Element-Selective Molecular Charge Transport Characteristics of Binuclear Copper(II)-Lanthanide(III) Complexes

Abstract

A series of isostructural dinuclear 3d-4f complexes, isolated as [CuLn(L·SMe)(OOCMe)(NO)]· xMeOH (Ln = Gd 1, Tb 2, Dy 3, and Y 4; x = 0.75-1) and comprising one acetate and two thioether-Schiff base (L·SMe) bridging ligands based on 4-(methylthio)aniline and 2-hydroxy-3-methoxybenzaldehyde (HL·SMe = CHNOS), was synthesized and fully characterized. The magnetic properties of the charge-neutral {CuLn} complexes are dominated by ferromagnetic Cu-Ln exchange interactions. Large-area electron transport studies reveal that the average conductivity of robust, self-assembled {CuLn} monolayers on a gold substrate is significantly lower than that of common alkanethiolates. Theoretical calculations of transmission spectra of individual complexes 1 and 4 embedded between two metallic electrodes show that the molecular current-voltage ( I- V) characteristics are strongly influenced by electron transport through the Cu centers and thus fully independent of the lanthanide ion, in excellent agreement with the experimental I- V data for 1-4. The β-polarized transmission indicated by calculations of 1 and 4 points out their potential as spin filters. In addition, the reactivity of the title compound 1 with Cu in a square-pyramidal coordination environment toward methanolate and azide was examined, resulting in the formation of a linear trinuclear complex, [CuNa(L·SMe)]NO·3MeOH (5), characterized by antiferromagnetic exchange interactions between the two copper ions.

Conductive Self-Assembled Monolayers of Paramagnetic {CoIICo4III} and {Co4IICo2III} Coordination Clusters on Gold Surfaces

“…By replotting J–V data in Fowler-Nordheim coordinates (transition voltage spectroscopy), it is possible to obtain information about energy level alignment inside the junction. All values of transition voltages ( V T ) coalesce to ~0.6 V (Table 1), ~0.3 V higher than those reported recently for the SAMs of {CuLn} complexes (Schmitz et al, 2018a). The featureless J–V curves shown in Figure 9 point out single-level, non-resonant tunneling, i.e., a single potential barrier and comparably weak coupling to either electrode.…”

“…V T is an approximate measure of the height of the potential barrier, which is related to level alignment, for example, the difference between the energy of the accessible frontier orbital of a molecule and the Fermi level of the electrode in an assembled junction. If a rectangular tunneling barrier is considered, the heights of the barriers in the SAMs of Co complexes are higher than those of {CuLn} complexes (Schmitz et al, 2018a).…”

“…The SAM's morphology, thickness, and current density–voltage characteristics were effectively interpreted using a combination of atomic force microscopy (AFM), scanning tunneling microscopy (STM), and ellipsometry. The findings obtained by these analyses are compared to those reported by us for thioether-Schiff base (L) supported [CuLn(L·SMe) 2 (OOCMe) 2 (NO 3 )] complexes with Ln = Gd, Tb, Dy, or Y (Schmitz et al, 2018a).…”

“…Self-assembled monolayers (SAMs) of 1 and 2 were physisorbed on template-stripped gold surfaces contacted by an eutectic gallium-indium (EGaIn) tip. The acquired current density-voltage ( I-V ) data revealed that the cobalt-based SAMs are more electrically robust than those of the previously reported dinuclear {Cu II Ln III } complexes with Ln = Gd, Tb, Dy, or Y (Schmitz et al, 2018a). In addition, between 170 and 220°C, the neutral, mixed-valence compound 1 undergoes a redox modification, yielding a {Co 5 }-based coordination cluster ( 1-A ) with five non-interacting, high-spin octahedral Co II centers as indicated by SQUID magnetometry analysis in combination with X-ray photoelectron spectroscopy and infrared spectroscopy.…”

“…At both these levels of molecule–surface interfaces, preservation of the main molecular characteristics (Mitcov et al, 2019) as identified in the bulk state, and avoidance of the agglomerative behavior of coordination compounds after their immobilization on solid support constitute important milestones in the controlled (micro-)spectroscopic addressing of the tunnel junction structures of molecule–electrode hybrid devices. The eventual step to practical devices (Sun et al, 2014) mandates an in-depth understanding of the adsorption, autonomous self-organization, electron charge-/spin-transport characteristics (Al-Owaedi et al, 2017; Bu et al, 2018) and switching mechanisms of mono- and polynuclear complexes of transition metals (Higgins and Nichols, 2018), lanthanides (Dreiser, 2015), and their mixed-metal derivatives (Schmitz et al, 2018a) on conductive and semi-conductive surfaces (Cornia et al, 2011).…”