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 LT) phase is prevalent consisting of low-spin Fe­(II) and low-spin Co­(III), [FeIILSCoIIILS]2. Temperature rise is the reason behind thermally-induced CTIST toward the high temperature (HT) phase consisting of low-spin Fe­(III) and high-spin Co­(II), [FeIIILSCoIIHS]2, being prevalent 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 Mössbauer 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.