From Helix to Macrocycle: Anion‐Driven Conformation Control of π‐Conjugated Acyclic Oligopyrroles

Abstract

Abstract Anion‐responsive pyrrole‐based linear receptor oligomers were newly synthesized and their anion‐driven dynamic conformation changes were investigated. Phenylene‐bridged dimers and a tetramer of dipyrrolyldiketone boron complexes as π‐conjugated acyclic anion receptors formed anion‐driven helical structures in the solid and solution states. In fact, single‐crystal X‐ray analyses of the receptor‐anion complexes exhibited various helical structures, such as [1+1]‐ and [1+2]‐type single helices and a [2+2]‐type double helix according to the lengths of oligomers and the existence of terminal aryl substituents. Anion‐binding modes and behaviors of the oligomers in solution state were also examined by 1 H NMR and UV/Vis spectra along with ESI‐TOF MS. Differences in the binding modes were observed in the solid and solution states. The oligomers showed augmented anion‐binding constants and anion‐tunable electronic and optical properties in comparison with the monomer receptor. A negative cooperative effect in the tetramer was observed in the second anion binding of the [1+2]‐type single helix due to electrostatic repulsion between two anions captured in the helix. Further, an anion‐template coupling reaction from the linear dimer provided a receptor macrocycle, which was obtained as a Cl − complex with distinct electronic and optical properties. The macrocycle exhibited extremely high anion‐binding constants (>10 10 m −1 in CH 2 Cl 2 ) through multiple hydrogen bonding.