Narrow Results

In this paper, we have investigated the structural, electronic and magnetic properties of Ga₁₂N₁₂ cluster doped with monodoped and bidoped Fe atoms within the density functional theory (DFT). Substitutional, exohedral and endohedral doping are considered. It is observed that both monodoped and bidoped clusters tend to be in exohedral doping. Mulliken population analysis is performed to obtain the charge transfer and magnetic moment. The magnetic moment is mainly derived from 3d orbitals of Fe atom for all isomers, while the magnetic properties would rely on the Fe–Fe distance.

Electronic structure calculations of all 10 unique base pair (bp) steps have been calculated to study the interaction energies of the bp steps, density of states (DOS), projected density of states (pDOS) using the density functional theory (DFT). Plane wave basis with ultrasoft pseudo-potential method has been used within the local density approximation (LDA) for the exchange correlation functional. Electron densities of the bp steps corresponding to HOMO and LUMO level have been calculated to understand the difference in stacking energies of the bp steps. The variation of HOMO–LUMO gap (g) of all possible bp steps on twist angle has been studied. We have observed that out of the 10 bp steps, 4 purine–purine bp steps (d(AA), d(GG), d(AG) and d(GA)), show significant variation of $g$ on twist angle. The observed variation on twist angle of d(AA) bp step has been explained by the calculated DOS and electron densities.

Single-molecule magnets (SMMs) have unique magnetic properties such as quantum tunneling of magnetization and quantum coherent oscillation, which have potential applications in quantum computation and information storage. In this paper, using the tip-deposition method, we have grafted individual Fe₄ molecules onto the semi-metallic Bi(111) surface. Low temperature scanning tunneling microscope (LT-STM) was used to characterize the molecular morphology and electronic structures. It was found that individual Fe₄ molecules reveal a triangle shape, which is consistent with the molecular structure of Fe₄. Scanning tunneling spectroscopy (STS) analysis indicated that the HOMO–LUMO gap is 0.49 eV. These studies provide direct information about the adsorption of individual SMMs on semi-metal surfaces.

To investigate the microscopic second-order nonlinear optical (NLO) behavior of the 1-salicylidene-3-thio-semicarbazone Schiff base compound, the electric dipole moments (μ), linear static polarizabilities (α) and first static hyperpolarizabilites (β) have been calculated using finite field second-order Møller-Plesset perturbation (FF MP2) theory. The ab-initio results on (hyper)polarizabilities show that the investigated molecule might have microscopic NLO properties with non-zero values. To understand the NLO behavior in the context of molecular orbital structure, we have also examined the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) and the HOMO-LUMO gap in the same theoretical framework as the (hyper)polarizability calculations. In addition to the NLO properties, the electronic transition spectra have been computed using a semi-empirical method (ZINDO). ZINDO calculation results show that the electronic transition wavelengths have been estimated to be shorter than 400 nm.

The stability, structural, electronic, and optical properties have been studied for most stable zinc sulfide nanoclusters Zn_xS_y (x + y = n = 2 to 4). A B3LYP-DFT/6-311G(3df) method is employed to optimize the geometries, and a TDDFT method is used for the study of the optical properties. The binding energies (BE), HOMO–LUMO gaps and the bond lengths have been obtained for all the clusters. We have considered also the zero point energy (ZPE) corrections ignored by the earlier workers. For a fixed value of n, we designate the most stable structure the one, which has maximum final binding energy per atom. The adiabatic and vertical ionization potentials (IP) and electron affinities (EA), charge on atoms, dipole moment, and optical properties have been investigated for the most stable structures. The nanoclusters containing large number of S atoms for each "n" are found to be most stable. Except for ZnS nanocluster, the HOMO–LUMO gap increases with the number of S atoms. Similarly, except for ZnS, IP and EA fluctuate with the cluster size but reveal downward trend. The optical absorption is quite weak in visible region but is strong in the ultraviolet region in most of the nanoclusters except a few. The growth of most stable nanoclusters may be possible in the experiments.

We have investigated the lowest-energy structures and electronic properties of the Cu_n(n = 2–10) nanoclusters based on density functional theory (DFT) in local density approximation. The total energies, binding energies per atom, bond lengths, HOMO-LUMO gaps and ionization potentials have been calculated. The results are compared well with other theoretical and available experimental results.

Facile synthesis of 3,17-disubstituted Ag(III) tritolylcorroles (2-5), R₂[TTC]Ag where R = methyl (2), phenyl (3), methyl acrylate (MA) (4) and phenylethynyl (PE) (5) using Pd-catalyzed reactions in good to excellent yields are reported. All synthesized corroles were characterized by various spectroscopic techniques and mass spectrometry. MA₂[TTC]Ag (4) and PE₂[TTC]Ag (5) exhibited highly red-shifted electronic spectral bands with considerable broadening due to extended $\pi$-conjugation and electron withdrawing effect of $\beta$-substituents. Geometry optimization of these corroles was performed using density functional theory (DFT). Among all, MA₂[TTC]Ag (4) exhibited very high dipole moment (10.31 D) which could be the potential candidate for nonlinear optical (NLO) applications. The redox tunability was achieved by substituting electron donating and withdrawing substituents at the $\beta$-positions. Particularly, corroles 4 and 5 exhibited lower HOMO–LUMO gap due to extended $\pi$-conjugation and electron withdrawing $\beta$-substituents.

Antipodal β-diformyl porphyrin (H₂TPP(CHO)₂) and its metal complexes (MTPP(CHO)₂, M = ${\text{Co}}^{\text{II}}$, ${\text{Ni}}^{\text{II}}$, ${\text{Cu}}^{\text{II}}$, and ${\text{Zn}}^{\text{II}})$ are synthesized and characterized by various spectroscopic techniques. Diformyl porphyrins possess two electron-withdrawing formyl substituents at their β-pyrrole positions making them electron-deficient macrocycles. UV-Vis absorption spectra of MTPP(CHO)₂ are red-shifted as compared to β-monoformyl/hydroxymethyl porphyrins and a further large anodic shift ($\mathrm{\Delta }$${\text{E}}_{1/2}$ = 0.15 - 0.52 V) was observed in their redox potentials. Hammett plots of MTPP(X)(Y) (X = CHO, CH₂OH; Y = H, CHO) showed that the HOMO-LUMO energy gap decreases with the increment in the Hammett parameter ($\sigma )$ of the substituents. DFT studies are performed for different positional isomeric diformyl porphyrins which reveal that the position of β-formyl groups affects the geometry of the porphyrin core while the symmetrical nature of 2,13-diformyl porphyrin and longer distance between two formyl groups bring quasiplanar conformation of the diformyl porphyrin macrocycle.

Two $\beta$-substituted Pd(II) porphyrins, (PdTPP(NO${}_2)$Ph₂ and PdTPP(Ph)${}_4)$, were synthesized and characterized by various spectroscopic techniques, and their spectral and electrochemical redox properties were investigated with respect to PdTPP. The single-crystal X-ray structure of PdTPP(NO${}_2)$-Ph₂ revealed saddle-shape conformation of the macrocyclic core of porphyrin. DFT studies revealed nonplanar saddle shape conformation of PdTPP(Ph)₄ with an average deviation of $\beta$-pyrrole carbon atoms from the porphyrin mean plane ($\mathrm{\Delta }$C${}_{\beta }$ = ± 0.751 Å) and also 24 core atoms ($\mathrm{\Delta }$24 = ± 0.368 Å) as compared to PdTPP(NO${}_2)$Ph₂ ($\mathrm{\Delta }$C${}_{\beta }$ = 0.649 Å and $\mathrm{\Delta }$24 = ± 0.317 Å) due to more $\beta$-pyrrole substitution. Both PdTPP(NO${}_2)$Ph₂ and PdTPP(Ph)₄ exhibited 13-19 nm and 49-59 nm red-shift in the Soret and Q${}_{x(0,0)}$ bands as compared to PdTPP, respectively. PdTPP(NO${}_2)$Ph₂ exhibited a very high dipole moment (7.32 D) as compared to PdTPP(Ph)₄ (0.239 D) due to the “push-pull” effect of electron donor (phenyl) and acceptor (-NO${}_2)$ groups at the $\beta$-pyrrole positions of the macrocycle. The redox potentials of PdTPP(NO${}_2)$Ph₂ anodically shifted ($\mathrm{\Delta }$E = 0.2-0.4V) as compared to PdTPP and PdTPP(Ph)₄ due to the presence of $\beta$-phenyl groups and the strong electron-withdrawing effect of the nitro group. Hence the HOMO-LUMO energy gap of PdTPP(NO${}_2)$Ph₂ decreased by 0.20 V and 0.28 V as compared to PdTPP(Ph)₄ and PdTPP, respectively.

The study of electron transport properties of molecular systems could be explained on the basis of the Landauer formalism. Unfortunately, due to the complexity of the experimental setup, most of these measurements have no control over the details of the electrode geometry, rotation of molecules, variation in angle of contacts, effect of fano resonances associated with side groups attached to rigid backbones, which results in a spectrum of IV-characteristics. Theoretical models can therefore help to understand and helps to develop new applications such as molecular sensors, etc. Thus we used simulation methods that generate the required structural ensemble, which is then analyzed with Green’s function methods to characterize the electronic transport properties. In present work we had discussed applications of this approach to understand the conductance in molecular system in the direction of controlling electron transport through molecules and studied the effect of rotation of sandwiched molecule.

Geometrical structures of (ZnSe)_n, $n=3x$, ($x=$ 1–4) and (Mn_xZn${}_{2x}$Se${}_{3x})$, ($x=1-4)$ clusters were calculated using density functional theory (DFT). Optical/absorption spectra, Raman spectra, HOMO–LUMO gap energy and binding energy of each cluster were calculated. The calculated results show the red shift of the optical/absorption spectra band caused by the manganese atoms doped in ZnSe clusters, and the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) gap energy value is decreased. Furthermore, we realized highly monodispersed manganese-doped zinc selenide quantum dots (Mn:ZnSe d-dots) experimentally by using a convenient route. The as-synthesized Mn:ZnSe d-dots were characterized by UV-Vis absorption, photoluminescence (PL), X-ray diffraction (XRD), TEM and HRTEM. The experimental results revealed that the as-prepared Mn:ZnSe d-dots with zinc-blende structure have an average size of about 3.9 nm.

The electrical conduction of isomers of anthracene molecule attached between two semi-infinite gold electrodes was simulated using extended Huckel theory (EHT)-based on semi-empirical model in this research work. The electron transport parameters were examined in two epochs by buffering anthracene and its isomer phenanthrene alternatively between gold electrodes using sulphur as an alligator clip, under variegated bias voltages. Differential NDR effect was observed in both the cases but phenanthrene exhibited more linear I–V curve than its counterpart, anthracene. The simulated results discovered phenanthrene as a better candidate than anthracene towards contributing to electrical conduction in molecular junctions. Phenanthrene reported maximum conductance of 0.74G₀ whereas anthracene exhibited 0.03G₀ at 0.8V.