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

A series of mono-, bi-, tri- and tetrametallic lanthanide formate frameworks incorporating Eu3+, Tb3+, Tm3+ and La3+ 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 C2221, forming 3D formate-bridged frameworks with Ln–Ln separations of 6.6–6.8 Å, favorable for interionic energy transfer. Controlled incorporation of Eu3+ and Tb3+ enabled continuous color tuning from green to red through an efficient Tb3+→Eu3+ energy-transfer process, reaching a maximum efficiency of 58.8% and quantum yields up to 70%. Introduction of Tm3+ provided blue emission, enabling RGB balance and near-white light generation in [(Fmd)Eu0.475Tb0.475Tm0.05(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, La3+ 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.