Fixed distance photoinduced electron transfer between Fe and Zn porphyrins encapsulated within the Zn HKUST-1 metal organic framework

Abstract

An attractive strategy for the development of photocatalytic metal organic framework (MOF) materials is to co-encapsulate a photoactive electron donor with a catalytic electron acceptor within the MOF. Here we report the co-encapsulation of both Zn(II) tetrakis(tetra 4-sulphonatophenyl)porphyrin (Zn4SP) and Fe(III) tetrakis(tetra 4-sulphonatophenyl)porphyrin (Fe4SP) into an HKUST-1 (Zn) MOF and demonstrate photoinduced electron transfer (ET) between the co-encapsulated guest. Photo-excitation of the Zn4SP results in fixed-distance inter-molecular ET between the encapsulated ³Zn4SP and the Fe(III)4SP as evident by the reduction in the encapsulated ³Zn4SP lifetime from 890 μs (k_obs = 1.1 × 10³ s⁻¹) to 83 μs (k_obs = 1.2 × 10⁴ s⁻¹) in the presence of Fe4SP giving a k_ET ∼ 1.1 × 10⁴ s⁻¹. The data are consistent with ET taking place between encapsulated porphyrins that are two cages apart in distance with a reorganizational energy of ∼1.65 eV, β = 1.25 and ΔG° = −0.97 eV (within a semi-classical Marcus theory framework).