Molar extinction coefficients of some carbohydrates in aqueous solutions

Molar extinction coefficients of some carbohydrates viz. l-arabinose (C₅H₁₀O₅), d-glucose (C₆H₁₂O₆), d-mannose (C₆H₁₂O₆), d-galactose (C₆H₁₂O₆), d(-) fructose (C₆H₁₂O₆) and maltose (C₁₂H₂₄O₁₂) in aqueous solutions have been determined at 81, 356, 511, 662, 1173 and 1332 keV by gamma ray transmission method in a narrow beam good geometry set-up. These coefficients have been found to depend upon the photon energy following a 4-parameter polynomial. These extinction coefficients for different sugars having the same molecular formula have same values varying within experimental uncertainty. Within concentration ranges studied, Beer-Lambert law is obeyed very well.     

Dichlorocarbene Addition to

In contrast to the terminal phosphinidene complex PhPW(CO)(5) (2), which adds to [5]metacyclophane (1) in a 1,4-fashion, dichlorocarbene preferentially adds in a 1,2-fashion to the formal "anti-Bredt" type double bond of the aromatic ring of 1 to afford the norcaradiene 11b, which immediately rearranges to the bridged cycloheptatriene 12b and further by a [1,5] sigmatropic chlorine migration to the isomeric 13b as the first observable product. More slowly, the latter isomerizes via a dissociative mechanism to give 15b. A computational study supports the notion that the [1,5] chlorine migration in the rearrangement 12b --> 13b, for which an activation barrier of 70.2 kJ mol(-)(1) was calculated, is essentially concerted with minor charge separation. In contrast, the analogous [1,5] chlorine migration in the flat model compound 7,7-dichlorocycloheptatriene (12a) displays features of a dissociative pathway.     

Ultrafast intramolecular exciton splitting dynamics in isolated low-band-gap polymers and their implications in photovoltaic materials design

Record-setting organic photovoltaic cells with PTB polymers have recently achieved ~8% power conversion efficiencies (PCE). A subset of these polymers, the PTBF series, has a common conjugated backbone with alternating thieno[3,4-b]thiophene and benzodithiophene moieties but differs by the number and position of pendant fluorine atoms attached to the backbone. These electron-withdrawing pendant fluorine atoms fine tune the energetics of the polymers and result in device PCE variations of 2-8%. Using near-IR, ultrafast optical transient absorption (TA) spectroscopy combined with steady-state electrochemical methods we were able to obtain TA signatures not only for the exciton and charge-separated states but also for an intramolecular ("pseudo") charge-transfer state in isolated PTBF polymers in solution, in the absence of the acceptor phenyl-C(61)-butyric acid methyl ester (PCBM) molecules. This led to the discovery of branched pathways for intramolecular, ultrafast exciton splitting to populate (a) the charge-separated states or (b) the intramolecular charge-transfer states on the subpicosecond time scale. Depending on the number and position of the fluorine pendant atoms, the charge-separation/transfer kinetics and their branching ratios vary according to the trend for the electron density distribution in favor of the local charge-separation direction. More importantly, a linear correlation is found between the branching ratio of intramolecular charge transfer and the charge separation of hole-electron pairs in isolated polymers versus the device fill factor and PCE. The origin of this correlation and its implications in materials design and device performance are discussed.     

Strong steric hindrance effect on excited state structural dynamics of Cu(I) diimine complexes

The metal-to-ligand-charge-transfer (MLCT) excited state of Cu(I) diimine complexes is known to undergo structural reorganization, transforming from a pseudotetrahedral D(2d) symmetry in the ground state to a flattened D(2) symmetry in the MLCT state, which allows ligation with a solvent molecule, forming an exciplex intermediate. Therefore, the structural factors that influence the coordination geometry change and the solvent accessibility to the copper center in the MLCT state could be used to control the excited state properties. In this study, we investigated an extreme case of the steric hindrance caused by attaching bulky tert-butyl groups in bis(2,9-di-tert-butyl-1,10-phenanthroline)copper(I), [Cu(I)(dtbp)(2)](+). The two bulky tert-butyl groups on the dtbp ligand lock the MLCT state into the pseudotetrahedral coordination geometry and completely block the solvent access to the copper center in the MLCT state of [Cu(I)(dtbp)(2)](+). Using ultrafast transient absorption spectroscopy and time-resolved emission spectroscopy, we investigated the MLCT state property changes due to the steric hindrance and demonstrated that [Cu(I)(dtbp)(2)](+) exhibited a long-lived emission but no subpicosecond component that was previously assigned as the flattening of the pseudotetrahedral coordination geometry. This suggests the retention of its pseudotetrahedral D(2d) symmetry and the blockage of the solvent accessibility. We made a comparison between the excited state dynamics of [Cu(I)(dtbp)(2)](+) with its mono-tert-butyl counterpart, bis(2-tert-butyl-1,10-phenanthroline)copper(I) [Cu(I)(tbp)(2)](+). The subpicosecond component assigned to the flattening of the D(2d) coordination geometry in the MLCT excited state was again present in the latter because the absence of a tert-butyl on the phenanthroline allows flattening to the pseudotetrahedral coordination geometry. Unlike the [Cu(I)(dtbp)(2)](+), [Cu(I)(tbp)(2)](+) exhibited no detectable emission at room temperature in solution. These results provide new insights into the manipulation of various excited state properties in Cu diimine complexes by certain key structural factors, enabling optimization of these systems for solar energy conversion applications.     

Thermodynamics of Chromium(VI) Anionic Species Sorption onto Surfactant-Modified Montmorillonite Clay

Batch sorption experiments performed on Cr(VI) species sorption showed a significantly enhanced removal of inorganic hexavalent chromium anionic species from aqueous solution by montmorillonite clays modified with quaternary amine, hexadecyltrimethylammonium (HDTMA) bromide. Unmodified clay had no affinity for chromium(VI) species. The sorption of Cr(VI) species has been carried out as a function of pH, contact time, adsorbate concentration (4.14x10(-5) to 8.62x10(-3) M), and temperature (5-45 degrees C). The surfactant-modified clay surface was stable when exposed to extremes in pH. The optimum pH for maximum sorption of Cr(VI) species was found to be at pH 1 and was constant between pH 2 and pH 6. The sorption data obtained was well described by DKR and Langmuir sorption isotherms. Sorption energy (E) for (i) surfactant sorption by montmorillonite clay and (ii) sorption of chromium(VI) species by surfactant modified clay have been computed from the DKR equation. Sorption energy evaluated for the sorption of both surfactant and Cr(VI) species showed that an ion-exchange mechanism was operative. The mechanism of retention appears to be replacement of counterion of the surfactant by Cr(VI) anionic species. Adsorbent capacity for the sorption of Cr(VI) species has been evaluated from the Langmuir sorption isotherm data. Thermodynamic parameters (Delta H degrees, Delta S degrees and Delta G degrees ) for surfactant sorption on montmorillonite clay and Cr(VI) sorption by modified clay have been evaluated. The specific rate constant for sorption of Cr(VI) species on modified montmorillonite was rapid during the first 10 min and equilibrium was found to be attained within 30 min. The sorption of Cr(VI) species onto modified montmorillonite clay followed first-order rate kinetics. Copyright 2000 Academic Press.     

Sorption of inorganic nanoparticles in woven cellulose fabrics

Titanium dioxide was deposited from aqueous suspension onto cellulosic surfaces.Titania was sourced from Degussa (P25TM,70:30 anatase:rutile).Dry uptake of particles was shown to be rapid and dominant with one-third of the deposition occurring in less than 30 s and over one-half in the first minute.Isotherms were recorded to compare the rate of titanium deposition on dry and pre-wetted cotton.In the dry case uptake reached a maximum in 30 min whereas in the pre-wetted case the uptake was seen to continue beyond 180 min.A broad trend of higher deposition occurring at lower pH was seen,corresponding to the region where surface charges were opposite and thus attractive.Dry pickup was less significant at high pH.The response to varying ionic strength was complex and was attributed to the combined effect of charge screening,particle aggregation and consequent particle entrapment or occlusion.Titania deposition into the interstices of woven cotton sheets resulted in the formation of inorganic,nanoparticulate skeletons which could be isolated by controlled combustion of the cellulose and thus cotton was suggested to have potential for the templated synthesis of high surface area semiconductor materials.