pH‐Switchable Excited‐State Dynamics of Iridium(III) Photosensitizers Bearing an N‐Heterocyclic Imine‐Functionalized 2,2’‐Bipyridine

ABSTRACT We report the design, synthesis, and comprehensive photophysical characterization of a new class of heteroleptic Ir III complexes featuring a strongly electron‐donating 4,4′‐bis(N‐heterocyclic imine)‐2,2′‐bipyridine (bpy‐NHI) ligand. The bpy‐NHI ligand was synthesized on a multigram scale via a seven‐step sequence and fully characterized. Reaction with the dimer [Ir(ppy) 2 Cl] 2 afforded the target complex [Ir(ppy) 2 (bpy‐NHI)] + , whose electronic structure was probed by (spectro)electrochemistry as well as by steady‐state and time‐resolved spectroscopy. Compared to the parent [Ir(ppy) 2 (bpy)] + complex, the new complex exhibits a significantly altered redox profile, including a novel irreversible oxidation at 0.4 V vs Fc + /Fc and the absence of the typical bpy‐centered reduction, consistent with the strongly electron‐donating nature of the NHI substituents. The high basicity of the NHI substituents enabled modulation of the excited‐state properties via protonation, leading to unusual excited‐state dynamics where the metal‐to‐ligand charge‐transfer state switches between bpy‐ and ppy‐based character. In the protonated form, the complex is able to bind chloride ions, which was qualitatively detected by changes in the ground‐state absorption of the complex. These findings establish bpy‐NHI as a powerful and sensitive electron‐rich ligand platform for modulating the photophysical and electrochemical properties of Ir(III) complexes, opening new opportunities for the design of photosensitizers with tailored excited‐state properties.