cyclic voltammograms were taken for these two carbons by varying the scan rate from 5 mV/s to 50 mV/s, the SNU-2 carbon kept the rectangular-shape up to a scan rate of 20 mV/s (Fig. 5b, solid line). In contrast, the MSC-25 carbon showed a deformed cyclic voltammogram at a scan rate of 10 mV/s and a completely collapsed one at a scan rate of 20 mV/s (Fig. 5b, dotted line). A detailed discussion on the electrochemical studies of the material will be presented in a forthcoming paper.In conclusion, we have made a new high surface area mesoporous carbon using Al-HMS as a template. From this research we discovered that the pores of HMS are 3D interconnected, unlike the originally proposed disordered hexagonal structure. The EDLC performance of the carbon material was superior to the commercially available carbon MSC-25 due to improved mesoporosity. The CV of the mesoporous carbon showed ideal rectangular shapes at a high scan rate of 20 mV/s.
Spin-coated films of an ethylhexyl derivative of polyfluorene can be converted on a pretreated polyimide substrate into highly oriented films by annealing in the liquid crystalline state. Together with improving orientation segregation of the wormlike molecules with respect to chain lengths and lamella formation proceeds. End groups are preferentially assembled in interlamellar regions. This morphological feature is thought to influence all measurements of intrinsic properties of polyfluorene films with similar histories. Electron diffraction patterns of the film are identical with X-ray fiber diagrams of fibers drawn from the melt and annealed in the liquid crystalline state. The experimental data show that the polymer molecules adopt a helical (5/q) conformation, packing in a trigonal unit cell. Molecular modeling based on ab initio MO calculations have been carried out to obtain independent estimates of chain geometry and conformation. These calculations are more in favor of a 5/2 rather than a 5/1 helix, with the argument of the observed packing of the individual PF chains and a plausibly low torsion angle of adjacent fluorene building blocks only for a 5/2 helix.
We have studied the dynamics of optically generated excitations in spin-coated glassy films of poly͑9,9-dioctylfluorene͒ ͑PFO͒ and in -phase PFO films using picosecond time resolved photoluminescence ͑PL͒ spectroscopy, performed both at room temperature ͑RT͒ and at 5 K. We also present measurements of the PL emission of PFO and -phase PFO at RT and 5 K following continuous wave ͑cw͒ excitation. We show that the cw emission from -phase PFO at 5 K is very highly resolved, permitting us to make an assignment of the different vibrational modes of the molecule that couple to the S 1 →S 0 transition. Via time-dependent spectroscopy measurements performed at 5 K, we are able to follow exciton diffusion and relaxation through an energetically broadened density of states to polymer chains having a longer conjugation length and lower energy gap. By comparing the relative emission intensity of the different vibronic transitions as a function of time, we are able to directly demonstrate that the lower energy emissive states are associated with longer conjugation length polymeric chains that have enhanced rigidity. At room temperature, we find that these relaxation processes occur faster than the resolution of our detector due to thermally assisted energy migration.
Liquid-crystalline polyfluorene (PF) homopolymers substituted with chiral alkyl side chains were synthesized, and their chiroptical properties in the solid state were investigated by means of circular dichroism (CD), circularly polarized photoluminescence (CPPL), and circularly polarized electroluminescence (CPEL) measurements. Polarization-selective scattering of light is shown to cause artifacts in the circularly polarized absorption and emission spectra in the wavelength range near or above the absorption edge, and a measurement scheme to avoid these is presented. For all derivatives, significant chiroptical effects appeared only after the solid layers have been annealed at elevated temperatures, preferably into the liquid-crystalline state of the polymer. The largest anisotropy factors were measured for a polyfluorene substituted with chiral (R)-2-ethylhexyl side chains, yielding absolute values of up to 0.28 for CPPL and up to 0.25 for CPEL. These are among the highest ever reported for a chiral conjugated polymer. Anisotropy factors for CD, CPEL, and CPPL were consistently found to follow an "odd-even effect" concerning the position of the chiral center in the alkyl side chain. If the chiral center is placed close to the polymer backbone, the CD is dominated by one peak with its maximum close to the maximum of the π-π* absorption band. This indicates that the chiroptical properties are most probably caused by intrachain effects rather than by pure interchain exciton coupling. This interpretation is supported by the results of time-dependent Hartree-Fock calculations for the isolated fluorene dimer and trimer. In both cases, the anisotropy factor depends strongly on the torsion angle between neighboring fluorene units. For the trimer, a maximum anisotropy factor of 0.25, close to the maximum values determined experimentally, is predicted for a torsion angle of ca. 105°. Both experimental and theoretical results indicate that the chiroptical properties of these chiral substituted polyfluorenes are mainly caused by a helical conformation of the conjugated polymer backbone.
A series of chirally substituted poly (p-phenyleneethynylene) copolymers was prepared by alkyne metathesis of mixtures of two different 2,5-dialkyl-1,4-dipropynylbenzenes. One of the monomers was the chiral bis-2,5-(S)-3,7-dimethyloctyl-1,4-dipropynylbenzene, and the second one was an achiral dipropynyl monomer. If the content of chiral monomer is 25-50 mol %, unusually large chiroptical effects, that is, optical dissymmetries, result in absorption (g = -0.38) and emission (g = -0.19) of these copolymers. The large dissymmetries can be explained by a supramolecular ordering of the PPEs into stranded features that are visualized by dark-field transmission electron microscopy. The strands of chirally substituted PPEs display a striated structure that suggests that the whole feature is a single chirally twisted crystallite.
An 8-kilobase HindlIl fragment carrying the histidase gene (hutH) and its regulatory region (hutP), from the Bacillus subtilis histidine utilization (hut) operon, was cloned in the temperate bacteriophage +105. Histidine utilization was restored in a hutHl mutant by the specialized transducing phage (+lO5hutH11). The histidase gene in +105hutH11 was inducible and was shown to be under catabolite repression. The nucleotide sequence of 3,932 base pairs including the hutH and hutP loci revealed three open reading frames (ORFs). The molecular weights of ORF1 and ORF2 proteins were calculated to be 16,576 (151 amino acid residues) and 55,675 (508 amino acid residues), respectively. Reverse transcriptase mapping experiments showed that the putative promoter for the hut operon could be recognized by RNA polymerase sigma 43. The transcript starts at an adenosine residue 32 base pairs upstream from the initiation codon of ORF1. hutH+-transforming activity was found in ORF2, indicating that ORF2 encoded the histidase. A hutPI mutation was determined as a substitution of an amino acid in ORF1. By using a specialized transducing phage containing the wild-type ORFi gene, it was demonstrated that the presence of ORF1 protein in trans was absolutely required for the induction of the hut operon in a hutPI mutant. These data strongly suggested that ORF1 encodes a positive regulator of the hut operon.The enzymes involved in histidine catabolism have been studied for many bacterial strains, such as Salmonella typhimurium, Klebsiella aerogenes, Pseudomonas species, and Bacillus subtilis. In all cases except the Pseudomonas species, histidine is catabolized to glutamic acid, ammonia, and formamide by the action of four enzymes (20); in the Pseudomonas species, five enzymes are involved (19). Genetic and biochemical studies have revealed that the genes responsible for histidine catabolism are organized as operons (hut operon) (13,18,20).hut genes in all cases are inducible and controlled by catabolite repression (19,20). The molecular basis of the regulation is well characterized for S. typhimurium and K. aerogenes. The expression of hut genes is regulated negatively by urocanic acid and positively by the catabolite activator protein with cyclic AMP (20). In addition, a third regulatory mechanism has been described for K. aerogenes and Pseudomonas aeruginosa, in which catabolite repression is relieved during nitrogen-limited growth (26,27
SummaryLevels of transcripts initiated at a hut promoter in Bacillus subtilis were analysed. The addition of histidine to the culture medium increased the level of the transcript sixfold. In the presence of histidine and glucose together, the level of the transcript was reduced to the level in the absence of Induction. Furthermore, addition of a mixture of 16 amino acids to cultures of induced cells and of catabolite‐repressed cells decreased levels of the transcript 16‐fold and 2.6‐fold, respectively. Thus, it appears that at least three regulatory mechanisms associated with induction, catabolite repression, and amino acid repression, control the transcriptional activity of the hut promoter. Expression of the hut promoter–lacZ fusions that contained various regions of the hutP gene and deletion analysis of the hutP region revealed a cis‐acting sequence associated with catabolite repression that was located between positions +204 and +231 or around position +203.
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