Photoinduced Dynamic Ligation in Metal-Organic Frameworks.

Metal organic frameworks (MOFs), a class of porous crystalline materials consisting of metal-based nodes and organic linkers, have emerged as a promising platform for photocatalysis due to their ultrahigh functional surface area, customizable topologies, and tunable energetics. While interesting photochemistry has been reported, the related photoinduced structural dynamics of MOFs remains unclear. The consensus is that the coordination bonds between MOF nodes and linkers are considered static during photoexcitation, while the open-metal sites on the nodes are taken as the key active sites for catalysis. In this work, through a complementary time-resolved visible and infrared (IR) spectroscopic investigation, along with computational studies, we report for the first time light-induced structural bond dissociation (COO-M) and reformation in an iron-oxo framework, MIL-101(Fe). The probed excited state displayed ligand-to-metal charge transfer (LMCT) characteristics and exhibited a ca. 30 µs lifetime. The incredibly long excited-state lifetime led us to probe potential structural rearrangements that facilitated charge separation in MIL-101(Fe). By probing the vibrational fingerprints of the carboxylate linker upon LMCT photoexcitation, we observed the reversible transition of the carboxylate-Fe bond from a bidentate bridging mode to a monodentate mode, indicating the partial dissociation of the carboxylate ligand. Importantly, the bidentate configuration is recovered on the same time scale of the excited state lifetimes as probed via visible transient absorption spectroscopy. The elucidated photoinduced configurational dynamics provides a foundation for an in-depth understanding of MOF-based photocatalytic mechanisms.

Synergistic Steric and Electronic Effects on the Photoredox Catalysis by a Multivariate Library of Titania Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) that display photoredox activity are attractive materials for sustainable photocatalysis. The ability to tune both their pore sizes and electronic structures based solely on the choice of the building blocks makes them amenable for systematic studies based on physical organic and reticular chemistry principles with high degrees of synthetic control. Here, we present a library of eleven isoreticular and multivariate (MTV) photoredox-active MOFs, UCFMOF-n, and UCFMTV-n-x% with a formula Ti6O9[links]3, where the links are linear oligo-p-arylene dicarboxylates with n number of p-arylene rings and x mol% of multivariate links containing electron-donating groups (EDGs). The average and local structures of UCFMOFs were elucidated from advanced powder X-ray diffraction (XRD) and total scattering tools, consisting of parallel arrangements of one-dimensional (1D) [Ti6O9(CO2)6]∞ nanowires connected through the oligo-arylene links with the topology of the edge-2-transitive rod-packed hex net. Preparation of an MTV library of UCFMOFs with varying link sizes and amine EDG functionalization enabled us to study both their steric (pore size) and electronic (highest occupied molecular orbital-lowest unoccupied molecular orbital, HOMO-LUMO, gap) effects on the substrate adsorption and photoredox transformation of benzyl alcohol. The observed relationship between the substrate uptake and reaction kinetics with the molecular traits of the links indicates that longer links, as well as increased EDG functionalization, exhibit impressive photocatalytic rates, outperforming MIL-125 by almost 20-fold. Our studies relating photocatalytic activity with pore size and electronic functionalization demonstrate how these are important parameters to consider when designing new MOF photocatalysts.

An Aluminum-Based Metal-Organic Cage for Cesium Capture.

Metal-organic cages are a class of supramolecular structures that often require the careful selection of organic linkers and metal nodes. Of this class, few examples of metal-organic cages exist where the nodes are composed of main group metals. Herein, we have prepared an aluminum-based metal-organic cage, H8[Al8(pdc)8(OAc)8O4] (Al-pdc-AA), using inexpensive and commercially available materials. The cage formation was achieved via solvothermal self-assembly of solvated aluminum and pyridine-dicarboxylic linkers in the presence of a capping agent, acetic acid. The obtained supramolecular structure was characterized by single-crystal X-ray diffraction (SCXRD), thermogravimetric analysis, and NMR spectroscopy. Based on crystal structure and computational analyses, the cage has a 3.7 Å diameter electron-rich cavity suitable for the binding of cations such as cesium (ionic radius of 1.69 Å). The host-guest interactions were probed with 1H and 133Cs NMR spectroscopy in DMSO, where at low concentrations, Cs+ binds to Al-pdc-AA in a 1:1 ratio. The binding site was identified from the crystal structure of CsH7[Al8(pdc)8(OAc)8O4] (Cs+⊂Al-pdc-AA), and a binding affinity of ∼106-107 M-1 was determined from NMR titration experiments. The Al-pdc-AA showed improved selectivity for cesium binding over alkali metal cations (Cs+ > Rb+ > K+ ≫ Na+ ∼ Li+). Collectively, the study reports a novel aluminum cage that can serve as a promising host for efficient and selective cesium removal.

Photoelectrochemical alcohol oxidation by mixed-linker metal-organic frameworks.

Metal-organic frameworks (MOFs) provide a suitable platform for stable and efficient heterogeneous photoelectrochemical oxidation catalysis due to their highly ordered structure, large surface area, and synthetic tunability. Herein, a mixed-linker MOF comprising of a photosensitizer [Ru(dcbpy)(bpy)2]2+ (bpy = 2,2'-bipyridine, dcbpy = 5,5'-dicarboxy-2,2'-bipyridine) and catalyst [Ru(tpy)(dcbpy)Cl]+ (tpy = 2,2':6',2''-terpyridine) that were incorporated into the UiO-67 framework and grown as thin films on a TiO2-coated, fluorine-doped tin oxide (FTO) electrode (RuB-RuTB-UiO-67/TiO2/FTO). When used as an electrode for the photoelectrochemical oxidation of benzyl alcohol, the mixed-linker MOF film showed a faradaic efficiency of 34%, corresponding to a 3-fold increase in efficiency relative to the RuB-UiO-67/TiO2/FTO control. This increase in catalytic efficiency is ascribed to the activation of RuTB moieties via oxidation by photogenerated RuIIIB. Transient absorption spectroscopy revealed the delayed appearance of RuIIITB* or RuIIITB formation, occurring with a lifetime of 21 ns, due to energy and/or electron transfer. The recovery kinetics of the charge separated state was increased (283 µs) in comparison to single-component control experiments (105 µs for RuB-UiO-67/TiO2/FTO and 7 µs for RuTB-UiO-67/TiO2/FTO) indicating a cooperative effect that could be exploited in chromophore/catalyst MOF motifs.

A Technical Guide for Performing Spectroscopic Measurements on Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) offer a unique platform to understand light-driven processes in solid-state materials, given their high structural tunability. However, the progression of MOF-based photochemistry has been hindered by the difficulty in spectrally characterizing these materials. Given that MOFs are typically larger than 100 nm in size, they are prone to excessive light scatter, thereby rendering data from valuable analytical tools like transient absorption and emission spectroscopy nearly uninterpretable. To gain meaningful insights of MOF-based photo-chemical and physical processes, special consideration must be taken toward properly preparing MOFs for spectroscopic measurements, as well as the experimental setups that garner higher quality data. With these considerations in mind, the present guide provides a general approach and set of guidelines for the spectroscopic investigation of MOFs. The guide addresses the following key topics: (1) sample preparation methods, (2) spectroscopic techniques/measurements with MOFs, (3) experimental setups, (3) control experiments, and (4) post-run stability characterization. With appropriate sample preparation and experimental approaches, pioneering advancements toward the fundamental understanding of light-MOF interactions are significantly more attainable.

Photoelectrochemical water oxidation by a MOF/semiconductor composite.

Artificial photosynthesis is one of the most promising forms of renewable fuel production, due to the abundance of water, carbon dioxide, and sunlight. However, the water oxidation reaction remains a significant bottleneck due to the high thermodynamic and kinetic requirements of the four-electron process. While significant work has been done on the development of catalysts for water splitting, many of the catalysts reported to date operate at high overpotentials or with the use of sacrificial oxidants to drive the reaction. Here, we present a catalyst embedded metal-organic framework (MOF)/semiconductor composite that performs photoelectrochemical oxidation of water at a formal underpotential. Ru-UiO-67 (where Ru stands for the water oxidation catalyst [Ru(tpy)(dcbpy)OH2]2+ (tpy = 2,2':6',2''-terpyridine, dcbpy = 5,5-dicarboxy-2,2'-bipyridine)) has been previously shown to be active for water oxidation under both chemical and electrochemical conditions, but here we demonstrate, for the first time, incorporation of a light harvesting n-type semiconductor as a base photoelectrode. Ru-UiO-67/WO3 is active for photoelectrochemical water oxidation at a thermodynamic underpotential (η ≈ 200 mV; Eonset = 600 mV vs. NHE), and incorporation of a molecular catalyst onto the oxide layer increases efficiency of charge transport and separation over bare WO3. The charge-separation process was evaluated with ultrafast transient absorption spectroscopy (ufTA) and photocurrent density measurements. These studies suggest that a key contributor to the photocatalytic process involves a hole transfer from excited to Ru-UiO-67. To our knowledge, this is the first report of a MOF-based catalyst active for water oxidation at a thermodynamic underpotential, a key step towards light-driven water oxidation.