• • PBP/MPTCF achieves NIR phosphorescence at 705 nm with an ultralong lifetime of 210.3 ms and ISC efficiency of 44.4%, enabling time-gated imaging with signal persistence >120 s in vivo.
• • Nanoparticles of PBP/MPTCF exhibit deep tissue penetration >2 mm and maintain cell viability >95% at 300 μM, confirming biocompatibility for clinical translation.
• • Tumor visualization is achieved within 4 h post-injection with a tumor-to-liver ratio of 2.8, demonstrating high-contrast precision diagnostics.
• • The host–guest dark triplet activation strategy converts non-emissive host triplets into guest NIR phosphorescence via Dexter-type TTET, providing a general design principle for efficient NIR RTP materials.