In Situ Confinement of 0D Halometallates Within Deep Eutectic Solvents: From Systematic Screening to Metal‐Tunable Luminescence for Anti‐Counterfeiting Eutectogels

Despite the promising microenvrionments of deep eutectic solvents (DESs), including their charge‐rich nature, tunable polarity, and microviscosity, the role of neat DESs as photonic media has remained largely underexplored, often relying on the addition of ex situ‐synthesized luminophores to produce photoluminescence (PL). Here, we report a photostable, metal ion (M n+ )‐tunable luminescent DES platform driven by the in situ formation of zero‐dimensional (0D) halometallates stabilized within a designed hydrogen bond network. Combinatorial screening of hydrogen bond acceptors and donors identifies an efficient pair for stabilizing highly emissive 0D chloroplumbate(II) complexes under ambient conditions, achieving a PL quantum yield (PLQY) of 60.2%. Moreover, this DES matrix generates versatile emission with customizable excitation and emission wavelengths, PL lifetimes, and PLQYs by accommodating other M n+ s, including the lanthanides Eu 3+ and Tb 3+ . Integrating these functional DESs into polymer networks yielded mechanically robust, luminescent eutectogels. By exploiting the independent emissive channels of these soft materials, we successfully constructed a highly multiplexed optical anti‐counterfeiting platform. This systematic work provides insights into the use of DESs as highly dynamic, functional matrices that directly govern the microenvironments for dopant coordination and speciation, leading to the discovery of emergent properties of materials.