1 |
Transglycosylation catalysed by the #beta#-glucosidase activity present in cellulase derived from Trichoderma reesei ATC 26921Baum, John Victor January 1998 (has links)
No description available.
|
2 |
Synthesis and Characterization of Triphenylene-BODIPY Paddle Wheel Conjugates for Ultra-Fast Light Induced Charge Separation Yielding High-Energy Charge-Separated StatesCantu, Robert 05 1900 (has links)
In the development of covalent organic frameworks (COFs), often the scaffold linkers are assumed to be electro- and photoinactive, and this was also to be the case for 2,3,6,7,10,11-hexahydroxytriphenylene, a tritopic linker. However, as demonstrated in the present study, the reaction product of this linker, hexaoxatriphenylene, is electron rich and when connected to a suitable photosensitizer engages itself in an efficient excited-state charge separation process. In the present study, we have employed BF2-chelated dipyrromethenes (BODIPYs) as sensitizers, which are connected to hexaoxatriphenylene through the center boron, rendering paddle-wheel-type structures. Systematic photophysical, electrochemical, computational, and photochemical studies involving pump-probe femtosecond transient spectroscopy have been performed to establish efficient charge separation in these novel supramolecular structures.
|
3 |
New Design Strategies for the Preparation and Linking of Cyclopenta-Fused Polycyclic Aromatic HydrocarbonsDu, Yachu 01 June 2021 (has links)
In this contribution, we demonstrate a new strategy to build electron deficient centers and electron accepting compounds through fusion of cyclopentene units. Two series of cyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAHs) were designed and synthesized as possible n-type semiconductors. One series of CP-PAHs were prepared by a palladium catalyzed cyclopentannulation reaction from precursors derived from the low-cost dye Vat Orange 3 (4,10-dibromoanthanthrone). The new cyclopenta-anthanthrene compounds possessed reduced optical and electrochemical gaps compared to anthanthrene derivatives and were electron acceptors with lowest unoccupied molecular orbital energies of ~ -3.4 eV to -3.9 eV. In a second research direction, CP-PAH chromophores were linked through two five-membered rings to create a new semiconductor linkage strategy. We show this new linker design can access planarized structures with reduced optical gaps and redox potentials. Two aceanthrylene chromophores were connected into dimer model systems with the chromophores either projected outward (2,2’-biaceanthrylene) or inward (1,1’-biaceanthrylene) and the optical and electronic properties were compared. Only the planar 2,2’-biaceanthrylene system showed significant reductions of the optical gaps (1 eV) and redox potentials in relation to the aceanthrylene monomer.
|
4 |
Magnetic quantum oscillations in organic metals based on the molecule bis(ethylenedithio)tetrathiafulvaleneCaulfield, Jason M. January 1994 (has links)
ET charge transfer salts (where ET is <en>bis(ethylenedithio)- tetrathiafulvalene) have relatively simple quasi two-dimensional Fermi surface topologies, making them ideal for the study of the relationship between bandstructure and properties such as superconductivity. Experimental studies of the Fermi surface areas and associated effective masses have been carried out using the Shubnikov-de Haas (SdH) and de Haas-van Alphen (dHvA) effects. By comparing the experimental results to theoretical bandstructure calculations the strength of many body interactions has been estimated. High pressure magnetotransport experiments have been carried out on the superconductor κ-ET<sub>2</sub>Cu(NCS)<sub>2</sub>. The observation of SdH and magnetic breakdown oscillations has allowed the pressure dependences of the Fermi surface topology and effective masses to be deduced and compared with simultaneous measurements of the superconducting critical temperature. The data strongly suggest that the enhancement of the effective mass and the superconducting behaviour are directly connected. The results are compared with several current theories of superconductivity. The dHvA effect has been used to probe the superconducting mixed state of κ-ET<sub>2</sub>Cu(NCS)<sub>2</sub>. A recent model of the superconducting mixed state is applied to the experimental data in an attempt to determine the value and symmetry of the superconducting energy gap. SdH measurements up to 30 T have been used to study spin densitywave formation in α-ET<sub>2</sub>KHg(SCN)<sub>4</sub>, and the reasons why a very slight increase of the unit cell volume (i.e. replacing the K in α-ET<sub>2</sub>KHg(SCN)<sub>4</sub> by NH<sub>4</sub>) stabilises a superconducting state. Galvanomagnetic techniques have been used to measure the quasi onedimensional Fermi surface orientation below the spin-density-wave transition, and to accurately determine the shape of the quasi twodimensional Fermi surface above it. The application of pressure has been used to gradually reduce the onset temperature of a metal-insulator transition and to eventually stabilise a superconducting state in ET<sub>3</sub>Cl<sub>2</sub>2H<sub>2</sub>O. The bandstructure of ET<sub>3</sub>C1<sub>2</sub>2H<sub>2</sub>O has been investigated using the SdH effect whilst hydrostatic pressure has been used to pass through the superconducting part of the phase diagram.
|
5 |
Carbon Nanostructure Based Donor-acceptor Systems for Solar Energy HarvestingDas, Sushanta Kumar 12 1900 (has links)
Carbon nanostructure based functional hybrid molecules hold promise in solarenergy harvesting. Research presented in this dissertation systematically investigates building of various donor-acceptor nanohybrid systems utilizing enriched single walled carbon nanotube and graphene with redox and photoactive molecules such as fullerene, porphyrin, and phthalocyanine. Design, synthesis, and characterization of the donor-acceptor hybrid systems have been carefully performed via supramolecular binding strategies. Various spectroscopic studies have provided ample information in terms of establishment of the formation of donor-acceptor hybrids and their extent of interaction in solution and eventual rate of photoinduced electron and/or energy transfer. Electrochemical studies enabled construction of energy level diagram revealing energetic details of the possible different photochemical events supported by computational studies carried out to establish the HOMO-LUMO levels in the donor acceptor systems. Transient absorption studies confirmed formation of charge separated species in the donor-acceptor systems which have been supported by electron mediation experiments. Based on the photoelectrochemical studies, IPCE of 8% was reported for enriched SWCNT(7,6)-ZnP donor-acceptor systems. In summary, the present investigation on the various nanocarbon sensitized donor-acceptor hybrids substantiates tremendous prospect, that could very well become the next generation of materials in building efficient solar energy harvesting devices andphotocatalyst.
|
6 |
PHOTO-INDUCED RADICAL COPOLYMERIZATIONS OF ELECTRON-RICH OLEFINS WITH ELECTRON-POOR OLEFINS.LEE, CHERYLYN. January 1987 (has links)
This study is a systematic investigation of the parameters and conditions necessary for photo-induced radical copolymerizations of donor olefins with acceptor olefins in the absence of an initiator. Very few cases have been previously reported and no mechanistic details of the initiation have been proposed in the literature. Our results show that the photoinitiation depends on the relative donor and acceptor strengths of the monomers, as well as the solvent. The highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor must be at the appropriate energy levels in order to produce a radical initiating species upon photoexcitation of the electron donor-acceptor (EDA) complex. If the donor-acceptor interaction is too weak, no copolymerization occurs. The excited complex (contact ion pair) presumably decays back to the ground state faster than producing an initiating species. If the donor-acceptor interaction is too strong, the excited complex dissociates to the free ions which could initiate ionic homopolymerization rather than radical copolymerization. The solvent may also determine the course of the reaction. In two cases, copolymerizations, which could be photo-induced in 1,2-dichloroethane, could not be photo-induced in acetonitrile. Dissociation of the excited complex (contact ion pair) is favored in polar solvents, such as acetonitrile, which are able to stabilize the ion radicals. This initiation method produces high molecular weight copolymers that may be cast into transparent films.
|
7 |
SYNTHESIS OF ELECTRON-POOR TETRASUBSTITUTED OLEFINS AND THEIR REACTIONS WITH ELECTRON-RICH COMONOMERS.SENTMAN, ROBERT CRAIG. January 1982 (has links)
Six electron-poor tetrasubstituted olefins were reacted with electron-rich comonomers. Of these, three {dimethyl dicyanofumarate (DDCF), dimethyl 1,1-dicyanoethylene-2,2-dicarboxylate (DDED), and dicarbomethoxymaleic anhydride (DCMA)} were found to polymerize with styrenes and vinyl ethers to form 1:1 alternating copolymers of low molecular weight. All polymerizations with vinyl ethers and DCMA required initiation, while the copolymerizations of DDED and DDCF with styrenes were spontaneous. Tetramethyl ethylenetetracarboxylate, diisopropylidene ethylenetetracarboxylate, and trimethyl cyanoethylenetricarboxylate failed to copolymerize under any conditions. The spontaneous reactions of these tetrasubstituted olefins can best be explained as proceeding via tetramethylene intermediates, resonance hybrids of biradicals and zwitterions. Spontaneous copolymerizations occur from biradical intermediates; cycloadduct formation occurs from both. Tetramethylene formation is electronically controlled during the reaction of DDED and electron-rich comonomers, as reflected by the structure of the isolated cyclobutanes. The orientation of this monomer is the copolymer with styrene is sterically controlled, as suggested by ('13)C NMR. Methyl 3,3-dicyanoacrylate, a new tetrasubstituted olefin, was found to spontaneously copolymerize with styrenes, and to form cyclobutanes with vinyl ethers. It could be copolymerized with vinyl ethers with radical initiation.
|
8 |
Some properties of charge transfer complexesMacFarlane, A. J. January 1968 (has links)
No description available.
|
9 |
Design, Synthesis, and Evaluation of Metal Cation Sensors with Donor-Acceptor ArchitectureCody, John W., Jr. 21 November 2006 (has links)
Copper is an essential trace element present in all living systems and is important for the function of many cellular enzymes. It ranks third in intracellular abundance behind only zinc and iron and plays a very important role as a catalytic cofactor in various cellular processes such as mitochondrial respiration, iron uptake, and the redox processes of a number of enzymes, including superoxide dismutase, lysyl oxidase, or tyrosinase. Any abnormality in copper trafficking pathways can lead to serious diseases such as Wilsons disease, Menkes syndrome and has been implicated in the neurodegenerative diseases amyotropic lateral sclerosis (ALS) and Alzheimers disease. While free copper in the cytoplasm would prove toxic, there is compelling evidence for the existence of a labile pool of copper that remains kinetically accessible. In order to investigate the existence of such a pool the development of Cu(I) selective probes is necessary.
Chapter I provides the background for the role of copper in biology and elucidates the main trafficking pathways discovered to date. This chapter also provides recent developments of fluorescent sensors for selective visualization of biologically relevant metals.
Chapter II discusses the exploration of a phenanthroline-based ligand for the selective detection of Cu(I). A series of derivatives incorporating chelating substituents in the 2- and 9-positions to enforce a 1:1 binding stoichiometry were synthesized and the properties of their respective Cu(I) complexes were characterized by x-ray structural analysis, and their photophysical properties were investigated by absorption and emission spectroscopy. Visible light excitation yielded metal-to-ligand charge-transfer (MLCT) excited states with luminescence lifetimes up to 155 ns. Electrochemical measurements further indicate that coordinative rearrangements are involved in nonradiative deactivation of the excited states. According to time-dependent density functional theory calculations (B3LYP/6-31G**), the major MLCT transitions are polarized along the C2 axis of the complex and originate predominantly from the dxz orbital.
In chapter III, the development of a ratiometric Cu(I) sensor based on a donor-acceptor functionalized biphenyl fluorophore platform is discussed. The fluorescence emission energy for such fluorophores is highly dependent upon the interannular twist angle and this property was harnessed to provide a ratiometric sensor selective for Cu(I). Coordination of Cu(I) leads to a flattening of the biphenyl backbone and was confirmed by absorbance and emission spectroscopy as well as 2D NOESY experiments. The peak emission energy was shifted by 39 nm towards higher energy upon metal cation binding with a concomitant 7 bathochromic shift in absorption energy. The photophysical data accompanied by 1H NMR analysis confirms a well-defined 1:1 binding stoichiometry between metal and ligand. The findings from this study showed ratiometric behavior for this probe, albeit with a lowered quantum yield.
While the quantum yield for the fluorophore discussed in chapter III was low (8.0%), the focus of chapter IV was the elucidation of the fluorescence quenching mechanism. To investigate the possibility of a twisted intramolecular charge transfer (TICT) state a donor-acceptor biphenyl fluorophore was synthesized incorporating a conformationally restricted amine donor group incapable of rotating out of plane in the excited state. Analysis of this derivative, as well as the sensor discussed in chapter III, reveals that fluorescence quenching is most likely due to hydrogen bonding to the acceptor subunit in they excited state.
Finally, in chapter V, a pyrazoline fluorophore library with varying numbers of fluorine substituents was synthesized. The photophysical and electrochemical properties of these fluorophores were measured in order to determine if careful tuning of the excited state electron transfer thermodynamics is possible. The compounds cover a broad range of excited state energies and reduction potentials, and the data suggest that selective and differential tuning of both the reduction potential of the acceptor as well as the excited state equilibrium energy. These findings show that the individual parameters involved in excited state electron transfer can be tuned by the modular architecture of the pyrazoline fluorophore.
|
10 |
Characterization of Reactive-Sputtered Copper doped ZnO Thin FilmsHuang, Shu-Chi 04 July 2006 (has links)
none
|
Page generated in 0.0534 seconds