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

Absorption and fluorescence spectra of alizarin (1,2-dihydroxyanthraquinone ) and purpurin (1,2,4-trihydroxyanthraquinone) were investigated as a funct ion of pH in water-dioxane (2:1, v/v) in the pH range 2-14. Absorbance chan ges with pH are accompanied by color changes; by increasing the pH, the spe ctra shift to the red. These molecules exhibit fluorescence emissions over the whole pH range explored; fluorescence quantum yields increase with incr easing pH and are greater for purpurin (Phi(F); = 10(-3)-10(-2)) than for a lizarin (Phi(F) = 10(-4)-10(-3)). The fluorescence lifetimes were measured by the phase-shift technique for the three species derived from purpurin (n eutral, tau = 150 ps; mono-anion, tau = 2600 ps and di-anion, tau = 380 ps) , and only for the di-anion of alizarin (tau = 80 ps). Both spectrophotomet ric and fluorimetric titrations gave pKs of the first and second dissociati on steps in the ground state. Excited-state pK*s were calculated by the For ster cycle. In the ground state, purpurin (pK(1) = 4.7; pK(2) = 9.5) is a s tronger acid than alizarin (pK(1) = 6.6; pK(2) = 12.3;) in both the first a nd second dissociation steps; in the excited state, purpurin is a weaker ac id in both dissociation steps. Copyright (C) 2000 John Wiley & Sons, Ltd.