Time-Resolved Spectroscopy of Photo-Induced Electron Transfer in Dinuclear and Tetranuclear Fe/Co Prussian Blue Analogues

The dynamics of photo-driven charge transfer-induced spin transition (CTIST) in two Fe/Co Prussian Blue Analogues (PBAs) is revealed by femtosecond IR and UV/vis pump-probe spectroscopy. Depending on temperature the known tetranuclear square-type complex [Co₂Fe₂(CN)₆(tp*)₂(4,4’-dtbbpy)₄](PF₆)₂ (1) exists in two electronic states. In acetonitrile solution at <240 K the low temperature (LT) phase is prevalent consisting of low-spin Fe(II) and low-spin Co(III), [Fe^II_LSCo^III_LS]₂. Temperature rise causes thermally induced CTIST towards the high temperature (HT) phase consisting of low-spin Fe(III) and high-spin Co(II), [Fe^III_LSCo^II_HS]₂, being prevalent at >300 K. Photo-excitation into the intervalence charge transfer (IVCT) band of the LT phase at 800 nm induces electron transfer in one Fe-Co edge of PBA 1 and produces a [Fe^III_LSCo^II_LS] intermediate which by spin crossover (SCO) is stabilized within 400 fs to a long-lived (>1 ns) [Fe^III_LSCo^II_HS]. In contrast, IVCT excitation of the HT phase at 400 nm generates a [Fe^II_LSCo^III_HS] species with a lifetime of 3.6 ps. Subsequent back-electron transfer populates the vibrationally hot ground state, which thermalizes within 8 ps. The newly synthesized dinuclear PBA, [CoFe(CN)₃(tp*)(pz*₄Lut)]ClO₄ (2), provides a benchmark of the HT phase of 1, i.e. [Fe^III_LSCo^II_HS], as verified by variable temperature magnetic susceptibility measurements and ⁵⁷Fe Mößbauer spectroscopy. The photo-induced charge transfer dynamics of PBA 2 indeed is almost identical to that of the HT phase of phase of PBA 1 with a lifetime of the excited [Fe^II_LSCo^III_HS] species of 3.8 ps.