Multi-level macrocyclic assembly achieving organic upconversion delayed fluorescence for targeted cell imaging

Purely organic room-temperature phosphorescence (PORTP) supramolecular materials have been widely explored in bioimaging, but their relatively short excitation wavelengths limit their further application. Herein, we report a purely organic phosphorescence resonance energy transfer (PRET) assembly, which was constructed by purely organic upconversion phosphor 6-bromoisoquinolium-modified permethylated β-cyclodextrin (BQ-PCD), cucurbit[8]uril (CB[8]) and tetra(4-sulfonatophenyl)porphyrin (TPPS), displaying not only CB[8]-confinement enhanced PORTP, but also multi-photon-mediated PRET achieving delayed deep-red luminescence for targeted cell imaging. Compared with BQ-PCD, the BQ-PCD⊂CB[8] supramolecular assembly remarkably prolongs PORTP lifetime from 13.8 ns to 640 μs, and further assembles with TPPS through strong host-guest interaction between PCD and TPPS, achieving delayed deep-red luminescence with a lifetime of around 11 μs through a highly efficient PRET process (94.5%). Taking advantage of the 1 : 2 stoichiometric ratio, BQ-PCD⊂CB[8] efficiently enhanced the upconversion process compared with BQ-PCD⊂CB[7], leading to delayed deep-red luminescence under multi-photon excitation at 920 nm in TPPS⊂BQ-PCD⊂CB[8]. With the further cascade assembly with tumor-targeting hyaluronic acid (HA), the upconversion supramolecular nanoparticle could be successfully applied for targeted cell imaging through an upconversion PRET process. This study offers a novel approach to fabricating long-lived deep-red fluorescence materials with upconversion properties via macrocyclic confinement assembly.