Water‐Stable 3D Cu ₄ I ₄ Framework With Near‐Unity Photoluminescence From Cluster‐Centered Self‐Trapped Excitons

Lead‐free alternatives to lead halide perovskites must achieve both high photoluminescence quantum yield (PLQY) and environmental stability for practical optoelectronic applications. While 0D copper halides demonstrate exceptional PLQY (>95%), their isolated clusters are vulnerable to moisture‐induced degradation. Conversely, 3D frameworks provide structural robustness but sacrifice luminescence efficiency. A water‐stable 3D Cu 4 I 4 single crystal framework (MpCuI) achieves 94% PLQY—the highest for 3D Cu 4 I 4 frameworks—through coordination‐driven assembly with 1‐methylpiperazine as a bifunctional N‐donor ligand. Cu─N coordination bonds (2.06–2.12 Å) extend Cu 4 I 4 cubane clusters into a close‐packed 3D network crystallizing in the highest‐symmetry orthorhombic space group ( Pnnn ) reported for this class. Spectroscopic and theoretical analyses reveal cluster‐centered self‐trapped exciton (CC‐STE) emission with strong exciton‐phonon coupling (Huang‐Rhys factor S = 22.54). MpCuI maintains 99.8% PLQY retention after 1‐week water immersion and demonstrates stable white‐light emission in proof‐of‐concept LED devices (CRI ∼84), establishing coordination bonding as a strategy for copper halide frameworks combining high efficiency with environmental robustness.