Femtosecond XANES Study of the Light-Induced Spin Crossover Dynamics in an Iron(II) Complex

TLDR

X‑ray absorption spectroscopy is a powerful probe of molecular structure, but it has previously been too slow to track the earliest dynamics after photoexcitation. The study investigates the ultrafast formation of the lowest quintet state of aqueous iron(II) tris(bipyridine) after excitation of the singlet metal‑to‑ligand‑charge‑transfer state. This was examined using femtosecond optical pump/x‑ray probe techniques based on XANES. The quintet state is populated in about 150 fs, confirmed by its full XANES spectrum at 300 fs, resolving the long‑standing question of its population mechanism, which follows a simple 1MLCT→3MLCT→5T cascade with the 3MLCT→5T relaxation matching the iron‑nitrogen stretch vibration period.

Abstract

X-ray absorption spectroscopy is a powerful probe of molecular structure, but it has previously been too slow to track the earliest dynamics after photoexcitation. We investigated the ultrafast formation of the lowest quintet state of aqueous iron(II) tris(bipyridine) upon excitation of the singlet metal-to-ligand-charge-transfer (1MLCT) state by femtosecond optical pump/x-ray probe techniques based on x-ray absorption near-edge structure (XANES). By recording the intensity of a characteristic XANES feature as a function of laser pump/x-ray probe time delay, we find that the quintet state is populated in about 150 femtoseconds. The quintet state is further evidenced by its full XANES spectrum recorded at a 300-femtosecond time delay. These results resolve a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which we identify as a simple 1MLCT-->3MLCT-->5T cascade from the initially excited state. The time scale of the 3MLCT-->5T relaxation corresponds to the period of the iron-nitrogen stretch vibration.