Low-band gap conjugated polymers with donor-acceptor (D-A) structures have emerged as second near-infrared (NIR-II) fluorescence probes for biological imaging. However, how to control the intramolecular charge transfer (ICT) to maintain the low band gap and improve the NIR-II fluorescence intensity is an urgent issue. Here, the quinoid polymers have been developed to effectively regulate the ICT for brighter NIR-II fluorescence signals. Thiophene repeat chain units of different lengths (T, 2T, and 3T) were utilized to link with electron-withdrawing ester-substituted thieno[3,4- b]thiophene (TT) to alter the density of the electron-withdrawing side groups for controlling the ICT. By increasing the thiophene chain length from TT-T to TT-3T, the density of the electron-withdrawing groups decreased and the ICT was weakened. In the case of NIR absorption and NIR-II emission, weakened ICT leads to brighter NIR-II fluorescence. After the preparation of the water-soluble quinoid polymer probes (CPs), TT-3T CPs with weak ICT exhibited the brightest NIR-II fluorescent signals among the three quinoid polymer probes. Several NIR-II biomedical imaging applications, including in vivo cell tracking, blood vascular system images, and lymphatic drainage mapping, show that the TT-3T CP-based nanoprobe had excellent characteristics of long-term stability and high NIR-II spatial resolutions in vivo.
Keywords: NIR-II Imaging; electron-withdrawing groups; enhanced NIR-II fluorescence; intramolecular charge transfer; quinoid polymers.