Photophysical properties of ruthenium(II) polypyridyl-gold(I) ethynyl dyads and triads containing mono- or diethynylphenanthroline incorporated into gold(I) triphenylphosphine organometallics

A new ruthenium(II)-gold(I) dyad, [Ru(bpy)₂{5-{(PPh ₃)-Au-C≡C}-phen}](PF₆)₂ (2), with a different substituted site compared to [Ru(bpy)₂{3-{(PPh ₃)-Au-C≡C}-phen}](PF₆)₂ (1), and a triad, [Ru(bpy)₂{3,6-bis{(PPh₃)-Au-C≡C}-phen}](PF ₆)₂ (3), with an unsymmetric diethynylphenanthroline relative to [Ru(bpy)₂{3,8-bis{(PPh₃)-Au-C≡C}-phen}] (PF₆)₂ (4) have been prepared. These four ruthenium(II)-gold(I) compounds showed typical metal-to-ligand charge-transfer (MLCT) absorption bands in the 400-550 nm region and a lowest energy π-π* absorption involved with the gold(I) perturbation in the 300-400 nm region. Broad emission bands assignable to the triplet MLCT transition were definitely observed in all compounds, indicating that the hybrid architecture constructed with Ru(II)-polypyridyl and Au(I)-ethynyl units converts the blue-green gold(I) perturbed π-π* phosphorescence into an orange MLCT-based emission. The transient absorption difference spectra of four compounds showed the difference in the electron transfer process between 2 and other compounds 1, 3, and 4 under the excited state. Ru(II)-Au(I) compounds except for 2 receive the supposed charge injection from a ruthenium center to an extended π-conjugated ethynyl-substituted phenanthroline, which contains one or two gold(I) organometallic unit(s), while 2 undergoes the electron transfer process from the ruthenium center not to the 5-ethynylphenanthroline but to one of the bipyridyl ligands under the excited state. This hypothesis is supported by the deflection of the spots of 2 and [Ru(bpy)₃](PF ₆)₂ from a linear correlation line in a plot of E(0-0) versus ΔE_1/2, which was based on the electrochemical and emission data of Ru(II)-Au(I) compounds and mononuclear ruthenium(II) polypyridyl complexes.