Discovery of Key TIPS-Naphthalene for Efficient Visible-to-UV Photon Upconversion under Sunlight and Room Light*.

While many studies have been done on triplet-triplet annihilation-based photon upconversion (TTA-UC) to produce visible light with high efficiency, the efficient TTA-UC from visible to UV light, despite its importance for a variety of solar and indoor applications, remains a challenging task. Here, we report the highest visible-to-UV TTA-UC efficiency of 20.5 % based on the discovery of an excellent UV emitter, 1,4-bis((triisopropylsilyl)ethynyl)naphthalene (TIPS-Nph). TIPS-Nph is an acceptor with desirable features of high fluorescence quantum yield and high singlet generation efficiency by TTA. TIPS-Nph has a low enough triplet energy level to be sensitized by Ir(C6)2 (acac), a superior donor that does not quench UV emission. The combination of TIPS-Nph and Ir(C6)2 (acac) realizes the efficient UV light production even with weak light sources such as an AM 1.5 solar simulator and room LEDs.