CHLORANIL-PHOTOSENSITIZED MONOMERIZATION OF DIMETHYLTHYMINE CYCLOBUTANE DIMERS AND EFFECT OF MAGNESIUM PERCHLORATE

Abstract— The photosensitized monomerization of the cyclobutane dimers of 1,3‐dimethylthymine by p‐chloranil was investigated by means of steady‐state irradiation and laser‐flash photolysis. Quantum yields for the monomerization are 0.34 for the cis, syn dimer, 0.39 for the trans, syn dimer, and < 10−2 for the cis, anti isomer. Formation of the chloranil anion radical associated with quenching of triplet chloranil by the dimers demonstrates that electron transfer from dimers to triplet chloranil occurs to initiate the monomerization. Kinetic analysis suggested that the swdimer cation radicals undergo the ring cleavage at 10o s−1 before escaping from the solvent cage, while the reactivity of the anti‐dimer cation radical is very low. The different reactivities of the syn and anti dimer cation radicals are discussed in terms of through‐bond coupling between the n orbitals of N(1) and N(1') involving the cyclobutane‐ring s̀ orbitals. In the cases of the syn‐dimers, the sensitizer‐dimer ion‐radical pairs undergo the rapid geminate recombination that works as a major energy dissipating channel responsible for the lower‐than‐unity quantum yields. It has been found that the presence of Mg(C1O4)2 at 0.1 M enhances 1.5 times either the monomerization of the syn dimers or the formation of the chloranil anion radical. A laser‐flash photolysis study shows that Mg2+ foms a complex with either the triplet or the anion radical of chloranil. The net salt effects are attributed to the retardation of the rapid geminate recombination by the participation of Mg2+ in the sensitizer‐dimer ion‐radical pairs. Copyright © 1990, Wiley Blackwell. All rights reserved