Comparison of electrochemical redox processes on silver electrode surfaces between meso-tetrakis(2-N-methylpyridyl)porphine and meso-tetrakis(4-N-methylpyridyl)porphine

Add time:07/12/2019    Source:sciencedirect.com

Surface-enhanced resonance Raman scattering (SERRS) spectra were measured for meso-tetrakis(2-N-methylpyridyl)porphine (TMPyP(2)) adsorbed on silver electrode surfaces from its 1 mol/l HCl solution and compared with the SERRS spectra of meso-tetrakis(4-N-methylpyridyl)porphine (TMPyP(4)). On adsorption at a silver surface, TMPyP(2) in the di-acid form is converted to a mixture of a phlorin-free base and a phlorin mono-cation, while TMPyP(4) on the same surface retains its di-acid form. This result suggests a difference in adsorption structure between the two porphines; i.e., TMPyP(2) takes on a structure in which reductive sites on the surface can attack the porphine ring, while TMPyP(4) does not take on such a structure. On sweeping the potential from 0 V (vs. A/AgCl) towards negative potential, the adsorbed TMPyP(2) shows a reduction sequence from the phlorin to a porphomethene and the adsorbed TMPyP(4) a sequence from the prophine di-acid to a porphomethene. On sweeping the potential back to 0 V, the porphomethenes of TMPyP(2) and TMPyP(4) are reoxidized to the corresponding phlorins. The conversion of the porphomethenes to the phlorins suggests that the silver electrode surface catalyzes the elimination reaction of electrons and protons from the porphomethenes, which causes the formation of the phlorins.

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