A group of photochemical nitric oxide (NO) delivery agents, O²-(4-hydroxyphenacyl) diazeniumdiolates 1–2 as well as O²-(3-(benzothiazole-2-yl)-4-hydroxyphenacyl) diazeniumdiolates 3–4, could be triggered by ultraviolet light (365–400 nm) and visible light (≥410 nm), respectively, to produce NO in cancer cells and generate potent antitumor activity. Importantly, with the photolysis of 3 being initiated, the accompanying distinct fluorescence color change (from green to blue) may act as a qualitative real-time optical indicator for NO release in cancer cells.