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Time-Resolved Spectroscopy of Photo-Induced Electron Transfer in Dinuclear and Tetranuclear Fe/Co Prussian Blue Analogues
preprintsubmitted on 30.10.2020, 17:37 and posted on 02.11.2020, 09:56 by Jennifer Zimara, Hendrik Stevens, Rainer Oswald, Serhiy Demeshko, Sebastian Dechert, Ricardo A. Mata, Franc Meyer, Dirk Schwarzer
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), [FeIILSCoIIILS]2. Temperature rise causes thermally induced CTIST towards the high temperature (HT) phase consisting of low-spin Fe(III) and high-spin Co(II), [FeIIILSCoIIHS]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 [FeIIILSCoIILS] intermediate which by spin crossover (SCO) is stabilized within 400 fs to a long-lived (>1 ns) [FeIIILSCoIIHS]. 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 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 [FeIILSCoIIIHS] species of 3.8 ps.