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 [Co2Fe2(CN)6(tp*)2(4,4'-dtbbpy)4](PF6)2 (1) exists in two electronic states. In acetonitrile solution, at 300 K. Photoexcitation 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 [FeIIILSCoIILS] intermediate which by spin-crossover (SCO) is stabilized within 400 fs to a long-lived (>1 ns) [FeIIILSCoIIHS] species. In contrast, IVCT excitation of the HT phase at 400 nm generates a [FeIILSCoIIIHS] 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., [FeIIILSCoIIHS], as verified by variable temperature magnetic susceptibility measurements and 57Fe Mossbauer spectroscopy. The photoinduced charge transfer dynamics of PBA 2 indeed are almost identical to that of the HT phase of PBA 1 with a lifetime of the excited [FeIILSCoIIIHS] species of 3.8 ps.