pH-Switchable Excited-State Dynamics of Iridium(III) Photosensitizers Bearing an N-Heterocyclic Imine-Functionalized...

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)₂Cl]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.