Geometry-dependent electronic properties of highly fluorescent conjugated molecules

We present a combined experimental/theoretical study of the electronic properties of conjugated para-phenylene type molecules under high pressure up to 80 kbar. Pressure is used as a tool to vary the molecular geometry and intermolecular interaction. The influence of the latter two on singlet and triplet excitons as well as polarons is monitored via optical spectroscopy. We have performed band structure calculations for the planar poly(para-phenylene) and calculated the dielectric function. By varying the intermolecular distances and the length of the polymer repeat unit the observed pressure effects can be explained.     

A new cleavage strategy for ester linked polymer supports: generation of a tertiary alcohol library

The addition of different Grignard reagents to polymer bound esters resulting in the generation of a tertiary alcohol library with concomitant cleavage is described.     

Magnetic and electrical properties of oxygen stabilized nickel nanofibers prepared by the borohydride reduction method

Fine nickel fibers have been synthesized by chemical reduction of nickel ions in aqueous medium with sodium borohydride. The thermal stability and relevant properties of these fibers, as-prepared as well as air-annealed, have been investigated by structural, magnetic and electrical measurements. As-prepared samples appear to have a novel crystal structure due to the presence of interstitial oxygen. Upon annealing in air, the fcc-Ni phase emerges out initially and develops into a nanocomposite subsequently by retaining its fiber-like structure in nano phase. The as-prepared sample is observed to be weakly magnetic at room temperature, but attains surprisingly high magnetization values at low temperatures. This is attributed to the modified spin structure, presumably due to the presence of interstitial oxygen in the lattice. Development of a weakly ferromagnetic and electrically conducting phase upon annealing in air is attributed to the formation of the fcc-Ni phase. The structural phase transformations corroborate well with magnetic and electrical measurements.     

Intramolecular copper(I)-catalyzed 1,3-dipolar cycloaddition of azido-alkynes: synthesis of triazolo-benzoxazepine derivatives and their biological evaluation

Synthesis of a series of [1,2,3] triazolo [5,1-c] [1,4]benzoxazepine derivatives have been accomplished by the intramolecular Cu(I)-catalyzed cycloaddition of azido-alkynes derived from salicylaldehyde. The biological profile of these heterocyclic structural scaffolds toward anti-bacterial as well as anti-fungal activity has also been illustrated.     

Flow chemistry approach for partial deuteration of alkynes: synthesis of deuterated taxol side chain

The partial deuteration of alkynes proceeds via a flow chemistry approach under Lindlar’s heterogeneous catalysis to provide cis-dideuterated olefins in good yields. Further, dideuterated olefin has been successfully utilized for Sharpless asymmetric dihydroxylation followed by Mitsunobu reaction for the synthesis of deuterated taxol side chain.     

A chiral pyrrolidine-pyrazole catalyst for the enantioselective Michael addition of carbonyls to nitroolefins

An enantioselective Michael reaction of carbonyl compounds to nitroolefins has been accomplished using a novel chiral pyrrolidine-pyrazole catalyst. This newly prepared catalyst was found to be very effective in providing good yields as well as good diastereo- and enantio-selectivities. The mechanism of the reaction has also been substantiated by mass spectral studies.     

Organostannoxane-supported multiferrocenyl assemblies: synthesis, novel supramolecular structures, and electrochemistry

Organostannoxane-based multiredox assemblies containing ferrocenyl peripheries have been readily synthesized by a simple one-pot synthesis, either by a solution method or by room-temperature solid-state synthesis, in nearly quantitative yields. The number of ferrocenyl units in the multiredox assembly is readily varied by stoichiometric control as well as by the choice of the organotin precursors. Thus, the reaction of the diorganotin oxides, R2SnO (R = Ph, nBu and tBu) with ferrocene carboxylic acid affords tetra-, di-, and mononuclear derivatives [{Ph2Sn[OC(O)Fc]2}2] (1), [{[nBu2SnOC(O)Fc]2O}2] (2), [nBu2Sn{OC(O)Fc}2] (3), [{tBu2Sn(OH)OC(O)Fc}2] (4), and [tBu2Sn{OC(O)Fc}2] (5) (Fc = eta(5)C5H4-Fe-eta(5)C5H5). The reaction of triorganotin oxides, R3SnOSnR3 (R = nBu and Ph) with ferrocene carboxylic acid leads to the formation of the mono-nuclear derivatives [Ph3SnOC(O)Fc] (6) and [{nBu3SnOC(O)Fc}(n)] (7). Molecular structures of the compounds 1-4 and 6 have been determined by single-crystal X-ray analysis. The molecular structure of compound 1 is new among organotin carboxylates. In this compound, ferrocenyl carboxylates are involved in both chelating and bridging coordination modes to the tin atoms to form an eight-membered cyclic structure. In all of these compounds, the acidic protons of the cyclopentadienyl groups are hydrogen bonded to the carboxylate oxygens (C-HO) to form rich supramolecular assemblies. In addition to this, pi-pi, T-shaped, L-shaped, and side-to-face stacking interactions involving ferrocenyl groups also occur. Compound 6 shows an interesting and novel intermolecular CO2-pi stacking interaction. Electrochemical analysis of the compounds 1-4, 6, and 7 shows a single, quasi-reversible oxidation peak corresponding to the simultaneous oxidation of four, two, and one ferrocenyl substituents, respectively. Compound 5 shows two quasi-reversible oxidation peaks. This is attributed to the positional difference among the ferrocenyl substituents on the tin atom. Additionally, while compounds 2 and 4 are electrochemically quite robust and do not decompose even after ten continuous CV cycles, compounds 1, and 3, 5-7 start to show decomposition after five cycles.     

Differentiation of Positional Isomers of Hybrid Peptides Containing Repeats of β-Nucleoside Derived Amino Acid (β-Nda-) and L-Amino Acids by Positive and Negative Ion Electrospray Ionization Tandem Mass Spectrometry (ESI-MS<Superscript><Emphasis Type="Italic">n</Emphasis></Superscript>)

A new class of positional isomeric pairs of -Boc protected oligopeptides comprised of alternating nucleoside derived β-amino acid (β-Nda-) and L-amino acid residues (alanine, valine, and phenylalanine) have been differentiated by both positive and negative ion electrospray ionization ion-trap tandem mass spectrometry (ESI-MS ⁿ ). The protonated dipeptide positional isomers with β-Nda- at the N-terminus lose CH₃OH, NH₃, and C₂H₄O₂, whereas these processes are absent for the peptides with L-amino acids at the N-terminus. Instead, the presence of L-amino acids at the N-terminus results in characteristic retro-Mannich reaction involving elimination of imine. A good correlation has been observed between the conformational structure of the peptides and the abundance of y_n⁺ and b_n⁺ ions in MS ⁿ spectra. In the case of tetrapeptide isomers that are reported to form helical structures in solution phase, no y_n⁺ and b_n⁺ ions are observed when the corresponding amide -NH- participates in the helical structures. In contrast, significant y_n⁺ and b_n⁺ ions are formed when the amide -NH- is not involved in the H-bonding. In the case of tetra- and hexapeptides, it is observed that abundant b_n⁺ ions are formed, presumably with stable oxazolone structures when the C-terminus of the b_n⁺ ions possessed L-amino acid and the β-Nda- at the C-terminus appears to prevent the cyclization process leading to the absence of corresponding b_n⁺ ions.     

Preparation and characterization of MgB2 superconductor

The MgB₂ superconductor, synthesized using solid-state and liquid-phase sintering methods, have been characterized for various properties. The upper critical field, irreversibility line and critical current density have been determined using magnetization data. The current-voltage characteristics recorded under an applied magnetic field revealed the existence of vortex glass transition. The surface analysis using X-ray photoelectron spectroscopy shows that MgB₂ is sensitive to atmospheric degradation.     

A new structural form of tin in a double O-capped cluster

A novel hexameric organooxotin cage [{(n-BuSn)3(PhO)3O}2{HPO3}4] 1 has been assembled by the reaction of [n-BuSnO(OH)] with (PhO)2P(O)H. The structure of 1 consists of two O-capped Sn3 clusters that are attached to each other by four bridging tripodal [HPO3]2- ligands.