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A novel sulfur-substituted naphthalimide (LSNI-S) was synthesized and investigated as a promising lysosome-targeting photosensitizer for photodynamic cancer therapy. The introduction of a dual-functional morpholine group to the 4-position of the naphthalimide backbone combined with the substitution of oxygen atoms by sulfur atoms could facilitate intersystem crossing from the excited singlet state to the reactive triplet state, leading to an excellent singlet oxygen generation efficiency of LSNI-S in organic solvents (Φ Δ ≈ 0.84 in air-saturated acetonitrile). Interestingly, LSNI-S could produce reactive oxygen species via both type-I and type-II mechanisms under physiological conditions. In particular, cell studies demonstrated that LSNI-S selectively localizes in lysosomes of cancer cells and exhibits excellent photodynamic therapy efficacy. Image 1 • A novel lysosome-targeting photosensitizer for selective photodynamic therapy was proposed. • LSNI-S could produce reactive oxygen species via both type-I and type-II mechanisms under physiological conditions. • LSNI-S exhibited excellent photodynamic therapy efficacy toward HeLa cells. [ABSTRACT FROM AUTHOR]