How complexing with a small number of transition metal ions can prevent dye fading

Abstract

In an earlier study of the effect of transition metal ions on the production of singlet oxygen by dyes it was found that four ions, namely Cu²⁺, Ni²⁺, Co²⁺ and Co³⁺ suppress singlet oxygen production altogether, provided the dyes are made ligands in coordination complexes of the ions. By this the fading half-life of the dyes is prolonged by at least 1000 times. This article is a mechanistic interpretation of the observed phenomena as well as an appreciation of their practical consequences. Enclosing the dyes together with the operative ions in coordination complexes circumnavigates the spin prohibition that separates the triplet excited state of the dye from its ground state. It does so by replacing the spin-forbidden step with two faster steps. The first is the transition of the highest energy electron of the excited state to a transient ionic state, the second is the transition from the ionic state to the ground state of the dye. The second transition is spin forbidden, but it is powerfully accelerated by the heavy-atom-effect of the ionic nucleus. This makes it possible to design colorants that are virtually fade-proof.