Femtosecond Extreme Ultraviolet Spectroscopy of an Iridium Photocatalyst Reveals Oxidation State and Ligand Field Specific Dynamics

Abstract

Femtosecond X-ray absorption spectroscopy at the Ir O3-edge and N6,7-edges is performed on the photocatalyst iridium(III) tris(2-phenylpyridine), Ir(III)(ppy)3 using a tabletop high-harmonic source. Extreme ultraviolet (XUV) absorption between 44 and 76 eV measures transitions from the Ir 5p3/2 and 4f5/2,7/2 core to 5d valence orbitals, and the position of these spectral features is shown to be sensitive to the oxidation state and ligand field of the metal center. Upon excitation of the singlet metal-to-ligand charge transfer (1MLCT) band at 400 nm, a shift in the spectra due to the formation of the Ir(IV) center is observed, as is the creation of a new spectral feature corresponding to transitions into the t2g hole. Vibrational cooling of the MLCT state on the 3 and 16 ps time scales is measured as changes in the intensity of the transient features. This work establishes XUV spectroscopy as a useful tool for measuring the electronic structure of third row transition metal photosensitizers and catalysts at ultrafast time scales.