Structural and Spin–Orbit Coupling Synergy Enables Tunable Afterglow in Cytosine‐Based Hybrid Materials

ABSTRACT Developing room‐temperature phosphorescent (RTP) materials with time‐dependent optical responses is of great interest for advanced information security applications. Herein, zinc‐based organic–inorganic metal halides, (Cyt) 2 ZnX 4 ·H 2 O (X = Cl, Br), were synthesized using cytosine as the organic cation and found to exhibit markedly different afterglow behaviors. Notably, (Cyt) 2 ZnCl 4 ·H 2 O shows a prolonged RTP lifetime up to 200 ms, substantially longer than that of the Cyt(Cl) system (78 ms), whereas no visible afterglow is observed for the Br analogue. Combined spectroscopic and theoretical studies reveal that the halide‐dependent RTP behavior is governed by the competition between triplet‐state population and nonradiative decay. Although (Cyt) 2 ZnBr 4 ·H 2 O exhibits stronger spin–orbit coupling and a smaller S 1 –T 1 energy gap, both of which favor intersystem crossing, its nonradiative decay rate (165.18 s −1 ) is nearly 50 times higher than that of (Cyt) 2 ZnCl 4 ·H 2 O (3.26 s −1 ). This accelerated triplet‐state deactivation suppresses observable phosphorescence. In contrast, the chloride‐based framework achieves a more favorable balance between efficient triplet‐state generation and reduced nonradiative losses, enabling persistent RTP emission. These findings provide quantitative insight into halide‐regulated triplet‐state dynamics and offer design principles for persistent phosphorescent materials and time‐resolved optical security technologies.