Efficient Red Organic Room-Temperature Phosphorescence Enabled by Fused-Cyclization-Induced Planarization for...

Long-wavelength organic room-temperature phosphorescence (RTP) holds great promise for time-resolved luminescence imaging due to its long lifetime and minimized background interference. However, achieving efficient red RTP remains challenging, mainly because of rapid nonradiative decay and insufficient triplet-state population. Here, we develop a fused-cyclization-induced planarization strategy combined with a host-guest doping approach to achieve highly efficient red RTP. Through systematic synthesis of a series of biquinoline-based guests with progressively locked conformations, we demonstrate that fused-ring formation significantly enhances molecular rigidity, suppresses nonradiative pathways, and lowers the triplet energy, leading to a remarkable increase in the phosphorescence efficiency (up to 47.53%) and lifetime (up to 702 ms). The optimized doped material exhibits a bright red emission centered at 616 nm under ambient conditions. Furthermore, the obtained phosphorescent nanoparticles show excellent dispersibility and biocompatibility, enabling high-contrast in vivo afterglow imaging with a signal-to-background ratio of 49.2. This work highlights fused-cyclization-induced planarization as an effective approach to designing efficient long-wavelength RTP materials for advanced bioimaging applications.