Rotor-Controlled Vibronic Coupling Enables Efficient Near-Infrared Phosphorescence in Iridium(III) Complexes for Enhanced Mitochondrial Photodynamic Therapy.

Near-infrared (NIR) phosphorescent iridium(III) complexes hold immense promise for biomedical imaging and optoelectronics but suffer from poor quantum yields due to the energy gap law, which amplifies nonradiative decay through vibrational quenching at lower energies. We introduce a molecular rotor engineering strategy that systematically modulates excited-state dynamics by varying the placement of the rotor on chromophoric and ancillary ligands. Restricting rotational motion upon aggregation or in rigid PMMA matrices dramatically suppresses nonradiative pathways, boosting quantum yields from ∼4% in solution to 65% in 10 wt% PMMA films, with further enhancements in the solid state (12.5%) and aggregates (14.5%). Structural and computational analyses reveal that molecular rotors govern excited-state behavior through configuration interaction between metal-to-ligand charge transfer (3MLCT) and ligand-centered (3LC) states, enabling dynamic population redistribution that tunes radiative and nonradiative channels without requiring discrete state conversion. This aggregation-induced phosphorescence (AIP) mechanism circumvents traditional energy gap limitations. The resulting efficient NIR emission demonstrates selective mitochondrial targeting and potent photodynamic anticancer activity, with aggregation-enhanced quantum yields proving essential for effective 1O2 generation. This rotor engineering approach establishes a versatile design framework for high-performance NIR emitters across therapeutic, imaging, and optoelectronic applications.