title: 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 (Ka11) 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 pi-conjugation, as compared to 2,6-dimethylphenyl (3c) and the previously reported derivatives (1a-c). Aryl-substitution at the alpha-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 pi-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 pi-planes controlled by external chemical stimuli.
authors.affiliation: Department of Bioscience and Biotechnology, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu 525-8577, Japan. maedahir@se.ritsumei.ac.jp
authors.forename: Hiromitsu
authors.initials: H
authors.lastname: Maeda
authors.forename: Yohei
authors.initials: Y
authors.lastname: Haketa
authors.forename: Takashi
authors.initials: T
authors.lastname: Nakanishi
mesh_terms: D000838:Anions / Q000737:chemistry; D001665:Binding Sites; D001880:Boranes / Q000737:chemistry; D001896:Boron Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D018360:Crystallography, X-Ray; D005782:Gels / Q000138:chemical synthesis* / Q000737:chemistry; D046911:Macromolecular Substances / Q000138:chemical synthesis* / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D010316:Particle Size; D044367:Phase Transition; D011758:Pyrroles / Q000138:chemical synthesis / Q000737:chemistry*; D012399:Rotation; D012680:Sensitivity and Specificity; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D013237:Stereoisomerism
chemical_list: D000838:Anions; D001880:Boranes; D001896:Boron Compounds; D005782:Gels; D046911:Macromolecular Substances; D011758:Pyrroles; C021274:boron trifluoride