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 pσâ†�dσ* transition. A much weaker band in the region 300–370 nm is assigned to a pσâ†�dδ* 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 3B1u[(dδ*)1(pσ)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)23+/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.