Novel azo compounds and their bioconjugates for phototherapy and/or photodiagnosis of tumors and other lesions. The azo derivatives of the present invention are designed to absorb at the low-energy ultraviolet, visible, or near-infrared (NIR) region of the electromagnetic spectrum. The phototherapeutic effect is caused by direct interaction of free radicals, the reactive intermediate produced upon photoexcitation of the azo compound, with the tissue of interest.