Coils,Rods and Rings in Hydrogen‐Bonded Supramolecular Polymers

The article discusses the development and properties of supramolecular polymers based on quadruple hydrogen bonds between self‐complementary ureidotriazine (UTr) and ureidopyrimidinone (UPy) functional groups. The high association constant with which these groups dimerize leads to polymers with a high degree of polymerization in isotropic solution. Application of these units for the functionalization of telechelic polymers results in new materials with mechanical properties approaching those of covalent polymers, but with a much stronger temperature‐dependent behavior. Solvophobic interactions between the hydrogen bonding moieties may be used to obtain supramolecular polymers with a well defined helical columnar architecture. Another consequence of the high dimerization constant of the UPy group is the phenomenon of a critical concentration in solutions of many bifunctional monomers. Below this concentration, only cycles are present, while above the critical concentration, the amount of cycles remains constant, and a polymer is formed. Conformational properties of the linker units are used to control the equilibrium between polymers and cycles, and are proposed to form a promising strategy toward tunable materials.

Supramolecular polymer material with elastomeric properties resulting from functionalization with UPy groups. (Reproduced with permission. © John Wiley & Sons, Inc.)     

Beyond Molecules: Mesoporous Supramolecular Frameworks Self‐Assembled from Coordination Cages and Inorganic Anions

Biological function arises by the assembly of individual biomolecular modules into large aggregations or highly complex architectures. A similar strategy is adopted in supramolecular chemistry to assemble complex and highly ordered structures with advanced functions from simple components. Here we report a series of diamond‐like supramolecular frameworks featuring mesoporous cavities, which are assembled from metal‐imidazolate coordination cages and various anions. Small components (metal ions, amines, aldehydes, and anions) are assembled into the hierarchical complex structures through multiple interactions including covalent bonds, dative bonds, and weak C? H???X (X=O, F, and π) hydrogen bonds. The mesoporous cavities are large enough to trap organic dye molecules, coordination cages, and vitamin B₁₂. The study is expected to inspire new types of crystalline supramolecular framework materials based on coordination motifs and inorganic ions.     

Thermally Stable Porous Hydrogen‐Bonded Coordination Networks Displaying Dual Properties of Robustness and Dynamics upon Guest Uptake

Two series of microporous lanthanide coordination networks of the general formula, {[Ln(ntb)Cl₃] ? x H₂O}_n (series 1 : monoclinic C2/c, Ln=Sm and Tb; series 2 : hexagonal P3₁/c, Ln=Sm and Eu; ntb=tris(benzimidazol‐2‐ylmethyl)amine, x=0–4) have been synthesized and characterized by IR, elemental analyses, thermal gravimetry, and single‐crystal and powder X‐ray diffraction methods. In both series, the monomeric [Ln(ntb)Cl₃] coordination units are consolidated by N? H???Cl or C? H???Cl hydrogen bonds to sustain three‐dimensional (3D) networks. However, the different modes of hydrogen bonding in the two series lead to crystallization of the same [Ln(ntb)Cl₃] monomers in different forms (monoclinic vs. hexagonal), consequently giving rise to distinct porous structures. The resulting hydrogen‐bonded coordination networks display high thermal stability and robustness in water removal/inclusion processes, which was confirmed by temperature‐dependent single‐crystal‐to‐single‐crystal transformation measurements. Adsorption studies with H₂, CO₂, and MeOH have been carried out, and reveal distinct differences in adsorption behavior between the two forms. In the case of MeOH uptake, the monoclinic network shows a normal type I isotherm, whereas the hexagonal network displays dynamic porous properties.     

Redox‐Controlled Hydrogen Bonding: Turning a Superbase into a Strong Hydrogen‐Bond Donor

Herein the synthesis, structures and properties of hydrogen‐bonded aggregates involving redox‐active guanidine superbases are reported. Reversible hydrogen bonding is switched on by oxidation of the hydrogen‐donor unit, and leads to formation of aggregates in which the hydrogen‐bond donor unit is sandwiched by two hydrogen‐bond acceptor units. Further oxidation (of the acceptor units) leads again to deaggregation. Aggregate formation is associated with a distinct color change, and the electronic situation could be described as a frozen stage on the way to hydrogen transfer. A further increase in the basicity of the hydrogen‐bond acceptor leads to deprotonation reactions.     

A Supramolecular Sorting Hat: Stereocontrol in Metal–Ligand Self‐Assembly by Complementary Hydrogen Bonding

A combination of self‐complementary hydrogen bonding and metal–ligand interactions allows stereocontrol in the self‐assembly of prochiral ligand scaffolds. A unique, non‐tetrahedral M₄L₆ structure is observed upon multicomponent self‐assembly of 2,7‐diaminofluorenol with 2‐formylpyridine and Fe(ClO₄)₂. The stereochemical outcome of the assembly is controlled by self‐complementary hydrogen bonding between both individual ligands and a suitably sized counterion as template. This hydrogen‐bonding‐mediated stereoselective metal–ligand assembly allows the controlled formation of nonsymmetric discrete cage structures from previously unexploited ligand scaffolds.     

Nonperipherally Octa(butyloxy)‐Substituted Phthalocyanine Derivatives with Good Crystallinity: Effects of Metal–Ligand Coordination on the Molecular Structure,Internal Structure,and Dimensions of Self‐Assembled Nanostructures

To investigate the effects of metal–ligand coordination on the molecular structure, internal structure, dimensions, and morphology of self‐assembled nanostructures, two nonperipherally octa(alkoxyl)‐substituted phthalocyanine compounds with good crystallinity, namely, metal‐free 1,4,8,11,15,18,22,25‐octa(butyloxy)phthalocyanine H₂Pc(α‐OC₄H₉)₈ ( 1 ) and its lead complex Pb[Pc(α‐OC₄H₉)₈] ( 2 ), were synthesized. Single‐crystal X‐ray diffraction analysis revealed the distorted molecular structure of metal‐free phthalocyanine with a saddle conformation. In the crystal of 2 , two monomeric molecules are linked by coordination of the Pb atom of one molecule with an aza‐nitrogen atom and its two neighboring oxygen atoms from the butyloxy substituents of another molecule, thereby forming a Pb‐connected pseudo‐double‐decker supramolecular structure with a domed conformation for the phthalocyanine ligand. The self‐assembling properties of 1 and 2 in the absence and presence of sodium ions were comparatively investigated by scanning electronic microscopy (SEM), spectroscopy, and X‐ray diffraction techniques. Intermolecular π–π interactions between metal‐free phthalocyanine molecules led to the formation of nanoribbons several micrometers in length and with an average width of approximately 100 nm, whereas the phthalocyaninato lead complex self‐assembles into nanostructures also with the ribbon morphology and micrometer length but with a different average width of approximately 150 nm depending on the π–π interactions between neighboring Pb‐connected pseudo‐double‐decker building blocks. This revealed the effect of the molecular structure (conformation) associated with metal–ligand (Pb? N_isoindole, Pb? N_aza, and Pb? O_butyloxy) coordination on the dimensions of the nanostructures. In the presence of Na⁺, additional metal–ligand (Na? N_aza and Na? O_butyloxy) coordination bonds formed between sodium atoms and aza‐nitrogen atoms and the neighboring butyloxy oxygen atoms of two metal‐free phthalocyanine molecules cooperate with the intrinsic intermolecular π–π interactions, thereby resulting in an Na‐connected pseudo‐double‐decker building block with a twisted structure for the phthalocyanine ligand, which self‐assembles into twisted nanoribbons with an average width of approximately 50 nm depending on the intertetrapyrrole π–π interaction. This is evidenced by the X‐ray diffraction analysis results for the resulting aggregates. Twisted nanoribbons with an average width of approximately 100 nm were also formed from the lead coordination compound 2 in the presence of Na⁺ with a Pb‐connected pseudo‐double‐decker as the building block due to the formation of metal–ligand (Na? N_aza and Na? O_butyloxy) coordination bonds between additionally introduced sodium ions and two phthalocyanine ligands of neighboring pseudo‐double‐decker building blocks.     

An AAA–DDD Triply Hydrogen‐Bonded Complex Easily Accessible for Supramolecular Polymers

For a complementary hydrogen‐bonded complex, when every hydrogen‐bond acceptor is on one side and every hydrogen‐bond donor is on the other, all secondary interactions are attractive and the complex is highly stable. AAA–DDD (A=acceptor, D=donor) is considered to be the most stable among triply hydrogen‐bonded sequences. The easily synthesized and further derivatized AAA–DDD system is very desirable for hydrogen‐bonded functional materials. In this case, AAA and DDD, starting from 4‐methoxybenzaldehyde, were synthesized with the Hantzsch pyridine synthesis and Friedländer annulation reaction. The association constant determined by fluorescence titration in chloroform at room temperature is 2.09×10⁷ M ^?1. The AAA and DDD components are not coplanar, but form a V shape in the solid state. Supramolecular polymers based on AAA–DDD triply hydrogen bonded have also been developed. This work may make AAA–DDD triply hydrogen‐bonded sequences easily accessible for stimuli‐responsive materials.     

Hydrogen‐Bonded Polymer–Porphyrin Assemblies in Water: Supramolecular Structures for Light Energy Conversion

In this study, a new type of functional, self‐assembled nanostructure formed from porphyrins and polyamidoamine dendrimers based on hydrogen bonding in an aqueous solution is presented. As the aggregates formed are promising candidates for solar‐energy conversion, their photocatalytic activity is tested using the model reaction of methyl viologen reduction. The self‐assembled structures show significantly increased activity as compared to unassociated porphyrins. Details of interaction forces driving the supramolecular structure formation and regulating catalytic efficiency are fundamentally discussed.

     

Complexation‐Induced Supramolecular Assembly Drives Metal‐Ion Extraction

Combining experiment with theory reveals the role of self‐assembly and complexation in metal‐ion transfer through the water–oil interface. The coordinating metal salt Eu(NO₃)₃ was extracted from water into oil by a lipophilic neutral amphiphile. Molecular dynamics simulations were coupled to experimental spectroscopic and X‐ray scattering techniques to investigate how local coordination interactions between the metal ion and ligands in the organic phase combine with long‐range interactions to produce spontaneous changes in the solvent microstructure. Extraction of the Eu³⁺–3(NO₃^?) ion pairs involves incorporation of the “hard” metal complex into the core of “soft” aggregates. This seeds the formation of reverse micelles that draw the water and “free” amphiphile into nanoscale hydrophilic domains. The reverse micelles interact through attractive van der Waals interactions and coalesce into rod‐shaped polynuclear Eu^III‐containing aggregates with metal centers bridged by nitrate. These preorganized hydrophilic domains, containing high densities of O‐donor ligands and anions, provide improved Eu^III solvation environments that help drive interfacial transfer, as is reflected by the increasing Eu^III partitioning ratios (oil/aqueous) despite the organic phase approaching saturation. For the first time, this multiscale approach links metal‐ion coordination with nanoscale structure to reveal the free‐energy balance that drives the phase transfer of neutral metal salts.     

Hydrogen‐Bond‐Guided Self‐Assembly of Nucleotides on a Receptor‐Array Surface

The hydrogen‐bond‐guided self‐assembly of 5′‐ribonucleotides bearing adenine(A), cytosine (C), uracil (U), or guanine (G) bases from aqueous solution on a lipid‐like surface decorated with synthetic bis(Zn^II–cyclen) (cyclen=1,4,7,10‐tetraazacyclodododecane) metal–complex receptor sites is described. The process was studied by using surface plasmon resonance spectroscopy. The data show that the mechanism of nucleotide binding to the 2D template is influenced by the chemistry of the bases and the pH value of the solution. In a neutral solution of pH 7.5, the process is cooperative and selective with respect to Watson–Crick pairs (A–U and C–G), which form stable double planes in accordance with the Chargaff rule. In a more acidic solution at pH 6.0, the interactions between complementary partners become non‐cooperative and the surface also stabilizes mismatched and wobble pairs due to the pH‐induced changes in the receptor coordination state. The results suggest that hydrogen bonding plays a key role in the self‐assembly of complementary nucleotides at the lipid‐like interface, and the cooperative character of the process stems from the ideal matching of the orientation and chemistry of all the interacting components with respect to each other in neutral solution.     

Photoinduced Triplet–Triplet Energy Transfer in a 2‐Ureido‐4(1H)‐Pyrimidinone‐Bridged,Quadruply Hydrogen‐Bonded Ferrocene–Fullerene Assembly

2‐Ureido‐4(1H)‐pyrimidinone‐bridged ferrocene–fullerene assembly I is designed and synthesized for elaborating the photoinduced electron‐transfer processes in self‐complementary quadruply hydrogen‐bonded modules. Unexpectedly, steady‐state and time‐resolved spectroscopy reveal an inefficient electron‐transfer process from the ferrocene to the singlet or triplet excited state of the fullerene, although the electron‐transfer reactions are thermodynamically feasible. Instead, an effective intra‐assembly triplet–triplet energy‐transfer process is found to be operative in assembly I with a rate constant of 9.2×10⁵ s^?1 and an efficiency of 73 % in CH₂Cl₂ at room temperature.     

Heterotapes: A Persistent,Dual‐Synthon Hydrogen‐Bonding Motif

The small dinitrile anion carbamoyldicyanomethanide, [C(CN)₂‐(CONH₂)]^? (cdm), reproducibly forms a hydrogen‐bonded tape containing two different supramolecular synthons: a “heterotape”. The tape incorporates both an amide dimer and a nitrile‐containing ring. The robustness of the motif is confirmed by its persistence from an isolated tape in a separated ion‐pair structure, [K(15c5)₂](cdm)? H₂O, to its incorporation into coordination complexes of octahedral metals, thus facilitating the formation of 2D sheets. Complexes containing coligands that occupy the equatorial coordination sites, [Cu(2,2′‐py₂NH)₂(cdm)₂]? 2MeOH, [Ni(cyclam)(cdm)₂], and [Cu(cyclam)(cdm)₂]?2MeOH (cyclam=1,4,8,11‐tetraazacyclotetradecane, 2,2′‐py₂NH=di(2‐pyridyl)amine), show retention of the heterotape motif, whilst the ethylene diamine complex [Cu‐(en)₂(cdm)₂] (en=ethylene diamine) displays an alternative hydrogen‐bonding motif due to interference from the diamine ligands.     

Charge‐Transfer‐Assisted Supramolecular 1 D Nanofibers through a Cholesteric Structure‐Directing Agent: Self‐Assembly Design for Supramolecular Optoelectronic Materials

Self‐assembly of pyrene butyric acid (PBA) and 2,4,7‐trinitro‐9H‐fluoren‐9‐one (TNF) directed by a pyridine‐linked cholesterol unit resulted in the formation of a conducting material (1.9472×10^?4 S Cm^?1) due to the formation of 1 D nanofibers. X‐ray diffraction, IR, and atomic force microscopic (AFM) techniques were used to establish the mechanism of the self‐assembly of the multicomponent gels. Results indicate efficient charge transfer in the 1 D nanofibers, assisted by hydrogen bonding.     

Two‐Dimensional Self‐Assembly of a Porphyrin–Polypyridyl Ruthenium(II) Hybrid on HOPG Surface through Metal–Ligand Interactions

The synthesis and self‐assembly behavior of porphyrin–polypyridyl ruthenium(II) hybrid, which consists of a flexible alkyl chain attached with two conjugated moieties is described. The electronic absorption spectrum and emission spectra show that the [C₈‐TPP‐(ip)Ru(phen)₂](ClO₄)₂, abbreviated as (C₈ip)TPPC has optical properties. Scanning tunneling microscopy (STM) studies found that the π–π interaction and metal–ligand interaction allow (C₈ip)TPPC to form self‐assembled structure and have an edge‐on orientation on the highly oriented pyrolytic graphite (HOPG) surface. The multidentate structure in (C₈ip)TPPC molecules act as linkers between the molecules and form metal–ligand coordination, which forces the assembly process in the direction of stable columnar arrays. In addition, although the sample was stored for two months in ambient conditions, STM experiments showed that the order of (C₈ip)TPPC self‐assembly only slightly decreased which indicates that the self‐assembled monolayer is stable. This work demonstrates that introducing a metal‐ligand in the porphyrin‐polypyridyl compound is a useful strategy to obtain novel surface assemblies.     

Supramolecular Porphyrin Copolymer Assembled through Host–Guest Interactions and Metal–Ligand Coordination

Bisporphyrin cleft molecule 1 Zn possessing a guest moiety assembled to form supramolecular polymers through host–guest interactions. Bispyridine cross‐linkers created interchain connections among the supramolecular polymers to form networked polymers in solution. Solution viscometry confirmed that the cross‐linked supramolecular polymers were highly entangled. Frequency‐dependent linear viscoelastic spectroscopy revealed that the supramolecular polymers generated well‐entangled solutions with associating and networking polymers, whereas the solid‐like aggregates moved individually without breaking and reforming structures below the transition temperature of 9.6 °C. Morphological transition of the supramolecular polymers was evidenced by AFM images; the non‐cross‐linked polymer resulted in wide‐spread thin networks, while the cross‐linked networks produced thicker worm‐like nanostructures. The supramolecular networks gelled in 1,1,2,2‐tetrachloroethane, and an elastic free‐standing film was fabricated with a Young’s modulus of 1 GPa.     

Hydrogen‐Bonding‐Induced Polymorphous Phase Transitions in 2D Organic Nanostructures

The 2D self‐assembly of various 2‐hydroxy‐7‐alkoxy‐9‐fluorenone (HAF) molecules has been investigated by scanning tunneling microscopy (STM) at the liquid/solid interface. A systematic study revealed that HAF molecules with different numbers of carbon atoms in their alkoxy chains could form two or three different kinds of nanostructures, that is, less‐ordered, flower‐like, and zig‐zag patterns, owing to the formation of different types of intermolecular hydrogen bonds. The observed structural transition was found to be driven by molecular thermodynamics, surface diffusion, and the voltage pulse that was applied to the STM tip. The zig‐zag pattern was the most stable of these configurations. An odd–even effect on the flower‐like structure, as induced by the odd and even number of carbon atoms in the side chain, was observed by STM. The influence of the odd–even effect on the melting point has a close relationship with the molecular self‐assembled pattern. Our results are significant for understanding the influence of hydrogen‐bonding interactions on the dominant adsorption behavior on the surface and provide a new visual approach for observing the influence of the odd–even effect on the phase transition.