Covalent Bond Locking in Semiconducting Oligomers Boosts Ultrabright NIR‐II Luminescence for Deep Brain Theranostics

Near-infrared (NIR)-II fluorescence imaging at 1000-1700 nm is widely used for deep-tissue visualisation and disease theranostics in the brain, with NIR-II theranostics greatly improving imaging resolution, imaging depth, and therapeutic efficacy. However, the extreme lack of molecular design in NIR-II fluorophores has slowed the discovery of bright candidates and restricted their efficacious application in brain theranostics. Here, we develop a covalent bond locking (CBL) strategy that enables the feasible design of bright NIR-II fluorophores by effectively restricting the twisted intramolecular charge transfer state. These spirofluorophores incorporate terminally spiro-donor groups, which leads to a higher molar extinction coefficient and improved quantum yield than non-spirofluorophores do. With bright and stable NIR-II fluorescence advantages, we demonstrate that CBL nanoparticles (NPs) of spirofluorophores achieve multiscale high-resolution NIR-II angiography via one-photon fluorescence and two-photon fluorescence bioimaging simultaneously. With apolipoprotein E (ApoE) modification, CBL@ApoE NPs achieve enhanced blood-brain barrier permeability, facilitating superior brain glioma theranostics. This work proposes a CBL strategy to engineer highly bright NIR-II fluorescent fluorophores, providing a reliable nanoplatform for deep brain theranostics that can be effectively delivered across biological barriers to target brain tumors.