At the “Peak” of vis-to-UV upconversion : clear advantages of TIPS substituents for a biphenyl annihilator

Abstract

Photon upconversion is a highly potent and versatile concept that can improve the performance of many contemporary technologies, such as photovoltaics, bioimaging, 3D printing, and various bond activation processes for chemical applications. Many different strategies like two-photon absorption (2PA), lanthanide-based upconversion nanoparticles (UCNPs), nonlinear optics, or sensitized triplet–triplet annihilation upconversion (sTTA-UC) can be utilized for the absorption of two low-energy photons and their conversion into one high-energy photon. Among these approaches, sTTA-UC shows great application potential since upconversion can operate under low incident excitation intensities utilizing noncoherent light sources. Owing to the need for high-energy photons in a multitude of applications, blue-to-UV upconversion is an especially relevant topic. The first blue-to-UV upconversion system via sensitized triplet–triplet annihilation upconversion was originally explored by Castellano et al. in 2006. A system consisting of Ir(ppy)3 as sensitizer (Sens) and 1,6-di-tert-butylpyrene as triplet energy acceptor and annihilator (An) was used, which laid the grounds for the implementation of blue-to-UV sTTA in chemical applications that would otherwise rely on high-energy UV excitation. The interest in such upconversion systems received a further boost by the discovery of TIPS-naphthalene as a benchmark annihilator by Kimizuka and Yanai, which, in combination with the sensitizer Ir(C6)2(acac), yielded the first highly efficient blue-to-UV upconversion system with achievable quantum yields ϕUC beyond 10%. With numerous recent application examples of blue-to-UV upconversion systems, such as in 3D printing, wastewater treatment, photocatalysis, and biomedical applications, the interest in the development of novel, even more efficient vis-to-UV upconversion systems is still increasing. Especially blue-to-UV upconversion systems generating wavelengths as short as possible are desired, where the high energy emission can photoexcite and activate a broad range of compounds and can overcome energy barriers that are inaccessible for visible-light driven systems. A few upconversion systems that utilize sTTA-UC to generate photons in the UV-B or even the UV-C region have been developed, but they lag behind in upconversion efficiency, leading to an enormous efficiency falloff for annihilators with emission wavelengths <365 nm. UV upconversion systems face challenges owing to the intrinsic nature of UV chemistry, such as quick degradation as well as increased filter effects. Moreover, the high excited-state energies seem to be responsible for inherent loss channels during the TTA process that still have to be understood. However, this spectral range is highly attractive because commercial high-power LEDs with peak emission wavelengths <365 nm are not available. This work extends the range of efficient upconversion (ϕUC > 10% in the linear region of the intensity dependence) systems to ∼350 nm in the UV region (emission onset at ∼325 nm), while simultaneously further exploring the influence of different substituents at the silylethynyl groups on potential annihilators.