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Acidichromic effects in 1,2‐di‐ and 1,2,4‐tri‐ hydroxyanthraquinones. A spectrophotometric and fluorimetric study
Author(s) -
Miliani C.,
Romani A.,
Favaro G.
Publication year - 2000
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/(sici)1099-1395(200003)13:3<141::aid-poc220>3.0.co;2-j
Subject(s) - alizarin , chemistry , fluorescence , dissociation (chemistry) , excited state , titration , absorbance , ground state , ion , analytical chemistry (journal) , photochemistry , nuclear chemistry , inorganic chemistry , organic chemistry , chromatography , physics , quantum mechanics , nuclear physics
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 (Φ F = 10 −3 –10 −2 ) than for alizarin (Φ F = 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 p K s of the first and second dissociation steps in the ground state. Excited‐state p K *s were calculated by the Förster cycle. In the ground state, purpurin (p K 1 = 4.7; p K 2 = 9.5) is a stronger acid than alizarin (p K 1 = 6.6; p K 2 = 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.