The phototransformation of 2-chloro, 6-chloro and 2,6-dichloropurines under UVC excitation (254 nm) has been studied and the major photoproducts have been identified using absorption spectroscopy, HPLC and mass spectrometry. It was shown that hydroxypurines were formed as the main products for all three investigated compounds both in the presence and absence of
oxygen. In the case of 6-chloro- and
2,6-dichloropurine, a photodimer is also formed as a minor photoproduct in the absence of
oxygen but is efficiently quenched in the presence of
oxygen. Nanosecond photolysis experiments also revealed significant intersystem crossing to the triplet state of the chloropurines which has been characterized (transient absorption spectra, triplet formation quantum yields and rate constants of quenching by
oxygen, Mn2+
ions and ground state). Experimental evidence allows to conclude that the triplet state is involved in photodimer formation whereas the hydroxypurine is formed from the reaction of the excited singlet state of chloropurines with the
solvent (water addition) through heterolytic C-Cl bond
rupture. Mass spectrometry and 1H NMR results allowed to propose a chemical pathway for dimer formation in the case of
2,6-dichloropurine in a two-step process: first a homolytic
rupture of C-Cl bond in the triplet state of the molecule with the formation of purinyl radicals, which subsequently react with an excess of ground state molecules and/or hydroxypurine primarily formed.