Metal Substitution as the Method of Modifying Electronic Structure of Metal-Organic Frameworks.

Targeted modification of electronic structure is an important step in the optimization of metal-organic frameworks (MOFs) for photovoltaic, sensing, and photocatalytic applications. The key parameters to be controlled include the band gap, the absolute energy position of band edges, the excited state charge separation, and degree of hybridization between metal and ligand sites. Partial metal replacement, or metal doping, within secondary building units is a promising, yet relatively unexplored route to modulate these properties in MOFs. Therefore, in the present study, a general method for selecting metal dopant is worked out in theory and validated by experiment, retaining MIL-125 and UiO-66 as the model systems. Metal mixing enables targeted optimization of key electronic structure parameters. This method is applicable to any MOF architecture and can serve as a roadmap for future synthesis of MOFs with predefined properties

Photoluminescent, upconversion luminescent and nonlinear optical metal-organic frameworks: From fundamental photophysics to potential applications

Metal-organic frameworks (MOFs) are a class of porous materials prepared by the self-assembly of metal ions or clusters with organic ligands. The unique characteristics of MOFs, including structural tunability, high surface areas, low densities and tailored pore surface functionalization, have made them leading contenders as high-performance porous materials, alongside the established zeolites and activated carbons. Consequently, the permanent porosity of MOFs has been extensively exploited for gas capture and separation and catalysis. In recent years, the field has been expanded towards new applications and many studies of MOFs are venturing into unexplored avenues. A large number of studies have been focused on photoluminescent, upconversion luminescent, and nonlinear optical MOFs for applications in areas such as white-light emission, bioimaging, sensing, and photocatalysis. Within the first half of this tutorial review, we present the fundamental principles of luminescence, including detailed scientific discussions on the luminescence origin of different materials such as organic dyes, transition metal complexes, quantum dots, and lanthanide compounds. Principles and important parameters for the applications of luminescent MOFs are introduced, followed by a summary of recent interesting publications for each application. In the second half, we introduce nonlinear optical effects including second harmonic generation and two-photon absorption, and upconversion of luminescence, followed by detailed examples of MOFs that exhibit these phenomena. Finally, insights about the remaining challenges and future directions are discussed. (C) 2018 Elsevier B.V. All rights reserved

Using genetic algorithms to systematically improve the synthesis conditions of Al-PMOF

The synthesis of metal-organic frameworks (MOFs) is often complex and the desired structure is not always obtained. In this work, we report a methodology that uses a joint machine learning and experimental approach to obtain the optimal synthesis of a MOF. A synthetic conditions finder was used to derive the experimental protocols and a microwave based high-throughput robotic platform was used for the synthesis of Al-PMOF ([H2TCPP[AlOH]2(DMF3(H2O)2)]). Al-PMOF was previously synthesized using a hydrothermal reaction, which gave a low throughput yield due to its relatively long reaction time (16 hours). In this work, we carried out a systematic search for the optimal reaction conditions using a microwave assisted reaction synthesis. For this search we used a genetic algorithm and we show that already in the 2nd generation we obtained conditions that give excellent crystallinity and yield close to 80% in much shorter reaction time (50 minutes). In addition, by analysing the failed and partly successful experiments, we could identify the most important experimental variables that determine the crystallinity and yield

Using genetic algorithms to systematically improve the synthesis conditions of Al-PMOF.

The synthesis of metal-organic frameworks (MOFs) is often complex and the desired structure is not always obtained. In this work, we report a methodology that uses a joint machine learning and experimental approach to optimize the synthesis conditions of Al-PMOF (Al2(OH)2TCPP) [H2TCPP = meso-tetra(4-carboxyphenyl)porphine], a promising material for carbon capture applications. Al-PMOF was previously synthesized using a hydrothermal reaction, which gave a low throughput yield due to its relatively long reaction time (16 hours). Here, we use a genetic algorithm to carry out a systematic search for the optimal synthesis conditions and a microwave-based high-throughput robotic platform for the syntheses. We show that, in just two generations, we could obtain excellent crystallinity and yield close to 80% in a much shorter reaction time (50 minutes). Moreover, by analyzing the failed and partially successful experiments, we could identify the most important experimental variables that determine the crystallinity and yield

Towards optimal photocatalytic hydrogen generation from water using pyrene-based metal-organic frameworks

Metal organic frameworks (MOFs) are promising materials for the photocatalytic H2 evolution reaction (HER) from water. To find the optimal MOF for a photocatalytic HER one has to consider many different factors. For example, studies have emphasized the importance of light absorption capability, optical band gap and band alignment. However, most of these studies have been carried on very different materials. In this work, we present a combined experimental and computation study of the photocatalytic HER performance, of a set of isostructural pyrene-based MOFs (M-TBAPy, where M = Sc, Al, Ti, and In). We systematically studied the effects of changing the metal in the node on the different factors that contribute to the HER rate (e.g., optical properties, the band structure, water adsorption). In addition, for Sc-TBAPy we also studied the effect of changes in crystal morphology on the photocatalytic HER rate. We used this understanding to improve photocatalytic HER efficiency of Sc-TBAPy, to exceed the one reported for Ti-TBAPy, in the presence of co-catalyst

Discovery of a self-healing catalyst for the hydrolytic dehydrogenation of ammonia borane

Sustainable catalysts based on earth-abundant elements are considered as economical alternatives to precious-metal-bearing catalysts and could be impactful for many applications. Self-healing sustainable catalysts, which in addition to their 'green' characteristic can spontaneously repair themselves without the need of applying heat, pressure or electrochemical bias, are particularly desirable for numerous large-scale chemical processes. Herein, we present the discovery of such a catalyst, named SION-X, for the hydrolytic dehydrogenation of ammonia borane (AB, NH3BH3). SION-X, with the chemical formula of CuII2[(BO)(OH)(2)](OH)(3), is the synthetic form of the mineral Jacquesdietrichite and, following in situ reduction, catalyzes the release of almost all 3 equivalents of hydrogen (H-2) from 1 equivalent of AB. During the reaction, the Cu-II ions in SION-X are reduced to Cu-0 nanoparticles, and after the reaction, following exposure to air, they are oxidized re-forming SION-X. As a consequence, the catalytic activity of SION-X toward the production of H-2 from AB remains unchanged over many cycles. The self-healing catalysis of SION-X in the absence of any extra energy input gives a new perspective in heterogeneous catalysis for energy-related applications

Toward Optimal Photocatalytic Hydrogen Generation from Water Using Pyrene-Based Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are promising materials for the photocatalytic H-2 evolution reaction (HER) from water. To find the optimal MOF for a photocatalytic HER, one has to consider many different factors. For example, studies have emphasized the importance of light absorption capability, optical band gap, and band alignment. However, most of these studies have been carried out on very different materials. In this work, we present a combined experimental and computation study of the photocatalytic HER performance of a set of isostructural pyrene-based MOFs (M-TBAPy, where M = Sc, Al, Ti, and In). We systematically studied the effects of changing the metal in the node on the different factors that contribute to the HER rate (e.g., optical properties, the band structure, and water adsorption). In addition, for Sc-TBAPy, we also studied the effect of changes in the crystal morphology on the photocatalytic HER rate. We used this understanding to improve the photocatalytic HER efficiency of Sc-TBAPy, to exceed the one reported for Ti-TBAPy, in the presence of a co-catalyst

Color-tunable and high quantum-yield luminescence from a biomolecule-inspired single species emitter of white light

Single-species light emitters with high photoluminescence quantum yields (PLQYs) and broad- spectrum color tunability are sought-after for applications ranging from bio-imaging to artificial lighting. We explore a new strategy to design such emitters, inspired by bioluminescent fireflies and click-beetles. These organisms use a single molecular substrate, D-Luciferin (LH2), to emit light ranging in color from green to red. By combining LH2 with metals, we synthesize new bio-analogous, color-tunable, luminescent metal complexes. The copper complex forms an organic molecule of intrinsic microporosity (OMIM), which crystallizes into a stable structure with intermolecular voids. By changing the composition of guest molecules in the voids, we can tune the emitted color. The optimum composition gives nearly perfect white light, with the highest PLQY reported for a single-species white- light emitter. Similarities between our OMIM and the luciferase active site provide a new approach to investigating the heavily-debated mechanisms underlying in-vivo bioluminescence color variations. Moreover, as a proof of principle, we show that these materials can be used in a new type of light- emitting device (LED). The current generation of LEDs requires at least two active layers to achieve color tunability. The tunability is intrinsic in our materials, and therefore may lead to simpler device fabrication