5 research outputs found

    The Influence of Linkers on Quantum Interference: A Linker Theorem

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    How heteroatomic substitutions affect electron transport through π-conjugated hydrocarbons has been the subject of some debate. In this paper we investigate the effect of heteroatomic linkers in a molecular junction on the electron-transmission spectrum, focusing on the occurrence of quantum interference (QI) close to the Fermi level, where conductivity can be significantly suppressed. We find that the substitution or addition of heteroatoms to a carbon skeleton at the contact positions does not change the main feature of QI due to the underlying carbon skeleton. QI in the overall system thus remains a robust feature. This empirical observation leads us to derive, in two mathematical ways, that these findings can be generalized. We note that addition or substitution of a carbon atom by a heteroatom at the contact positions will increase or decrease the number of electrons in the π-system, which will lead to a change in the alignment of the molecular orbitals of the isolated system relative to the electrode Fermi level. Both Hückel and density functional theory calculations on model systems probe the effect of this Fermi level change and confirm qualitatively the implications of the underlying mathematical proofs

    Dioxygen: What Makes This Triplet Diradical Kinetically Persistent?

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    Experimental heats of formation and enthalpies obtained from G4 calculations both find that the resonance stabilization of the two unpaired electrons in triplet O<sub>2</sub>, relative to the unpaired electrons in two hydroxyl radicals, amounts to 100 kcal/mol. The origin of this huge stabilization energy is described within the contexts of both molecular orbital (MO) and valence-bond (VB) theory. Although O<sub>2</sub> is a triplet diradical, the thermodynamic unfavorability of both its hydrogen atom abstraction and oligomerization reactions can be attributed to its very large resonance stabilization energy. The unreactivity of O<sub>2</sub> toward both these modes of self-destruction maintains its abundance in the ecosphere and thus its availability to support aerobic life. However, despite the resonance stabilization of the π system of triplet O<sub>2</sub>, the weakness of the O–O σ bond makes reactions of O<sub>2</sub>, which eventually lead to cleavage of this bond, very favorable thermodynamically

    Qualitative Insights into the Transport Properties of Hückel/Möbius (Anti)Aromatic Compounds: Application to Expanded Porphyrins

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    Expanded porphyrins have been recently identified as promising candidates for conductance switching based on aromaticity and molecular topology changes. However, the factors that control electron transport switching across the metal–molecule–metal junction still need to be elucidated. For this reason, the transport properties of Hückel/Möbius (anti)­aromatic compounds are investigated thoroughly in this work to gain qualitative understanding into the conductivity of these unique macrocycles. Starting from a polyene model, a simple counting rule is developed to predict the occurrence of quantum interference around the Fermi level at the Hückel level of theory. Next, the different approximations of Hückel theory are lifted, enabling the exploration of the influence of each of these approximations on the transport properties of expanded porphyrins. Along the way, a detailed study on the relationship between the conductance and aromaticity/topology has been undertaken. Even though it has been proposed that the π-conjugated systems of expanded porphyrins can be approximated as polyene macrocycles based on the “annulene model”, it turns out that the distortion induced by the pyrrole rings to the electronic structure of the expanded porphyrins causes the simple counting rule for the prediction of quantum interference developed for polyenes to fail in some specific situations. Nevertheless, our back-of-the-envelope approach enables an intuitive rationalization of most of the transport properties of expanded porphyrins. Our conclusions cast further doubt on the proposed negative relationship between conductance and aromaticity and highlight the importance of the connectivity on determining the shape of the transmission functions of the different states. We hope that the new insights provided here will offer experimentalists a road map toward the design of functional, multidimensional electronic switches based on expanded porphyrins

    Captodative Substitution: A Strategy for Enhancing the Conductivity of Molecular Electronic Devices

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    We explore a new strategy to tune the conductivity of molecular electronic devices: captodative substitution. We demonstrate that a careful design of such substitution schemes on a benzene parental structure can enhance the conductivity by almost an order of magnitude under small bias. Once this new strategy has been established, we apply it to molecular wires and demonstrate that it enables the unprecedented anti-Ohmic design of wires whose conductivity increases with the length. Overall, the captodative substitution approach provides a very promising pathway toward full chemical control of the conductivity of molecules which opens up the possibility to design molecular switches with an improved on/off ratio among others

    Analysis of Aromaticity in Planar Metal Systems using the Linear Response Kernel.

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    The linear response kernel is used to gain insight into the aromatic behavior of the less classical metal aromatic E<sub>4</sub><sup>2–</sup> and CE<sub>4</sub><sup>2–</sup> (E = Al, Ga) clusters. The effect of the systematic replacement of the aluminum atoms in Al<sub>4</sub><sup>2–</sup> and CAl<sub>4</sub><sup>2–</sup> by germanium atoms is studied using, Al<sub>3</sub>Ge<sup>–</sup>, Al<sub>2</sub>Ge<sub>2</sub>, AlGe<sub>3</sub><sup>+</sup>, Ge<sub>4</sub><sup>2+</sup>, CAl<sub>3</sub>Ge<sup>–</sup>, CAl<sub>2</sub>Ge<sub>2</sub>, CAlGe<sub>3</sub><sup>+</sup>, and CGe<sub>4</sub><sup>2+</sup>. The results are compared with the values of the delocalization index (δ<sup>1,3</sup>) and nucleus independent chemical shifts (NICS<sub><i>zz</i></sub>). Unintegrated plots of the linear response, computed for the first time on molecules, are used to analyze the delocalization in these clusters. All aromaticity indices studied, the linear response, δ<sup>1,3</sup>, and NICS<sub><i>zz</i></sub>, predict that the systems with a central carbon are less aromatic than the systems without a central carbon atom. Also, the linear response is more pronounced in the σ-electron density than in the π-density, pointing out that the systems are mainly σ-aromatic
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