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Theoretical Insights into the Origin of Photoluminescence of Au25(SR)18– Nanoparticles

Author:
Weerawardene, K. L. Dimuthu M., Aikens, Christine M.
Source:
Journal of the American Chemical Society 2016 v.138 no.35 pp. 11202-11210
ISSN:
1520-5126
Subject:
geometry, image analysis, ligands, nanoparticles, photoluminescence, spectroscopy
Abstract:
Understanding fundamental behavior of luminescent nanomaterials upon photoexcitation is necessary to expand photocatalytic and biological imaging applications. Despite the significant amount of experimental work into the luminescence of Au₂₅(SR)₁₈– clusters, the origin of photoluminescence in these clusters still remains unclear. In this study, the geometric and electronic structural changes of the Au₂₅(SR)₁₈– (R = H, CH₃, CH₂CH₃, CH₂CH₂CH₃) nanoclusters upon photoexcitation are discussed using time-dependent density functional theory (TD-DFT) methods. Geometric relaxations in the optimized excited states of up to 0.33 Å impart remarkable effects on the energy levels of the frontier orbitals of Au₂₅(SR)₁₈– nanoclusters. This gives rise to a Stokes shift of 0.49 eV for Au₂₅(SH)₁₈– in agreement with experiments. Even larger Stokes shifts are predicted for longer ligands. Vibrational frequencies in the 75–80 cm–¹ range are calculated for the nuclear motion involved in the excited-state nuclear relaxation; this value is in excellent agreement with vibrational beating observed in time-resolved spectroscopy experiments. Several excited states around 0.8, 1.15, and 1.25 eV are calculated for the Au₂₅(SH)₁₈– nanocluster. Considering the typical underestimation of DFT excitation energies, these states are likely responsible for the emission observed experimentally in the 1.15–1.55 eV range. All excited states arise from core-based orbitals; charge-transfer states or other “semi-ring” or ligand-based states are not implicated.
Agid:
5491452