Substrate-Binding Reactions of the3[dσ*pσ] Excited State of Binuclear Gold(I) Complexes with Bridging Bis(dicyclohexylphosphino)methane Ligands: Emission and Time-Resolved Absorption Spectroscopic Studies

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Abstract
The complexes [Au2(dcpm)2]Y2 (dcpm=bis(dicyclohexylphosphino)methane; Y=ClO4− (1), PF6− (2), CF3SO3− (3), Au(CN)2− (4), Cl− (5), SCN− (6) and I− (7)) were prepared, and the structures of 1 and 4–7 were determined by X‐ray crystallography. Complexes 1–4 display intense phosphorescence with λmax at 360–368 nm in the solid state at room temperature as well as in glassy solutions at 77 K. The solid‐state emission quantum yields of the powdered samples are 0.37 (1), 0.74 (2), 0.53 (3) and 0.12 (4). Crystalline solid 5 displays both high‐energy UV (λmax=366 nm) and low‐energy visible emissions (λmax=505 nm) at room temperature, whereas either 6 or 7 shows only an intense emission with λmax at 465 or 473 nm, respectively. All the complexes in degassed acetonitrile solutions exhibit an intense phosphorescence with λmax ranging from 490 to 530 nm. The high‐energy UV emission is assigned to the intrinsic emission of the 3[dσ*pσ] excited state of [Au2(dcpm)2]2+, whereas the visible emission is attributed to the adduct formation of the triplet excited state with the solvent/counterion. The quenching rate constants of the visible emission of [Au2(dcpm)2]2+ in acetonitrile by various anions are 6.08×105 (ClO4−), 9.18×105 (PF6−), 1.55×107 (Cl−) and 4.06×109 (I−) mol−1 dm3 s−1. The triplet‐state difference absorption spectra of 1–4 in acetonitrile show an absorption band with λmax at 350 nm and a shoulder/absorption maxima at 395–420 nm; their relative intensities are dependent upon the halide ion present in solution. Substrate binding reactions of the 3[dσ*pσ] excited state with halide (X−) to give [Au2(dcpm)2X]+* would account for the lower energy absorption maxima in the triplet‐state difference absorption spectra. With iodide as the counterion, complex 7 undergoes a photoinduced electron‐transfer reaction with I− to give the radical anion I2−.

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