Thermally Activated Delayed Fluorescence Sensitization of Bis(phenylethynyl)benzenes for Visible-to-UV Upconversion

Sensitized triplet–triplet annihilation can be utilized to upconvert low-intensity visible (Vis) light into ultraviolet (UV) light, making it useful for applications powered by short-wavelength sunlight. However, dye systems for this wavelength range are limited. Here, we report that Vis-to-UV upconversion (UC) is achieved with dye pairs that include the thermally activated delayed fluorescence (TADF) luminophores 2,3,5,6-tetrakis(carbazol-9-yl)benzonitrile (4CzBN) or 2,4,5,6-tetrakis(9H-carbazol-9-yl) isophthalonitrile (4CzIPN) as sensitizer and bis(phenylethynyl)benzene (BPEB) or an alkoxylated BPEB derivative as emitter. The photophysical characterization of degassed toluene solutions of the four possible combinations of these dyes reveals efficient triplet energy transfer efficiencies exceeding 80%, upconverted UV emission centered at ca. 380 nm with an anti-Stokes shift of up to 0.64 eV from the blue range of the optical spectrum, excitation threshold intensities as low as ca. 25 mW·cm−2, and UC quantum yields up to 4.5%, depending on the dye combination. These results establish pairs of TADF sensitizers and BPEB emitters as viable candidates for Vis-to-UV (UC), broadening the molecular design space for next-generation photonic and solar-powered UV applications.