Luminescence metal nanoclusters (NCs), such as gold, copper and silver, exhibit a strong photoluminescence with unusual properties and, thus, have long been the focus of intense research. Nonetheless, the origin of their photoluminescence is still unclear and the subject of scientific debates. By judiciously manipulating the delicate balance between the nature of Ag metal centers, carboxylate ligands, and coordination modes, we herein report the remarkable solvent-stimulated luminescence enhancement of the core-shell structured Ag NCs. Emission studies in the mixed DMSO/water solution show that the Ag NCs exhibit dual emission at room temperature, with a short-lived (τ~ 1.0 ns) and low quantum yield (φ~1%) emission at 460 nm, and a long-lived component at 530 nm (τ~ 80 µs and φ~40%), in which the latter band correlated well to the solvent-triggered emission. The strong phosphorescence at the lower-energy band is originated from the triplet carboxylate ligand-centered transitions (³n→π* from carbonyl) together with a heavy silver atom contribution due to a strong surface coordination interaction between the carboxylate ligand and low-coordinated Ag atoms on the core of Ag NCs. We suggest that the unusually short C=O…Ag surface interaction leads to a combination of effects that promote triplet emission. This short bond may also prevent any vibrational freedom of the carboxylate groups by packing it tightly induced by solvent effect, preventing loss of the triplet to heat, and ultimately leading to an unprecedently high room-temperature phosphorescent quantum yield of metal NCs (up to 40%). The custom-tailored emitting channel conversion from singlet to triplet excited state was first achieved at the molecule level, which constitutes a step further into the understanding of the mechanism involved in the stimuli-responsive luminescent properties of metal-based NCs.

Fig. 1: Illustration of dual Luminescence of a core-shell structured Ag nanoclusters induced by delicate structure and configuration change of surface carboxylate ligand at nanoscale interfaces

REFERENCES,

[1] Chen, Y.; Yang, T.; Pan, H.; Yuan, Y.; Chen, L.; Liu, M.; Zhang, K.; Zhang, S.; Wu, P.; Xu, J. J. Am. Chem. Soc. 2014, 136, 1686.