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, noncoherent 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 of novel 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 give 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 nonradiative 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 that of 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 must be understood for obtaining optimized systems.