Visible-to-UV triplet-triplet annihilation upconversion from a thermally activated delayed fluorescence/pyrene pair in an air-saturated solution
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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%).
KeywordsUpconversion Triplet-Triplet Annihilation Anti-Stokes Emission UV Generation Thermally Activated Delayed Fluorescence
energy difference between S1 and T1 states
integrated fluorescence intensity
refractive index of solvent
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This work; was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1D1A3B07049650) and by the Korean government (MSIT) through GCRC-SOP (No. 2011-0030013).
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