**5. Activity control based on the electron transfer**

Electron transfer can be controlled by the surrounding environment. For example, pH is an important factor to control the photoinduced electron transfer [29, 30, 48, 80, 81]. Since it has been reported that cancer cells are slightly acidic (pH 6 ~ 7) against normal tissues (pH 7 ~ 7.4) [82–85], control of the electron transfer of the photosensitizer by pH can be applied for the development of cancer-selective PDT. In the cases of pH-dependent 1 O2 photosensitizers, the redox control [30, 86–88], the structure change [89], and the control of intersystem crossing [90] by pH have been reported as the important concepts. Several types of pH-activatable-porphyrin photosensitizers [30, 88], including a phosphorus(V) porphyrin [48, 81], have been reported. In addition, a self-quenching of the photoexcited molecules can be also used to control the activity [47]. In this section, several examples about the activity control of electron transfer-photosensitizers are introduced.
