Engineering Light-Mediated Bistable Azobenzene Switches Bearing Urea D-Aminoglucose Units for Chiral Discrimination of Carboxylates

The symmetrical molecular receptors 1a and 1b consisting of a photochemically addressable azobenzene tether functionalized with urea hydrogen-bonding groups and D-carbohydrates as chiral selectors were developed to achieve control over the chiral recognition of alpha-amino acid-derived carboxylates. The photo- and thermally interconvertible planar E-1 and concaved Z-1 were found to exhibit different affinities, selectivities, and binding modes toward these biologically important anions in a highly polar medium (DMSO + 0.5% H2O). Binding affinity for the same enantiomerically pure guest was up to 3 times higher for E-1 than for Z-1 (cf. parameter beta). In addition, the rate of thermal Z -> E isomerization was found to depend on the chiral binding ability of Z-1, i.e., more strongly bound carboxylate enantiomer as well as higher enantiomer concentration caused faster relaxation to E-1.