A self-assembled monolayer of an alkanoic acid-derivatized porphyrin on gold surface: A structural investigation by surface plasmon resonance, ultraviolet-visible, and infrared spectroscopies

We describe in this paper the preparation of a self-assembled monolayer (SAM) film of 5,10,15,20-tetra(N-10-carboxydecyl-pyridinium-4-yl) porphyrin on an Au substrate by chemisorption of the multi-COOH terminal groups onto the metal surface. A film thickness of 19 Angstrom is deduced from surface plasmon resonance measurement. A significant red shift of the Soret band in the UV-vis spectrum of the porphyrin SAM compared with that of the porphyrin in solution suggests the formation of "head-to-tail" type aggregates in the SAM. The absence of the N-H in-plane mode and the presence of the out-of-plane mode in the infrared reflection-absorption spectrum (IRAS) and the surface-enhanced infrared absorption spectrum (SEIRAS) further suggest that the porphyrin macrocycle is oriented flatly with respect to the surface of the metal substrate. In addition, there may be one of the four acid groups that is unbound to the metal surface, as evidenced by the presence of a very weak band at 1707 cm(-1) due to the C=O stretching mode of the COOH group in the SEIRA spectrum of the porphyrin monolayer on the Au island film. (C) 2001 Academic Press.