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

Abstract

Titanium-based metal−organic frameworks (Ti-MOFs) have attracted intense research attention because they can store charges in the form of Ti 3+ 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 (Cd 2+ , Sr 2+ , and Ba 2+ ) 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.

Determining Optical Band Gaps of MOFs

“…Because of the great expense of GW-methods and because of the practical utility of estimating E opt for understanding MOF photophysical properties, we employed relatively inexpensive hybrid-GGA approaches, in particular the tightly converged HSE06/HSEsol06 functional (see Supporting Information for computational methods) because of its broad implementation in standard quantum chemical software packages. Furthermore, this level of DFT theory has been shown to yield remarkable agreement with experimental optical gaps, especially in MOFs. , …”

Highly Strained Bi‐MOF on Bismuth Oxyhalide Support with Tailored Intermediate Adsorption/Desorption Capability for Robust CO2 Photoreduction

“…Recently, porous crystalline metal‐organic frameworks (MOFs) is of considerable interest in fields of energy or charge transfer owing to their flexible coordination environments, tuneable electronic structure and robust adsorption ability towards small molecules [7–9] . MOFs could be used directly as photocatalysts once with proper band structure [10, 11] .…”

“…Recently, porous crystalline metal-organic frameworks (MOFs) is of considerable interest in fields of energy or charge transfer owing to their flexible coordination environments, tuneable electronic structure and robust adsorption ability towards small molecules. [7][8][9] MOFs could be used directly as photocatalysts once with proper band structure. [10,11] However, the category as such is of great limitation, and the indeed charge separation efficiency is far less than that of traditional photocatalysts, simultaneously with partial intrinsic functions masked or not fully integrated.…”

A Double‐Helix Metal‐Chain Metal‐Organic Framework as a High‐Output Triboelectric Nanogenerator Material for Self‐Powered Anticorrosion

“…Expectedly, ZUT-8(C = C) has the highest HOMO, so electrons have more opportunities to move from the original structure to produce CEs. [39,40] Additionally, the band gaps (E g ) of the three ZUT-8 structures were 3.05, 2.52, and 2.08 eV, respectively. Similarly, the band gaps measured via ultraviolet absorption spectroscopy conform to the rule ZUT-8(C = C) < ZUT-8 (N=N) < ZUT-8(C-C).…”