“Strong Assisting Weak” Effect for Long‐Lived Room Temperature Phosphorescence in Host–Guest‐Doped Systems

Abstract Organic room temperature phosphorescence (RTP) materials, with long lifetime, large Stokes shift, and high signal‐to‐noise ratio, have great potential in numerous fields. And now, the host–guest‐doped systems have become an effective approach for facilitating RTP, regardless of the still unclear inherent mechanism. In this work, a series of host–guest‐doped RTP materials were constructed, and based on experimental results, the “strong assisting weak” effect was proposed. That is, the host with strong spin‐orbit coupling (SOC) effect can effectively enhance the phosphorescence of the guest with weak intersystem crossing (ISC), while maintaining the guest's relatively long lifetime. With 4‐dimethylaminopyridine (DMAP) as the host and naphthalene (NL) as the guest, the long RTP lifetime of 1.79 s could be achieved, which is obviously longer than 0.5 s of NL@ polymethyl methacrylate (PMMA). Triplet–triplet energy transfer (TTET) process from the dark triplet excitons of DMAP to NL was found to play the key role in triggering RTP of NL. These findings deepen our understandings of RTP mechanisms and provide practical guidance for the creation of highly efficient and long‐lived RTP materials.