Photoluminescence properties of four-coordinate gold(I)-phosphine complexes of the types [Au(diphos)2]PF6 and [Au2(tetraphos)2](PF6)2

Numerous reports describe the photoluminescence of two- and three-coordinate gold(I)-phosphine complexes, but emission in their analogous four-coordinate complexes is almost unknown. This work examines the luminescence of tetrahedral gold(I) complexes of the types [Au(diphos)(2)]PF6 (diphos = 1,2-bis(diphenylphosphino)ethane, 1) and [Au-2(tetraphos)(2)](PF6)(2) (tetraphos = (R*,R*)-(+/-)/(R*,S*)-1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane, (R',R')-(+/-)1(R*,S*)-2). Although nonemitting in solution, these complexes luminesce with an intense yellow color (lambda(max) 580-620 nm) at 293 K in the solid state or when immobilized as molecular dispersions within solid matrixes. The excited-state lifetimes of the emissions (tau 4.1-9.4 mus) are markedly dependent on the inter- and intramolecular phenyl-phenyl pairing interactions present. At 77 K in an ethanol glass, two transitions are observed: a minor emission at lambda(max) 415-450 nm and a major emission at lambda(max) 520-595 nm. For [Au(1)(2)]PF6, lifetimes of tau 251.0 +/- 20.5 mus were determined for the former transition and tau 14.9 +/- 4.6 mus for the latter. Density functional theory (DFT) calculations and comparative studies indicate that the former of these emissions involves triplet LMCT pi*(Ph) --> Au(d)-P(p) transitions associated with individual P-phenyl groups. The latter emissions, which are the only ones observed at 293 K, are assigned to LMCT pi*(Ph-Ph) --> Au(d)-P(p) transitions associated with excited P-phenyl dimers. Other tetrahedral gold (I)-phosphine complexes containing paired P-Ph substituents display similar emissions. The corresponding phosphine ligands, whether free, protonated, or bound to Ag(I), do not exhibit comparable emissions. Far from being rare, luminescence in four-coordinate Au(l)-phosphine complexes appears to be general when stacked P-phenyl groups are present.