Spin‐Orbit Coupling‐Modulated Ultrafast Spin‐Flip of Singlet/Triplet Self‐Trapped Exciton Near‐Infrared‐II Emission in Highly Distorted Manganese Iodide Dimer for Optical Multiplexing

ABSTRACT Self‐trapped excitons (STEs) are generating significant interest due to their broadband emission and self‐absorption‐free advantages. However, achieving high‐efficiency singlet/triplet STE near‐infrared (NIR) emissive tuning remains challenging issues that originate from energy gap law and large Stokes shift. Herein, novel manganese iodide dimers have been demonstrated in CsI crystalline matrix with high photoluminescence quantum yields of 18% and 25% for singlet and triplet STE emissions up to 1200 nm, respectively, where ultrafast spin‐flip process from triplet to singlet excited states is realized via Pb 2+ ‐doping strategy. Temperature‐dependent steady‐state, electron paramagnetic resonance, femtosecond transient absorption spectroscopic techniques and theoretical calculations verify intersystem crossing, and reverse intersystem crossing (RISC) processes are governed by the interplay between spin‐orbit coupling (SOC) and Jahn–Teller (JT) effect. RISC is accelerated by enhanced SOC due to heavy‐atom effects (Pb and I), suppressed JT distortions, and reduced excited‐state structural reorganization, leading to RISC rate as fast as 6.7 × 10 11 s ‒1 , more than two‐order‐of‐magnitude enhancement before Pb doping. Moreover, a unified framework is developed including Mn 2+ ‐Mn 2+ ion pair, molecular orbital, and configurational coordinate diagram to interpret STE‐based NIR emissions in 0D systems. These findings gain deep insights into ultrafast STE dynamics for designing highly emissive NIR materials toward photonic applications.