The self-assembly of lipophilic guanosine derivatives in solution and on solid surfaces

The self-assembly of lipophilic deoxyguanosine derivatives 1 and 2 has been studied in solution by NMR spectroscopy and ESI-MS (electrospray ionization mass spectrometry). NMR data show the existence of two types of self-assembled, ribbonlike structures (A and B), which are connected at the guanine moieties through two different H-bonded networks. The first species (A), which is stable in the solid state and characterised by cyclic NH(2)-O(6) and NH(1)-N(7) hydrogen bonds, is detected soon after dissolving the polycrystalline powder in rigorously anhydrous CDCl3. In solution it slowly undergoes a structural transition towards a thermodynamically stable ribbon characterised by NH(1)-O(6) and NH(2)-N(3) cyclic hydrogen bonds (B). On the other hand, at surfaces, self-assembled ribbon nanostructures have been grown from solutions of derivative 1 both on mica and at the graphite-solution interface. They have been investigated by means of tapping mode scanning force microscopy (SFM) and scanning tunnelling microscopy (STM), respectively. SFM revealed dry, micrometer-long nanoribbons with a molecular cross-section. while STM imaging at submolecular resolution indicates a molecular packing of type A, like the one detected in the solid state. This indicates that, upon adsorption at the solid-liquid interface, the guanosine moieties undergo a structural rearrangement from a B-type to an A-type ribbon.

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.

A Poly(para-phenylene) with Hydrophobic and Hydrophilic Dendrons: Prototype of an Amphiphilic Cylinder with the Potential to Segregate Lengthwise.

Langmuir monolayers are formed from an amphiphilically decorated poly(para-phenylene), which indicates that its hydrophilic and hydrophobic parts segregate lengthwise along the polymer backbone in this nanometer-sized cylinder as illustrated in A. This polymer differs from known amphiphiles in that it consists of a linear, covalently bound sequence of "little" amphiphiles. It is much more rigid than common "polysoaps", which should increase its potential to aggregate, for example, into channels.

Morphological Characterization of the Molecular Superstructure of Polyphenylene Ethynylene Derivatives.

Two different polyphenylene ethynylene derivatives, one partly hydrophobic and one hydrophilic, were investigated with a combination of X-ray and light scattering techniques and hydrodynamic techniques, as well as scanning force microscopy and transmission electron microscopy to elucidate their molecular structure and aggregation behavior in tetrahydrofuran and water, respectively. It turns out that both polymers possess a rod-like molecular architecture which, however, is the basis of a cascade of aggregation steps. Both, electron microscopy and X-ray analysis support the concept of a primary back-to-back aggregation of polymer chains into cylindrically shaped aggregates with high anisometry. The thickness of these aggregates was between 4.0 and 4.5 nm. The aggregates of the hydrophobic polymer further associate into fibrils and these fibrils form clusters of globular shape, though with high internal anisometry. Copyright 1999 Academic Press.