Hydrogen-Bonding Induced Alternate Stacking of Donor (D) and Acceptor (A) Chromophores and their Supramolecular Switching to Segregated States

This paper reports comprehensive studies on the mixed assembly of bis-(trialkoxybenzamide)-functionalized dialkoxynaphthalene (DAN) donors and naphthalene-diimide (NDI) acceptors due the cooperative effects of hydrogen bonding, charge-transfer (CT) interactions, and solvophobic effects. A series of DAN as well as NDI building blocks have been examined (wherein the relative distance between the two amide groups in a particular chromophore is the variable structural parameter) to understand the structure-dependent variation in mode of supramolecular assembly and morphology (organogel, reverse vesicle, etc.) of the self-assembled material. Interestingly, it was observed that when the amide functionalities are introduced to enhance the self-assembly propensity, the mode of co-assembly among the DAN and NDI chromophores no longer remained trivial and was dictated by a relatively stronger hydrogen-bonding interaction instead of a weak CT interaction. Consequently, in a highly non-polar solvent like methylcyclohexane (MCH), although kinetically controlled CT-gelation was initially noticed, within a few hours the system sacrificed the CT-interaction and switched over to the more stable self-sorted gel to maximize the gain in enthalpy from the hydrogen-bonding interaction. In contrast, in a relatively less non-polar solvent such as tetrachloroethylene (TCE), in which the strength of hydrogen bonding is inherently weak, the contribution of the CT interaction also had to be accounted for along with hydrogen bonding leading to a stable CT-state in the gel or solution phase. The stability and morphology of the CT complex and rate of supramolecular switching (from CT to segregated state) were found to be greatly influenced by subtle structural variation of the building blocks, solvent polarity, and the DAN/NDI ratio. For example, in a given D-A pair, by introducing just one methylene unit in the spacer segment of either of the building blocks a complete change in the mode of co-assembly (CT state or segregated state) and the morphology (1D fiber to 2D reverse vesicle) was observed. The role of solvent polarity, structural variation, and D/A ratio on the nature of co-assembly, morphology, and the unprecedented supramolecular-switching phenomenon have been studied by detail spectroscopic and microscopic experiments in a gel as well as in the solution state and are well supported by DFT calculations.     

Selective recognition of fluoride and acetate by a newly designed ruthenium framework: experimental and theoretical investigations

An effective anion sensor, [Ru(II)(bpy)(2)(H(2)L(-))](+) (1(+)), based on a redox and photoactive {Ru(II)(bpy)(2)} moiety and a new ligand (H(3)L = 5-(1H-benzo[d]imidazol-2-yl)-1H-imidazole-4-carboxylic acid), has been developed for selective recognition of fluoride (F(-)) and acetate (OAc(-)) ions. Crystal structures of the free ligand, H(3)L and [1](ClO(4)) reveal the existence of strong intramolecular and intermolecular hydrogen bonding interactions. The structure of [1](ClO(4)) shows that the benzimidazole N-H of H(2)L(-) is hydrogen bonded with the pendant carboxylate oxygen while the imidazole N-H remains free for possible hydrogen bonding interaction with the anions. The potential anion sensing features of 1(+) have been studied by different experimental and theoretical (DFT) investigations using a wide variety of anions, such as F(-), Cl(-), Br(-), I(-), HSO(4)(-), H(2)PO(4)(-), OAc(-) and SCN(-). Cyclic voltammetry and differential pulse voltammetry established that 1(+) is an excellent electrochemical sensor for the selective recognition of F(-) and OAc(-) anions. 1(+) is also found to be a selective colorimetric sensor for F(-) or OAc(-) anions where the MLCT band of the receptor at 498 nm is red shifted to 538 nm in the presence of one equivalent of F(-) or OAc(-) with a distinct change in colour from reddish-orange to pink. The binding constant between 1(+) and F(-) or OAc(-) has been determined to be logK = 7.61 or 7.88, respectively, based on spectrophotometric titration in CH(3)CN. The quenching of the emission band of 1(+) at 716 nm (λ(ex) = 440 nm, Φ = 0.01 at 298 K in CH(3)CN) in the presence of one equivalent of F(-) or OAc(-), as well as two distinct lifetimes of the quenched and unquenched forms of the receptor 1(+), makes it also a suitable fluorescence-based sensor. All the above experiments, in combination with (1)H NMR, suggest the formation of a 1:1 adduct between the receptor (1(+)) and the anion (F(-) or OAc(-)). The formation of 1:1 adduct {[1(+)·F(-)] or [1(+)·OAc(-)]} has been further evidenced by in situ ESI-MS(+) in CH(3)CN. Though the receptor, 1(+), is comprised of two N-H protons associated with the coordinated H(2)L(-) ligand, only the free imidazole N-H proton participates in the hydrogen bonding interactions with the incoming anions, while the intramolecularly hydrogen bonded benzimidazole N-H proton remains intact as evidenced by the crystal structure of the final product (1). The hydrogen bond mediated anion sensing mechanism, over the direct deprotonation pathway, in 1(+) has been further justified by a DFT study and subsequent NBO analysis.     

Self-assembled one dimensional functionalized metal-organic nanotubes (MONTs) for proton conduction

Two self-assembled isostructural functionalized metal-organic nanotubes have been synthesized using 5-triazole isophthalic acid (5-TIA) with In(III) and Cd(II). In- and Cd-5TIA possess one-dimensional (1D) nanotubular architecture and show proton conductivity along regular 1D channels, measured as 5.35 × 10(-5) and 3.61 × 10(-3) S cm(-1) respectively.     

Probing platinum degradation in polymer electrolyte membrane fuel cells by synchrotron X-ray microscopy

Synchrotron-based scanning transmission X-ray spectromicroscopy (STXM) was used to characterize the local chemical environment at and around the platinum particles in the membrane (PTIM) which form in operationally tested (end-of-life, EOL) catalyst coated membranes (CCMs) of polymer electrolyte membrane fuel cells (PEM-FC). The band of metallic Pt particles in operationally tested CCM membranes was imaged using transmission electron microscopy (TEM). The cathode catalyst layer in the beginning-of-life (BOL) CCMs was fabricated using commercially available catalysts created from Pt precursors with and without nitrogen containing ligands. The surface composition of these catalyst powders was measured by X-ray Photoelectron Spectroscopy (XPS). The local chemical environment of the PTIM in EOL CCMs was found to be directly related to the Pt precursor used in CCM fabrication. STXM chemical mapping at the N 1s edge revealed a characteristic spectrum at and around the dendritic Pt particles in CCMs fabricated with nitrogen containing Pt-precursors. This N 1s spectrum was identical to that of the cathode and different from the membrane. For CCM samples fabricated without nitrogen containing Pt-precursors the N 1s spectrum at the Pt particles was indistinguishable from that of the adjacent membrane. We interpret these observations to indicate that nitrogenous ligands in the nitrogen containing precursors, or decomposition product(s) from that source, are transported together with the dissolved Pt from the cathode into the membrane as a result of the catalyst degradation process. This places constraints on possible mechanisms for the PTIM band formation process.     

Sequential [2+2] Photocycloaddition and Rearrangement to Substituted Cyclopentanone. A formal Synthesis of Planococcyl Acetate

An extremely short synthesis of 2,2-dimethyl-3-hydroxymethyl cyclopentanone 2 (R¹ = R² = Me), an advanced intermediate in the synthesis of the pheromone planococcyl acetate 3, is described.     

Cyclisation of Ortho -Cyclohexenyl Phenols via Epoxidation #

A number of ortho -cyclohexenyl phenols 2(a–i) have been cyclised by treatment with one equivalent of m-chloroper-oxybenzoic acid in refluxing benzene for 4 h to furnish linearly fused 1-hydroxy-1,2,3,4,4a,9a-hexahydrodibenzofurans 3(a–i) (70–80%) and bicyclic 3-hydroxy-1,3-ethanochromans 4(a–f) (10–20%). Products 3(a–i) were oxidised with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (excess) in refluxing dry xylene for 6 h to give 2,3-dihydrodibenzofuran-1 (2H)-ones 6(a–i) (85–95%).

     

Electronic phase separation in transition metal oxide systems

Electronic phase separation is increasingly getting recognized as a phenomenon of importance in understanding the magnetic and electron transport properties of transition metal oxides. The phenomenon dominates the rare-earth manganates of the formula Ln(1-x)A(x)MnO(3)(Ln = rare earth and A = alkaline earth) which exhibit ferromagnetism and metallicity as well as charge-ordering, depending on the composition, size of A-site cations and external factors such as magnetic and electric fields. We discuss typical phase separation scenarios in the manganates, with particular reference to Pr(1-x)Ca(x)MnO(3)(x= 0.3-0.4), (La(1-x)Ln(x))(0.7)Ca(0.3)MnO(3)(Ln = Pr, Nd, Gd and Y) and Nd(0.5)Sr(0.5)MnO(3). Besides discussing the magnetic and electron transport properties, we discuss electric field effects. Rare-earth cobaltates of the type Pr(0.7)Ca(0.3)CoO(3) and Gd(0.5)Ba(0.5)CoO(3) also exhibit interesting magnetic and electron transport properties which can be understood in terms of phase separation.     

The adiabatic‐to‐diabatic transformation angle and topological phases for strongly interacting states: Solution with four states

A few years back T. Vértesi et al. (J Phys Chem A 2005, 109, 3476) gave an elegant procedure to derive the adiabatic‐to‐diabatic transformation angle (ADT‐angle) for lowest two states, γ₁₂, in presence of their interaction with the higher states. In this article, we explicitly solved the coupled differential equations involving the ADT‐angles describing the mixing of four interacting states for the first time to get all the ADT‐angles, γ_ij's (including the γ₁₂) associated with well‐defined topological phases. In application to a suitable configuration space (CS) of a molecular system, we have demonstrated well‐defined Berry phases (Berry M. V., R Soc London Ser A 1984, 45, 392) for an isolated system with as many as four strongly interacting states. © 2012 Wiley Periodicals, Inc.