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 <240 K, the low temperature (LT) phase is prevalent consisting of low-spin Fe(II) and low-spin Co(III), [(FeLsCoLsIII)-Co-II](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), [(FeLsCoHsII)-Co-III], 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 [(FeLsCoLsII)-Co-III] intermediate which by spin-crossover (SCO) is stabilized within 400 fs to a long-lived (>1 ns) [(FeLsCoHsII)-Co-III] species. In contrast, IVCT excitation of the HT phase at 400 nm generates a [(FeLsCoHsII)-Co-III] 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*(4)Lut)]ClO4 (2), provides a benchmark of the HT phase of 1, i.e., [(FeLsCoHsII)-Co-III], as verified by variable temperature magnetic susceptibility measurements and Fe-57 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 [(FeLsCoHsIII)-Co-II] species of 3.8 ps.

Automated summary

The study reveals the dynamics of photodriven charge transfer-induced spin transition in two Fe/Co Prussian Blue Analogues using femtosecond IR and UV/vis pump-probe spectroscopy. The different temperature-dependent electronic states of the complexes lead to electron transfer and spin crossover processes.