Natural pore-forming proteins act as viral helical coats and transmembrane channels, exhibit antibacterial activity and are used in synthetic systems, such as for reversible encapsulation or ...stochastic sensing. These diverse functions are intimately linked to protein structure. The close link between protein structure and protein function makes the design of synthetic mimics a formidable challenge, given that structure formation needs to be carefully controlled on all hierarchy levels, in solution and in the bulk. In fact, with few exceptions, synthetic pore structures capable of assembling into periodically ordered assemblies that are stable in solution and in the solid state have not yet been realized. In the case of dendrimers, covalent and non-covalent coating and assembly of a range of different structures has only yielded closed columns. Here we describe a library of amphiphilic dendritic dipeptides that self-assemble in solution and in bulk through a complex recognition process into helical pores. We find that the molecular recognition and self-assembly process is sufficiently robust to tolerate a range of modifications to the amphiphile structure, while preliminary proton transport measurements establish that the pores are functional. We expect that this class of self-assembling dendrimers will allow the design of a variety of biologically inspired systems with functional properties arising from their porous structure.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
X-ray measurements reveal a crystalline monolayer at the surface of the eutectic liquid Au₈₂Si₁₈, at temperatures above the alloy's melting point. Surface-induced atomic layering, the hallmark of ...liquid metals, is also found below the crystalline monolayer. The layering depth, however, is threefold greater than that of all liquid metals studied to date. The crystallinity of the surface monolayer is notable, considering that AuSi does not form stable bulk crystalline phases at any concentration and temperature and that no crystalline surface phase has been detected thus far in any pure liquid metal or nondilute alloy. These results are discussed in relation to recently suggested models of amorphous alloys.
The discovery of electrically conducting organic crystals and polymers has widened the range of potential optoelectronic materials, provided these exhibit sufficiently high charge carrier mobilities ...and are easy to make and process. Organic single crystals have high charge carrier mobilities but are usually impractical, whereas polymers have good processability but low mobilities. Liquid crystals exhibit mobilities approaching those of single crystals and are suitable for applications, but demanding fabrication and processing methods limit their use. Here we show that the self-assembly of fluorinated tapered dendrons can drive the formation of supramolecular liquid crystals with promising optoelectronic properties from a wide range of organic materials. We find that attaching conducting organic donor or acceptor groups to the apex of the dendrons leads to supramolecular nanometre-scale columns that contain in their cores π-stacks of donors, acceptors or donor-acceptor complexes exhibiting high charge carrier mobilities. When we use functionalized dendrons and amorphous polymers carrying compatible side groups, these co-assemble so that the polymer is incorporated in the centre of the columns through donor-acceptor interactions and exhibits enhanced charge carrier mobilities. We anticipate that this simple and versatile strategy for producing conductive π-stacks of aromatic groups, surrounded by helical dendrons, will lead to a new class of supramolecular materials suitable for electronic and optoelectronic applications.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Electron microscopy methods have been used to visualize individual spherical and cylindrical supramolecular dendrimers, providing definitive confirmation of the structures suggested by previous x-ray ...diffraction analysis that assumed a microsegregated model. These dendrimers are self-assembled, self-organized, and aligned spontaneously and simultaneously in hexagonal columnar or cubic thermotropic liquid-crystal phases with high uniformity. Homeotropic and planar ordering of the hexagonal columnar liquid crystal was precisely controlled by a variety of surfaces. The stiffness of these cylinders was evaluated by examining their planar texture and its defects.
The rational design, the synthesis via a new convergent approach, and the characterization of four generations of monodendrons based on the AB3 building block methyl 3,4,5-trishydroxybenzoate are ...described. The first generation monodendrons are crystalline. The second generation methyl 3,4,5-tris-3‘,4‘,5‘-tris(n-dodecyloxy)benzyloxybenzoate (12Gn-AG-CH 3 , n = 2), the corresponding benzyl alcohol (12G2-AG-CH 2 OH) and benzoic acid (12G2-AG), and all higher generation monodendrons with n = 3 and 4 exhibit in addition to the crystal phase an isomorphous cubic liquid-crystalline (LC) phase of Pm3̄n space group. The cubic unit cell parameters of 12G2-AG, 12G3-AG, and 12G4-AG are 68.3, 79.2, and 84.0 Å, respectively. Computed electron density profiles demonstrated that in the cubic phase 12, 6, and 2 monodendrons of generation 2, 3 and 4 are self-assembled in supramolecular dendrimers resembling spherical micelles when they are centered at the corners and the body-center and respectively highly rounded tetrahedra when they are at 1/4 and 3/4 distance along one of the bisectors of each face of the cubic cell. This novel thermotropic LC phase is similar to that of the lyotropic Pm3̄n phase of biological lipids. These supramolecular dendrimers contain a poly(benzyl ether) core dispersed in an aliphatic matrix of nearly uniform density which is made up of the melted terminal long alkyl chains of the monodendrons. The size of the supramolecular dendrimer and of the entire unit cell increases with generation number as a result of a disproportionately large increase in the number of AB3 repeat units relative to the number of terminal alkyl chains in a monodendron which decreases its average cone angle. The present experiments provide the first examples of monodendrons which self-assemble into spherical supramolecular dendrimers forming a thermotropic cubic LC phase.
S m, Thermoelectric power studies have also been carried out on the molecular wires that show a reversal of the sign on going from
the p–type to the n–type system (positive for p–type and negative ...for n–type). This is in conformity with the expected nature
of the charge carriers, namely, holes in HHTT + TNF (p–type) and electrons in HPA+anthracene (n–type). It is pointed out that
since the doped systems behave nearly as one–dimensional conductors unusual physical properties might be anticipated., or more relative to that in undoped samples; σ∥ attains a value of 10, reaches the insulating regime. Thus the columnar phases behave nearly as one–dimensional conductors and may be described
as ‘molecular wires’., We have studied the electrical conductivity of well–aligned samples of hexahexylthiotriphenylene (HHTT) and hexapentyloxyanthraquinone
(HPA) in the pure, as well as doped, states. The former compound was doped with 0.62 mol % by weight of the electron acceptor,
trinitrofluorenone (TNF), the latter with 0.60 mol % by weight of the electron donor, anthracene. In the columnar phases,
doping causes the AC (1 kHz) conductivity along the columnar axis (σ∥) to increase by a factor of 10, Finally, we discuss very briefly some further studies that are under way.We propose to adopt the four–probe method to increase
the highest measurable conductivity to the metallic range (after eliminating the lead and contact resistances) with a view
to finding metallic conductivity in discotic liquid crystals. We also intend to carry out Hall–effect studies and to explore
the possibility of observing superconductivity by looking for the Meissner effect in magnetic susceptibility measurements., ≥10, which is seven orders of magnitude higher than that reported for any liquid crystalline system, and, to our knowledge, the
largest observed in an organic conductor. Further, the perpendicular conductivity, σ, which was the maximum measurable limit of our experimental set–up. On the other hand, in the isotropic phase, doping makes
hardly any difference to the conductivity. The DC conductivity of doped HHTT exhibits an enormous anisotropy, σ
We report here the synthesis and characterization of an unsymmetrically substituted, monofunctionalized anthraquinone-based discotic liquid crystal, namely ...5-hydroxy-1-(4-nitrobenzyloxy)2,3,6,7-tetrapentyloxy-9,10-anthraquinone. Interestingly, this compound, which has only four alkyloxy chains attached to the anthraquinone core, shows a hexagonal columnar phase (Col
h
) of much wider temperature range compared with analogues having six alkyloxy chains. Incidentally, among the anthraquinone-based disc-like molecules exhibiting columnar phases known so far, this is the first molecule, which does not possess any C
2
-axis of symmetry. Besides its unsymmetrical structure and interesting mesomorphic behaviour, monofunctionalized monomers of this kind, unlike the available C
2
-symmetric difunctionalized tetraethers, can serve as potential synthons for obtaining dimers in a unique way. In addition, various anthraquinone-based oligomers, metallomesogens, and side group liquid crystalline polymers can be obtained from these monomers.
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