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

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 5.2×10⁵ 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.