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

Weerawardene, K. L. Dimuthu M., Aikens, Christine M.
Journal of the American Chemical Society 2016 v.138 no.35 pp. 11202-11210
geometry, image analysis, ligands, nanoparticles, photoluminescence, spectroscopy
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.