A new type of porphyrin photosensitizer capable of generating singlet oxygen upon irradiation, storing it through binding to pyridone subunits, followed by slow release at 20–40 °C, is reported. The timescale of singlet oxygen release can be varied depending on the pyridone group substitution pattern, forming endoperoxides of different stabilities. Modified tetra- and octa-substituted pyridone-porphyrins showed solubility in water, allowing for straightforward delivery into cells. The effect of delayed singlet oxygen formation due to endoperoxide decomposition was demonstrated on cancer cells in vitro.