A fluorescent probe based on scopoletin-3-carboxylic acid: pH and micellization evaluation

Coumarins are a class of compounds known for their synthetic and structural versatility, as well as their properties of great interest, such as biological and photophysical activities, and their application in the analysis of molecular microenvironments and their characteristics, such as pH. In this work, the synthesis of 7-hydroxy-6-methoxy-2-oxo-2H-chromene-3-carboxylic acid (3-SCA) was reported, and its absorption, emission and excitation spectral properties, along with photophysical properties such as molar absorption coefficient, lifetime, and Stokes Shift (Δν) in different solvents and pHs, were evaluated for the first time. High values of fluorescence quantum yield were observed for 3-SCA in solvents of different polarities, with a lifetime of approximately 5 ns. The Stokes shift values for 3-SCA were high, with these values greater in aqueous medium. Additionally, the acid equilibrium constant values of 3-SCA were evaluated through absorption, emission, and excitation measurements, resulting in two pKa values of 3.7 for the carboxylic acid group at C-3 and 6.6 for the hydroxyl group at C-7. 3-SCA was successfully applied in determining the critical micellar concentration of the surfactants: anionic (SDS), zwitterionic (SB3-14), neutral (F-127), and cationic (CTAB). The high sensitivity of the spectral properties of 3-SCA as a function of pH, combined with the high Stokes shift values in different media, indicates that 3-SCA is an interesting probe with applications in evaluating local pH and micelle formation.