Node-Dependent Photoinduced Electron Transfer in Third-Generation 2D MOFs Containing Earth-Abundant Metal Ions

Abstract

Five isostructural 2D metal–organic frameworks (MOFs), based on a photoactive CuI metallolinker and mixed mono-/dinuclear secondary building units (SBUs), are reported. The MOFs 1(M) (M = Mn, Co, Cu, Zn, and Cd) exhibit broad absorption across the visible-light spectrum and emission centered at ca. 730 nm. Upon photoexcitation, the rigidity of the framework hinders the pseudo-Jahn–Teller distortion of the metallolinker’s excited state, providing efficient intersystem crossing into the triplet state. Rapid luminescence quenching in 1(Cu) and 1(Co) suggests photoinduced electron transfer (PET) to the SBUs, whereas lifetimes of up to 22.2 ns are observed in 1(Zn). The quantum yields relative to the parent photosensitizer (PS) decrease for metal nodes containing transition metal ions with partially occupied d-orbitals but increase for the d10 systems CdII and ZnII by a factor of up to 6. Importantly, the excited state decay rates directly correlate with the occupancy of the [MII(OH2)]x moieties in the MOFs providing nonradiative decay pathways via O–H oscillators. Cyclovoltammetry reveals minor changes in CuI/II oxidation potential, with excited-state reduction potentials for 1(M) rivalling Ru analogues. These results establish bis(diimine)copper(I) photosensitizers as viable metallolinkers for MOFs and present a rare example of an isostructural series obtained from a photosensitive metallolinker.