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 substrate s was examined. For all of the substrates studied the rate constant of fluorescence quenching (k(q)) increases as the excited state oxidatio n 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. F its of the data to this relationship allow for the estimation of the r eduction potentials of the substrates (E-red) The pyrimidines 1,3-dime thylthymine, 1,3-dimethyluracil, and 1,3,6-trimethyluracil give E-red values (in CH3CN) ranging from -2.06 (vs SCE) to -2.14 V. Their dihydr o derivatives, 1,3-dimethyl-5,6-dihydrothymine, 1,3-dimethyl-5,6-dihyd rouracil, and 1,3,6-trimethyl-5,6-dihydrouracil gave E-red values rang ing from -1.90 to -2.07 V. The higher E-red values for the dihydropyri midines compared with their unsaturated derivatives is attributed to a romatic 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 ex amined 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 di mer anion radical suffers from an unfavorable charge-dipole interactio n between the added electron and the O-4 carbonyl group in the remaini ng pyrimidine ring. In contrast, the trans-syn dimer anion radical sho ws mainly a stabilizing inductive electron-withdrawing effect of the r emaining O-4 carbonyl group. Solvent effects on E-red were also examin ed. It is shown that the protic solvent, CH3OH, significantly stabiliz es the anion radicals, raising E-red by ca. 400 mV over the value in C H3CN.