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.     

DNA binding, nuclease activity and cytotoxicity studies of Cu(II) complexes of tridentate ligands

Cu(II) complexes of three tridentate ligands, L(1), L(2) and L(3), [L(1), N-((1H-imidazole-2-yl)methyl)-2-(pyridine-2-yl)ethanamine; L(2), N-((1-methyl-1H-imidazole-2-yl)methyl)-2-(pyridine-2-yl)ethanamine; L(3), 2-(pyridine-2-yl)-N-((pyridine-2-yl)methyl)ethanamine] respectively, were synthesized and characterized. The single crystal X-ray structure of complex 1 reveals the pseudo octahedral coordination geometry around the copper center. Absorption and fluorescence experimental evidence show good DNA binding propensity (in the order of 10(5) M(-1)) of the complexes. Thermal denaturation and circular dichroism (CD) analyses reveal minor structural changes of calf thymus (CT) DNA in presence of complexes and groove and/or surface binding of the complexes to CT-DNA. Kinetic DNA cleavage assay shows pseudo-first-order kinetic reaction between the complex and supercoiled (SC) DNA. In addition, mechanistic SC DNA cleavage results show higher DNA cleavage activity in presence of reducing agent, due to the presence of hydroxyl radicals. In vitro cytotoxicity assay of the complexes demonstrate that the complexes have low toxicity for different cancer cell lines and IC(50) values were between 37 and 156 μM.     

Spectroscopic Studies of a New Multi-Element Sensitive Fluorescent Probe Derived from 2-(2-pyridyl)benzimidazole: Selective Discrimination of Zn2+ from Its Congeners

ABSTRACT

A new multielement sensitive fluorescent probe, 1-(2-(phenylthio)ethyl)-2-(pyridin-2-yl)-1H-benzo[d]imidazole (L), has been synthesized by the reaction between 2-(pyridyl) benzimidazole and 2-chloroethyl phenyl sulfide. Excitation and emission wavelength of L are at 330 and 371 nm, respectively. Among various transition and nontransition metal ions, it can selectively read Zn²⁺ ion as the emission wavelength of L undergoes a red shift by 31 nm upon binding with Zn²⁺ in methanol. In the presence of Cd²⁺, Hg²⁺, and other common cations, the emission wavelength of L remains unchanged, and thus allows us to discriminate Zn²⁺ from its congeners. Both L and its Zn²⁺ complex are well characterized by different spectroscopic techniques like ¹H-NMR, ESI-TOF (+) mass, FT-IR, and elemental analysis data. The binding constant value of the complexation reaction between L and Zn²⁺ is found as 724.6 M^?1 in methanol. Density functional theoretical (DFT) studies nicely demonstrate the red shift in the emission wavelength of L upon binding with Zn^2+.     

One-dimensional coordination polymer of copper(I) thiocyanate with the Schiff base ligand N,N′-bis(3,4-dimethoxybenzylidene)butane-1,4-diamine: synthesis, crystal structure, spectroscopic and thermal properties

Abstract  

A new one-dimensional polymeric copper(I)–thiocyanate complex with the Schiff base ligand N,N′-bis(3,4-dimethoxybenzylidene)butane-1,4-diamine, {Cu₂((μ _N,N′ -3,4-MeO-ba)₂bn)(μ_1,3-NCS)₂} _n , was synthesized and characterized by elemental analysis, ¹H and ¹³C NMR, FT–IR spectroscopy, and thermal analysis. The thermal behavior of the complex was studied using thermogravimetry in order to evaluate thermal stability and thermal decomposition pathways. The molecular structure of the complex was determined by single-crystal X-ray diffraction which revealed that the coordination geometry around the copper(I) ion is distorted trigonal. The Schiff base ligand (3,4-MeO-ba)₂bn acts as a bis-monodentate and bridging ligand (μ _N,N′ ) and coordinates via two N atoms to the metal centers and adopts an E,E conformation. The coordination spheres of the metal atoms are completed by the N and S atoms from two thiocyanate anion bridges (μ_1,3-NCS), forming a zigzag chain propagating along [001].     

Multiple topologies from glycopolypeptide-dendron conjugate self-assembly: nanorods, micelles, and organogels

Glycopolypeptides (GPs) were synthesized by ring-opening polymerization of glycosylated N-carboxyanhydride monomer and attached to hydrophobic dendrons at one chain end by "click" reaction to obtain amphiphilic anisotropic macromolecules. We show that by varying polypeptide chain length and dendron generation, an organogel was obtained in dimethylsulfoxide, while nanorods and micellar aggregates were observed in aqueous solutions. Assemblies in water were characterized by electron microscopy and dye encapsulation. Secondary structure of the GP chain was shown to affect the morphology, whereas the chain length of the poly(ethylene glycol) linker between the GP and dendron did not alter rod-like assemblies. Bioactive surface chemistry of these assemblies displaying carbohydrate groups was demonstrated by interaction of mannose-functionalized nanorods with ConA.     

Bivariate Birnbaum–Saunders distribution and associated inference

Univariate Birnbaum–Saunders distribution has been used quite effectively to model positively skewed data, especially lifetime data and crack growth data. In this paper, we introduce bivariate Birnbaum–Saunders distribution which is an absolutely continuous distribution whose marginals are univariate Birnbaum–Saunders distributions. Different properties of this bivariate Birnbaum–Saunders distribution are then discussed. This new family has five unknown parameters and it is shown that the maximum likelihood estimators can be obtained by solving two non-linear equations. We also propose simple modified moment estimators for the unknown parameters which are explicit and can therefore be used effectively as an initial guess for the computation of the maximum likelihood estimators. We then present the asymptotic distributions of the maximum likelihood estimators and use them to construct confidence intervals for the parameters. We also discuss likelihood ratio tests for some hypotheses of interest. Monte Carlo simulations are then carried out to examine the performance of the proposed estimators. Finally, a numerical data analysis is performed in order to illustrate all the methods of inference discussed here.     

Synthesis,spectral characterisation and electrochemical studies of dichloro-{1-benzyl-2-(arylazo)imidazole}platinum(II) complexes and their dioxolene derivatives

1-Benzyl-2-(arylazo)imidazoles, p-RC₆H₄N=NC₃H₂-N₂1-CH₂Ph [RaaiBz (2); R=H(a), Me(b), Cl(c)], react with K₂PtCl₄ in boiling MeCN–H₂O (1:1 v/v) to give brownish-red Pt(RaaiBz)Cl₂ (3) complexes. Addition of dioxolene in the presence of Et₃N to a CHCl₃–MeOH solution of Pt(RaaiBz)Cl₂ yields green mixed complexes of composition [Pt(RaaiBz)(O,O)] [O,O = catecholate (cat) (4); 4-tert-butylcatecholate (tbcat), (5); 3,5-di-tert-butylcatecholate (dtbcat), (6); tetracholorocatecholate (tccat), (7)] which were characterised by elemental analyses, i.r., u.v.–vis.–near i.r. and ¹H-n.m.r. spectral data. The solution electronic spectra exhibit ligand-to-ligand charge-transfer (l.l.c.t) transitions in the red to near i.r. region; the position and symmetry of the band depend upon the substituent on the dioxolene and arylazoimidazole. This effect is qualitatively assigned as HOMO(dioxolene) LUMO(RaaiBz). A cyclic voltammogram of the dioxolene complex reveals two consecutive oxidative couples corresponding to catechols to semiquinones and semiquinone to quinone, respectively and the reductive couples represent azo reductions.     

Atom-efficient cross-coupling reactions of triarylbismuths with acyl chlorides under Pd(0) catalysis

The atom-efficient cross-coupling reaction of triarylbismuths with a variety of aliphatic, aromatic, and hetero-aromatic acyl chlorides was demonstrated to afford high yields of cross-coupled ketones under palladium catalysis. The corresponding cross-coupling reaction with diacid chlorides also furnished bis-coupled ketones in good yields.     

Nucleophilic ligand displacement in dichloro[2-(arylazo)heterocycle]palladium(II) complexes by benzimidazole – a kinetic and mechanistic study

The reactions of benzimidazole (Bimz) with dichloro{2-(arylazo)pyridine}palladium(II), [Pd(aap)Cl₂ (3), aap = RC₆H₄N= N-2-C₅H₄N; R = H(a), o-Me(b), m-Me(c), p-Me(d), p-Cl(e)] and dichloro{2-(arylazo)pyrimidine}palladium(II), [Pd(aapm)Cl₂ (4), aapm = RC₆H₄N=N-2-C₆H₃N₂; R = H(a), o-Me(b), m-Me(c), p-Me(d), p-Cl(e)] were followed separately. The kinetics were examined under pseudo-first order conditions with reference to Bimz in MeCN at 385 and 390 nm, respectively and at 298 K using a u.v.–vis. spectrophotometer. The product was isolated and characterised as trans-Pd(Bimz)₂Cl₂. The reaction between Pd(aap)Cl₂ and Bimz follows the rate law, rate = K[Bimz]²[Pd(aap)Cl₂], a single step process, whereas biphasic behaviour is observed for the reaction between Pd(aapm)Cl₂ and Bimz; each step is first order with respect to the concentration of complex and to Bimz. The rate data support a nucleophilic association path and the rate decreases upon addition of Cl^–(LiCl). The aryl ring substituent of the arylazoheterocycle influences the substitution rate as follows: k(e) > k(a) > k(d) > k(c) > k(b). The k-values are linearly correlated with Hammett constants with usual deviations for the m-Me and o-Me substituents because of their steric crowding. The rate follows the order: k(aap) > k(aapm). This is unusual with reference to the -acidity order of the heterocycles, pyridine < pyrimidine, as the increased -acidity will enhance the nucleophilic association. The charge density calculation by MNDO method shows that in the pyrimidine ring of arylazopyrimidine, the peripheral m-N(uncoordinated) carries a high negative charge which may retard the nucleophilic association rate. This effect is absent in the pyridine ring of arylazopyridine and may be the reason for the faster single step nucleophilic substitution in Pd(aap)Cl₂.