Rigid‐and‐Flexible Molecular Bows With High NIR Photothermal Performance

ABSTRACT Ideal near‐infrared organic small molecules (NIR‐OSMs) should possess high molar extinction coefficients (ε) and photothermal conversion efficiency (PTCE) to achieve desirable photothermal effects at low doses. However, existing NIR‐OSMs rarely combine both properties, resulting in low photothermal performance (ε × PTCE). Herein, we propose a “rigid‐and‐flexible” molecular design strategy: bridging the two ends of a high‐ε rigid π‐conjugated molecule (quaterrylene diimides, QDI ) with flexible alkyl chains bends the molecule, thereby imparting high PTCE. Experimental results demonstrate that bending significantly reduces the fluorescence quantum yield and lifetime, increasing the nonradiative decay rate by nearly one order of magnitude. Simultaneously, bending greatly weakens intermolecular π–π interactions, leading to a loose packing mode favorable for photothermal conversion. More importantly, adjusting the bending curvature allows precise tuning of the maximum absorption wavelength within the range of 780–825 nm to match the 808 nm laser. Leveraging these advantages, QDI[10] nanoparticles exhibit the highest PTCE of 77.4%, while QDI[12] nanoparticles achieve a record‐high photothermal performance of 93,400 M − 1 cm − 1 at 808 nm. This strategy can be extended to other rigid π‐conjugated systems with high ε but low PTCE, promoting the development of efficient photothermal therapy using NIR‐OSMs.