Spectral Properties of Fluorophores Combining the Boronic Acid Group with Electron Donor or Withdrawing Groups. Implication in the Development of Fluorescence Probes for Saccharides

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Abstract
We evaluated the spectral properties of four stilbene derivatives containing the boronic acid group [−B(OH)2]:stilbene-4-boronic acid (STBA), 4‘-cyanostilbene-4-boronic acid (CSTBA), 4‘-methoxystilbene-4-boronic acid (MSTBA), and 4‘-(dimethylamino)stilbene-4-boronic acid (DSTBA). The emission spectrum of DSTBA displays a large solvent-polarity dependence showing the formation of a photoinduced charge transfer state (CT). This state is weakly present in MSTBA and not present for CSTBA and STBA for the neutral form of the boronic acid group. These results show the donor withdrawing property of the neutral form of the boronic acid group. At higher pH, the boronic acid group is present in the anionic form [−B(OH)3-], resulting in a change of the configuration around the boron atom from the triangular planar (sp2 hybridization) to the tetrahedral conformation (sp3 hybridization). This change induced a blue shift of about 50 nm and an increase of intensity in the emission spectrum of DSTBA because of the loss of the electron-withdrawing properties for the anionic form of the boronic acid group, leading to the loss of the CT effect. The same effect is also observed for MSTBA. In contrast, a red shift of about 35 nm and a decrease of intensity are observed for CSTBA from the neutral to the anionic forms of the boronic acid group. These observations lead to the conclusion that the anionic form of the boronic acid group acts as an electron donor group and a photoinduced CT state can be formed when an electron withdrawing group is present on the fluorophore. The usefulness of this effect for the development of saccharide probes is also demonstrated. After addition of sugar, the emission spectra of DSTBA and MSTBA showed a blue shift and an increase of the intensity. On the other hand, a red shift and a decrease of the intensity are observed in the emission spectra of CSTBA after addition of sugar. A change from the neutral to the anionic form of the boronic acid group is used to explain these changes. These results show that the use of the combination of electron donor or withdrawing groups with the boronic acid group is a new and promising way to develop ratiometric fluorescent probes for glucose and other saccharides.

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