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

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.

Coinage Metal Superatomic Cores: Insights into Their Intrinsic Stability and Optical Properties from Relativistic DFT Calculations

“…In this concern, we aim to evaluatet he characteristic fingerprints given by the cluster cores, and the differences between the coinage metals. [72] Thus, supporting the approachtom odel the opticalspectra. For one of the most prominent ligand-protected gold clusters, namely [Au 25 (SR) 18 ] À (R = Me) containing the 8-je [Au 13 ] 5 + core, the employed level of TD-DFT theory (TZ2P-PBE/ZORA) exhibits similar results to recent calculations, [68,69,80] denoting af irst peak calculated at 1.41 eV,w ith am ainly 1P!1D transition character.…”

“…Moreover,i nt he case of [M 32 ] 14 + ,t he larger STO-QZ4P basis set (quadruple-x plus four polarization functions) was found to be necessary.V ibrational frequencies were computed to ascertain energy minima. [14,[70][71][72] Spin-orbit calculations were done at the TZ2P/PBE level by using the two-component ZORA Hamiltonian. Time-dependent DFT (TD-DFT) calculations were employed at the same level, but by using the Perdew-Burke-Ernzerhof( PBE) exchange-correlation functional [66] because of its improved performance on longrange interactions and relativelyl ow computational cost.…”

Evaluation of bonding, electron affinity, and optical properties of M@C28 (M = Zr, Hf, Th, and U): Role of d‐ and f‐orbitals in endohedral fullerenes from relativistic DFT calculations

“…For comparison, the optical properties were calculated with the PBE functional (Supporting Information) which gave similar results. Calculation of optical properties with PBE and LB94, among other GGA functional, has been shown to provide reliable results in comparison to experimental data in several cluster structures …”

“…Calculation of optical properties with PBE and LB94, among other GGA functional, has been shown to provide reliable results in comparison to experimental data in several cluster structures. [44][45][46][47][48][49] The EDA [50] on the basis of the Morokuma2Ziegler scheme, [33][34][35] was used to evaluate the nature of the endohedral bonding. The dissociation energy (D e ) can be directly related to the total interaction energy (DE tot ) by taken into account both preparation energy, from the structural desestabilization of the fullerene host, DE prep , and the instantaneous interactions between the selected fragments, DE int , which leads to the following relation:…”

[Au12(SR)6]2−, As Smaller 8‐Electron Gold Nanocluster Retaining an SP3‐Core. Evaluation of Bonding and Optical Properties from Relativistic DFT Calculations

“…In this sense, the optical absorption spectra up to 3.0 eV was simulated and compared to the prototypical 8‐ ce [Au 25 (SMe) 18 ] − cluster (Figure ). As previously shown experimentally,,,, [Au 25 (SMe) 18 ] − exhibits a first low‐energy electronic HOMO→LUMO transition, ascribed to a 1P→1D transition. The corresponding peak is found at 1.41 eV, in agreement with recent theoretical analysis .…”