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 long-lived charge separation and transport 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 nonpolar 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 density functional theory (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 τ ∼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.