Kinetic and energetic aspects of the photodimerisation of some systemic pyrimidine fungicides

Abstract

The photodimerisation of 2-dimethylamino-4-hydroxy-6-methyl-5-n-butyl-(1), 2-ethylamino-4-hydroxy-6-methyl-5-n-butyl-(2), and 4-hydroxy-5,6-dimethyl-2-dimethylamino-pyrimidine (3) has been investigated in acetonitrile. Quenching experiments have shown that the formation of photodimers involves a triplet state of (1)–(3), the energies of which have been estimated. The quantum yields for intersystem crossing from the singlet to triplet manifolds of (1)–(3) fall in the range 0.09–0.11, and the quantum yields for photodimerisation fall in the range 0.07–0.08. Rate constants for radiationless decay of the triplet state pyrimidines have been determined (2.1–3.0 × 10⁵ s^–1), and also the rate constants for reaction of the triplet states of (1)–(3) with the corresponding ground state pyrimidine (1.5–2.0 × 10⁸ l mol^–1 s^–1). The efficiency of the triplet state dimerisation reactions of (1)–(3) is high compared with that of the analogous triplet state reactions of the biologically important pyrimidines, thymine and uracil.