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Photoluminescent materials are more than just the glow-in-the-dark stars your parents glued to your bedroom ceiling as a child. Photoluminescent materials are widely used in applications ranging from medical imaging to TV screens. However, improving their efficiency remains a key challenge.

Researchers at Tohoku University, the Tokyo University of Science and the Indian Institute of Technology Madras have successfully enhanced the phosphorescence quantum yield of silver (Ag) clusters by utilizing the heavy atom effect—an approach which had not been fully explored in metal clusters until now.

The results are in Small.

Developing luminescent materials and sensitizers utilizing metal clusters is a crucial challenge in materials science. Ligand-protected metal clusters exhibit some phosphorescence at room temperature, however, more work needs to be done to discover ways to enhance their emission efficiency.

"The first clue for how to improve efficiency in phosphorescent materials was from a principle that has long been applied to organic fluorescent dyes," explains Yuichi Negishi (Tohoku University).

"By incorporating heavy elements such as iodide and bromine into the dyes, it creates a heavy atom effect that enhances phosphorescence. However, its potential in ligand-protected metal clusters was largely unexplored."

In this study, the researchers successfully demonstrated that the internal heavy atom effect, induced by an encapsulated iodide ion, plays a vital role in improving phosphorescence. The heavy atom effect facilitates a desirable process called intersystem crossing (ISC) in excited states, leading to enhanced phosphorescence.

Using precise synthetic techniques, the research team successfully designed metal clusters using Ag with either an encapsulated sulfide (S^2−) ion or iodide (I^−) anion.

Structural analysis via single-crystal X-ray diffraction confirmed the successful formation of these S@Ag₅₄ and I@Ag₅₄ clusters. Photophysical measurements revealed a dramatic increase in phosphorescence efficiency upon substitution of S^2− with I^−.

This discovery provides a new design strategy for next-generation luminescent materials and triplet sensitizers, which can be applied to areas such as bioimaging, photonic devices, and energy conversion technologies. The research team aims to further explore how encapsulated species (in this case, the added ions) influence other photophysical and electronic properties.