title: Recognition, encapsulation, and selective fluorescence sensing of nitrate anion by neutral C3-symmetric tripodal podands bearing amide functionality.
abstract: A series of neutral C(3)-symmetric acyclic artificial receptors incorporating amide functionality has been designed, synthesized, and fully characterized. Upon protonation, these conformationally flexible N-bridged tripodal podands 1-5 form in situ cone shape conformation through hydrogen bonding and C-H···π interactions. The protonation-induced interior preorganized cavity is capable of entrapping nitrate anions through the amide N-H bonds to form discrete nitrate complexes (1a-5a), which were fully characterized by NMR, HRESI mass spectra, and single crystal structures. By incorporating suitable fluorophores at each branch of the tripod receptor, the resulting fluorescent receptor 5 selectively recognized nitrate anions by fluorescent quenching in a DMSO solution and displayed one of the highest binding affinities for nitrate anions reported so far in polar media. Receptor 5 represents a unique example of a neutral receptor for the recognition of nitrate anions in polar solvent media by its zwitterionic form. The possible mechanism of proton-induced preorganization of these flexible, acyclic receptors in a convergent cone conformation followed by nitrate complexation has been proposed to rationalize the effective nitrate recognition.
authors.affiliation: Institute of Chemistry, Academia Sinica, Taipei, 115, Taiwan, Republic of China.
authors.forename: Ashutosh S
authors.initials: AS
authors.lastname: Singh
authors.forename: Shih-Sheng
authors.initials: SS
authors.lastname: Sun
mesh_terms: D000577:Amides / Q000737:chemistry*; D000838:Anions / Q000032:analysis; D015374:Biosensing Techniques / Q000379:methods*; D018360:Crystallography, X-Ray; D004121:Dimethyl Sulfoxide / Q000737:chemistry; D005456:Fluorescent Dyes; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D009281:Naphthalenes / Q000138:chemical synthesis*; D009566:Nitrates / Q000032:analysis*; D011522:Protons; D012996:Solutions; D013050:Spectrometry, Fluorescence
chemical_list: D000577:Amides; D000838:Anions; D005456:Fluorescent Dyes; D009281:Naphthalenes; D009566:Nitrates; D011522:Protons; D012996:Solutions; D004121:Dimethyl Sulfoxide