Photoluminescence Origin of Au38(SR)24 and Au22(SR)18 Nanoparticles: A Theoretical Perspective

Photoluminescence of metal nanoparticles has drawn considerable research interest due to their potential fundamental and industrial applications in optoelectronics and biomedicine. However, the origin and underlying mechanism of photoluminescence in these clusters still need to be explored. Herein, the geometrical and electronic structural changes upon photoexcitation in the Au38(SH)24 and Au22(SH)18 nanoclusters are discussed using time-dependent density functional theory (TD-DFT) methods. Geometric relaxations in the Au23 core of Au38(SH)24 up to a maximum of 0.05 Å lead to slight electronic structure changes in the optimized singlet excited states with different state symmetries. The observed geometric and electronic structure variations upon photoexcitation are minor compared to the previously studied Au25(SH)18– nanoparticle. These small distortions can be correlated with small Stokes shifts calculated in the range of 0.06–0.09 eV in comparison to 0.49 eV for the Au25(SH)18– nanoparticle. Compared to Au38(SH)24, the optimized first singlet and triplet excited states of Au22(SH)18 nanoparticle show larger structural flexibility in the Au7 core, which leads to significant electronic structure modifications and large Stokes shifts. These states are predicted to have microsecond-scale or longer lifetimes, in agreement with available experimental data.