Cucurbit[8]Uril Driven Upconversion Near-Infrared Delayed Fluorescence for Targeted Cell Imaging

Pure organic upconversion systems have opened new possibilities for deep-tissue bioimaging based on efficient two or three-photon excitation. Herein, a pure organic room-temperature phosphorescence (RTP) upconversion supramolecular assembly is reported that exhibits near-infrared (NIR) delayed fluorescence by upconverted excitation at 940 nm. This system is constructed by co-assembling cucurbit[8]uril (CB[8])-confined indole-bridged xanthene derivative (CyBr) with the complex of CB[8] encapsulated two sulfonated bromophenylpyridinium (PySO₃⊂CB[8]), guided by β-cyclodextrin-grafted hyaluronic acid (HACD). First, the spatial confinement of CB[8] 10-fold enhances the fluorescence of CyBr. Subsequently, as the tumor-targeting agent, HACD realizes cascade fluorescence enhancement, boosting the quantum yield (QY) of CyBr by 22 times. Moreover, the cavities of β-CDs in HACD provide assembly sites for multivalent interactions with the sulfonic acid group of PySO₃⊂CB[8], achieving highly efficient phosphorescence resonance energy transfer (PERT) with an efficiency of 84.49%. This leads to NIR delayed fluorescence at 710 nm with a lifetime of 99.16 µs. Notably, this system exhibits exceptional upconversion capability under 940 nm three-photon excitation, enabling targeted A549 cancer cell imaging with deep-tissue penetration and long-lived NIR emission for low-background detection.