Synthesis of 6-carboxy-2,2-dimethylchromans and chromenes

The condensation of hydroxybenzoic acids and their methyl esters with isoprene in the presence of orthophosphoric acid gives corresponding 2,2-dimethylchromans, which can be dehydrogenated with DDQ or NBS to give the corresponding 6-carboxy-2,2-dimethylchromenes. The synthesis of β-tubaic acid, a natural compound, has also been achieved.     

Remarkably stable amphiphilic random copolymer assemblies: A structure–property relationship study

Synthesis of a library of amphiphilic random copolymers from a single reactive pre‐polymer and their self‐assembly is reported. Post‐polymerization modifications of the parent polymer containing pendant N‐hydroxy succinimide (NHS) ester groups with various oligooxyethylene (OE) amines produce amphiphilic random copolymers with same degree of polymerization and equal extent of randomness. ¹H‐NMR and FT‐IR data indicate quantitative substitution in all cases. The critical aggregation concentration (CAC) for all the polymers is estimated to be in the range of 10^?5 M. Stability of these nano‐aggregates is studied by photoluminescence using time dependent F—rster Resonance Energy Transfer (FRET) between co‐encapsulated lipophilic dyes namely DiO and DiI in the hydrophobic pocket of the aggregates. These studies suggest remarkably high stability for all systems. However those with shorter hydrophilic pendant chains are found to be even more robust. Morphology is examined by high resolution transmission electron microscopy (HRTEM) which reveals multi‐micellar clusters and vesicles for polymers containing short and longer OE segments, respectively. Encapsulation efficacy is tested with both hydrophobic and hydrophilic guest molecules. All of them can encapsulate hydrophobic guest pyrene while a hydrophilic dye Calcein can be sequestered only in vesicle forming polymers. Lower critical solution temperature (LCST) is exhibited by only one polymer that contains the shortest OE chains. All polymers exhibit excellent cell viability as determined by MTT assay. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4932–4943     

Numerical study of vortex effects in superconducting microstrip lines

A high transition temperature superconducting microstrip structure is modeled using the theoretical approach developed by Coffey and Clem for elucidating the vortex effects in propagation. Impedance type Green’s functions are derived for the electric field around the strip and the propagation characteristics are computed for a wide range of applied field, reduced temperature and superconducting strip thickness in a Galerkin procedure. The increase of static field and temperature result in increased vortex motion, which in turn, causes a corresponding change in the propagation characteristics of the transmission line. Numerical results are presented for propagation parameters and quality factor.     

Static and synchronized switching noise management of replicated optical pulse trains

Demonstration is provided for the generation of high quality replicated optical pulse trains, from a single pulse. This uses the storage capability of an electro-optically switched recirculating fibre loop. For better understanding, the first step involves the development of general purpose derivations related to the evolution of the noise with near-unity loop gains. This is a basic tool to evaluate the propagation of the noise and the temporal evolution of the signal to noise ratio during the replication process. In a second step, the comparison of theoretical and experimental results will be shown to evidence good agreement, up to more than 1000 replica. Finally, experiments are performed on synchronised gain-switching inside the loop to help optimise the dynamic range of replication at the output of the system.     

Third harmonic generation in layered media in presence of optical bistability of the fundamental

We study third harmonic generation in layered configuration when the fundamental exhibits bistable response. We consider two geometries, namely, a Fabry-Perot cavity with reflection coatings and a distributed feedback structure with alternate nonlinear layers. In both the cases for suitable choice of frequency, the power response at the fundamental frequency is bistable. We show that bistability of the fundamental leads to a multivalued character of the generated third harmonic in both the forward and backward directions. Moreover, we study frequency response in the case of the Fabry-Perot cavity and show that additional structures arise in the generated third harmonic due to frequency bistability of the fundamental. Our calculations suggest the possibility of an all optical switch at third harmonic frequency controlled by the parameters (like intensity, frequency) of the fundamental.     

On computing the long-time solution of the two-dimensional Navier-Stokes equations

The long-time behavior of the incompressible Navier-Stokes equations on a 2-torus is computed using several discretizations in space and in time. We compare several nonlinear Galerkin methods (NGMs) with the standard Galerkin method (SGM) in both their ability to capture the dynamic and geometric behavior of a turbulent flow accurately, and the computational efficiency with which they are solved using variable time-step integration schemes. To measure the convergence of the chaotic attractor we employ Poincaré section plots as well as density functions of instantaneous Lyapunov exponents. As expected, for the smooth (single mode) force used, the NGMs fare no better than the SGM in reproducing the correct behavior. As the number of modes for each method is increased, however, a distinct advantage in computational cost emerges for the NGMs. This trend suggests that the correct solution may be computed at considerable savings for a nonsmooth force, at higher Reynolds numbers.This work was supported in part by the U.S. Department of Energy under Grant No. DE-FG02-92ER25120, and by the U.S. Office of Naval Research under Grant No. N00014-91-J-1140. Computations were mainly performed on the Gray Y-MP at the Ohio Supercomputer Center under project PAS620. Those on the CM-200 were carried out with resources supported in part by the Army Research Office contract with the University of Minnesota Army High Performance Computing Research Center.     

Hopping of chaotic dynamics in a doubly resonant parametric oscillator

We study a doubly resonant optical parametric oscillator where the pump can feed two pairs of signal-idler modes. We assume the presence of gain at the pump frequency. We investigate the various oscillation states of interest, namely, when only the first pair oscillates with the other pair having null amplitudes and vice versa. We demonstrate the exchange of dynamics between the mode pairs when the relevant parameters of the cavity, namely, the phase mismatch factors or the decay rates switch because of fluctuations. The exchange of dynamics is shown to be independent of the nature of dynamics, i.e. independent of whether the motion isn-periodic or chaotic. We also investigate the case where both the pairs can exhibit chaotic dynamics though these states are difficult to realize because of fluctuations.     

Simulation of microscopic chemical processes. III. Deactivation and dissociation of bromine in argon at high pressure

Previous numerical simulation of the vibrational relaxation of highly excited bromine in argon at different pressures is here extended in several ways.