Light-Harvesting Fullerene Dyads as Organic Triplet Photosensitizers for Triplet–Triplet Annihilation Upconversions

Visible light-harvesting C₆₀–bodipy dyads were devised as universal organic triplet photosensitizers for triplet–triplet annihilation (TTA) upconversion. The antennas in the dyad were used to harvest the excitation energy, and then the singlet excited state of C₆₀ will be populated via the intramolecular energy transfer from the antenna to C₆₀ unit. In turn with the intrinsic intersystem crossing (ISC) of the C₆₀, the triplet excited state of the C₆₀ will be produced. Thus, without any heavy atoms, the triplet excited states of organic dyads are populated upon photoexcitation. Different from C₆₀, the dyads show strong absorption of visible light at 515 nm (C-1, ε = 70400 M^–1 cm^–1) or 590 nm (C-2, ε = 82500 M^–1 cm^–1). Efficient intramolecular energy transfer from the bodipy moieties to C₆₀ unit and localization of the triplet excited state on C₆₀ were confirmed by steady-state and time-resolved spectroscopy as well as DFT calculations. The dyads were used as triplet photosensitizers for TTA upconversion, and an upconversion quantum yield up to 7.0% was observed. We propose that C₆₀–organic chromophore dyads can be used as a general molecular structural motif for organic triplet photosensitizers, which can be used for photocatalysis, photodynamic therapy, and TTA upconversions.