNT significantly with respect to defect-free nanotubes. Our results indicate that the huge discrepancies in hydrogen storage capacities of SWCNT reported in the literature could be attributed to the nature of defects present in nanotubes. DFT calculations also reveal that the adsorption energy depends not only on the nature and number of defects present on the surface of nanotube but also on the equilibrium structure of rings.Item Hydrogen storage in Titanium-doped single-walled carbon nanotubes with Stone-Wales defects(Elsevier, 2017) Ghosh, SarbaniThe hydrogen storage capacity of Titanium-doped single-walled carbon nanotubes (SWCNT) containing the Stone-Wales 5,8 defects was studied using molecular dynamics simulations. The equilibrium doping sites and their stability were estimated using density functional theory. Although introduction of structural defects and dopant atom decreases the formation and cohesive energies, the drop in these energies is not large enough to hinder the thermodynamic feasibility of formation of these structures. Moreover, we observed that the stability of SWCNTs, where Ti is doped by replacing two carbon atoms is similar to that of the defect-free nanotube. This particular novel configuration (D5) was also obtained by rearranging the bonds in the 5 and 8-member rings of the Stone-Wales defect. Doping Ti on the defective rings has a more significant effect on the adsorption of hydrogen than doping on the regular 6-member rings. The D5 SWCNT showed the highest gravimetric and volumetric storage capacities at a temperature of 298K and a moderate pressure of 140atm. We also compared the performance of the D5 SWCNT with a recently reported Ti-doped porphyrin SWCNT and observed that the storage capacity of the D5 SWCNT was significantly higher at similar conditions. Our results suggest that Ti-doped SWCNTs with the Stone-Wales (5,8) defects show a promising potential to meet the ultimate goal set by the US Department of Energy for hydrogen storage.Item Hydrogen adsorption in pyridine bridged porphyrin-covalent organic framework(Elsevier, 2019) Ghosh, SarbaniCovalent organic frameworks (COFs), a class of carbon-based polymeric materials have the potential to be used as hydrogen adsorbent. Three dimensional (3D) COFs, due to their low density and high surface area, although have higher hydrogen adsorption, they have less stability than two dimensional (2D) COFs. Here we studied porphyrin group containing 2D COF, namely H2,P-COF for hydrogen storage using density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations and the results were compared with the most common 2D COFs, COF-1 and COF-5. Cylindrical shaped 2D COFs where isolated unit blocks are stacked in multiple layers due to van der Waals interactions between individual layers, increase the effective surface area for hydrogen storage. A further modification has been done by bridging the inter-layer gap by pyridine molecules. Insertion of pyridine increases the separation distance of layers of 2D COFs as well as the free volume. Feasibility of the structure formation and stability of all the structures were checked using DFT study. To ensure the structural stability of bridged COFs after hydrogen loading, alternating layers of COF were bridged. Single, bi, tri and tetra -pyridine molecules were chemically bonded with the existing carbon ring present in between two C2O2B rings to form pyridine bridged H2,P-COFs. Our GCMC results show a significant increase in storage capacity which is mainly due to an increase in the free volume of the material. The highest capacity of 5.1 wt% and 20 g H2/L at 298 K and 100 bar, above the gravimetric DOE goal, has been found at room temperature for tetra-pyridine doped porphyrin COF structure.Item qPlus atomic force microscopy of the Si(100) surface: Buckled, split-off, and added dimers(AIP, 2009-08) Gangopadhyay, SubhashisDimer configurations at the Si(100) surface have been studied with noncontact atomic force microscopy in the qPlus mode at 77 K, using both large (10 nm peak to peak) and small (0.5 nm peak to peak) oscillation amplitudes. In addition to the , , and reconstructions of the pristine surface, a variety of defect types including ad-dimers, vacancies, and split-off dimers have been imaged. Our data appear at odds with the currently accepted structural model for split-off dimers. At low oscillation amplitudes the degree of apparent dimer buckling can be “tuned” by varying the frequency shift set point.Item First-Principal Study of the Effect of Ti Doping on Hydrogenation of Mgn (N=2-8) Nanoclusters(SSRN, 2023-02) Bandyopadhyay, DebashisWe report the effect of single Ti atom doping Mgn (n=2-8) clusters on hydrogenation kinetics using density functional theory (DFT). During growth process, variations of different energetic parameters, such as binding energy, stability, etc., with the size of the clusters, Mg4 and TiMg5, are found as thermodynamically stable. From chemisorption energies in hydrogenated Mgn and TiMgn clusters, we have estimated the catalytic effect of Ti doping in Mgn clusters to improve the dose of hydrogen. It is found that the activation barrier reduces by about 32.8% after Ti doping, as evident from the IRC study. ELF mapping indicates that the Ti doping increase the reactive sites in the Mg5 cluster, and the TiMg5 can absorb extra hydrogen molecule to attain the average binding energy of Mg5-2H. So, Ti doping in the Mg5 cluster improves the hydrogenation process as well as the dose concentration because of its catalytic activitiesItem The role of oxygen defects in the electronic, optical and phonon dispersion of the LAGO perovskite: a density functional theory investigation(RSC, 2023-10) Bandyopadhyay, DebashisThe study aims to investigate the electronic, optical and phonon dispersion properties of a pure and 2.5% O-defect induced LAGO perovskite, using density functional theory (DFT) with generalized gradient approximation (GGA) and the PBE functional. The research reveals a significant reduction in the band gap from 3.27 eV in pure LAGO to 2.18 eV in defect-induced LAGO. The defect-induced LAGO exhibits relatively strong light absorption in the visible region compared to pure LAGO. The phonon-dispersion analysis identifies one acoustic and two transverse optical mode branches. The calculated Debye temperatures for pure and defect-induced systems are 469.92 K and 463.69 K, respectively, attributed to weaker bonds in defect-induced LAGO. The findings offer fundamental insights into the impact of oxygen vacancies on the electronic, optical, and phonon properties of the LAGO perovskite that can potentially improve the electronic and optoelectronic devices operating across a wide range of spectral frequencies.Item Insights into the electronic structure and stability of TiMgn (n = 1–12) clusters: Validation of electron counting rule(Elsevier, 2022-08) Bandyopadhyay, DebashisThe present study reports the investigation of the electronic structure and stability of TiMgn (n = 1–12) nanoclusters in the framework of the linear combination of atomic orbital density functional theory (DFT) together with a spin-polarized generalized gradient approximation (GGA). We have calculated different thermodynamic and chemical parameters, such as binding energy (BE), embedding energy (EE), fragmentation energy (FE), HOMO-LUMO gap, vertical ionization potential (VIP), vertical electron affinity (VEA), etc. during the growth process of the cluster to understand their stability. From the study, the TiMg8 cluster comes out to be the most stable cluster. Further, the positive charge (Mulliken charge) on the Ti in TiMg8 cluster indicates that the Mg8-cage behaves as an electron acceptor during hybridization with the Ti. The absence of any reactive site (obtained from NBO), minimum Mg-Ti bond length, and negative NICS in TiMg8 ground state cluster support it as a globally stable cluster. Low ELF index value revealing the non-reactivity of the TiMg8 cluster. Finally, orbital analysis shows that TiMg8 clusters follow closed-cell electronic orbital sequence 1S21P61D102S2 and satisfy the 20-electron counting rule. IR and Raman spectrum calculations are also presented to understand the vibrational nature of the clusters.Item Magnetic behavior in (1≤n≤12) clusters: A density functional investigation(AIP, 2014-04) Bandyopadhyay, DebashisWith a goal to produce magnetic moment in Doped clusters which will be useful for practical applications, we have considered the structure and magnetic properties of Pure Germanium clusters and substitutionally doped it with Cr dimer to produce clusters. As the first step of calculation, geometrical optimizations of the nanoclusters have been done. These optimized geometries have been used in calculate the average binding energy per atom (BE), HOMO-LUMO gap and hence the relative stability of the clusters. These parameters have been demonstrated as structural and electronic properties of the clusters. Gap between highest occupied molecular orbital and lowest unoccupied molecular orbital indicate cluster to be a potential motif for generating magnetic cluster assembled materials. Based on these values a comparative study on different sized clusters has been done in order to understand the origin of structures, electronic and magnetic properties of nanoclusters.