Highly Efficient Upconverted Fluorescence from Coulombically Assembled Ion Pair

Abstract An ion pair consisting of a dicationic Os(II) complex and two monoanionic fluorophores is designed and prepared. The ion pair exhibited strong upconverted (UC) greenish blue fluorescence emission upon photoexcitation of the Os(II) entity at a wavelength of 785 nm, with a high quantum yield of 13% (its theoretical limit of 50%) and a low threshold excitation intensity of 5.9 mW cm⁻². This threshold intensity is at least an order of magnitude smaller than those obtained for previous systems, demonstrating the high sensitivity of the ion pair system for UC fluorescence emission. The notable improvements of the results over previous systems are attributed to diffusion‐free acceleration of bimolecular processes, including triplet‐triplet energy transfer and triplet‐triplet annihilation, within the Coulombically bound ion complex containing the Os(II) photosensitizer and the fluorescent annihilators in a 1:2 stoichiometry, which is ideal for triplet‐triplet annihilation‐based upconversion.