An Ester‐substituted Semiconducting Polymer with Efficient Nonradiative Decay Enhances NIR‐II Photoacoustic Performance for Monitoring of Tumor Growth

Photoacoustic agents have been of vital importance for improving the imaging contrast and reliability against self-interference from endogenous substances. Herein, we synthesized a series of thiadiazoloquinoxaline (TQ)-based semiconducting polymers (SPs) with a broad absorption covering from NIR-I to NIR-II regions. Among them, the excited s-BDT-TQE, a repeating unit of SPs, shows a large dihedral angle and narrow adiabatic energy as well as low radiative decay, attributing to its strongly electron-deficient ester-substituted TQ-segment. In addition, its more vigorous molecular motions trigger a higher reorganization energy that further yields an efficient photoinduced nonradiative decay, which has been carefully examined and understood by theoretical calculation. Thus, BDT-TQE SP-cored nanoparticles with twisted intramolecular charge transfer (TICT) feature exhibit a high NIR-II photothermal conversion efficiency (61.6%) and preferable photoacoustic tracking of in situ hepatic tumor growth for more than 20 days. This study highlights a unique strategy for constructing efficient NIR-II PA agents via TICT-enhanced PNRD effect, further advancing their applications for in vivo bioimaging.