Rational Design of Semiconducting Oligomer for Third Harmonic Generation Bioimaging of Ultradeep Brain Imaging with NIR-IIb Excitation

Abstract

Multiphoton microscopy (MPM) has unparalleled promise in high-spatiotemporal bioimaging within the tissue-transparent window of 1500 to 1700 nm, commonly referred to as the near-infrared-IIb (NIR-IIb) region. However, so far, surprisingly few cases of non-fluorescent MPM probes have been reported, and their imaging performances are relatively limited. Herein, this study introduces a highly efficient third harmonic generation (THG) probe based on semiconducting oligomer derivatives (BTICs), which exhibit strong THG responses under NIR-IIb (1700 nm) excitation. Leveraging halogen chemistry, semiconducting oligomers with varying halogen substitutions and nanoparticles (NPs) exhibit unexpectedly high THG performance across different aggregation states upon NIR-IIb excitation. The BTICs NPs exhibit a large THG conversion efficiency (1215 × 10⁻⁸⁴ cm⁶ s² photon⁻²) and exceptional resistance to photobleaching. Furthermore, the biocompatibility and in vivo THG angiography capabilities of BTICs NPs are validated, achieving the visualisation of deep-brain vasculature with unprecedented spatial resolution at a record-high imaging depth of 1745 µm. The pioneering exploitation of semiconducting oligomer-based THG probes establishes a new class of high-performance materials, enabling ultra-deep THG imaging of the brain and advancing the design of next-generation THG imaging platforms.