Charge Transfer and Recombination Pathways through Fullerene Guests in Porphyrin-based MOFs

Porphyrin-based metal organic frameworks (MOFs) offer a unique platform for building porous donor-acceptor networks that exhibit conductive behavior and/or long-lived charge separation upon incorporation of electron acceptor guest species. Here, porphyrin-based MOFs, PCN-222(H2) and PCN-222(Zn), synthesized as nanoparticle suspensions, are successfully infiltrated with fullerene acceptor molecules, C60 or PC61BM, in both polar and non-polar solvent environments. The location and relative binding strength of these guest species are evaluated through a combination of N2 physisorption measurements, photoluminescence quenching and UV-vis absorption titration experiments. Semiempirical tight binding calculations are used to screen potential locations of the fullerene guest within the MOF pores, and hybrid DFT computed interaction energies confirm the energetically favorable positions. The fundamental photophysics of these donor-acceptor host-guest combinations are probed using ultrafast transient absorption spectroscopy. Sub-picosecond electron transfer involving initial exciplex population is observed, with slow charge recombination lifetimes on the order of ~1 ns for all systems in both dimethylformamide and 1,4-dioxane. Charge recombination occurs through population of fullerene and/or framework porphyrin triplet states depending on the porphyrin metalation status. The photophysics of the fullerene-loaded MOFs are discussed in the context of relevant porphyrin-fullerene donor-acceptor molecules to highlight the unique role of the framework environment in dictating photoinduced electron transfer and decay pathways.