© 2021 Elsevier LtdPhotodynamic therapy (PDT) is an alternative way to treat the cancer non-surgically with minimum harm, in which a specific light and light sensitive photosensitizer (PS) is used. This study deals with the utilization of zinc photosensitizers (λmax ∼700 nm) from the group of phthalocyanines in photodynamic therapy. The photosensitizers differed in bearing either tetra- or octa-mercaptopropionic acid groups. The phthalocyanines (L1ZnPc and DipZnPc) were tested in cancer cell line, HeLa cell (epitheloid cervix carcinoma), by dark and light cytotoxicity. The photophysico-chemical properties of the resulting zinc photosensitizers were analyzed and to investigate the singlet oxygen's efficiency, photochemical properties were determined. For L1ZnPc, the singlet oxygen quantum yield was ΦΔ = 0.41 and for DipZnPc, the value was ΦΔ = 0.27. In addition to the experimental outcomes, this study handles detailed calculations of the absorbed light dose by the photosensitizer at a specific wavelength. In the light of this study, the researchers could get the insight of the precise amount of energy delivered to the cells by a laser beam for critical applications. The absorbed dose of the laser irradiation was calculated within a true wavelength range that is the most detailed explanation in literature.