Dependence of Electron Transfer Directionality on the Composition and Orientation of a Copper(II) Aminomethyl-Pyridine Surfactant in a Monolayer on Gold Electrodes

Abstract

We investigate the chemical transformations that occur with the metallosurfactant [CuII(LPy16)2ClO4]ClO4(1) during its deposition as a Langmuir–Blodgett monolayer film in between the gold electrodes of a Au|LB|Au junction. Species 1 was anticipated as an insulator because of mismatching energies between the Au electrode Fermi levels and the singly occupied 3dx2–y2 metal-based MO in a 3d9 configuration expected to enable tunneling. It incorporates softer amine and pyridine donors in a [N2N′2] square-pyramidal geometry, differs from a previous insulator in which the CuII ion is surrounded by a harder [N2O2]2– iminophenolate environment, and was intended to probe whether the ligand field around the CuII ion suffices to enable unidirectional electron transport. Interestingly, rather than insulation or unidirectional transport, we observe a molecular orientation-dependent sigmoidal I/V curve characteristic of bidirectional conductivity. Because this is unusual for similar asymmetric molecules asymmetrically placed among electrodes, we proceeded to a meticulous evaluation of the film using X-ray photoelectron spectroscopy and infrared spectroscopy to conclude that the ligand is susceptible to reduction, reoxidation, imine/iminium transformation, and hydrolysis yielding 2-pyridinecarboxyaldehyde (pca) so that the initial [CuII(LPy16)2]2+ and the hydrolyzed [CuII(pca)2]2+, among other possible species, are deposited on the Au electrode surface.