Transferable Semiempirical Models for Judd-Ofelt Parameters and Multiphonon Relaxation in Eu³⁺ Complexes: Toward an...

The rational design of lanthanide-based luminescent systems using purely theoretical approaches remains challenging due to the reliance of existing models on experimental data. Herein, we propose a theoretical protocol for the design of new Eu³⁺ based luminescent systems, starting from a previously synthesized and experimentally characterized complex. The approach is based on two semiempirical models: (i) a combination of the QDC and BOM models for the calculation of Judd-Ofelt intensity parameters (Ω₂ and Ω₄) and (ii) a model based on the van Dijk-Schuurmans equation and Fermi's golden rule to estimate nonradiative emission rates (A_NR) from radiative rates (A_R). The model for calculating the Judd-Ofelt intensity parameters was parameterized using the [Eu₂(Ibf)₆(bpy)₂] complex as a reference and validated with the analogous complexes [Eu₂(Ibf)₆(4,4'-dmbpy)₂] and [Eu₂(Ibf)₆(5,5'-dmbpy)₂]. In contrast, due to its mathematical structure, the model for calculating A_NR required the inclusion of all three systems in the parameterization set. The models reproduce the intensity parameters with average errors below 4% and the nonradiative rate with errors below 0.2%. The protocol was applied to eight new complexes obtained by substituting the bipyridine methyl group with NH₂, NO₂, OCH₃, and OH at the 4,4'-and 5,5'-positions. The theoretical predicted luminescent properties indicate that methoxy-substituted systems are predicted to exhibit superior performance, with predicted quantum yields of ~93.8%. This protocol provides a computationally efficient strategy for the rational design of Eu³⁺ based luminescent materials.