The X-C center dot center dot center dot Y (X = O/F, Y = O/S/F/Cl/Br/N/P) `carbon bond' and hydrophobic interactions

While the tetrahedral face of methane has an electron rich centre and can act as a hydrogen bond acceptor, substitution of one of its hydrogens with some electron withdrawing group (such as -F/OH) can make the opposite face electron deficient. Electrostatic potential calculations confirm this and high level quantum calculations show interactions between the positive face of methanol/methyl fluoride and electron rich centers of other molecules such as H2O. Analysis of the wave functions of atoms in molecules shows the presence of an unusual C center dot center dot center dot Y interaction, which could be called `carbon bonding'. NBO analysis and vibrational frequency shifts confirm the presence of this interaction. Given the properties of alkyl groups bonded to electronegative elements in biological molecules, such interactions could play a significant role, which is yet to be recognized. This and similar interactions could give an enthalpic contribution to what is called the `hydrophobic interactions'.

Triarylborane conjugated acacH ligands and their BF2 complexes: facile synthesis and intriguing optical properties

A facile synthetic route for a new class of organoborane compounds (Mes)(2)B-arene-acacH and (Mes)(2)Barene-acacBF2 (Mes = mesityl and arene = C6H4 or C6Me4) is reported. The new dyads exhibit intriguing photophysical properties. A small structural change in spacer connecting the two chromophores leads to fine tuning of photophysical properties. The dyad containing 2,3,5,6-tetramethyl phenyl spacer acts as a selective ``turn-on'' chemodosimetric sensor for cyanide ion. Steric crowding around the boron centre significantly alters anion binding events. From NMR titration studies it is established that fluoride and cyanide follow different binding mechanisms which lead to intriguing optical properties in the reported probes.

Triarylborane-Appended New Triad and Tetrad: Chromogenic and Fluorogenic Anion Recognition

Facile synthesis of triad 3 and tetrad 4 incorporating -B(Mes)(2) (Mes = mesityl (2,4,6-trimethylphenyl)), boron dipyrromethene (BODIPY), and triphenylamine is reported. Introduction of two dissimilar acceptors (triarylborane and BODIPY) on a single donor resulted in two distinct intramolecular charge transfer processes (amine-to-borane and amine-to-BODIPY). The absorption and emission properties of the new triad and tetrad are highly dependent on individual building units. The nature of electronic communication among the individual fluorophore units has been comprehensively investigated and compared with building units. Compounds 3 and 4 showed chromogenic and fluorogenic responses for small anions such as fluoride and cyanide.

Going beyond Red with a Tri- and Tetracoordinate Boron Conjugate: Intriguing Near-IR Optical Properties and Applications in Anion Sensing

The design and synthesis of a new tri- and tetracoordinate boron conjugate is reported. The conjugate shows broad near-IR emission (similar to 625-850 nm) and is found to be a selective colorimetric and ratiometric sensor for fluoride ions.

1,9-Pyrazoloanthrone as a Colorimetric and ``Turn-On'' Fluorometric Chemosensor: Structural Implications

A colorimetric and ``turn-on'' fluorescent chemosensor based on 1,9-pyrazoloanthrone specifically for cyanide and fluoride ion detection shows a remarkable solid state reaction when crystals of tetrabutylammonium cyanide and fluoride are brought in physical contact with 1,9-pyrazoloanthrone. X-ray crystal structures of 1,9-pyrazoloanthrone and complexes have been determined, and the ion sensing activity (detection limit of 0.2 and 2 ppb) has been inferred based on spectroscopic and structural features.

Colorimetric anion sensor based on receptor having indole- and thiourea-binding sites

A new colorimetric sensor L containing nitro-substituted indole and bisthiocarbonohydrazone units for selective fluoride and acetate ions is designed and synthesized. The receptor L shows well-defined color change in the visible region of the spectrum with an absorption band at similar to 515 nm and 506 nm, respectively, for the F- and CH3COO- ions in an acetonitrile solution. Job's plots indicated the formation of 1 : 1 (L with CH3COO-) and 1 : 2 (L with F-) complexes. The interaction of L with the F- ion undergoes a deprotonation process and release of HX2](-), whereas with the CH3COO- ion, it forms a stable LH2(...)X](-) complex. The relative affinities of the anions with L are rationalized using computational studies.

Process induced electroactive beta-polymorph in PVDF: effect on dielectric and ferroelectric properties

The effects of various processing conditions, like annealing, poling, mechanical rolling and their combinations, on the dielectric and ferroelectric properties of PVDF poly(vinylidene fluoride)] were systematically studied in this work. Further, the effect of processing sequence on the structure and properties was investigated. While all the processing conditions adopted here resulted in phase transformation of the alpha- to electroactive beta-polymorph in PVDF, the fraction of beta-phase developed was observed to be strongly contingent on the adopted process. The transformation of alpha- to electroactive beta-polymorph was determined by X-ray diffraction and FTIR. The neat PVDF showed only beta-phase, whereas mechanically rolled samples exhibited the highest ca. 85% beta-phase in PVDF. Both the permittivity and the loss tangent decreased in the samples which had undergone different processing conditions. The polarization-electric field (P-E) loops for all the samples were evaluated. Interestingly, the energy density, estimated from the electrical displacement-electric field (D-E) loops, was observed to be highest for the poled samples which were initially rolled. The results indicate that various processing conditions can influence the dielectric and the ferroelectric properties differently.

Unique nanoporous antibacterial membranes derived through crystallization induced phase separation in PVDF/PMMA blends

In this study, a unique method was adopted to design porous membranes through crystallization induced phase separation in PVDF/PMMA (poly(vinylidene fluoride)/poly(methyl methacrylate)) blends. By etching out PMMA, which segregates either in the interlamellar and/or in the interspherulitic regions of the blends, nanoporous hierarchical structures can be derived. Different nanoparticles like titanium dioxide (TiO2), silver nanoparticle (Ag) decorated carbon nanotubes (Ag-CNTs), TiO2 decorated CNTs (TiO2-CNTs), Ag decorated TiO2 (Ag-TiO2) and Ag-TiO2 decorated CNTs (Ag@TiO2-CNTs) were synthesized and melt mixed with 80/20 PVDF/PMMA blends to render antibacterial activity to the membranes. Scanning electron microscopy (SEM) was used to study the crystalline morphology of the membranes. A significant improvement in the trans-membrane flux was obtained in the blends with Ag@TiO2 decorated CNTs as compared to the membranes derived from the neat blends, which can be attributed to the interconnected pores in these membranes. Both qualitative and quantitative studies of antifouling and antibacterial activity (using E. coli as a model bacterium) were performed using the standard plate count method. SEM micrographs clearly showed that the antifouling activity of the membranes was improved with addition of Ag@TiO2-CNTs. In the quantitative standard plate count method, the bacterial colony significantly decreased with the addition of Ag@TiO2-CNTs as against neat blends. This study opens a new avenue in the fabrication of polymer blend based membranes for water filtration.

A rhodamine-based `turn-on' Al3+ ion-selective reporter and the resultant complex as a secondary sensor for F- ion are applicable to living cell staining

A newly designed fluorescent aluminum(III) complex (L'-Al; 2) of a structurally characterized non-fluorescent rhodamine Schiff base (L) has been isolated in pure form and characterized using spectroscopic and physico-chemical methods with theoretical density functional theory (DFT) support. On addition of Al(III) ions to a solution of L in HEPES buffer (1 mM, pH 7.4; EtOH-water, 1 : 3 v/v) at 25 degrees C, the systematic increase in chelation-enhanced fluorescence (CHEF) enables the detection of Al(III) ions as low as 60 nM with high selectivity, unaffected by the presence of competitive ions. Interestingly, the Al(III) complex (L'-Al; 2) is specifically able to detect fluoride ions by quenching the fluorescence in the presence of large amounts of other anions in the HEPES buffer (1 mM, pH 7.4) at 25 degrees C. On the basis of our experimental and theoretical findings, the addition of Al3+ ions to a solution of L helps to generate a new fluorescence peak at 590 nm, due to the selective binding of Al3+ ions with L in a 1 : 1 ratio with a binding constant (K) of 8.13 x 10(4) M-1. The Schiff base L shows no cytotoxic effect, and it can therefore be employed for determining the intracellular concentration of Al3+ and F-ions by 2 in living cells using fluorescence microscopy.

Unusual Fragility and Cooperativity in Glass-Forming and Crystalline PVDF/PMMA Blends in the Presence of Multiwall Carbon Nanotubes

Poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA) are completely miscible below 50 wt % PVDF in the blends. In this work, an attempt was made to understand the fragility/cooperativity relation in glass-forming and crystalline blends of PVDF/PMMA and in the presence of a heteronucleating agent, multiwall carbon nanotubes (CNTs). Hence, three representative blends were chosen: a completely amorphous (10/90 by wt, PVDF/PMMA), on the verge of amorphous miscibility (50/50 by wt, PVDF/PMMA), and crystalline (60/40 by wt, PVDF/PMMA) blends. The intermolecular cooperativity/coupling, fragility, and configurational entropy near the glass transition temperature (T-g) were studied using differential scanning calorimetry (DSC) and broadband dielectric relaxation spectroscopy (DRS). It was observed that the blends with higher concentration of PMMA were more fragile (fragility index m = 141) and those with higher concentration of PVDF were more strong (m = 78). Interestingly, the coupling was less in the glass-forming blends (10/90 by wt, PVDF/PMMA) than the crystalline blends as manifested from DRS. This observation was also supported by DSC measurements which reflected that the cooperative rearranging region (CRR) existed over a smaller length scales in fragile blends as compared to strong blends, possibly due to restricted amorphous mobility. This effect was more prominent in the presence of CNTs, in particular for 50/50 (by wt) and 60/40 (by wt) PVDF/PMMA blends. Further, the configurational entropy, as manifested from DRS, decreased significantly in the strong blends in striking contrast to the fragile blends, supported by DSC, which manifested in an increase in the volume of cooperativity in the strong blends. The higher coupling in the crystalline blends can be attributed to good packing of the amorphous regions. While this is understood for crystalline blends (60/40 by wt, PVDF/PMMA), it is envisaged that enhanced dynamic heterogeneity is accountable for increased coupling in the case of blends which are on the verge of amorphous miscibility (50/50 by wt, PVDF/PMMA). The latter is also supported by broad relaxations near the T-g in DRS. Interestingly, the intermolecular coupling in the blends in the presence of CNTs has reduced, though the potential energy barrier hindering the rearrangement of CRR is lower than the blends without CNTs. In addition, the amorphous packing is not as effective as the blends without CNTs. This is manifested from reduced volume of cooperativity in particular, for 50/50 (by wt) and 60/40 (by wt) blends.

The X-C---Y (X = O/F, Y = O, S, F, Cl, Br, N, P,) Carbon bond and hydrophobic interactions

While the tetrahedral face of methane has an electron rich centre and can act as a hydrogen bond acceptor, substitution of one of its hydrogens with some electron withdrawing group (such as -F/OH) can make the opposite face electron deficient. Electrostatic potential calculations confirm this and high level quantum calculations show interactions between the positive face of methanol/methyl fluoride and electron rich centers of other molecules such as H2O. Analysis of the wave functions of atoms in molecules shows the presence of an unusual C···Y interaction, which could be called 'carbon bonding'. NBO analysis and vibrational frequency shifts confirm the presence of this interaction. Given the properties of alkyl groups bonded to electronegative elements in biological molecules, such interactions could play a significant role, which is yet to be recognized. This and similar interactions could give an enthalpic contribution to what is called the 'hydrophobic interactions'.

Solid-state optical absorption from optimally tuned time-dependent range-separated hybrid density functional theory

We present a framework for obtaining reliable solid-state charge and optical excitations and spectra from optimally tuned range-separated hybrid density functional theory. The approach, which is fully couched within the formal framework of generalized Kohn-Sham theory, allows for the accurate prediction of exciton binding energies. We demonstrate our approach through first principles calculations of one- and two-particle excitations in pentacene, a molecular semiconducting crystal, where our work is in excellent agreement with experiments and prior computations. We further show that with one adjustable parameter, set to produce the known band gap, this method accurately predicts band structures and optical spectra of silicon and lithium fluoride, prototypical covalent and ionic solids. Our findings indicate that for a broad range of extended bulk systems, this method may provide a computationally inexpensive alternative to many-body perturbation theory, opening the door to studies of materials of increasing size and complexity.

New physical insights into the electromagnetic shielding efficiency in PVDF nanocomposites containing multiwall carbon nanotubes and magnetic nanoparticles

This work attempts to bring critical insights into the electromagnetic shielding efficiency in polymeric nanocomposites with respect to the particle size of magnetic nanoparticles added along with or without a conductive inclusion. To gain insight, various Ni-Fe (NixFe1-x; x = 10, 20, 40; Ni: nickel, Fe: iron) alloys were prepared by a vacuum arc melting process and different particle sizes were then achieved by a controlled grinding process for different time scales. Poly(vinylidene fluoride), PVDF based composites involving different particle sizes of the Ni-Fe alloy were prepared with or without multiwall carbon nanotubes (MWNTs) by a wet grinding approach. The Ni-Fe particles were thoroughly characterized with respect to their microstructure and magnetization; and the electromagnetic (EM) shielding efficiency (SE) of the resulting composites was obtained from the scattering parameters using a vector network analyzer in a broad range of frequencies. The saturation magnetization of Ni-Fe nanoparticles and the bulk electrical conductivity of PVDF/Ni-Fe composites scaled with increasing particle size of NiFe. Interestingly, the PVDF/Ni-Fe/MWNT composites showed a different trend where the bulk electrical conductivity and SE scaled with decreasing particle size of the Ni-Fe alloy. A total SE of similar to 35 dB was achieved with 50 wt% of Ni60Fe40 and 3 wt% MWNTs. More interestingly, the PVDF/Ni-Fe composites shielded the EM waves mostly by reflection whereas, the PVDF/Ni-Fe/MWNT shielded mostly by absorption. A minimum reflection loss of similar to 58 dB was achieved in the PVDF/Ni-Fe/MWNT composites in the X-band (8-12 GHz) for a particular size of Ni-Fe alloy nanoparticles. This study brings new insights into the EM shielding efficiency in PVDF/magnetic nanoparticle based composites in the presence and absence of conducting inclusion.

Attenuating microwave radiation by absorption through controlled nanoparticle localization in PC/PVDF blends

Nanoscale ordering in a polymer blend structure is indispensable to obtain materials with tailored properties. It was established here that controlling the arrangement of nanoparticles, with different characteristics, in co-continuous PC/PVDF (polycarbonate/poly(vinylidene fluoride)) blends can result in outstanding microwave absorption (ca. 90%). An excellent reflection loss (RL) of ca. -71 dB was obtained for a model blend structure wherein the conducting (multiwall carbon nanotubes, MWNTs) and the magnetic inclusions (Fe3O4) are localized in PVDF and the dielectric inclusion (barium titanate, BT) is in PC. The MWNTs were modified using polyaniline, which facilitates better charge transport in the blends. Furthermore, by introducing surface active groups on BT nanoparticles and changing the macroscopic processing conditions, the localization of BT nanoparticles can be tailored, otherwise BT nanoparticles would localize in the preferred phase (PVDF). In this study, we have shown that by ordered arrangement of nanoparticles, the incoming EM radiation can be attenuated. For instance, when PANI-MWNTs were localized in PVDF, the shielding was mainly through reflection. Now by localizing the conducting inclusion and the magnetic lossy materials in PVDF and the dielectric materials in PC, an outstanding shielding effectiveness of ca. -37 dB was achieved where shielding was mainly through absorption (ca. 90%). Thus, this study clearly demonstrates that lightweight microwave absorbers can be designed using polymer blends as a tool.

Tailor-Made Distribution of Nanoparticles in Blend Structure toward Outstanding Electromagnetic Interference Shielding

Engineering blend structure with tailor-made distribution of nanoparticles is the prime requisite to obtain materials with extraordinary properties. Herein, a unique strategy of distributing nanoparticles in different phases of a blend structure has resulted in >99% blocking of incoming electromagnetic (EM) radiation. This is accomplished by designing a ternary polymer blend structure using polycarbonate (PC), poly(vinylidene fluoride) (PVDF), and poly(methyl methacrylate) (PMMA) to simultaneously improve the structural, electrical, and electromagnetic interference shielding (EMI). The blend structure was made conducting by preferentially localizing the multi-wall nanotubes (MWNTs) in the PVDF phase. By taking advantage of pp stacking MWNTs was noncovalently modified with an imidazolium based ionic liquid (IL). Interestingly, the enhanced dispersion of IL-MWNTs in PVDF improved the electrical conductivity of the blends significantly. While one key requisite to attenuate EM radiation (i.e., electrical conductivity) was achieved using MWNTs, the magnetic properties of the blend structure was tuned by introducing barium ferrite (BaFe) nanoparticles, which can interact with the incoming EM radiation. By suitably modifying the surface of BaFe nanoparticles, we can tailor their localization under the macroscopic processing condition. The precise localization of BaFe nanoparticles in the PC phase, due to nucleophilic substitution reaction, and the MWNTs in the PVDF phase not only improved the conductivity but also facilitated in absorption of the incoming microwave radiation due to synergetic effect from MWNT and BaFe. The shielding effectiveness (SE) was measured in X and K-u band, and an enhanced SE of -37 dB was noted at 18 GHz frequency. PMMA, which acted as an interfacial modifier in PC/PVDF blends further, resulting in a significant enhancement in the mechanical properties besides retaining high SE. This study opens a new avenue in designing mechanically strong microwave absorbers with a suitable combination of materials.