Low-Resistance Molecular Wires Propagate Spin-Polarized Currents

Abstract

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.

The Importance of Spin State in Chiral Supramolecular Electronics

“…The spin of the transmitted electrons is detected using a Mott polarimeter ( Göhler et al, 2011 ). Conductive-probe atomic force microscopy (cp-AFM) is another technique that measures the spin-dependent conduction through single molecules ( Xie et al, 2011 ; Kettner et al, 2015 ; Bullard et al, 2019 ). With this technique, the current-voltage (J-V) curves are registered on a SAM of chiral molecules adsorbed on a ferromagnetic substrate (normally nickel), while gold nanoparticles are attached to the tail of some of these molecules.…”

“…Spin polarization Hall devices measure Hall voltage and cyclic voltammetry response on chiral SAMs ( Kumar et al, 2017 ; Bullard et al, 2019 ). The sign of the observed Hall voltage depends on the chirality of the molecule.…”

“…The spin selectivity in electron transfer was also noted when 12-mer oligopeptides were attached to ferrocene, where oxidation or reduction were favored depending on the L - or D -enantiomer and the direction of the magnetic field ( Tassinari et al, 2018 ). Importantly, a polyproline chiral system (Pro 8 ) conjugated to zinc porphyrins showed that the spin-polarized generated currents were further transmitted over distances surpassing the length of chiral molecules ( Bullard et al, 2019 ). Very recently, ds peptide nucleic acids (PNAs) have been proposed to study the CISS effect.…”

“…They followed the kinetics of the enantioselective adsorption of dsDNA on a magnetized Ni/Au surface, finding that the rate of absorption was considerably different for up and down magnetization of the substrate ( Banerjee-Ghosh et al, 2018 ). Additionally, researchers have investigated the effect of oxidative damage on the spin transport through monolayers of dsDNA using a Hall device ( Bullard et al, 2019 ). Unexpectedly, dsDNA having one and two oxidative damages in the base pairs had higher spin polarization than undamaged dsDNA films.…”

Review of porphyrin-based photodynamic therapy materials

“…The reaction between DPBF and 1 O2 can induce an absorbance loss of DPBF [32,33]. Similarly the fluorescence enhancement of singlet oxygen sensor green (SOSG) can be employed to quantify singlet oxygen generation [34,35]. The intracellular 1 O2 generation can also be estimated by using 2,7-dichlorofluorescein diacetate (DCFH-DA) as an indicator because DCFH-DA can be oxidized by 1 O2 to dichlorofluorescein (DCF) yielding green fluorescence [36,37].…”