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

The dynamics of the photodriven charge transfer-induced spin transition (CTIST) in two Fe/Co Prussian Blue Analogues (PBAs) are revealed by femtosecond IR and UV/vis pump-probe spectroscopy. Depending on temperature, the known tetranuclear square-type complex CoFe(CN)(tp*)(4,4'-dtbbpy) () exists in two electronic states. In acetonitrile solution, at <240 K, the low temperature () phase is prevalent consisting of low-spin Fe(II) and low-spin Co(III), [FeCo]. Temperature rise is the reason behind thermally-induced CTIST toward the high temperature () phase consisting of low-spin Fe(III) and high-spin Co(II), [FeCo], being prevalent at >300 K. Photoexcitation into the intervalence charge transfer (IVCT) band of the phase at 800 nm induces electron transfer in one Fe-Co edge of PBA and produces a [FeCo] intermediate which by spin-crossover (SCO) is stabilized within 400 fs to a long-lived (>1 ns) [FeCo] species. In contrast, IVCT excitation of the phase at 400 nm generates a [FeCo] 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)(tp*)(pz*Lut)]ClO (), provides a benchmark of the phase of , i.e., [FeCo], as verified by variable temperature magnetic susceptibility measurements and Fe Mössbauer spectroscopy. The photoinduced charge transfer dynamics of PBA indeed are almost identical to that of the phase of PBA with a lifetime of the excited [FeCo] species of 3.8 ps.