In search of structure-function relationships in transition-metal based rectifiers

Heterometallic chains have been proposed as potential current rectifiers in molecular electronics, their left-right asymmetry providing, at least in principle, a mechanism for differentiation of current flow in forward and reverse directions. Here we compare two known extended metal atom chains (EMACs), Ru2Ni(dpa)(4)(NCS)(2) and Ru2Cu(dpa)(4)(NCS)(2), both of which meet the first criterion for rectification in so much as they are physically asymmetric. In both cases the dominant transport channel is a doubly degenerate pi* orbital localised, to a first approximation, on the Ru-2 unit. As a result, current is limited by tunnelling across the Au-SCN-Ni/Cu junction. The paramagnetic Ni centre tunes the left-right delocalisation of the channel, making the minority-spin (beta) channel more transparent than its spin-alpha counterpart and this difference provides the basis for asymmetry in the current under forward and reverse bias.