On the photolysis of bis[2-( o-chlorophenyl)-4,5-diphenylimidazole] sensitized by 2-isopropylthioxanthone or Michler's ketone

Abstract

The triplet states of 2-isopropylthioxanthone (ITX) and 4,4′-bis( N,N′-dimethylamino)benzophenone (Michler's ketone, MK) react very rapidly with bis[2-( o-chlorophenyl)-4,5-diphenylimidazole] (I 1) as inferred from flash photolysis studies. The rate constants of the reaction of 3ITX * and 3MK * with I 1 (determined at room temperature in acetonitrile and benzene solution) are about 1 × 10 10 dm 3 mol −1 s −1. Imidazolyl radicals (R 1 ) are formed in these reactions as the major product (φ(R 1 ) = 1.1). By contrast, the quantum yield is higher (φ(R 1 ) ≈ 2 in the direct photolysis of I 1. With respect to the reaction mechanism, triplet—triplet energy transfer can be excluded since E T(I 1) > E T(ITX) and E T(I 1) > E T(MK). Electrical conductivity measurements reveal that, in the system MK—I 1, free ions are formed with a very low yield (φ(ion) ⩽ 0.01), which indicates that charge transfer complexes do not play a prominent role as intermediates in the reaction mechanism. It is postulated that, in the reaction of the sensitizer triplets with I 1, an encounter complex is formed which decomposes very quickly, the mutual orientation of T and I 1 determining the path of the decomposition. The latter involves either the formation of imidazolyl radicals or the formation of the starting compounds.