Comparative Study of Charge-Assisted Hydrogen- and Halogen-Bonding Capabilities in Solution of Two-Armed Imidazolium Receptors toward Oxoanions
Two-armed imidazolium-based anion receptors have been prepared. The central 2,7-disubstituted naphthalene ring features two photoactive anthracene end-capped side arms with central 2-bromoimidazolium or hydrogen-bonding imidazolium receptors. Combined emission and 1H and 31P NMR studies carried out in the presence of a wide variety of anions reveal that only HP2O73–, H2PO4–, SO42–, and F– anions promoted noticeable changes. The halogen receptor 62+·2PF6– acts as a selective fluorescent molecular sensor for H2PO4– anions, since only this anion promotes the appearance of the anthracene excimer emission band, whereas it remains unchanged in the presence of the other tested anions. In addition this halogen receptor behaves as a chemodosimeter toward HP2O73– anion, through its transformation into the corresponding bis-imidazolone after debromination by the action of the basic anion. The association constant values of the halogen-bonding complexes in a competitive solvent CD3CN/MeOD (8/2) mixture with H2PO4– and SO42– anions are higher than those found for the hydrogen-bonding counterpart. In contrast, in the less competitive CH3CN solvent higher binding affinity for anions corresponds to the hydrogen-bonding receptor 72+·2PF6–. In addition, the receptor 62+·2PF6– represents a useful alternative as an imaging agent in living cells in a wide range of emission wavelengths.