Spectroscopy and redox properties of the luminescent excited state of [Au₂(dppm)₂]²⁺ (dppm = Ph₂PCH₂PPh₂)

The electronic absorption spectrum of [Au₂(dppm)₂]²⁺ [dppm = bis(diphenylphosphino)methane] in acetonitrile exhibits an absorption band at 290 nm, attributable to a p_σ ← d_σ* transition. A much weaker band in the region 300-370 nm is assigned to a p_σ ← d_δ* transition. Excitation of [Au₂(dppm)₂]²⁺ in degassed acetonitrile at 310-390 nm at room temperature results in phosphorescence centred at 575 nm, which is most likely to be derived from the ³B_1u [(d_δ*)¹(p_σ)¹] state. The phosphorescence of [Au₂(dppm)₂]^2+* is found to be quenched by a number of electron acceptors and donors, as well as energy-transfer acceptors. The powerful reducing nature of [Au₂(dppm)₂]^2+* is revealed by its excited-state redox potential E°[Au₂(dppm)₂ ^3+/2+*] of -1.6(1) V vs. a saturated sodium chloride calomel electrode (s.s.c.e.) which was determined through studies of quenching by a series of pyridinium acceptors.