Fluorescent Anion Receptor with Two Methylene Bridged Bis-imidazolium Ring on Anthracene

Abstract

Artificial receptors for selective anion recognition is an area of intensive investigation as anions play a fundamental role in a wide range of chemical and biological process. Receptors based on guest anion induced changes of color or fluorescence seems to be attractive due to their high sensitivity and low detection limit. In many cases, hydrogen bonds are utilized as anion recognition element as host-guest complementarity could be achieved from the directionality of hydrogen bonds. The orientation of hydrogen bonds can differentiate anionic guests with different geometries. While most of hydrogen bond anion receptors utilize N-H---X− or O-H---X− hydrogen bonds, 1,3-disubstituted imidazolium groups are recently introduced as new anion binding hydrogen bond moiety by forming (C-H)--X hydrogen bonds between C(2)-H in imidazolium rings and the guest anion. Depending on the spatial arrangement of 1,3-disubstituted imidazolium groups, halide, dihydrogen phosphate, dicarboxylate, sulfate selective receptors have been reported. To extend the scope of imidazolium anion receptor, we have designed and synthesized the receptor 1, bearing two methylene bridged bis-imidazolium ring on 1,8-positions of anthracene. For the synthesis of anthracene receptor 1, methylene bridged bis-imidazole 2 was synthesized in 64% yield from the reaction of imidazole and bromochloromethane. Then the bis-imidazole 2 was reacted with 0.3 equivalents of ethyl bromide to give imidazolium ion 3 in 37% yield. This intermediate was refluxed in acetonitrile for 4 hours with 1,8-bis(bromomethyl)anthracene. Anion exchange with ammonium hexafluorophosphate gave receptor 1 bearing two methylene bridged bis-imidazolium ring on 1,8-positions of anthracene (Scheme 1). All compounds were characterized by H NMR, C NMR and high resolution mass spectrum. The complexation abilitiy of receptor 1 to the halides was measured by standard H NMR titration experiments in 10% DMSO-d6 in CD3CN using a constant host concentration(4 mM) and increasing concentrations of anions (1-10 equivalents). The chemical shift data were analyzed by EQNMR. The addition of tetrabutylammonium halide salts to the solution of 1 in 10% DMSO-d6 in CD3CN resulted in downfield shifts in two C(2) proton peaks of imidazolium ring. In case of iodide ion, two C(2) protons originally resonating at δ = 9.10 and 9.05 were shifted to δ = 9.50 and 9.47 upon the addition of 5 equivalents of iodide ion, which indicates that all C(2) protons are involved in binding events. Assuming 1 : 1 binding stoichiometry, the association constant calculated from the chemical shift change of C(2)H of imidazolium ring was 103 ± 11. The anthracene receptor 1 displayed strong fluorescence emission in 10% DMSO in acetonitrile solution as shown in Figure 1. The excitation and emission wavelength were 365 nm and 413 nm respectively. The associations between the anthracene receptor 1 and spherically shaped halides were investigated by fluorescence titration. The fluorescence change of the receptor 1 was monitored in 10% DMSO in acetonitrile. The intensity of emission spectrum from 10 μM solution of the anthracene receptor 1 decreased as the concentration of tetrabutylammonium halides salts was increased, which indicates the association between the receptor 1 and halides. The stoichiometry between host and guest was determined by fluorescence Job plot, which