Excitation dynamics in disubstituted polyacetylene

Journal ArticleWe studied the excitation dynamics in films of disubstituted polyacetylene, a degenerate ground-state conjugated polymer, using psec transient and steady-state spectroscopies. The polymer is found to support charged and neutral topological soliton excitations concurrent with a strong intrinsic photoluminescence band with quantum efficiency, η ≈50%. This leads to stimulated emission in thin films and lasing in cylindrical µ cavities. The seeming contradiction of a degenerate ground-state polymer with high η is explained by the lowest excited-state ordering

Photophysics of pristine and C60-doped disubstituted polyacetylene

Journal ArticleUsing a variety of steady-state spectroscopies we studied the long-lived photoexcitations and electronic excited states of poly disubstituted acetylene (PDPA-nBu), the backbone structure of which is a disubstituted trans-polyacetylene, as well as PDPA-nBu/C60 blends. The cw spectroscopies include absorption, photoluminescence (PL), photoinduced absorption (PA), and PA-detected magnetic resonance (PADMR). Although Using a variety of steady-state spectroscopies we studied the long-lived photoexcitations and electronic excited states of poly disubstituted acetylene (PDPA-nBu), the backbone structure of which is a disubstituted trans-polyacetylene, as well as PDPA-nBu/C60 blends. The cw spectroscopies include absorption, photoluminescence (PL), photoinduced absorption ~PA!, and PA-detected magnetic resonance (PADMR). Although PDPA-nBu is a degenerate ground-state polymer, nevertheless, and in contrast to trans-polyacetylene, we found that it has a strong PL band with quantum efficiency larger than 60%. From polarized PL studies on stretched oriented films we conclude that the PL emission originates from intrachain excitons rather than the polymer side groups. This shows that the lowest-lying exciton in PDPA-nBu is a Bu state rather than an Ag state, in contrast to the order of the lowest-lying excitonic states in trans-polyacetylene. The polarized absorption in PDPA-nBu contains three distinctly different bands with different polarization properties, which are interpreted according to the model of Shukla and Mazumdar

Theory of nonlinear optical properties of phenyl-substituted polyacetylenes

In this paper we present a theoretical study of the third-order nonlinear optical properties of poly(diphenyl)polyacetylene (PDPA) pertaining to the third-harmonic-generation (THG) process. We study the aforesaid process in PDPA's using both the independent electron Hueckel model, as well as correlated-electron Pariser-Parr-Pople (P-P-P) model. The P-P-P model based calculations were performed using various configuration interaction (CI) methods such as the the multi-reference-singles-doubles CI (MRSDCI), and the quadruples-CI (QCI) methods, and the both longitudinal and the transverse components of third-order susceptibilities were computed. The Hueckel model calculations were performed on oligo-PDPA's containing up to fifty repeat units, while correlated calculations were performed for oligomers containing up to ten unit cells. At all levels of theory, the material exhibits highly anisotropic nonlinear optical response, in keeping with its structural anisotropy. We argue that the aforesaid anisotropy can be divided over two natural energy scales: (a) the low-energy response is predominantly longitudinal and is qualitatively similar to that of polyenes, while (b) the high-energy response is mainly transverse, and is qualitatively similar to that of trans-stilbene.Comment: 13 pages, 7 figures (included), to appear in Physical Review B (April 15, 2004

Prediction of infrared light emission from pi-conjugated polymers: a diagrammatic exciton basis valence bond theory

There is currently a great need for solid state lasers that emit in the infrared, as this is the operating wavelength regime for applications in telecommunications. Existing $\pi$--conjugated polymers all emit in the visible or ultraviolet, and whether or not $\pi$--conjugated polymers that emit in the infrared can be designed is an interesting challenge. On the one hand, the excited state ordering in trans-polyacetylene, the $\pi$--conjugated polymer with relatively small optical gap, is not conducive to light emission because of electron-electron interaction effects. On the other hand, excited state ordering opposite to that in trans-polyacetylene is usually obtained by chemical modification that increases the effective bond-alternation, which in turn increases the optical gap. We develop a theory of electron correlation effects in a model $\pi$-conjugated polymer that is obtained by replacing the hydrogen atoms of trans-polyacetylene with transverse conjugated groups, and show that the effective on-site correlation in this system is smaller than the bare correlation in the unsubstituted system. An optical gap in the infrared as well as excited state ordering conducive to light emission is thereby predicted upon similar structural modifications.Comment: 15 pages, 15 figures, 1 tabl

The roots of the halophyte <em>Salicornia brachiata</em> are a source of new halotolerant diazotrophic bacteria with plant growth-promoting potential.

Soil salinity is the major cause limiting plant productivity worldwide. Nitrogen-fixing bacteria were enriched and characterised from roots of Salicornia brachiata, an extreme halophyte which has substantial economic value as a bioresource of diverse and valuable products. Nitrogen-free semisolid NFb medium with malate as carbon source and up to 4% NaCl were used for enrichment and isolation of diazotrophic bacteria. The isolates were tested for plant growth-promoting traits and 16S rRNA, nifH and acdS genes were analysed. For selected strains, plant growth-promoting activities were tested in axenically grown Salicornia seedlings at different NaCl concentrations (0&ndash;0.5M). New halotolerant diazotrophic bacteria were isolated from roots of S. brachiata. The isolates were identified as Brachybacterium saurashtrense sp. nov., Zhihengliuella sp., Brevibacterium casei, Haererehalobacter sp., Halomonas sp., Vibrio sp., Cronobacter sakazakii, Pseudomonas spp., Rhizobium radiobacter, and Mesorhizobium sp. Nitrogen fixation as well as plant growth-promoting traits such as indole acetic acid (IAA) production, phosphate solubilisation, and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity were demonstrated. For Brachybacterium saurashtrense and Pseudomonas sp., significant plant growth-promoting activities were observed in Salicornia in salt stress conditions. Salicornia brachiata is a useful source of new halotolerant diazotrophic bacteria with plant growth-promoting potential

Cloning of GST Gene in Plant Expression Vector

Abstract Tau class glutathione transferases (GSTU) genes are plant specific, induced by different abiotic stresses and protect plants against oxidative stress. GST gene of 258 A

<em>Brachybacterium saurashtrense</em> sp. nov., a halotolerant root-associated bacterium with plant growth promoting potential.

A Gram-positive, aerobic, non-motile, coccoid shaped, halotolerant bacterium (JG 06 T) was isolated from roots of Salicornia brachiata, an extreme halophyte. Phylogenetic analysis based on 16S rRNA gene sequence showed sequence similarities of 99.2% to Brachybacterium paraconglomeratum, 99.0% to B. conglomeratum, and 98.2% to B. faecium. DNA-DNA hybridization with the close relatives Brachybacterium paraconglomeratum (DSM 46341T), B. conglomeratum (DSM 10241 T), B. faecium (DSM 4810T), B. tyrofermentans (DSM 10673 T), B. alimentarium (DSM 10672 T), B. fresconsis (DSM 14564 T), B. sacelli (DSM 14566 T) and B. muris (DSM 15460 T) resulted in re-association values of 36.2%, 36.5%, 35.8%, 27.6%, 27.9%, 28.2%, 28.7% and 11.2%, respectively. The peptidoglycan type of strain JG 06 T was variant A4&gamma;. The menaquinone content was MK7 (100%). The polar lipid profile consisted of diphosphatidyglycerol, phosphatidyglycerol, monogalactosyl diglyceride and three unidentified phospholipids and three glycolipids. The predominant fatty acid was anteiso-C15:0 (52.07%); significant amounts of iso-C16:0(12.38%), iso-C15:0(8.59%) and anteiso-C17:0(10.03%) were also present. The G+C content of DNA was 73.0 mol%. The strain formed a growth pellicle in nitrogen-free semisolid NFb medium containing NaCl up to 4% and reduced acetylene to ethylene, characteristic for N2 fixation. In nutrient broth medium it grew up to 15% NaCl. It also had the ability to produce IAA, siderophore, utilized ACC as sole source of nitrogen and possessed ACC deaminase enzyme. On the basis of physiological, biochemical data and phylogeny, strain JG 06 T should be placed in the genus Brachybacterium. This bacterium represents a novel species of genus Brachybacterium for which the name Brachybacterium saurashtrense sp. nov. is proposed, with type strain JG 06 T (= DSM 23186T = IMCC 252T)