Wavelength-ratiometric probes for saccharides based on donor-acceptor diphenylpolyenes

The spectroscopic and photophysical properties of two donor-acceptor derivatives of 1,4-diphenylbuta-1,3-diene and 1,6-diphenylhexa-1,3,5-diene are described. Both compound posses a dimethylamino and a boronic acid groups as electron-donor and electron-withdrawing groups, respectively. Solvent polarity effects on the steady-state and fluorescence intensity decay are presented and show the formation of an excited-state charge-transfer (CT) state for both compounds. The formation of the anionic form of the boronic acid group at high pH induces a blue shift and an increase of the intensity in the emission spectra for both compounds. These spectral changes are interpreted as the lost of the electron-withdrawing property of the anionic form of the boronic acid group. The observed pK _a of both compounds is around 8.8 and decrease to ~6 and ~7 in presence of fructose and glucose, respectively. Both compounds display a decrease of the mean lifetime at higher pH. Effects of the sugars on the fluorescence spectra and fluorescence lifetimes are also presented. For both compounds, a blue shift and an increase of the intensity are observed. These spectral changes lead to a wavelength-ratiometric method for the sugar recognition and analysis. Titration curves against fructose, galactose and glucose and dissociation constants are presented. Both compounds show a higher affinity for fructose. The affinity decreases for galactose and for glucose, respectively. Sugar effects on the fluorescence intensity decay are also presented. Both compounds display a decrease of the mean lifetime after addition of sugar.