Takehiro Hirao

Supramolecular Polymerization Triggered by Molecular Recognition between Bisporphyrin and Trinitrofluorenone

Self-assembled supramolecular polymers consist of molecular components that are held together through noncovalent interactions. The reversible noncovalent interactions can be used to produce healable, stimuli-responsive, and switchable supramolecular polymers. This new class of intelligent polymer materials, with macroscopic properties that might be turned on and off by external stimuli, has helped supramolecular polymer chemistry to gain momentum within the field of polymer science. The design of wellorganized polymer architectures requires the integration of certain supramolecular components that must be capable of creating the strong noncovalent interactions necessary for producing an appreciable degree of polymerization. Multiple hydrogen-bonding, hydrophobic, cation–dipole, CH/p, and aromatic electron donor–acceptor interactions are often employed in the synthesis of functional supramolecular polymers. Supramolecular porphyrin polymers have recently attracted attention because of their creative applications in photoactive devices. Coordination-driven self-assembly is one of the most useful approaches for building large and elaborate porphyrin architectures. However, self-assembly of porphyrins in organic media, driven by weak noncovalent forces, such as van der Waals and CH/p interactions, is very limited, even though the porphyrin moiety possesses a flat and electron-rich surface that creates the possibility of attractive van der Waals, stacking, and charge-transfer interactions. Recently, we have developed a bisporphyrin cleft connected by a pyridine dicarboxamide linker that assembles to form a unique complementary dimer in organic media. The competitive complexation of a flat, electron-deficient aromatic guest into the bisporphyrin cleft leads to a p donor–acceptortype host–guest complex. These supramolecular motifs should be useful for the synthesis of supramolecular porphyrin polymers. To investigate this strategy, a p donor– acceptor-type host–guest motif was incorporated into the heteroditopic monomer 1. The electron-deficient guest moiety, 4,5,7-trinitrofluorenone-2-carboxylate (TNF), can bind within the bisporphyrin cleft through a charge-transfer interaction, and iterative head-to-tail host–guest complexation should produce a new supramolecular polymer (Figure 1). Herein, we report the novel molecular recognition-directed supramolecular polymerization of monomer 1 in solution and solid state.

Supramolecular Fullerene Polymers and Networks Directed by Molecular Recognition between Calix[5]arene and C60

A biscalix[5]arene-C60 supramolecular structure was utilized for the development of supramolecular fullerene polymers. Di- and tritopic hosts were developed to generate the linear and network supramolecular polymers through the complexation of a dumbbell-shaped fullerene. The molecular association between the hosts and the fullerene were carefully studied by using (1) H NMR, UV/Vis absorption, and fluorescence spectroscopy. The formation of the supramolecular fullerene polymers and networks was confirmed by diffusion-ordered (1) H NMR spectroscopy (DOSY) and solution viscometry. Upon concentrating the mixtures of di- or tritopic hosts and dumbbell-shaped fullerene in the range of 1.0-10 mmol L(-1) , the diffusion coefficients of the complexes decreased, and the solution viscosities increased, suggesting that large polymeric assemblies were formed in solution. Scanning electron microscopy (SEM) was used to image the supramolecular fullerene polymers and networks. Atomic force microscopy (AFM) provided insight into the morphology of the supramolecular polymers. A mixture of the homoditopic host and the fullerene resulted in fibers with a height of (1.4±0.1) nm and a width of (5.0±0.8) nm. Interdigitation of the alkyl side chains provided secondary interchain interactions that facilitated supramolecular organization. The homotritopic host generated the supramolecular networks with the dumbbell-shaped fullerene. Honeycomb sheet-like structures with many voids were found. The growth of the supramolecular polymers is evidently governed by the shape, dimension, and directionality of the monomers.

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 Youngs modulus of 1 GPa.

Sequence-controlled supramolecular terpolymerization directed by specific molecular recognitions

Nature precisely manipulates primary monomer sequences in biopolymers. In synthetic polymer sequences, this precision has been limited because of the lack of polymerization techniques for conventional polymer synthesis. Engineering the primary monomer sequence of a polymer main chain represents a considerable challenge in polymer science. Here, we report the development of sequence-controlled supramolecular terpolymerization via a self-sorting behavior among three sets of monomers possessing mismatched host–guest pairs. Complementary biscalix[5]arene-C60, bisporphyrin-trinitrofluorenone (TNF), and Hamilton’s bis(acetamidopyridinyl)isophthalamide-barbiturate hydrogen-bonding host–guest complexes are separately incorporated into heteroditopic monomers that then generate an ABC sequence-controlled supramolecular terpolymer. The polymeric nature of the supramolecular terpolymer is confirmed in both solution and solid states. Our synthetic methodology may pave an avenue for constructing polymers with tailored sequences that are associated with advanced functions.Nature can precisely control monomer sequences in biopolymers, but this is somewhat problematic in the formation of synthetic polymers. Here the authors show sequence-controlled supramolecular terpolymerization via self-sorting behavior among three sets of monomers possessing mismatched host-guest pairs.

Liposome collapse resulting from an allosteric interaction between 2,6-dimethyl-β-cyclodextrins and lipids

Although heptakis(2,6-di-O-methyl)-β-cyclodextrin (DMe-β-CDx) has been reported to exhibit higher cytotoxicity than many other cyclodextrins because of the way in which it abstracts cholesterols from liposomes, we have identified another reason for its cytotoxicity based on its interaction with lipids. These interactions exhibited nonlinear sigmoidal responses with Hill coefficient values (n) in the range of 3.0–3.6, which indicated that this phenomenon involves positive allosterism. Furthermore, analysis by mass spectroscopy revealed that the lipid–DMe-β-CDx complexes had stoichiometric ratios in the range of 1 : 1–1 : 4.

Control over multiple molecular states with directional changes driven by molecular recognition

Recently, ligand–metal coordination, stimuli-responsive covalent bonds, and mechanically interlinked molecular constructs have been used to create systems with a large number of accessible structural states. However, accessing a multiplicity of states in sequence from more than one direction and doing so without the need for external energetic inputs remain as unmet challenges, as does the use of relatively weak noncovalent interactions to stabilize the underlying forms. Here we report a system based on a bispyridine-substituted calix[4]pyrrole that allows access to six different discrete states with directional control via the combined use of metal-based self-assembly and molecular recognition. Switching can be induced by the selective addition or removal of appropriately chosen ionic guests. No light or redox changes are required. The tunable nature of the system has been established through a combination of spectroscopic techniques and single crystal X-ray diffraction analyses. The findings illustrate a new approach to creating information-rich functional materials.Multistate molecular systems usually rely on external energy inputs to switch between states. Here, the authors show that a bispyridyl calixpyrrole system directed by only weak noncovalent interactions and metal coordination can access six discrete structural states, with directional and sequential control.