Optical Spectra of the Special Au₁₄₄ Gold-Cluster Compounds: Sensitivity to Structure and Symmetry

Abstract

This report concerns the remarkable fine structure reported recently in the optical absorption spectrum of the ubiquitous icosahedral Au144(SR)60 cluster compounds when measured under cryogenic conditions. The theoretical explanation of the spectrum relied upon an I-symmetrized variant of the conventional Pd145-type structure-model; real-time TDDFT calculations revealed that, in contradistinction to the prior state of knowledge, the spectrum is profoundly structured and rich in quantum-state information. Reported herein is an investigation of the sensitivity of the theoretical electronic absorption spectra of this compound to variations in the structure. Both I-symmetric as well as asymmetric structure-models are considered; having the same core structure and connectivity, these differ in the mutual configurations about the pyramidal S atoms, which produce significant structure differences penetrating into the gold core. As R-groups, both methyl (R=CH3) and hydrogen (R=H) are considered. The effects on the structure and spectra of local optimizations employing different exchange-correlation (xc-) functionals are also considered. The results may be summarized as follows: all computed spectra show a rich fine-structure when computed at a similar level of resolution (∼0.16 eV, transform limited); the I-symmetric structure with R=CH3 has more pronounced features than the asymmetric structure with the same rest group. This is consistent with the high degree of symmetry-imposed degeneracy in the electronic states of the former. These spectral differences between the I-symmetric and the unsymmetrical models are reduced when the CH3 R group is replaced by the smaller R=H. Many other systematic differences are noted. In particular, we show explicitly the differences caused by changing the R group, the exchange-correlation functional in the geometry relaxation, and the charge state. The present study contains clear indications as to what factors need to be well-controlled in order to achieve good agreement with available experiment. They will be useful for understanding ligand effects on the optical characteristics of thiolate-protected (and other) noble-metal clusters in this interesting size-range where the plasmon (LSPR) emerges.

Classical and ab Initio Plasmonics Meet at Sub-nanometric Noble Metal Rods

“…18,19 These advances open up a different length scale for plasmonics, where genuine quantum effects like quantization of electron energy levels, electron-density spill-out, and reduced surface d-electron screening become important. An example of the peculiar LSPR phenomenology emerging in this regime is the suppression of transverse plasmonic modes due to the quantization of electron energy levels in ultrathin Au rods.…”

Bonding and properties of superatoms. Analogs to atoms and molecules and related concepts from superatomic clusters

“…The structure and related properties of Faradaurates, that is, a gold cluster with a large nuclearity, has also been well exploited by DFT calculations. Lopez‐Acevedo and coworkers . have proven the sensitivity of the optical absorption spectrum to both structure and symmetry of the optical spectrum for the Au 144 (SR) 60 gold‐cluster, which appears to be almost unaltered when different groups are included.…”

A new time‐dependent density‐functional method for molecular plasmonics: Formalism, implementation, and the Au144(SH)60 case study

“…Another study pointed out the richness of spectral features of the experimental absorption of Au 144 (SC 6 H 13 ) 60 , modelled by the simpler Au 144 (SCH 3 ) 60 compound: a fair qualitative agreement was obtained between theory and experiment. A further systematic computational study addressed the effect of structure, symmetry, anisotropy, exchange‐correlation functional, R group, and charge state on the optical spectrum of Au 144 (SR) 60 species. Very recently new low temperature (25 K) absorption experiments have been published as well …”