Cancer is a leading cause of death worldwide. Despite progress in therapy, new approaches to cure cancer are urgently needed. Such an approach is the topic of this project: It investigates small molecules that are completely non-toxic for cells. However, if they are activated by light, they fragment into smaller parts, which are very toxic, killing cells quickly. For activation, special light guides are needed, which precisely deliver the light to the desired spot – here, the cancer cells. Such lights are developed in this project, too. As the molecules are not toxic before activation and cannot be activated by e.g. daylight, the patient is not affected by them. The toxicity is confined to the cancer. Therefore, the small, activatable molecules and the light guides are together a very promising new strategy for cancer therapy.