Photoinduced Electron Transfer to Pyrimidines and 5,6-Dihydropyrimidine Derivatives: Reduction Potentials Determined by Fluorescence Quenching Kinetics

Abstract

The dynamics of flourescence quenching of excited state electron donor sensitizers by various pyrimidine and 5,6-dihydropyrimidine substrates was examined. For all of the substrates studied the rate constant of fluorescence quenching (kq) increases as the excited state oxidation potential (Eox* ) becomes more negative. The dependence of kq on Eox* in each case is well described by the Rehm−Weller relationship. Fits of the data to this relationship allow for the estimation of the reduction potentials of the substrates (Ered). The pyrimidines 1,3-dimethylthymine, 1,3-dimethyluracil, and 1,3,6-trimethyluracil give Ered values (in CH3CN) ranging from −2.06 (vs SCE) to −2.14 V. Their dihydro derivatives, 1,3-dimethyl-5,6-dihydrothymine, 1,3-dimethyl-5,6-dihydrouracil, and 1,3,6-trimethyl-5,6-dihydrouracil gave Ered values ranging from −1.90 to −2.07 V. The higher Eredvalues for the dihydropyrimidines compared with their unsaturated derivatives is attributed to aromatic stabilization in the pyrimidines, which i...