Mechanically driven reorganization of thermoresponsive diblock copolymer assemblies in water

Controlled formation of a variety of 3D structures was observed at high polymer weight fractions in water from a single diblock, consisting of poly(N-isopropylacrylamide), PNIPAM, and polystyrene, PSTY segments. The structures form through a mechanical process driven by swelling of hydrophilic polymer segments upon a change in temperature (see picture, SDS=sodium dodecylsulfate).     

Poly(p-phenylene ethynylene)s with facially amphiphilic pendant groups: solvatochromism and supramolecular assemblies

Novel functionalized poly(p-phenylene ethynylene)s (PPEs) bearing facially amphiphilic cholic and deoxycholic acid units are synthesized by a Pd-catalyzed Sonogashira cross-coupling reaction. Some interesting properties, particularly their optical and self-assembly characteristics, are unraveled. The PPEs that carry bile acid substituents exhibit remarkable solvatochromism in a wide range of solvent systems, and judicious choice of the solvents can adjust the size and morphology of the formed nanoscale supramolecular aggregates. The incorporation of these naturally occurring building blocks can also impart biocompatibility to the conjugated system and stimulate the growth of living cells.     

Stabilizing bolaform amphiphile interfacial assemblies by introducing mesogenic groups

We describe the synthesis and characterization of the mesogen-bearing bolaform amphiphile 4,4'-dihydroxybiphenylbis(11-pyridinium-N-yl-undecanoic ester) dibromide (BP-10) and its solid/liquid interfacial self-assembly. Cylindrical micelles are directly observed by atomic force microscopy (AFM) at the interface between mica and the aqueous solution above the critical micelle concentration (cmc). In situ and ex situ AFM studies indicate that the cylindrical micelles are stable both at the mica/solution interface and in the dry state. The enhanced stability of the micellar structures enables a detailed investigation of their self-assembly behavior and supramolecular structures at the interface. The adsorption model proposed here is supported by the variation of the interfacial self-assemblies on changing the solution concentration and substrate temperature.     

Supramolecular [6]chochin and "Big Mac" made from chiral Piedfort assemblies

Facile chemical synthesis of the natural chiral-pool-derived host 1 and its subsequent crystallization ("supramolecular synthesis") from different solvents yielded crystalline assemblies. Crystal structure determinations of five of the so formed solvent-inclusion compounds (1 a-1 e) reveal hexagonal symmetries in four cases. The structural characteristics of these chiral host-guest ensembles with varying stoichiometries can be best described as assemblies formed through intra-pair hydrogen bridges of host molecules into Piedfort pairs of differing complexity. Hitherto undescribed, these Piedfort pairs also form even larger regular assemblies that we designate "Big Mac"-like shapes. In the only nonhexagonal case, six independent host molecules form a huge supramolecular analogue of [6]benzocyclophane, also known as [6]chochin, extending this giant supermolecule through intermolecular hydrogen bonds into macroscopic (mm-size) dimensions. As all these crystals are inherently chiral, and new model systems for solid-state applications can be envisaged.     

Synthesis of supramolecular polymers by ionic self-assembly of oppositely charged dyes

A new type of supramolecular polymer was prepared by ionic self-assembly (ISA) from two oppositely charged dyes; a perylenediimide and a copper-phthalocyanine derivative. Coulomb coupling stabilizes the whole structure, and a combination of charge-transfer interactions and discotic stacking facilitates the exclusive formation of one-dimensional polymeric chains. The supramolecular dye-polymers have a large association constant (2.4 x 10(7) L mol(-1)), high molecular weight, and high mechanical stability. The use of cryo-transmission electron microscopy (cryo-TEM) confirmed the existence of extended fibers of width 2.4 nm. Further image analysis revealed slight undulation and faint segmentation of the fibers, and density maxima were observed at a regular interval of 3.6 nm along the fiber axis. The fiber-like structure (and aggregate of fibers) is also found in the solid state, as shown by the results of mineralization contrasting experiments, atomic force microscopy (AFM), and X-ray analyses. A structural model is proposed, in which the structural subunits, arranged in a side-by-side conformation, form a stacked structure.     

A new class of low-molecular-weight amphiphilic gelators

A new powerful class of low-molecular-weight amphiphilic compounds has been synthesized and their structure-property relationships with respect to their gelation ability of organic solvents have been investigated. These compounds are able to gel organic solvents over a broad range of polarity. Especially polar solvents such as valeronitrile and gamma-butyrolactone can be gelled even at concentrations far below 1 wt %. It was found that the gelation ability of these asymmetrically substituted p-phenylendiamines depends on a well-balanced relation of the terminal head group, the units involved in hydrogen bonding (amide or urea groups), and on the length of the alkyl chain. With this class of new gelators it is possible to tailor thermal and mechanical properties in different organic solvents and open various application possibilities.     

Smart materials based on self-assembled hydrogen-bonded comb-shaped supramolecules

Block copolymer self-assembly and supramolecular chemistry can be combined most naturally to prepare smart polymer nanomaterials. An attractive route is based on comb-shaped supramolecules, obtained by attaching side chains to (co)polymers by physical (non-covalent) interactions. Hydrogen bonding is a key element of our approach. It combines an ease of synthesis with other important approach-specific elements, such as hierarchical self-assembly, strongly enhanced processability, swelling, and cleaving. Functional properties discussed include anisotropic proton conductivity, switching proton conductivity, electronically conducting nanowires, polarized luminance, dielectric stacks (optical reflectivity), functional membranes, and nano objects.     

A general concept for the preparation of hierarchically structured pi-conjugated polymers

Typical biopolymers exhibit structures and order on different length scales. By contrast, the number of synthetic polymers with a similar degree of hierarchical structure formation is still limited. Starting from recent investigations on the structures of amyloid proteins as well as research activities toward nanoscopic scaffolds from synthetic oligopeptides and their polymer conjugates, a general strategy toward hierarchically structured pi-conjugated polymers can be developed. The approach relies on the supramolecular self-assembly of diacetylene macromonomers based on beta-sheet forming oligopeptides equipped with hydrophobic polymer segments. Polymerization of these macromonomers proceeds under retention of the previously assembled hierarchical structure and yields pi-conjugated polymers with multi-stranded, multiple-helical quaternary structures.     

From molecular to macroscopic engineering: shaping hydrogen-bonded organic nanomaterials

The self‐assembly and self‐organization behavior of chromophoric acetylenic scaffolds bearing 2,6‐bis(acetylamino)pyridine ( 1 , 2 ) or uracyl‐type ( 3 – 9 ) terminal groups has been investigated by photophysical and microscopic methods. Systematic absorption and luminescence studies show that 1 and 2 , thanks to a combination of solvophilic/solvophobic forces and π–π stacking interactions, undergo self‐organization in apolar solvents (i.e., cyclohexane) and form spherical nanoparticles, as evidenced by wide‐field optical microscopy, TEM, and AFM analysis. For the longer molecular module, 2 , a more uniform size distribution is found (80–200 nm) compared to 1 (20–1000 nm). Temperature scans in the range 283–353 K show that the self‐organized nanoparticles are reversibly formed and destroyed, being stable at lower temperatures. Molecular modules 1 and 2 were then thoroughly mixed with the complementary triply hydrogen‐bonding units 3 – 9 . Depending on the specific geometrical structure of 3 – 9 , different nanostructures are evidenced by microscopic investigations. Combination of modules 1 or 2 with 3 , which bears only one terminal uracyl unit, leads to the formation of vesicular structures; instead, when 1 is combined with bis‐uracyl derivative 4 or 5 , a structural evolution from nanoparticles to nanowires is observed. The length of the wires obtained by mixing 1 and 4 or 1 and 5 can be controlled by addition of 3 , which prompts transformation of the wires into shorter rods. The replacement of linear system 5 with the related angular modules 6 and 7 enables formation of helical nanostructures, unambiguously evidenced by AFM. Finally, thermally induced self‐assembly was studied in parallel with modules 8 and 9 , in which the uracyl recognition sites are protected with tert‐butyloxycarbonyl (BOC) groups. This strategy allows further control of the self‐assembly/self‐organization process by temperature, since the BOC group is completely removed on heating. Microscopy studies show that the BOC‐protected ditopic modules 8 self‐assemble and self‐organize with 1 into ordered linear nanostructures, whereas BOC‐protected tritopic system 9 gives rise to extended domains of circular nano‐objects in combination with 1 .