Metalloporphyrin loaded semiconducting polymer dots as potent photosensitizers via triplet-triplet energy transfer

Photodynamic therapy (PDT) is a potent treatment for clinical therapy of superficial tumors. This study describes the metallophorphyrin loaded semiconducting polymer dots (Pdots) for PDT applications. We use conjugated polymers as light-harvesting matrices and metalloporphyrin as triplet photosensitizers to prepare small nanoparticles. Energy transfer occurred from conjugated polymers to metalloporphyrin along with the quenching of polymer fluorescence and amplified singlet oxygen generation. Notably, both spectroscopy and energy level analysis indicated a triplet-triplet energy transfer mechanism in metalloporphyrin loaded Pdots yielding singlet oxygen. The singlet states of metalloporphyrin were first populated by energy transfer, and then transferred to the triplet states through efficient intersystem crossing. We postulate that energy transfer occur from the triplet states of metallophorphyrin to the triplet states of conjugated polymers, which interact with dissolved oxygen and give high singlet oxygen generation. This mechanism is different from the porphyrin-doped Pdots, which sensitize oxygen through triplet states of dyes. We evaluate the singlet oxygen yield and photodynamic effect of metalloporphyrins doped Pdots by various cellular assays. The results show that metalloporphyrins doped Pdots exhibit high singlet oxygen generation and effectively destroy cancer cells at a low nanoparticle concentration and light irradiation dose, indicating their promising application in photodynamic cancer treatment.