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Electronic Absorption Properties (electronic + absorption_property)
Selected AbstractsStructural Effects on the Electronic Absorption Properties of 5,6-Dihydroxyindole Oligomers: The Potential of an Integrated Experimental and DFT Approach to Model Eumelanin Optical Properties,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 3 2008Marco D'Ischia Elucidation of the relationships between structural features and UV,visible absorption properties of 5,6-dihydroxyindole oligomers is an essential step towards an understanding of the unique optical properties of eumelanins. Herein, we report the first combined experimental and density functional theory (DFT) investigation of the 5,6-dihydroxyindole oligomers so far isolated. 2,2,-Biindolyl 2 and the 2,4,-biindolyl 3 absorb at longer wavelengths relative to 2,7,-biindolyl 4 and their spectra were well predicted by DFT analysis. The absorption bands of 2,4,:2,,4,,- and 2,4,:2,,7,,-triindolyls 5 and 6 also fall at different wavelengths and can be interpreted by DFT simulations as being due to a combination of two main separate transitions. Tetramer 7, in which two 2,4,-biindolyl units are linked through a 2,3,-connection, exhibits a broad chromophore extending over the entire UV range without well defined absorption maxima. Within the dimer,tetramer range examined, three key points emerge: (1) an increase in oligomer chain length does not result in any regular and predictable bathochromic shift; (2) a marked broadening of the absorption bands occurs when going from the monomer to the tetramer structure; and (3) the mode of coupling of the indole units is a crucial, hitherto unrecognized, structural parameter affecting the electronic absorption properties of 5,6-dihydroxyindole oligomers. It is concluded that use of experimentally characterized oligomeric scaffolds as a basis for DFT calculations is a most promising approach to building reliable structural models for studies of eumelanins optical properties. [source] Pyrazole and Pyrazolyl Complexes of cis -Bis(2,2,-bipyridine)chlororuthenium(II): Synthesis, Structural and Electronic Characterization, and Acid-Base ChemistryEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 5 2009Hershel Jude Abstract Complexes of the type cis -[Ru(bpy)2(Cl)(L)]+ [bpy = 2,2,-bipyridine; with L = pyrazole (1H), 4-methylpyrazole (2H), and 3,5-dimethylpyrazole (3H)] were synthesized and isolated as hexafluorophosphate salts. The molecular structures of these new complexes were fully characterized by 1H NMR spectroscopy and ESI mass spectrometry, and the crystal structure of 3H·PF6 was determined by X-ray crystallography. Compound 3H·PF6 (C25H24ClF6N6PRu) crystallizes in the monoclinic space group P21/n with a = 12.102(2) Å, b = 16.826(3) Å, c = 13.016(2) Å, , = 92.606(2)°, V = 2647.6(8) Å3, and Z = 4. The crystal structure of 3H reveals the formation of an intramolecular hydrogen bond (2.562 Å) between the pyrazole N(2),H site and the chloride ligand. The redox and electronic absorption properties of 1H, 2H, and 3H, as well as their deprotonated counterparts [L = pyrazolate (1), 4-methylpyrazolate (2), and 3,5-dimethylpyrazolate (3)], were investigated by cyclic voltammetry and UV/Vis spectroscopy. For detailed analysis of the electronic nature of this series of pyrazolyl ligands, the results are discussed along with other relevant cis -[Ru(bpy)2(X)(Y)]n+ complexes. From spectrophotometric pH titrations, the basicity associated with the coordinated pyrazole/pyrazolate couple in water was found in all three cases to be unusually high, partly owing to the N,H···Cl hydrogen bond that stabilizes the protonated, azole state. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Thermodynamic and kinetic analysis of the isolated FAD domain of rat neuronal nitric oxide synthase altered in the region of the FAD shielding residue Phe1395FEBS JOURNAL, Issue 12 2004Adrian J. Dunford In rat neuronal nitric oxide synthase, Phe1395 is positioned over the FAD isoalloxazine ring. This is replaced by Trp676 in human cytochrome P450 reductase, a tryptophan in related diflavin reductases (e.g. methionine synthase reductase and novel reductase 1), and tyrosine in plant ferredoxin-NADP+ reductase. Trp676 in human cytochrome P450 reductase is conformationally mobile, and plays a key role in enzyme reduction. Mutagenesis of Trp676 to alanine results in a functional NADH-dependent reductase. Herein, we describe studies of rat neuronal nitric oxide synthase FAD domains, in which the aromatic shielding residue Phe1395 is replaced by tryptophan, alanine and serine. In steady-state assays the F1395A and F1395S domains have a greater preference for NADH compared with F1395W and wild-type. Stopped-flow studies indicate flavin reduction by NADH is significantly faster with F1395S and F1395A domains, suggesting that this contributes to altered preference in coenzyme specificity. Unlike cytochrome P450 reductase, the switch in coenzyme specificity is not attributed to differential binding of NADPH and NADH, but probably results from improved geometry for hydride transfer in the F1395S, and F1395A,NADH complexes. Potentiometry indicates that the substitutions do not significantly perturb thermodynamic properties of the FAD, although considerable changes in electronic absorption properties are observed in oxidized F1395A and F1395S, consistent with changes in hydrophobicity of the flavin environment. In wild-type and F1395W FAD domains, prolonged incubation with NADPH results in development of the neutral blue semiquinone FAD species. This reaction is suppressed in the mutant FAD domains lacking the shielding aromatic residue. [source] Roles of rhenium diimine complexes as metal-ion probes and photosensitizers in functional polyurethanesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 11 2003Ching Sum Hui Abstract Polyurethanes incorporated with rhenium diimine complexes were synthesized. The polymers exhibited interesting morphologies and solution properties. Results from gel permeation chromatography suggested the formation of polymer aggregates in solutions. The polymers could act as probes for metal ions. The addition of metal ions to polymer solutions led to significant changes in the electronic absorption properties of the polymer solutions. This was attributed to the interactions between the polyether moieties and metal ions. The metal complexes could also act as efficient photosensitizers. After doping with charge-transport viologens, the photoconductivity of the polymers was greatly enhanced. The experimental quantum efficiency was simulated with Onsager's theory. The thermalization distances and the primary yields were typically 12,14 Å and 10,3, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1708,1715, 2003 [source] Structural Effects on the Electronic Absorption Properties of 5,6-Dihydroxyindole Oligomers: The Potential of an Integrated Experimental and DFT Approach to Model Eumelanin Optical Properties,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 3 2008Marco D'Ischia Elucidation of the relationships between structural features and UV,visible absorption properties of 5,6-dihydroxyindole oligomers is an essential step towards an understanding of the unique optical properties of eumelanins. Herein, we report the first combined experimental and density functional theory (DFT) investigation of the 5,6-dihydroxyindole oligomers so far isolated. 2,2,-Biindolyl 2 and the 2,4,-biindolyl 3 absorb at longer wavelengths relative to 2,7,-biindolyl 4 and their spectra were well predicted by DFT analysis. The absorption bands of 2,4,:2,,4,,- and 2,4,:2,,7,,-triindolyls 5 and 6 also fall at different wavelengths and can be interpreted by DFT simulations as being due to a combination of two main separate transitions. Tetramer 7, in which two 2,4,-biindolyl units are linked through a 2,3,-connection, exhibits a broad chromophore extending over the entire UV range without well defined absorption maxima. Within the dimer,tetramer range examined, three key points emerge: (1) an increase in oligomer chain length does not result in any regular and predictable bathochromic shift; (2) a marked broadening of the absorption bands occurs when going from the monomer to the tetramer structure; and (3) the mode of coupling of the indole units is a crucial, hitherto unrecognized, structural parameter affecting the electronic absorption properties of 5,6-dihydroxyindole oligomers. It is concluded that use of experimentally characterized oligomeric scaffolds as a basis for DFT calculations is a most promising approach to building reliable structural models for studies of eumelanins optical properties. [source] | ||||||||||