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 (A 4 L 6 ) and helicate (A 2 L 3 ), 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 ⊂ ([18]crown-6)} ⊂ 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.