Redox‐Addressable Single‐Molecule Junctions Incorporating a Persistent Organic Radical**

Abstract

Integrating radical (open‐shell) species into non‐cryogenic nanodevices is key to unlocking the potential of molecular electronics. While many efforts have been devoted to this issue, in the absence of a chemical/electrochemical potential the open‐shell character is generally lost in contact with the metallic electrodes. Herein, single‐molecule devices incorporating a 6‐oxo‐verdazyl persistent radical have been fabricated using break‐junction techniques. The open‐shell character is retained at room temperature, and electrochemical gating permits in situ reduction to a closed‐shell anionic state in a single‐molecule transistor configuration. Furthermore, electronically driven rectification arises from bias‐dependent alignment of the open‐shell resonances. The integration of radical character, transistor‐like switching, and rectification in a single molecular component paves the way to further studies of the electronic, magnetic, and thermoelectric properties of open‐shell species.

Redox-controlled conductance of polyoxometalate molecular junctions

“…55,58,59 This is likely because the SOMO is localized on the Mo and therefore it is embedded inside the molecule and partly protected from a too strong interaction with the metal electrodes in a similar way as recently demonstrated for a verdazyl radical, which was stable in its open-shell configuration at room temperature in molecular junctions. 56 We note that I-V measurements on a reference sample without the POMs, i.e. TS AU-C6//Pt junctions, give a higher value ε 0 ≈ 0.9 eV (section 8 in the ESI †), in good agreement with the previous results for the LUMO of alkyl chains on Au.…”

“…been recently observed when switching a molecule between open-shell (radical) and closed-shell configurations. 54,56 It is also likely that the details on how these dipoles are organized (ordered vs. disordered), where they are located in the SAM, as well as the molecular ordering of the POMs influence the large dispersion of the I-V measurements observed here. This feature calls for more experiments, e.g.…”

“…For the one-electron reduced PMo 12 ( i ), the added electron is localized on a SOMO located near the electrode Fermi level as also reported when open-shell radicals are incorporated into molecular junctions. 51–56 Generally speaking, it is known from ab initio calculations that the SOMO–SUMO gap of a series of one-electron reduced POMs is low at around ∼0.2 eV, 57 and thus both levels can now easily contribute to ET in the molecular junctions, leading to the experimentally observed enhanced conductance (Fig. 6b).…”

“…55,58,59 This is likely because the SOMO is localized on the Mo and therefore it is embedded inside the molecule and partly protected from a too strong interaction with the metal electrodes in a similar way as recently demonstrated for a verdazyl radical, which was stable in its open-shell configuration at room temperature in molecular junctions. 56…”

“…, between (PMo 12 ) 4− and NH 3 + , (PMo 12 ) 4− and TBA + , charge transfer at the electrodes) is likely to change the energy landscape in the junction 67 and consequently the ET properties, especially the direction of the asymmetry of the I – V s. 68–70 Finally, we note that a change of the shape of the I – V curves (asymmetric vs. symmetric) has also been recently observed when switching a molecule between open-shell (radical) and closed-shell configurations. 54,56…”