Enhancing Triplet Properties of Phthalocyanines with Heavy Atoms: Iodine Substitution or Palladium Complexation?

The introduction of heavy atoms onto photosensitizers is a common strategy to enhance their triplet-state properties, yet systematic comparisons between different approaches remain limited. Herein, tetra-iodine substitution and palladium complexation are comparatively evaluated as different heavy-atom strategies to improve the triplet properties and singlet oxygen generation of phthalocyanines. Combined experimental and computational approaches were used to distinguish the respective effects of peripheral tetra-iodination and Zn vs Pd metal complexation. Experimentally, both strategies resulted in significantly similarly higher singlet oxygen quantum yields measured either with the indirect/DPBF method or the direct 1O2 phosphorescence detection method, and complete fluorescence quenching with the palladium complexation. On the other hand, computational spin-orbit coupling analyses using the time-dependent density functional theory reveal that while palladium complexation provides the strongest spin-orbit coupling effects, iodination offers a more balanced improvement between triplet enhancement and experimental accessibility. These findings provide practical guidelines for the rational design of phthalocyanine-based photosensitizers for applications relying on triplet sensitization.