White Light Emission and Energy Transfer in Mixed‐Metal Europium/Terbium/Thulium Formate Metal–Organic Frameworks: Effect of Lanthanum Dilution

ABSTRACT A series of mono‐, bi‐, tri‐ and tetrametallic lanthanide formate frameworks incorporating Eu 3+ , Tb 3+ , Tm 3+ and La 3+ were synthesized under solvothermal conditions and comprehensively characterized in the solid state. The compounds [(Fmd)Ln(HCOO) 4 ] n (Ln = Tb, Eu, Tm) are isostructural and crystallize in the orthorhombic space group C222 1 , forming 3D formate‐bridged frameworks with Ln–Ln separations of 6.6–6.8 Å, favorable for interionic energy transfer. Controlled incorporation of Eu 3+ and Tb 3+ enabled continuous color tuning from green to red through an efficient Tb 3+ →Eu 3+ energy‐transfer process, reaching a maximum efficiency of 58.8% and quantum yields up to 70%. Introduction of Tm 3+ provided blue emission, enabling RGB balance and near‐white light generation in [(Fmd)Eu 0.475 Tb 0.475 Tm 0.05 (HCOO) 4 ] n , which exhibits CIE coordinates of (0.3198, 0.3027), a CCT of 6276 K and a CRI of 91. However, cross‐relaxation processes reduced the quantum yield to 4%. To suppress concentration quenching, La 3+ was introduced as a non‐emissive diluent, yielding a trigonal La(HCOO) 3 ‐based phase with improved quantum yield (26%) while preserving warm‐white emission (CIE: 0.3389, 0.3591; CCT: 5256 K; CRI: 83). These results demonstrate the potential of lanthanide formate frameworks for studying energy‐transfer phenomena and tailoring white‐light emission through compositional engineering and rare‐earth dilution strategies.