Lattice Modulation on Singlet‐Triplet Splitting of Silver Cluster Boosting Near‐Unity Photoluminescence Quantum Yield

Developing thermally activated delayed fluorescence (TADF)‐active silver clusters with near‐unity quantum efficiency is of practical importance in cutting‐edge optoelectronic devices, but remains a tremendous challenge due to the difficulty of de novo synthesis and uncertainty of properties. Herein, we demonstrate a lattice modulation on parent TADF‐activate silver cluster, acheving TADF‐driven photoluminescence quantum yield (PLQY) from 12% to near‐unity. Systematic experimental and calculated results reveal that the lattice modulation effectively lower the singlet‐triplet splitting (ΔEST) from 718 to 549 cm‐1, thereby facilitating thermally activated reverse intersystem crossing: T5→S5, leading to extremely efficient TADF by surpassing both phosphorescence and non‐radiative decay, thus boosting the near‐unity PLQY. Such high PLQY are extremely rare in the TADF‐active silver clusters and even in the whole noble‐metal clusters. This research showcases an unparalleled example of lattice modulation to realize near unity PLQY of TADF‐active silver clusters.