Reduced Radical Characteristics of 7-Aminocoumarin Dyes Studied by Pulse Radiolysis Technique

Abstract

Characteristics of the reduced radicals of a series of 7-aminocoumarin dyes (C) have been investigated in different solvents using pulse radiolysis (PR) technique. Since these coumarin dyes are almost insoluble in water, a mixed solvent (MS) system containing 5.0 mol dm-3 of 2-propanol (2PNL) and 1.0 mol dm-3 of acetone (ACT) in water has been used in the present work to substitute the aqueous solvent. In MS, for a wide pH range of ∼1−9, the reduced coumarin radicals are formed in the neutral form, CH•. These radicals show two absorption bands, one in the 300−400 nm region and the other in the 500−600 nm region. As the coumarin dyes undergo hydrolysis in strongly alkaline solutions, PR studies in MS could not be extended beyond pH ∼9. In 2PNL, the reduced coumarin radicals are also formed in the CH· form, even in the presence of ∼1 mol dm-3 of a strong proton acceptor like triethylamine (TEA). Since the pKb value of TEA is 3.19, it is indicated that the acid dissociation constant of the CH· radicals (pKaR) must be >11. The PR results in MS and 2PNL clearly indicate that the anionic form of the reduced coumarin radicals (C•-) formed by initial one-electron reduction of the dyes undergo very fast protonation to give the CH• form in the solution. To avoid protonation of C•-, PR experiments were carried out in acetonitrile (ACN) solutions in the presence of ∼1 mol dm-3 of aniline (AN). With this high concentration, AN efficiently scavenges the cation radicals from the solvent ion pairs (ACN+···e-) formed following the electron pulse, and thus the equivalent amount of electrons become available to reduce C to C•-. It is seen that the C•- radicals of all the coumarin dyes studied have unusually weak absorption bands in the 500−800 nm region. Since the coumarin dyes are good electron acceptors, present results on CH• and C•- radicals will be useful in elucidating the ET mechanism using coumarin dyes as the electron acceptors.