Benzofuranyl-derived flavylium salts, chalcones and chalconates: Insights from X-ray structure, DFT calculations and spectroscopic pH response

Abstract

In this contribution, we report the synthesis of benzofuranyl-derived flavylium salts whose structure have been confirmed by mass spectrometry and by NMR. Their structural and electronic properties have been studied by absorption and fluorescence spectroscopies and DFT calculations. Theoretical structural parameters of the calculated compounds were validated by the experimentally obtained X-Ray structure of 2-(2-benzofuranyl)-7-hydroxy-1-benzopyrylium perchlorate. The absorption spectra of the benzofuranyl-derived flavylium salts were simulated with good accuracy with respect to the experimental data. In equilibrated buffered solutions, the pH-driven multi-state evolution reaction of novel benzofuranyl-flavyliums afforded trans-chalcone and chalconate derivatives as the predominant species. The elusive nature of the species formed in solution was corroborated by chemical quantum calculations. The fluorescence bands of the flavylium, trans-chalcone and chalconate forms were assigned to their fluorophoric units by selective excitation of each one of the species present in solution. Fluorescence spectroscopy strongly suggests that 2-(2-benzofuranyl)-7-hydroxy-1-benzopyrylium perchlorate displays excited state proton transfer (ESPT).