Increasing Molecular Planarity through Donor/Side‐Chain Engineering for Improved NIR‐IIa Fluorescence Imaging and NIR‐II Photothermal Therapy under 1064 nm

Developing conjugated small molecules (CSM) with intense NIR‐II (1000–1700 nm) absorption for phototheranostic is highly desirable but remains a tremendous challenge due to a lack of reliable design guidelines. This study reports a high‐performance NIR‐II CSM for phototheranostic by tailoring molecular planarity. A series of CSM show bathochromic absorption extended to the NIR‐II region upon the increasing thiophene number, but an excessive number of thiophene results in decreased NIR‐IIa (1300–1400 nm) brightness and photothermal effects. Further introduction of terminal nonconjugated alkyl chain can enhance NIR‐II absorption coefficient, NIR‐IIa brightness, and photothermal effects. Mechanism studies ascribe this overall enhancement to molecular planarity stemming from the collective contribution of donor/side‐chain engineering. This finding directs the design of NIR‐II CSM by rational manipulating molecular planarity to perform 1064 nm mediated phototheranostic at high efficiency. We propose a simple strategy based on molecular planarity to design a high‐performance donor‐acceptor‐donor‐type conjugated small molecule with both intense NIR‐II absorption and robust energy output. The resulting probe was used for 1064 nm laser‐triggered NIR‐II (1000–1700 nm) fluorescence/photoacoustic imaging and dual‐mode imaging‐guided NIR‐II photothermal therapy in vivo.