Pentacene Nanorods as Effective Photosensitizer for Photodynamic Therapy of Tumor via Singlet Fission

Singlet fission (SF), whereby a singlet exciton is converted into a pair of triplet excitons, can improve the efficiency of solar cells. Pentacene has been extensively studied as the most promising SF compound, owing to its 200% yield of triplet states. However, the easy degradation of pentacene in the presence of light and air owing to photooxidation cannot be explained by the classical O-1(2) generation mechanism. To address this issue, in the present study, pentacene nanorods (Pc NRs) are prepared as a novel photosensitizer (PS); self-carried Pc NRs exhibited higher O-1(2) generation capacity. Thus, a novel O-1(2) generation mechanism is proposed based on the SF effect. The initial photon absorption occurs to access single-exciton states, S1-S3. Excited-state Pc pairs accelerate the SF effect in pentacene NRs, leading to a non-adiabatic transition to the dark D state. Dark D state is a singlet state by two triplets coupled overall, and it can transfer its energy to O-3(2) for generating O-1(2). Using Pc NRs as PSs, photodynamic therapy (PDT) inhibits tumor growth in 4T1 tumor-bearing mice upon 405-nm-wavelength and 650-nm-wavelength laser irradiations. This study paves the way to discover novel PSs that are not considered with classical O-1(2) generation mechanisms.