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The geometry and bonding of N-heterocyclic carbenes to metal surfaces depends on the substituents on the N-atoms.

The stochastic nature of single-molecule charge transport measurements requires collection of large data sets to capture the full complexity of a molecular system. Data analysis is then guided by certain expectations, for example, a plateau feature in the tunnelling current distance trace, and the molecular conductance extracted from suitable histogram analysis. However, differences in molecular conformation or electrode contact geometry, the number of molecules in the junction or dynamic effects may lead to very different molecular signatures. Since their manifestation is a priori unknown, an unsupervised classification algorithm, making no prior assumptions regarding the data is clearly desirable. Here we present such an approach based on multivariate pattern analysis and apply it to simulated and experimental single-molecule charge transport data. We demonstrate how different event shapes are clearly separated using this algorithm and how statistics about different event classes can be extracted, when conventional methods of analysis fail.

Cyclic oligomers comprising strongly interacting redox-active monomer units represent an unknown, yet highly desirable class of nanoscale materials. Here we describe the synthesis and properties of the first family of molecules belonging to this compound category-differently sized rings comprising only 1,1'-disubstituted ferrocene units (cyclo[n], n = 5-7, 9). Due to the close proximity and connectivity of centres (covalent Cp-Cp linkages; Cp = cyclopentadienyl) solution voltammograms exhibit well-resolved, separated 1e(-) waves. Theoretical interrogations into correlations based on ring size and charge state are facilitated using values of the equilibrium potentials of these transitions, as well as their relative spacing. As the interaction free energies between the redox centres scale linearly with overall ring charge and in conjunction with fast intramolecular electron transfer (∼10(7) s(-1)), these molecules can be considered as uniformly charged nanorings (diameter ∼1-2 nm).

Ferrocenes are ubiquitous organometallic building blocks that comprise a Fe atom sandwiched between two cyclopentadienyl (Cp) rings that rotate freely at room temperature. Of widespread interest in fundamental studies and real-world applications, they have also attracted some interest as functional elements of molecular-scale devices. Here we investigate the impact of the configurational degrees of freedom of a ferrocene derivative on its single-molecule junction conductance. Measurements indicate that the conductance of the ferrocene derivative, which is suppressed by two orders of magnitude as compared to a fully conjugated analog, can be modulated by altering the junction configuration. Ab initio transport calculations show that the low conductance is a consequence of destructive quantum interference effects that arise from the hybridization of metal-based d-orbitals and the ligand-based π-system. By rotating the Cp rings, the hybridization, and thus the quantum interference, can be mechanically controlled, resulting in a conductance modulation that is seen experimentally. File list (2) download file view on ChemRxiv ferrocene_main.pdf (0.97 MiB) download file view on ChemRxiv ferrocene_SI.pdf (8.17 MiB) Mechanically-tunable Quantum Interference in Ferrocene-based Single-Molecule Junctions

We report the large scale syntheses and 'oxidative purification' of fcI 2 , fcBr 2 and FcBr (fc = ferrocene-1,1'diyl, Fc = ferrocenyl). These valuable starting materials are typically laborious to separate via conventional techniques, but can be readily isolated by taking advantage of their increased E 1/2 relative to FcH/FcX contaminants. Our work extends this methodology towards a generic tool for the separation of redox active mixtures. Scheme 1 Synthesis and oxidative purification of (a) FcI (Goeltz and Kubiak), 21 and fcBr, and (b) fcI 2 and fcBr 2 (this work) (A − = Cl − , [FeCl 3 ] − or [FeCl 4 ] − ).

We have applied a new, robust and unsupervised approach to data collection, sorting and analysis that provides fresh insights into the nature of single-molecule junctions. Automation of tunneling current-distance (I(s)) spectroscopy facilitates the collection of very large data sets (up to 100,000 traces for a single experiment), enabling comprehensive statistical interrogations with respect to underlying tunneling characteristics, noise and junction formation probability (JFP). We frequently observe unusual low-to-high through-molecule conductance features with increasing electrode separation, in addition to numerous other "plateau" shapes, which may be related to changes in interfacial or molecular bridge structure. Furthermore, for the first time we use the JFP to characterize the homogeneity of functionalized surfaces at the nanoscale.

Self-assembled monolayers (SAMs) formed using N-heterocyclic carbenes (NHCs) have recently emerged as thermally and chemically ultrastable alternatives to those formed from thiols. The rich chemistry and strong σ-donating ability of NHCs offer unique prospects for applications in nanoelectronics, sensing, and electrochemistry. Although stable in SAMs, free carbenes are notoriously reactive, making their electronic characterization challenging. Here we report the first investigation of electron transport across single NHC-bound molecules using the scanning tunneling microscope-based break junction (STM-BJ) technique. We develop a series of air-stable metal NHC complexes that can be electrochemically reduced in situ to form NHC-electrode contacts, enabling reliable single-molecule conductance measurements of NHCs under ambient conditions. Using this approach, we show that the conductance of an NHC depends on the identity of the single metal atom to which it is coordinated in the junction. Our observations are supported by density functional theory (DFT) calculations, which also firmly establish the contributions of the NHC linker to the junction transport characteristics. Our work demonstrates a powerful method to probe electron transfer across NHC-electrode interfaces; more generally, it opens the door to the exploitation of surface-bound NHCs in constructing novel, functionalized electrodes and/or nanoelectronic devices.