A Crown Ether-Ammonium Supramolecular Platform for Luminescent [Cu₄I₆]^2- Cluster Phosphors

Control over the supramolecular environment of emissive metal halide clusters offers an attractive route to tune the photophysical behavior of hybrid phosphors. Among these systems, ionic [Cu₄I₆]^2- cluster phosphors are promising blue excitable emitters that exhibit broadband cluster-centered luminescence; however, rational strategies for modulating their local environment remain limited. Here, a supramolecular host-guest approach was employed to stabilize and tune [Cu₄I₆]^2- clusters using 18-crown-6 (18C6) in combination with three ammonium-derived cations of increasing steric and hydrophobic character (NH₄ ⁺, CH₃NH₃ ⁺ (MA), and C₈H₁₂N⁺ (PEA)). This design enabled systematic variation of the organic framework while preserving the same emissive cluster motif. Structural analysis, combined with Hirshfeld surface mapping, revealed that the cluster's structural distortion is governed by the topology and spatial distribution of intermolecular contacts arising from crystal packing. Dense and anisotropic I···H and Cu···H contacts in NH₄(18C6) and PEA-(18C6) imposed uneven constraints on the [Cu₄I₆]^2- units, increasing I-Cu-I bond angle distortion and strengthening exciton-phonon coupling, resulting in broader emission. In contrast, MA-(18C6) exhibited fewer host-cluster contacts but significant I···I intercluster interactions, leading to reduced lattice relaxation and narrower photoluminescence. Density functional theory calculations confirmed strong electronic confinement within the [Cu₄I₆]^2- clusters and revealed that variations in band dispersion arise from differences in the supramolecular environment. Among the series, PEA-(18C6) combined high PLQY (∼80%), improved environmental stability, and efficient blue-driven white light emission. These results demonstrate that supramolecular host-guest design provides a modular platform for tuning structure property relationships in copper halide cluster phosphors and for developing stable, lead free materials for solid-state lighting.