Tuning the Photoinduced Magnetic Coupling in Terbium Porphyrazines: The Effect of Thiadiazole Electron-Withdrawing Groups and an Asymmetric Secondary Coordination Environment

Modulating intramolecular magnetic interactions between localized 4f electrons and delocalized photoexcited macrocyclic π-systems (J–L interaction) provides a strategy for designing magneto-optical materials. However, the effects of ligand electronic structure and asymmetric coordination environments remain poorly understood. Here, we report the synthesis and magneto-optical properties of an asymmetric terbium(III) complex featuring a thiadiazole electron-withdrawing group (EWG)-containing porphyrazine ligand and an acetylacetonate ligand, [TbTTDPz(acac)(DMF)(H2O)]. Variable-temperature, variable-field magnetic circular dichroism (VTVH-MCD) spectroscopy at the Q-band confirms parallel-type J–L coupling. A comparative analysis with the symmetric [TbPc(cyclen)]Cl analogue suggests that the asymmetric environment contributes to reduced ligand-centered orbital angular momentum (Lz(π) = 1.5), while the thiadiazole substitution enhances the J–L interaction (ΔJL = 1.5 cm–1). Multiconfigurational and DFT calculations demonstrate that explicitly including acac π orbitals in the active space is essential for accurately evaluating Lz(π) and ΔJL, as its highest occupied orbital resides in close energetic proximity to the TTDPz HOMO, facilitating hybridization. Spatial distribution analyses reveal that the thiadiazole EWG creates an electropositive core, forcing the excited-state LUMOs to contract inward. This maximizes spatial overlap with the lanthanide center to enhance the J–L interaction. Thus, modifying the primary π-framework and the asymmetric secondary coordination environment enables systematic tuning of J–L interactions.