Photon-Upconverting Ionic Liquids: Effective Triplet Energy Migration in Contiguous Ionic Chromophore Arrays

Inspired by the bicontinuous ionic‐network structure of ionic liquids (ILs), we developed a new family of photofunctional ILs which show efficient triplet energy migration among contiguously arrayed ionic chromophores. A novel fluorescent IL, comprising an aromatic 9,10‐diphenylanthracene 2‐sulfonate anion and an alkylated phosphonium cation, showed pronounced interactions between chromophores, as revealed by its spectral properties. Upon dissolving a triplet sensitizer, the IL demonstrated photon upconversion based on triplet–triplet annihilation (TTA‐UC). Interestingly, the TTA‐UC process in the chromophoric IL was optimized at a much lower excitation intensity compared to the previous nonionic liquid TTA‐UC system. The superior TTA‐UC in this IL system is characterized by a relatively high triplet diffusion constant (1.63×10−6 cm2 s−1) which is ascribed to the presence of ionic chromophore networks in the IL. On a collision course: Fluorescent ionic liquids (ILs) that show triplet–triplet‐annihilation‐based photon upconversion (TTA‐UC) are reported. The presence of nanostructured ionic chromophore networks explains the fast triplet energy migration and consequent efficient TTA‐UC emission detected at a low excitation power comparable to solar irradiance.