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

Abstract

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.

Approaching the Spin-Statistical Limit in Visible-to-Ultraviolet Photon Upconversion

“…This value is to the best of our knowledge the highest vis-to-UV Φ UC,g reported to date and a significant improvement on the previous record. 28 − 30 The remaining systems yielded Φ UC,g values ranging from 4 to 14%, and full results are presented in Figure 2 C and Table 3 . It should be noted that the values achieved for TP (12.6%) and PPD (5.8%), which both emit from singlet states just shy of 4 eV, are multifold improvements on that previously reported 30 , 63 and likely result from more efficient TET and, subsequently, more efficient TTA between triplets.…”

“…Significant progress has however been made recently, with reports on a Φ UC of around 10% for three different systems. 28 − 30 Still, there is no fundamental reason as to why much higher efficiencies would not be possible.…”

“… 30 The 10% limit has also been surpassed by pairing an iridium complex or a ketocoumarin derivative with 1,4-bis((triisopropylsilyl)ethynyl)naphthalene (TIPS-Naph), systems which also demonstrate low threshold excitation intensities ( I th ). 28 , 29 Other annihilators previously investigated include other naphthalene and oxazole derivatives, 8 , 30 , 36 species from the terphenyl family, 13 , 37 and as of recently also a biphenyl derivative with the capability to emit light beyond 4 eV. 38 …”

“…Specifically, employing TIPS-Naph as the annihilator species yields a record-setting internal Φ UC of 16.8% (out of a 50% maximum), which is a significant improvement on the previously best performing vis-to-UV TTA-UC systems. 28 − 30 High internal Φ UC values are also obtained for PPO (14.0%), 2,5-diphenylfuran (PPF, 13.0%), a compound never used for vis-to-UV TTA-UC before, and for p -terphenyl (TP, 12.6%), a compound which emits much deeper in the UV region. The performances of the remaining systems are also evaluated, and the intrinsic properties governing the TTA-UC process are obtained and analyzed.…”

Low power threshold photochemical upconversion using a zirconium(iv) LMCT photosensitizer

“…17,18 However, realizing high normalized upconversion efficiency values (hUC), where hUC ³ 20% at or below solar irradiance, remains a significant scientific challenge. 19 Synthetic advances enabling control of the photophysical properties of transition metal complexes and materials are revolutionizing triplet sensitization. [20][21][22][23][24][25][26][27][28][29] Numerous molecular triplet sensitizers featuring long-lived excited states contain rare-earth and precious second-or third-row transition metals, including Ru(II) [30][31][32][33] , Ir(III) [34][35][36][37] , Re(I) 38,39 , Pt(II) or Pd(II).…”