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

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 H-1 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 (K-1 and K-2) were determined as 1.53 x 10(6) and 4.84 X 10(4) 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.