Femtosecond dynamics of metal-centered and ligand-to-metal charge-transfer (t2g-based) electronic excited states in various solvents: A comprehensive study of IrBr62-

The photophysical properties of intraconfigurational metal-centered (MC) and ligand-to-metal charge transfer (LMCT) states were studied in a prototype low spin heavy d(5) transition metal complex, IrBr62-. The femtosecond-to-picosecond dynamics of this complex was investigated in solutions of drastically different polarity (acetonitrile, chloroform, and water) by means of ultrafast broadband transient absorption spectroscopy. We observed that the system, when excited into the third excited [second LMCT, U-2(u)'(T-1u)] state, undergoes distortion from the Franck-Condon geometry along the t(2g) vibrational mode as a result of the Jahn-Teller effect, followed by rapid internal conversion to populate (90 fs) the second excited [first LMCT, U-2(g)'(T-1g)] state. Vibrational decoherence and vibrational relaxation (similar to 400 fs) in U-2(g)'(T-1g) precede the decay of this state via internal conversion (time constants, 2.8 and 3 ps in CH3CN and CHCl3 and 0.76 ps in water), which can also be viewed as back electron transfer and which leads into the intraconfigurational MC U-2(g)'(T-2g) state. This is the lowest-excited state, from which the system returns to the ground state. This MC state is metastable in both CH3CN and CHCl3 (lifetime, similar to 360 ps), but is quenched via OH-mediated energy transfer in aqueous environments, with the lifetime shortening up to 21 ps in aqueous solutions. The cascade relaxation mechanism is the same upon excitation into the second excited state. Excitation of IrBr62- in chloroform into higher U-2(u)'(T-2u), E-2(u)''(T-2u), and E-2(g)'(T-1g) states is observed to populate the third excited U-2(u)'(T-1u) state within 100 fs. These experiments allow us to resolve the ultrafast relaxation coordinate and emphasize that the excited-state Jahn-Teller effect is a driving force in the ultrafast dynamics, even for heavy transition metal complexes with very significant spin-orbit interactions. Published under license by AIP Publishing.