Single-molecule topological insulators are promising candidates as conducting wires over nanometer length scales. In past, most conjugated molecular wires exhibit low conductance that decays as the wire length increases. To overcome this limitation, we studied a family of oligophenylene-bridged bis(triarylamines) with tunable and stable (mono-/di-)radicaloid character. The wires can undergo one-and two-electron chemical oxidations to the corresponding monocation and dication, respectively. We found that the oxidized wires exhibit high reversed conductance decay with increasing length, consistent with the expectation for the Su-Schrieffer-Heeger-type one-dimensional (1D) topological insulators. The champion 2.6 nm long dication displays a significantly high conductance greater than 0.1 G0 (2e 2 /h, the conductance quantum), 5400-fold greater than the neutral form. The observed conductance-length relationship is similar between monocation and dication series. DFT calculations elucidate how the frontier orbitals and delocalization of radicals facilitate the observed nonclassical quasi-metallic behavior. These findings offer new insights into molecular design of highly conducting 1D topological insulators.

Aryl halides are ubiquitous functional groups in organic chemistry, yet despite their obvious appeal as surface-binding linkers and as precursors for controlled graphene nanoribbon synthesis, they have seldom been used as such in molecular electronics. The confusion regarding the bonding of aryl iodides to Au electrodes is a case in point, with ambiguous reports of both dative Au−I and covalent Au−C contacts. Here we form single-molecule junctions with a series of oligophenylene molecular wires terminated asymmetrically with iodine and thiomethyl to show that the dative Au−I contact has a lower conductance than the covalent Au−C interaction, which we propose occurs via an in situ oxidative addition reaction at the Au surface. Furthermore, we confirm the formation of the Au−C bond by measuring an analogous series of molecules prepared ex situ with the complex Au I (PPh 3 ) in place of the iodide. Density functional theorybased transport calculations support our experimental observations that Au−C linkages have higher conductance than Au−I linkages. Finally, we demonstrate selective promotion of the Au−C bond formation by controlling the bias applied across the junction. In addition to establishing the different binding modes of aryl iodides, our results chart a path to actively controlling oxidative addition on an Au surface using an applied bias.

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The creation of stable molecular monolayers on metallic surfaces is a fundamental challenge of surface chemistry. N-Heterocyclic carbenes (NHCs) were recently shown to form self-assembled monolayers that are significantly more stable than the traditional thiols on Au system. Here we theoretically and experimentally demonstrate that the smallest cyclic carbene, cyclopropenylidene, binds even more strongly than NHCs to Au surfaces without altering the surface structure. We deposit bis(diisopropylamino)cyclopropenylidene (BAC) on Au(111) using the molecular adduct BAC−CO 2 as a precursor and determine the structure, geometry, and behavior of the surface-bound molecules through high-resolution X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Our experiments are supported by density functional theory calculations of the molecular binding energy of BAC on Au(111) and its electronic structure. Our work is the first demonstration of surface modification with a stable carbene other than NHC; more broadly, it drives further exploration of various carbenes on metal surfaces.

Chemical crosslinkers are commonly used to stabilize both natural and synthetic macromolecules, while providing opportunities to install functionality and modulate polymer architecture. Here, we introduce the aromatic cyclopropenium cation as a tri-functional crosslinker of secondary aminecontaining polymers. The one-step crosslinking reaction is rapid and requires no subsequent purification. When dispersed in aqueous media, the crosslinked polymers form spherical nanoparticles with highly positive charge that is maintained even in alkaline conditions. This synthetic strategy will enable the incorporation of cyclopropenium into a wide variety of macromolecules.