New Fluorescence Domain “Excited Multimer” Formed upon Photoexcitation of Continuously Stacked Diaroylmethanatoboron Difluoride Molecules with Fused π‐Orbitals in Crystals

The crystal‐packing structures of seven derivatives of diaroylmethanatoboron difluoride ( 1 a – gBF₂ ) are characterized by no overlap of the π‐conjugated main units of two adjacent molecules (type I), overlap of the benzene ring π‐orbitals of two adjacent molecules (type II), and overlap of the benzene and dihydrodioxaborinine rings π‐orbitals of adjacent molecules (type III). The crystal‐packing structures govern the fluorescence (FL) properties in the crystalline states. The FL domain that is present in type I crystals, in which intermolecular orbital interactions are absent, leads to excited monomer‐like FL properties. In the case of the type II crystals, the presence of intermolecular overlap of the benzene rings π‐orbitals generates new FL domains, referred to as “excited multimers”, which possess allowed S₀–S₁ electronic transitions and, as a result, similar FL lifetimes at longer wavelengths than the FL of the type I crystals. Finally, intermolecular overlap of the benzene and dihydrodioxaborinine ring π‐orbitals in the type III crystals leads to “excited multimer” domains with forbidden S₀–S₁ electronic transitions and longer FL lifetimes at similar wavelengths as that in type I crystals.     

Evidence for Dual Pathway in Through‐Space Singlet Energy Transfers in Flexible Cofacial Bisporphyrin Dyads

Flexible “pacman” scaffolds built upon a calix[4]arene platform bearing a [18]crown‐6 ether and either two OH functions or two OPr groups at the lower rim have been used to generate donor–acceptor (D–A) dyads incorporating a zinc–porphyrin donor and a free‐base porphyrin acceptor. Through‐space singlet energy transfer (SET) in the D–A dyads was studied by time‐resolved fluorescence spectroscopy. Although the effects of conformational changes are well documented when the chromophores switch from a non‐cofacial to a cofacial arrangement, little is known about flexible pacman scaffolds in which the changes are limited to the distance between the chromophores. The known SET rates for reported, geometrically well‐defined, rigid pacman D–A dyads were used as calibration to estimate the D–A distances in the flexible pacman dyads. Due to the flexibility of the calix[4]arene spacer, the D–A dyads adopt a “closed” or “open” geometry that is tuned by intramolecular hydrogen bonds (O? H???[18]crown‐6 ether) and by solvent interactions. Changes in the SET rates between the open and closed geometries were surprisingly less dramatic than expected, and are explained by a dual SET pathway that is specific to the calix[4]arene platform. Time‐resolved fluorescence studies support the hypothesis that, for the “open” conformer, the preferred through space SET pathway (i.e., at the shortest distance) is located within the calix[4]arene cavity through the cofacial phenyl groups. For the “closed” conformer, the preferred through space SET route is located between the zinc and free‐base porphyrins.     

A Mechanochemically Triggered “Click” Catalyst

“Click” chemistry represents one of the most powerful approaches for linking molecules in chemistry and materials science. Triggering this reaction by mechanical force would enable site‐ and stress‐specific “click” reactions—a hitherto unreported observation. We introduce the design and realization of a homogeneous Cu catalyst able to activate through mechanical force when attached to suitable polymer chains, acting as a lever to transmit the force to the central catalytic system. Activation of the subsequent copper‐catalyzed “click” reaction (CuAAC) is achieved either by ultrasonication or mechanical pressing of a polymeric material, using a fluorogenic dye to detect the activation of the catalyst. Based on an N‐heterocyclic copper(I) carbene with attached polymeric chains of different flexibility, the force is transmitted to the central catalyst, thereby activating a CuAAC in solution and in the solid state.     

Asararenes—A Family of Large Aromatic Macrocycles

We announce the establishment of a new family of macrocycles—the asararenes, which are based on para‐methylene linked “asarol methyl ether” (1,2,4,5‐tetramethoxybenzene) units. Macrocycles with 6–12 aromatic units have been synthesized and isolated in a single step from asarol methyl ether and paraformaldehyde. Even larger rings, with up to 15 asarol methyl ether units, have been observed by high‐resolution mass spectrometry. Single‐crystal X‐ray structures of asar[6]‐, asar[7]‐, asar[8]‐, asar[9]‐, asar[10]‐ and asar[11]arene highlight the diverse structural features of this family of macrocycles. While the cavities of the asar[6–8]arene macrocycles are mostly filled with methoxyl groups, the asar[9]‐ and asar[10]arene rings contain accessible cavities and self‐assemble into infinite channels filled with solvent molecules in the solid state. These solid‐state structures highlight the potential of this family of macrocycles for a wide range of potential applications.     

“Dumbbell”‐ and “Clackers”‐Shaped Dimeric Derivatives of Monocarba‐closo‐dodecaborate

We designed, synthesized, and characterized two types of dimeric forms of monocarba‐closo‐dodecaborate, namely, a “dumbbell”‐shaped dianion having a C?C bond and a “clackers”‐shaped monoanion having an iodonium linker. The unique architectures of these anionic molecules were established by X‐ray analysis. Spectroscopic analysis, DFT calculations, and reactivity experiments revealed high anionic and chemical stability of both anions, which are crucial properties for weakly coordinating anions.     

On the “Wolfsberg–Helmholz Conjecture” of “Extended‐Hückel Theory”

After having reviewed some pioneer integral approximations closely related to Rüdenberg's expansions of one‐ and two‐electron orbital products, we apply the previously described “Implicit Multi‐Center Integration” techniques on Roothaan's “restricted” Fock‐matrix components over standard atomic orbital bases. The resulting compact forms are very similar to the well‐known “Wolfsberg–Helmholz Conjecture” of “Extended‐Hückel Theory,” which relates the various off‐diagonal matrix elements of “restricted” Fock‐type to their corresponding diagonal counterparts. In this way, a “nonempirical Extended‐Hückel Theory” can be created. © 2012 Wiley Periodicals, Inc.     

Direct synthesis of terminally “clickable” linear and hyperbranched polyesters

1,3‐Dipolar cycloaddition of an organic azide and an acetylenic unit, often referred to as the “click reaction”, has become an important ligation tool both in the context of materials chemistry and biology. Thus, development of simple approaches to directly generate polymers that bear either an azide or an alkyne unit has gained considerable importance. We describe here a straightforward approach to directly prepare linear and hyperbranched polyesters that carry terminal propargyl groups. To achieve the former, we designed an AB‐type monomer that carries a hydroxyl group and a propargyl ester, which upon self‐condensation under standard transesterification conditions yielded a polyester that carries a single propargyl group at one of its chain‐ends. Similarly, an AB₂ type monomer that carries one hydroxyl group and two propargyl ester groups, when polymerized under the same conditions yielded a hyperbranched polymer with numerous “clickable” propargyl groups at its molecular periphery. These propargyl groups can be readily clicked with different organic azides, such as benzyl azide, ω‐azido heptaethyleneglycol monomethylether or 9‐azidomethyl anthracene. When an anthracene chromophore is clicked, the molecular weight of the linear polyester could be readily estimated using both UV–visible and fluorescence spectroscopic measurements. Furthermore, the reactive propargyl end group could also provide an opportunity to prepare block copolymers in the case of linear polyesters and to generate nanodimensional scaffolds to anchor a variety of functional units, in the case of the hyperbranched polymer. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3200–3208, 2010     

From Packed “Sandwich” to “Russian Doll”: Assembly by Charge‐Transfer Interactions in Cucurbit[10]uril

As the host possessing the largest cavity in the cucurbit[n]uril (CB[n]) family, CB[10] has previously displayed unusual recognition and assembly properties with guests but much remains to be explored. Herein, we present the recognition properties of CB[10] toward a series of bipyridinium guests including the tetracationic cyclophane known as blue box along with electron‐rich guests and detail the influence of encapsulation on the charge‐transfer interactions between guests. For the mono‐bipyridinium guest (methylviologen, MV ²⁺), CB[10] not only forms 1:1 and 1:2 inclusion complexes, but also enhances the charge‐transfer interactions between methylviologen and dihydroxynaphthalene ( HN ) by mainly forming the 1:2:1 packed “sandwich” complex (CB[10] ? 2 MV ²⁺ ?HN ). For guest 1 with two bipyridinium units, an interesting conformational switching from linear to “U” shape is observed by adding catechol to the solution of CB[10] and the guest. For the tetracationic cyclophane‐blue box, CB[10] forms a stable 1:1 inclusion complex; the two bipyridinium units tilt inside the cavity of CB[10] according to the X‐ray crystal structure. Finally, a supramolecular “Russian doll” was built up by threading a guest through the cavities of both blue box and CB[10].     

The “Precursor Approach” to Patterned Fluorescence Images in Polymer Films

In contrast to the conventional two‐step method, which involves the generation of reactive functional groups followed by incubation in a dye solution (a wet developing process), the “precursor approach” enables the rapid and cost‐effective generation of patterned images in one step, without the need for an additional wet process. By using the “precursor approach”, the fluorescence of precursor molecules in polymer films can be effectively manipulated by: (1) photoinduced removal of transient protecting groups; (2) photoinduced protonation or intramolecular proton transfer; (3) photochromism; (4) photoinduced formation of fluorophores; (5) photoinduced oxidative degradation or molecular orientation.

     

Using “Threading Followed by Shrinking” to Synthesize Highly Stable Dialkylammonium‐Ion‐Based Rotaxanes

Herein, we report a “threading followed by shrinking” approach for the synthesis of rotaxanes by using an “oxygen‐deficient” macrocycle that contained two arylmethyl sulfone units and the dumbbell‐shaped salt bis(3,5‐dimethylbenzyl)ammonium tetrakis(3,5‐trifluoromethylphenyl)borate as the host and guest components, respectively. The extrusion of SO₂ from both of the arylmethyl sulfone units of the macrocyclic component in the corresponding [2]pseudorotaxane resulted in a [2]rotaxane that was sufficiently stable to maintain its molecular integrity in CD₃SOCD₃ at 393 K for at least 5 h.     

Novel calix[4]cryptands: “Mappemonde II” and “Mill II”

Two novel calix[4]cryptands were synthesized from 1,3‐alternate calix[4]bis‐azacrown. “Mappemonde II” consists of one 1,3‐calix[4]bis‐azacrown wrapped by a benzo‐crown ether loop. “Mill II” is composed of two 1,3‐calix[4]bis‐azacrowns linked by two benzo‐crown ether strands.     

Stoichiometrically Controlled Revocable Self‐Assembled “Spiro” versus Quadruple‐Stranded “Double‐Decker” Type Coordination Cages

The simple combination of Pd^II with the tris‐monodentate ligand bis(pyridin‐3‐ylmethyl) pyridine‐3,5‐dicarboxylate, L , at ratios of 1:2 and 3:4 demonstrated the stoichiometrically controlled exclusive formation of the “spiro‐type” Pd₁L₂ macrocycle, 1 , and the quadruple‐stranded Pd₃L₄ cage, 2 , respectively. The architecture of 2 is elaborated with two compartments that can accommodate two units of fluoride, chloride, or bromide ions, one in each of the enclosures. However, the entry of iodide is altogether restricted. Complexes 1 and 2 are interconvertible under suitable conditions.     

“Printing” DNA Strand Patterns on Small Molecules with Control of Valency,Directionality, and Sequence

The incorporation of synthetic molecules as corner units in DNA structures has been of interest over the last two decades. In this work, we present a facile method for generating branched small molecule‐DNA hybrids with controllable valency, different sequences, and directionalities (5′–3′) using a “printing” process from a simple 3‐way junction structure. We also show that the DNA‐imprinted small molecule can be extended asymmetrically using polymerase chain reaction (PCR) and can be replicated chemically. This strategy provides opportunities to achieve new structural motifs in DNA nanotechnology and introduce new functionalities to DNA nanostructures.     

Dipyrenylcalix[4]arene—A Fluorescence‐Based Chemosensor for Trinitroaromatic Explosives

A new chemosensor‐based approach to the detection of nitroaromatics is described. It involves the analyte‐induced quenching of excimer emission of a dipyrenyl calix[4]arene ( L ). The chemical and photophysical properties of the complexes formed between L and mono‐, di‐, and trinitrobenzene, and di‐ and trinitrotoluene were studied in acetonitrile and chloroform by using ¹H NMR, UV/Vis, and fluorescence spectroscopy. Fluorescence spectroscopy revealed that the trinitroaromatics engendered the largest response among the various substrates tested, with the sensitivity for these analytes being correspondingly high. Quantitative analysis of the fluorescence titration profile generated from the titration of L with TNT provided evidence that this particular functionalized calix[4]arene receptor allows for the detection of TNT down to the low ppb level in CH₃CN. A single‐crystal X‐ray diffraction analysis revealed that in the solid state the complex L? TNT consists of a supramolecular crystalline polymeric structure, the formation of which appears to be driven by intermolecular π–π interactions between two pyrene units and a TNT molecule held at a distance of 3.2–3.6 Å, as well as by intra‐ and intermolecular hydrogen‐bonds among the amide linkages. Nevertheless, the changes in the ¹H NMR, UV/Vis, and fluorescence spectrum, including sharp color changes, are ascribed to a charge‐transfer interaction arising from complementary π–π overlap between the pyrene subunits and the bound trinitroaromatic substrates. A number of ab initio calculations were also carried out and, considered in concert, they provide further support for the proposed charge‐transfer interactions, particularly in the case of L? TNT.     

A Biomimetic Nanoparticle to “Lure and Kill” Phospholipase A2

Inhibition of phospholipase A2 (PLA2) has long been considered for treating various diseases associated with an elevated PLA2 activity. However, safe and effective PLA2 inhibitors remain unavailable. Herein, we report a biomimetic nanoparticle design that enables a “lure and kill” mechanism designed for PLA2 inhibition (denoted “L&K‐NP”). The L&K‐NPs are made of polymeric cores wrapped with modified red blood cell membrane with two inserted key components: melittin and oleyloxyethyl phosphorylcholine (OOPC). Melittin acts as a PLA2 attractant that works together with the membrane lipids to “lure” in‐coming PLA2 for attack. Meanwhile, OOPC acts as inhibitor that “kills” PLA2 upon enzymatic attack. Both compounds are integrated into the L&K‐NP structure, which voids toxicity associated with free molecules. In the study, L&K‐NPs effectively inhibit PLA2‐induced hemolysis. In mice administered with a lethal dose of venomous PLA2, L&K‐NPs also inhibit hemolysis and confer a significant survival benefit. Furthermore, L&K‐NPs show no obvious toxicity in mice. and the design provides a platform technology for a safe and effective anti‐PLA2 approach.     

“Push‐pull” supramolecular chromophores supported on cyclopolymers

Well‐defined macromolecules have been obtained through free‐radical cyclopolymerization and cyclocopolymerization of difunctional and acrylic‐like monomers, which contained “push‐pull” supramolecular chromophores, able to form 1:1 complexes with Eu³⁺ ions in solution. The monomeric molecular modules are built around bismalonate crown ethers in a convergent fashion, in which one of the malonate moiety is derivatized as the ylidene malonate push‐pull fragment, and the other malonate moiety is elaborated to introduce two polymerizable and acrylic‐like substituents. The free‐radical induced cyclopolymerization of these monomers, or their cyclocopolymerization with UV/Vis “silent” but structurally related monomers, afforded macromolecular architectures characterized by GPC, NMR and DSC techniques. UV/Vis titration studies, performed with Eu(OTf)₃ as the supramolecular probe, revealed how adjacent chromophores within the polymeric backbone are virtually independent from each other, and how the binding ability towards the probe of these multivalent, highly packed cyclopolymeric architectures, although reduced, is still clearly detectable. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5202–5213, 2008     

“Printing” DNA Strand Patterns on Small Molecules with Control of Valency,Directionality, and Sequence

The incorporation of synthetic molecules as corner units in DNA structures has been of interest over the last two decades. In this work, we present a facile method for generating branched small molecule‐DNA hybrids with controllable valency, different sequences, and directionalities (5′–3′) using a “printing” process from a simple 3‐way junction structure. We also show that the DNA‐imprinted small molecule can be extended asymmetrically using polymerase chain reaction (PCR) and can be replicated chemically. This strategy provides opportunities to achieve new structural motifs in DNA nanotechnology and introduce new functionalities to DNA nanostructures.     

To What Extent are “Atoms in Molecules” Structures of Hydrocarbons Reproducible from the Promolecule Electron Densities?

The “atoms in molecules” structures of 225 unsubstituted hydrocarbons are derived from both the optimized and the promolecule electron densities. A comparative analysis demonstrates that the molecular graphs derived from these two types of electron densities at the same geometry are equivalent for almost 90 % of the hydrocarbons containing the same number and types of critical points. For the remaining 10 % of molecules, it is demonstrated that by inducing small perturbations, through the variation of the used basis set or slight changes in the used geometry, the emerging molecular graphs from both densities are also equivalent. Interestingly, the (3, ?1) critical point between two “non‐bonded” hydrogen atoms, which triggered “H?H bonding” controversy is also observed in the promolecule densities of certain hydrocarbons. Evidently, the topology of the electron density is not dictated by chemical bonds or strong interactions and deformations induced by the interactions of atoms in molecules have a quite marginal role, virtually null, in shaping the general traits of the topology of molecular electron densities of the studied hydrocarbons, whereas the key factor is the underlying atomic densities.     

“Inverted” Deep Eutectic Solvents Based on Host‐Guest Interactions

Herein, the concept of “inverted” (the mode “molecules mainly interact with cations”) deep eutectic solvents (DESs) is proposed. A strategy to form inverted DESs by host‐guest interactions was developed, and thus numerous DESs could be designed and formed by a combination of host and guest molecules. These liquids are expected to be used as nonaqueous electrolytes in potassium‐ion batteries or other fields for further exploration.     

Stereodynamics of Metal–Ligand Assembly: What Lies Beneath the “Simple” Spectral Signatures of C2‐Symmetric Chiral Chelates

A series of C₂‐symmetric chiral tetra‐dentate ligands were prepared by using [4,5]‐ or [5,6]‐pinene‐fused 2,2′‐bipyridyl units that are supported across a rigid arylene–ethynylene backbone. These conformationally pre‐organised chelates support stable 1:1 metal complexes, which were fully characterised by UV/Vis, fluorescence, circular dichroism (CD), and ¹H NMR spectroscopy. A careful inspection of the exciton‐coupled circular dichroism (ECCD) and ¹H NMR spectra of the reaction mixture in solution, however, revealed the evolution and decay of intermediate species en route to the final 1:1 metal–ligand adduct. Consistent with this model, mass spectrometric analysis revealed the presence of multiple metal complexes in solution at high ligand‐to‐metal ratios, which were essentially unobservable by UV/Vis or fluorescence spectroscopic techniques. Comparative studies with a bi‐dentate model system have fully established the functional role of the π‐conjugated ligand skeleton that dramatically enhances the thermodynamic stability of the 1:1 complex. In addition to serving as a useful spectroscopic handle to understand the otherwise “invisible” solution dynamics of this metal–ligand assembly process, temperature‐dependent changes in the proton resonances associated with the chiral ligands allowed us to determine the activation barrier (ΔG^≠) for the chirality switching between the thermodynamically stable but kinetically labile (P)‐ and (M)‐stereoisomers.