Fabrizio Kevin, Lazarou Konstantinos A, Payne Lillian I, Twight Liam P, Golledge Stephen, Hendon Christopher H, Brozek Carl K
J Am Chem Soc. 2021 Aug 18;143(32):12609-12621. doi: 10.1021/jacs.1c04808. Epub 2021 Aug 9.
Titanium-based metal-organic frameworks (Ti-MOFs) have attracted intense research attention because they can store charges in the form of Ti 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, Sr, and Ba) 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 . 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.
基于钛的金属有机框架材料（Ti-MOFs）因其能以钛的形式存储电荷，并通过在MOF-液体界面发生的非均相光氧化还原反应作为光催化剂与助催化剂协同作用，而备受研究关注。电荷存储和电荷转移均取决于MOF和分子底物的氧化还原电位，但控制这些能量方面的因素尚未得到很好的理解。此外，涉及Ti-MOFs的光催化作用依赖于助催化剂而非钛的固有反应活性，部分原因是具有开放金属位点的Ti-MOFs较为罕见。在此，我们报道了一类名为MUV-10的Ti-MOFs可以通过合成修饰，引入一系列具有灵活配位环境的氧化还原惰性离子，这些离子能够控制光活性轨道的能量。安装在MOF簇中的路易斯酸性阳离子（Cd、Sr和Ba）或引入孔道中的阳离子（H、Li、Na、K）可调节MOF的电子结构和带隙。通过使用光学氧化还原指示剂，我们首次直接测量了光激发MOF的费米能级（氧化还原电位）。综合来看，这些结果解释了Ti-MOFs存储电荷的能力，并为通过静电控制实现非均相光氧化还原化学提供了设计原则。
