Probing the Fate of Lowest-Energy Near-Infrared Metal-Centered Electronic Excited States: CuCl₄^2– and IrBr₆^2–

Ultrafast transient absorption spectroscopy is used to investigate the radiationless relaxation dynamics of CuCl₄^2– and IrBr₆^2– complexes directly promoted into their lowest-energy excited metal-centered states upon near-infrared femtosecond excitation at 2000 nm. Both the excited CuCl₄^{2– 2}E and IrBr₆^{2– 2}U_g′(T_2g) states undergo internal conversion to the ground electronic states, yet with significantly different lifetimes (55 fs and 360 ps, respectively) despite the fact that the ²E and ²U_g′(T_2g) states are separated by the same energy gap (∼5000 cm^–1) from the respective ground state. This difference likely arises from the predominance of the Jahn–Teller effect in a Cu²⁺ ion and the spin–orbit coupling effect in an Ir⁴⁺ ion. The approach documented in this work may be used for elucidating the role of low-energy metal-centered states in relaxation cascades of a number of coordination compounds, allowing for design of efficient light-triggered metal complexes.