A New Chiral Probe for Sulfate Anion: UV, CD, Fluorescence, and NMR Spectral Studies of 1:1 and 2:1 Complex Formation and Structure of Chiral Guanidinium−p-Dimethylaminobenzoate Conjugate with Sulfate Anion

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Abstract
A new chiral chromophoric host 1, possessing a 4-(N,N-dimethylamino)benzoate (DMAB) group tethered to a chiral bicyclic guanidinium subunit, was synthesized and applied to the probe for sulfate anion. Host 1 showed typical successive 1:1 and 2:1 host:guest complexation behavior toward the divalent sulfate anion, as revealed by UV−vis, CD, fluorescence, and 1H NMR spectroscopic studies. The DMAB chromophore was shown to be a sensitive CD spectral probe for assessing not only the complexation behavior but also complex stoichiometry and structure. The stepwise 1:1 and 2:1 complexation constants (K1 and K2) were determined as 1.53 × 106 and 4.84 × 104 M-1, respectively, by NMR titration in CD3CN. The CD exciton chirality method allowed us to determine the chiral sense (spatial arrangement) of the two DMAB moieties in the 2:1 complex as negative (counterclockwise). The dual fluorescence behavior of DMAB was employed for elucidating the role of the countercation upon complexation of host 1 with sulfates possessing lipophilic countercation(s) such as tetrabutylammonium.