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

Abstract

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 ΔG_CS 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.