Comparative Electrochemical Behaviors of a Series of SH-Terminated-Functionalized Porphyrins Assembled on a Gold Electrode by Scanning Electrochemical Microscopy (SECM)

The synthetic sulfhydryl porphyrin compounds (abbreviated as H(2)TPPO(CH)(n)SH, n = 3, 4, 6, 9, 10, 12), with a systematic series of spacers containing large end groups-porphyrin rings and different length alkyl chains, were prepared to investigate the electron transport (ET) kinetics between a gold electrode modified with H(2)TPPO(CH)(n)SH and a redox mediator (ferricyanide). The three ET kinetics were clearly distinguishable by scanning electrochemical microscopy (SECM). As shown by the rate constants (k(ox)) of bimolecular ET between H(2)TPPO(CH)(n)SH and the redox mediator, the rate constants (k(b)) of tunneling ET between the underlying gold electrode and the redox mediator, and the rate constants (k') of pinhole or defects, ET depended upon alkyl chain length, surface coverage, and the concentration of the redox mediator. Among all of them, the bimolecular reaction which contained electron donor (mediators) and electron acceptor (SAMs) was a dynamic cyclic process. That strongly suggested that the bimolecular reaction should mimic ET of photosystem II in chloroplast.