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.