PMID: 12940723;Abstract:
The disassembly of dendritic structures was realized by a cascade cleavage reaction triggered by an initially stimulated group in the dendrimer periphery. A depolymerizable backbone was engineered into prototypical dendritic structures. Evidence for the completion of the disassembly process is provided by the absorbance peak of the p-nitrophenoxide ion that was intentionally installed at the focal point of the dendrons. Observation of the UV spectra during the disassembly process supports a stepwise cascade cleavage proceeding from the periphery into the core. Copyright © 2003 American Chemical Society.
The effect of the molecular orientation distribution of the first monolayer of donor molecules at the hole-harvesting contact in an organic photovoltaic (OPV) on device efficiency was investigated. Two zinc phthalocyanine (ZnPc) phosphonic acids (PA) deposited on indium tin oxide (ITO) electrodes are compared: ZnPc(PA)4 contains PA linkers in all four quadrants, and ZnPcPA contains a PA linker in one quadrant. ZnPcPA monolayers exhibited a broad distribution of molecular orientations whereas ZnPc(PA)4 adsorption produced a monolayer with a narrower orientation distribution with the molecular plane more parallel to the ITO surface. We used potential-modulated attenuated total reflectance spectroelectrochemistry (PM-ATR) to characterize the charge-transfer kinetics of these films and show that the highest rate constants correspond to ZnPc subpopulations that are oriented more parallel to the ITO surface plane. For ZnPc(PA)4, rate constants exceeded 104 s–1 and are among the highest ever reported for a surface-confined redox couple, which is attributable to both its orientation and the small ZnPc–electrode separation distance. The performance of OPVs with ITO hole-harvesting contacts modified with ZnPc(PA)4 was comparable to that achieved with highly activated bare ITO contacts, whereas for ZnPcPA-modified contacts, the OPV performance was similar to that observed with (hole-blocking) alkyl-PA modifiers. These results demonstrate the synergism between molecular structure, energetics, and dynamics at interfaces in OPVs.
Abstract:
A series of amine-imine bidentate ligands based on a trans-2,5-disubstituted pyrrolidine and pyridine moieties have been prepared. The use of these ligands in the palladium-catalyzed allylic alkylation reaction of rac-(E)-1,3-diphenylprop-2-enyl acetate is reported. The results suggest that these ligands are good catalyst precursors for the reaction. Electronic modification on the pyridine ring of the ligands does not have a significant effect on the enantioselectivity of the reaction but does on the reaction rate, while structural modification on either the pyridine or the pyrrolidine moiety affords dramatic changes on the outcome of the stereochemistry. Evidence from various studies suggested that during the palladium-catalyzed allylic alkylation reaction, nucleophilic attack onto the 1,3-diphenylallyl moiety in the transition state occurs mainly trans to the pyridine ring of the less stable conformation of the palladium complexes. © 2009 Elsevier Ltd. All rights reserved.
Abstract:
Grazing incident X-ray diffraction and X-ray reflectivity have been performed on Langmuir monolayers of low generation monodendrons containing a crown-ether polar group, azobenzene spacer, and varying number of peripheral alkyl chains of 1, 2, 4, and 8. We observe that the cross-sectional mismatch between the bulky polar head and the alkyl tails has a profound effect on the local ordering of the alkyl tails. It is found that the alkyl chains in a single-tail molecule are significantly tilted away from the surface normal. The tilt is eliminated in molecules with two or more alkyl chains where the cross-sectional mismatch is in favor of the peripheral tails. The molecule with one tail possesses a supercell orthorhombic packing caused by structural nonequivalency on the neighboring tails. The two- and four-tail molecules form a mixed structure best described by a quasi-hexagonal unit cell, and the eight-tail molecule forms a more stable hexagonal unit cell. Peripheral tails for these molecules are in standing-off orientation. We suggest that the steric constraints cause lower correlations and a staggered packing structure of monolayers from the eight-tail molecule. We suggest that branching alkyl tails off the same phenyl ring and the presence of the phenyl rings in the vicinity of the branching are limiting factors on the chain packing at the air-water interface in monodendrons with multiple peripheral tails. We conclude that a significant portion of the molecules is submerged in the water subphase and possesses a "kink" shape.
