Unimolecular Sonoafterglow Probes for Biomarker-Activated Imaging of Senescent Cancer Cells

Abstract Sonoafterglow imaging is a promising deep-tissue molecular imaging modality that effectively suppresses autofluorescence and offers enhanced tissue penetration depth compared to conventional optical methods. However, its potential is constrained by the lack of unimolecular, self-sustaining sonoafterglow luminophores. Herein, we report the first unimolecular sonoafterglow luminophores for the activatable imaging of senescent cancer cells in vivo. Structure–activity screening of 12 unimolecular phenoxy-adamantylidene-based luminophores identifies two benzoxazolium-substituted derivatives, SL6 (ortho) and SL12 (para), as the best-performing candidates, exhibiting intense red emission, long afterglow half-lives, and good rechargeability. Mechanistic investigations reveal that under ultrasound irradiation, SL6 and SL12 generate singlet oxygen, which undergo cycloaddition with the adamantylidene moieties to form metastable endoperoxides that subsequently rearrange into 1,2-dioxetane intermediates. These intermediates then undergo chemically initiated electron-exchange luminescence-mediated chemiexcitation cascades, wherein intramolecular energy transfer to the aryl-benzoxazolium gives rise to the sonoafterglow emission. The para-substituted luminophore is further developed into a β-galactosidase-activatable probe, enabling real-time, longitudinal imaging of DOX-induced senescent tumor in vivo. This work not only establishes the first unimolecular scaffold for self-sustaining sonoafterglow but also offers a versatile platform for biomarker-activatable sonoafterglow imaging.