Acidichromic effects in 1,2-di- and 1,2,4-tri- hydroxyanthraquinones. A spectrophotometric and fluorimetric study

Abstract

Absorption and fluorescence spectra of alizarin (1,2-dihydroxyanthraquinone) and purpurin (1,2,4-trihydroxyanthraquinone) were investigated as a function of pH in water–dioxane (2:1, v/v) in the pH range 2–14. Absorbance changes with pH are accompanied by color changes; by increasing the pH, the spectra shift to the red. These molecules exhibit fluorescence emissions over the whole pH range explored; fluorescence quantum yields increase with increasing pH and are greater for purpurin (ΦF = 10−3–10−2) than for alizarin (ΦF = 10−4–10−3). The fluorescence lifetimes were measured by the phase-shift technique for the three species derived from purpurin (neutral, τ = 150 ps; mono-anion, τ = 2600 ps and di-anion, τ = 380 ps), and only for the di-anion of alizarin (τ = 80 ps). Both spectrophotometric and fluorimetric titrations gave pKs of the first and second dissociation steps in the ground state. Excited-state pK*s were calculated by the Förster cycle. In the ground state, purpurin (pK1 = 4.7; pK2 = 9.5) is a stronger acid than alizarin (pK1 = 6.6; pK2 = 12.4) in both the first and second dissociation steps; in the excited state, purpurin is a weaker acid in both dissociation steps. Copyright © 2000 John Wiley & Sons, Ltd.