Origin of Photoluminescence of Ag25(SR)18– Nanoparticles: Ligand and Doping Effect

Recent theoretical insights into the origin of photoluminescence of thiolate-protected gold nanoclusters raise the question of whether the observed luminescence mechanism is valid for related silver nanoparticles. To this end, we perform density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations on the Ag 25 (SR) 18 – (R = H, PhMe 2 ) nanocluster, which is currently the only thiolate-protected silver cluster that has a matching analogue in gold. The geometric and electronic structural modifications of Ag 25 (SH) 18 – upon photoexcitation are found to be similar but less pronounced than those of Au 25 (SH) 18 – at the same level of theory. The Stokes shift is calculated to be 0.37 eV and the replacement of R = H model ligands by R = PhMe 2 decreases the Stokes shift in contrast to the increase in Stokes shift for aliphatic ligands previously observed in the Au 25 system. The calculated emission energy agrees well with the experimental photoluminescence energy when the typical underestimation of DFT calculations is considered. Similar to Au 25, luminescence of Ag 25 arises from a HOMO–LUMO transition where core-based superatomic P and D orbitals are involved. Heteroatom doping of Au 25 and Ag 25 clusters with Ag/Au does not affect the origin of the photoemission of these systems. However, the radiative lifetimes of the lowest singlet excited states (S 1 ) and emission energies are slightly affected.