Ultrafast Photophysics of Ni(I)-Bipyridine Halide Complexes: Spanning the Marcus Normal and Inverted Regimes.

Owing to their light-harvesting properties, nickel-bipyridine (bpy) complexes have found wide use in metallaphotoredox cross-coupling reactions. Key to these transformations are Ni(I)-bpy halide intermediates that absorb a significant fraction of light at relevant cross-coupling reaction irradiation wavelengths. Herein, we report ultrafast transient absorption (TA) spectroscopy on a library of eight Ni(I)-bpy halide complexes, the first such characterization of any Ni(I) species. The TA data reveal the formation and decay of Ni(I)-to-bpy metal-to-ligand charge transfer (MLCT) excited states (10-30 ps) whose relaxation dynamics are well described by vibronic Marcus theory, spanning the normal and inverted regions as a result of simple changes to the bpy substituents. While these lifetimes are relatively long for MLCT excited states in first-row transition metal complexes, their duration precludes excited-state bimolecular reactivity in photoredox reactions. We also present a one-step method to generate an isolable, solid-state Ni(I)-bpy halide species, which decouples light-initiated reactivity from dark, thermal cycles in catalysis.

Tracking nuclear motion in single-molecule magnets using femtosecond X-ray absorption spectroscopy.

The development of new data storage solutions is crucial for emerging digital technologies. Recently, all-optical magnetic switching has been achieved in dielectrics, proving to be faster than traditional methods. Despite this, single-molecule magnets (SMMs), which are an important class of magnetic materials due to their nanometre size, remain underexplored for ultrafast photomagnetic switching. Herein, we report femtosecond time-resolved K-edge X-ray absorption spectroscopy (TR-XAS) on a Mn(III)-based trinuclear SMM. Exploiting the elemental specificity of XAS, we directly track nuclear dynamics around the metal ions and show that the ultrafast dynamics upon excitation of a crystal-field transition are dominated by a magnetically active Jahn-Teller mode. Our results, supported by simulations, reveal minute bond length changes from 0.01 to 0.05 Å demonstrating the sensitivity of the method. These geometrical changes are discussed in terms of magneto-structural relationships and consequently our results illustrate the importance of TR-XAS for the emerging area of ultrafast molecular magnetism.