Enabling Thermally Stimulated Delayed Phosphorescence in Cu(I) Cyclic Trinuclear Complexes with Near-Unity Quantum Yield

Thermally stimulated delayed phosphorescence (TSDP) emission has recently been discovered in several Au(I)/Au(III) complexes, featuring thermally enhanced emission intensities and notable quantum yields (QYs). Developing earth-abundant metal-based TSDP emitters with further increased QYs holds significant promise for practical applications. Herein, we present a halogen bonding approach to achieve TSDP emission in bromo-substituted Cu(I) cyclic trinuclear complexes (CTCs). Photophysical analysis and theoretical calculations reveal the crucial role of halogen bonding in suppressing the excited-state distortions and reducing energy differences between the first and second triplet excited states (T₁ and T₂). This enables efficient spin-allowed reverse internal conversion, leading to the TSDP behavior. Additionally, the low internal reorganization energy and rigid halogen-bonded network in bromo-substituted Cu(I) CTCs result in significantly suppressed nonradiative decay and high QYs, with one approaching near-unity. This work provides an innovative approach to extend the TSDP behavior from Au(I)/Au(III) to Cu(I) complexes with high QYs.