Crystal-to-Crystal synthesis of photocatalytic MOFs for visible-light reductive coupling and mechanistic investigations.

: Post-modification of reticular materials with well-defined catalysts is an appealing approach to produce new catalytic functional materials with improved stability and recyclability, but also to study the catalysis phenomena in confined spaces. A promising strategy to this end is the post-functionalization of crystalline and robust metal-organic frameworks (MOFs) exploiting the potential of crystal-to-crystal transformations for further the characterisation of catalysts. In this regard, two new photocatalytic materials, MOF-520-PC1 and MOF-520-PC2, are straightforwardly obtained by the post-functionalization of MOF-520 with perylene-3-carboxylic acid (PC1) and perylene-3-butyric acid (PC2). The single crystal-to-crystal transformation yielded the X-ray diffraction structure of catalytic MOF-520-PC2. The well-defined disposition of the perylenes inside the MOF served as suitable model systems to get insights into the photophysical properties and mechanism by combining steady-state, time-resolved and transient absorption spectroscopy. The resulting materials are active organophotoredox catalysts in the reductive dimerisation of aromatic aldehydes, benzophenones, and imines, under mild reaction conditions. Moreover, MOF-520-PC2 can be applied for synthesising gram-scale quantities of products in continuous-flow conditions under steady-state light irradiation. This work provides an alternative approach for the construction of well-defined metal-free MOF based catalysts.