Identifying the Triplet Intermediate State in the Ultrafast Photoinduced Spin-Transition of Low-Spin Iron(ii) Compounds Using Femtosecond M2,3-Edge Absorption Spectroscopy

Abstract

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 ∆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.