Comparative electrochemical investigations on series of SH-terminated-functional porphyrins

The electrochemical behaviors of self-assembled substituted porphyrins (SH-terminated, abbreviated as H2TPPO(CH2)nSH, n = 3, 12) on a gold electrode were investigated using the steady-state scanning electrochemical microscopy (SECM). The different electron-transfer (ET) kinetics, including the bimolecular ET between the porphyrin self-assembled monolayers (SAMs) and the redox mediator [K3Fe(CN)6], the tunneling ET between the underlying gold electrode and [K3Fe(CN)6], and pinholes or defects, were clearly distinguishable. The SECM strategy was developed to deal with the two types of porphyrin SAMs. First, a model using alkanethiols [(CH2)nSH, n = 3, 12] as the functional template was proposed to change the conformation of porphyrin SAMs in a unit area of the electrode. Second, the porphyrin SAMs were directly prepared by inserting a metal (cobalt) into the center of the porphyrin ring. The results show the distinct effect of the presence of alkanethiols on the kinetics of the different-chain length porphyrins. In addition, the rate constants of the bimolecular ET significantly increased after the insertion of cobalt. The results are in agreement with the density functional theory (DFT).

► Alkanethiols as the functional template was proposed to change the conformation of large end groups-porphyrins SAMs. ► Metal porphyrin SAMs was prepared to study the electron transport (ET) pathways. ► Distinguished different electron transport (ET) pathways and calculate relative rate constants by SECM. ► Bimolecular reaction as a model could be used to mimic ET process in chloroplasts or the important bio-molecular reaction in nature.