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 [Co<sub>2</sub>Fe<sub>2</sub>(CN)<sub>6</sub>(tp*)<sub>2</sub>(4,4’-dtbbpy)<sub>4</sub>](PF<sub>6</sub>)<sub>2</sub>
(<b>1</b>) exists
in two electronic states. In acetonitrile solution at <240 K the low
temperature (<b>LT</b>) phase is prevalent
consisting of low-spin Fe(II) and low-spin Co(III), [Fe<sup>II</sup><sub>LS</sub>Co<sup>III</sup><sub>LS</sub>]<sub>2</sub>.
Temperature rise causes thermally induced CTIST towards the high temperature (<b>HT</b>) phase consisting of low-spin
Fe(III) and high-spin Co(II), [Fe<sup>III</sup><sub>LS</sub>Co<sup>II</sup><sub>HS</sub>]<sub>2</sub>,
being prevalent at >300 K. Photo-excitation into the intervalence charge
transfer (IVCT) band of the <b>LT</b> phase
at 800 nm induces electron transfer in one Fe-Co edge of PBA <b>1</b> and produces a [Fe<sup>III</sup><sub>LS</sub>Co<sup>II</sup><sub>LS</sub>]
intermediate which by spin crossover (SCO) is stabilized within 400 fs to a
long-lived (>1 ns) [Fe<sup>III</sup><sub>LS</sub>Co<sup>II</sup><sub>HS</sub>].
In contrast, IVCT excitation of the <b>HT</b>
phase at 400 nm generates a [Fe<sup>II</sup><sub>LS</sub>Co<sup>III</sup><sub>HS</sub>] 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)<sub>3</sub>(tp*)(pz*<sub>4</sub>Lut)]ClO<sub>4</sub>
(<b>2</b>), provides a benchmark of the <b>HT</b> phase of <b>1</b>, i.e. [Fe<sup>III</sup><sub>LS</sub>Co<sup>II</sup><sub>HS</sub>], as
verified by variable temperature magnetic susceptibility measurements and <sup>57</sup>Fe
Mößbauer spectroscopy. The photo-induced charge transfer dynamics of PBA <b>2</b> indeed is almost identical to that of
the <b>HT</b> phase of phase of PBA <b>1</b> with a lifetime of the excited [Fe<sup>II</sup><sub>LS</sub>Co<sup>III</sup><sub>HS</sub>]
species of 3.8 ps.
Time-Resolved Spectroscopy of Photo-Induced Electron Transfer in Dinuclear and Tetranuclear Fe/Co Prussian Blue Analogues
