Enhanced optoelectronic properties of UV-light-induced photodegraded TPD

We report UV-light-induced photodegradation of N,N’-diphenyl-N,N’-bis(3-methylphenyl)-1,1’-biphenyl-4,4’ diamine (TPD) dye. When a halomethane solution of TPD is exposed to UV light it first oxidizes TPD and subsequently with further exposure leads to irreversible transformation of TPD to an acridine derivative as photoproduct. Exclusive use of halomethane solvents like chloroform and dichloromethane suggests solvent radical initiated photochemical transformation of TPD to a stable acridine-based photoproduct. The resultant photoproduct of TPD showed broad-band dual (both blue and green) emissions in various solvents and also from the sublimed thin film. Based on solvent- and concentration-dependent photoluminescence (PL) of the photoproduct, PL excitation and lifetimes of blue and green bands, the origin of emission in the green region is due to an intermolecular excimer/aggregate of acridine-based photoproduct. A film of acridine-based photoproduct dispersed in polystyrene also showed substantial improvement in electrical conductivity compared to the parent compound TPD. The technique of UV-light-induced molecule structure transformation and material processing under specific environmental conditions may be considered as an efficient means of obtaining materials with enhanced optoelectronic properties. PACS 61.82.Pv; 72.80.Le; 82.50.-m; 82.30.Lp; 33.50.-j     

Kinetic and mechanistic studies on the interaction of adenosine with hydroxopentaaquarhodium(III) ion

The kinetics of the title reaction have been studied spectrophotometrically as a function of pH, [substrate], [adenosine] and temperature (50–65°C) by monitoring the appearance of a characteristic peak of the adenosine substituted product (λ_max = 289 nm). The reaction rate is pH dependent in the 3.0–4.3 range. With increase in [adenosine] the rate was found to increase and approached a limit at a higher adenosine concentration. The following rate law has been established at pH 4.3: d[Rh(H₂O)₃(OH)(adenosine)²⁺]/dt = k _a K _E[Rh(H₂O)₅(OH)²⁺]_total[adenosine]/ (1 + K _E[adenosine]) Rate and activation parameters are consistent with an associative interchange mechanism. Experimental results are discussed with reference to literature data for analogous systems. This revised version was published online in June 2006 with corrections to the Cover Date.     

Imaging characteristics of a birefringent lens under broadband illumination

This paper reports the imaging characteristics of an optical system consisting of a lens made of a uniaxial birefringent crystal sandwiched between two linear polarizers under polychromatic input illumination. The optic axis of the birefringent crystal is perpendicular to the lens axis and for an initial investigation, the spectral profile of the input beam is considered to be uniform for all wavelengths. The study reveals that if the variation of the birefringence parameters of the device with the wavelength is less, a larger bandwidth may be used without considerably deviating from the imaging characteristics under strictly monochromatic illumination. The nonlinear variation of birefringence with wavelength results in an asymmetric axial irradiance distribution. This obviates an optimal operating wavelength as well as a proper choice of the lens material. It is found that no appreciable change in image quality under diffraction-limited condition results for an input beam having 632 nm operating wavelength and 60 nm bandwidth. Therefore, it may be feasible to use this system with a white light source in conjunction with a wavelength-selective filter.     

Velocity selective optical pumping and repumping effects with counter and copropagating laser radiations for D<Subscript>2</Subscript> lines of rubidium

The effects of a strong control or pump laser, counter propagating or copropagating with the probe beam, on the probe absorption spectra of ⁸⁵Rb and ⁸⁷Rb-D₂ transitions have been investigated inside a room temperature Rb vapour cell. In both cases a set of strong velocity selective resonance dips are observed at different velocities. Their movements across the Doppler broadened probe absorption profile have been studied for different lock frequencies of the control laser. These spectra are modified by optical pumping effects due to the presence of another hyperfine component of the ground state. A repumping laser, from the dark hyperfine component of the ground level transfers almost 75% of the atoms from the dark state to the pump probe cycle hence reducing the optical pumping effect. A numerical simulation is done to explain the observed spectra. The effect of a control laser on the Lamb dip spectrum of the probe laser has also been investigated. The control beam is used to improve the strength of a weak hyperfine dip on the Doppler broadened probe spectrum. The strength of the hyperfine dip increases by a factor of 3.2 in presence of the control laser. The observed dips show that pump-probe spectroscopy can be used as velocity selectors of atoms.     

Laser damage studies in nonlinear optical crystal sodium p-nitrophenolate dihydrate

We report the surface laser damage threshold in sodium p-nitrophenolate dihydrate, a nonlinear optical crystal. The experiment is performed with a pulsed Nd:YAG laser in TEM₀₀ mode. The single shot damage thresholds are 11.16±0.28 GW cm^-2 and 1.25±0.02 GW cm^-2 for 1064 nm and 532 nm laser wavelengths respectively. A close correlation between the laser damage threshold and mechanical hardness is observed. A possible mechanism of laser damage is discussed. PACS 42.70.Mp; 61.80.Ba; 81.10.Dn     

The physics of accelerator driven sub-critical reactors

In recent years, there has been an increasing worldwide interest in accelerator driven systems (ADS) due to their perceived superior safety characteristics and their potential for burning actinides and long-lived fission products. Indian interest in ADS has an additional dimension, which is related to our planned large-scale thorium utilization for future nuclear energy generation. The physics of ADS is quite different from that of critical reactors. As such, physics studies on ADS reactors are necessary for gaining an understanding of these systems. Development of theoretical tools and experimental facilities for studying the physics of ADS reactors constitute important aspect of the ADS development program at BARC. This includes computer codes for burnup studies based on transport theory and Monte Carlo methods, codes for studying the kinetics of ADS and sub-critical facilities driven by 14 MeV neutron generators for ADS experiments and development of sub-criticality measurement methods. The paper discusses the physics issues specific to ADS reactors and presents the status of the reactor physics program and some of the ADS concepts under study.      

Charge Ordering,Competing Magnetic Interactions,and Magneto-Resistance Effects in Layered Iron(IV)-Based Oxides

Hyperfine Interactions - Iron(IV)-based Ruddlesden-Popper-type oxides Sr3Fe2?x Co x O7?y (0≤x≤1) have been synthesized and studied by various techniques. It is shown that...     

Studies in chlorin chemistry. II. A versatile synthesis of dihydrodipyrrins

[reaction: see text] Dihydrodipyrrins are key building blocks for the synthesis of hydroporphyrins, many of which have important biological activity. The title compounds were prepared in stereo- and regioselective fashion by a three-step sequence consisting of (1) Pd(0)-catalyzed coupling-cyclization of 2-iodopyrroles with gamma-alkynoic acids to afford enelactones of the desired substitution pattern, (2) methylenation at the lactone carbonyl group employing the Petasis reagent, and (3) in situ enol-ether hydrolysis and amination of the resultant 1,4-diketone to close the pyrroline ring (nine examples). Yields for each step were generally high, although in substrates not blocked by geminal substitution aromatization to a dipyrromethane is a competing side reaction.     

Intramolecular 1,3-dipolar nitrone cycloaddition route to bicyclic fused enediyne

Bicyclic isooxazolidinyl enediynes 1 and 2 have been prepared by intramolecular nitrone cycloaddition between the two arms of an acyclic enediyne precursor 13. The reaction is highly regioselective and the two configurational isomers have similar onset temperatures for BC indicating no influence of bridgehead stereochemistry on the kinetics of cyclization.     

Catalytic metathesis of carbon dioxide with heterocumulenes mediated by titanium isopropoxide

The insertion of an isopropoxide ligand of titanium isopropoxide into heterocumulenes gives a product that carries out metathesis at elevated temperatures by undergoing insertion of a second heterocumulene in a head to head fashion, followed by an extrusion reaction.