BODIPY-Based Photodynamic Agents for Exclusively Generating Superoxide Radical over Singlet Oxygen

Developing Type-I photosensitizers is considered as an efficient approach to overcome the deficiency of traditional photodynamic therapy (PDT) for hypoxic tumors. However, it remains a challenge to design photosensitizers for generating reactive oxygen species by the Type-I process. Herein, we report a series of alpha,beta-linked BODIPY dimers and a trimer that exclusively generate superoxide radical (O-2(-.)) by the Type-I process upon light irradiation. The triplet formation originates from an effective excited-state relaxation from the initially populated singlet (S-1) to triplet (T-1) states via an intermediate triplet (T-2) state. The low reduction potential and ultralong lifetime of the T-1 state facilitate the efficient generation of O-2(-.) by inter-molecular charge transfer to molecular oxygen. The energy gap of T-1-S-0 is smaller than that between O-3(2) and O-1(2) thereby precluding the generation of singlet oxygen by the Type-II process. The trimer exhibits superior PDT performance under the hypoxic environment.