Multi-Level Confinement Single-Molecule Charge Transfer Activated PRET for Near-Infrared Targeted Cell Imaging

This work reports an interesting phosphorescent resonance energy transfer (PRET) system with a large Stokes shift (320 nm) and near-infrared (NIR) delayed fluorescence emission for high-performance bioimaging, where a tailored charge transfer supramolecular assembly was constructed from dibenzyl-bridged pyridinium derivative (G1), cucurbit[8]uril (CB[8]), and β-cyclodextrin-grafted hyaluronic acid (HACD). The rigid dibenzyl linker of G1 induces a unique zigzag conformation, enabling G1 to form an antiparallel charge transfer complex with CB[8], giving a binding constant as high as 5.49 × 10⁶ m^-1 and triggering a nanofiber-to-nanorod topological transformation that boosts energy transfer efficiency. Significantly, multi-level confinement by CB[8] and HACD aligns G1 into donor-acceptor arrays, realizing single-molecule PRET from bromophenyl pyridinium (phosphor donor) to anthracenyl pyridinium (acceptor). This unprecedented process expands 500 nm phosphorescence to 650 nm NIR delayed fluorescence with a lifetime recorded as 11.20 µs, overcoming traditional PRET's short-wavelength limitation. Crucially, the system achieves successful NIR channel labeling/detection in living cells, demonstrating its bioimaging potential. This work advances supramolecular PRET design via rational structural tailoring and confinement engineering, holding broad significance for cell imaging, information encryption, and anti-counterfeiting.