Raman and FTIR spectroscopies of fluorescein in solutions

Raman and Fourier transform-infra red (FT-IR) spectroscopies of fluorescein in aqueous solutions have been investigated in the pH range from 9.1 to 5.4. At pH 9.1 fluorescein is in the dianion form. At pH 5.4, fluorescein is a mixture of monoanion (∼85%), dianion and neutral forms (together ∼15%). The fluorescence quantum yield drops from 0.93 for the dianion form to 0.37 for the monoanion form. The Raman and FT-IR studies focused on the frequency range from 1000 to 1800 cm −1 which contains the skeletal vibrational modes of the xanthene moiety of fluorescein. At pH 9.1, the spectroscopic feature of fluorescein dianion are consistent with a picture of an electron delocalized among the xanthene moiety and two identical oxygens attached to opposite ends of the xanthene moiety, forming a very symmetric structure. The characteristic of fluorescein dianion is the presence of the phenoxide-like stretch at 1310 cm −1. At pH 5.4, fluorescein monoanion has lost the symmetric structure characteristic of the dianion. The spectra of the monoanion have distinctive contributions from the phenolic bend at 1184 cm −1. The assignments of the vibrational bands shown in Raman and FT-IR spectra are given based on both literature and the ab initio calculations at the Hartree–Fock level with HF/6-31++G* basis set. Excellent correlation is found between the experimental and calculated spectra.