Molecular assembly of manganese mesoporphyrin derivatives on a gold electrode and their electron transfer activity

Disulfide-linked manganese mesoporphyrin derivatives with spacer methylene-chain groups (Cn), (MnMP-Cn-S)2 [n=2, 6, 12] and one fluorinated mesoporphyrin derivative, (MnMPF4-C12-S)2 (Scheme 1) were synthesized and assembled on a gold electrode to study the nature of their electron transfer with respect to catalytic activity. The electron transfer of the mesoporphyrins from and to the self-assembled monolayers (SAMs) was measured in aqueous solution as well as in dimethyl sulfoxide (DMSO) using cyclic voltammetry and potential modulation reflectance, i.e., electroreflectance. Redox reaction of the mesoporphyrins on the electrode occurred between Mn(III) and Mn(II) occluded in the porphyrin ring. The rate constants of the electron transfer from and to the manganese mesoporphyrins on gold electrodes, estimated from the ER vs. modulation frequency relation, increased with the decreasing chain lengths but decreased due to fluorination of the porphyrin ring, which is in agreement with the rates for tetraphenyl porphyrins and its halogenated derivatives. Interestingly, the rate constants for the manganese mesoporphyrins in DMSO were greater than the rates for the tetraphenyl porphyrins and were enhanced by additional insertion of alkanethiols into the SAMs in aqueous solution.