Ultrafast dynamics in LMCT and intraconfigurational excited states in hexahaloiridates(iv), models for heavy transition metal complexes and building blocks of quantum correlated materials.

The population and structural dynamics of IrCl is studied in acetonitrile and aqueous solutions in comparison to isoelectronic IrBr using ultrafast broadband, dispersed transient absorption, with both octahedra excited with 85 fs pulses at four different wavelengths, encompassing the first seven t-based electronic states. Ligand-to-metal charge transfer (LMCT) 420 or 490 nm excitation of IrCl into U'(T) + E''(T) states, superimposed due to Ham effect, or U'(T), respectively, leads to symmetry lowering due to Jahn-Teller effect in these excited states with the subsequent 100 fs decay into U'(T). This first LMCT state is formed vibrationally coherent in the 104 cm t (scissor) or 243 cm e (out-of-phase-stretch) Jahn-Teller modes for the respective excitation wavelength. Direct excitation into U'(T) at 600 nm and the intraconfigurational lowest excited U'(T) state at 1900 nm helped to establish that U'(T) decays via back electron transfer into U'(T) (time constants, 3.55 ps in acetonitrile and 0.9 ps in water), and the decay of U'(T) into the ground state is the rate-limiting relaxation step. The relaxation cascade of IrBr is similar with short-lived (≤100 fs) higher LMCT states, but the vibrational coherence is only observed in the Jahn-Teller t mode. Faster back electron transfer for IrBr is explained by the energy gap law. The intraconfigurational U'(T) states, which are ∼5100 cm above the ground state for both complexes, have a sub-nanosecond lifetime largely independent of the ligand nature (∼350 ps, acetonitrile).