Visible-to-UV triplet-triplet annihilation upconversion from a thermally activated delayed fluorescence/pyrene pair in an air-saturated solution

Abstract

Despite increasing use of triplet-triplet annihilation upconversion (TTA-UC) of low-energy visible light, the generation of ultraviolet (UV) photons by TTA remains challenging because of the difficulty in finding sensitizers and acceptors with suitable energy levels. Here, we report efficient, photostable visible-to-UV TTA-UC in an air-saturated solution using a new pair with suitable energy levels: a thermally activated delayed fluorescence (TADF) molecule and pyrene. 4CzIPN, which has extremely small energy difference ΔEST (0.083 eV), was used as the TADF sensitizer to promote effective triplet energy transfer to the acceptor. When oleic acid was used as an effective singlet oxygen receptor in an air-saturated solution, the 4CzIPN/pyrene pair exhibited bright upconverted emission at 370–430 nm under 445 nm laser excitation, but no noticeable upconverted emission was observed when 4CzIPN was paired with previously reported UV-emitting acceptors [2,5-diphenyloxazole (PPO), p-terphenyl, and p-quaterphenyl]. TTA was confirmed by the quadratic dependence of the upconverted emission intensity on the 445 nm laser power density. The maximum quantum yield of the upconverted emission from the 4CzIPN/pyrene pair was 0.66%, which is considerable when compared with that of a previously reported visible-to-UV TTA-UC system with a biacetyl/PPO pair (0.58%).