Systematic Understanding of Freely Optimized Excited-State Evolution in Ir(III) Photocatalysts

Polypyridyl Ir-(III) complexes have dominated the field of transition metal photocatalysis due to their diverse tunability, photostability, and microsecond excited state lifetimes. Numerous studies have addressed structure-property and structure-activity relationships of Ir-(ppy)₃ and [Ir-(ppy)₂(bpy)]⁺ parent complexes, using density functional theory to explain experimental excited-state reactivity of the photoactive triplet states, but few works characterize the relationship of the competing charge-transfer and metal-centered triplet states and fewer still explore photoactive archetypes beyond the Ir-(ppy)₃ and Ir-(ppy)₂(bpy) classes. This work employs density functional theory to characterize and quantify the excited state surfaces of six Ir-(III) parent complexes and their isomers to gain a systematic understanding of how coordination at the metal center impacts photoactivity. The photophysical impact of (1) increasing the number of Ir-N contacts with bpy-type ligands, (2) altering through-metal C-Ir-N contact orientation and (3) increasing ligand rigidity and conjugation with fused aromatic rings is explored by plotting projected potential energy surfaces along quantitative energetic and structural axes. This investigation provides a theoretical understanding to support Ir-(III) fame and versatility as a photocatalyst, providing quantum mechanical characterization of design principles that augment excited state lifetimes.