Uncapping energy transfer pathways in metal-organic frameworks through heterogeneous structures.

Metal-organic frameworks (MOFs) are porous, crystalline materials known for their structural versatility and high thermal stability, making them promising candidates for light-emitting diode applications. Distinct classes of MOFs, such as multivariate (MTV)-MOFs and MOF-on-MOFs, introduce heterogeneity by incorporating multiple ligands within a single unit cell (MTV-MOFs) or by stacking different MOFs on top of each other (MOF-on-MOF). Although these strategies improve their properties, the mechanisms of energy transfer between their heterogeneous components and their effects on optical properties, such as quantum yields, remain poorly understood. In this study, we demonstrate that MOF heterostructures significantly improve quantum yield compared to single-ligand-based MOFs. Using Zr-UiO-67 as the base MOF, we generated an array of MOF-on-MOFs and analogous MTV-MOFs utilizing other Zr-MOFs constructed from 4,4'-azobenzenedicarboxylate or 4,4'-stilbenedicarboxylate linkers. Our results demonstrate that, although the emission color coordinates for the stilbene-based materials are identical, the MTV-MOFs increase the quantum yield from 8.2% to 10.2%, whereas the MOF-on-MOFs reach a quantum yield of up to 40.0%.