Near-Infrared Emissive\nπ‑Conjugated Oligomer\nNanoparticles for Three- and Four-Photon Deep-Brain Microscopic Imaging\nBeyond 1700 nm Excitation

High-resolution visualization of the deep brain is still\na challenging\nand very significant issue. Multiphoton microscopy (MPM) holds great\npromise for high-spatiotemporal deep-tissue imaging under NIR-III\nand NIR-IV excitation. However, thus far, their applications have\nbeen seriously restricted by the scarcity of efficient organic probes.\nHerein, we designed and synthesized two donor–acceptor–donor-type\nconjugated small molecules (<b>TNT</b> and <b>TNS</b>)\nfor <i>in vivo</i> mouse deep-brain imaging with three-\nand four-photon microscopy under 1700 and 2200 nm excitation. With\na selenium (Se) substitution, we synthesized two conjugated small\nmolecules to promote their emission into the deep near-infrared region\nwith high quantum yields of 55% and 20% in THF solvent, respectively,\nand their water-dispersive nanoparticles have relatively large absorption\ncross-sections in the 1700 and 2200 nm windows, respectively, with\ngood biosafety. With these superiorities, these organic NPs achieve\nhigh-resolution deep-brain imaging <i>via</i> three-photon\nand four-photon microscopy with excitation at 1700 and 2200 nm windows,\nand 1620 μm deep in the brain vasculature can be visualized\n <i>in vivo</i>. This study demonstrates the efficiency\nof NIR-emissive conjugated small molecules for high-performance MPM\nimaging in the NIR-III and NIR-IV window and provides a route for\nthe future design of organic MPM probes.