Triplet Excitation Energy Dynamics in Metal–Organic Frameworks

Metal–organic frameworks (MOFs) are appealing candidates for use in energy harvesting and concentrating because of their high chromophore density and structural tunability. The ability to engineer electronic excitation energy transport pathways is of particular interest for designing energy harvesting materials. In this study, theoretical analysis was performed on energy transfer in MOFs that contain light absorbing ruthenium complexes that serve as hopping intermediates for energy transfer kinetics and energy trapping osmium complexes. We find that the excitation transport kinetics is well described by a Dexter (exchange) triplet-to-triplet energy transfer mechanism with multistep incoherent exciton hopping. The modeling combines ab initio electronic structure theory with kinetic network analysis. The sensitivity of Dexter mechanism energy transfer to framework structure establishes different kinds of energy transport paths in the different structures. For example, the mixed Ru/Os MOF structures described here establish one or three-dimensional hopping networks. As such, Dexter mechanism energy harvesting materials may be amenable to designing structures that can spatially direct exciton energy along specific pathways for energy delivery to reaction centers.