Abstract: We select a [5-para-alkyloxyphenyl-10, 15, 20-tri(m-ethoxyphenyl)]porphyrin hydroxy rare earth complexes(HoOH) and 4, 4'-bipyridine(44BPY) as the blocking units, and successfully assemble a HoOH monolayer on the 44BPY-modified silver surface based on axial ligation. The structure and orientation of the SAMs were studied by UV-Vis and SERS spectroscopy. The results reveal that 44BPY is assembled onto the silver surface with an end-on adsorption via 4-position nitrogen. The other side pyridine groups coordinate into the central metal ions of HoOH. Therefore, a HoOH monolayer is ligated to the underlying 44BPY layer along its axial direction, forming new alternative films. In the alternative films, the oblique angle between the molecular long axis of 44BPY and the normal of the substrate increased, while the molecular plane of HoOH was nearly parallel to the substrate.

Key words: SERS, Porphyrin, Axial ligation, Self-assembled film