Remarkable luminescence properties of gold and silver nanoclusters attract researchers due to potential applications in photocatalysis, optoelectronics, and especially in biomedicine such as bioimaging, sensing, and cancer therapy. Extensive experimental investigations on luminescence properties of ligand-protected gold and silver nanoclusters during the past decade have failed to unravel their exact photoluminescence mechanism. Herein, density functional and time-dependent density functional theory (DFT and TDDFT) calculations are performed to elucidate electronic-level details of several such systems upon photoexcitation. Thiolate-protected Au25–, Ag25–, Au38, Au22, and Ag444– as well as phosphine- and chloride-protected Au133+ nanoclusters are considered. Multiple excited states are found to be involved in photoemission from Au25(SR)18– nanoclusters, and their energies agree well with experimental emission energies. The Au13 core-based excitations arising due to electrons excited from superatom P orbitals into the lowest two superatom D orbitals are responsible for all of these states. The large Stokes shift is attributed to significant geometrical and electronic structure changes in the excited state. The origin of photoluminescence of Ag25(SR)18– nanocluster is analogous to its gold counterpart and heteroatom doping of each cluster with gold and silver respectively does not affect their luminescence mechanism. Other systems have been examined to determine how widespread these observations are. We observe a very small Stokes shift for Au38(SH)24 that correlates with a relatively rigid structure with small bond length changes in its Au23 core and a large Stokes shift for Au22(SH)18 with a large degree of structural flexibility in its Au7 core. This suggests a relationship between the Stokes shifts of gold−thiolate nanoparticles and their structural flexibility upon photoexcitation. To compare isoelectronic systems, excited state relaxations of the [Au13(dppe)5Cl2]3+ nanocluster are compared with those of Au25(SR)18–. Similarities in luminescence mechanism and Stokes shift are observed.

Luminescence properties of ligand-protected gold and silver nanoclusters

2019 Spring National Meeting

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