Molecular Engineering of NIR‐II AIE Luminogen Excited at 1700 nm for Ultradeep Intravital Brain Two‐Photon Fluorescence Imaging

Abstract The limited tissue penetration depth and spatial resolution are the major bottlenecks for deep‐brain imaging. In this study, molecular engineering by tailoring electron donors is conducted to develop for the first time an NIR‐II (second near‐infrared) emissive fluorescence probe, namely DCTBT, for effective deep‐brain two‐photon fluorescence imaging. Benefiting from its good biocompatibility, high photostability, bright NIR‐II emission as aggregates and large two‐photon fluorescence action cross section at the 1700 nm excitation window, DCTBT offers the imaging depths of 2180 and 1135 µm in mouse brain with removed and intact skull, respectively. These results are the record depths for brain imaging, compared to all kinds of fluorescent probes and all modalities of multiphoton microscopy at all demonstrated excitation wavelengths. Moreover, with DCTBT labeling, hemodynamic imaging of blood flow in mouse brain vessels down to a depth of 714 µm with the intact skull is achieved. Multiphoton fluorescence imaging with the NIR‐II probe DCTBT excited at the 1700 nm window may readily provide methodology for deep‐brain structural and hemodynamic research.