Efficient Red Organic Room-Temperature Phosphorescence Enabled by Fused-Cyclization-Induced Planarization for Time-Resolved Luminescence Imaging

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.