Multifunctional Tricarbazolo Triazolophane Macrocycles: One‐Pot Preparation,Anion Binding,and Hierarchical Self‐Organization of Multilayers

Programming the synthesis and self‐assembly of molecules is a compelling strategy for the bottom‐up fabrication of ordered materials. To this end, shape‐persistent macrocycles were designed with alternating carbazoles and triazoles to program a one‐pot synthesis and to bind large anions. The macrocycles bind anions that were once considered too weak to be coordinated, such as PF₆^?, with surprisingly high affinities (β₂=10¹¹ M ^?2 in 80:20 chloroform/methanol) and positive cooperativity, α=(4 K₂/K₁)=1200. We also discovered that the macrocycles assemble into ultrathin films of hierarchically ordered tubes on graphite surfaces. The remarkable surface‐templated self‐assembly properties, as was observed by using scanning tunneling microscopy, are attributed to the complementary pairing of alternating triazoles and carbazoles inscribed into both the co‐facial and edge‐sharing seams that exist between shape‐persistent macrocycles. The multilayer assembly is also consistent with the high degree of molecular self‐association observed in solution, with self‐association constants of K=300 000 M ^?1 (chloroform/methanol 80:20). Scanning tunneling microscopy data also showed that surface assemblies readily sequester iodide anions from solution, modulating their assembly. This multifunctional macrocycle provides a foundation for materials composed of hierarchically organized and nanotubular self‐assemblies.     

A set of homologous hetarylenediyne macrocycles by oxidative acetylene-acetylene coupling

The synthesis of a set of bipyridine-containing macrocycles by oxidative acetylene-acetylene dimerization is described. The cycles are separated by preparative GPC, and the smallest homologue is analyzed by single-crystal X-ray diffraction, which shows a layered structure with channels that are partially filled with parts of the flexible chains of adjacent macrocycles. The cyclic trimer has a D3h symmetry and is a possible candidate for the formation of metal organic supramolecular assemblies on surfaces.     

Sterically‐Limited Self‐Assembly of Pt4 Macrocycles into Discrete Non‐covalent Nanotubes: Porous Supramolecular Tetramers and Hexamers

We report a template‐free strategy based on steric repulsion for the isolation of discrete columnar aggregates of macrocycles. Specifically, introduction of sterically‐demanding trityl‐derived substituents at the periphery of Pt₄ Schiff base macrocycles limits the otherwise infinite one‐dimensional columnar aggregation to discrete tetrameric and hexameric assemblies. Single crystal X‐ray diffraction studies of these compounds reveal discrete nanotubes of finite length that pack inefficiently resulting in three‐dimensional networks of interconnected void space. The discrete assemblies were studied by N₂ adsorption and show enhanced surface area when stacked. In the absence of bulky substituents the macrocycles are nonporous. This strategy for engineering discrete supramolecular macrocyclic aggregates may be generalized to other columnar assembling systems.     

Self-assembly, binding, and dynamic properties of heterodimeric porphyrin macrocycles

A series of heterodimeric tetralactam macrocycles have been self-assembled using two kinetically labile zinc porphyrin-pyridine interactions. The stability constants have been determined by UV-vis titrations in CHCl3. The stability constants depend on the degree of preorganization of the linker units connecting the interacting groups. The ability of the self-assembled macrocycles to bind a terephthalamide guest was also investigated. One of the macrocycles was used for the construction of a [2]rotaxane. The dynamic properties of this system provide insight into the exchange mechanisms that operate in complex noncovalent assemblies.     

Coordination-driven self-assembly of predesigned supramolecular triangles

The design and self-assembly of three supramolecular triangles is described. A novel 60 degrees corner unit directs the exclusive formation of triangular assemblies that are not in detectable equilibrium with other macrocycles. The resulting triangles have sides ranging from 2.7 to 3.5 nm in length and molecular masses as high as 5396 amu. The crystal structure of one of the assemblies shows an approximately 1.4 nm cavity; the crystal packing forms open, triangular channels. The characterization of the supramolecular triangles by multinuclear NMR, elemental analysis, and electrospray mass spectrometry is also reported.     

Unprecedented selectivity in the formation of large-ring oligoimines from conformationally bistable chiral diamines

[reaction: see text] Stereoselective formation of large macrocycles in "click-type" reactions is a current challenge. Chiral macrocycles of differing size and shape (e.g., rectanglimine or loopimine) were selectively obtained by cyclocondensation of terephthalaldehyde or isophthalaldehyde with conformationally bistable chiral diamines derived from trans-1,2-diaminocyclohexane and aromatic dianhydrides. This opens new opportunities for the programmed synthesis of large-ring molecular assemblies.     

Aza-beta3-cyclotetrapeptides

The cyclization of aza-beta(3)-tetrapeptides gives access to new CTP (cyclotetrapeptide) analogues. These stereocontrolled templates are assembled without any asymmetric synthesis. X-ray crystallographic structure and NMR analysis show that the macrocyclic scaffold is characterized by a fully cooperative intramolecular H-bond network, in sharp contrast with the nanotubular assemblies observed for beta(3)-cyclotetrapeptides. This folding property reduces considerably the polarity of aza-beta(3)-tetrapeptides and should be useful in addressing intracellular targets.     

Nanotube and three-way nanotube formation with nonionic amphiphilic block peptides

Amphiphilic block polypeptides having a helical hydrophobic block with a uniform chain length and a hydrophilic nonionic block were newly synthesized and self-assembled into homogeneous nanotubes with ca. 60 nm diameter and ca. 200 nm length. The tubular assembly was shown to be elongated by heating over micrometer length without changing the diameter. Notably, a distinctive three-way nanotube was obtained just by mixing two kinds of amphiphilic polypeptides with the same helical hydrophobic block but different chain lengths of the hydrophilic block. The morphology of the molecular assemblies was shown to be tunable from a curved sheet-shaped assembly to a long or short nanotubular assembly and a three-way nanotubular assembly by suitable molecular design of the hydrophobic block, selection of the chain length of the hydrophilic block, mixing two-type block peptides, and processing such as heating.     

Robust Ordered Bundles of Porous Helical Nanotubes Assembled from Fully Rigid Ionic Benzene‐1,3,5‐tricarboxamides

Size‐controlled and ordered assemblies of artificial nanotubes are promising for practical applications; however, the supramolecular assembly of such systems remains challenging. A novel strategy is proposed that can be used to reinforce intermolecular noncovalent interactions to construct hierarchical supramolecular structures with fixed sizes and long‐range ordering by introducing ionic terminals and fully rigid arms into benzene‐1,3,5‐tricarboxamide (BTA) molecules. A series of similar BTA molecules with distinct terminal groups and arm lengths are synthesized; all form hexagonal bundles of helical rosette nanotubes spontaneously in water. Despite differences in molecular packing, the dimensions and bundling of the supramolecular nanotubes show almost identical concentration dependence for all molecules. The similarities of the hierarchical assemblies, which tolerate certain molecular irregularities, can extend to properties such as the void ratio of the nanotubular wall. This is a rational strategy that can be used to achieve supramolecular nanotubes in aqueous environments with precise size and ordering at the same time as allowing molecular modifications for functionality.     

Arene–perfluoroarene interactions confer enhanced mechanical properties to synthetic nanotubes

Supramolecular nanotubes prepared through macrocycle assembly offer unique properties that stem from their long-range order, structural predictability, and tunable microenvironments. However, assemblies that rely on weak non-covalent interactions often have limited aspect ratios and poor mechanical integrity, which diminish their utility. Here pentagonal imine-linked macrocycles are prepared by condensing a pyridine-containing diamine and either terephthalaldehyde or 2,3,5,6-tetrafluoroterephthalaldehyde. Atomic force microscopy and synchrotron in solvo X-ray diffraction demonstrate that protonation of the pyridine groups drives assembly into high-aspect ratio nanotube assemblies. A 1 : 1 mixture of each macrocycle yielded nanotubes with enhanced crystallinity upon protonation. UV-Vis and fluorescence spectroscopy indicate that nanotubes containing both arene and perfluoroarene subunits display spectroscopic signatures of arene–perfluoroarene interactions. Touch-spun polymeric fibers containing assembled nanotubes prepared from the perhydro- or perfluorinated macrocycles exhibited Young''s moduli of 1.09 and 0.49 GPa, respectively. Fibers containing nanotube assemblies reinforced by arene–perfluoroarene interactions yielded a 93% increase in the Young''s modulus over the perhydro derivative, up to 2.1 GPa. These findings demonstrate that tuning the chemical composition of the monomeric macrocycles can have profound effects on the mechanical strength of the resulting assemblies. More broadly, these results will inspire future studies into tuning orthogonal non-covalent interactions between macrocycles to yield nanotubes with emergent functions and technological potential.

Arene–perfluoroarene interactions resulted in enhanced crystallinity between analogous perhydro- and perfluoro macrocycles in a supramolecular nanotube assembly.     

Equipping metallo-supramolecular macrocycles with functional groups: assemblies of pyridine-substituted urea ligands

A series of di-(m-pyridyl)-urea ligands were prepared and characterized with respect to their conformations by NOESY experiments and crystallography. Methyl substitution in different positions of the pyridine rings provides control over the position of the pyridine N atoms relative to the urea carbonyl group. The ligands were used to self-assemble metallo-supramolecular M(2)L(2) and M(3)L(3) macrocycles which are generated in a finely balanced equilibrium in DMSO and DMF according to DOSY NMR experiments and ESI FTICR mass spectrometry. Again, crystallography was used to characterize the assemblies. Methyl substitution in positions next to the pyridine nitrogen prevents coordination, while the other ligands form small metallo-supramolecular macrocycles. The incorporated urea carbonyl groups provide hydrogen bonding sites which converge towards the center of the assemblies.     

Anion-templated synthesis

In contrast to the well-studied templating properties of cationic and neutral species there are relatively few examples of anion-templated syntheses. This is attributed to some of the intrinsic properties of anions such as their diffuse nature, pH sensitivity, and their relative high solvation free energies. However, the increasing number of anion-templated assemblies reported over the past few years demonstrates that these limitations are not as critical as first thought. This review summarizes the most important results on the use of anions as directing agents for the syntheses of a wide range of inorganic and organic assemblies (such as macrocycles, cages, helicates, rotaxanes, and extended structures). It is hoped that this will stimulate a closer look into this emerging area of supramolecular chemistry.     

Assemblies of carbon and boron-nitrogen nanotubes and fullerenes: Structure and properties

This review concerns assemblies of the main carbon nanostructures (fullerenes and nanotubes) generated by interactions between similar and dissimilar species. The two major families of these nanomaterials are reviewed: (1) assemblies with weak (van der Waals) bonds between fullerenes and/or nanotubes (fullerites, nanopeapods, nanotube bundles, and nanotubular crystals) and (2) assemblies formed by covalently bonded (polymerized) fullerenes and/or nanotubes (covalent crystals of small fullerenes C _{n < 60} , nanobuds, ropes of polymerized nanotubes, and covalent networks built of nanotubes). Data are systematized on their atomic structures, stability factors, electronic structures, chemical bonding, physical and chemical properties, and potential fields of application. Related heteronanomaterials (assemblies of boron-nitrogen fullerene-like molecules and/or nanotubes) are reviewed briefly.     

Theoretical study on tertiary structural elements of beta-peptides: nanotubes formed from parallel-sheet-derived assemblies of beta-peptides

Parallel or polar strands of beta-peptides spontaneously form nanotubes of different sizes in a vacuum as determined by ab initio calculations. Stability and conformational features of [CH3CO-(beta-Ala)k-NHCH3]l (1 < or = k < or = 4, 2 < or = l < or = 4) models were computed at different levels of theory (e.g., B3LYP/6-311++G(d,p)// B3LYP/6-31G(d), with consideration of BSSE). For the first time, calculations demonstrate that sheets of beta-peptides display nanotubular characteristics rather than two-dimensional extended beta-layers, as is the case of alpha-peptides. Of the configurations studied, k = l = 4 gave the most stable nanotubular structure, but larger assemblies are expected to produce even more stable nanotubes. As with other nanosystems such as cyclodextrane, these nanotubes can also incorporate small molecules, creating a diverse range of applications for these flexible, biocompatible, and highly stable molecules. The various side chains of beta-peptides can make these nanosystems rather versatile. Energetic and structural features of these tubular model systems are detailed in this paper. It is hoped that the results presented in this paper will stimulate experimental research in the field of nanostructure technology involving beta-peptides.     

One-dimensional assembly of chalcogenide nanoclusters with bifunctional covalent linkers

Even though different approaches have been developed to achieve various 1D assemblies of nanocrystals, few studies have been done on the assembly of crystallographically well-defined chalcogenide nanoclusters. Here, by using bifunctional organic ligands as the directional linker, a series of one-dimensional assemblies of semiconducting chalcogenide nanoclusters have been prepared and characterized. The synthetic method allows for the preparation of differently sized tetrahedral nanoclusters that are joined together with organic linkers of different length and rigidity. Multiple linking modes between nanoclusters and organic ligands are revealed in four different assemblies that also exhibit size-dependent optical properties.     

Controlling the Structure and Length of Self‐Synthesizing Supramolecular Polymers through Nucleated Growth and Disassembly

Directing self‐assembly processes out‐of‐equilibrium to yield kinetically trapped materials with well‐defined dimensions remains a considerable challenge. Kinetically controlled assembly of self‐synthesizing peptide‐functionalized macrocycles through a nucleation–growth mechanism is reported. Spontaneous fiber formation in this system is effectively shut down as most of the material is diverted into metastable non‐assembling trimeric and tetrameric macrocycles. However, upon adding seeds to this mixture, well‐defined fibers with controllable lengths and narrow polydispersities are obtained. This seeded growth strategy also allows access to supramolecular triblock copolymers. The resulting noncovalent assemblies can be further stabilized through covalent capture. Taken together, these results show that self‐synthesizing materials, through their interplay between dynamic covalent bonds and noncovalent interactions, are uniquely suited for out‐of‐equilibrium self‐assembly.     

Intramolecular energy transfer within butadiyne-linked chlorophyll and porphyrin dimer-faced, self-assembled prisms

The synthesis and photophysical properties of butadiyne-linked chlorophyll and porphyrin dimers in toluene solution and in several self-assembled prismatic structures are described. The butadiyne linkage between the 20-positions of the macrocycles results in new electronic transitions polarized along the long axes of the dimers. These transitions greatly increase the ability of these dimers to absorb the solar spectrum over a broad wavelength range. Femtosecond transient absorption spectroscopy reveals the relative rate of rotation of the macrocycles around the butadiyne bond joining them. Following addition of 3-fold symmetric, metal-coordinating ligands, both macrocyclic dimers self-assemble into prismatic structures in which the dimers comprise the faces of the prisms. These structures were confirmed by small-angle X-ray scattering experiments in solution using a synchrotron source. Photoexcitation of the prismatic assemblies reveals that efficient, through-space energy transfer occurs between the macrocyclic dimers within the prisms. The distance dependence of energy transfer between the faces of the prisms was observed by varying the size of the prismatic assemblies through the use of 3-fold symmetric ligands having arms with different lengths. These results show that self-assembly of discrete macrocyclic prisms provides a useful new strategy for controlling singlet exciton flow in antenna systems for artificial photosynthesis and solar cell applications.     

Exploiting the dithiocarbamate ligand in metal-directed self-assembly

The dithiocarbamate (dtc) ligand has proved to be an extremely versatile and robust motif for metal-directed self-assembly. Its ease of formation and wide ranging coordination chemistry has led to the formation of an array of novel and complex supramolecular architectures. Well-defined structures such as macrocycles, cages, catenanes and nanodimensional assemblies can be generated using a variety of oligomeric dithiocarbamate constructs in combination with transition metals. Polymetallic assemblies containing appropriately designed host cavities have allowed the binding of cationic, anionic and neutral guest species to be investigated. The use of the dithiocarbamate ligand has recently expanded to stabilising gold nanoparticles and preparing multimetallic wires and arrays. This perspective highlights the considerable potential that this simple and versatile ligand has to offer.     

Aldolase Cascade Facilitated by Self-Assembled Nanotubes from Short Peptide Amphiphiles

Early evolution benefited from a complex network of reactions involving multiple C−C bond forming and breaking events that were critical for primitive metabolism. Nature gradually chose highly evolved and complex enzymes such as lyases to efficiently facilitate C−C bond formation and cleavage with remarkable substrate selectivity. Reported here is a lipidated short peptide which accesses a homogenous nanotubular morphology to efficiently catalyze C−C bond cleavage and formation. This system shows morphology-dependent catalytic rates, suggesting the formation of a binding pocket and registered enhancements in the presence of the hydrogen-bond donor tyrosine, which is exploited by extant aldolases. These assemblies showed excellent substrate selectivity and templated the formation of a specific adduct from a pool of possible adducts. The ability to catalyze metabolically relevant cascade transformations suggests the importance of such systems in early evolution.     

Complexation behavior of a highly preorganized 7,7-diphenylnorbornane-derived macrocycle: towards the design of molecular clocks

The syntheses of two new cyclophane hosts, 4 and 6, are described. The main difference between them is the higher degree of preorganization of 4 as a consequence of the inclusion of the 7,7-diphenylnorbornane (DPN) subunit. The inner cavity of 4 adopts a belt-shaped structure, while 6 has a twisted geometry. In the solid state, the molecules of macrocycle 6 are stacked along an axis to form nanotubular structures. Compounds 4 and 6 form two of the strongest complexes between arene cyclophanes and Ag(+) reported up to date. The silver cation is located inside the cavity of the macrocycles. The stability of 4.Ag(+) is considerably higher than that of 6.Ag(+). The additional stabilization of 4.Ag(+) is attributed to higher preorganization of macrocycle 4. DNMR experiments as well as theoretical calculations carried out with 4.Ag(+) show evidence of Ag(+)-hopping between two different binding sites inside the macrocycle. This phenomenon could be the basis for the design of molecular clocks.