Photophysics of 7-hydroxycoumarin 3-carboxylic acid in micelles

Abstract

7-Hydroxy coumarin-3-carboxylic acid (7-HCCA) is a photoacid that exists in three distinct forms: neutral, anionic and di-anionic. It falls under the category of hydroxy aromatic and carboxy aromatic molecules. This molecule exhibits strong fluorescence and is commonly employed in the detection of hydroxyl radicals. In our study, we utilised absorption and fluorescence spectroscopy to investigate the photophysical behaviour of 7-HCCA in different micellar systems. Through our experiments, we observed that as we varied the pH within the ranges of 2–3, 4–6, 7–10, the three aforementioned forms of 7-HCCA are emerged. Specifically, in cationic surfactants, the dye displayed an equilibrium between two forms of the 7-HCCA in the ground state at any pH. Furthermore, our steady-state excitation studies also proved the presence of an equilibrium between two forms of the 7-HCCA in all cationic systems. By examining the steady-state emission spectra, we also observed the presence of two distinct species in the excited state. When cationic surfactants were introduced, we observed a decrease in the fluorescence intensity and quantum yield compared to that observed in the water medium. It was evident that the photophysical properties of 7-HCCA were modulated with variations of the chain length and head group of the surfactants. In summary, our investigation shed light on the photophysical characteristics of 7-HCCA in different micellar systems with varying the pH. By employing the absorption and fluorescence spectroscopy, we successfully identified the coexistence of multiple forms of 7-HCCA and observed how the cationic surfactants influenced its fluorescence emission properties, ultimately contributing to a deeper understanding of its behaviour in these systems.