Stepwise 1D Growth of Luminescent Au(I)−Ag(I) Phosphine−Alkynyl Clusters: Synthesis, Photophysical, and Theoretical Studies

Reactions between the diphosphino-gold cationic complexes [Au2(PPh2-C2-(C6H4)n-C2-PPh2)2]2+ (n = 0, 1, 2, 3) and polymeric acetylides (AuC2Ph)n and (AgC2Ph)n lead to the formation of a new family of heterometallic clusters with the general formula [Au8+2nAg6+2n(C2Ph)8+4n(PPh2C2(C6H4)nC2PPh2)2]2+, n = 0 (1), 1 (2), 2 (3), 3 (4). Compounds 1−4 were characterized in detail by NMR and ESI-MS spectroscopy. Complex 1 (n = 0) crystallizes in two forms (orange (1a) and yellow (1b)), one of which (1a) has been analyzed by X-ray crystallography. The luminescence behavior of 1−4 has been studied. Compounds 2 and 3 exhibited orange-red phosphorescence with quantitive quantum efficiency in both aerated and degassed CH2Cl2, implying O2-independent phosphorescence due to efficient protection of the emitting chromophore center by the organic ligands. Complex 3 exhibits reasonable two-photon absorption (TPA) property with a cross section of σ ≈ 45 GM (800 nm), which is comparable to the value of commercially available TPA dyes such as coumarin 151. Computational studies have been performed to correlate the structural and photophysical features of the complexes studied. The metal-centered triplet emission within the heterometallic core is suggested to play a key role in the observed phosphorescence. The luminescence spectrum of 1 in CH2Cl2 shows dual phosphorescence maximized at 575 nm (the P1 band) and 770 nm (the P2 band). Both P1 and P2 bands possess identical excitation spectra, i.e., the same ground-state origin, and the same relaxation dynamics throughout the temperature range of 298−200 K. The dual emission of 1 arises from fast structural fluctuation upon excitation, perhaps forming two geometry isomers, which exhibit distinctly different P1 and P2 bands. The scrambling dynamics might require large-amplitude motion and, hence, is hampered in rigid media, as evidenced by the single emission for 1a (610 nm) and 1b (570 nm) observed in solid.