De Novo Synthesis of Acridone-Based Zn-Metal–Organic Framework (Zn-MOF) as a Photocatalyst: Application for Visible Light-Mediated Oxidation of Sulfides and Enaminones

Acridone, a cyclic analogue of benzophenone that undergoes efficient intersystem crossing (ISC) to the triplet-excited state with near-unity quantum yield, was elaborated as a 3-connecting triacid linker, i.e., H₃AcTA, to develop a photocatalytic metal-organic framework (MOF) for energy transfer applications; the triacid linker inherently features concave shapes, an attribute that is important for the construction of MOFs with significant porosity. Metal ion (Zn²⁺)-assisted self-assembly of the triacid yielded a Zn-MOF, i.e., Zn-AcTA, with a solvent-accessible volume of ca. 31%. The protection of the acridone chromophore in the MOF in conjunction with a wider cross-section of its absorption in the visible region renders the MOF an excellent heterogeneous photosensitizer for singlet oxygen (¹O₂) generation by energy transfer to the ground-state triplet oxygen (³O₂). It is shown that the Zn-MOF can be applied as a photosensitizing catalyst for visible light-mediated oxidation of various sulfides to sulfoxides and enaminones to amino-esters via 1,2-acyl migration. It is further demonstrated that the photocatalyst can be easily recycled without any loss of catalytic activity and structural integrity. Based on mechanistic investigations, ¹O₂ is established as the reactive oxygen species in photocatalytic oxidation reactions. The results constitute the first demonstration of rational development of a photocatalytic MOF based on acridone for heterogeneous oxidations mediated by ¹O₂.