Application of a bodipy–C70 dyad in triplet–triplet annihilation upconversion of perylene as a metal-free photosensitizer

A bodipy–C70 dyad was synthesized and applied in triplet–triplet annihilation (TTA) upconversion of perylene as a novel metal-free organic photosensitizer. The photophysical processes were investigated by the methods of steady-state UV-Vis absorption and fluorescence spectroscopy, nanosecond time-resolved transient absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. The bodipy–C70 dyad showed an increased molar extinction coefficient up to 82 300 mol−1 cm−1 at 518 nm compared with the C70 monomer. With photo-excitation of the bodipy moiety at 532 nm, the intramolecular singlet–singlet energy transfer between bodipy and C70 units was efficient with a quantum yield of nearly 100%, and the lowest triplet state of the dyad was subsequently populated via ISC of the C70 moiety, with a lifetime of ca. 80 μs in toluene. Electrochemical investigation suggested that the intramolecular electron transfer of the excited dyad was thermodynamically prohibited in toluene due to the positive ΔGCS for charge-separation. With the presence of perylene in solution as the triplet energy acceptor and emitter, the TTA upconverted fluorescence was observed with a maximum quantum yield of 10.3%. The overall upconversion capability of 4417 M−1 cm−1 exceeded that of C70 approximately two-fold. Moreover, the bodipy–C70 dyad also exhibited an enhanced optical stability under intense irradiation. All data indicated that the dyad was another ideal photosensitizer for TTA upconversion of perylene in the fullerene derivative family.