Hydrogen Bond-Assisted Excited State Switching and Fluoride Responsive Behavior of Orthogonal Spiroborate Ester Derived from Naturally Occurring α-Mangostin

Abstract

This study presents a sustainable and efficient method for synthesizing a spiroborate ester (MBO) from α-mangostin (MN), a natural xanthonoid isolated from the pericarp of mangosteen fruit. The synthetic process, structural characterization and photophysical properties of MBO are thoroughly discussed. MBO exhibits an absorption band at 370 nm, with a shoulder around 410–450 nm in non-polar solvents, which shifts to 424 nm in polar protic solvents. The compound displays green to blue emission, with intensity decreasing in polar protic environments. These spectral changes result from hydrogen bond-assisted excited state switching between n-π* and π-π* states, as confirmed by computational studies. MBO also shows excellent anion binding properties, functioning as a dual-channel probe for both colorimetric and turn-on fluorescence detection of fluoride ions. This approach not only adds value to the residual mangosteen pericarp by converting it into a fluorescent sensor but also offers a semisynthetic, environmentally benign alternative to conventional organic receptors for fluoride detection.