A dibasic acid with reversed order of the stepwise ionization constants: 2,7‐dichlorofluorescein in the ternary solvent mixture benzene–ethanol–water

Abstract

AbstractThe ionization constants of 2,7‐dichlorofluorescein were determined and the molecular structures of the equilibrium forms were established in the ternary solvent system benzene–ethanol–water (mass ratio 47:47:6) by using visible spectroscopy. In this solvent mixture with relative dielectric constant εr = 12.8 and normalized Reichardt's parameter E = 0.587, an inversion of the stepwise ionization constants of the dye, Ka1 and Ka2, occurs. The Ka1/Ka2 ratio changes from 16 in water and 4.8 in 50 mass % aqueous ethanol to 0.1 in the ternary solvent system with ionic strength of 0.002 mol dm−3, where pKa1 = 9.23 ± 0.08 and pKa2 = 8.22 ± 0.08 (25 °C). Such an extraordinary interrelation between the Ka constants is in line with the molecular structure of ionic and nonionic species of 2,7‐dichlorofluorescein deduced from their vis absorption spectra, as well as with the expressed salt effects and the probable ion association. The dianion R2− (5 in Scheme 1) possesses a band with λmax = 512 nm and a molar absorptivity of E = 84.3 × 103 cm−1 mol−1 dm3. The tautomeric equilibrium of the neutral form, H2R, is strongly shifted toward the colorless lactone (2 in Scheme 1). The (very intensive) absorption band of the HR− species singled out from the vis spectra at different acidity, is red‐shifted by Δλ ≈ 20 nm against the band of the dianion R2−. Hence, the monoanion HR− is found to be completely converted into the ‘phenolate’ tautomer 4 with groups COOH and O−, while in aqueous solutions the ‘carboxylate’ tautomer 3 with groups COO− and OH predominates. In 50 mass % aqueous ethanol, the two tautomers 3 and 4 exist in commensurable concentrations. Emission and fluorescence excitation spectra and fluorescence quantum yield of the dianion R2− in the ternary solvent mixture were determined. Copyright © 2006 John Wiley & Sons, Ltd.