Subpicosecond intersystem crossing in mono- and Di(organophosphine)gold(I) naphthalene derivatives in solution

Abstract

Femtosecond-to-microsecond broadband transient absorption experiments are reported for Cy3PAu(2-naphthyl) (1), (Cy3PAu) 2(2,6-naphthalenediyl) (2), and (Cy3PAu) 2(2,7-naphthalenediyl) (3), where Cy = cyclohexyl. Global and target analyses of the data, based on a sequential kinetic model, reveal four spectral components. These components are assigned to (1) excited state absorption (ESA) of the ligand-centered S1 state; (2) ESA of a receiver ligand-to-metal or metal-to-ligand charge transfer triplet state (τ1 ≤ 300 fs); (3) ESA of the vibrationally excited, ligand-centered T1 state (τ3 = 7-10 ps); and (4) ESA of the relaxed T1 state. Intersystem crossing (ISC) occurs in hundreds of femtoseconds, while internal conversion (IC) in the triplet manifold is slow (τ2 ≈ 2 ps). The relaxed T1 state shows biphasic decay kinetics in 2 and 3 with lifetimes of hundreds of picoseconds and hundreds of nanoseconds in air-saturated conditions, while only monophasic decay is observed in 1 under identical conditions. The primary decay pathway of the T1 state is assigned to quenching by O2, while the secondary channel is tentatively assigned to self-quenching or triplet-triplet annihilation. The ISC rate in 1 is not modulated significantly by the incorporation of a second heavy-atom group effecter. Instead, the position at which the second Au(I)-phosphine group is attached plays a noticeable role in the ISC rate, showing a 3-fold decrease in that of 2 compared to that of 3. The results challenge the conventional view that the rate of IC is larger than that of ISC, lending further support to the emerging kinetic model proposed for other transition-metal complexes. Gold(I) now joins the exclusive group of transition metals known to form organometallic complexes exhibiting excited-state nonequilibrium dynamics. © 2012 American Chemical Society.