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 10(6) 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 10(3) 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. (C) 2003 Elsevier B.V. All rights reserved.