IDENTIFYING THE TRIPLET INTERMEDIATE STATE IN THE ULTRAFAST PHOTOINDUCED SPIN-TRANSITION OF LOW-SPIN IRON(II) COMPOUNDS USING FEMTOSECOND M2,3-EDGE ABSORPTION SPECTROSCOPY

In this work, we perform femtosecond M$_{2,3}$-edge XANES on low-spin iron complexes using a high harmonic generation (HHG) light source. Low-spin iron complexes hold promise as affordable alternatives to ruthenium-based photosensitizers. Conventional iron complexes with polypyridyl ligands are unsuitable because the excitation energy is dissipated in 200 fs by ultrafast population of the low-lying quintet metastable state. This formally $\Delta$S=2 process is conjectured to proceed through a metal-centered triplet intermediate state, but spectroscopic evidence of such an intermediate has remained elusive. Here we use femtosecond M$_{2,3}$-edge XANES to elucidate the role of metal-centered intermediate states. The transient response of Fe(phen)$_{3}$(SCN)$_{2}$ following excitation in the MLCT band clearly shows the presence of a metal-centered $^{3}$T$_{1}$ state. The transient response also shows oscillations consistent with the evolution of a vibrational wave-packet on the quintet surface. We also recorded the M$_{2,3}$-edge transient response of Fe[(CF$_{3}$)$_{2}$bpca]$_{2}$, a complex with a weaker ligand field. A comparison of the photophysical behaviors of Fe(phen)$_{3}$(SCN)$_{2}$ and Fe[(CF$_{3}$)$_{2}$bpca]$_{2}$ sheds light on the influence of crystal field strength on the relaxation mechanism of the MLCT state. Our findings add a hitherto missing piece to the existing picture of photophysics of iron complexes