Layer-by-layer assembly of Zn(ii) and Ni(ii) 5,10,15,20-tetra(4-ethynylphenyl)porphyrin multilayers on Au using copper catalyzed azide-alkyne cycloaddition

Abstract

We have developed a versatile layer-by-layer (LbL) fabrication method to assemble porphyrin based multilayer thin-films on electron-beam evaporated Au surfaces utilizing copper(I) catalyzed azide-alkyne cycloaddition (CuAAC) as both a means of anchoring the films to the Au surface and coupling the individual molecular layers together. The molecular based multilayer films are comprised of Zn(II) and Ni(II) 5,10,15,20-tetra(4-ethynylphenyl)porphyrin and a bis-azido linker layer. Herein, we describe the fabrication and characterization of multilayer films on Au surfaces modified with an azido-terminated alkanethiol self assembled monolayer. The absorbance growth trends, as followed by UV-vis absorption, show a consistent linear increase that extends over tens of bilayers. Multilayer film thicknesses were obtained from spectroscopic ellipsometry, using a Cauchy model applied over the transparent range, and resulted in a consistent linear growth trend. Optical constants, index of refraction and extinction coefficients, were then determined using an oscillator model over the entire visible region. The resulting extinction coefficients were consistent with those typical of Zn(II) and Ni(II) porphyrin absorption spectra. The topology of the films and surface roughness was analyzed by tapping mode atomic force microscopy (TM-AFM) and confirmed the continuous nature of the films. X-Ray photoelectron spectroscopy (XPS) was consistent with the expected elemental composition of the porphyrin based films assembled on Au surfaces. Additionally, XPS was used to examine the utility of ethylenediaminetetraacetic acid disodium salt (Na₂EDTA) as a Cu chelator to remove adventitious catalyst following multilayer fabrication.