At the “Peak” of vis-to-UV Upconversion: Clear Advantages of TIPS Substituents for a Biphenyl Annihilator

Sensitized triplet–triplet annihilation systems enable the efficient conversion of two low-energy photons from a low-intensity, non-coherent light source into one high-energy photon, opening avenues for diverse applications. The attachment of triisopropylsilylethynyl (TIPS-ethynyl) groups to aromatic compounds has led to the development of many annihilators with high upconversion quantum yields. Here, we synthesized a series novel of symmetrical biphenyls bearing silylethynyl substituents of varying sizes and evaluated them as annihilators in visible-to-UV upconversion systems. While all of these systems do not suffer from excimer formation issues, their upconversion performances differ significantly. Small substituents like trimethylsilylethynyl gave similar upconversion quantum yields (up to ~ 12 %) as the TIPS-ethynyl groups, but the larger triphenylsilylethynyl substituents reduce the achievable quantum yields by half, which is likely due to additional non-radiative loss channels arising from altered energies of higher excited triplet states. The UV emission of the novel annihilators is hypsochromically shifted by about 20 nm relative to the TIPS-naphthalene benchmark UV annihilator, thereby reaching a highly attractive spectral region for bond activation photochemistry. Using the most efficient annihilator, bTIPS-BP, we achieved blue-to-UV upconversion-driven release of fluorescein from a photocage. In the greater context of photon upconversion and in view of other recent reports on substituent effects, our results indicate that several chromophore-specific effects have to be understood for obtaining optimized systems.