Bifunctional molecules that receive two electrons sequentially and reversibly through only one of their two reducible groups

The electrochemical reduction mechanism of four bifunctional molecules (2, 5–7) has been investigated. Each of these compounds contains a kinetically fast electron-accepting group (p-cyanobenzoate function) and a kinetically slow electron-accepting group (2-nitroprop-2-yl), but each of these groups has an almost identical formal potential for reduction to the radical anion. Compounds 2, 5 and 6 are reduced in the following pathway: (1) initial introduction of an electron into the “fast” end; (2) intramolecular electron transfer from the “fast” to the “slow” end and (3) introduction of the second electron into the fast end. For 2 and 5, the rate of the intramolecular electron transfer reaction is too fast to measure, its rate constant exceeding 106 s−1. However, for 6, which features a long steroidal group that separates the “fast” and “slow” ends, the rate constant was of the order of 103 s−1 at 298 K and the effect of the slower intramolecular electron transfer is clearly evident in the cyclic voltammograms. The reduction of 7 is complicated by a relatively rapid double cleavage reaction occurring at the state of the diradical dianion.