Zinc Oxide Nano‐ and Microfabrication from Coordination‐Polymer Templates

Hex nut : An emerging synthetic approach based on metal–organic coordination‐polymer templates has been used to fabricate micro‐ and nanoscale crystals. By using a diverse range of molecular building blocks coupled with conventional synthetic techniques, it is possible to synthesize ZnO crystals with tailored sizes, shapes (such as hexagonal rings; see figure), and surface properties.

     

New Insights into the Hexaphenylethane Riddle: Formation of an α,o‐Dimer

Upon reduction of a 1H‐cyclobuta[de]naphthalene‐4,5‐diylbis(diarylmethylium) species, a new C? C bond is formed between the C_α and C_ortho atoms of the two chromophores, which presents an unprecedented coupling pattern for the dimerization of two trityl units. By attaching an annulated cyclobutane ring at the opposite peri position of the naphthalene core, the distance between the C_α carbon atoms was elongated beyond the limit of σ‐bond formation through “scissor effects”. The suppression of C_α? C_α bond formation, which would lead to hexaphenylethane‐type compounds, is key to the first successful isolation of the α,o‐adducts. The 5‐diarylmethylene‐6‐triarylmethyl‐1,3‐cyclohexadiene unit in the α,o‐adducts is stable, and isomerization of the cyclohexadiene unit into an aromatic system was not observed. The newly formed C_α? C_ortho bond was cleaved upon two‐electron oxidation to regenerate the dicationic dye.     

Light‐Driven Coordination‐Induced Spin‐State Switching: Rational Design of Photodissociable Ligands

The bistability of spin states (e.g., spin crossover) in bulk materials is well investigated and understood. We recently extended spin‐state switching to isolated molecules at room temperature (light‐driven coordination‐induced spin‐state switching, or LD‐CISSS). Whereas bistability and hysteresis in conventional spin‐crossover materials are caused by cooperative effects in the crystal lattice, spin switching in LD‐CISSS is achieved by reversibly changing the coordination number of a metal complex by means of a photochromic ligand that binds in one configuration but dissociates in the other form. We present mathematical proof that the maximum efficiency in property switching by such a photodissociable ligand (PDL) is only dependent on the ratio of the association constants of both configurations. Rational design by using DFT calculations was applied to develop a photoswitchable ligand with a high switching efficiency. The starting point was a nickel–porphyrin as the transition‐metal complex and 3‐phenylazopyridine as the photodissociable ligand. Calculations and experiments were performed in two iterative steps to find a substitution pattern at the phenylazopyridine ligand that provided optimum performance. Following this strategy, we synthesized an improved photodissociable ligand that binds to the Ni–porphyrin with an association constant that is 5.36 times higher in its trans form than in the cis form. The switching efficiency between the diamagnetic and paramagnetic state is efficient as well (72 % paramagnetic Ni–porphyrin after irradiation at 365 nm, 32 % paramagnetic species after irradiation at 440 nm). Potential applications arise from the fact that the LD‐CISSS approach for the first time allows reversible switching of the magnetic susceptibility of a homogeneous solution. Photoswitchable contrast agents for magnetic resonance imaging and light‐controlled magnetic levitation are conceivable applications.     

Coordination‐Driven Folding and Assembly of a Short Peptide into a Protein‐like Two‐Nanometer‐Sized Channel

Short peptide helices have attracted attention as suitable building blocks for soft functional materials, but they are rarely seen in crystalline materials. A new artificial nanoassembly of short peptide helices in the crystalline state is presented in which peptide helices are arranged three‐dimensionally by metal coordination. The folding and assembly processes of a short peptide ligand containing the Gly‐Pro‐Pro sequence were induced by silver(I) coordination in aqueous alcohol, and gave rise to a single crystal composed of polyproline II helices. Crystallographic studies revealed that this material possesses two types of unique helical nanochannel; the larger channel measures more than 2 nm in diameter. Guest uptake properties were investigated by soaking the crystals in polar solutions of guest molecules; anions, organic chiral molecules, and bio‐oligomers are effectively encapsulated by this peptide‐folded porous crystal, with moderate to high chiral recognition for chiral molecules.     

Creating Coordination‐Based Cavities in a Multiresponsive Supramolecular Gel

Creating cavities in varying levels, from molecular containers to macroscopic materials of porosity, have long been motivated for biomimetic or practical applications. Herein, we report an assembly approach to multiresponsive supramolecular gels by integrating photochromic metal–organic cages as predefined building units into the supramolecular gel skeleton, providing a new approach to create cavities in gels. Formation of discrete O‐Pd₂L₄ cages is driven by coordination between Pd²⁺ and a photochromic dithienylethene bispyridine ligand (O‐PyFDTE). In the presence of suitable solvents (DMSO or MeCN/DMSO), the O‐Pd₂L₄ cage molecules aggregate to form nanoparticles, which are further interconnected through supramolecular interactions to form a three‐dimensional (3D) gel matrix to trap a large amount of solvent molecules. Light‐induced phase and structural transformations readily occur owing to the reversible photochromic open‐ring/closed‐ring isomeric conversion of the cage units upon UV/visible light radiation. Furthermore, such Pd₂L₄ cage‐based gels show multiple reversible gel–solution transitions when thermal‐, photo‐, or mechanical stimuli are applied. Such supramolecular gels consisting of porous molecules may be developed as a new type of porous materials with different features from porous solids.     

Steric effect and mobility of the alkyl chain in regio‐irregular poly‐3‐alkylthiophenes

The physicochemical properties of polyalkylthiophenes with various side‐chain length were widely investigated in order to reveal the functions of alkyl side‐chains in these polymers. The effects of the side‐chains on the properties of polyalkylthiophenes can be explained by their steric hindrance and mobility. The steric hindrance of alkyl chain affected not only the polymerization mechanism of the monomers but also the redox potential, interchain distance, charge transport properties, and film morphology. The mobility of the side‐chain influences the rate of dedoping, heat of transitions of polymers. The structure regio‐regularity, stability of polarons/bipolarons, film morphologies, and interchain interactions determine the optical and electric properties of polyalkylthiophenes. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1763–1772, 1999     

A Smart Sensing Method for Object Identification Using Circularly Polarized Luminescence from Coordination‐Driven Self‐Assembly

The potential use of circularly polarized luminescence for object identification in a sensor application is demonstrated. New luminescence probes using pyrene derivatives as sensor luminophores were developed. (R,R)‐Im₂Py and (S,S)‐Im₂Py contain two chiral imidazole moieties at 1,6‐positions through ethynyl spacers (angle between spacers ca. 180°). The probe molecules spontaneously self‐assemble into chiral stacks (P or M helicity) upon coordination to metal ions with tetrahedral coordination (Zn²⁺). The chiral probes display neither circular dichroism (CD) nor circularly polarized luminescence (CPL) without metal ions. However, (R,R)‐Im₂Py and (S,S)‐Im₂Py exhibit intense chiroptical activity (CD and CPL) upon self‐assembly with Zn²⁺ ions. (R,R)‐Im₂Py and (S,S)‐Im₂Py with chemical stimuli‐responsibility allow sensing using the CPL signal as detection output, enabling us to discriminate between a signal from the target analyte and that from non‐target species.     

Thermal Cleavage of Cyclobutane Rings in Photodimerized Coordination‐Polymeric Sheets

Three coordination polymers, [Cd₂(pvba)₂(tbdc)(dmf)₂] ( 1 ), [Co₂(pvba)₂(tbdc)(dmf)₂(H₂O)₂] ( 2 ), and [Ni₂(pvba)₂(tbdc)(dmf)₂(H₂O)₂] ( 3 ) (H₂tbdc=2,3,5,6‐tetrabromobenzenedicarboxylic acid, Hpvba=trans‐2‐(4′‐pyridyl)vinylbenzoic acid), were synthesized by solvothermal methods. The solid‐state structures of compounds 1 and 2 were determined by X‐ray crystallography. In compounds 1 and 2 , the bimetallic cores acted as secondary building units that connected the tbdc ligands in one direction and a pair of pvba ligands, which were aligned in a head‐to‐tail parallel manner, in the orthogonal direction to form sheet structures. The C?C bonds in these pvba ligand pairs in all three compounds were well‐aligned to undergo quantitative [2+2] cycloaddition reactions in the solid state under UV irradiation, thereby yielding their cyclobutane derivatives. This photochemical reaction appeared to facilitate structural transformations from one 2D structure into another in the solid state. The photoreactive Co^II‐ and Ni^II coordination polymers exhibited a reversible dehydration–rehydration reaction that was accompanied by color changes from pink to purple and green to yellow, respectively, owing to a change in coordination number from six to five. Magnetic studies showed that compound 2 was an antiferromagnet, which displayed a field‐dependent transition with a critical field (H_c) of 40 kOe at 2 K; the antiferromagnetic interaction between the Co₂ units was strengthened and weakened by dehydration and UV irradiation, respectively. The cyclobutane ligand in the photodimerized products was cleaved on heating to yield a mixture of trans‐ and cis‐isomers of pvba, as monitored by ¹H NMR spectroscopy. The Cd^II coordination polymer underwent quantitative cleavage of the cyclobutane ring whilst the other two underwent partial cleavage.     

Self‐Assembly of Chiral Propeller‐like Supermolecules with Unusual “Sergeants‐and‐Soldiers” and “Majority‐Rules” Effects

Chiral amplification is an interesting phenomenon in supramolecular chemistry mainly observed in complicated systems in which cooperative effect dominate. Herein, chiral, supramolecular, propeller‐like architectures have been constructed through coassembly of an achiral disk‐shaped molecule and chiral amino acid derivatives driven by intermolecular hydrogen bonding. Both the “sergeants‐and‐soldiers” principle and “majority‐rules” effect are applicable in these discrete four‐component supermolecules, which are the simplest supramolecular system ever reported that exhibit chiral amplification.     

Coordination‐Driven Self‐Assembly of a Molecular Knot Comprising Sixteen Crossings

Molecular knots have become highly attractive to chemists because of their prospective properties in mimicking biomolecules and machines. Only a few examples of molecular knots from the billions tabulated by mathematicians have been realized and molecular knots with more than eight crossings have not been reported to date. We report here the coordination‐driven [8+8] self‐assembly of a higher‐generation molecular knot comprising as many as sixteen crossings. Its solid‐state X‐ray crystal structure and multinuclear 2D NMR findings confirmed its architecture and topology. The formation of this molecular knot appears to depend on the functionalities and geometries of donor and acceptor in terms of generating appropriate angles and strong π‐π interactions supported by hydrophobic effects. This study shows coordination‐driven self‐assembly offers a powerful potential means of synthesizing more and more complicated molecular knots and of understanding differences between the properties of knotted and unknotted structures.     

Synthesis and Application of a Bidentate Ligand Based on Decafluoro‐3‐phenyl‐3‐pentanol: Steric Effect of Pentafluoroethyl Groups on the Stereomutation of O‐Equatorial C‐Apical Spirophosphoranes

1,1,1,2,2,4,4,5,5,5‐Decafluoro‐3‐phenyl‐3‐pentanol was prepared by a Cannizzaro‐type disproportionation reaction, and the dimetallated compound was used as a bidentate ligand, which is bulkier than the Martin ligand (1,1,1,3,3,3‐hexafluoro‐2‐phenyl‐2‐propanol). A P? H spirophosphorane was synthesized by utilizing the new bidentate ligand, and the structure of the product was essentially the same as that of the P? H phosphorane with Martin ligands. Phosphoranes that exhibit reversed apicophilicity (O‐equatorial) were also synthesized and could be converted into the corresponding stable stereoisomers (O‐apical). The crystal structures of O‐equatorial phosphoranes and the O‐apical isomers were slightly affected by the steric repulsion of pentafluoroethyl groups. Kinetic measurements revealed that the stereomutation of O‐equatorial methylphosphorane to the O‐apical isomer was slowed. The activation enthalpy for the stereomutation of the former to the latter was higher than that of the phosphorane with Martin ligands by 5.1 kcal mol^?1.     

Construction of Chlorosomal Rod Self‐Aggregates in the Solid State on Any Substrates from Synthetic Chlorophyll Derivatives Possessing an Oligomethylene Chain at the 17‐Propionate Residue

Chlorosomes are one of the most unique natural light‐harvesting antennas and their supramolecular nanostructures are still under debate. Chlorosomes contain bacteriochlorophyll (BChl)‐c, d and e molecules and these pigments self‐aggregate under a hydrophobic environment inside a chlorosome. The self‐aggregates are mainly constructed by the following three interactions: hydrogen bonding, coordination bonding and π–π stacking. Supramolecular nanostructures of self‐aggregated BChls have been widely investigated by spectroscopic and microscopic techniques. Model compounds of such chlorosomal BChl molecules have been synthesized and the effects of esterified long alkyl chains at the 17‐propionate residue for their self‐aggregation have been studied. Structurally simple zinc chlorophyll derivatives possessing an oligomethylene chain as the esterifying group at the 17‐propionate residue were prepared as chlorosomal BChl models. The synthetic zinc BChls self‐aggregated in nonpolar organic solvents to give precipitates. The resulting insoluble self‐aggregated solids were investigated on a variety of substrates, including hydrophobic, neutral and hydrophilic substrates, by visible absorption, circular dichroism and polarized light absorption spectroscopies, as well as atomic force, transmission electron and scanning electron microscopies. The self‐aggregates of synthetic Zn‐BChls formed rods with an approximately 5 nm diameter and wires with further elongated growth of the rods (aspect ratio >200). The diameter size was consistent with that estimated for natural chlorosomal rods in a filamentous anoxygenic phototroph, Chloroflexus aurantiacus. The supramolecular formation and stability of the rod on the examined substrates depended on the length of an oligomethylene chain at the 17‐propionate residue as well as on the surface properties. Especially, the number of the 5 nm rods on the substrates increased with an elongation of the chain.     

Detoxifying Polyhalogenated Catechols through a Copper‐Chelating Agent by Forming Stable and Redox‐Inactive Hydrogen‐Bonded Complexes with an Unusual Perpendicular Structure

The use of selective metal chelating agents with preference for binding of a specific metal ion to investigate its biological role is becoming increasingly common. We found recently that a well‐known copper‐specific chelator 2,9‐dimethyl‐1,10‐phenanthroline (2,9‐Me₂OP) could completely inhibit the synergistic toxicity induced by tetrachlorocatechol (TCC) and sodium azide (NaN₃). However, its underlying molecular mechanism is still not clear. Here, we show that the protection by 2,9‐Me₂OP is not due to its classic copper‐chelating property, but rather due to formation of a multiple hydrogen‐bonded complex between 2,9‐Me₂OP and TCC, featuring an unusual perpendicular arrangement of the two binding partners. The two methyl groups at the 2,9 positions in 2,9‐Me₂OP were found to be critical to stabilize the 2,9‐Me₂OP/TCC complex due to steric hindrance, and therefore completely prevents the generation of the reactive and toxic semiquinone radicals by TCC/NaN₃. This represents the first report showing that an unexpected new protective mode of action for the copper “specific” chelating agent 2,9‐Me₂OP by using its steric hindrance effect of the two CH₃ groups not only to chelate copper, but also to “chelate” a catechol through multiple H‐bonding. These findings may have broad biological implications for future research of this widely used copper‐chelating agent and the ubiquitous catecholic compounds.