Orchestrating Anion–π + Interactions to Accelerate Metal‐Free Room‐Temperature Phosphorescence

ABSTRACT Overcoming the long‐standing trade‐off between efficiency and radiative rate in metal‐free phosphorescence remains a central challenge in molecular photophysics and is pivotal for the next‐generation optoelectronic technologies. Here, we demonstrate that tuning the strength of anion–π + interactions in pyridinium‐based contact ion pairs (CIPs) enables exceptionally fast room‐temperature phosphorescence (RTP) spanning 540 to 605 nm and, with radiative rates reaching 5.2×10 5 s −1 , places this crystalline material among the fastest known organic phosphors. By systematically tuning electron deficiency, steric confinement, and CIP organization through mono‐ and dicarboxyester‐functionalized pyridinium iodides, we achieved unusually short and highly polarized I − –π + tunable contacts that approach 3.50 Å. Structural analysis reveals not only tightly bound, sandwich‐like, and columnar‐organized ion pairs but also unexpected dynamic behavior, including reversible, thermally activated anion drift in the solid state. Collectively, these solid state features engender strongly coupled iodide → π + charge‐transfer states with enhanced spin–orbit coupling, thereby promoting ultrafast intersystem crossing. Spectroscopic and theoretical studies identify an intensity‐borrowing mechanism within the charge‐transfer manifold that underlies the accelerated phosphorescence and/or thermally stimulated delayed phosphorescence. The resulting materials show promising X‐ray scintillation performance. Our work establishes anion–π + engineering as a general and previously underexploited strategy for achieving high‐rate, metal‐free phosphorescence.