Novel Saccharide-Photoinduced Electron Transfer Sensors Based on the Interaction of Boronic Acid and Amine

Two boronic acid systems, monoboronic acid 3 and diboronic acid 8, were synthesized. When saccharides form cyclic boronate esters with these boronic acids, the Lewis acid–base interaction between the boronic acid moiety and tertiary amine is strengthened; when saccharides form cyclic boronate esters with boronic acids the acidity of the boronic acid is enhanced. The strength of this acid–base interaction modulates the photoinduced electron transfer (PET) from the amine to anthracene. Both of these compounds show increased fluoresecence at pH 7.77 through supression of the photoinduced electron transfer from nitrogen to anthracene on saccharide binding, a direct result of the stronger boron–nitrogen bond. Compound 3 shows the typical selectivity of monoboronic acids towards saccharides. Compound 8 which has a cleftlike structure is particularly selective and sensitive for glucose due to the formation of an intramolecular 1:1 complex between the two boronic acids and the 1,2- and 4,6-hydroxyls of glucose. This is the first example in which ditopic recognition of monosaccharides is achieved in a PET sensor system.