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

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 ( k q ) increases as the excited state oxidation potential ( E ox * ) becomes more negative. The dependence of k q on E ox * 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 ( E red ). The pyrimidines 1,3-dimethylthymine, 1,3-dimethyluracil, and 1,3,6-trimethyluracil give E red values (in CH 3 CN) 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 E red values ranging from −1.90 to −2.07 V. The higher E red values for the dihydropyrimidines compared with their unsaturated derivatives is attributed to aromatic stabilization in the pyrimidines, which is not present in the dihydro derivatives. In addition, the E red for both the trans- syn and cis-syn diastereomers of the dimethylthymine cyclobutane dimer was examined using the same method. The trans-syn dimer gives an E red of −1.73 V and the cis-syn dimer gives an E red of −2.20 V. This remarkable difference is attributed to a stereoelectronic effect. The cis-syn dimer anion radical suffers from an unfavorable charge−dipole interaction between the added electron and the O 4 carbonyl group in the remaining pyrimidine ring. In contrast, the trans- syn dimer anion radical shows mainly a stabilizing inductive electron-withdrawing effect of the remaining O 4 carbonyl group. Solvent effects on E red were also examined. It is shown that the protic solvent, CH 3 OH, significantly stabilizes the anion radicals, raising E red by ca. 400 mV over the value in CH 3 CN.