Spin‐Orbit Coupling of Ln ³⁺ Induces and Modulates Thermally Activated Delayed Fluorescence in Heterometallic LnAgP ₃ Clusters

The rational design of Ag(I)-based thermally activated delayed fluorescence (TADF) materials requires fundamental understanding of structure-property relationship governing their emission characteristics. In this work, coordination with Ln³⁺ ions endows the resulting compounds LnAgP₃ (Ln = Gd/Eu/Y) with pronounced photoluminescence. Temperature-dependent emission spectra and decay lifetime measurements reveal that Y³⁺ and Gd³⁺ incorporation induces distinct TADF activity in the [AgP₃] moiety. In EuAgP₃, combined the experimental results supports efficient energy transfer from [AgP₃] to Eu³⁺ via both singlet energy transfer (SET) and triplet energy transfer (TET) pathways. Modulating the Eu:Gd molar ratio within a lattice enables precise control over the TADF performance of the [AgP₃] unit. At a Eu:Gd ratio of 0.5:0.5, optimal TADF performance of the [AgP₃] moiety is observed with a larger k(S₁→S₀) value of 1.05 × 10⁷ s^-1 and a shorter TADF decay time of 6.45 µs. Theoretical calculations further reveal that the SOC of the 4f orbitals perturbs the electronic structure of [AgP₃], compressing ΔE(S₁-T₁) to <0.2 eV, which enables reverse intersystem crossing (RISC) and thus TADF. Consequently, varying the Eu:Gd ratio provides an indirect handle over the SOC-mediated Ag↔Eu energy-transfer pathway, offering an effective route to regulate the TADF performance of the [AgP₃] moiety.