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

Abstract

Temperature-dependent absorption and ultrafast luminescence dynamics of [Au25(PPh3)10(SC6)5Cl2 ]2+ (Au25-rod) was studied and compared with [Au25(SC6)18]− (Au25-sphere) and Au38(SC2Ph)24 (Au38-rod) to understand the influence of the crystal structure on the optical properties of monolayer protected gold clusters. The temperature-dependent absorption of Au25-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 Au25-sphere and Au38-rod. There is an increase in the oscillator strength with a decrease in temperature of about 40% for Au25-rod while it is nearly 250% for Au25-sphere and more than 180% for Au38-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 Au25-rod is consistent with its structure that possesses no shell-gold. Femtosecond luminescence measurements carried out on Au25-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, Au25-sphere and Au38-rod clusters show specific wavelength-dependent luminescence growth and decay, representing the relaxation of core-gold states to shell-gold states.

Electron localization in rod-shaped triicosahedral gold nanocluster

“…With temperature decreasing from 330 K down to 77 K, three features were observed: (i) both ESA and GSB peaks became sharper and additional peaks emerged, (ii) both the maxima of ESA and GSB evolve to higher energies, and (iii) the intensities of both ESA and GSB increased significantly. The steady-state absorption spectra of Au 25 (SR) 18 , Au 38 (SR) 24 , and rod-shaped Au 25 were reported to exhibit similar behaviors as the temperature decreased (47,48), which was explained by electron-phonon (e-p) coupling and lattice expansion interactions. Here, the blue shift and enhancement of ESA at low temperatures suggest that the ESA is also strongly affected by the e-p coupling effect.…”

Ultrabright Luminescence from Gold Nanoclusters: Rigidifying the Au(I)–Thiolate Shell

“…In a previous study, we have shown that the luminescence from quantum-sized gold clusters consists of two components: a weak, low quantum yield visible luminescence followed by near infrared luminescence. [28][29][30] The visible luminescence relaxes in ultrafast time scales and was assigned to core-gold luminescence. To monitor the visible luminescence dynamics of Au 22 clusters, femtosecond measurements were carried out after excitation at 400 nm and monitoring at 520 to 550 nm.…”

“…Other quantum-sized gold clusters have also shown similar ultrafast luminescence at this wavelength region, which was assigned to the relaxation of core-gold to shellgold. [28][29][30] Femtosecond luminescence anisotropy measurements have shown zero anisotropy for the visible luminescence indicating that the excitation and luminescence decay are dominantly localized on core-gold states.…”

Dependence of ultrafast dynamics in gold–silver alloy nanoclusters on the proportion of the metal content

“…The number of Ag atoms in Au NCs affects the S 1 state significantly. Moleculelike efficient internal conversion (IC) (~ 1 ps) was observed before in Au NCs accompanied by core-shell charge transfer [65,66]. Sfeir et al attributed the faster decaying component to rapid IC [67].…”