Identification of Metal-Centered Excited States in Cr(III) Complexes with Time-Resolved L-edge X-ray Spectroscopy

Abstract

New coordination complexes of 3d metals that possess photoactive metal-centered (MC) excited states are promising targets for optical applications and photocatalysis. Ultrafast spectroscopy plays an important role in elucidating the photophysical mechanisms that underlie photochemical activity. However, it can be difficult to assign transient signals to specific electronic excited states and mechanistic information is often inferred from kinetics. Here it is demonstrated that 3d L-edge X-ray absorption spectroscopy is highly selective for MC excited states. This is accomplished by probing the ²E spin-flip excited state in Cr(acac)₃ using synchrotron-based picosecond time-resolved XAS in solution. This excited state of Cr(III) has the property that its potential is nested with the ground state, which allows for the assessment of purely electronic changes upon excited state formation. Combining the measurements with ligand field and ab initio theory shows that the observed spectral changes between the ⁴A₂ ground and ²E excited states are due to an intensity redistribution among the core-excited multiplets. Extrapolating these results to higher-lying MC excited states predicts that Cr L₃-edge XAS can distinguish two states separated by ~0.1 eV despite the L₃-edge resolution being limited by the 0.27 eV lifetime width of the 2p core-hole. This highlights the potential of L-edge XAS as a sub-natural linewidth probe of electronic state identity.