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

• Published in: Journal of the American Chemical Society 2016-09, Vol.138 (35), p.11202-11210
• Main Authors: Weerawardene, K. L. Dimuthu M, Aikens, Christine M
• Format: Article
• Language: English
• Subjects: INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.6b05293

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.