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In this study, two chlorin photosensitizers (PSs) having long-wavelength absorbing (λ max 707 and 709 nm) and mitochondria or lysosome organelle-targeting abilities have been developed for use in photodynamic therapy (PDT). We simply modified two PSs, PS1 (N -propyl unit as a flexible terminus) and PS2 (N -propargyl unit as a rigid terminus), which displayed near-infrared absorption and fluorescence emission and high 1O 2 quantum yield (0.720 and 0.593), indicating a suitable PS for PDT, resulting in high-performance photodynamic activity with low IC 50 values (0.31–0.36 μM against A549 and HeLa cells). Importantly, PS1 and PS2 exhibited excellent mitochondrial or lysosomal organelle-targeting ability with high Pearson's correlation coefficient values (∼0.9). The excellent PDT effect of PS1 and PS2 was confirmed by fluorescence images of intracellular reactive oxygen species generation as well as live/dead staining images. Therefore, these two PSs have great potential as fluorescence imaging materials and PDT reagents. [Display omitted] • Developed PSs having long-wavelength absorption (λ max 707 and 709 nm) for deep penetration of light into the tumour tissue. • PS1 and PS2 are useful for cancer diagnosis with strong fluorescence imaging due to their NIR fluorescence (600–860 nm). • PSs display high 1O 2 quantum yields (0.72 and 0.59) and high performance PDT against cancer cells. • Mitochondria or lysosome organelle-targeting with high PCC values (∼0.9) inducing apoptosis and necrosis of cancer cells. [ABSTRACT FROM AUTHOR]

[Display omitted] • Synthesis of transition metal complexes of second-generation chlorin photosensitizer. • Highly improved photostability for all tested metal complexes. • In vitro heavy-atom effect for palladium(II) complex of free-base compound. • Unusual inhibition of 1O 2 generation after coordination with cobalt(II) In the present study, a chlorin derivative photosensitizer (PS), purpurin-18 methyl ester (P18ME), and its transition metal complexes were synthesized and evaluated for their in vitro photodynamic therapy (PDT) efficacy. The effect of introducing transition metals into the free-base PS was assessed, and the heavy-atom effect of two divalent transition metals, palladium(II) and cobalt(II), was compared. All metal complexes exhibited hypsochormic shifts of the Q-bands and demonstrated significantly improved photostability. In addition, the palladium(II) complex of P18ME (Pd-P18ME) exhibited increased singlet oxygen (1O 2) photogeneration and enhanced efficacy in vitro , owing to the heavy-atom effect. However, the cobalt(II) complex of P18ME displayed reduced 1O 2 photogeneration and decreased photodynamic activity. These results showed that the heavy-atom effect might depend on the compound structure and the transition metal used. Therefore, Pd-P18ME displayed enhanced photodynamic activity based on the heavy-atom effect to induce increased photostability and 1O 2 photogeneration, indicating a potential PS candidate for PDT. [ABSTRACT FROM AUTHOR]