Synthesis and Properties of Metalloporphyrin Monolayers and Stacked Multilayers Bound to an Electrode via Site Specific Axial Ligation to a Self-Assembled Monolayer

A general method has been developed for the immobilization of metalloporphyrins at a gold electrode surface coated with a self-assembled monolayer (SAM). SAMs containing imidazole-terminated adsorbates are shown to bind a series of metalloporphyrins, including bis-acetonitrile octaethylporphyrinatoruthenium(II), Ru(OEP)(CH3CN)2; bis-acetonitrile octaethyltetraazaporphyrinato ruthenium(II), Ru(OETAP)(CH3CN)2; bis-acetonitrile tetra-(p-chlorophenyl)porphyrinatoruthenium(II), Ru(Tp-ClPP)(CH3CN)2; bis-acetonitrile octaethylporphyrinatoosmium(II), Os(OEP)(CH3CN)2; and carbonyl meso-tetramesitylporphyrinatoruthenium(II), Ru(TMP)(CO). The SAM/metalloporphyrin films have been characterized by optical ellipsometry, contact angle goniometry, X-ray photoelectron spectroscopy, grazing angle FT-IR spectroscopy, transmission visible spectroscopy, and electrochemistry. The results indicate that the metalloporphyrins are chemisorbed via axial ligand substitution of the metal center with the porphyrin ring parallel to the surface and the second axial ligand position normal to the surface. Scanning tunneling microscopy images of Ru(TMP)(CO) bound to the SAM corroborate this model. Axial ligation of metalloporphyrins to SAMs serves as the basis for an iterative, defined approach to the preparation of stacked single component and mixed metalloporphyrin multilayers on SAMs. In these materials, the bidentate ligand pyrazine serves as a bridge between successive metalloporphyrins in the stacks.