Theoretical Insights into Excitation-Induced Oxygen Activation on a Tetrahedral Ag8 Cluster.

Research on small-molecule dissociation on plasmonic silver nanoparticles is on the rise. Herein, we investigate the effect of various parameters of light, i.e., field strength, polarization direction, and energy of oscillation, on the dynamics of oxygen upon photoexcitation of the O2@Ag8 composite using real-time time-dependent density functional theory calculations with Ehrenfest dynamics. From our excited-state dynamics calculations, we found that increasing the strength of the external electric field brings a significant contribution to the O-O dissociation. In addition, the polarization direction of the incident light becomes important, especially at weaker field strengths. The light that is polarized along the direction of charge transfer from the metal to adsorbate and the light that is polarized along the long axis of molecular oxygen were found to enhance the bond breaking of O2 significantly. We also found that at the weakest electric field strength, the oxygen molecule stays adsorbed to the silver cluster when the incident light resonates with low-energy excited states and desorbs away from the metal cluster with high-energy excitations. With strong electric fields, oxygen either desorbs or dissociates.

Ultrafast Nonradiative Decay of a Dipolar Plasmon-like State in Naphthalene.

Motivated by the uncertainty in our understanding of ultrafast plasmon decay mechanisms, we examine the effect of nuclear vibrations on the dynamical behavior of the strong plasmon-like dipole response of naphthalene, known as the ß peak. The real-time time-dependent density functional (RT-TDDFT) method coupled with Ehrenfest molecular dynamics is used to describe the interconnected nuclear and electronic motion. Several vibrational modes promote drastic plasmon decay in naphthalene. The most astonishing finding of this study is that activation of one particular vibrational mode (corresponding to the B1u representation in D2h point group symmetry) leads to a continuous drop of the dipole response corresponding to the ß peak into a totally symmetric, dark, quadrupolar electronic state. A second B1u mode provokes the sharp plasmon-like peak to split due to the breaking of structural symmetry. Nonadiabatic coupling between a B2g vibrational mode and the ß peak (a B1u electronic state) gives rise to a B3u vibronic state, which can be identified as one of the p-band peaks that reside close in energy to the ß peak energy. Overall, strong nonadiabatic coupling initiates plasmon decay into nearby electronic states in acenes, most importantly into dark states. These findings expand our knowledge about possible plasmon decay processes and pave the way for achieving high optical performance in acene-based materials such as graphene.

Geometrical and Electronic Structure, Stability, and Optical Absorption Spectra Comparisons between Thiolate- and Chloride-Stabilized Gold Nanoclusters.

Thiolate-stabilized gold nanoclusters have drawn significant attention for their extraordinary properties and their applications in many fields such as catalysis, sensing, biomedicine, etc. However, due to the size, complexity, and conformational flexibility of thiolate ligands, accurate structure prediction can be a challenge using computational approaches. Substitution of thiolate ligands with chloride ligands provides a possible alternative. In this work, the stabilities of a series of gold thiolate and chloride clusters with 1:1 stoichiometry (AunLn; L = SH, Cl; n = 2-9) and the analogs of some experimentally observed gold nanoclusters (AunLm; L = SH, Cl; n = 18-133) are examined, and binding energies, HOMO-LUMO gaps, and absorption spectra are determined using density functional theory (DFT). We observed that the optimized geometries of gold nanoclusters for both types of ligands converged to the same local minimum structure as the experimentally observed structures. The average binding energy per gold atom in gold clusters converges after Au4L4. The binding energies of chloride-stabilized gold clusters and nanoclusters average 87.5% and 95.7% of the binding energies of thiolate-stabilized systems for the clusters and nanoclusters, respectively. Typically, thiolates are found to be more stable than the chlorides. However, higher HOMO-LUMO gaps in Au2Cl2, Au38Cl24, and Au102Cl44 compared to their thiolate analogs suggest systems of particular interest for investigating the possible existence of chloride-based gold nanoclusters. Absorption spectra are very similar regardless of the ligand used. This study also demonstrates that in theoretical studies on large nanoclusters, complex thiolate ligands can be replaced by Cl ligands to predict structural and electronic properties with reasonable accuracy and reduced computational effort.