A Cyanine with 83.2% Photothermal Conversion Efficiency and Absorption Wavelengths over 1200 nm for Photothermal Therapy

Abstract Developing small‐molecule photothermal agents (PTAs) with good near‐infrared‐II (NIR‐II) response for deeper tissue penetration and minimizing damage to healthy tissues has attracted much attention in photothermal therapy (PTT). However, concentrating ultra‐long excitation wavelengths and high photothermal conversion efficiencies (PCEs) into a single organic small molecule is still challenging due to the lack of suitable molecular structures. Here, six polymethine cyanine molecules based on the structure of indocyanine green are synthesized by increasing the conjugated structure of the two‐terminal indole salts and the number of rigid methine units, and incorporating longer alkyl side chains into the indole salts. Ultimately, IC‐1224 is obtained with an absorption wavelength of more than 1200 nm, which has a high PCE up to 83.2% in the NIR‐II window and exhibits excellent PTT tumor ablation performance. This provides a high‐performance NIR‐II‐responsive PTA, and offers further possibilities for the application of PTT in biomedical fields.