STRUCTURE‐REACTIVITY RELATIONSHIPS IN REDOX‐PHOTOSENSITIZED SPLITTING OF PYRIMIDINE DIMERS AND UNUSUAL ENHANCING EFFECT OF MOLECULAR OXYGEN *

Abstract

Abstract Redox photosensitization using the phenanthrene‐ p ‐dicyanobenzene pair in acetonitrile has been applied to the respective four isomeric dimers of N.N′‐dimethylthymine (DMT) and N,N′‐dimethyluracil (DMU) as well as to several related cyclobutane compounds. The head‐to‐head ( syn ) dimers of both DMT and DMU can undergo photosensitized splitting in the following order of efficiency: cis, syn dimer of DMT > cis, syn dimer of DMU > trans, syn dimer of DMT. On the other hand, the head‐to‐tail ( anti ) dimers are totally unreactive and have higher oxidation potentials than the corresponding syn dimers. It is suggested that the key mechanistic pathway is the formation of π complexes between the dimers and the photo‐generated cation radical of phenanthrene by way of which splitting of the cyclobutane ring catalytically occurs without the formation of the discrete cation radical of the dimers. Structure‐reactivity relationships are interpreted in terms of through‐bond interactions between the n orbitals of N(l) and N(l′) involving the C(6)‐C(6′) bond, as well as in terms of steric repulsion. It was found that aeration of solution greatly enhances the quantum yields of photosensitized splitting; the limiting quantum yield for splitting of the cis, syn dimer of DMT is 100.