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

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