Near Infrared Absorption/Emission of B ← N Embedded Polymers with Aromatic Substituents: A Judicious Balance Among Absorption Coefficient, Emission Quantum Yield, and Photothermal Conversion Efficiency

For semiconducting polymers (SPs) used in biomedical fields, e.g., bioimaging and photothermal therapy, three critical parameters, that is absorption coefficient (ε), emission quantum yield (ΦFL), and photothermal conversion efficiency (PCE) in near-infrared (NIR) region should be taken into account but reaching judicious balance among the three parameters is difficult. In this work, three B ← N embedded polymers, namely, BN–Cl-V, BN–Cl-S, and BN–Cl-N anchoring chlorinated phenyl as side groups with NIR absorption/emission properties, are synthesized. Due to the rigid backbone and steric substituents, these polymers not only exhibit high absorption coefficients but also maintain moderate fluorescence intensities. Moreover, high PCE values are tested owing to the free motion of phenyl side groups, which suppress the radiative transitions. Impressively, BN–Cl-V displays the highest ε of 60.2 L·g–1·cm–1, ΦFL of 4.25%, and PCE of 81% because of its best backbone coplanarity and D–A conjugation. Cytotoxicity experiments in vitro manifest that BN–Cl-V triggers more than 80% cell apoptosis under 808 nm laser irradiation (0.3 W/cm2) for 5 min. We propose an effective strategy to design SPs with balanced and satisfactory ε, ΦFL, and PCE values, which is enlightening for the preparation of polymer biomedical agents.