Design Considerations for Oligo(p-phenyleneethynylene) Organic Radicals in Molecular Junctions

Abstract

Spin polarization in the electron transmission of radicals is important for understanding single-molecule conductance experiments focusing on shot noise, Kondo properties, or magnetoresistance. We study how stable radical substituents can affect such spin polarization when attached to oligo(p-phenyleneethynylene) (OPE) backbones. We find that it is not straightforward to translate the spin density on a stable radical substituent into spin-dependent transmission for the para-connected wires under study here, owing to increased steric interactions compared with meta-connected wires, and a resulting twisting of the radical substituent and OPE π systems. The most promising example is a t-butyl nitroxide substituent, which, despite little pronounced spin delocalization onto the backbone, yields a spindependent transmission feature, which one might be able to shift toward the Fermi energy by additional substituents. We also find that for bulkier substituents, dispersion interactions with the substituent can lead to twisting of one of the outer OPE rings, reducing the overall conductance. As a further potential design consideration, attaching radicals via linkers might increase the possibilities for spin-dependent intermolecular and molecule− electrode interactions.

A review of oligo(arylene ethynylene) derivatives in molecular junctions

“…It was noted that larger spin polarisation (and potentially enhanced Kondo features, see below) might be achieved by using meta-OPE analogues, where reduced intramolecular steric hindrance would be expected. 267 This would lead to a more coplanar backbone and hence increased spin delocalisation along the OPE wire, although as discussed in section 3.2 lower overall conductance would be expected. Incorporating radical functionality directly into the molecular backbone may not be straightforward either; a (non-OAE) molecular wire with an otherwise stable Blatter radical in its backbone appeared to oxidise to a closed-shell species when bound between electrodes in STM-BJ experiments.…”

“…266 The limited spin filtering behaviour of 78 is supported by a recent computational study of other structurally similar radical-bearing OPE3s, which found that good orbital overlap between the π-systems of the OPE backbone and radical is needed to achieve appreciable spin filtering. 267 Unfortunately this is difficult to achieve experimentally as significant steric shielding, which promotes divergence from coplanarity, is usually required to stabilise organic radicals. It was noted that larger spin polarisation (and potentially enhanced Kondo features, see below) might be achieved by using meta-OPE analogues, where reduced intramolecular steric hindrance would be expected.…”

Large Enhancement of the Single‐Molecular Conductance of a Molecular Wire through a Radical Substituent

“…[8][9][10] In particular, the molecular conductance of organic radicals has recently started attracting interest in conjunction with their characteristic spintronic behaviors represented by magnetoresistance and the Kondo resonance. [11][12][13][14][15][16][17][18] Hayakawa, Scheer, and co-workers have observed 1.6 � 1.9-fold larger conductance of TEMPO-OPE compared with nonradical OPE at zero field in the course of investigating magnetoresistance (Figure 1) by the MCBJ method. [13] It is intriguing that the remote radical induced non-negligible enhancement.…”

“…Since the flourishing of the mechanically controllable break junction (MCBJ) and scanning tunneling microscopy‐break junction (STM‐BJ) techniques, [1,2] molecular structure‐conductance relationships have been disclosed from various viewpoints such as dihedral angle, [3,4] substituents, [5–7] and molecular orbital [8–10] . In particular, the molecular conductance of organic radicals has recently started attracting interest in conjunction with their characteristic spintronic behaviors represented by magnetoresistance and the Kondo resonance [11–18] . Hayakawa, Scheer, and co‐workers have observed 1.6±1.9‐fold larger conductance of TEMPO‐OPE compared with nonradical OPE at zero field in the course of investigating magnetoresistance (Figure 1) by the MCBJ method [13] .…”

The syntheses, structures and spectroelectrochemical properties of 6-oxo-verdazyl derivatives bearing surface anchoring groups

“…16,17 More generally, radical molecular species have been identified as exceptionally promising compounds for use in the development of the field of molecular electronics. [18][19][20][21][22][23] The incorporation of a molecular system featuring an unpaired electron within an electrode | molecule | electrode junction leads to a host of fascinating electronic properties, 24 including enhanced rectification, 25 spin-gating, 26 Kondo effects, 27 magnetoresistive effects 28 and quantum interference, 29 whilst the redox-activity of such systems allows for an exciting range of electrochemically gated applications. [30][31][32][33] This has prompted exploration of a small number of perchlorotriphenylmethyl radicals 25,[34][35][36][37] as well as radical species generated within a molecular junction by in situ redox [38][39][40][41][42][43][44] or chemical 29 processes.…”