Periodic Mesoporous Organosilica with Molecular‐Scale Ordering Self‐Assembled by Hydrogen Bonds

Nanoporous materials with functional frameworks have attracted attention because of their potential for various applications. Silica‐based mesoporous materials generally consist of amorphous frameworks, whereas a molecular‐scale lamellar ordering within the pore wall has been found for periodic mesoporous organosilicas (PMOs) prepared from bridged organosilane precursors. Formation of a “crystal‐like” framework has been expected to significantly change the physical and chemical properties of PMOs. However, until now, there has been no report on other crystal‐like arrangements. Here, we report a new molecular‐scale ordering induced for a PMO. Our strategy is to form pore walls from precursors exhibiting directional H‐bonding interaction. We demonstrate that the H‐bonded organosilica columns are hexagonally packed within the pore walls. We also show that the H‐bonded pore walls can stably accommodate H‐bonding guest molecules, which represents a new method of modifying the PMO framework.     

Charge Separation in a Multifunctionalized Framework of Hydrogen‐Bonded Periodic Mesoporous Organosilica

Integration of functional molecular parts into nanoporous materials in a state that allows intermolecular charge or energy transfer is one of the key approaches to the development of photofunctional and electroactive materials. Herein, we report charge separation in a functionalized framework of a periodic mesoporous organosilica (PMO) self‐assembled by hydrogen bonds. Electroactive π‐conjugated organic species with different electron‐donating and electron‐accepting properties were selectively fixed onto the external surface of a nanoparticulate PMO, within the pore wall, and onto the surface of the internal mesopore. UV irradiation of the modified PMO resulted in photoinduced electron transfer and charge separation from the external surface to the pore wall and from the pore wall to the surface of the internal mesopores. These results suggest the high potential of multifunctionalized PMOs in the construction of photocatalytic reaction fields.     

Unidirectional Threading into a Bowl‐Shaped Macrocyclic Trimer of Boron–Dipyrrin Complexes through Multipoint Recognition

Bowl‐shaped macrocycles have the distinctive feature that their two sides are differentiated, and thus can be developed into elaborate hosts that fix a target molecule in a controlled geometry through multipoint interactions. We now report the synthesis of a bowl‐shaped macrocyclic trimer of the boron–dipyrrin (BODIPY) complex and its unidirectional threading of guest molecules. Six polarized B^δ+?F^δ‐ bonds are directed towards the center of the macrocycle, which enables strong recognition of cationic guests. Specifically, the benzylbutylammonium ion is bound in a manner in which the benzyl group is located at the convex face of the bowl and the butyl group at its concave face. Furthermore, adrenaline was strongly captured on the convex side of the bowl by hydrogen bonding, Coulomb forces, and C?H???π interactions.     

Two‐Component Supramolecular Gels Derived from Amphiphilic Shape‐Persistent Cyclo[6]aramides for Specific Recognition of Native Arginine

A unique supramolecular two‐component gelation system was constructed from amphiphilic shape‐persistent cyclo[6]aramides and diethylammonium chloride (or triethylammonium chloride). This system has the ability to discriminate native arginine from 19 other amino acids in a specific fashion. Cyclo[6]aramides show preferential binding for the guanidinium residue over ammonium groups. This specificity was confirmed by both experimental results and theoretical simulations. These results demonstrated a new modular displacement strategy, exploring the use of species‐binding hydrogen‐bonded macrocyclic foldamers for the construction of two‐component gelation systems for selective recognition of native amino acids by competitive host–guest interactions. This strategy may be amenable to developing a variety of functional two‐component gelators for specific recognition of various targeted organic molecular species.     

Synthesis,Structure, and Molecular Recognition of S6‐ and (SO2)6‐Corona[6](het)arenes: Control of Macrocyclic Conformation and Properties by the Oxidation State of the Bridging Heteroatoms

We report herein the synthesis, structure, and molecular recognition of S₆‐ and (SO₂)₆‐corona[6](het)arenes, and demonstrate a unique and efficient strategy of regulating macrocyclic conformation and properties by adjusting the oxidation state of the heteroatom linkages. The one‐pot nucleophilic aromatic substitution reaction of 1,4‐benzenedithiol derivatives, biphenyl‐4,4′‐dithiol and 9,9‐dipropyl‐9H‐fluorene‐2,7‐dithiol with 3,6‐dichlorotetrazine afforded S₆‐corona[3]arene[3]tetrazines. These compounds underwent inverse‐electron‐demand Diels–Alder reaction with enamines and norbornadiene to produce S₆‐corona[3]arene[3]pyridazines. Facile oxidation of sulfide linkages yielded (SO₂)₆‐corona[3]arene[3]pyridazines. All corona[6](het)arenes adopted generally hexagonal macrocyclic ring structures; however, their electronic properties and conformation could be fine‐tuned by altering the oxidation state of the sulfur linkages. Whereas (SO₂)₆‐corona[3]arene[3]pyridazines were electron‐deficient, S₆‐corona[3]arene[3]pyridazines acted as electron‐rich macrocyclic hosts that recognized various organic cations in both aqueous and organic solutions.     

D3h‐Symmetrical Shape‐Persistent Macrocycles Consisting of Pyridine–Acetylene–Phenol Conjugates as an Efficient Host Architecture for Saccharide Recognition

Hexagonal shape‐persistent macrocycles (SPMs) consisting of three pyridine and three phenol rings linked with acetylene bonds were developed as a preorganized host for saccharide recognition by push–pull‐type hydrogen bonding. Three tert‐butyl or 2,4,6‐triisopropylphenyl substituents were introduced on the host to suppress self‐aggregation by steric hindrance. In spite of the simple architecture, association constants K_a of the host with alkyl glycoside guests reached the order of 10⁶ m ^?1 on the basis of UV/Vis titration experiments. This glycoside recognition was much stronger than that in the cases of acyclic equivalent hosts because of the entropic advantage brought by preorganization of the hydrogen‐bonding sites. Solid–liquid extraction and liquid–liquid transport through a liquid membrane were demonstrated by using native saccharides, and much preference to mannose was observed.     

Triptycene‐Based Chiral Macrocyclic Hosts for Highly Enantioselective Recognition of Chiral Guests Containing a Trimethylamino Group

A new class of chiral macrocyclic arene composed of three chiral 2,6‐dihydroxyltriptycene subunits bridged by methylene groups was designed and synthesized. Structural studies showed that the macrocyclic molecule adopts a hex‐nut‐like structure with a helical chiral cavity and highly fixed conformation. Efficient resolution was achieved through the introduction of chiral auxiliaries to give a couple of enantiopure macrocycles, which exhibited high enantioselectivity towards three pairs of chiral compounds containing a trimethylamino group.     

Manganese‐Doped Highly Ordered Mesoporous Silicate with High Efficiency for Oxidation Suppression

Herein, we demonstrate a facile approach to manganese‐doped highly ordered mesoporous silicate with oxidation‐suppression function. As biocompatible supports of guest ions, the ordered mesoporous silicate was synthesized by evaporation‐induced self‐assembly. The phase‐transition from disordered to lamellar structures in the highly ordered mesoporous structure of these porosity‐tuned materials was controlled by adjusting the concentration of a lab‐made polystyrene‐b‐polyethylene oxide copolymer. Manganese was successfully incorporated as a guest in the hexagonally packed mesoporous silicate by using an ultrasound‐assisted technique. The incorporation of manganese ions into the pores of a mesoporous silicate support could be induced for host–guest functional applications. Manganese‐doped mesoporous silicate structures have been examined for their use as antioxidizing agents by electron spin resonance (ESR) measurements and radical‐scavenging tests. The manganese atoms in the mesoporous structures could act in a free‐radical‐scavenging capacity, much like manganese nanoparticles. The high efficiency of their oxidation‐suppression function is extended for application to catalytic products.     

Synthesis and Molecular Recognition of Water‐Soluble S6‐Corona[3]arene[3]pyridazines

We report the efficient and scalable synthesis and molecular‐recognition properties of novel and water‐soluble S₆‐corona[3]arene[3]pyridazines. The synthesis comprises a one‐pot nucleophilic aromatic substitution reaction between diesters of 2,5‐dimercaptoterephthalate and 3,6‐dichlorotetrazine followed by the inverse electron‐demand Diels–Alder reaction of the tetrazine moieties with an enamine and exhaustive saponification of esters. The resulting S₆‐corona[3]arene[3]pyridazines, which adopt a 1,3,5‐alternate conformation in the crystalline state, are able to selectively form stable 1:1 complexes with dicationic guest species in water with association constants ranging from (1.10±0.06)×10³ M ^?1 to (1.18±0.06)×10⁵ M ^?1. The easy availability, large cavity size, strong and selective binding power render the water‐soluble S₆‐corona[3]arene[3]pyridazines useful macrocyclic hosts in various disciplines of supramolecular chemistry.     

Light‐Harvesting Three‐Chromophore Systems Based on Biphenyl‐Bridged Periodic Mesoporous Organosilica

Three‐chromophore systems with light‐harvesting behavior were prepared, which are based on periodic mesoporous organosilica (PMO) with crystal‐like ordered structure. The organic bridges of biphenyl‐PMO in the pore walls act as donors and two types of dye are incorporated in the one‐dimensional channels. Consecutive two‐step‐Förster resonance energy transfer is observed from the biphenyl moieties to mediators (diethyl‐aminocoumarin or aminoacridone), followed by energy transfer from mediators to acceptors (dibenzothiacarbocyanine, indodicarbocyanine, sulforhodamine G). High energy‐transfer efficiencies ranging from 70 to 80 % are obtained for two‐step‐FRET, indicating that the mesochannel structure with one‐dimensional ordering provides spatial arrangement of chromophore pairs for an efficient direct energy transfer. The emission wavelength can be tuned by a choice of acceptor dye: 477 nm (diethylaminocoumarin), 519 nm (aminoacridone), 567 nm (sulforhodamine G), 630 nm (dibenzothiacarbocyanine), and 692 nm (indodicarbocyanine).     

Modulation of Excited‐State Proton Transfer of 2‐(2′‐Hydroxyphenyl)benzimidazole in a Macrocyclic Cucurbit[7]uril Host Cavity: Dual Emission Behavior and pKa Shift

The effect of the macrocyclic host, cucurbit[7]uril (CB7), on the photophysical properties of the 2‐(2′‐hydroxyphenyl)benzimidazole (HPBI) dye have been investigated in aqueous solution by using ground‐state absorption and steady‐state and time‐resolved fluorescence measurements. All three prototropic forms of the dye (cationic, neutral, and anionic) form inclusion complexes with CB7, with the largest binding constant found for the cationic form (K≈2.4×10⁶ M ^?1). At pH≈4, the appearance of a blue emission band upon excitation of the HPBI cation in the presence of CB7 indicates that encapsulation into the CB7 cavity retards the deprotonation process of the excited cation, and hence reduces its subsequent conversion to the keto form. Excitation of the neutral form (pH≈8.5), however, leads to an increase in the keto form fluorescence, indicating an enhanced excited‐state intramolecular proton‐transfer process for the encapsulated dye. In both the ground and excited states, the two pK_a values of the HPBI dye show upward shifts in the presence of CB7. The prototropic equilibrium of the CB7‐complexed dye is represented by a six‐state model, and the pH‐dependent changes in the binding constants have been analyzed accordingly. It has been observed that the calculated pK_a values using this six‐state model match well with the values obtained experimentally. The changes in the pK_a values in the presence of CB7 have been corroborated with the modulation of the proton‐transfer process of the dye within the host cavity.     

Size‐Selective Complexation and Extraction of Endohedral Metallofullerenes with Cycloparaphenylene

A new strategy for the non‐chromatographic extraction of metallofullerenes from solutions of arc‐processed raw soot is based on the size‐selective complexation with cycloparaphenylene (CPP). [11]CPP has a high affinity for M_x@C₈₂ (x=1, 2); for example, Gd@C₈₂ can be selectively extracted from a fullerene mixture by the addition of [11]CPP. This approach should open new opportunities in metallofullerene chemistry, including for the bulk extraction of metallofullerenes.     

Recognition of 1,3‐Butadiene by a Porous Coordination Polymer

The separation of 1,3‐butadiene from C₄ hydrocarbon mixtures is imperative for the production of synthetic rubbers, and there is a need for a more economical separation method, such as a pressure swing adsorption process. With regard to adsorbents that enable C₄ gas separation, [Zn(NO₂ip)(dpe)]_n (SD‐65; NO₂ip=5‐nitroisophthalate, dpe=1,2‐di(4‐pyridyl)ethylene) is a promising porous material because of its structural flexibility and restricted voids, which provide unique guest‐responsive accommodation. The 1,3‐butadiene‐selective sorption profile of SD‐65 was elucidated by adsorption isotherms, in situ PXRD, and SSNMR studies and was further investigated by multigas separation and adsorption–desorption‐cycle experiments for its application to separation technology.     

Molecular Recognition: Influence of Para‐Versus‐Meta‐Substituted Phenyl Moieties on the Swelling Degree of Macroscopic Polymeric Gels in Aqueous Cyclodextrin Solutions

Free radical terpolymerization of (N,N)‐dimethylacrylamide, ethylene‐glycol‐dimethacrylate and N‐(p‐ or m‐ethyl‐phenyl)acrylamide leads to para‐ and meta‐ethyl‐phenyl‐modified hydrophilic polymer networks. Polymeric networks of different molar ratios are prepared in special molds to give water swellable disc‐ shaped samples. The swelling behavior in water and aqueous cyclodextrin (CD) solution of the obtained samples is described while a distinctive differentiation between the para‐ and meta‐ethyl‐phenyl containing networks in CD solution can be found.

     

Periodic Mesoporous Organosilica Materials: Self‐Assembly of Carbamothioic Acid‐Bridged Organosilane Precursors

A new series of carbamothioic acid‐containing periodic mesoporous organosilica (PMO) materials has been synthesized by a direct cocondensation method, in which an organosilica precursor N,S‐bis[3‐(triethoxysilyl)propyl]carbamothioic acid (MI) is treated with tetraethyl orthosilicate (TEOS), and the nonionic surfactant Pluronic 123 (P123) is used as a template under acidic conditions in the presence of inorganic additives. Moreover, the synthesis of the PMO material consisting of the MI precursor without TEOS has been realized. These novel PMO materials have large surface areas, well‐ordered mesoporous structures, hollow fiberlike morphologies, and thick walls. They are also structurally well‐ordered with a high organosilica precursor content, and the carbamothioic acid groups are thermally stable up to 250 °C. Furthermore, the organosilica materials exhibit hydrothermal stability in basic solution.