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 (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 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 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 O4 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 O4 carbonyl group. Solvent effects on E red were also examined. It is shown that the protic solvent, CH3OH, significantly stabilizes the anion radicals, raising E red by ca. 400 mV over the value in CH3CN.

Model Studies on a Carprofen Derivative as Dual Photosensitizer for Thymine Dimerization and (6–4) Photoproduct Repair

“…[38] By using the Rehm-Weller equation, the excited-state oxidation potential of PPMe was estimated from its excited-state energy and the steady-state oxidation potential of carbazole. [39] It was found to be E 1 * ox = À2.30 V versus NHE (singlet) and E 3 * ox = À1.54 V versus NHE (triplet).…”

Promotional effect of ionic liquids in electrophilic fluorination of methylated uracils

“…carbonates. 37,38 The preparation of compounds 3 and 4 was described in ref. It is noteworthy that this method is distinguished by (1) use only of minimum amount of ILs; (2) good potential for recycling of ILs; (3) use of an ecologically acceptable uorination reagent; (4) operational simplicity; (5) use of MeOH and EtOH as 'green solvents'.…”

Investigation of the fluorescence quenching of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) by certain substituted uracils

“…Indeed, the 5-halo uracil derivatives have demonstrated antitumor and antiviral properties [15,16]. The dynamics of fluorescence quenching of excited-state electron donor sensitizers by various pyrimidine substrates was examined by Scannell et al [17]. Rak et al [18] have investigated the effect of complementary pyrimidines (cytosine, thymine) using the twostate model of electron transfer by theoretical methods.…”