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

Temperature-dependent absorption and ultrafast luminescence dynamics of [Au-25(PPh3)(10)(SC6)(5)Cl-2](2+) (Au-25-rod) was studied and compared with [Au-25(SC6)(18)](-) (Au-25-sphere) and Au-38(SC2Ph)(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.