The effect of ligand structure on surface enhanced Raman scattering by Fe(II) macrocyclic complexes: [Fe IITPC] 2+ and [Fe IIDPC] 2+

Abstract

This paper reports studies on surface-enhanced Raman scattering (SERS) by two ferrous macrocyclic complexes, one with the pentadentate ligand, DPC (DPC) = N, N′-di-2-picolyl-4,7-diaza-1-oxacyclononane), and the other with the hexadentate ligand, TPC (TPC) = N, N′, N″-tri-2-picolyl-1,4,7-triazacyclononane). The purpose of this work is to show how the ligand structure changes Raman scattering by the Fe(II) complexes on a silver hydrosol protected by sodium citrate, Ag/Cit 3−. It is demonstrated that SERS by [Fe IITPC] 2+ on Ag/Cit 3− is enhanced by the factor of E = 3.5 ± 0.5 10 3, if the concentration of the sol is five times of the complex. However, the relative intensities and frequencies in the SERS spectrum are parallel to those obtained upon resonance Raman experiments with λ ex = 441.2 nm. In contrast to [Fe IITPC] 2+, Raman scattering by [Fe IIDPC] 2+ on Ag/Cit 3− is quite different to that obtained from the aqueous solution with no silver sol, and includes resonance Raman-forbidden modes. These observations are considered in terms of the electromagnetic and chemical mechanisms of surface-enhanced Raman scattering. It is postulated that the chemical mechanisms is predominant in Raman scattering by the [Fe IIDPC] 2+ complex with the open coordination site on the Fe II cation, whereas the electromagnetic mechanism provides a better fit to the behavior of [Fe IITPC] 2+ on Ag/Cit 3−, as there is no opportunity for an Ag[Fe IITPC] 2+ surface complex formation.