Probing the Fate of Lowest-Energy Near-Infrared Metal-Centered Electronic Excited States: CuCl42- and IrBr62-

Ultrafast transient absorption spectroscopy is used to investigate the radiationless relaxation dynamics of CuCl42- and IrBr62- complexes directly promoted into their lowest energy excited metal centered states upon near infrared femtosecond excitation at 2000 nm. Both the excited CuCl42- E-2 and IrBr62- 2Ug'(T-2g) states undergo internal conversion to the ground electronic states, yet with significantly different lifetimes (55 Is and 360 ps, respectively) despite the fact that the E-2 and 2Ug'(T-2g) states are separated by the same energy gap (similar to 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.