Truxene-linked di-coumarin-mono-corrole tetrad: Synthesis, photophysical properties and application in Triplet−Triplet annihilation upconversion

Triplet−triplet annihilation upconversion (TTA-UC) has drawn widespread interest because of its great potential applications. However, it is still a challenge to design and synthesize efficient triplet photosensitizers without heavy atoms for TTA-UC. In this work, a triplet photosensitizer tetrad 2C-T-Cor based on two coumarin moieties and one corrole moiety with a truxene as the bridge was synthesized and used in TTA-UC as a novel heavy atom-free organic photosensitizer (perylene bisimide (PBI) was used as triplet acceptor). The photophysical properties were investigated by means of steady-state UV–Vis absorption and fluorescence spectroscopy, nanosecond time-resolved transient absorption spectroscopy, three-dimensional excitation emission matrix fluorescence spectra, and cyclic voltammetry. With photo-excitation of the coumarin unit at 420 nm, the intramolecular singlet–singlet energy transfer takes place from the coumarin moiety to the corrole unit with a transfer efficiency of 63.9%. Electrochemical study indicated that the intramolecular electron transfer was thermodynamically prohibited in toluene owing to the positive ΔGCS for charge-separation. A triplet excited state was investigated for 2C-T-Co via nanosecond time-resolved transient absorption and the triplet lifetime is 118 μs. 2C-T-Co can act as the triplet photosensitizer for the application of TTA-UC, and the upconversion quantum yield of 0.98% was observed.