A Deep-Red Emissive Cage-in-Rings Complex for Lysosome Imaging

Bioimaging technology is a fundamental platform for visualizing biological processes and guiding clinical interventions. Consequently, developing effective strategies for constructing biocompatible, long-wavelength emissive, and intrinsically selective bioprobes has long been a central goal in chemistry. Herein, we develop a stepwise assembly protocol to construct an exotic cage-in-rings bioimaging probe, TPBCage⁶⁺⊂3CB[8], through noncovalent association of a hexacationic cage (TPBCage⁶⁺) with cucurbit[8]uril (CB[8]). The complex adopts a thermodynamically favored C₂-symmetrical conformation rather than the expected C₃-symmetrical analogue. It preserves two exposed pyridinium units, providing a structural basis for efficient cellular uptake, while the other four pyridinium units are partially shielded by CB[8], reducing nonspecific interactions in bioimaging to some extent compared with the free cage. Encapsulation by CB[8] effectively suppresses π-π stacking of the cage, improving its aqueous solubility. Concomitantly, CB[8] encapsulation narrows the energy gap of the cage, resulting in a red shift in emission from 552 to 652 nm and an enhanced fluorescence quantum yield. Benefiting from enhanced water solubility, good biocompatibility, and deep-red emission, the complex enables lysosome-selective imaging in deep-red region. This work establishes an alternative supramolecular strategy for subcellular-selective imaging, in which cage-in-rings confinement enables control over excited-state properties, enabling the development of intrinsically selective bioimaging probes.