Tailoring of solid state electrical conductivity and optical electron transfer activation on of dioxygen in solution through supramolecular charge-transfer interaction in ion pairs

Abstract

Redoxactive and light-sensitive ion pair complexes of the type {A 2+[ML 2] −2}, wherein A 2+ is a bipyridinium acceptor and [ML 2] −2, MNi, Pd, Pt, a planar dithiolene metalate, according to their solid state structure can be divided into two classes. Class I consists of compounds which contain an approximately planar acceptor and which therefore can form mixed stacks of slightly slipped coplanar components. Class II complexes contain a strongly twisted acceptor which gives rise to a chain-like structure in which only each a half of a component can adopt a coplanar arrangement. While application of the Hush-Marcus model reveals that both classes possess the same mean reorganization energy for electron transfer from the dianion to the dication, only for class 1 compounds does the logarithm of the specific electrical conductivity increase linearly with the free activation energy of this process, as calculated from the energy of the ion pair charge transfer band and the redox potentials. For class II complexes there is no similar relation. Proper selection of the acceptor reduction potential and the central metal allows to tailor optical electron transfer-induced activation of dioxygen in solution. The primary electron transfer step affords the oxidized donor, [ML 2] −, and the reduced acceptor, A •+, which reduces dioxygen to superoxide. The latter reaction can compete with back-electron transfer only when the reduction potential of A 2+ is more negative than −0.6V. The overall reaction proceeds only in the case of MPt, suggesting that the photoreactive state has ion pair charge transfer character.