Meso-electron-withdrawing pentamethine cyanines achieve large anti-Stokes blue-emitting triplet-triplet annihilation upconversion.

Developing metal-free triplet-triplet annihilation upconversion systems that operate under far-red excitation and produce efficient blue emission is challenging due to an inherent trade-off between a large anti-Stokes shift and a high upconversion quantum yield. Here we show that introducing electron-withdrawing groups at the meso-position of a pentamethine cyanine scaffold narrows the singlet-triplet energy gap, suppresses photoisomerization, and promotes the intersystem crossing. These meso-electron-withdrawing pentamethine cyanines possess higher triplet state energy levels than their meso-unsubstituted counterparts. In particular, the meso-hemicyanine-derived sensitizer exhibits characteristic low-temperature phosphorescence. When paired with perylene, it extends the excitation wavelength to 721 nm and yields blue emission at 470 nm, yielding an upconversion quantum yield of 9.6% (upper limit 50%). This study demonstrates that molecular engineering of pentamethine cyanines via meso-position electron-withdrawing substitution effectively reduces the singlet-triplet energy gap, enabling the design of organic triplet-triplet annihilation upconversion materials and advancing their utility in photonics and biophotonics.