Pressure-Dependent Optical and Vibrational Properties of Mono layer Molybdenum Disulfide

Controlling the band gap by tuning the lattice structure through pressure engineering is a relatively new route for tailoring the optoelectronic properties of two-dimensional (2D) materials. Here, we investigate the electronic structure and lattice vibrational dynamics of the distorted monolayer 1T-MoS2 (1T') and the monolayer 2H-MoS2 via a diamond anvil cell (DAC) and density functional theory (DFT) calculations. The direct optical band gap of the monolayer 2H-MoS2 increases by 11.7% from 1.85 to 2.08 eV, which is the highest reported for a 2D transition metal dichalcogenide (TMD) material. DFT calculations reveal a subsequent decrease in the band gap with eventual metallization of the monolayer 2H-MoS2, an overall complex structureproperty relation due to the rich band structure of MoS2. Remarkably, the metastable 1T'-MoS2 metallic state remains invariant with pressure, with the J(2), A(1g), and E(2)g modes becoming dominant at high pressures. This substantial reversible tunability of the electronic and vibrational properties of the MoS2 family can be extended to other 2D TMDs. These results present an important advance toward controlling the band structure and optoelectronic properties of monolayer MoS2 via pressure, which has vital implications for enhanced device applications.

Molecular effects of encapsulation of glucose oxidase dimer by graphene

Knowing the nature of the enzyme-graphene interface is critical for a design of graphene-based biosensors. Extensive contacts between graphene and enzyme could be obtained by employing a suitable encapsulation which does not impede its enzymatic reaction. We have performed molecular dynamics simulations to obtain an insight on many forms of contact between glucose oxidase dimer and the single-layer graphene nano-sheets. The unconnected graphene sheets tended to form a flat stack regardless of their initial positions around the enzyme, whereas the same graphene sheets linked together formed a flower-like shape engendering different forms of wrapping of the enzyme. During the encapsulation no core hydrophobic residues of the enzyme were exposed. Since the polar and charged amino acids populated the enzyme's surface we also estimated, using DFT calculations, the interaction energies of individual polar and charged amino acid residues with graphene. It was found that the negatively charged residues can bind to graphene unexpectedly strongly; however, the main effect of encapsulation comes from the overlap of adjacent edges of graphene sheets.

Remarkable visible light-triggered cytotoxicity of mitochondria targeting mixed-ligand cobalt(III) complexes of curcumin and phenanthroline bases binding to human serum albumin

Six new mixed-ligand cobalt(III) complexes of formulation Co(N-N)(2)(O-O)](ClO4)(2) (1-6), where N-N is a N,N-donor phenanthroline base, namely, 1,10-phenanthroline (phen in 1, 2), dipyrido3,2-d:2',3'-f] quinoxaline (dpq in 3, 4), and dipyrido3,2-a:2',3'-c]phenazine (dppz in 5, 6), O-O is acetylacetonate (acac in 1, 3, 5) or curcumin (bis(4-hydroxy-3-methoxyphenyl)-1,6-diene-3,5-dione, cur in 2, 4, 6), have been synthesized and characterized. The X-ray crystal structures of complex 1 (as PF6- salt, 1a) and 3 show distorted octahedral geometries formed by the CoN4O2 core. The complexes 1, 3 and 5 having the simple acac ligand are prepared as control species to understand the role of curcumin. The optimized geometries and the frontier orbitals of the curcumin complexes 2, 4, and 6 are obtained from the DFT calculations. The complexes 2, 4, and 6 having the photoactive curcumin moiety display an absorption band in the visible region near 420 nm and show remarkable photocytotoxicity in HeLa cancer cells with respective IC50 values of 7.4 mu M, 5.1 mu M and 1.6 mu M while being much less toxic in dark. MTT assay using complex 6 shows that it is not significantly photocytotoxic to MCF-10A normal cells. The control complexes having the acac ligand are non-toxic both in the presence and absence of light. The cell death is apoptotic in nature and triggered by the photogeneration of reactive oxygen species. Fluorescence imaging experiments on HeLa cells reveals that complex 6 accumulated primarily inside the mitochondria. Human serum albumin (HSA) binding experiments show that the complexes bind HSA with good affinity, but 6 binds with the highest affinity, with a K-b value of 9.8 x 10(5) M-1. Thus, complex 6 with its negligible toxicity in the dark and in normal cells but remarkable toxicity in visible light holds significant photochemotherapeutic potential.

Al3+-Ion-Triggered Conformational Isomerization of a Rhodamine B Derivative Evidenced by a Fluorescence Signal - A Crystallographic Proof

A newly designed rhodamine B anisaldehyde hydrazone exhibits Al3+-ion-induced cis (L) to trans (L) conformational isomerization with respect to the xanthene moiety through a rotation about a N-N bond; the isomerization is indicated by a detectable naked-eye color change and a turn-on red fluorescence in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer (EtOH/Water 1:9 v/v; pH 7.4) at 25 degrees C. In support of this observation, detailed spectroscopic and physicochemical studies along with density function theory (DFT) calculations have been performed. This cis-to-trans conformational isomerization is due to Al3+ ion coordination, which induces this visual color change and the turn-on fluorescence response. To strengthen our knowledge of the conformational isomerization, detailed structural characterizations of the cis and trans isomers in the solid state were performed by single-crystal X-ray diffraction. To the best of our knowledge, this is the first structural report of both cis and trans conformational isomers for this family of compounds. Moreover, this noncytotoxic probe could be used to image the accumulation of Al3+ ions in HeLa and MCF-7 cell lines.

Implication of a sigma-Methane Complex en Route to Elimination of Methane from a Ruthenium Complex: An Experimental and Theoretical Investigation

The five-coordinated 16-electron complex Ru(Me)(dppe)(2)]OTf] (3) undergoes methane elimination at room temperature to afford the ortho-metalated species (dppe){(C6H5)(C6H4)PCH2CH2P(C6H5)(2)}Ru]OTf] (7). Methane elimination, monitored using NMR spectroscopy, revealed no intermediate throughout the reaction. The NOE between Ru-Me protons and ortho phenyl protons and an agostic interaction trans to the methyl group were found in complex 3 by NMR spectroscopy, which form the basis for three plausible pathways for methane elimination and ortho metalation: pathway I (through spatial interaction), pathway II (through oxidative addition and reductive elimination), and pathway III (through agostic interaction). Methane elimination from complex 3 via pathway I was discounted, since it involves interactions through space and not through bonds. Moreover, the calculated energy barrier for the pathway I transition state was quite high (71.3 kcal/mol), which also indicates that this pathway is very unlikely. Furthermore, no spectroscopic evidence for oxidatively added seven-coordinated Ru(IV) species was found and the computed energy barrier of the transition state for pathway II was moderately high (41.1 kcal/mol), which suggests that this cannot be the right pathway for methane elimination and ortho-metalation of complex 3. On the other hand, indirect evidence in the form of chemical reactions point to the most plausible pathway for methane elimination, pathway III, via the intermediacy of a sigma-CH4 complex that could not be found spectroscopically. DFT calculations at several levels on this pathway showed an initial low-barrier rearrangement through TS1 to a square-pyramidal intermediate wherein methyl and agostic C-H are cis to each other. Migration of hydrogen from agostic C-H and elimination of methane proceed through the transition state TS2, which retains a weak metal-H bonding through most parts of the reaction coordinate. Upon comparison of all three pathways, pathway III was found to be the most likely for methane elimination and ortho-metalation of complex 3.

Synthesis and characterization of monometallic rhenium(I) complexes and their application as selective sensors for copper(II) ions

Novel imine functionalized monometallic rhenium(I) polypyridine complexes (1-4) comprising two phenol moieties attached to 2,20-bipyridine ligands L1-L4 have been synthesized and characterized. These complexes exhibit selective and sensitive detection towards copper(II) ions and this is observed through changes in UV-visible absorption, luminescence and time-resolved spectroscopic techniques. An enormous enhancement is observed in emission intensity, quantum yield and luminescence lifetime with the addition of copper(II) ions, and this can be attributed to the restriction of C=N isomerization in the Re(I) complexes. The strong binding between copper(II) ions and these complexes reveals that the binding constant values are in the range of 1.1 x 10(3)-6.0 x 103 M-1. The absorption spectral behavior of the complexes is supported by DFT calculations.

Polyfunctional Lewis Acids: Intriguing Solid-State Structure and Selective Detection and Discrimination of Nitroaromatic Explosives

Synthesis and crystal structures of three porphyrin-based polyfunctional Lewis acids 1-3 are reported. Intermolecular HgClHgCl (linear and -type) interactions in the solid state of the peripherally ArHgCl-decorated compound 3 lead to a fascinating 3D supramolecular architecture. Compound3 shows a selective fluorescence quenching response to picric acid and discriminates other nitroaromatic-based explosives. For the first time, an electron-deficient polyfunctional Lewis acid is shown to be useful for the selective detection and discrimination of nitroaromatic explosives. The Stern-Volmer quenching constant and detection limits of compound3 for picric acid are the best among the reported small-molecular receptors for nitroaromatic explosives. The electronic structure, Lewis acidity, and selective sensing characteristics of 3 are well corroborated by DFT calculations.

Achieving Highly Efficient, Selective, and Stable CO2 Reduction on Nitrogen-Doped Carbon Nanotubes

The challenge in the electrosynthesis of fuels from CO2 is to achieve durable and active performance with cost-effective catalysts. Here, we report that carbon nanotubes (CNTs), doped with nitrogen to form resident electron-rich defects, can act as highly efficient and, more importantly, stable catalysts for the conversion of CO2 to CO. The unprecedented overpotential (-0.18 V) and selectivity (80%) observed on nitrogen-doped CNTs (NCNTs) are attributed to their unique features to facilitate the reaction, including (i) high electrical conductivity, (ii) preferable catalytic sites (pyridinic N defects), and (iii) low free energy for CO2 activation and high barrier for hydrogen evolution. Indeed, DFT calculations show a low free energy barrier for the potential-limiting step to form key intermediate COOH as well as strong binding energy of adsorbed CON and weak binding energy for the adsorbed CO. The highest selective site toward CO production is pyridinic N, and the NCNT-based electrodes exhibit no degradation over 10 h of continuous operation, suggesting the structural stability of the electrode.

Introduction of a catalytic triad increases the glutathione peroxidase-like activity of diaryl diselenides

Reactive oxygen species (ROS)-mediated diseased states are of major concern in modern day life. Under oxidative stress conditions, the cellular antioxidants deplete, leading to several biological disorders. Small molecule mimics of different antioxidant enzymes are found to be useful in supplementing the biological systems to detoxify ROS. In this study, we have synthesized a series of amine or amide-based diselenides containing an additional amino group as glutathione peroxidase (GPx) mimetics. These diselenides act as a catalytic triad model of the native GPx featuring two basic amino groups near the selenium centre. A comparison of the catalytic activities reveals that the additional amino group increases the activity significantly in the presence of aromatic thiols. Deprotonation of thiol by an additional amine either stabilizes the selenolate intermediate or facilitates the nucleophilic attack of thiol in other intermediates. The Se-77 NMR experiments and DFT calculations show that the amino group does not have any significant effect on the catalytic intermediates. Although the amino moiety increases the nucleophilicity of the thiol, it does not prevent the thiol exchange reactions that take place in the selenenyl sulfide intermediates.

Effect of Chalcogens on Electronic and Photophysical Properties of Vinylene-Based Diketopyrrolopyrrole Copolymers

Three vinylene linked diketopyrrolopyrrole based donor acceptor (D-A) copolymers have been synthesized with phenyl, thienyl, and selenyl units as donors. Optical and electronic properties were investigated with UV-vis absorption spectroscopy, cyclic voltammetry, near edge X-ray absorption spectroscopy, organic field effect transistor (OFET) measurements, and density functional theory (DFT) calculations. Optical and electrochemical band gaps decrease in the order phenyl, thienyl, and selenyl. Only phenyl-based polymers are nonplanar, but the main contributor to the larger band gap is electronic, not structural effects. Thienyl and selenyl polymers exhibit ambipolar charge transport but with higher hole than electron mobility. Experimental and theoretical results predict the selenyl system to have the best transport properties, but OFET measurements prove the thienyl system to be superior with p-channel mobility as high as 0.1 cm(2) V-1 s(-1).

Direct Observation of Thermal Equilibrium of Excited Triplet States of 9,10-Phenanthrenequinone. A Time-Resolved Resonance Raman Study

The photochemistry of aromatic ketones plays a key role in various physicochemical and biological processes, and solvent polarity can be used to tune their triplet state properties. Therefore, a comprehensive analysis of the conformational structure and the solvent polarity induced energy level reordering of the two lowest triplet states of 9,10-phenanthrenequinone (PQ) was carried out using nanosecond-time-resolved absorption (ns-TRA), time-resolved resonance Raman (TR3) spectroscopy, and time dependent-density functional theory (TD-DFT) studies. The ns-TRA of PQ in acetonitrile displays two bands in the visible range, and these two bands decay with similar lifetime at least at longer time scales (mu s). Interestingly, TR3 spectra of these two bands indicate that the kinetics are different at shorter time scales (ns), while at longer time scales they followed the kinetics of ns-TRA spectra. Therefore, we report a real-time observation of the thermal equilibrium between the two lowest triplet excited states of PQ assigned to n pi* and pi pi* of which the pi pi* triplet state is formed first through intersystem crossing. Despite the fact that these two states are energetically close and have a similar conformational structure supported by TD-DFT studies, the slow internal conversion (similar to 2 ns) between the T-2(1(3)n pi*) and T-1(1(3)pi pi*) triplet states indicates a barrier. Insights from the singlet excited states of PQ in protic solvents J. Chem. Phys. 2015, 142, 24305] suggest that the lowest n pi* and pi pi* triplet states should undergo hydrogen bond weakening and strengthening, respectively, relative to the ground state, and these mechanisms are substantiated by TD-DFT calculations. We also hypothesize that the different hydrogen bonding mechanisms exhibited by the two lowest singlet and triplet excited states of PQ could influence its ISC mechanism.

Ru (II)-Catalyzed C-H Activation: Ketone-Directed Novel 1,4-Addition of Ortho C-H Bond to Maleimides

A 1,4-addition with the nucleophilic center generated at the ortho carbon atom of an aromatic ketone in the presence of the highly reactive alpha-C-H bond, using a directing group strategy, is presented. The reaction yields pharmaceutically useful 3-arylated succinimide derivatives. In order to gain understanding of this redox neutral reaction, despite the presence of copper acetate, and to substantiate the lack of Heck-type products, DFT calculations have been carried out.

Graphene-oxide-supported ultrathin Au nanowires: efficient electrocatalysts for borohydride oxidation

We report stable ultrathin Au nanowires supported on reduced graphene oxide with outstanding electrocatalytic activity for borohydride oxidation. Electrochemical impedance spectroscopy measurements showed abnormal inductive behavior, indicative of surface reactivation. DFT calculations indicate that the origin of the high activity stems from the position of the Au d-band center.

Molecular dynamics and simulations study on the vibrational and electronic solvatochromism of benzophenone

Solvent plays a key role in diverse physico-chemical and biological processes. Therefore, understanding solute-solvent interactions at the molecular level of detail is of utmost importance. A comprehensive solvatochromic analysis of benzophenone (Bzp) was carried out in various solvents using Raman and electronic spectroscopy, in conjunction with Density Functional Theory (DFT) calculations of supramolecular solute-solvent clusters generated using classical Molecular Dynamics Simulations (c-MDSs). The >C=O stretching frequency undergoes a bathochromic shift with solvent polarity. Interestingly, in protic solvents this peak appears as a doublet: c-MDS and ad hoc explicit solvent ab initio calculations suggest that the lower and higher frequency peaks are associated with the hydrogen bonded and dangling carbonyl group of Bzp, respectively. Additionally, the dangling carbonyl in methanol (MeOH) solvent is 4 cm(-1) blue-shifted relative to acetonitrile solvent, despite their similar dipolarity/polarizability. This suggests that the cybotactic region of the dangling carbonyl group in MeOH is very different from its bulk solvent structure. Therefore, we propose that this blue-shift of the dangling carbonyl originates in the hydrophobic solvation shell around it resulting from extended hydrogen bonding network of the protic solvents. Furthermore, the 1(1)n pi* (band I) and 1(1)pi pi* (band II) electronic transitions show a hypsochromic and bathochromic shift, respectively. In particular, these shifts in protic solvents are due to differences in their excited state-hydrogen bonding mechanisms. Additionally, a linear relationship is obtained for band I and the >C=O stretching frequency (cm(-1)), which suggests that the different excitation wavelengths in band I correspond to different solvation states. Therefore, we hypothesize that the variation in excitation wavelengths in band I could arise from different solvation states leading to varying solvation dynamics. This will have implications for ultrafast processes associated with electron-transfer, charge transfer, and also the photophysical aspects of excited states. (C) 2016 AIP Publishing LLC.

Simultaneous tunability of the electronic and phononic gaps in SnS2 under normal compressive strain

Controlled variation of the electronic properties of. two-dimensional (2D) materials by applying strain has emerged as a promising way to design materials for customized applications. Using density functional theory (DFT) calculations, we show that while the electronic structure and indirect band gap of SnS2 do not change significantly with the number of layers, they can be reversibly tuned by applying biaxial tensile (BT), biaxial compressive (BC), and normal compressive (NC) strains. Mono to multilayered SnS2 exhibit a reversible semiconductor to metal (S-M) transition with applied strain. For bilayer (2L) SnS2, the S-Mtransition occurs at the strain values of 17%,-26%, and -24% under BT, BC, and NC strains, respectively. Due to weaker interlayer coupling, the critical strain value required to achieve the S-Mtransition in SnS2 under NC strain is much higher than for MoS2. From a stability viewpoint, SnS2 becomes unstable at very low strain values on applying BC (-6.5%) and BT strains (4.9%), while it is stable even up to the transition point (-24%) in the case of NC strain. In addition to the reversible tuning of the electronic properties of SnS2, we also show tunability in the phononic band gap of SnS2, which increases with applied NC strain. This gap increases three times faster than for MoS2. This simultaneous tunability of SnS2 at the electronic and phononic levels with strain, makes it a potential candidate in field effect transistors (FETs) and sensors as well as frequency filter applications.