Lanthanide-Doped Gold Nanocluster Assemblies Exhibiting Dopant-Directed Size Evolution, Symmetry-Controlled Eu(3+) Emission, and Selective Chemical Recognition.

The controlled doping of metal ions into nanoscale assemblies offers a powerful yet underexplored route to engineer emergent optical and chemical functionality. A central challenge lies in understanding how dopant identity simultaneously governs (i) the structural evolution of size-dependent nanocluster assemblies, (ii) the symmetry-sensitive photophysics of the dopant ion, and (iii) the collective chemical reactivity of the hybrid system. Addressing these interconnected questions is essential for rationally programming multifunctional nanoscale materials. Here, we demonstrate the controlled incorporation of Eu into zinc-mediated gold nanocluster (Zn-Au NCs) assemblies and systematically elucidate the resulting structural, photophysical, and chemical evolution. Eu doping generates a dominant emission band at 570 nm while preserving the intrinsic absorption features of the Zn-Au NCs framework, confirming electronic integration without disruption of the quantum-confined metallic core. Density functional theory calculations further reveal that Eu incorporation induces local structural distortion, introduces localized Eu-derived electronic states within the HOMO-LUMO gap, and promotes charge localization through enhanced ionic Eu-O interactions. X-ray photoelectron spectroscopy verifies retention of the Eu oxidation state, indicating incorporation without redox transformation. The selective amplification of a single Eu transition reveals coordination-symmetry modulation imposed by the nanocluster environment, establishing a strategy for regulating lanthanide emission within metallic assemblies. Structurally, increasing Eu content Zn:Eu = 200:1 to 200:20 induces a systematic enlargement of the assemblies, highlighting dopant identity as a decisive factor in directing nanocluster growth. Chemically, the embedded Eu functions as a hard Lewis acidic site, enabling selective molecular recognition through hard-hard interactions and producing a ratiometric fluorescence response while maintaining stable host emission. Collectively, this work establishes lanthanide doping as a general approach to couple symmetry control, structural evolution, and chemical responsiveness in nanocluster assemblies, advancing the design of programmable hybrid photonic materials.