Does a Blatter Radical Retain Its Open-Shell Character When Incorporated into Gold–Molecule–Gold Junctions?

Abstract

One of the key issues for the use of organic radicals in high-performance molecular electronic, spintronic and thermoelectric devices is the retention, at room temperature, of their open-shell character when they are in contact with the metal electrodes of a junction. By means of first-principles quantum transport calculations, we have investigated the stability and the electronic transport properties of single-molecule junctions incorporating a Blatter radical (BR) with thiomethyl linker groups. Our calculations suggest that the BR can unequivocally retain its open-shell nature even when bound to gold electrodes through undercoordinated gold adatoms. Such a state cannot be destroyed by the surrounding solvent molecules and the applied bias. The calculated low-bias conductance values of junctions with BR, an oxidized BR derivative, and a closed-shell stilbene molecule are in quantitative agreement with the available experimental measurements. The small conductance of the BR can be associated with the localization of its π-type singly occupied and singly unoccupied molecular orbitals (SOMO and SUMO), leading to an asymmetrically weak electronic coupling to the two gold electrodes, even though these two orbitals lie very close to the Fermi energy. Furthermore, the nonmonotonic modulation of the junction conductance controlled by a charge gate can be used in experiments to further verify the presence of the unpaired electron. This proof may be realized in practice with an electrostatic solidstate or an electrochemical gate. Our findings deepen the current understanding of the radical−metal interfacial properties and facilitate the design of future radical-based multifunctional molecular devices.

Au-Thiolate Interfacial Coordination: The Key to Determining the Spin State of a Blatter Radical When Incorporated into Gold–Molecule–Gold Junctions

“…According to the Hirshfeld charge analysis, the difference between the majority and minority spins distributed on the BR increases to 0.88, implying that the open-shell electronic configuration of this radical is maintained. At variance with our previous finding that the BR molecule functionalized with thiomethyl linker groups can unequivocally retain its open-shell nature even when interacting with two gold electrodes via undercoordinated Au adatoms, it appears that the stability of BR with the thiol linkers in junctions depends heavily on the details of the Au–S interfacial configuration.…”

“…When compared to our previous study of a BR coordinated to two gold electrodes through thiomethyl linker groups, the spin state of BR with thiol groups is very sensitive to the electrode–molecule interfacial configurations due to the much stronger Au–S covalent bonds. However, the localization of the BR SOMO and SUMO still restricts the formation of efficient conducting channels around the Fermi energy and thus hinders the appearance of the expected high conductance no matter whether the open-shell character is maintained or not . Our findings elucidate the influential effects of gold–thiolate interfacial configurations on the stability of BRs within junctions and provide a new perspective to prevent the quenching of the open-shell state within a junction; namely, increasing the Au–S interfacial coordination number appears to be more effective than introducing electron-withdrawing substituents.…”

“…In a recent work, we have reported that a BR attached to two gold electrodes with undercoordinated gold adatoms through thiomethyl linker groups can unequivocally retain its open-shell nature, even in the presence of surrounding solvent molecules and applied bias . This feature is closely related to the dative nature of the gold–thiomethyl bond at the radical–gold interfaces, which cannot significantly affect the electronic and spin state of the BR in the junction.…”