Unraveling the optoelectronic and photochemical behavior of Zn ₄O-based metal organic frameworks

Zn₄O clusters corresponding with those present in MOF-5, IRMOF-8, and IRMOF-9 have been synthesized as monounits with the respective monodentate organic linkers attached. Ultraviolet (UV) absorption analysis of synthesized clusters and metal organic frameworks (MOFs) reveals that the organic moieties in the MOFs lose their individual absorption and emission characteristics, in contrast with their subunits which retain these characteristics. Observed photoluminescence (PL) quantum yields of the MOFs are higher than those of their corresponding subunits. This is explained by the isolation of clusters in monounits, where linkers are only attached to one cluster, resulting in greater charge localization compared to MOFs. The photocatalytic activity of the clusters in the nonporous monounits crystals turned out to be higher than that of their MOF counterparts and that of zinc oxide in the oxidation of propene at room temperature under UV illumination. Clearly porosity does not offer in this case any significant advantage in contrast to charge localization at Zn₄O clusters. This observed difference in photocatalytic activity was found to be inversely proportional to the difference in PL quantum yields between monounits and MOFs, highlighting competition between decay of excited states by photon emission and electron transfer. Detection of the same products of partial oxidation with similar selectivities for the crystals of the synthesized monounits as well as for the MOFs led to the conclusion that Zn₄O clusters in the monounits are in fact photochemical equivalents of those in their corresponding MOFs.