Dark Triplet State Activation to Construct Near-Infrared Host–Guest Organic Room Temperature Phosphorescence Materials for In Vivo Bioimaging
Organic room temperature phosphorescence (RTP) materials, particularly those emitting in the near-infrared (NIR) region, hold great promise for bioimaging due to their deep-tissue penetration and minimal autofluorescence interference. However, achieving efficient NIR RTP with long lifetimes remains challenging due to inefficient triplet exciton utilization. Herein, we propose a dark triplet state activation strategy to achieve efficient NIR RTP by leveraging host–guest energy transfer. Using benzophenone derivatives (BP, OBP, MBP, PBP) as rigid host matrices with high intersystem crossing (ISC) efficiency and an NIR fluorophore (MPTCF) as the guest, we achieve efficient Dexter-type triplet-triplet energy transfer (TTET) that converts non-emissive host triplets into guest-centered NIR phosphorescence. Systematic optimization of the host–guest system has shown that PBP/MPTCF exhibits exceptional performance, including long phosphorescence centered at 705 nm, an ultralong phosphorescence lifetime (210.3 ms), and high ISC efficiency (44.4%). When fabricated into nanoparticles (NPs), PBP/MPTCF exhibits superior performance, featuring prolonged phosphorescence signals (>120 s), deep tissue penetration capability (>2 mm), and excellent biocompatibility (cell viability >95% at 300 μM). In addition, this system enables high-contrast subcutaneous imaging with excellent dispersibility and stable in vivo imaging capability. More importantly, PBP/MPTCF NPs demonstrate precise lymph node mapping through time-gated phosphorescence imaging and efficient tumor visualization within 4 h post-injection with a high tumor-to-liver ratio of 2.8. The successful activation of dark triplet states through this host–guest approach provides a general design principle for developing high-performance NIR RTP materials, while the demonstrated biomedical applications highlight their significant potential for advanced bioimaging and precision diagnostics.