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 pK b value of TEA is 3.19, it is indicated that the acid dissociation constant of the CH· radicals (pK a R) 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.

Photoinduced intermolecular electron transfer from aromatic amines to coumarin dyes in sodium dodecyl sulphate micellar solutions

“…55 Thus, in saturated aqueous solutions, the concentrations of the coumarin dyes were estimated to be only in the range of ϳ5ϫ10 Ϫ7 mol dm Ϫ3 , using spectrophotometric measurements. In SDS micellar solutions, however, the dyes are reasonably soluble.…”

Temperature Effect on the Fluorescence Anisotropy Decay Dynamics of Coumarin-153 Dye in Triton-X-100 and Brij-35 Micellar Solutions¶

“…Because the dye C153 is almost insoluble in water (22) and because its solubility increases substantially in micellar solutions, it is expected that C153 will be mostly solubilized in the micellar phase. Steady-state absorption spectra were recorded using a JASCO (Tokyo, Japan) model V530 spectrophotometer.…”

Tuning of electron transfer reactions in pluronic–surfactant supramolecular assemblies

“…[10][11][12][13][14][15][16][17][18][19][20]27 To be mentioned that in the literature there are extensive reports, suggesting and demonstrating experimentally the involvement of photoinduced ET (PET) from ground state amine donors to excited coumarin acceptors as the mechanism for the strong fluorescence quenching observed in the coumarin-amine systems. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]67,68,[76][77][78][79] Drawing an analogy, thus, we attribute the observed fluorescence quenching in the present C343-DMAN system is due to the PET process.…”