Complexation of Porphyrin with a Pyridine Moiety in Self-Assembled Monolayers on Metal Surfaces

Abstract

The preparation of ordered porphyrin monolayers on the solid surfaces was pursued by noncovalent bonding. Four kinds of self-assembled monolayers (SAM) that carried pyridine moieties were constructed on the surface of silver colloids (4Py-SAM from 4,4‘-dithiodipyridine, 2Py-SAM from 2,2‘-dithiodipyridine, DTPA-4Py-SAM from dithiodi[N-(4-pyridylethyl)propylamide], and DTPA-2Py-SAM from dithiodi[N-(2-pyridylethyl)propylamide]. The structure of the SAMs was observed by surface-enhanced Raman spectroscopy. When a nitrogen atom in the pyridine ring of the SAM was directed to the solution phase (in the cases of 4Py-SAM, DTPA-4Py-SAM, and DTPA-2Py-SAM), incubation of the SAM with a low concentration (<22 nM) of water-soluble porphyrin into which a zinc ion had been incorporated (zinc(II) 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphine tetra-p-toluenesulfonate: TMPyP Zn(II)) resulted in a definite Raman scattering corresponding to the porphyrin ring, in addition to that of the SAM itself. In the absence of zinc ion (5,10,15,20-tetrakis(1-methylpyridinium-4-yl)-21H,23H-porphine tetra-p-toluenesulfonate: TMPyP H2), on the other hand, only the scattering of the SAM was observed. Furthermore, the 2Py-SAM, in which a nitrogen atom of the pyridine ring was directed to the silver surface, showed no scattering corresponding to the porphyrin moiety, either. These results indicated that the porphyrin ring was attached to the SAM by the axial ligand linkage between the pyridine ring of the SAM and the zinc ion of the porphyrin. It was suggested that the SAM, on which porphyrin groups were fixed in the ordered manner, would be useful as molecular optical devices.