830 nm-Excitable Triplet–Triplet Annihilation Upconversion Achieved by a Selenium-Integrated Heptamethine Cyanine Sensitizer

We demonstrated that the selenium-integrated heptamethine cyanine acts as a near-infrared photosensitizer when combined with rubrene (annihilator), achieving triplet-triplet annihilation upconversion (TTA-UC) with 830 nm excitation and 557 nm emission and an upconversion quantum yield of up to 1.0% (out of 50%). Compared to the previously reported TTA-UC pair (IR806/rubrene), the upconversion quantum yield of Cy2/rubrene improves 50-fold. Photophysical characterization indicates that the integration of selenium into the cyanine skeleton in Cy2 increases the intersystem crossing rate and triplet excited state quantum yield to 3.22 × 10⁸ s^-1 and 23%, respectively, which is significantly enhanced 3.5-fold and 2.6-fold compared to iodine-substituted heptamethine cyanine (Cy1). The superior triplet excited state properties of Cy2 are crucial factors underlying the exceptional upconversion performance. Eventually, via spin coating the mixture of Cy2/rubrene solution, we fabricated the 830 nm responsive single-layer TTA-UC film, which exhibited excellent transmittance and bright upconversion luminescence. These findings are for the longest wavelength light-excitable metal-free small-molecule-based TTA-UC system, significantly promoting photon upconversion applications in solar energy harvesting and biophotonics.