Temperature-Dependent Absorption and Ultrafast Luminescence Dynamics of Bi-Icosahedral Au₂₅ Clusters

Temperature-dependent absorption and ultrafast luminescence dynamics of [Au 25 (PPh 3 ) 10 (SC 6 ) 5 Cl 2 ] 2+ (Au 25 -rod) was studied and compared with [Au 25 (SC 6 ) 18 ] − (Au 25 -sphere) and Au 38 (SC 2 Ph) 24 (Au 38 -rod) to understand the influence of the crystal structure on the optical properties of monolayer protected gold clusters. The temperature-dependent absorption of Au 25 -rod shows a shift in the absorption maximum to high energies and a small increase in the oscillator strength with decrease in temperature. The energy shift was modeled via the O’Donnell and Chen relationship, which yielded average phonon energy of 160 ± 80 cm –1, quite smaller than the 350 cm –1 observed for Au 25 -sphere and Au 38 -rod. There is an increase in the oscillator strength with a decrease in temperature of about 40% for Au 25 -rod while it is nearly 250% for Au 25 -sphere and more than 180% for Au 38 -rod. The oscillator strength increase is attributed to the coupling of core-gold exciton and shell-gold phonons. The smaller increase in the oscillator strength for Au 25 -rod is consistent with its structure that possesses no shell-gold. Femtosecond luminescence measurements carried out on Au 25 -rod clusters show wavelength-independent ultrafast luminescence decay traces. The lifetimes from the analysis are consistent with the relaxation of higher energy states. In contrast, Au 25 -sphere and Au 38 -rod clusters show specific wavelength-dependent luminescence growth and decay, representing the relaxation of core-gold states to shell-gold states.