A rational design of polymers through donor modulation to weaken the aggregation-caused quenching effect for NIR-II fluorescence imaging

The great tissue penetration depth and low tissue autofluorescence of NIR-II fluorescence imaging make it attractive for in vivo diagnosis. However, the aggregation-caused quenching (ACQ) effect is among the dominant obstacles that weaken NIR-II imaging and restrict its application. Herein, the donor unit, 2,8-dibromo-6H,12H-5,11-methanodibenzo[b,f] [1,5]diazocine with a V-configuration, was introduced to prepare the donor-acceptor (D-A) polymer P-TB with a twisted backbone, while the planar D-A polymer P-TP was used as a control. P-TB and P-TP were prepared by Stille Coupling with DPP as the acceptor. The main absorption peaks of P-TB and P-TP are located at 610 nm and 640 nm, and the emission peaks of P-TB and P-TP are 1060 nm and 930 nm, respectively. Significantly, the V-shaped P-TB showed no obvious ACQ effect within 600 μM, and the same phenomenon was demonstrated during in vivo NIR-II imaging in mice, which proves that the introduction of V-configuration donor units is beneficial for weakening the ACQ effect. This work outlines a prospective tactic for the design of conventional NIR-II fluorescent polymers by modulating the configuration of the donor units.