Based on the cyanine dye scaffold, two photosensitizers (PSs) (C1 and C2) were successfully synthesized. These PSs exhibited a maximum absorption wavelength of 660 nm, falling within the near-infrared window. Compared to the commercially available photosensitizer ICG, the newly developed PSs demonstrated enhanced reactive oxygen species (ROS) generation under lower light doses. Notably, these PSs can simultaneously produce both singlet oxygen (¹O₂) and superoxide anion radicals (O₂˙⁻), classifying them as dual type I/II PSs. Compared with C1, the ¹O₂ quantum yield of C2 was higher, as high as 10.6 times that of ICG. Theoretical calculations revealed that the molecules of C2 possessed a small singlet–triplet energy gap (ΔE_S–T), which facilitated more efficient intersystem crossing (ISC) from the singlet to triplet state, thereby promoting greater ROS generation. Experiments showed that C1 and C2 were located in mitochondria and could cause a decrease in mitochondrial membrane potential, leading to cell death. Animal experiments have shown that C2 effectively suppressed tumor growth without side effects.

Stable organic π-conjugated diradcialoids with tunable diradical characters can profoundly affect emerging technology. Over the past years, great efforts have been devoted to studying the structure-diradical character relationship in diradicaloids. Herein, a series of quinoidal isoindigo (IID) compounds with different attached terminal end groups were designed. Detailed analysis focuses on elucidating the driving force for evoking and enhancing the diradical character in the quinoidal IID systems. The arylene units of the IID core and the bridged aromatic units determine the contribution of the open-shell diradical form in the ground state. Diradical character y₀ correlates well with bond length alternation (BLA), the total HOMA, and the total NICS(1)zz, and it is tuned by bridged aromatic units and terminal end groups in symmetric systems. The zwitterionic character weakens the diradical character in asymmetric systems to different extents. This work contributes to the deep understanding of evoking and enhancing the diradical character in quinoidal IID-based diradcialoids, providing useful guidelines to produce new molecules with desirable properties.