The development of porous well-defined hybrid materials (e.g., metal-organic frameworks or MOFs) will add a new dimension to a wide number of applications ranging from supercapacitors and electrodes to "smart" membranes and thermoelectrics. From this perspective, the understanding and tailoring of the electronic properties of MOFs are key fundamental challenges that could unlock the full potential of these materials. In this work, we focused on the fundamental insights responsible for the electronic properties of three distinct classes of bimetallic systems, MM'-MOFs, MM'-MOFs, and M(ligand-M')-MOFs, in which the second metal (M') incorporation occurs through (i) metal (M) replacement in the framework nodes (type I), (ii) metal node extension (type II), and (iii) metal coordination to the organic ligand (type III), respectively. We employed microwave conductivity, X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, powder X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, pressed-pellet conductivity, and theoretical modeling to shed light on the key factors responsible for the tunability of MOF electronic structures. Experimental prescreening of MOFs was performed based on changes in the density of electronic states near the Fermi edge, which was used as a starting point for further selection of suitable MOFs. As a result, we demonstrated that the tailoring of MOF electronic properties could be performed as a function of metal node engineering, framework topology, and/or the presence of unsaturated metal sites while preserving framework porosity and structural integrity. These studies unveil the possible pathways for transforming the electronic properties of MOFs from insulating to semiconducting, as well as provide a blueprint for the development of hybrid porous materials with desirable electronic structures.

We report the first study of a gas‐phase reaction catalyzed by highly dispersed sites at the metal nodes of a crystalline metal–organic framework (MOF). Specifically, CuRhBTC (BTC3−=benzenetricarboxylate) exhibited hydrogenation activity, while other isostructural monometallic and bimetallic MOFs did not. Our multi‐technique characterization identifies the oxidation state of Rh in CuRhBTC as +2, which is a Rh oxidation state that has not previously been observed for crystalline MOF metal nodes. These Rh2+ sites are active for the catalytic hydrogenation of propylene to propane at room temperature, and the MOF structure stabilizes the Rh2+ oxidation state under reaction conditions. Density functional theory calculations suggest a mechanism in which hydrogen dissociation and propylene adsorption occur at the Rh2+ sites. The ability to tailor the geometry and ensemble size of the metal nodes in MOFs allows for unprecedented control of the active sites and could lead to significant advances in rational catalyst design.

Pt–Re clusters supported on titania have shown promise as catalysts for the low temperature water–gas shift reaction. However, the enhanced activity of the bimetallic Pt–Re catalyst versus pure Pt is not well understood. In this work, exclusively bimetallic clusters were grown on TiO2(110) by vapor-deposition of Pt on 2 ML Re clusters and Re on 2 ML Pt clusters. Temperature programmed desorption experiments with CO were used to determine the concentration of Re at the surface, given that CO dissociates on Re but not on Pt. Deposition of 2 ML Pt on 2 ML Re resulted in Re core–Pt shell structures, whereas deposition of low coverages (<0.5 ML) of Re on 2 ML Pt resulted in complete diffusion of Re into the Pt clusters. Both of these Pt on Re bimetallic clusters are thermodynamically favored by the lower surface free energy of Pt compared to Re, and both are also more active than pure Pt clusters in the WGS reaction. Postreaction XPS experiments indicate that Re in the Pt on Re clusters is not oxidized under WGS conditions (130–190 °C). Furthermore, preoxidized Pt–Re clusters exhibit lower activity than both pure Pt and the unoxidized Pt–Re clusters, demonstrating that ReO x does not provide active sites in the WGS reaction. Density functional theory calculations show that CO binds less strongly to the Pt on Re surface alloy compared to pure Pt, and infrared absorption–reflection spectroscopy studies on a Pt–Re surface alloy confirm that the coverage of CO after WGS reaction is lower on the Pt–Re alloy surface. Thus, decreased CO poisoning on Pt–Re could explain the higher WGS activity of the bimetallic clusters.

The effect of donor (D)–acceptor (A) alignment on the materials electronic structure was probed for the first time using novel purely organic porous crystalline materials with covalently bound two‐ and three‐dimensional acceptors. The first studies towards estimation of charge transfer rates as a function of acceptor stacking are in line with the experimentally observed drastic, eight‐fold conductivity enhancement. The first evaluation of redox behavior of buckyball‐ or tetracyanoquinodimethane‐integrated crystalline was conducted. In parallel with tailoring the D‐A alignment responsible for “static” changes in materials properties, an external stimulus was applied for “dynamic” control of the electronic profiles. Overall, the presented D–A strategic design, with stimuli‐controlled electronic behavior, redox activity, and modularity could be used as a blueprint for the development of electroactive and conductive multidimensional and multifunctional crystalline porous materials.

We report the first examples of purely organic donor-acceptor materials with integrated π-bowls (πBs) that combine not only crystallinity and high surface areas but also exhibit tunable electronic properties, resulting in a four-orders-of-magnitude conductivity enhancement in comparison with the parent framework. In addition to the first report of alkyne-azide cycloaddition utilized for corannulene immobilization in the solid state, we also probed the charge transfer rate within the Marcus theory as a function of mutual πB orientation for the first time, as well as shed light on the density of states near the Fermi edge. These studies could foreshadow new avenues for πB utilization for the development of optoelectronic devices or a route for highly efficient porous electrodes.

MoS2 clusters have been grown on a TiO2(110) substrate to provide a model surface for better understanding the adsorbate interactions and chemical activity on titania-supported MoS2 clusters. Scanning tunneling microscopy experiments show that clusters with elongated shapes and flat tops are formed, and the long axes of the clusters have specific orientations with respect to the [001] direction on TiO2(110). In contrast, deposition of Mo in the absence of H2S results in a high density of smaller, round clusters that cover the majority of the surface. The morphologies of the MoS2 clusters do not change after exposure to various gases (D2, CO, O2, H2O, and methanol) in ultrahigh vacuum. However, exposure to higher pressures of O2, H2O, or methanol (10 Torr), as well as exposure to air, causes the clusters to disintegrate as Mo in the clusters becomes oxidized. Temperature-programmed desorption studies with CO on the MoS2 clusters show a distinct desorption peak at 280 K, which is not observed on metallic Mo or titania. Density functional theory calculations suggest that these new adsorption sites for CO are at the edges of the elongated MoS2 clusters, particularly along the (101̅0) edge containing sulfur vacancy sites.

Metal node engineering in combination with modularity, topological diversity, and porosity of metal–organic frameworks (MOFs) could advance energy and optoelectronic sectors.

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