Iminyl-Radical-Mediated Formation of Covalent Au–N Bonds for Molecular Junctions

Abstract

The interaction between organic radicals and transition metals plays a crucial role in radical-mediated chemical reactions, functional devices, and biocatalysis. Characterizing such interactions, however, remains a long-standing challenge due to the inherently high reactivity of radical species. Here, using a scanning tunneling microscope breaking junction (STM-BJ) technique, we are able to detect the interaction mode between iminyl radicals and the gold surface at a single molecule level. We show that the free iminyl radicals generated through photochemical N−O bond homolysis of oxime esters react toward the gold electrode surface and produce covalent Au−N bonds. Intriguingly, we find that the Au−N bonding reactions lead to the formation of robust and highly conductive single-molecule junctions. These findings provide not only insights into the mechanism of iminyl-radical-involved reactions but also a facile photolysis method to create a new type of covalent electrode−molecule bonding contact for molecular devices.

Photooxidation driven formation of Fe-Au linked ferrocene-based single-molecule junctions

“…We note that Au electrodes undergo relaxation and reorganization upon rupture of the Au-Au contact, resulting in a difference between the plateau length and actual molecular junction length, known as the snapback distance [30][31][32] . The reported snapback distances for various molecular structures and solvents are around 5-8 Å [33][34][35][36] . After taking into account this snapback distance, the shorter plateau length for the oxidized molecule is consistent with the distance between sulfur and iron in 1, ~7.4 Å [33][34][35] .…”

Evolution of Single-Molecule Electronic Interfaces

“…100,101 After that, Zang et al constructed a covalent Au−N bond between the gold electrode interface and free iminyl radicals through photochemical N−O bond homolysis of oxime esters. 102 Furthermore, electrochemical reactions provide an alternative to construct covalent interfaces in a controllable manner. Venkataraman et al achieved the electrochemical conversion of Au ← N bonds into novel Au−N contacts in situ, reducing the contact resistance of the molecule−electrode interface.…”

“…In 2019, Darwish et al. reported a precursor diazonium salt moiety with covalent anchoring ability via a free-radical reaction, which significantly improves the lifetime of single-molecule devices by forming covalent bonds with gold and silicon electrodes. , After that, Zang et al constructed a covalent Au–N bond between the gold electrode interface and free iminyl radicals through photochemical N–O bond homolysis of oxime esters . Furthermore, electrochemical reactions provide an alternative to construct covalent interfaces in a controllable manner.…”