One of the critical issues for the realization of molecular electronics is the development of ideal molecule-electrode contacts that render efficient charge transportation and thus attenuate the unwanted voltage drop and power loss. The conductance at the single-molecule level has long been expected to be correlated strongly with the electrode materials. However, other than gold, systematic studies of a homologous series of molecules to extract the headgroup-metal contact conductance (G(n=0)) have not been reported. Carefully examined herein are the conductances of alkanedithiols anchored onto electrode materials of Au and Pt as well as the conductances of alkanediisothiocyanates on Au, Pd, and Pt by utilizing the method of STM-BJ (scanning tunneling microscopy break junction). In comparison with Au substrate, Pd and Pt are group 10 elements with stronger d-orbital characteristics, and larger local density of states near the Fermi level. The model compounds, SCN(CH(2))(n)NCS (n = 4, 6, and 8), are studied because the isothiocyanate (-NCS) headgroup is a versatile ligand for organometallics, an emerging class of molecular wires, and can bind to substrates of noble metals to complete a metal-molecule-metal configuration for external I-V measurements. Also studied include alkanedithiols, one of the most scrutinized systems in the field of single-molecule conductance. The results show that the conductance for single molecules bridged between a pair of Pt electrodes is about 3.5-fold superior to those between Au electrodes. On all electrode materials, observed are two sets of conductance values, with the smaller set being 1 order of magnitude less conductive. These findings are ascribed to the degree of electronic coupling between the headgroup and the electrode.

Spin based properties, applications, and devices are typically related to inorganic ferromagnetic materials. The development of organic materials for spintronic applications has long been encumbered by its reliance on ferromagnetic electrodes for polarized spin injection. The discovery of the chirality-induced spin selectivity (CISS) effect, in which chiral organic molecules serve as spin filters, defines a marked departure from this paradigm because it exploits soft materials, operates at ambient temperature, and eliminates the need for a magnetic electrode. To date, the CISS effect has been explored exclusively in molecular insulators. Here we combine chiral molecules, which serve as spin filters, with molecular wires that despite not being chiral, function to preserve spin polarization. Self-assembled monolayers (SAMs) of right-handed helical (L-proline) 8 (Pro 8 ) and corresponding peptides, N-terminal conjugated to (porphinato) zinc or meso-to-meso ethyne-bridged (porphinato)zinc structures (Pro 8 PZn n ), were interrogated via magnetic conducting atomic force microscopy (mC-AFM), spin-dependent electrochemistry, and spin Hall devices that measure the spin polarizability that accompanies the charge polarization. These data show that chiral molecules are not required to transmit spin-polarized currents made possible by the CISS mechanism. Measured Hall voltages for Pro 8 PZn 1−3 substantially exceed that determined for the Pro 8 control and increase dramatically as the conjugation length of the achiral PZn n component increases; mC-AFM data underscore that measured spin selectivities increase with an increasing Pro 8 PZn 1−3 N-terminal conjugation. Because of these effects, spindependent electrochemical data demonstrate that spin-polarized currents, which trace their genesis to the chiral Pro 8 moiety, propagate with no apparent dephasing over the augmented Pro 8 PZn n length scales, showing that spin currents may be transmitted over molecular distances that greatly exceed the length of the chiral moiety that makes possible the CISS effect.

Understanding factors that underpin the signs and magnitudes of electron spin–spin couplings in biradicaloids, especially those that are integrated into highly delocalized electronic structures, promises to inform the design of molecular spintronic systems. Using steady-state and variable temperature electron paramagnetic resonance (EPR) spectroscopy, we examine spin dynamics in symmetric, strongly π-conjugated bis­[(porphinato)­copper] (bis­[PCu]) systems and probe the roles played by atom-specific macrocycle spin density, porphyrin-to-porphyrin linkage topology, and orbital symmetry on the magnitudes of electronic spin–spin couplings over substantial Cu–Cu distances. These studies examine the following: (i) meso-to-meso-linked bis­[PCu] systems having oligoyne spacers, (ii) meso-to-meso-bridged bis­[PCu] arrays in which the PCu centers are separated by a single ethynyl unit or multiple 5,15-diethynyl­(porphinato)­zinc­(II) units, and (iii) the corresponding β-to-β-bridged bis­[PCu] structures. EPR data show that, for β-to-β-bridged systems and meso-to-meso-linked bis­[PCu] structures having oligoyne spacers, a through σ-bond coupling mechanism controls the average exchange interaction (J avg). In contrast, PCu centers separated by a single ethynyl or multiple 5,15-diethynyl­(porphinato)­zinc­(II) units display a phenomenological decay of ln­[J avg] versus Cu–Cu σ-bond separation number of ∼0.115 per bond, half as large as for these other compositions, congruent with the importance of π-mediated spin–spin coupling. These disparities derive from effects that trace their origin to the nature of the macrocycle–macrocycle linkage topology and the relative energy of the Cu d x 2–y 2 singly occupied molecular orbital within the frontier orbital manifold of these electronically delocalized structures. This work provides insight into approaches to tune the extent of spin exchange interactions and distance-dependent electronic spin–spin coupling magnitudes in rigid, highly conjugated biradicaloids.

This study develops the first heteropentametal extended metal atom chain (EMAC) in which a string of nickel cores is incorporated with a diruthenium unit to tune the molecular properties. Spectroscopic, crystallographic, and magnetic characterizations show the formation of a fully delocalized Ru2(5+) unit. This [Ru2]-containing EMAC exhibits single-molecule conductance four-fold superior to that of the pentanickel complex and results in features of negative differential resistance (NDR), which are unobserved in analogues of pentanickel and pentaruthenium EMACs. A plausible mechanism for the NDR behavior is proposed for this diruthenium-modulated EMAC.

Significance In contrast to conventional electronics, spintronics devices exploit the electron spin as an additional degree of freedom. The chirality-induced spin selectivity (CISS) effect, in which chiral molecules act as spin filters in electron transport, provides a pathway to control spins in molecules. We describe an approach that integrates both spin-polarizing and spin-propagating functionality into organic structures that feature low charge transport resistances. Binding chiral ligands to molecular wires controls polarized spin handedness, regulates spin currents, generates large NIR rotational strengths, and provides a mechanism to flip the favored spin orientation for spin transmission through chiral organic molecules. This work points the way to materials that provide both high spin selectivity and large-magnitude spin currents via the CISS mechanism.

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