Abstract:
Second generation dendrimer 2 with single azobenzene moieties attached to the periphery of each dendron is prepared by using protection-deprotection methodology. This modified convergent method using key intermediates methyl 3-benzyloxy-5-hydroxybenzoate (4) and methyl 3-allyloxy-5-hydroxybenzoate (8) is used widely in precise dendrimer synthesis. Preparation of higher generation of dendrimers with only one azobenzene moiety in the exterior of each dendron is ongoing. The complete comparison of physical properties between the two types of dendrimers 1 and 2 is investigated.
Abstract:
Amphiphilic monodendrons of lower generations, AD12-N, containing a benzyl-15-crown-5 polar focal point, photochromic spacer and different number of dodecyl tails as peripheral groups (n = 1-8) have been investigated for their ability to form uniform monolayers at solid surfaces. The surface pressure-area behavior, photomechanical behavior and the morphology of the monomolecular films were investigated. We observed that all compounds studied are capable of forming stable Langmuir and Langmuir Blodgett monolayers, with virtually flat packing of molecules. Higher generation dendrimers form very uniform monolayers, without the usual domain microstructure. For AD12-4 monolayer on solid support, we observed stripped microstructure with several layers (3-6) bundled together. The periodicity of this structure of 8 nm was close to layered spacing, obtained from X-ray data for bulk material. For this compound, we proposed the model of double-layered packing of the molecules, with partial overlapping of the central segments and suggested that deposition on a solid substrate resulted in changing orientation of molecular fragments. Fast reversible photochromic response was observed for all monolayers with a conversion level of 50%. © 2002 Elsevier Science B.V. All rights reserved.
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.
