Photoreversible Supramolecular Polymerisation and Hierarchical Organization of Hydrogen‐Bonded Supramolecular Co‐polymers Composed of Diarylethenes and Oligothiophenes

Diarylethene 1 equipped with two monotopic melamine hydrogen‐bonding sites and oligothiophene‐functionalized ditopic cyanurate (OTCA) were mixed in a nonpolar solvent to form AA‐BB‐type supramolecular co‐polymers (SCPs) bearing photoswitchable moieties in their main chains and extended π systems as side chains. UV/Vis, fluorescence, dynamic light scattering (DLS), TEM, and AFM studies revealed that the two functional co‐monomers formed flexible quasi‐one‐dimensional SCPs in solution that hierarchically self‐organized into helical nanofibers through H‐aggregation of the oligothiophene side chains. Upon irradiating the SCPs with UV light, a transition occurred from the H‐aggregated state to non‐aggregated monomeric oligothiophene side chains, as shown by spectroscopic studies, which indicates the formation of small oligomeric species held together only by hydrogen‐bonding interactions. TEM and AFM visualized unfolded fibrils corresponding to elongated single SCP chains formed upon removal of solvent. The helical nanofibers were regenerated upon irradiating the UVirradiated solution with visible light. These results demonstrated that the supramolecular polymerisation followed by hierarchical organization can be effectively controlled by proper supramolecular designs using diarylethenes and π‐conjugated oligomers.     

Amine‐Directed Hydrogen‐Bonded Two‐Dimensional Supramolecular Structures

Utilizing pure amine hydrogen bonding is a novel approach for constructing two‐dimensional (2D) networks. Further, such systems are capable of undergoing structural modifications due to changes in pH. In this study, we designed a 2D network of triaminobenzene (TAB) molecules that by varying the pH from neutral to acidic, form either ordered or disordered structures on Au(111) surface as revealed in scanning tunneling microscopy images. In near‐neutral solution (pH ≈5.5), protonation of TAB generates charged species capable of forming H‐bonds between amine groups of neighboring molecules resulting in the formation of a 2D supramolecular structure on the electrified surface. At lower pH, due to the protonation of the amine groups, intermolecular hydrogen bonding is no longer possible and no ordered structure is observed on the surface. This opens the possibility to employ pH as a chemical trigger to induce a phase transition in the 2D molecular network of triaminobenzene molecules.     

A Supramolecular Hydrogen‐Bonded Network as a Diffusion Barrier for Metal Adatoms

Confined in a molecular corral : A supramolecular network changes the mechanism by which underpotential deposition (UPD) of copper proceeds on a gold electrode modified by a self‐assembled monolayer (SAM). Lateral diffusion of Cu adatoms is suppressed between adjacent cells of a network/SAM hybrid structure. Instead, UPD occurs by direct deposition into the SAM filled pores of the network, where the Cu adatoms are confined.

     

Synthesis,Self‐Assembly,and Multi‐Stimuli Responses of a Supramolecular Block Copolymer

A supramolecular block copolymer is prepared by the molecular recognition of nucleobases between poly(2‐(2‐methoxyethoxy)ethyl methacrylate‐co‐oligo(ethylene glycol) methacrylate)‐SS‐poly(ε‐caprolactone)‐adenine (P(MEO₂MA‐co‐OEGMA)‐SS‐PCL‐A) and uracil‐terminated poly(ethylene glycol) (PEG‐U). Because the block copolymer is linked by the combination of covalent (disulfide bond) and noncovalent (A U) bonds, it not only has similar properties to conventional covalently linked block copolymers but also possesses a dynamic and tunable nature. The copolymer can self‐assemble into micelles with a PCL core and P(MEO₂MA‐co‐OEGMA)/PEG shell. The size and morphologies of the micelles/aggregates can be adjusted by altering the temperature, pH, salt concentration, or adding dithiothreitol (DTT) to the solution. The controlled release of Nile red is achieved at different environmental conditions.

     

“Sandglass”‐Shaped Self‐Assembly of Coil–rod–coil Triblock Copolymer Containing Rigid Aniline‐Pentamer

The amphiphilic PEG_{1 500}‐b‐EM AP‐b‐PEG_{1 500} (EM PAP) triblock copolymer of poly(ethylene glycol) (PEG) and emeraldine aniline‐pentamer (EM AP) in its concentrated solution can self‐assemble into a special shape like “sandglass”, as observed by transmission electron microscopy (TEM), field emission scanning electron microscopy (ESEM) and atomic force microscopy (AFM). This “sandglass”‐shaped assembly is composed of several “rods” aggregated in the middle, with every “rod” being about 8 µm in length and 300 nm in diameter. We conclude that the special “sandglass”‐shaped assembly may come into being because of the inducement effect of the crystallization of EM AP segments, by studying electron diffraction (ED) results and wide‐angle X‐ray diffusion (WAXD) characterization of the EM PAP triblock copolymer.

     

High‐Fidelity Noncovalent Synthesis of Hydrogen‐Bonded Macrocyclic Assemblies

A hydrogen‐bonded cyclic tetramer is assembled with remarkably high effective molarities from a properly designed dinucleoside monomer. This self‐assembled species exhibits an impressive thermodynamic and kinetic stability and is formed with high fidelities within a broad concentration range.     

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.     

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.)     

STM Insight into Hydrogen‐Bonded Bicomponent 1 D Supramolecular Polymers with Controlled Geometries at the Liquid–Solid Interface

Bicomponent supramolecular polymers , consisting of two alternating molecules bridged through six H‐bonds, are observed by STM at the solid–liquid interface. Control of the geometry of the 1D architecture was obtained by using two different connecting molecules with different conformational rigidity, affording either linear (see picture, left) or zigzag (right) motifs.

     

Inversion of Supramolecular Helicity in Oligo‐p‐phenylene‐Based Supramolecular Polymers: Influence of Molecular Atropisomerism

The helical organization of oligo‐p‐phenylene‐based organogelators has been investigated by atomic force microscopy, circular and vibrational circular dichroism, and Raman techniques. Whilst OPPs with more than two phenyl rings in the core self‐assemble into left‐handed helices, that with a biphenyl core shows an inversion of the supramolecular helicity depending on the formation conditions through the atropisomerism of the biphenyl central unit. The results presented herein outline a new example of kinetically controlled modulation of supramolecular helicity.     

Two‐Dimensional Nanoporous Networks Formed by Liquid‐to‐Solid Transfer of Hydrogen‐Bonded Macrocycles Built from DNA Bases

We present an approach that makes use of DNA base pairing to produce hydrogen‐bonded macrocycles whose supramolecular structure can be transferred from solution to a solid substrate. A hierarchical assembly process ultimately leads to two‐dimensional nanostructured porous networks that are able to host size‐complementary guests.     

Unique Hydrogen‐Bonded Complex of Hydronium and Hydroxide Ions

H₃O⁺ and OH^?, formed by the self‐ionization of two coordinating water molecules during the crystal growing of a host molecule [1,3,5‐tris(hydroxymethyl)2,4,6‐triethylbenzene ( 1 )], could be effectively stabilized by hydrogen‐bonding interactions with the preorganized hydroxy groups of three molecules of 1. The binding motifs observed in the complex ( 1 )₃?H₃O⁺?HO^? show remarkable similarity to those postulated for the hydrated hydronium and hydroxide ion complexes, which play important roles in various chemical, biological, and atmospheric processes, but their molecular structures are still not fully understood and remain a subject of intensive research.     

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.     

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.

     

Hydrogen Bonding Directed Supramolecular Polymerisation of Oligo(Phenylene‐Ethynylene)s: Cooperative Mechanism,Core Symmetry Effect and Chiral Amplification

The design of supramolecular motifs with tuneable stability and adjustable supramolecular polymerisation mechanisms is of crucial importance to precisely control the properties of supramolecular assemblies. This report focuses on constructing π‐conjugated oligo(phenylene ethynylene) (OPE)‐based one‐dimensional helical supramolecular polymers that show a cooperative growth mechanism. Thus, a novel set of discotic molecules comprising a rigid OPE core, three amide groups, and peripheral solubilising wedge groups featuring C₃ and C₂ core symmetry was designed and synthesised. All of the discotic molecules are crystalline compounds and lack a columnar mesophase in the solid state. In dilute methylcyclohexane solution, one‐dimensional supramolecular polymers are formed stabilised by threefold intermolecular hydrogen bonding and π–π interactions, as evidenced by ¹H NMR measurements. Small‐angle X‐ray and light scattering measurements reveal significant size differences between the columnar aggregates of C₃‐ and C₂‐symmetrical discotics, that is, the core symmetry strongly influences the nature of the supramolecular polymerisation process. Temperature‐dependent CD measurements show a highly cooperative polymerisation process for the C₃‐symmetrical discotics. In contrast, the self‐assembly of C₂‐symmetrical discotics shows a smaller enthalpy release upon aggregation and decreased cooperativity. In all cases, the peripheral stereogenic centres induce a preferred handedness in the columnar helical aggregates. Moreover, one stereogenic centre suffices to fully bias the helicity in the C₂‐symmetrical discotics. Finally, chiral amplification studies with the C₃‐symmetrical discotics were performed by mixing chiral and achiral discotics (sergeants‐and‐soldiers experiment) and discotics of opposite chirality (majority‐rules experiment). The results demonstrate a very strong sergeants‐and‐soldiers effect and a rather weak majority‐rules effect.     

Supramolecular template‐directed synthesis of stable and high‐efficiency photoluminescence 9,10‐diphenylanthryl‐bridged ladder polysiloxane

A soluble well‐defined 9,10‐diphenylanthryl‐bridged ladder polysiloxane ( DPAnLP ) was prepared via supramolecular assembly‐directed condensation polymerization of silanols. The ladder superstructure ( LS ) was obtained via a synergistic interaction of H‐bonding and π–π stacking between polymerizable precursor 2‐tert‐butyl‐9,10‐bis(methyldihydroxylsilyl)anthracene in organic solvent. The resultant LS was then used as template to direct the condensation of silanol groups to obtain DPAnLP with high regularity. It was found that DPAnLP can emit blue light (430 nm) with great stability and high efficiency in both solution and solid film, which indicated a well organizing of fluorophore within confined environment (ladder structure). TGA and DSC measurements showed that DPAnLP had good thermal stability, and cyclic voltammetry detection gave a low‐lying highest occupied molecular orbital level. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2491–2497, 2010