Bis(phenylethynyl)benzenes enable stable visible-to-ultraviolet sensitized triplet-triplet annihilation upconversion

Ultraviolet (UV) light plays a central role in applications ranging from photochemistry to sterilization and water treatment. However, its low abundance in sunlight (∼10%) limits the direct solar use of UV-driven processes. Sensitized triplet-triplet annihilation upconversion (TTA-UC) offers a promising route to generate UV light from visible light under low-power excitation. Yet, molecular systems capable of efficient visible-to-UV TTA-UC remain scarce. Here, we demonstrate that 1,4-bis(phenylethynyl)benzene (BPEB) and its alkoxylated derivative serve as efficient UV-emitting annihilators when paired with the visible-light sensitizer Ir(ppy)₃ in toluene solution. These systems achieve upconverted emission centered at 380 nm, with anti-Stokes shifts exceeding 0.6 eV with respect to excitation energy and threshold excitation intensities as low as 11.5 mW cm^-2. Spectroscopic studies suggest that modulation of high-energy excited-state dynamics plays a key role in optimizing upconversion performance. By broadening the molecular design space of UV-emitting annihilators beyond traditional polycyclic aromatics, this study provides a foundation for future development of low-intensity visible-to-UV TTA-UC systems. These findings expand the molecular toolkit for photonic applications where UV emission from ambient light is required.