Coordination-driven self-assembly of 2D-metallamacrocycles using a shape-selective Pt(2)(II)-organometallic 90 degrees acceptor: design, synthesis and sensing study

Synthesis of a series of two-dimensional metallamacrocycles via coordination-driven self-assembly of a shape-selective Pt(2)(II)-molecular building unit incorporating carbazole-ethynyl functionality is described. An equimolar (1 : 1) combination of a Pt(2)(II)-organometallic 90 degrees acceptor, 1, with rigid linear ditopic donors (L(a) and L(b)) afforded [4 + 4] self-assembled octanuclear molecular squares, 2 and 3, in quantitative yields, respectively [L(a) = 4,4'-bipyridine; L(b) = trans-1,2-bis(4-pyridyl)ethylene]. Conversely, a similar treatment of 1 with an amide-based unsymmetrical flexible ditopic donor, L(c), resulted in the formation of a [2 + 2] self-sorted molecular rhomboid (4a) as a single product [L(c) = N-(4-pyridyl)isonicotinamide]. Despite the possibility of several linkage isomeric macrocycles (rhomboid, triangle and square) due to the different connectivity of L(c), the formation of a single and symmetrical molecular rhomboid (4a) as the only product is an interesting observation. All the self-assembled macrocycles (2, 3 and 4a) were fully characterized by multinuclear NMR ((1)H and (31)P) and ESI-MS analysis. Further structural insights about the size and shape of the macrocycles were obtained through energy minimization using density functional theory (DFT) calculations. Decoration of the starting carbazole building unit with Pt-ethynyl functionality enriches the assemblies to be more p-electron rich and luminescent in nature. Macrocycles 2 and 3 could sense the presence of electron deficient nitroaromatics in solution by quenching of the initial intensity upon gradual addition of picric acid (PA). They exhibited the largest quenching response with high selectivity for nitroaromatics compared to several other electron deficient aromatics tested.

Cu(II)-azide polymers with various molar equivalents of blocking diamine ligands: synthesis, structures, magnetic properties with DFT studies

Four new neutral copper-azido polymers [Cu(4)(N(3))(8)(Me-hmpz)(2)](n) (1), [Cu(4)(N(3))(8)(men)(2)](n) (2), [Cu(5)(N(3))(10)(N,N-dmen)(2)](n) (3) and [Cu(5)(N(3))(10)(N,N'-dmen)(5)](n) (4) [Me-hmpz = 1-methylhomopiperazine; men = N-methylethylenediamine; N, N-dmen = N, N-dimethylethylenediamine and N, N'-dmen = N, N'-dimethylethylenediamine] have been synthesized by using various molar equivalents of the chelating diamine ligands with Cu(NO(3))(2)center dot 3H(2)O and an excess of NaN(3). Single-crystal X-ray structures show that the basic asymmetric units of 1 and 2 are very similar, but the overall 1D structures were found to be quite different. Complex 3 with a different composition was found to be 2D in nature, while the 1D complex 4 with 1 : 1 metal to diamine ratio presented several new structural features. Cryomagnetic susceptibility measurements over a wide range of temperature were corroborated with density functional theory calculations (B3LYP functional) performed on the complexes 1-3 to provide a qualitative theoretical interpretation of their overall magnetic behavior.

The interaction of halogen molecules with SWNTs and graphene

The interaction of halogen molecules of varying electron affinity, such as iodine monochloride (ICl), bromine (Br(2)), iodine monobromide (IBr) and iodine (I(2)) with single-walled carbon nanotubes (SWNTs) and graphene has been investigated in detail. Halogen doping of the two nanocarbons has been examined using Raman spectroscopy in conjunction with electronic absorption spectroscopy and extensive theoretical calculations. The halogen molecules, being electron withdrawing in nature, induce distinct changes in the electronic states of both the SWNTs and graphene, which manifests with a change in the spectroscopic signatures. Stiffening of the Raman G-bands of the nanocarbons upon treatment with the different halogen molecules and the emergence of new bands in the electronic absorption spectra, both point to the fact that the halogen molecules are involved in molecular charge-transfer with the nanocarbons. The experimental findings have been explained through density functional theory (DFT) calculations, which suggest that the extent of charge-transfer depends on the electron affinities of the different halogens, which determines the overall spectroscopic properties. The magnitude of the molecular charge-transfer between the halogens and the nanocarbons generally varies in the order ICl > Br(2) > IBr > I(2), which is consistent with the expected order of electron affinities.

Two new 1D chains of Ni(2)Na(2) heterometallic double half-cubane building units: Synthesis, structures and variable temperature magnetic study

An equimolar mixture of Ni(NO(3))(2)center dot 6H(2)O and pyridine-2-aldehyde with two equivalents of NaN(3) in methanol in the presence of NaOMe resulted in the formation of light green precipitate which upon crystallization from dimethylformamide (DMF) yielded light green single crystals [{Ni(2)Na(2)(pic)(4)(N(3))(2)(H(2)O)(2)(MeOH)}center dot MeOH center dot 3H(2)O](n) (1) and [{Ni(2)Na(2)(pic)(4)(N(3))(2)(H(2)O)(4)}center dot 2DMF center dot H(2)O](n) (2) (pic = pyridine-2-carboxylate) at room temperature and high temperature (100 degrees C), respectively. Variable temperature magnetic studies revealed the existence of overall ferromagnetic behaviour with J approximate to + 10 cm(-1) and D approximate to -2 to -7 cm(-1) for 1 and 2, respectively. Negative D values as well as variation of D upon slight distortion of structure by varying reaction temperature were observed. The X-band Electron Paramagnetic Resonance (EPR) spectra of both 2 and 3 were recorded below 50 K. The structural distortion was also implicated from the EPR spectra. Density Functional Theory (DFT) calculations on both complexes were performed in two different ways to corroborate the magnetic results. Considering only Ni(2)(II) dimeric unit, results were J = + 20.65 cm(-1) and D = -3.16 cm(-1) for 1, and J = +24.56 cm(-1) and D = -4.67 cm(-1) for 2. However, considering Ni(2)(II)Na(2)(I) cubane as magnetic core the results were J = +16.35 cm(-1) (1), +19.54 cm(-1) (2); D = -3.05 cm(-1) (1), -4.25 cm(-1) (2).

Study on the structure and vibrational spectra of efavirenz conformers using DFT: Comparison to experimental data

Efavirenz, (S)-6-chloro-4-(cyclopropylethynyl)-1,4-dihydro-4-(trifluoromethyl)-2H-3 ,1-benzoxazin-2-one, is an anti HIV agent belonging to the class of the non-nucleoside inhibitors of the HIV-1 virus reverse transcriptase. A systematic quantum chemical study of the possible conformations, their relative stabilities and vibrational spectra of efavirenz has been reported. Structural and spectral characteristics of efavirenz have been studied by vibrational spectroscopy and quantum chemical methods. Density functional theory (DFT) calculations for potential energy curve, optimized geometries and vibrational spectra have been carried out using 6-311++G(d,p) basis sets and B3LYP functionals. Based on these results, we have discussed the correlation between the vibrational modes and the crystalline structure of the most stable form of efavirenz. A complete analysis of the experimental infrared and Raman spectra has been reported on the basis of wavenumber of the vibrational bands and potential energy distribution. The infrared and the Raman spectra of the molecule based on OFT calculations show reasonable agreement with the experimental results. The calculated HOMO and LUMO energies shows that charge transfer occur within the molecule. (C) 2011 Elsevier B.V. All rights reserved.

Symmetry-dependent phonon renormalization in monolayer MoS2 transistor

A strong electron-phonon interaction which limits the electronic mobility of semiconductors can also have significant effects on phonon frequencies. The latter is the key to the use of Raman spectroscopy for nondestructive characterization of doping in graphene-based devices. Using in situ Raman scattering from a single-layer MoS2 electrochemically top-gated field-effect transistor (FET), we show softening and broadening of the A(1g) phonon with electron doping, whereas the other Raman-active E-2g(1) mode remains essentially inert. Confirming these results with first-principles density functional theory based calculations, we use group theoretical arguments to explain why the A(1g) mode specifically exhibits a strong sensitivity to electron doping. Our work opens up the use of Raman spectroscopy in probing the level of doping in single-layer MoS2-based FETs, which have a high on-off ratio and are of technological significance.

Coupled phonons, magnetic excitations, and ferroelectricity in AlFeO3: Raman and first-principles studies

We determine the nature of coupled phonons and magnetic excitations in AlFeO3 using inelastic light scattering from 5 to 315 K covering a spectral range from 100 to 2200 cm(-1) and complementary first-principles density functional theory-based calculations. A strong spin-phonon coupling and magnetic ordering-induced phonon renormalization are evident in (1) anomalous temperature dependence of many modes with frequencies below 850 cm(-1), particularly near the magnetic transition temperature T-c approximate to 250 K, and (2) distinct changes in band positions of high-frequency Raman bands between 1100 and 1800 cm(-1); in particular, a broad mode near 1250 cm(-1) appears only below T-c, attributed to the two-magnon Raman scattering. We also observe weak anomalies in the mode frequencies similar to 100 K due to a magnetically driven ferroelectric phase transition. Understanding of these experimental observations has been possible on the basis of first-principles calculations of the phonons' spectrum and their coupling with spins.

Two Novel Heterometallic Chains Featuring Mn-II and Na-I Ions in Trigonal-Prismatic Geometries Alternately Linked to Octahedral Mn-IV Ions: Synthesis, Structures, and Magnetic Behavior

Two new one-dimensional heterometallic complexes, Mn3Na(L)(4)(CH3CO2)(MeOH)(2)]-(ClO4)(2)center dot 3H(2)O (1), Mn3Na(L)(4)(CH3CH2CO2)-(MeOH)(2)](ClO4)(2)center dot 2MeOH center dot H2O (2) LH2 = 2-methyl-2-(2-pyridyl)propane-1,3-diol], have been synthesized and characterized by X-ray crystallography. Both complexes feature Mn-II and Na-I ions in trigonal-prismatic geometries that are linked to octahedral Mn-IV ions by alkoxy bridges. Variable-temperature direct- and alternating-current magnetic susceptibility data indicated a spin ground state of S = 11/2 for both complexes. Density functional theory calculations performed on 1 supported this conclusion.

Study of molecular structure and vibrational spectra of poly (beta,-1-malic acid) PMLA] using DFT approach

Poly (beta-L-malic acid) (PMLA) is a biodegradable polymer and it has various important applications in the biomedical field. In the present work the structural and spectral characteristics of PMLA have been studied by methods of infrared. Raman spectroscopy and quantum chemistry. Electrostatic potential surface, optimized geometry, harmonic vibrational frequencies, infrared intensities and activities of Raman scattering were calculated by density functional theory (DFT) using oligomeric approach employing B3LYP with complete relaxation in the potential energy surface using 6-311++G (d, p) basis set. Based on results, we have discussed the correlation between the vibrational modes and the structure of the PMLA. A complete analysis of the experimental infrared and Raman spectra has been reported on the basis of wavenumber of the vibrational bands and potential energy distribution. The calculated HOMO and LUMO energies shows that charge transfer occur within the molecule. The calculated infrared and the Raman spectra of the polymer based on DFT calculations show reasonable agreement with the experimental results. (c) 2012 Elsevier Ltd. All rights reserved.

Determination of C-13 Chemical Shift Anisotropy Tensors and Molecular Order of 4-Hexyloxybenzoic Acid

4-Alkoxy benzoic acids belong to an important class of thermotropic liquid crystals that are structurally simple and often used as starting materials for many novel mesogens. 4-Hexyloxybenzoic acid (HBA) is a homologue of the same series and exhibits an enantiotropic nematic phase. As this molecule could serve as an ideal model compound, high resolution C-13 NMR studies of HEA in solution, solid, and liquid crystalline phases have been undertaken. In the solid state, two-dimensional separation of undistorted powder patterns by effortless recoupling (2D SUPER) experiments have been carried out to estimate the magnitude of the components of the chemical shift anisotropy (GSA) tensor of all the aromatic carbons. These values have been used subsequently for calculating the orientational order parameters in the liquid crystalline phase. The GSA values computed by density functional theory (DFT) calculations showed good agreement with the 2D SUPER values. Additionally, C-13-H-1 dipolar couplings in the nematic phase have been determined by separated local field (SLF) spectroscopy at various temperatures and were used for computing the order parameters, which compared well with those calculated by using the chemical shifts. It is anticipated that the CSA values determined for MBA would be useful for the assignment of carbon chemical shifts and for the study of order and dynamics of structurally similar novel mesogens in their nematic phases.

A Rare Phenoxido/Acetato/Azido Bridged Trinuclear and an Unprecedented Phenoxido/Azido Bridged One-Dimensional Polynuclear Nickel(II) Complexes: Synthesis, Crystal Structure, and Magnetic Properties with Theoretical Investigations on the Exchange Mechanism

The reaction of a tridentate Schiff base ligand HL (2-(3-dimethylaminopropylimino)-methyl]-phenol) with Ni(II) acetate or perchlorate salts in the presence of azide as coligand has led to two new Ni(II) complexes of formulas Ni3L2(OAc)(2)(mu(1,1)-N-3)(2)(H2O)(2)]center dot 2H(2)O (1) and Ni2L2(mu(1,1)-N-3) (mu(1,3)-N-3)](n)(2). Single crystal X-ray structures show that complex 1 is a linear trinuclear Ni(II) compound containing a mu(2)-phenwddo, an end-on (EO) azido and a syn-syn acetato bridge between the terminal and the central Ni(II) ions. Complex 2 can be viewed as a one-dimensional (1D) chain in which the triply bridged (di-mu(2)-phenoxido and EO azido) dimeric Ni-2 units are linked to each other in a zigzag pattern by a single end-to-end (EE) azido bridge. Variable-temperature magnetic susceptibility studies indicate the presence of moderate ferromagnetic exchange coupling in complex 1 with J value of 16.51(6) cm(-1). The magnetic behavior of 2 can be fitted in an alternating ferro- and antiferromagnetic model J(FM) = +34.2(2.8) cm(-1) and J(AF) = -21.6(1.1) cm(-1)] corresponding to the triple bridged dinuclear core and EE azido bridge respectively. Density functional theory (DFT) calculations were performed to corroborate the magnetic results of 1 and 2. The contributions of the different bridges toward magnetic interactions in both compounds have also been calculated.

Electrical properties of buckled multiwalled carbon nanotube arrays in the diffusive regime

We report the geometrical effect of graded buckled multiwalled carbon nanotube arrays on the electrical transport properties in the diffusive regime, via successive breakdown caused by the Joule heating. This breakdown occurs in the straighter region. Empirical relations involving the current-carrying ability, resistance, breakdown power, threshold voltage, diameter and length of carbon nanotube arrays are discussed on the basis of an extensive set of experimental data along with justification. The experimental results are corroborated by the density functional tight-binding calculations of electronic band structure. The band gap decreases as buckleness increases leading to the enhancement in the current-carrying ability and elucidating the role of buckleness in carbon nanotubes. Copyright (c) EPLA, 2012

A new diketopyrrolopyrrole-based co-polymer for ambipolar field-effect transistors and solar cells

A new thieno3,2-b]thiophenediketopyrrolopyrrole-benzo1,2-b:4,5-b']dithio phene based narrow optical gap co-polymer (PTTDPP-BDT) has been synthesized and characterized for field-effect transistors and solar cells. In field-effect transistors the polymer exhibited ambipolar charge transport behaviour with maximum hole and electron mobilities of 10(-3) cm(2) V-1 s(-1) and 10(-5) cm(2)V(-1) s(-1), respectively. The respectable charge transporting properties of the polymer were consistent with X-ray diffraction measurements that showed close molecular packing in the solid state. The difference in hole and electron mobilities was explained by density functional theory calculations, which showed that the highest occupied molecular orbital was delocalized along the polymer backbone with the lowest unoccupied molecular orbital localized on the bis(thieno3,2-b]thiophene)diketopyrrolopyrrole units. Bulk heterojunction photovoltaic devices with the fullerene acceptor PC70BM were fabricated and delivered a maximum conversion efficiency of 3.3% under AM1.5G illumination. (C) 2012 Elsevier B.V. All rights reserved.

Unraveling siRNA unzipping kinetics with graphene

Using all atom molecular dynamics simulations, we report spontaneous unzipping and strong binding of small interfering RNA (siRNA) on graphene. Our dispersion corrected density functional theory based calculations suggest that nucleosides of RNA have stronger attractive interactions with graphene as compared to DNA residues. These stronger interactions force the double stranded siRNA to spontaneously unzip and bind to the graphene surface. Unzipping always nucleates at one end of the siRNA and propagates to the other end after few base-pairs get unzipped. While both the ends get unzipped, the middle part remains in double stranded form because of torsional constraint. Unzipping probability distributions fitted to single exponential function give unzipping time (tau) of the order of few nanoseconds which decrease exponentially with temperature. From the temperature variation of unzipping time we estimate the energy barrier to unzipping. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.4742189]

Methane and carbon dioxide adsorption on edge-functionalized graphene: a comparative DFT study

With a view towards optimizing gas storage and separation in crystalline and disordered nanoporous carbon-based materials, we use ab initio density functional theory calculations to explore the effect of chemical functionalization on gas binding to exposed edges within model carbon nanostructures. We test the geometry, energetics, and charge distribution of in-plane and out-of-plane binding of CO2 and CH4 to model zigzag graphene nanoribbons edge-functionalized with COOH, OH, NH2, H2PO3, NO2, and CH3. Although different choices for the exchange-correlation functional lead to a spread of values for the binding energy, trends across the functional groups are largely preserved for each choice, as are the final orientations of the adsorbed gas molecules. We find binding of CO2 to exceed that of CH4 by roughly a factor of two. However, the two gases follow very similar trends with changes in the attached functional group, despite different molecular symmetries. Our results indicate that the presence of NH2, H2PO3, NO2, and COOH functional groups can significantly enhance gas binding, making the edges potentially viable binding sites in materials with high concentrations of edge carbons. To first order, in-plane binding strength correlates with the larger permanent and induced dipole moments on these groups. Implications for tailoring carbon structures for increased gas uptake and improved CO2/CH4 selectivity are discussed. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.4736568]