Rectification in Nanoscale Devices Based on an Asymmetric Five‐Coordinate Iron(III) Phenolate Complex

Abstract

Rectification consists of an asymmetric flow of electric current. In a macroscopic electrical circuitry, rectifiers, such as vacuum tubes or solid-state diodes, control the mobility of current, enabling it to flow in one direction and preventing reversibility. This directionality is fundamental to the conversion of alternating into direct current. Molecular rectification, which was proposed in the celebrated Aviram-Ratner ansatz, [1] anticipates the feasibility of a current flow in one direction that takes place in an electrode j molecule j electrode junction. Central to this paradigm is the existence of asymmetric molecules that incorporate electron-donor and electron-acceptor moieties, [DA], with an excited state] of higher, but accessible, energy.[2] Usually, donor and acceptor are separated by a s-or p-bridge to decrease electronic coupling, [3] and if the requirements are fulfilled, rectification occurs with contributions from Schottky, asymmetric, and/or unimolecular mechanisms. Schottky rectification is based on interfacial dipoles from electrode contact or on covalent bonding between the molecule and the electrode.[4] Asymmetric and unimolecular mechanisms rely on the use of frontier molecular orbitals of the molecule; whereas the former relies on an asymmetric placement of the HOMO or the LUMO in the electrode j molecule j electrode assembly, the latter is based on small HOMO-LUMO gaps that allow for through-molecule current flow. [4a, 5] Although experimental distinction between asymmetric and unimolecular contributions can be ambiguous, there is consensus that electroactive molecules with local low symmetry [6] constitute good candidates for this enterprise, and welldocumented cases of molecular rectification heavily rely on the formation of high-quality Langmuir-Blodgett (LB) films. [7] Although it has been shown that self-assembled monolayers of polypyridine-cobalt(II) complexes in octahedral environments [8] can act as single-electron transistors and induce increased resistance (Coulomb blockade) at cryogenic temperatures, the incorporation of transition-metal complexes into electrode j molecule junctions has generally employed symmetric molecules and has been rather slow in development. Examples involve assemblies based on metalloporphyrins, [9] terpyridine-ruthenium(II) complexes, [10] as well as trivalent cobalt [11] and rhodium [12] azo-containing species in octahedral environments that are capable of the symmetric conductance that is relevant for memory-switching devices. An example of rectification based on an octahedral bipyridine/acac-ruthenium(II) system has been reported (acac = acetylacetonate), [13] but the effect of lowering global symmetries around the metal center is yet to be tested.Our group is engaged in an effort to integrate bioinspired asymmetry principles into new molecular materials, with the aim of developing redox-responsive metallosurfactants with topologies that display unique structural, spectroscopic, and surface patterning behavior. We recently reported on the red...

Electronic Modulation of the SOMO–HOMO Energy Gap in Iron(III) Complexes towards Unimolecular Current Rectification

“…Complex 4 also displayed two quasi‐reversible oxidative processes at 0.54 and 0.8 V Fc+/Fc . In this case the anodic waves are attributed exclusively to phenolate/phenoxyl oxidation, in good agreement with the literature …”

Efficient water oxidation with electromodified Langmuir–Blodgett films of procatalytic [CoIII(N2O3)] metallosurfactants on electrodes

“…Besides providing superior uniform coverage with minute amounts of material, the LB method provides direct control over the thickness of the deposited films by varying the number of layers. [14] Over the years we have developed redox-active metallosurfactants aiming at molecular diodes, [14,15] molecular displays, [16] and other high-end applications.…”

“…We have designed a new redox-active, amphiphilic Co(III) complex shown in Scheme 1. The ligand H 3 L N2O3 was synthesized according to a previously published procedure, [14] and Please do not adjust margins Please do not adjust margins During metallation the ligand is oxidized from amine to imine while cobalt(II) is oxidized to cobalt(III). We have investigated this oxidation in detail for both Fe and Co complexes.…”

“…We have investigated this oxidation in detail for both Fe and Co complexes. [11,14,18] The FTIR spectra showed symmetric and asymmetric C-H vibrations around 2869-2954 cm -1 and a shoulder peak about 1590 cm -1 belonging to the stretching mode of the imine C=N.…”

“…[11,18a] The first two processes are quasireversible, and the third is ill-defined. [14,15,21] was observed at ca. 42 mN/m.…”