Red Phosphorescence at Elevated Temperatures Enabled by Dexter Energy Transfer in Polyaromatic Hydrocarbon‐Xanthone Systems

Abstract Organic materials with red persistent phosphorescence hold immense promise for biotechnology due to their excellent tissue permeability and high signal‐to‐background ratios. However, inefficient spin‐orbit coupling, high triplet susceptibility, and narrow energy gapspromoted nonradiative deactivations, pose a formidable obstacle to achieving efficient red phosphorescence. This study addresses these challenges by introducing xanthone (Xan)‐based host–guest systems. Utilizing polyaromatic hydrocarbons (PAHs) as guests, efficient red to near‐infrared (NIR) phosphorescent materials with ultralong lifetimes and high quantum yields of up to 821 ms and 2.32%, respectively, are successfully developed. Ultrafast spectroscopy and theoretical studies reveal that Dexter energy transfer (DET) is the dominant mechanism responsible for red phosphorescence. This DET process between Xan and PAHs not only effectively utilizes the dark triplet state of the Xan host but also significantly enhances the triplet generation of the PAH guests, transforming them into potent phosphorescent luminophores. Furthermore, the inherent rigidity of Xan and PAHs endows the resulting materials with excellent phosphorescence performance, even at elevated temperatures (e.g., 423 K). This strategy, proven to be general, paves the way for designing efficient red/NIR phosphorescent materials through the DET mechanism, enabling their applications in molecular imaging and advanced high‐temperature encryption.