Photoinduced Charge Transfer with a Small Driving Force Facilitated by Exciplex-like Complex Formation in Metal–Organic Frameworks

Photoinduced charge transfer (PCT) is a key step in the light-harvesting (LH) process producing the redox equivalents for energy conversion. However, like traditional macromolecular donor–acceptor assemblies, most MOF-derived LH systems are designed with a large ΔG 0 to drive PCT. To emulate the functionality of the reaction center of the natural LH complex that drives PCT within a pair of identical chromophores producing charge carriers with maximum potentials, we prepared two electronically diverse carboxy-terminated zinc porphyrins, BFBP­(Zn)-COOH and TFP­(Zn)-COOH, and installed them into the hexagonal pores of NU-1000 via solvent-assisted ligand incorporation (SALI), resulting in BFBP­(Zn)@NU-1000 and TFP­(Zn)@NU-1000 compositions. Varying the number of trifluoromethyl groups at the porphyrin core, we tuned the ground-state redox potentials of the porphyrins within ca. 0.1 V relative to that of NU-1000, defining a small ΔG 0 for PCT. For BFBP­(Zn)@NU-1000, the relative ground- and excited-state redox potentials of the components facilitate an energy transfer (EnT) from NU-1000* to BFBP­(Zn), forming BFBP­(Zn)S1* which entails a long-lived charge-separated complex formed through an exciplex-like [BFBP­(Zn)S1*–TBAPy] intermediate. Various time-resolved spectroscopic data suggest that EnT from NU-1000* may not involve a fast Förster-like resonance energy transfer (FRET) but rather through a slow [NU-1000*–BFBP­(Zn)] intermediate formation. In contrast, TFP­(Zn)@NU-1000 displays an efficient EnT from NU-1000* to [TFP­(Zn)–TBAPy], a complex that formed at the ground state through electronic interaction, and thereon showed the excited-state feature of [TFP­(Zn)–TBAPy]*. The results will help to develop synthetic LHC systems that can produce long-lived photogenerated charge carriers with high potentials, i.e., high open-circuit voltage in photoelectrochemical setups.