Tuning the Photophysical Properties of Metal-Free Room Temperature Organic Phosphors via Compositional Variations in Bromobenzaldehyde/Dibromobenzene Mixed Crystals

Spurred by several recent discoveries and a broad and largely unexplored design space, purely organic phosphorescent materials are starting to garner interest for potential applications in organic optoelectronics and sensors. One particularly promising class of purely organic phosphor is the family consisting of bromobenzaldehyde emitters doped into crystals of dibromobenzene hosts. These stand out by featuring bright, robust, and color-tunable room temperature phosphorescence. However, despite these attractive qualities, the mixed crystal motif defining these materials puts complex demands on crystal packing, chemical structure, and sample preparation in ways that are not yet well understood. Here we report a detailed study on these materials to optimize emission efficiency and fine-tune color. Overall, data suggest that achieving efficient inclusion of the emitter into the host crystal is critical to optimizing quantum efficiency. Phosphorescent emission from the mixed crystals is polarized, indicating that bromobenzaldehyde guest is ordered in the dibromobenzene host crystal. While host compounds tolerate a surprising variety of emitter sizes, both oversized and undersized, maximum quantum efficiency is reached when emitters and hosts are identically sized and the former is present at 110 wt % of total solids. The optimum quantum efficiency for these systems appears, empirically, to be in the range of 4555%. To fine-tune emission color, altering the halogen substitution of the emitter molecule affords sequential 530 nm changes to emission maxima within the green region. The relatively minor impact these alterations have on the overall chemical structure affords color tuning with minimal detriment to mixing efficiency.