Nanoparticle-cored dendrimers: synthesis and characterization

The synthesis and characterization of a group of new dendrimers-namely, nanoparticle-cored dendrimers (NCDs)-are described. These materials were obtained by the reduction of hydrogen tetrachloroaurate phase-transferred into toluene in the presence of Fréchet-type polyaryl ether dendritic disulfide wedges of generation 1-5. These materials, possessing nanometer-sized gold clusters at the core and dendritic wedges radially connected to the core by Au-S bonds, were analyzed by TEM and TGA, and by UV, IR, and NMR spectroscopies. The number of branching units connected to the core decreased with the generation of the dendritic wedge, and this number changed from 2.18/nm(2) for Au-G-2 to 0.27/nm(2) for Au-G-5. This result suggests that, in the higher-generation NCDs, a large fraction of the surface area of the metal cluster is not passivated and is therefore available for catalytic activity.     

Bicyclooctanes: cis-bicyclo[3.3.0]octa-2,6 and 2,7-diene-2-carboxylic acids

Two independent routes to each of the title acids, useful in the synthesis of natural products, are presented. The sequences commence with readily available materials which are amenable to the preparation of multigram quantities. Complete ¹³C spectral data is supplied for intermediates and products.     

Pulse radiolysis studies of dendritic macromolecules with biphenyl peripheral groups and a ruthenium tris-bipyridine core.

Electron-transfer reactions in Fréchet-type dendrimers with biphenyl peripheral groups and a ruthenium core were investigated by pulse radiolysis techniques. Fast electron-transfer rates found in the two ruthenium dendrimers suggest a very efficient electronic coupling between the peripheral donor groups and the core acceptor.     

Intramolecular charge transfer in 1:1 Cu(II)/pyrenylcyclam dendrimer complexes

Two newly synthesized pyrenylcyclam dendrons (1 and 2) exhibit a new emission band centered at 450 nm when coordinated with copper triflate. Observed fluorescence shifts induced by coordinative metalation indicate an unusual intramolecular charge transfer from a pyrenyl excited state to the coordinated metal ion that competes with pyrene excimer formation. This interaction likely proceeds by photoexcitation of pi-complex of the appended arene, followed by intramolecular charge transfer within the dendritic 1:1 cyclam/metal complex, effecting reduction of the bound Cu(II) metal ion. The appended dendritic groups not only decrease the equilibrium binding constant with Cu(II) but also participate in a new excited-state pathway as an alternative to energy-dissipative excimer formation.     

Novel photoswitchable receptors: synthesis and cation-induced self-assembly into dimeric complexes leading to stereospecific [2+2]-photocycloaddition of styryl dyes containing a 15-crown-5 ether unit

Styryl dyes 4a-e containing a 15-crown-5 ether unit and a quinoline residue with a sulfonatoalkyl or sulfonatobenzyl N-substituent were synthesized. The relationship between the photochemical behavior of these dyes and their aggregates derived from complexation with Mg(2+) in MeCN was studied using (1)H NMR and absorption spectroscopy. The E-isomers of 4a-e were shown to form highly stable dimeric (2:2) complexes with Mg(2+). Upon irradiation with visible light, the dimeric complexes undergo two competing photoreactions, viz., geometric E --> Z isomerization, resulting in an anion-capped 1:1 complex of the Z-isomer with Mg(2+) and stereospecific syn-head-to-tail [2+2]-cycloaddition, affording a single isomer of bis-crown-containing cyclobutane. The N-substituent in the dye has a dramatic effect on the photochemical behavior of the dimeric complex. Molecular dynamics and semiempirical quantum-chemical calculations were carried out to interpret the observed photocycloaddition in the dimer. Conformational equilibria for the dimer of (E)-4b were analyzed using (1)H NMR spectroscopy.     

Metal-core--organic shell dendrimers as unimolecular micelles

The synthesis and characterization of nanoparticle-cored dendrimers (NCDs), consisting of a metal core capped by arylpolyethers terminated with ester or carboxylate groups, are reported. These NCDs, comprising nanometer-sized gold clusters at the core and organic dendrons radially connected to the gold core by gold-sulfur bonds, were analyzed by TEM, TGA, UV, IR, and NMR spectroscopies. The density of the branching units connected to the core decreased from 1.90/nm(2) for a first-generation NCD (Au-G1(CO(2)Me)) to 0.80/nm(2) for a fourth-generation NCD (Au-G4(CO(2)Me)). Although the ester-terminated NCDs were stable and resisted aggregation, they were easily hydrolyzed to the corresponding water-soluble sodium salts. Aqueous solutions of (Au-Gn(CO(2)Na)) exhibited micellar properties. Since these NCDs possess a relatively unpassivated metal core and an organic aryl ether shell with micellar and dendritic properties, they are expected to have important potential applications in catalysis.