On the limit of proton-coupled electronic doping in a Ti(iv)-containing MOF

Photoinitiated Single-Crystal to Single-Crystal Redox Transformations of Titanium-Oxo Clusters

Titanium-oxo clusters can undergo photochemical reactions under UV light, resulting in the reduction of the titanium-oxo core and oxidation of surface ligands. This is an important step in photocatalytic processes in light-absorbing Ti/O-based clusters, metal-organic frameworks, and (nano)material surfaces; however, studying the direct outcome of this photochemical process is challenging due to the fragility of the immediate photoproducts. In this report, titanium-oxo clusters [TiO(OiPr)(L)]n (n = 4, L = O2PPh2, or n = 6, L = O2CCH2tBu) undergo a two-electron photoredox reaction in the single-crystal state via an irreversible single-crystal to single-crystal (SC-SC) transformation initiated by a UV laser. The process is monitored by single crystal X-ray diffraction revealing the photoreduction of the cluster with coproduction of an (oxidized) acetone ligand, which is retained in the structure as a ligand to Ti(3+). The results demonstrate that photochemistry of inorganic molecules can be studied in the single crystal phase, allowing characterization of photoproducts which are unstable in the solution phase.

Gram-scale synthesis of MIL-125 nanoparticles and their solution processability

Although metal-organic framework (MOF) photocatalysts have become ubiquitous, basic aspects of their photoredox mechanisms remain elusive. Nanosizing MOFs enables solution-state techniques to probe size-dependent properties and molecular reactivity, but few MOFs have been prepared as nanoparticles (nanoMOFs) with sufficiently small sizes. Here, we report a rapid reflux-based synthesis of the photoredox-active MOF Ti8O8(OH)4(terephthalate)6 (MIL-125) to achieve diameters below 30 nm in less than 2 hours. Whereas MOFs generally require ex situ analysis by solid-state techniques, sub-30 nm diameters ensure colloidal stability for weeks and minimal light scattering, permitting in situ analysis by solution-state methods. Optical absorption and photoluminescence spectra of free-standing colloids provide direct evidence that the photoredox chemistry of MIL-125 involves Ti3+ trapping and charge accumulation onto the Ti-oxo clusters. Solution-state potentiometry collected during the photochemical process also allows simultaneous measurement of MOF Fermi-level energies in situ. Finally, by leveraging the solution-processability of these nanoparticles, we demonstrate facile preparation of mixed-matrix membranes with high MOF loadings that retain the reversible photochromism. Taken together, these results demonstrate the feasibility of a rapid nanoMOF synthesis and fabrication of a photoactive membrane, and the fundamental insights they offer into heterogeneous photoredox chemistry.

Ligand field tuning of d-orbital energies in metal-organic framework clusters

Linker functionalization is a common route used to affect the electronic and catalytic properties of metal-organic frameworks. By either pre- or post-synthetically installing linkages with differing linker moieties the band gap, workfunction, and exciton lifetimes have been shown to be affected. One overlooked aspect of linker functionalization, however, has been the impact on the metal d-orbital energies to which they are bound. The ligand field differences should result in substantial changes in d-splitting. In this study we use density functional theory (DFT) to study the energetics of d-orbital energy tuning as a function of linker chemistry. We offer a general descriptor, linker pK_a, as a tool to predict resultant band energies in metal-organic frameworks (MOFs). Our calculations reveal that simple functionalizations can affect the band energies, of primarily metal d lineage, by up to 2 eV and illustrate the significance of this band modularity using four archetypal MOFs: UiO-66, MIL-125, ZIF-8, and MOF-5. Together, we show that linker functionalization dramatically affects d-energies in MOF clusters and highlight that linker functionalization is a useful route for fine-tuning band edges centered on the metals, rather than linkers themselves.

Tunable Band Gaps in MUV-10(M): A Family of Photoredox-Active MOFs with Earth-Abundant Open Metal Sites

Titanium-based metal-organic frameworks (Ti-MOFs) have attracted intense research attention because they can store charges in the form of Ti³⁺ and they serve as photosensitizers to cocatalysts through heterogeneous photoredox reactions at the MOF-liquid interface. Both the charge storage and charge transfer depend on the redox potentials of the MOF and the molecular substrate, but the factors controlling these energetic aspects are not well understood. Additionally, photocatalysis involving Ti-MOFs relies on cocatalysts rather than the intrinsic Ti reactivity, in part because Ti-MOFs with open metal sites are rare. Here, we report that the class of Ti-MOFs known as MUV-10 can be synthetically modified to include a range of redox-inactive ions with flexible coordination environments that control the energies of the photoactive orbitals. Lewis acidic cations installed in the MOF cluster (Cd²⁺, Sr²⁺, and Ba²⁺) or introduced to the pores (H⁺, Li⁺, Na⁺, K⁺) tune the electronic structure and band gaps of the MOFs. Through the use of optical redox indicators, we report the first direct measurement of the Fermi levels (redox potentials) of photoexcited MOFs in situ. Taken together, these results explain the ability of Ti-MOFs to store charges and provide design principles for achieving heterogeneous photoredox chemistry with electrostatic control.