Host‐Guest Doping Enables Room Temperature Phosphorescence from Triarylboranes

Abstract Revisiting our previous report of RTP in triarylboranes, we found that pure crystalline triarylboranes do not exhibit RTP as initially reported, although all but one purified compound shows RTP in annealed PMMA matrices. Inspired by the impurity responsible for the previously observed RTP, we designed a family of binary host–guest RTP systems constructed by pairing distinct triarylboranes. By tuning the host and guest structures and doping ratios, afterglow visible to the naked eye for up to 8 s can be achieved in crystalline materials under ambient conditions, at doping levels as low as one part per billion. The ultra‐long afterglow makes these materials promising for information encryption, while their high sensitivity to oxygen in non‐annealed PMMA films highlights their potential for sensing applications. Trends in the phosphorescence intensities and lifetimes, dependent on guest molecular structure, are rationalized by computed triplet‐triplet energy transfer rates between the hosts and guests, providing a concrete design strategy for optimizing host‐guest RTP. Overall, this work underscores the critical importance of material purity and advances understanding of the host‐guest interactions governing efficient organic RTP.