Near‐Infrared Heptamethine Cyanine Photosensitizers with Efficient Singlet Oxygen Generation for Anticancer Photodynamic Therapy

Near‐infrared photosensitizers are valuable tools to improve treatment depth in photodynamic therapy (PDT). However, their low singlet oxygen (1O2) generation ability, indicated by low 1O2 quantum yield, presents a formidable challenge for PDT. To overcome this challenge, the heptamethine cyanine was decorated with biocompatible S (Scy7) and Se (Secy7) atom. We observe that Secy7 exhibits a redshift in the main absorption to ~840 nm and an ultra‐efficient 1O2 generation capacity. The emergence of a strong intramolecular charge transfer effect between the Se atom and polymethine chain considerably narrows the energy gap (0.51 eV), and the heavy atom effect of Se strengthens spin–orbit coupling (1.44 cm−1), both of which greatly improved the high triplet state yield (61%), a state that determines the energy transfer to O2. Therefore, Secy7 demonstrated excellent 1O2 generation capacity, which is ~24.5‐fold that of indocyanine green, ~8.2‐fold that of IR780, and ~1.3‐fold that of methylene blue under low‐power‐density 850 nm irradiation (5 mW cm−2). Secy7 exhibits considerable phototoxicity toward cancer cells buried under 12 mm of tissue. Nanoparticles formed by encapsulating Secy7 within amphiphilic polymers and lecithin, demonstrated promising antitumor and anti‐pulmonary metastatic effects, exhibiting remarkable potential for advancing PDT in deep tissues.