Synthesis, characterization and photophysical properties of PPh2–C2–(C6H4)n–C2–PPh2 based bimetallic Au(I) complexes

A family of the diphosphines PPh2C2(C6H4)nC2PPh2 (n = 0–3), which possess a dialkynyl-arene spacer between the phosphorus atoms, was used for the synthesis of a series of bimetallic gold(I) complexes 1–7. Unlike the corresponding polynuclear Au(I) clusters, which show unique phosphorescence, 1–7 reveal dual emissions consisting of fluorescence and phosphorescence. The results are rationalized, in a semi-quantitative manner, by the trace (1–3) to zero (4–7) contribution of MLCT varying with the number of conjugated phenylene rings. As a result, unlike typical polynuclear Au(I) clusters with 100% triplet state population, the rate constant of the S1→T1 intersystem crossing is drastically reduced to 109 s−1 (4–7)–1010 s−1 (1–3), so that the fluorescence radiative decay rate can compete or even dominates. The drastic O2 quenching of phosphorescence demonstrates the unprotected nature of the emission chromophores in 1–7, as opposed to the well protected, O2 independent phosphorescence in most multimetallic Au(I) clusters.