Visible‐to‐UV Photon Upconversion in Nanostructured Chromophoric Ionic Liquids

Visible‐to‐ultraviolet (vis‐to‐UV) triplet‐triplet annihilation based photon upconversion (TTA‐UC) is achieved in a non‐volatile chromophoric ionic liquid (IL) for the first time. A novel IL is synthesized by combining UV‐emitting anion 4‐(2‐phenyloxazol‐5‐yl)benzenesulfonate (PPOS) and trihexyltetradecylphosphonium cation (P(66614)). The nanostructured organization of chromophoric anions is demonstrated by synchrotron X‐ray and optical measurements. When the IL is doped with a triplet sensitizer tris(2‐phenylpyridinato)iridium(III) (Ir(ppy)(3)), the visible‐to‐UV TTA‐UC with a relatively low threshold excitation intensity of 61 mW cm(−2) is achieved. This is due to a large triplet diffusion coefficient in the IL (1.4×10(−7) cm(2) s(−1)) as well as a high absorption coefficient 15 cm(−1) and a long PPOS triplet lifetime of 1.55 ms, all implemented in the condensed IL system. This work demonstrates the unique potential of ILs to control chromophore arrangements for desired functions.