Key Takeaways & Executive Findings
- •• • 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.
Abstract
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.
1. Introduction
Organic room temperature phosphorescence (RTP) materials have attracted significant attention for bioimaging due to their large Stokes shift, ultralong luminescence lifetime, and tunable optical properties. However, most existing RTP materials emit at wavelengths below 700 nm, which restricts tissue penetration depth due to strong photon scattering and absorption by biological tissues. Near-infrared (NIR) emission (700–900 nm) is essential for deep-tissue imaging, yet achieving efficient NIR RTP with long lifetimes remains challenging because of inefficient triplet exciton utilization. Conventional NIR fluorophores, quantum dots, and lanthanide-doped nanophosphors cannot intrinsically achieve time-gated imaging contrast, as their prompt emission is not temporally separated from autofluorescence.
This work addresses the bottleneck by proposing a dark triplet state activation strategy. Using benzophenone derivatives as rigid host matrices with high intersystem crossing efficiency and an NIR fluorophore as the guest, the system leverages Dexter-type triplet-triplet energy transfer to convert non-emissive host triplets into guest-centered NIR phosphorescence. The optimized PBP/MPTCF system achieves a phosphorescence lifetime of 210.3 ms and an ISC efficiency of 44.4%, enabling deep tissue penetration (>2 mm) and high-contrast in vivo imaging. This host–guest approach provides a general design principle for developing high-performance NIR RTP materials, overcoming the limitations of existing RTP systems and advancing precision diagnostics.
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TUO Yanyan, WANG Hongbo, HAN Mingyang, LU Chen, JIANG Guoyu, GONG Jianye, LI Chunbin, FENG Lina, DING Dan, WANG Jianguo (2026). Dark Triplet State Activation to Construct Near-Infrared Host–Guest Organic Room Temperature Phosphorescence Materials for In Vivo Bioimaging. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3864-4
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Frequently Asked Questions
What is the mechanism behind the dark triplet state activation and how does it improve NIR RTP efficiency?
The dark triplet state activation leverages Dexter-type triplet-triplet energy transfer (TTET) from benzophenone derivative hosts (e.g., PBP) with high intersystem crossing (ISC) efficiency to an NIR fluorophore guest (MPTCF). This converts non-emissive host triplets into guest-centered NIR phosphorescence, achieving an ISC efficiency of 44.4% and a phosphorescence lifetime of 210.3 ms at 705 nm.
How does the PBP/MPTCF nanoparticle system perform in terms of tissue penetration and biocompatibility for in vivo imaging?
PBP/MPTCF nanoparticles exhibit deep tissue penetration capability exceeding 2 mm and maintain cell viability above 95% at a concentration of 300 μM, demonstrating excellent biocompatibility. These properties enable high-contrast subcutaneous imaging and stable in vivo imaging.
What are the specific imaging capabilities of PBP/MPTCF NPs for tumor visualization and lymph node mapping?
PBP/MPTCF NPs enable precise lymph node mapping through time-gated phosphorescence imaging and efficient tumor visualization within 4 hours post-injection, achieving a high tumor-to-liver ratio of 2.8, which indicates high specificity for tumor tissue.
How does the phosphorescence lifetime and signal duration of PBP/MPTCF compare to existing NIR RTP materials?
PBP/MPTCF exhibits an ultralong phosphorescence lifetime of 210.3 ms and prolonged phosphorescence signals exceeding 120 seconds in vivo, which is superior to many existing NIR RTP materials that typically have shorter lifetimes and weaker tissue penetration.
What is the significance of the host–guest design strategy for developing NIR RTP materials?
The host–guest design strategy provides a general principle for activating dark triplet states to achieve efficient NIR RTP. By optimizing host matrices and guest fluorophores, it enables high ISC efficiency and long-lived NIR emission, overcoming the limitations of traditional RTP materials and expanding their applications in deep-tissue bioimaging and precision diagnostics.
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