Electronic relaxation dynamics in [Au25(SR)18]−1 (R = CH3, C2H5, C3H7, MPA, PET) thiolate-protected nanoclusters

Abstract

We investigate the excited electron dynamics in [Au₂₅(SR)₁₈]⁻¹ (R = CH₃, C₂H₅, C₃H₇, MPA, PET) [MPA = mercaptopropanoic acid, PET = phenylethylthiol] nanoparticles to understand how different ligands affect the excited state dynamics in this system. The population dynamics of the core and higher excited states lying in the energy range 0.00–2.20 eV are studied using a surface hopping method with decoherence correction in a real-time DFT approach. All of the ligated clusters follow a similar trend in decay for the core states (S₁–S₆). The observed time constants are on the picosecond time scale (2–19 ps), which agrees with the experimental time scale, and this study confirms that the time constants observed experimentally could originate from core-to-core transitions and not from core-to-semiring transitions. In the presence of higher excited states, R = H, CH₃, C₂H₅, C₃H₇, and PET demonstrate similar relaxations trends whereas R = MPA shows slightly different relaxation of the core states due to a smaller gap between the LUMO+1 and LUMO+2 gap in its electronic structure. The S₁ (HOMO → LUMO) state gives the slowest decay in all ligated clusters, while S₇ has a relatively long decay. Furthermore, separate electron and hole relaxations were performed on the [Au₂₅(SCH₃)₁₈]⁻¹ nanocluster to understand how independent electron and hole relaxations contribute to the overall relaxation dynamics.