Employing N-phenyl-tribenzo[b,d,f]azepine as a new electron-donating group to construct high-emissive AIE molecules for NIR-II fluorescence imaging.

Near-infrared-II (NIR-II) fluorophores with the aggregation-induced emission (AIE) character have attracted extensive attention due to their significant potential in bioimaging applications. While such molecules typically adopt a donor-π-acceptor-π-donor (D-π-A-π-D) molecular architecture, the limited variety of donor units (e.g., triphenylamine or tetraphenylethylene derivatives) directly constrains the development of new NIR-II AIE molecules. To address this issue, herein for the first time N-phenyl-tribenzo[b,d,f]azepine (PTZ) is employed as a novel donor group for constructing NIR-II AIE molecules. In detail, two new AIE molecules, TT-PTZ and ST-PTZ, are synthesized using PTZ as the donor, hexylthiophene as the π-bridge, and benzobisthiadiazole or Se-substituted benzobisthiadiazole as the acceptor. Photophysical studies reveal that both molecules exhibit strong NIR-II emission in nanoparticles (NPs). Particularly, compared to the benchmark molecule 2TT-oC6B (NPs: fluorescence quantum yield Φ of 8.4 %) which employs a classical triphenylamine donor, the counterpart analog TT-PTZ with the PTZ donor displays much stronger emission (NPs: Φ of 26.1 %), highlighting the superiority of this saddle-shaped PTZ donor. Transient absorption spectroscopy and molecular dynamics simulations indicate that the non-radiative decays are significantly suppressed in the aggregated state, thus resulting in strong emission. The application of ST-PTZ NPs in NIR-II vascular imaging enables clear observation of capillaries, achieving a high resolution with a full width at half maximum (FWHM) of 57.9 μm, which represents one of the best performances reported to date for NIR-II macroscopic fluorescence imaging. Furthermore, ST-PTZ NPs are employed for NIR-II fluorescence imaging-guided tumor resection, demonstrating precise fluorescence navigation capabilities.