Published in Chemical Science, and also highlighted in C&EN, the Lin Group prepared three metal–organic frameworks (MOFs) of the UiO-68 network topology and investigated for sorption of uranium from water and artificial seawater. The stable and porous phosphorylurea-derived MOFs were shown to be highly efficient in sorbing uranyl ions, with saturation sorption capacities as high as 217 mg U g−1 which is equivalent to binding one uranyl ion for every two sorbent groups.
Coordination modes between uranyl groups and simplified phosphorylurea motifs were investigated by DFT calculations, revealing a thermodynamically favorable monodentate binding of two phosphorylurea ligands to one uranyl ion. Convergent orientation of phosphorylurea groups at appropriate distances inside the MOF cavities is believed to facilitate their cooperative binding with uranyl ions. This work represents the first application of MOFs as novel sorbents to extract actinide elements from aqueous media.
Researchers in the Lin Group have constructed a 6,8-connected 3-dimensional metal–organic framework, MOF, of the tph topology from a new aromatic-rich, tetraphenylmethane-derived octa-carboxylate bridging ligand and trizinc cluster secondary building units, SBUs, which exhibited exceptionally high hydrogen and methane uptake capacities. This MOF has the highest hydrogen uptake among all of the MOFs that have been examined to date.
As described in the journal Chemical Science, the gravimetric and volumetric methane uptake capacities for this new framework are the highest among thousands of MOFs that have been evaluated. The Lin Group attribute the exceptionally high gas uptake capacities to the highly branched, aromatic-rich nature of the bridging ligand, optimal pore size, and the open metal sites in the trizinc SBUs in a stable high-connectivity MOF.
The Lin Group, in collaboration with Dr. Shubin Lin of the UNC Research Computing Center, reports in Chemical Science the first enzyme-like catalysis by a metal-organic framework, MOF. They constructed a pair of highly porous chiral MOFs, 1 and 2, from chiral 1,1'-binaphthyl-2,2'-phosphoric acid-derived 3,3',6,6'- and 4,4'',6,6'-tetra(benzoate) ligands, respectively. Both MOFs are active catalysts for Friedel–Crafts reactions between indole and imines.
Interestingly, the 1-catalyzed asymmetric reactions yielded the major enantiomers of the opposite chirality to those afforded by the corresponding homogeneous catalyst. Structural analyses and Quantum Mechanics/Molecular Mechanics calculations revealed that the flip of product handedness results from the chiral environment of the MOF cavity, similar to enzymatic catalysis in which the product stereoselectivity is determined by the enzyme pocket.
Metal-organic frameworks, MOFs, are an interesting class of porous crystalline materials that can be easily functionalized at the molecular level. The Lin Group is interested in using photoactive MOFs as a new platform to integrate different functional components that are needed for solar energy conversion.
The group reports in JACS on the design of synergistic hydrogen evolution photocatalysts based on platinum nanoparticle MOF assemblies. Platinum nanoparticles were selectively loaded to the cavities of phosphorescent MOFs (1 and 2) to enable efficient photocatalytic H2 evolution via photo-injection of electrons from the light-harvesting MOF frameworks into the platinum nanoparticles. The Pt@2 assembly showed a hydrogen evolution efficiency increase by approximately five times, compared to the homogeneous control, and could be readily recycled and reused by centrifugation.
As described in Advanced Materials, the Lin Group has developed a new metal-organic framework (MOF)-templated method for the synthesis of a mixed metal oxide nanocomposite with interesting photophysical properties. Fe-containing nanoscale MOFs are coated with amorphous titania, which are then are calcined to produce crystalline composite octahedral nanoshells with hematite Fe2O3 nanoparticles embedded in anatase TiO2.
This material enables photocatalytic hydrogen production from water using visible light, which cannot be achieved by either Fe2O3 or TiO2 alone or a mixture of the two. This versatile MOF-templated nanocomposite synthesis procedure could be readily modified, by varying the type of MOF and the coating material, to prepare new materials with desirable synergistic properties.
Hydrogen production from water splitting provides a potential solution to storing harvested solar energy in chemical fuels, but this process requires active and robust catalysts that can oxidize water to provide a source of electrons for proton reduction. As reported in ACS Applied Materials and Interfaces, the Lin Group, in collaboration with Bruce Hinds's Group at the University of Kentucky, has developed a new way to study molecular water oxidation catalysts by grafting Ir complexes, directly and covalently, onto carbon electrodes.
Carbon-grafted Ir complexes electrochemically oxidize water with a turnover frequency of up to 3.3 s-1 and a turnover number of at least 644 during the first hour. Electrochemical water oxidation with grafted catalysts gave enhanced rates and stability compared to chemically driven water oxidation with the corresponding molecular catalysts. This strategy provides a way to systematically evaluate catalysts under tunable conditions, potentially providing new insights into electrochemical water oxidation processes and water oxidation catalyst design.
Mesoporous silica nanospheres (MSNs) are a promising material for magnetic resonance imaging (MRI) contrast enhancement, as they can carry high loadings of Gd(III) complexes. MSN-based MRI contrast agents can circumvent many of the limitations of small molecule contrast agents such as low contrast enhancement efficiency, potential toxicity, and the inability to specifically target disease tissues. Nanoparticle-based MRI contrast agents must be cleared in a timely fashion to avoid the long-term toxicity.
Researchers in the Lin Group, as published in the journal Small, report the incorporation of a cleavable Gd(III) chelate into the MSN material such that the chelate is rapidly cleared after injection. The material was further functionalized with poly(ethylene glycol) and a targeting ligand to impart biocompatibility and target specificity. The effectiveness of this material as a MRI contrast agent was effectively demonstrated in vivo with human colon and pancreatic adenocarcinoma cells; the chelate was successfully cleaved and cleared via the renal excretion pathway.
A new type of nanoparticle developed in the laboratories of the Lin Group, has shown potential for more effective delivery of chemotherapy to treat cancer. Kenan Distinguished Professor of Chemistry and Pharmacy, Wenbin Lin and his graduate students Joseph Della Roca, Rachel Huxford, and Erica Comstock-Duggan report their findings in Angewandte Chemie. The new nanoparticle is able to deliver larger amounts of a drug and will not leak the drug as the particle circulates through the blood stream on its way to the target.
In the proof-of-concept experiments, the researchers tested the nanoparticle's ability to deliver therapeutic doses of the chemotherapy drug oxaliplatin to colon and pancreatic tumors. The oxaliplatin-based particles showed significant growth inhibition of pancreatic tumors that are extremely difficult to treat. The nanoparticle has two to three times therapeutic efficacy over oxaliplatin.
This new particle is different from other nanoparticles in its very high drug loading and in the ability to release the chemotherapeutics in a controlled fashion. The release of therapeutic cargoes depends on the naturally occurring molecules that are more abundant in many tumors.
Metal-organic frameworks (MOFs) have been shown to provide an excellent platform for the rational design of solid catalysts for a variety of organic transformations through the direct incorporation of catalytic building blocks into the framework. As published in Angewandte Chemie, researchers in the Lin Group have developed the first MOF catalyst activated by reversible single-crystal to single-crystal reduction.
The chiral MOFs synthesized from redox-active Ru-salen bridging ligands were reversibly reduced from their catalytically inactive Ru(III) form (green) to their active Ru(II) form (red). The Ru(II)-salen MOFs were demonstrated to be effective catalysts for the asymmetric cyclopropanation of styrene and other substituted alkenes with high diastereo- and enantioselectivites.
Metal-organic frameworks, MOFs, have emerged as an ideal platform for engineering functional materials. In particular, MOFs have recently provided a tunable platform for designing solid catalysts for a number of organic transformations. As published in Chemical Communications, researchers in the Lin Group introduced different metal centers into the MOFs both as the primary catalytic sites and as the metal connecting points. The disparate metal centers in such MOFs can be used to catalyze different chemical reactions to allow multiple-step manipulations of organic substrates with a single solid.
Chiral MOFs based on Mn-Salen derived bridging ligands have been previously demonstrated as effective heterogeneous asymmetric catalysts for alkene epoxidation reactions. On the other hand, [Zn4(µ4-O)(O2CR)6] secondary building units (SBUs), one of the most established connecting nodes in MOFs, were reported to be responsible for several cases of Lewis acid catalysis. In their work, the Mn-Salen based dicarboxylate ligands and [Zn4(µ4-O)(O2CR)6] SBUs in a chiral MOF of the lcy topology catalyzed stereoselective alkene epoxidation and epoxide ring-opening reactions, respectively, without deleterious interference from each other, as a result of catalytic site isolation in a solid. This is the first chiral MOF reported for regio- and stereo-selective sequential catalysis.
Published in JACS, investigators in the Lin Group present the first quantitative study of the dynamics of molecular diffusion into solvent-filled Metal Organic Framework, MOF, channels. They studied diffusion-controlled luminescence quenching of a phosphorescent MOF built from the Ru(bpy)32+-derived bridging ligand, using a series of amines of different sizes as quenchers. The dynamics of amine diffusion into these solvent-filled channels was probed by modeling time-dependent luminescence quenching data, which provide quantitative diffusion coefficients for the amine quenchers.
Triethylamine, tripropylamine, and tributylamine were found to follow Fickian diffusion with a diffusivity of (1.1 ± 0.2) × 10-13, (4.8 ± 1.2) × 10-14, and (4.0 ± 0.4) × 10-14 m2/s, respectively. Diisopropylethylamine, DIPEA, on the other hand, was found to be too large to enter the MOF channels. Despite its size, 4-MeOPhNPh2 can enter the MOF channels via a slow, complicated framework/guest intercalation process to result in extensive framework distortion as revealed by powder X-ray diffraction. This quantitative information on molecular diffusion in MOFs is of fundamental importance to many potential applications, for example heterogeneous catalysis.
Scientists in the Lin Group, as published in JACS and soon to be featured in C&E News, have obtained porous cross-linked polymers (PCPs) with phosphorescent ruthenium and iridium building blocks via cobalt-catalyzed alkyne trimerization reactions. The resultant Ru- and Ir-PCPs are highly porous and thermally stable in air. They do not dissolve or decompose in any solvent tested, including concentrated hydrochloric acid.
The photoactive PCPs were shown to be highly effective, recyclable, and reusable heterogeneous photocatalysts for a number of organic transformations, including aza-Henry reactions, α-arylation of bromomalonate, and oxyamination of an aldehyde. The photocatalytic activities of the PCPs are comparable to those of homogeneous ruthenium and iridium photocatalysts. This work highlights the potential of developing photoactive PCPs as highly stable, molecularly tunable, recyclable, and reusable heterogeneous photocatalysts to drive a variety of important organic transformations by harvesting energy from sunlight.
Metal-organic frameworks, MOFs, have attracted a great deal of recent interest owing to the ability to fine-tune their properties at the molecular level. Researchers in the Lin Group, as published in JACS, demonstrate the systematic design of a family of isoreticular chiral metal-organic frameworks (CMOFs 1-5) of the primitive cubic network (pcu) topology, constructed from [Zn4(µ4-O)(O2CR)6] secondary building units (SBUs) and systematically elongated dicarboxylate struts that are derived from chiral Mn-Salen catalytic subunits.
Although the CMOFs 1 vs. 2 and CMOFs 3 vs. 4 pairs were constructed from the same building blocks, they exhibit 2-fold interpenetrated or non-interpenetrated structures, respectively, depending on the steric sizes of the solvents that were used to grow the MOF crystals. The open channel and pore sizes of the CMOF series vary systematically owing to the tunable dicarboxylate struts and controllable interpenetration patterns. CMOFs 1-5 were shown to be highly effective catalysts for asymmetric epoxidation of a variety of unfunctionalized olefins with up to 92% e.e. The rates of epoxidation reactions strongly depend on the CMOF open channel sizes, and the catalytic activities of CMOFs-2 and -4 approach that of a homogeneous control catalyst.
In a collaborative work involving the Lin, Papanikolas and Meyer Groups, isomorphous metal−organic frameworks, MOFs, were designed and synthesized to study the classic Ru to Os energy transfer process that has potential applications in light-harvesting with supramolecular assemblies. Published in JACS, the researchers show how the crystalline nature of the MOFs allows precise determination of the distances between metal centers by X-ray diffraction, thereby facilitating the study of the Ru→Os energy transfer process.
The mixed-metal MOFs with 0.3, 0.6, 1.4, and 2.6 mol % Os doping were also synthesized in order to study the energy transfer dynamics with a two-photon excitation at 850 nm. The Ru lifetime at 620 nm decreases from 171 ns in the pure Ru MOF to 29 ns in the sample with 2.6 mol % Os doping. In the mixed-metal samples, energy transfer was observed with an initial growth in Os emission corresponding with the rate of decay of the Ru excited state. These results demonstrate rapid, efficient energy migration and long distance transfer in isomorphous MOFs.
Joseph Falkowski , a graduate student in the Lin Group, has been awarded a Department of Energy Office of Science Fellowship. Joseph was selected based on his outstanding academic accomplishments and graduate education. The DOE will support Joseph's research as well as provide him with opportunities to attend conferences and symposia internationally. Joseph is one of only 150 students nationally and the only one in our department to receive this award.
Joseph's research focuses on creating new mesoporous materials for asymmetric catalysis and energy conversion. Using metal-organic frameworks, MOFs, the gap between traditional homogeneous and heterogeneous catalysis systems is bridged. This allows for the synthesis of solid state catalysts, which have the controlled active centers that are the hallmark of homogeneous catalysts as well as the recyclability inherent in heterogeneous systems.
In the first of two back-to-back communications in the same issue of Angewandte Chemie, the Lin Group describes the design of Nanoscale Coordination Polymers, NCPs, as potential contrast agents for computed tomography, CT, a biomedical imaging technique based on X-ray attenuation of a specimen.
The NCPs were synthesized using an iodinated ligand to carry high payloads of iodine, ca. 63 wt%, which allows for high X-ray attenuation as demonstrated by phantom studies. This type of nanomaterial offers a new strategy for designing efficient CT contrast agents that do not suffer from the inherent drawbacks of small-molecule agents. The work was carried out in collaboration with Professor Otto Zhou and his group in the Physics Department at UNC-Chapel Hill.
The platinum-containing chemotherapeutic cisplatin is the first-line treatment for many types of cancer, but results in a myriad of disparaging dose-limiting side effects, such as nephrotoxicity and neurotoxicity. Nanomaterials have shown great promise in selectively delivering chemotherapeutics to tumors to reduce these side effects and to increase therapeutic indices. As reported in JACS, the Lin Group has developed a novel nanovector platform based on nanoscale metal-organic frameworks, NMOFs, for delivering chemotherapeutics and imaging contrast agents.
NMOFs are materials crafted from metals and organic bridging ligands, and can be engineered to contain reactive functional groups. In this study, the amino groups incorporated into the NMOFs were used to graft optical imaging contrast agents or platinum-containing chemotherapeutics. These modified NMOFs were coated in silica to reduce premature release of imaging contrast agents or chemotherapeutics before arriving at the tumor sites. Preliminary in vitro tests showed that these NMOFs could effectively cause cell death in human colon cancer cell cultures with an efficacy similar to cisplatin. The Lin Group hopes to further modify this strategy to deliver other cancer drugs and imaging contrast agents.
In a paper published in J. Am. Chem. Soc., Dr. Liqing Ma and Dr. David J. Mihalcik in the Lin Group reports highly porous and robust MOFs by increasing the network connectivity. New (4,8)-connected MOFs were built from new octa-carboxylate ligands, which exhibit remarkable framework stability as evidenced by perfect agreement between experimental N2 adsorption isotherms and GCMC simulations as well as by identical PXRD patterns between pristine and evacuated MOFs.
The porosity and functionality of the MOFs can be tuned by modification of the orthogonal functional groups, leading to a hydrogen uptake of 2.5 wt% at 1atm and 77K. This work represents a new approach toward designing highly porous, robust, tunable, and functional MOFs using multidentate bridging ligands of high connectivity.
Nanomaterials have shown great promise in selectively delivering imaging and therapeutic agents to diseased tissues. Currently available nanoparticle formulations are either purely inorganic (e.g., gold nanoparticles) or purely organic (e.g., polymer nanoparticles). The Lin Group is developing new strategies for formulating hybrid nanomaterials that combine both inorganic and organic components for imaging and therapeutic applications. In an upcoming "hot" paper in Angew. Chem. (DOI: 10.1002/anie.200802911), Taylor, Jin, and Lin described the surfactant-assisted synthesis of two different gadolinium nanoscale metal-organic frameworks (NMOFs) from the same building blocks by simply controlling the pH values of the reaction media. The Gd NMOFs were shown to be excellent contrast enhancement agents for magnetic resonance imaging and optical imaging. In a separate paper published in J. Am. Chem. Soc. and highlighted in Chemical & Engineering News (August 18, 2008, page 35), the Lin group demonstrated the utility of hybrid nanoparticles as an efficient vehicle for delivering anticancer drugs specifically to cancer cells.
Rieter, Pott, Taylor, and Lin formulated highly degradable nanoparticles based on Pt-containing nanoscale coordination polymers (NCPs). The release of the Pt drug was controlled by encapsulating the NCPs in shells of amorphous silica. The in vitro anticancer efficacies of Pt-based NCPs were controlled by conjugating the core-shell nanoparticles to peptides that bind specifically to cell surface receptors over-expressed by cancer cells. The generality of the hybrid nanoparticle approaches developed in the Lin group should allow for the design of effective delivery vehicles for a variety of other biologically and medically important cargoes.
Cheng "Wave" Wang, a graduate student in the Lin Group, has been selected for a 2013 Young Investigator Awards given by the ACS Division of Inorganic Chemistry.
As an awardee, Wave will be giving an award presentation at the 2013 Fall ACS National meeting in Indianapolis, where he will receive $1000 and a plaque at the special oral presentation session. Wave's research focuses on developing novel metal organic frameworks for solar energy related applications and heterogeneous asymmetric catalysis.
Nanoscale metal–organic frameworks, NMOFs, of the UiO-66 structure containing high Zirconium and Hafnium content, 37% and 57% by weight respectively, were synthesized and characterized in work by the Lin Group, published in and featured on the cover of the Journal of Materials Chemistry.
The potential of these NMOFs as contrast agents for X-ray computed tomography, CT, imaging was evaluated. Hf-NMOFs of different sizes were coated with silica and poly(ethylene glycol), PEG, to enhance biocompatibility, and were used for in vivo CT imaging of mice, showing increased attenuation in the liver and spleen.
Researchers in the Lin Group, published in JACS, have built a highly porous and fluorescent metal–organic framework, MOF-1, from a chiral tetracarboxylate bridging ligand derived from 1,1′-bi-2-naphthol, BINOL, and a cadmium carboxylate infinite-chain secondary building unit. The fluorescence of MOF-1 can be effectively quenched by amino alcohols via H-bonding with the binaphthol moieties decorating the MOF, leading to a remarkable chiral sensor for amino alcohols with greatly enhanced sensitivity and enantioselectivity over BINOL-based homogeneous systems.
The higher detection sensitivity of MOF-1 is due to a preconcentration effect by which the analytes are absorbed and concentrated inside the MOF channels, whereas the higher enantioselectivity of MOF-1 is believed to result from enhanced chiral discrimination owing to the cavity confinement effect and the conformational rigidity of the BINOL groups in the framework. MOF-1 was quenched by four chiral amino alcohols with unprecedentedly high Stern–Volmer constants of 490–31200 M–1 and enantioselectivity ratios of 1.17–3.12.
Artificial photosynthesis provides a promising and revolutionary approach toward green chemical transformations, but requires molecular systems or materials that can not only act as antenna for photon capture but are also able to transfer the energies to the reaction centers to drive desired chemical transformations. Multifunctional framework materials with appropriate hierarchical architectures provide a potential platform to integrate light-harvesting and catalytic centers for solar energy utilization.
As reported in ACS Applied Materials and Interfaces, the Lin Group has synthesized nonporous cross-linked polymers (CPs) based on the tetra(p-ethynylphenyl)¬methane tetrahedral nodes and a stoichiometric amount of linear linkers that are derived from photoactive [Ru(bpy)3]2+ or [Ir(ppy)2(bpy)]+ (bpy is 2,2'-bipyridine and ppy is 2-phenylpyridine) via Sonogashira cross-coupling reactions. In spite of their nonporous nature, these CPs are highly active and recyclable heterogeneous photocatalysts for a range of organic transformations such as aza-Henry reaction, aerobic amine coupling, and dehalogenation of benzyl bromoacetate. The very high catalytic activities of these nonporous CPs result from their light-harvesting ability, which allows collection of photons by exciting the 3MLCT states of the phosphors via framework sensitization and migration of the excited states to the particle surface to drive the redox catalysis.
Metal-organic frameworks, MOFs, represent a new class of structurally ordered hybrid materials whose properties can be fine-tuned at the molecular level to suit many applications. In particular, recent works from the Lin and Meyer groups have demonstrated rapid energy migration over long distances and efficient electron transfer quenching at the interfaces of emitting MOFs.
MOFs with triplet metal-to-ligand charge transfer excited-states offer a promising scaffold for amplified quenching, a signal gain as a result of interactions between a sensing material and analytes accompanied by rapid energy migration. A remarkable example of MOF-based amplified quenching has been published in JACS using Ru(II)-bpy based MOFs and cationic quenchers. The MOF surface is partially terminated with carboxylate groups that have strong non-covalent interactions with cationic quenchers and lead to quenching enhancements of up to 7000-fold compared to a model complex. This work points to the potential of designing MOF-based sensors for sensitive and selective sensing of many analytes via amplified quenching.
The Lin Group is developing Nanoscale Coordination Polymers, NCPs, as a new platform for delivering biologically and biomedically important imaging contrast agents and chemotherapeutics. Published in Chemical Communications, the group reports a general strategy to deliver nitrogen-containing bisphosphonates, MBPs, to cancer cells by incorporation into NCPs. MBPs are clinically used to treat bone-resorption related diseases such as osteoporosis.
Although MBPs also exhibit antitumor activities, they do not have favorable pharmacokinetics for in vivo applications. The Lin Group has demonstrated that bisphosphonate-based NCPs display superior anti-tumor efficacy when compared to free drugs against human lung and pancreatic cancer cells. These NCPs are currently being evaluated as anticancer therapeutics.
Nanoscale coordination polymers (NCPs) have been demonstrated as an interesting platform for the delivery of methotrexate (MTX), an antifolate cancer drug, as they possess many potential advantages over small-molecule chemotherapeutics such as high payloads, lower systemic toxicity, tunability, and enhanced tumor uptake. NCPs also overcome the limitations of existing nanoparticle formulations that have very low drug loadings.
Researchers in the Lin Group, published in Chemical Science, report the incorporation of MTX as a building block in an NCP formulation with exceptionally high drug loadings (up to 79.1 wt%) and the selective delivery of the NCP to cancer cells. Encapsulation of the NCP in a functionalized lipid bilayer allows for targeted delivery and controlled release to cancer cells. A phosphor can be doped into the NCPs for monitoring particle uptake by optical imaging. The lipid-coated and anisamide-targeted NCPs have superior in vitro efficacy against acute lymphoblastic leukemia cells when compared to the free drug.
Metal-organic frameworks (MOFs) have emerged as an interesting class of functional porous solids. Researchers in the Lin Group are interested in utilizing MOFs as a platform to integrate individual functional components for solar energy utilization. As published in JACS, they have developed the first MOF-based heterogeneous catalytic systems for water oxidation, photocatalytic CO2 reduction, and visible light-driven organic photocatalysis.
Incorporating molecular functionality into framework structures is a well-established method to achieve synthetically tunable functional materials. However, most MOF structures tend to lack stability under the conditions of a water oxidation reaction. Researchers selected an exceptionally stable framework called UiO-67, which is built from Zr6O4(OH)4(CO2)12 secondary building units and linear dicarboxylate linkers. They successfully incorporated three iridium-based water oxidation catalysts by doping the framework to yield highly stable MOFs that demonstrated catalytic water oxidation. They also doped a rhenium-based catalyst and iridium/ruthenium-based phosphors into the framework to afford heterogeneous photocatalysts for CO2 reduction and organic transformations, respectively.
A collaboration between the Lin, Meyer, and Papanikolas groups, published in JACS, describes how microscale metal–organic frameworks (MOFs) were synthesized from photoactive Ru(II)-bpy building blocks with strong visible light absorption and long-lived triplet metal-to-ligand charge transfer (3MLCT) excited states. These MOFs underwent efficient luminescence quenching in the presence of either oxidative or reductive quenchers.
Up to 98% emission quenching was achieved with either an oxidative quencher (1,4-benzoquinone) or a reductive quencher (N,N,N′,N′-tetramethylbenzidine), as a result of rapid energy migration over several hundred nanometers followed by efficient electron transfer quenching at the MOF/solution interface. The photoactive MOFs act as an excellent light-harvesting system by combining intraframework energy migration and interfacial electron transfer quenching.
Optical imaging has the potential to become a powerful imaging modality for early diagnosis of human diseases. The effectiveness of optical imaging processes can be significantly improved with suitable dyes used as contrast agents. Now, in the journal Angewandte Chemie, researchers in the Lin Group have introduced a novel contrast agent that marks tumor cells in vitro. The new approach takes advantage of a nanoscale coordination polymer built from a phosphorescent ruthenium complex, which allows for an extraordinarily high level of dye loading.
The ruthenium complexes undergo phosphorescence instead of fluorescence, hence they do not suffer from self-quenching even if the dye molecules are close to each other. In a human cell culture, it was possible to selectively image a sample of cancer cells with the phosphorescent nanoparticles. This method highly improves upon other methods using fluorescent dyes, and the researchers hope that it will be possible to develop contrast agents for tumor detection based on these new nanomaterials.
Carolina Chemistry professors Wenbin Lin and Maurice Brookhart are included in the "Top Chemists of the Past Decade" list. The list ranks the world's top 100 chemists by the impact of their published research, and contains Nobel Laureates and other chemistry luminaries who have each published more than 50 influential papers in the past ten years.
Thomson Reuters published the table in support of the International Year of Chemistry, and to celebrate the acomplishments of chemists who achieved the highest citation impact scores for chemistry papers, articles and reviews, published since January 2000.
Metal-organic framework, MOF, catalysts provide a unique opportunity to interrogate their structure/function relationships via single crystal X-ray diffraction studies. Published in Angewandte Chemie, the Lin Group has determined the workings of Ti-Binolate based chiral Lewis acid catalysts by carrying out single crystal to single crystal transformation of an interpenetrated MOF. The structure of a typically un-isolatable Ti-bis(Binolate) catalyst was unambiguously determined via postsynthesis crosslinking of the MOF framework with titanium tetra(isopropoxide).
In the post-synthetically modified MOF, the Ti centers bridge intermolecularly between two binolate moieties, instead of chelating with a single binolate ligand intramolecularly as seen in earlier works by the Lin Group. The Ti catalytic centers in the resulting MOF experience less steric influence of the chiral binaphthyl moieties, leading to modest enantio-discrimination in diethylzinc addition reactions to aromatic aldehydes. This work demonstrates the ability to obtain high-precision structural information of MOF-derived catalysts, thus facilitating the rational design of next generation high performance solid catalysts.
Professor Wenbin Lin is one of two Principal Investigators who will lead a team of UNC scientists awarded a five-year $2,308,800 grant from the National Cancer Institute's Cancer Nanotechnology Platform Partnerships to address the critical need for early diagnosis of, and more effective treatments for, pancreatic cancer.
Using targeted nano-particle technology, based on nano-materials developed in the Lin lab, the scientists will design nanoscale metal-organic frameworks –a new class of hybrid nano-materials- capable of carrying both imaging and therapeutic cargoes or multiple drugs to increase therapeutic effect.
"Pancreatic cancer is difficult to detect early and to treat," says Lin. "By developing a more targeted delivery system for imaging, we hope to be able to detect tumors earlier. And by using the hybrid nano-materials to deliver drugs directly to the tumor, we could lessen side effects for patients."
Metal–organic frameworks, MOFs, built by bridging metal ions with organic linkers, represent a new class of porous hybrid materials with attractive tunability in compositions, structures and functions. In particular, the mild conditions typically employed for their synthesis allow for the functionalization of their building blocks, and thus the rational design of novel materials.
Researchers in the Lin Group, as published in Nature Chemistry, demonstrate the systematic design of eight mesoporous chiral metal–organic frameworks, with the framework formula [LCu2(solvent)2] (where L is a chiral tetracarboxylate ligand derived from 1,1'-bi-2-naphthol), that have the same structures but channels of different sizes. Chiral Lewis acid catalysts were generated by postsynthesis functionalization with Ti(OiPr)4, and the resulting materials proved to be highly active asymmetric catalysts for diethylzinc and alkynylzinc additions, which converted aromatic aldehydes into chiral secondary alcohols. The enantioselectivities of these reactions can be modified by tuning the size of the channels, which alters the diffusion rates of the organic substrates.
As reported in JACS, scientists in the Lin Group have designed Phosphorescent cyclometalated iridium tris(2-phenylpyridine) derivatives and incorporated them into coordination polymers as tricarboxylate bridging ligands. Three different crystalline coordination polymers were synthesized using a solvothermal technique and were characterized using a variety of methods, including single-crystal X-ray diffraction, PXRD, TGA, IR spectroscopy, gas adsorption measurements, and luminescence measurements. The coordination polymer built from Ir[3-(2-pyridyl)benzoate]3, 1, was found to be highly porous with a nitrogen BET surface area of 764 m2/g, whereas the coordination polymers built from Ir[4-(2-pyridyl)benzoate]3, 2 and 3, were nonporous.
The 3MLCT phosphorescence of each of the three coordination polymers was quenched in the presence of O2. However, only 1 showed quick and reversible luminescence quenching by oxygen, whereas 2 and 3 exhibited gradual and irreversible luminescence quenching by oxygen. The high permanent porosity of 1 allows for rapid diffusion of oxygen through the open channels, leading to efficient and reversible quenching of the 3MLCT phosphorescence. This work highlights the opportunity of designing highly porous and luminescent coordination polymers for sensing other important analytes.
In the second of two back-to-back communications in the same issue of Angewandte Chemie, the Lin Group describes how a simple freeze-drying technique was devised to preserve the porosity of Metal-Organic Frameworks, MOFs, upon solvent removal. High-boiling solvents inside the channels are replaced by benzene, which is then frozen and removed under vacuum by sublimation.
Bypassing the liquid phase in the freeze-drying procedure prevents pore collapse due to surface tension, leading to much enhanced permanent porosity and hydrogen uptake as well as interesting structural transformations in the evacuated MOFs. Such a simple and effective freeze drying method can be extended to the processing of other porous materials for many applications in the future.
Metal-organic frameworks (MOFs) are a new class of molecule-based hybrid materials that have shown great promise for a number of applications including nonlinear optics, gas storage, catalysis, biomedical imaging, and drug delivery. Recent efforts on MOF research are steered toward synthesizing materials based on elaborately designed organic bridging ligands. As the organic bridging ligands become more elaborate and large, the resulting MOFs tend to experience significant framework distortion upon solvent removal.
As published in Angewandte Chemie, Dr. Liqing Ma in the Wenbin Lin Group reports a novel strategy to rigidify MOFs via unusual interlocking and interpenetration of networks of different dimensionality. His results have significant implications on enhancing gas uptake by MOFs, as such rigidification drastically increases permanent porosity and robustness of MOFs.
Metal-organic frameworks (MOFs), a new class of highly porous crystalline solids with infinite network structures, have attracted a great deal of recent interest owing to their potential applications in a number of areas, including gas storage, nonlinear optics, and catalysis. The Lin Group is particularly interested in designing homochiral porous MOFs for heterogeneous asymmetric catalysis and chiral separations. These applications require MOFs with open channels that are several nanometers in dimensions. The synthesis of such mesoporous MOFs presents a significant challenge owing to their tendency to interpenetrate which can drastically reduce the porosity and sizes of open channels. The elimination of framework interpenetration (catenation) thus is the key to constructing MOFs with very large open channels
Dr. Liqing Ma in the Lin Group has demonstrated that the catenation of MOFs can be suppressed by controlling either the charily of organic ligands or by the nature of the solvents involved in the MOF synthesis. When an elongated chiral tetra-carboxylate ligand was reacted with copper nitrate, they obtained a non-catenated homochiral MOF with open channels of 3.2 nm. When the racemic ligand was used, the structure of the MOFs depended on the size of solvent molecules. While dimethylformamide (DMF) gave the catenated structure, the larger diethylformamide (DEF) afforded non-catenated MOFs. A significant uptake of a nano-sized dye molecule of 1.8×2.2 nm in dimensions into the non-catenated framework unambiguously demonstrates the maintenance of open channels and their accessibility to nano-sized molecules in solution. This work opens up a new direction in designing mesoporous MOFs for heterogeneous catalysis and chemical separations.