Photophysical Tuning of Organic Ionic Crystals from Ultralong Afterglow to Highly Efficient Phosphorescence by Variation of Halides

Manipulation of photophysical properties of pure organic materials via simple alteration is attractive but extremely challenging because of the lack of valid design strategies for achieving ultralong afterglow or efficient room-temperature phosphorescence. Herein, we report a first photophysical manipulation of organic ionic crystals from ultralong afterglow to highly efficient phosphorescence by variation of halides in the crystals. Crystal structural analysis reveals ultralong organic afterglow of tetraphenylphosphonium chloride is promoted by strong intermolecular electronic coupling in the crystal, and theoretical analysis demonstrates that the tremendous boost of the phosphorescence of tetraphenylphosphonium iodide is caused by the coupling effects of significant heavy atom effect from iodine atoms and a small energy difference between the first singlet and triplet states. This work contributes to regulating long-lived emissive behaviors of pure organic ionic crystals in a controlled way and will promote the development of optical switches controlled by external stimuli.