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.     

A Robust Nanoporous Supramolecular Metal–Organic Framework Based on Ionic Hydrogen Bonds

Hydrogen‐bond assembly of tripod‐like organic cations [H₃‐MeTrip]³⁺ (1,2,3‐tri(4′‐pyridinium‐oxyl)‐2‐methylpropane) and the hexa‐anionic complex [Zr₂(oxalate)₇]^6? leads to a structurally, thermally, and chemically robust porous 3D supramolecular framework showing channels of 1 nm in width. Permanent porosity has been ascertained by analyzing the material at the single‐crystal level during a sorption cycle. The framework crystal structure was found to remain the same for the native compound, its activated phase, and after guest resorption. The channels exhibit affinities for polar organic molecules ranging from simple alcohols to aniline. Halogenated molecules and I₂ are also taken up from hexane solutions by this unique supramolecular framework.     

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.     

Simulations of Guest Transport in Clathrates of Dianin’s Compound and Hydroquinone

Clathrates have been proposed for use in a variety of applications including gas storage, mixture separation and catalysis due to the potential for controlled guest diffusion through their porous lattices. Here molecular dynamics simulations are employed to study guest transport in clathrates of hydroquinone (HQ) and Dianin’s compound (DC). Systems investigated were HQ with methanol and acetonitrile, and DC with methanol and ethanol. Simulations were set up with one guest in the pore, two guests in the pore and one vacancy in the pore and a filled pore, and free‐energy barriers for movement between cavities of the pore were estimated for all cases. Comparison between these simulations indicates that guest transport most likely proceeds by molecules moving from full to empty cavities consecutively, one by one, rather than in a concerted manner. Thus, the presence of empty cavities is very important for guest transport, which becomes more energetically demanding in fully loaded systems. Flexibility of the host can assist guest transport. In the studied DC clathrates transport occurs via an intermediate conformation in which the hydroxyl group of the alcohol guest molecule participates in the hydrogen‐bonded ring of the host. We also address the issue of the number of methanol guest molecules that DC accommodates, for which conflicting information exists. We found that this is likely to be temperature dependent and suggest that under some conditions the system is most likely non‐stoichiometric.     

Versatile Benzimidazole/Triphenylamine Hybrids: Efficient Nondoped Deep‐Blue Electroluminescence and Good Host Materials for Phosphorescent Emitters

Two new bipolar compounds, N,N,N′,N′‐tetraphenyl‐5′‐(1‐phenyl‐1H‐benzimidazol‐2‐yl)‐1,1′:3′,1′′‐terphenyl‐4,4′′‐diamine ( 1 ) and N,N,N′,N′‐tetraphenyl‐5′‐(1‐phenyl‐1H‐benzimidazol‐2‐yl)‐1,1′:3′,1′′‐terphenyl‐3,3′′‐diamine ( 2 ), were synthesized and characterized, and their thermal, photophysical, and electrochemical properties were investigated. Compounds 1 and 2 possess good thermal stability with high glass‐transition temperatures of 109–129 °C and thermal decomposition temperatures of 501–531 °C. The fluorescence quantum yield of 1 (0.52) is higher than that of 2 (0.16), which could be attributed to greater π conjugation between the donor and acceptor moieties. A nondoped deep‐blue fluorescent organic light‐emitting diode (OLED) using 1 as the blue emitter displays high performance, with a maximum current efficiency of 2.2 cd A⁻¹ and a maximum external efficiency of 2.9 % at the CIE coordinates of (0.17, 0.07) that are very close to the National Television System Committee’s blue standard (0.15, 0.07). Electrophosphorescent devices using the two compounds as host materials for green and red phosphor emitters show high efficiencies. The best performance of a green phosphorescent device was achieved using 2 as the host, with a maximum current efficiency of 64.3 cd A⁻¹ and a maximum power efficiency of 68.3 lm W⁻¹; whereas the best performance of a red phosphorescent device was achieved using 1 as the host, with a maximum current efficiency of 11.5 cd A⁻¹, and a maximum power efficiency of 9.8 lm W⁻¹. The relationship between the molecular structures and optoelectronic properties are discussed.     

Effect of Guest–Host Hydrogen Bonding on the Structures and Properties of Clathrate Hydrates

To provide improved understanding of guest–host interactions in clathrate hydrates, we present some correlations between guest chemical structures and observations on the corresponding hydrate properties. From these correlations it is clear that directional interactions such as hydrogen bonding between guest and host are likely, although these have been ignored to greater or lesser degrees because there has been no direct structural evidence for such interactions. For the first time, single‐crystal X‐ray crystallography has been used to detect guest–host hydrogen bonding in structure II (sII) and structure H (sH) clathrate hydrates. The clathrates studied are the tert‐butylamine (tBA) sII clathrate with H₂S/Xe help gases and the pinacolone + H₂S binary sH clathrate. X‐ray structural analysis shows that the tBA nitrogen atom lies at a distance of 2.64 Å from the closest clathrate hydrate water oxygen atom, whereas the pinacolone oxygen atom is determined to lie at a distance of 2.96 Å from the closest water oxygen atom. These distances are compatible with guest–water hydrogen bonding. Results of molecular dynamics simulations on these systems are consistent with the X‐ray crystallographic observations. The tBA guest shows long‐lived guest–host hydrogen bonding with the nitrogen atom tethered to a water HO group that rotates towards the cage center to face the guest nitrogen atom. Pinacolone forms thermally activated guest–host hydrogen bonds with the lattice water molecules; these have been studied for temperatures in the range of 100–250 K. Guest–host hydrogen bonding leads to the formation of Bjerrum L‐defects in the clathrate water lattice between two adjacent water molecules, and these are implicated in the stabilities of the hydrate lattices, the water dynamics, and the dielectric properties. The reported stable hydrogen‐bonded guest–host structures also tend to blur the longstanding distinction between true clathrates and semiclathrates.     

“Breathing” in Adsorbate‐Responsive Metal Tetraphosphonate Hybrid Materials

Breathe easy : Reversible H₂O and NH₃ gas uptake by 2D calcium tetraphosphonates (see figure) is accompanied by framework structural changes similar to those previously reported for some carboxylate‐based hybrids. This breathing mechanism is accompanied by a volume increase of 55 %, while maintaining the topology and crystallinity of the material.

     

Amphiphilic Inclusion Spaces for Various Guests and Regulation of Fluorescence Intensity of 1,8‐Bis(4‐aminophenyl)anthracene Crystals

A host framework for inclusion of various guest molecules was investigated by preparation of inclusion crystals of 1,8‐bis(4‐aminophenyl)anthracene (1,8‐BAPA) with organic solvents. X‐ray crystallographic analysis revealed construction of the same inclusion space incorporating 1,8‐BAPA and eight guest molecules including both non‐polar (benzene) and polar guests (N,N‐dimethylformamide, DMF). Fluorescence efficiencies varied depending on guest molecule polarity; DMF inclusion crystals exhibited the highest fluorescence intensity (Φ_F=0.40), four times as high as that of a benzene inclusion crystal (Φ_F=0.10). According to systematic investigations of inclusion phenomena, strong host–guest interactions and filling of the inclusion space led to a high fluorescence intensity. Temperature‐dependent fluorescence spectral measurements revealed these factors effectively immobilised the host framework. Although hydrogen bonding commonly decreases fluorescence intensity, the present study demonstrated that such strong interactions provide excellent conditions for fluorescence enhancement. Thus, this remarkable behaviour has potential application toward sensing of highly polar molecules, such as biogenic compounds.     

A Dynamic and Responsive Host in Action: Light‐Controlled Molecular Encapsulation

The rational design of a flexible molecular box, oAzoBox ⁴⁺, incoporating both photochromic and supramolecular recognition motifs is described. We exploit the E?Z photoisomerization properties of azobenzenes to alter the shape of the cavity of the macrocycle upon absorption of light. Imidazolium motifs are used as hydrogen‐bonding donor components, allowing for sequestration of small molecule guests in acetonitrile. Upon E→Z photoisomerization of oAzoBox ⁴⁺ the guest is expelled from the macrocyclic cavity.     

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.     

Encapsulation‐Induced Remarkable Stability of a Hydrogen‐Bonded Heterocapsule

Remarkably enhanced stability of the self‐assembled hydrogen‐bonded heterocapsule 1?2 by the encapsulation of 1,4‐bis(1‐propynyl)benzene 3 a was found with K_a=1.14×10⁹ M ^?1 in CDCl₃ and K_a2=1.59×10⁸ M ^?2 in CD₃OD/CDCl₃ (10 % v/v) at 298 K. The formation of 3 a @( 1?2 ) was enthalpically driven (ΔH°<0 and ΔS°<0) and there was a unique inflection point in the correlation between ΔH° versus ΔS° as a function of polar solvent content. The ab initio calculations revealed that favorable guest–capsule dispersion and electrostatic interactions between the acetylenic parts (triple bonds) of 3 a and the aromatic inner space of 1?2 , as well as less structural deformation of 1?2 upon encapsulation of 3 a , play important roles in the remarkable stability of 3 a @( 1?2 ).     

Interaction between Anions and Molybdenum Allyl Dicarbonyl Complexes of 1,4,7‐Trithiacyclononane

The labile complex [MoCl(η³‐methallyl)(CO)₂(NCMe)₂] reacts with the ligand 1,4,7‐trithiacyclononane ([9]aneS₃) and the salt NaBAr′₄ to afford [Mo(η³‐methallyl)(CO)₂([9]aneS₃)][BAr′₄] ( 1?BAr′₄ ). An analogous reaction of [MoBr(η³‐allyl)(CO)₂(NCMe)₂] yields [Mo(η³‐allyl)(CO)₂([9]aneS₃)][BAr′₄] ( 2?BAr′₄ ). The new compounds 1?BAr′₄ and 2?BAr′₄ were characterized by IR and NMR spectroscopic analysis and X‐ray diffraction studies. Both compounds feature the cyclic thioether [9]aneS₃ coordinated as a tridentate ligand to the molybdenum center. The allyl ligand in 2?BAr′₄ is aligned with the middle of the OC‐Mo‐CO angle, which is acute. Both of these features are typical of most pseudo‐octahedral allyl dicarbonyl molybdenum complexes. In contrast, the allyl group is rotated in 1?BAr′₄ , which is attributed to steric hindrance between the methyl substituent and the ligated thioether, and the OC‐Mo‐CO angle is obtuse. Compound 1?BAr′₄ undergoes rapid substitution of [9]aneS₃ by either chloride and fluoride ions in dichloromethane, and the products include the known species [{Mo(η³‐methallyl)(CO)₂}₂(μ‐Cl)₃]^? and a structurally similar new anionic complex with two fluoro and one hydroxo bridging ligands, respectively. Stable supramolecular adducts were formed in the reactions of 1?BAr′₄ and 2?BAr₄ with bromide, iodide, hydrogensulfate, and methanesulfonate compounds. The binding constants of these adducts in dichloromethane were calculated from ¹H NMR spectroscopic titration data, and the solid‐state structures of the 1?Br , 1?HSO₄ , 1?I , and 2?I adducts were determined by X‐ray diffraction studies. The surprising slightly higher stability of the iodide adduct relative to that of bromide was investigated theoretically, with the results pointing to an effect of the differential solvation of the halide ions.     

Controlled Dynamic Helicity of a Folded Macrocycle Based on a Bisterephthalamide with a Twofold Z‐Shaped Structure

Dynamic helicity in a folded macrocycle and control of the helical preference are described. We designed macrocycle 1 with a dual mode of folding through the integration of two flexible units that are arranged twice to form a cyclic structure. As a folding unit, we used a terephthalamide skeleton and a Z‐shaped hydrocarbon: the former acted as a control unit to induce a preference of a particular sense of dynamic helicity and the latter was just a spacer. A terephthalamide unit provided a binding site for capturing a ditopic hydrogen‐bonding guest when it adopted helically folded syn forms (M/P). Thus, only the terephthalamide unit controlled the helical sense of dynamic helicity in a folded macrocycle through the supramolecular transmission of chirality upon complexation with a chiral ditopic guest. In addition, chirality on a host could also contribute to the control of the helical preference in a folded macrocycle, which led to exceptionally enhanced chiroptical signals.     

Highly Phosphorescent Supramolecular Hydrogels Based on Platinum Emitters

We have synthesised a neutral, water‐soluble, Pt^II complex able to aggregate more efficiently in aqueous solutions than in organic solvents. The aggregates are luminescent and are not quenched by molecular oxygen. Further, we have prepared phosphorescent hydrogels utilising host–guest interactions between cyclodextrins and the tetraethylene glycol tails of the Pt^II complex. The soft assemblies feature host‐dependent emission properties.