Aromaticity is a fundamental concept in chemistry. It is described by Hückel’s rule that states that a cyclic planar π-system is aromatic when it shares 4n+2 π-electrons and antiaromatic when it possesses 4n π-electrons. Antiaromatic compounds are predicted to exhibit remarkable charge transport properties and high redox activities. However, it has so far only been possible to measure compounds with reduced aromaticity but not antiaromatic species due to their energetic instability. Here, we address these issues by investigating the single-molecule charge transport properties of a genuinely antiaromatic compound, showing that antiaromaticity results in an order of magnitude increase in conductance compared with the aromatic counterpart. Single-molecule current–voltage measurements and ab initio transport calculations reveal that this results from a reduced energy gap and a frontier molecular resonance closer to the Fermi level in the antiaromatic species. The conductance of the antiaromatic complex is further modulated electrochemically, demonstrating its potential as a high-conductance transistor.
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.
We propose a novel platform for stable and highly conducting single molecule electronics and characterize its mechanical, electronic and conducting properties using ab initio simulations. We study a biphenylene-based molecular architecture on gold and consider that the antiaromatic instability of biphenylene leads to the breaking of internal carbon-carbon bonds and subsequent formation of Au-C covalent bonds with the substrate. In the resulting conformation the conjugated rings have a large twist angle and stand almost upright on the surface. The top contact is realized by functionalizing one end of the biphenylene unit with a chemical linker group, which in the adsorbed geometry is positioned far from the surface. We consider several linker terminations for this top contact, which is approached in our simulations by a gold tip. Using Density-Functional Theory (DFT) and Non-Equilibrium Green's Function (NEGF) methods, we quantify the mechanical and electron transport properties of the molecular junction and discuss their relationship with the nature of the linker group. Our results show that this biphenylene-based platform is very stable and provides high electronic transparency to current flow, demonstrating its potential in single molecule conductance studies.
Antiaromatic molecules have been predicted to exhibit increased electron transport properties when placed between two nanoelectrodes compared to their aromatic analogues. While some studies have demonstrated this relationship, others have found no substantial increase. We use atomistic simulations to establish a general relationship between the electronic spectra of aromatic, antiaromatic, and quinoidal molecules and illustrate its implications for electron transport. We compare the electronic properties of a series of aromatic‐antiaromatic counterparts and show that antiaromaticity effectively p‐dopes the aromatic electronic spectra. As a consequence, the conducting properties of aromatic‐antiaromatic analogues are closely related. For similar attachment points to the electrodes, an interference feature is expected in the HOMO‐LUMO gap of one whenever it is absent in the other one. We demonstrate how the relative conductance of aromatic‐antiaromatic pairs can be tuned and even reversed through the choice of chemical linker groups. Our work provides a general picture relating connectivity, (anti)aromaticity, and quantum interference and establishes new design rules for single molecule circuits.
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