Deprotonation and protonation studies of some substituted 1,4-and 9,10-anthraquinones

Ground and excited state deprotonation and protonation pK(a) values of anthraquinones with amino, hydroxy and chloro substituents were determined by means of spectrophotometric titrations and Forster cycle calculations. In the mixed amino-hydroxy 9,10-anthraquinones a second stage of deprotonation and protonation was not registered, and only when the primary amino function is attached to an electron-withdrawing group (-COCH2Cl) a second ionization takes place. Acidity of the hydroxy-1,4-anthraquinones in the ground state is about 2.5 pK(a-) units higher than that of the related 9,10-anthraquinones. The determined values of pK(a+) of protonation are within the limits of -4.46 up to -8.39. The calculated values of the excited state of deprotonation are in the range 1.15-2.57, while the excited state values for protonation vary from 0.91 to 2.57 correspondingly. It was concluded that a proton transfer in the excited state is possible for all the investigated compounds. (C) 2002 Published by Elsevier Science Ltd.