Search

We report a multi-component synthetic strategy on a two-dimensional crystalline covalent organic framework (COF) by connecting acetonitrile with aromatic aldehyde and acetaldehyde moieties to form an unprecedented cyano-substituted buta-1,3-diene linkage. Different from most of the COFs that were crystallized from the condensations from two components, the presented COF is generated from two competitive and reversible reactions among three moieties. The buta-1,3-diene COF exhibits remarkable photoactivity with a low exciton binding energy of 44.4 ± 1.5 meV for promoted charge separation, which enables the buta-1,3-diene-linked COF as an efficient photocatalyst for various aerobic oxidation reactions under visible light. Our multi-component synthesis strategy may provide new sights for synthesizing COFs with structural diversity and functional variability that are hard to achieve by traditional COF synthesis.

Three-dimensional covalent organic frameworks (3D-COFs) are emerging as designable porous materials because of their unique structural characteristics and porous features. However, because of the lack of 3D organic building units and the less reversible covalent bonds, the topologies of 3D-COFs to date have been limited to dia, ctn, ffc, bor, rra, srs, pts, lon, stp, acs, tbo, bcu, and fjh. Here we report a 3D-COF with the ceq topology utilizing a D3h-symmetric triangular prism vertex with a planar triangular linker. The as-synthesized COF displays a twofold-interpenetrated structure with a Brunauer-Emmett-Teller surface area of 1148.6 m2 g-1. Gas sorption measurements revealed that 3D-ceq-COF could efficiently absorb CO2, CH4, and H2 under a moderate surface area. This work provides new building units and approaches for structural and application exploration of 3D-COFs.

Hydroxide anion transport is essential for alkaline fuel cells, but hydroxide anion has an inherently low conductivity owing to its small diffusion coefficient and high mass. Ordered open channels found in covalent organic frameworks are promising as pathways to enable hydroxide anion transport, but this remains to be explored. Here we report designed synthesis of anionic covalent organic frameworks that promote hydroxide anion transport across the one-dimensional channels. Engineering cationic chains with imidazolium termini onto the pore walls self-assembles a supramolecular interface of single-file hydroxide anion chains in the channels. The frameworks facilitate hydroxide anion transport to achieve an exceptional conductivity of 1.53 × 10-2 S cm-1 at 80 °C, which is 2-6 orders of magnitude higher than those of linear polymers and other porous frameworks. Impedance spectroscopy at different temperatures and studies on deuterated samples reveal that hydroxide anions transport via a proton-exchange hopping mechanism. These results open a way to design framework materials for energy conversions via engineering an anionic interface.

Three-dimensional covalent organic frameworks (3D-COFs) are emerging as designable porous materials because of their unique structural characteristics and porous features. However, because of the lack of 3D organic building units and the less reversible covalent bonds, the topologies of 3D-COFs to date have been limited to

We report the synthesis of crystalline two-dimensional covalent organic frameworks (COFs) by connecting hydroxybenzene-aldehyde and acetonitrile building blocks to form cyano-substituted benzofuran linkages. Unlike the majority of COFs that were synthesized based on condensation reactions, the COFs in this report are crystallized from irreversible cascade reactions involving consecutive cyanide migration, ring-closure, and oxidation reactions. The irreversible property endows the COFs with high chemical stability in both acid and base and allows them to be postsynthetically modified to install predesigned functionality under harsh conditions. We highlight that the functionalized COFs serve as exceptional vehicles for superprotonic conductions.

The pre-designable porous structures found in covalent organic frameworks (COFs) render them attractive as a molecular platform for addressing environmental issues such as removal of toxic heavy metal ions from water. However, a rational structural design of COFs in this aspect has not been explored. Here we report the rational design of stable COFs for Hg(II) removal through elaborate structural design and control over skeleton, pore size, and pore walls. The resulting framework is stable under strong acid and base conditions, possesses high surface area, has large mesopores, and contains dense sulfide functional termini on the pore walls. These structural features work together in removing Hg(II) from water and achieve a benchmark system that combines capacity, efficiency, effectivity, applicability, selectivity, and reusability. These results suggest that COFs offer a powerful platform for tailor-made structural design to cope with various types of pollution.

A series of two-dimensional covalent organic frameworks (2D COFs) locked with intralayer hydrogen-bonding (H-bonding) interactions were synthesized. The H-bonding interaction sites were located on the edge units of the imine-linked tetragonal porphyrin COFs, and the contents of the H-bonding sites in the COFs were synthetically tuned using a three-component condensation system. The intralayer H-bonding interactions suppress the torsion of the edge units and lock the tetragonal sheets in a planar conformation. This planarization enhances the interlayer interactions and triggers extended π-cloud delocalization over the 2D sheets. Upon AA stacking, the resulting COFs with layered 2D sheets amplify these effects and strongly affect the physical properties of the material, including improving their crystallinity, enhancing their porosity, increasing their light-harvesting capability, reducing their band gap, and enhancing their photocatalytic activity toward the generation of singlet oxygen. These remarkable effects on the structure and properties of the material were observed for both freebase and metalloporphyin COFs. These results imply that exploration of supramolecular ensembles would open a new approach to the structural and functional design of COFs.

With the aim of achieving surface-mediated enantioselective adsorption, the self-assembly of chiral oligo(p-phenylenevinylene) (OPV3T) with nucleosides is investigated at the liquid/solid interface by means of scanning tunneling microscopy and molecular modeling. OPV3T enantiomers form mirror related hexameric rosette patterns. The DNA nucleoside, thymidine, does not self-assemble into stable adlayers but coadsorbs with OPV3T on the surface, leading to a pattern transformation of OPV3T from rosettes to dimers, and a change in chiral expression as well. Diastereoselective recognition between OPV3T and thymidine enantiomers can be used to resolve thymidine enantiomers at an achiral surface with an OPV3T enantiomer as the resolving agent. The impact of molar ratio and concentration on the self-assembly and chiral resolution is systematically investigated. Because there is no interaction between OPV3T and thymidine in solution, the liquid/solid interface acts as the platform for the chiral resolution of thymidine enantiomers.

A synthetic strategy was developed for the preparation of porphyrins containing between one and four stereogenic centers, such that their molecular weights vary only as a result of methyl groups which give the chiral forms. The low-dimensional nanoscale aggregates of these compounds reveal the profound effects of this varying molecular chirality on their supramolecular structure and optical activity. The number of stereogenic centers influences significantly the self-assembly and chiral structure of the aggregates of porphyrin molecules described here. A scanning tunneling microscopy study of monolayers on graphite shows that the degree of structural chirality with respect to the surface increases almost linearly with the number of stereogenic centers, and only one handedness is formed in the monolayers, whereas the achiral compound forms a mixture of mirror-image domains at the surface. In solution, four hydrogen bonds induce the formation of an H-aggregate, and circular dichroism measurements and theoretical studies indicate that the compounds self-assemble into helical structures. Both the chirality and stability of the aggregates depend critically on the number of stereocenters. The chiral porphyrin derivatives gelate methylcyclohexane at concentrations dependent on the number and position of chiral groups at the periphery of the aromatic core, reflecting the different aggregation forces of the molecules in solution. Increasing the number of stereogenic centers requires more material to immobilize the solvent, in all likelihood because of the greater solubility of the porphyrins. The vibrational circular dichroism spectra of the gels show that all compounds have a chiral environment around the amide bonds, confirming the helical model proposed by calculations. The morphologies of the xerogels (studied by scanning electron microscopy and scanning force microscopy) are similar, although more fibrous features are present in the molecules with fewer stereogenic centers. Importantly, the presence of only one stereogenic center, bearing a methyl group as the desymmetrizing ligand, in a molecule of considerable molecular weight is enough to induce single-handed chirality in both the one- and two-dimensional supramolecular self-assembled structures.

The structures and energies of isolobal (CH)n and (BCO)n polyhedral species, computed at the B3LYP density functional theory level, reveal contrasts in behavior. The strain energies of the (BCO)n cages are much smaller. Also unlike the (CH)n cages, the most stable (BCO)n polyhedra (n > or = 10) prefer structures with the largest number of three-membered rings. The planar (or nearly planar) faces of the cage systems were modeled by computations on planar, isoelectronic (CH2)n (Dnh) and (HBCO)n (Cnv) rings. While the strain energies of all the planar carbon rings, relative to the most stable D5h (CH2)5, were large, the strain energies of all the planar (HBCO)n (Cnv) rings were small. Remarkably, the three-membered (HBCO)3 (C3v) ring was the most stable. Finally, large (BCO)n systems prefer tubelike rather than cage structures.

The molecule in which a methyl viologen (MV2+) moiety is coupled through a three-carbon chain to one bipyridine in Ru(bpy)32+ (bpy = 2,2‘bipyrine), [Ru(bpy)2(4-(2-(1‘-methyl-4−4‘-bipyridinediium-1-yl)propyl)-4‘-methyl-2,2‘-bipyridine), abbreviated RuII(bpy)2(MV2+-bpy)4+], does not emit upon photoexcitation because of rapid quenching of the MLCT state by the MV2+. The integrated steady-state photoemission intensity is

Immobilization of single-stranded (ss) DNA on an electrode covered with an aluminum alkanebisphosphonate film by immersion in an ss-DNA solution was demonstrated with ss-DNA labeled with R~(bpy)3~+. This was detected by observing the electrogenerated chemiluminescence (ECL) produced upon oxidation in a solutior containing trin-propylamine. Surface hybridization was shown by immobilization of unlabeled ss-DNA, followed by exposure to a complementary labeled strand of DNA to produce R~(bpy)3~+-labeled double-stranded (ds) DNA on the surface. The extent of DNA hybridization was determined by ECL. Hybridization of unlabeled DNA, e.g., poly(dA) and poly(dT), could be detected by treatment with R~(phen)3~+ and ECL of this species associated with the ds-DNA. In a similar way, an eight-base single strand of DNA was determined by immobilizing it on the film followed by hybridization of the eight-base complementary strand. The surface-immobilized eight-base pair ds-DNA was also detected by observing the ECL of intercalated Ru(phen)3*+. The immobilization and hybridization of DNA on a selfassembled thin film is of interest both in studies of molecular recognition of DNA and in various application^.'-^ Apart from providing a predictable surface that binds a complementary strand of DNA on substrates for studying DNA double-helix formation and its interactions with proteins and other molecules: a film with single-stranded (ss) DNA can form the basis of a DNA biosensor, e.g., by serving as a designed surface modified electrode. DNA diagnostics has become an important area in molecular biology and biotechnology studies, with applications to the determination of disease-causing and food-contaminating organisms and in forensic and environmental investigations. However, traditional methods for DNA sequencing have several disadvantages, e.g., they are labor intensive, expensive, time consuming, and difficult to a u t ~ m a t e . ~ . ~ The development of new DNA biosensors has involved the application of optical methods (e.g., luminescence, ellipsometry, and pseudo-Brewster angle reflectometry), piezoelectric devices (e.g., surface acoustic wave, quartz crystal microbalance), and electrochemical techniques (e.g., cyclic voltammetry and square wave voltammeElectrogenerated chemiluminescence (ECL) has also been applied to DNA analysis, e.g., of DNA produced by the polymerase chain reaction by capturing R~(bpy)3~+-labeled DNA on magnetic beads, transporting the beads to an electrochemical cell, and detecting emission under electrochemical exc i t a t i~n .~ ,~ 'Abstract published in Advance ACS Abstracts, February 15, 1995. (1) Furuta, H.; Magda, D.; Sessler, J. L. J. Am. Chem. SOC. 1991, 113, 978. In a previous study, we demonstrated that an aluminum(1II) alkanebisphosphonate film can be used to immobilize calf thymus double-stranded (ds) DNA based on the interaction of the film metal center (A13+) with the phosphate group of DNA and that the immobilized DNA could be determined by ECL detection upon oxidation of a solution of R~(phen)3~+ intercalated with DNA and tri-n-propylamine.10 The technique for the fabrication of the aluminum alkanebisphosphonate, AL2(CdBP), film by sequential adsorptionheaction steps has been described previously.lo,'l We report here that the film can also be used to immobilize ss-DNA (e.g., poly(dA), poly(dT), poly(dC), and eight-base and 30-base synthesized DNA chains). After the immobilization, the film can be employed to recognize a complementary strand of DNA in a solution followed by ECL detection. The strategy employed is shown in Figure 1. Experimental Section Apparatus. The Plexiglas cell designed for ECL and electrochemical studies of the film immobilized on the gold surface supported on the silicon wafer was described previously. A saturated calomel reference electrode (SCE) and a platinum wire counterelectrode were used for all measurements. The electrochemical measurements coupled with ECL experiments were camed out with a Model 175 universal programmer, a Model 173 potentiostat (Princeton Applied Research, PAR, Princeton, NJ), and an Omnigraphic 2000 x-y recorder (Houston Instruments, Houston, TX). The ECL emission was detected with a Model C1230 single-photon-counting system (Hamamatsu Corp., Bridgewater, NJ) utilizing a Hamamatsu R928P PMT, cooled to -20

The formation of DNA nucleoside-assisted p-conjugated nanostructures was studied by means of scanning tunneling microscopy (STM) and force field simulations. Upon adsorption of the achiral oligo(p-phenylenevinylene) (OPV) derivative at the liquid/solid interface, racemic conglomerates with mirror related rosettes are formed. Addition of the DNA nucleosides d- and l-thymidine, which act as "chiral handles", has a major effect on the supramolecular structure and the expression of chirality of the achiral OPV molecules. The influence of these "chiral handles" on the expression of chirality is probed at two levels: monolayer symmetry and monolayer orientation with respect to the substrate. This was further explored by tuning the molar ratio of the building blocks. Molecular modeling simulations give an atomistic insight into the monolayer construction, as well as the energetics governing the assembly. Thymidine is able to direct the chirality and the pattern of OPV molecules on the surface, creating chiral lamellae of p-conjugated dimers.

Various well-defined Ni-Pt(111) model catalysts are constructed at atomic-level precision under ultra-high-vacuum conditions and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. Subsequent studies of CO oxidation over the surfaces show that a sandwich surface (NiO(1-x)/Pt/Ni/Pt(111)) consisting of both surface Ni oxide nanoislands and subsurface Ni atoms at a Pt(111) surface presents the highest reactivity. A similar sandwich structure has been obtained in supported Pt-Ni nanoparticles via activation in H(2) at an intermediate temperature and established by techniques including acid leaching, inductively coupled plasma, and X-ray adsorption near-edge structure. Among the supported Pt-Ni catalysts studied, the sandwich bimetallic catalysts demonstrate the highest activity to CO oxidation, where 100% CO conversion occurs near room temperature. Both surface science studies of model catalysts and catalytic reaction experiments on supported catalysts illustrate the synergetic effect of the surface and subsurface Ni species on the CO oxidation, in which the surface Ni oxide nanoislands activate O(2), producing atomic O species, while the subsurface Ni atoms further enhance the elementary reaction of CO oxidation with O.

Upon deposition of an achiral solution of diarylethene 1 on the achiral surface of graphite, a highly ordered monolayer with large homachiral domains is formed by self-assembly. Scanning tunneling micro ropy and molecolar dynamics show that the building blocks forming this monolayer are chiral dinners. We suggest that the mechanism of formation of these chiral dinners is first, surface-Induced atropoisomerism, which provides diarylethene 1 with a fixed helicity, and second, shape recognition between atropoisomers having similar helicites.

Monodispersed, hydrophilic, superparamagnetic magnetic nanospheres with a high fraction of magnetite were synthesized by combining modified miniemulsion/emulsion polymerization and sol-gel technique for the first time. The surface of the nanospheres was coated by a silica layer with controlled thickness. Transmission electron microscopy experimental results showed well-proportioned, equal-sized, magnetite/polystyrene (Fe3O4/PS) nanospheres with a thin silica shell. Based on the TGA data, the fraction of magnetite in the Fe3O4/PS nanospheres core was estimated to be 80 wt %. Magnetization measurements indicated that the superparamagnetic nature of the nanospheres had high saturation magnetization of 40 emu/g at 300 K. The procedures of the novel synthesis are described in detail. Also discussed are the mechanisms of the novel combined miniemulsion/emulsion polymerization processes.