Time-resolved spectroscopy of a photoactive dinuclear W/Ru complex: spectroscopic evidence for a metastable intermediate with side-on coordinated carbonyl ligand

Abstract

The photo-induced dynamics of a redox-active dinuclear W(II)/Ru(II) complex, [Tp*W(CO)Br(PyCCCH₂)–Ru(bpy)₂](PF₆) (2-PF₆), is revealed by femtosecond infrared and UV-vis pump-probe spectroscopy in combination with quantum chemical calculations. The use of the mononuclear tungsten alkyne complex [Tp*W(CO)Br(PyCCCH₃)] (1) as a benchmark allowed an in-depth analysis of the excited state kinetics of 2-PF₆. Excitation of the dinuclear complex at 400 nm produces predominantly a triplet metal-to-ligand charge transfer state localised at the Ru(bpy)₂ chromophore (³MLCT_bpy) with a lifetime of 6 ps. The following transformation into a tungsten-centered triplet state (³MC_W) is accompanied by significant charge transfer and rearrangement of the W–C–O geometry. Subsequent intersysten crossing back to the ground state on a timescale of 12 ps produces a vibrationally excited molecule with up to 3 quanta in the CO stretching vibration. A minor fraction of 10% of the population reacts to an intermediate exhibiting a lifetime of 140 ps and a CO stretching frequency of 1703 cm⁻¹. Our quantum chemical calculations disclose that this species corresponds to an isomer trapped in a metastable state of the S₀ potential surface, with the CO bound side-on to the W centre.