Dendronized Polymers: Synthesis, Characterization, Assembly at Interfaces, and Manipulation

Dendrimers are presently one of the most intensely studied classes of compounds because of their unusual structure. They can be described as a jungle of entangled branches traversed by winding trails which lead to sweet fruits and bright blossoms. On these trails one can reach the thicket's interior as well as find a way out. Expressed less lyrically, this thicket stands for regularly branched, densely packed structures, and the trails represent voids and channels not filled by bent back branches but by solvent. The fruit and blossoms are photochemically, electrochemically, or synthetically addressable units, catalytically active sites, etc., and the back and forth on the trails stands for transport processes. In a mathematical sense dendrimers are enveloped by an interface, which defines what is either in or out. This interface is shaped like a sphere if the trails are filled to bursting. Otherwise dendrimers are more flattened like amoeba, especially if in contact with a surface. The high density of the functional groups, the expansion of these compounds to a range of several nanometers, the existence of usable "surface" and transport possibilities in and with them have made dendrimers interesting candidates for many applications. This review describes how dendrimer construction and polymer synthesis were combined and used to move from fully or flattened spherical shapes to cylindrical ones. The shape-inducing influence of dendritic substituents can be driven to create nanoobjects with a cylindrical shape, which not only considerably widens the range of applications for the dendrimer class but also opens up new perspectives for supramolecular and polymer chemistry. Because of the sheer size of the described objects and complexity of shape-related properties, research in this area must necessarily be interdisciplinary. This article tries to mirror this by giving special attention not only to synthesis but also the characterization and behavior of these compounds in bulk and at interfaces. Furthermore, potential application fields are described.     

Double-helical ultrastructure of polycationic dendronized polymers determined by single-particle Cryo-TEM

The ultrastructure of cationic dendronized polymers (denpols) of third and fourth generations (PG3 and PG4) in water was determined by using single-particle cryo-transmission electron microscopy (cryo-TEM). At concentrations in the region of 50 mg L(-1), networks of double-stranded fibers were revealed that exhibit well-defined diameters of 5.9 nm+/-0.4 nm for PG3 and 7.4 nm+/-0.4 nm for PG4. The structure varies with progression along the fibers, and includes a double helix with a pitch of 7.0+/-0.4 nm for PG3 and 9.0+/-0.4 nm for PG4. The formation of the double strands is attributed to the hydrophobic effect and limited crowding in the dendron shell of the third and fourth generation denpols investigated. From solutions of lower concentrations (around 10 mg L(-1)), isolated molecular fibers were adsorbed onto high-energy surfaces and examined by performing scanning force microscopy (SFM) on mica, and after staining, TEM on glow-discharged carbon films. In both cases, characteristic undulations of single strands were observed, which are attributed largely to the adsorption process.     

Supramolecular helices via self-assembly of 8-oxoguanosines

The cooperative effect of solvophobic interactions and hydrogen bonding has been exploited to self-assemble supramolecular helical architectures of 8-oxoguanosines in different environments. This self-assembly into helical structures is completely different from that of the parent guanosines which, in the same experimental conditions, form flat, ribbonlike structures. While optical microscopy and X-ray diffraction suggest a chiral columnar aggregate in the LC phase, NMR and Circular Dichroism reveal the presence of a helical structures in solution. Scanning Tunneling Microscopy made it possible to visualize hexagonally arranged G-quartets on graphite, which are sections of the helices packed with their long axis perpendicular to the basal plane of the substrate. Due to their rectifying electrical properties, such helices are interesting for fabricating (opto)electronic biodevices.     

Quantum theory of chemical bond formation processes in condensed systems: studies towards exploration of electrochemical dark and photoprocesses on semiconductors

Quantum theoretical models of chemical bond formation and partial charge transfer processes in condensed systems, and their extension to processes in electronic non-equilibrium are investigated in this paper with a view to further exploration of electrochemical and photoelectrochemical kinetics on semiconductors. Electrochemical dark and photocurrents on n-III-V-semiconductors are correlated with calculated transition probabilities for atom-group transfer over larger distances (80 to 240 pm), leading to a first estimate of potential surface shapes compatible with experiment. Some specific problems connected with transition probability calculations for heavy-particle transfer in strong anharmonic potentials are considered in detail, including approximation of Franck-Condon transitions in arbitrary potentials by their classical limit.     

Terphenyl cyclooctatetraenyl compounds of samarium

The syntheses and molecular structures of a number of terphenyl-based compounds of the lanthanide element samarium are reported. Reaction of 2 equiv of DppLi (Dpp = 2,6-diphenylphenyl) with 1 equiv of SmCl(3) in tetrahydrofuran at room temperature yields (Dpp)(2)SmCl(micro-Cl)Li(THF)(3) (1). The one-pot reaction of 1 equiv of K(2)COT (COT(2)(-) = cyclooctatetraenyl dianion) with 1 equiv SmCl(3) in tetrahydrofuran at room temperature followed by addition of 1 equiv of terphenyllithium salt DppLi, DmpLi (Dmp = 2,6-dimesitylphenyl), or DanipLi (Danip = 2,6-di(o-anisol)phenyl) produces DppSmCOT(micro-Cl)Li(THF)(3) (2), DmpSm(THF)COT (3), and DanipSm(THF)COT (4), respectively. In the case of the Danip-based compound 4 the order of addition of reagents can be reversed producing the same compound, however, in considerably lower yield. Compound 2 can also be prepared by reaction of 1 with 1 equiv of K(2)COT in tetrahydrofuran. The molecular structure of the bis(terphenyl) compound 1 exhibits a formally four-coordinate metal atom. The molecular structures of the terphenyl COT compounds 2-4 feature monomeric complexes which are obtained either as a lithium chloride adduct (2) or as tetrahydrofuran adducts (3, 4). In 4 the Danip ligand adopts the meso form.     

1,4-Diazobicyclo[2.2.2]octane-catalyzed coupling of aldehydes and activated double bonds. Part 3. A short ans practical synthesis of mikanecic acid (4-vinyl-1-cyclohexene-1,4-dicarboxylic acid)

The title dicarboxylic acid 1d has been prepared in 24% overall yield via, 1,4-diazabicyclo[2.2.2]octane (DABCO)-catalyzed coupling of ethanal and tert-butyl propenoate ( 3 ) to 4 , S_N2′-reaction to tert-butyl (Z)-2-romomethyl-2-butenoate ( 5a ), dehydrobrominatin to tert-butyl 2-methylidene-3-butenoate ( 2c ), dimerizatoin to di-tert-butyl 4-vinyl-1-cyclohexene-1,4-dicarboxylate ( 1c ) and acidic ester cleavage. Acidic cleavage of easily obtainable 5a affords (Z)-2-bromomethyl-2-butenoic acid ( 5a ) in 68% yield with respect to ethanal.     

Reaction intermediates in the catalytic Gif-type oxidation from nuclear inelastic scattering

Nuclear inelastic scattering (NIS) of synchrotron radiation, also known as nuclear resonant vibrational spectroscopy (NRVS), has been shown to provide valuable insights into metal-centered vibrations at Mössbauer-active nuclei. We present a study of the iron-centered vibrational density of states (VDOS) during the first step of the Gif-type oxidation of cyclohexene with a novel trinuclear Fe₃(μ₃-O) complex as catalyst precursor. The experiments were carried out on shock-frozen solutions for different combinations of reactants: Fe₃(μ₃-O) in pyridine solution, Fe₃(μ₃-O) plus Zn/acetic acid in pyridine without and with addition of either oxygen or cyclohexene, and Fe₃(μ₃-O)/Zn/acetic acid/pyridine/cyclohexene (reaction mixture) for reaction times of 1 min, 5 min, and 30 min. The projected VDOS of the Fe atoms was calculated on the basis of pseudopotential density functional calculations. Two possible reaction intermediates were identified as [Fe^(III)(C₅H₅N)₂(O₂CCH₃)₂]⁺ and Fe^(II)(C₅H₅N)₄(O₂CCH₃)₂, yielding evidence that NIS (NRVS) allows to identify the presence of iron-centered intermediates also in complex reaction mixtures.