Light-Harvesting “Antenna” Behavior in NU-1000

Structural or conceptual synthetic analogues of natural photosynthetic light-harvesting complexes hold promise as entities capable of both efficient collection of visible-region solar photons and rapid and efficient delivery of stored solar energy to energy-converting electrodes or chemical catalysts. Porous, crystalline metal–organic frameworks (MOFs) are promising supramolecular scaffolds for this purpose. Here, we report on the "antenna behavior" of chromophore assemblies comprising aligned organic linkers within a zirconium-based MOF, NU-1000. The behavior was probed primarily via amplified emission quenching using either an electron donor (ferrocene carboxylate) or an electron acceptor (3,5-dinitrobenzoate) as a redox quencher. We used solvent-assisted ligand incorporation (SALI) as a versatile means of siting/immobilizing desired quencher molecules directly within the extended chromophoric structures. We find that a photogenerated molecular exciton (spanning about four linkers) can sample ∼300 chromophoric linkers—behavior which translates to a single-step energy-transfer or exciton-hopping time of a few picoseconds. These findings clearly suggest that NU-1000 and similar MOFs can offer an "antenna" of significant size and that with a suitable catalyst immobilized on the MOF node, can find application in solar energy conversion devices.