Optical and excited state properties of Au30(S-Adm)18 and Au38S2(S-Adm)20 nanoclusters: Accuracy from TD-DFT and TDDFT + TB approaches

Gold nanoclusters display intriguing photoluminescence properties that are vital for advancements in biomedical imaging and optoelectronic technologies. However, the fundamental mechanisms underlying their emission remain elusive, largely due to the complex relationship of structural and electronic factors intrinsic to the cluster architecture. Here, we investigate the role of photo-induced geometrical and electronic variations in the resulting excited and emissive states for the representative medium-sized atomically precise ligand-protected Au30(S-Adm)18 and Au38S2(S-Adm)20 clusters, by using time-dependent density functional theory (TD-DFT) and TDDFT + TB methods. Our results indicate that approximate methods, integrating a comprehensive DFT ground state description with the tight binding approximation in linear response calculations, serve as a cost-effective alternative for precise predictions of size and ligand effects on the ground and excited states, providing a foundation for extending such analyses to larger systems. The results reveal a strong dependence of optical behavior on cluster size and ligand environment, capturing key features such as Stokes shifts and singlet-triplet energy gaps (∆EST = 0.04-0.16 eV). Both clusters demonstrate efficient reverse intersystem crossing facilitated by thermally activated delayed fluorescence (TADF), attributed to the small difference between potential minima of S1 and T1 states. Furthermore, silver doping is shown to modulate excited-state lifetimes without altering the emission origin, offering precise control over photophysical characteristics. These findings are consistent with experimental observations and provide a rational strategy for the design of nanoclusters with tailored luminescence properties, relevant to applications in sensing, catalysis, and optoelectronic devices.