Photoresponsive Circular Supramolecular Polymers: A Topological Trap and Photoinduced Ring‐Opening Elongation

Topological features of one‐dimensional macromolecular chains govern the properties and functionality of natural and synthetic polymers. To address this issue in supramolecular polymers, we synthesized two topologically distinct supramolecular polymers with intrinsic curvature, circular and helically folded nanofibers, from azobenzene‐functionalized supramolecular rosettes. When a mixture of circular and helically folded nanofibers was exposed to UV light, selective unfolding of the latter open‐ended supramolecular polymers was observed as a result of the curvature‐impairing internal force produced by the trans‐to‐cis photoisomerization of the azobenzene. This distinct sensitivity suggests that the topological features of supramolecular polymers define their mechanical stability. Furthermore, the exposure of circular supramolecular polymers in more polar media to UV irradiation resulted in ring opening followed by chain elongation, thus demonstrating that the circular supramolecular polymer can function as a topological kinetic trap.     

Ligand-redox assisted nickel catalysis toward stereoselective synthesis of (n+1)-membered cycloalkanes from 1,n-diols with methyl ketones

A well-defined, bench-stable nickel catalyst is presented here, that can facilitate double alkylation of a methyl ketone to realize a wide variety of cycloalkanes. The performance of the catalyst depends on the ligand redox process comprising an azo-hydrazo couple. The source of the bis electrophile in this double alkylation is a 1,n-diol, so that (n+1)-membered cycloalkanes can be furnished in a stereoselective manner. The reaction follows a cascade of dehydrogenation/hydrogenation reactions and adopts a borrowing hydrogen (BH) method. A thorough mechanistic analysis including the interception of key radical intermediates and DFT calculations supports the ligand radical-mediated dehydrogenation and hydrogenation reactions, which is quite rare in BH chemistry. In particular, this radical-promoted hydrogenation is distinctly different from conventional hydrogenations involving a metal hydride and complementary to the ubiquitous two-electron driven dehydrogenation/hydrogenation reactions.

A homogeneous nickel catalyst is described that forms (n+1)-membered cycloalkane rings from ketones and 1,n-diols following a radical-promoted pathway.     

Spectroscopic and Photophysical Characterization of Fluorescent Chemosensors for Monosaccharides Based on N-Phenylboronic Acid Derivatives of 1,8-Naphthalimide

Spectroscopic and photophysical properties of two fluorescent probes for monosaccharides are presented. Probes are based on the N-phenyl-1,8-naphthalimide structure having the boronic acid group [R-B(OH)₂] in ortho in one case, and meta in the other case, positions of the N-phenyl group. Formation of the anionic form of the boronic acid group [R-B(OH)^– ₃] induced a substantial decrease of the steady-state fluorescence of both compounds. Because no change in the fluorescence lifetime from the neutral to the anionic forms is observed, static quenching resulting from photoinduced electron transfer from the anionic form of the boronic acid is used to explain the decrease of the emission intensity. Both compounds show substantial decreases of their fluorescence intensity in the presence of sugars. In addition, this decrease of the fluorescence intensity is associated with an increase of the fluorescence lifetime for the ortho derivative while no effect on the lifetime is observed for the meta derivative. Both photoinduced electron transfer and steric hindrance are discussed to correlate the observed results.     

Fourier grid Hamiltonian‐based multidimensional Floquet propagator method for continuous,pulsed, and bichromatic laser field: Application to the multiphoton dissociation dynamics of HCN molecule

We present a Fourier grid Hamiltonian‐based multidimensional propagator method for multiphoton dissociation dynamics of triatomic molecule in continuous, pulsed, and bichromatic laser field. The method has been successfully applied to the photodissociation dynamics of HCN molecule, i.e., collinear geometry. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Grass fiber reinforced phenol formaldehyde resin composite: preparation,characterization and evaluation of properties of composite

There is a growing interest in the development of new materials through utilization of natural resources. This paper describes evaluation of water leached and alkali treated chopped grass fiber reinforced phenol formaldehyde composite. Here alkali treatment of grass fiber was carried out by varying the concentration of sodium hydroxide. The thermal stability of the composite was assessed by thermogravimetric analysis (TGA). Fourier transformation infrared spectroscopic study of both water leached and alkali treated grass fiber‐phenolic resin composite was also performed. Water absorption and swelling behavior of grass fiber phenolic resin composites in water were studied and the alkali treated grass fiber‐resin composite showed less water absorption and swelling. A composite prepared from 1% alkali treated grass fiber and 55% resin, showed the highest tensile strength whereas a composite prepared from 5% alkali treated grass fiber and 55% resin, showed maximum flexural properties. Copyright © 2006 John Wiley & Sons, Ltd.     

Conductometric determination of cerium(III) as metavanadate

Summary Conductometric determination of Ce³⁺ has been carried out successfully at 30±0.5°C by titrating it against standard sodium metavanadate solution in 15% ethanol. An overall effect of ethanol concentration on the results has also been studied. The titration curves obtained show a sharp break at the end point and lend support to the formation of Ce(VO₃)₃. The method has the advantage of being a simple and precise titrimetric procedure for the determination of Ce³⁺ in microquantities.

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Zusammenfassung Es wird eine einfache und genaue Methode zur konduktometrischen Bestimmung von Cer(III) beschrieben. Die Titration erfolgt mit Natriummetavanadat in 15%iger äthanolischer Lösung bei 30±0,5°C. Der Einfluß der Äthanolkonzentration auf die Ergebnisse wurde untersucht. Die Titrationskurven weisen am Endpunkt einen scharfen Knick auf und deuten auf die Bildung von Ce(VO₃)₃ hin.

     

A quantum-classical approach to the photoabsorption spectrum of pyrazine

We have used the time-dependent discrete variable representation (TDDVR) method to simulate the photoabsorption spectrum of pyrazine. The time-dependent molecular dynamics of pyrazine after excitation to the S2 electronic state is considered as a benchmark to investigate the S2 absorption spectrum. We have carried out the dynamics on a basic four-mode model of pyrazine with the inclusion of five major modes as well as the rest of the vibrational modes as bath modes. Investigations reveal the effect of bath modes such as energy and population transfer from the subsystem to the bath. Calculated results demonstrate excellent agreement with traditional quantum-mechanical findings during the entire propagation and converge to the exact quantum results when enough gridpoints are used. It appears that TDDVR, as a numerical quantum dynamics methodology, is a good compromise between accuracy and speed.