Aryl-Substituted C3-Bridged Oligopyrroles as Anion Receptors for Formation of Supramolecular Organogels

Abstract

BF₂ complexes of aryl-substituted dipyrrolyldiketones (3a−c, 5a−d) have been synthesized by the condensation of arylpyrroles obtained by Suzuki cross-coupling reactions with malonyl chloride, followed by treatment with BF₃·OEt₂. The binding constants (K_a11) of the BF₂ complexes (3a−c) for various anions (Cl^-, Br^-, CH₃CO₂^-, H₂PO₄^-, and HSO₄^-) in CH₂Cl₂ decrease in the order Ph (3a) > o-tolyl (3b) > 2,6-Me₂Ph (3c), possibly because of differences in the planarity and the number of interacting o-CH units at the binding sites. Aryl-substituted receptors exhibit a [1+1] binding mode with Cl^- as well as a [2+1] binding mode under conditions of high concentration and low temperature, as suggested by ¹H NMR studies in CD₂Cl₂. These receptors, especially phenyl-substituted (3a) and o-tolyl (3b), exhibit drastic colorimetric and fluorescent changes in the presence of F^- due to extended π-conjugation, as compared to 2,6-dimethylphenyl (3c) and the previously reported derivatives (1a−c). Aryl-substitution at the α-positions of pyrrole is an excellent means for the introduction of various substituents at the periphery of the anion receptors. For example, derivatives with long alkoxy chains at 3,4,5-positions of the substituent aryl rings (5b−d) afford emissive gel structures in hydrocarbon solvents, such as octane, based on the stacking of slipped H- and J-aggregates at the core π-plane. The structural organization of the supramolecular gels was investigated by AFM, SEM, and XRD measurements as well as by considering the solid-state packing of crystalline derivatives. The slow transformation of the gel to the solution phase by the addition of various anions, possibly except for F^-, is correlated with the unique properties of these acyclic receptors where inversions of pyrrole rings are required for anion binding. Boron complexes of 1,3-dipyrrolyl-1,3-propanediones with aryl-substituents, as a new class of acyclic anion receptors, have shown efficient binding due to the interacting o-CH units and, in the case of the derivative with long aliphatic chains, afforded the emissive supramolecular organogels using stacking of core π-planes controlled by external chemical stimuli.