Light-Triggered High-Affinity Binding of Tetramethylammonium over Potassium Ions by [18]crown-6 in a Tetrahedral Anion Cage

An azobenzene-functionalized bis-bis(urea) ligand (L^azo) and phosphate anion assemble to give the first photoactive “aniono” constructs, tetrahedron (A₄L₆) and helicate (A₂L₃), which readily undergo interconversion through cis-trans isomerization of the azo group under irradiation/heating. Most strikingly, the tetrahedral cage can accommodate an [18]crown-6 molecule, which can capture two tetramethylammonium (TMA⁺) ions with an unusually high affinity, even capable of replacing K⁺ in [K([18]crown-6)]⁺ to form a {(TMA)₂ (Formula presented.) ([18]crown-6)} (Formula presented.) cage “Russian doll” structure. Thus, the current work may provide a model for the light-driven binding and exchange of the biologically important K⁺ and TMA⁺ ions.