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

Visible light-harvesting C-60-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 C-60 will be populated via the intramolecular energy transfer from the antenna to C-60 unit. In turn with the intrinsic intersystem crossing (ISC) of the C-60, the triplet excited state of the C-60 will be produced. Thus, without any heavy atoms, the triplet excited states of organic dyads are populated upon photoexcitation. Different from C-60, the dyads show strong absorption of visible light at 515 nm (C-1, epsilon = 70400 M-1 cm(-1)) or 590 nm (C-2, epsilon = 82500 M-1 cm(-1)). Efficient intramolecular energy transfer from the bodipy moieties to C-60 unit and localization of the triplet excited state on C-60 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 C-60-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.