Bimetallic Pd(ii)/Fe(ii)-mediated self-assembly of three-dimensional hybrid multilayers with a terpyridine-contained poly(vinylpyridine) derivative as a linker on substrate surface

Abstract

Pd(II)/Fe(II)-mediated three-dimensional bimetal-organic multilayers have been constructed on the substrate surfaces with the use of a polymeric ligand of terpyridine-contained poly(vinylpyridine) (PVPTPy) derivative as a linker as well as inorganic salts of Na₂PdCl₄ and Fe(BF₄)₂ as connectors. The assembly process was monitored by measuring the absorption spectra, which revealed two main absorption bands at wavelengths of 250∼340 nm and about 580 nm corresponding to the electron transition of pyridyl or terpyridyl substituents as well as metal–ligand charge transfer, respectively. X-Ray photoelectron spectra revealed that the multilayer of (Pd)Fe-PVPTPy was composed of the elements of B(1s), Cl(2p), C(1s), Pd(3d), N(1s), O(1s), F(1s) and Fe(2p), which confirmed formation of the bimetal-organic multilayers. Morphologies of the multilayers were characterized by using scanning electron microscope and atomic force microscopy. Cyclic voltammograms revealed a couple of irreversible redox waves with cathodic peaks appearing in the potential range from −0.2 to −0.4 V (vs. Ag/AgCl), which related to the scan rates and layer numbers of the multilayers. This redox wave was designated as one electron transfer of Fe(II)-PVPTPy/Fe(III)-PVPTPy in the multilayers. The current density of the redox wave decreased with increasing layer numbers of the films due to an increase of the film resistance. Both the polymeric ligand and its bimetal-organic multilayers showed a broad emission at wavelengths of 390∼450 nm. A red shift for the maximum emission band was recorded in the (Pd)Fe-PVPTPy films. Because the transition metal ions can selectively coordinate with either pyridyl or terpyridyl coordinative site, the present method may be developed to design and construct the desired multi-functional metal–organic hybrid materials.