Electronic Properties of p‐Xylylene and p‐Phenylene Chains Subjected to Finite Bias Voltages: A New Highly Conducting Oligophenyl Structure

Abstract

Recently, experimental and theoretical determination of electric currents induced by finite bias voltages in p-xylylene chains attached to gold contacts revealed higher conductance of these systems in comparison with p-phenylene homologous chains. To gain more insight into the conducting properties of these oligophenyl structures, ab initio studies were carried out on the electronic properties of two different p-xylylene-like chains (pX1 and pX2) and the p-phenylene (pP) chain attached to gold contacts, with molecular formulas AuCH2 (C6 H4 )n CH2 Au (n=1-5), Au2 C(C6 H4 )n CAu2 (n=1-5), and Au(C6 H4 )n Au (n=1-5), respectively. The molecules were subjected to finite bias voltages ranging from 0 to 5 V. Analysis of the intramolecular electron transfer and electron delocalization revealed a completely opposite response to electric perturbation of pX2 in comparison with pX1 and pP. Thus, in pX2 the applied voltage causes an increase in the electron delocalization within the rings together with a large electron transfer and energetic stabilization. On the contrary, the same voltages partially destroy the electron delocalization in pX1 and pP, produce a large local electron polarization in the benzene rings, and a smaller energetic stabilization. These differences can be rationalized in terms of the role played by polarized valence bond structures in the total wave function. Theoretical estimation of the I/V profiles indicates that pX2 chains are much better electronic conductors than pX1 and pP.

Study of electron transport in polybenzenoid chains covalently attached to gold atoms through unsaturated methylene linkers

“…(4) within the range of electric potential employed, as it is reflected by the values of the Pearson coefficient also shown in the Table. Moreover, α ≈ 3 for all the systems, displaying deviations smaller than 0.03. This is not only a characteristic of the molecular chains considered here since the same value of α was recently found for other molecular devices such as benzene-1,4-dithiolate [24] and other oligophenyl chains [28]. Then, the values of β may be employed alone as a measure of the relative ease of different molecular junctions to conduct electrons through the central molecular chain at a certain applied voltage.…”

“…In this work, as in our previous works [24,28], we have analysed the I/V profiles, confirming that for all the molecular devices investigated the current-voltage curves satisfy the following non-ohmic relation, giving rise to the following approximated expression for G,…”

“…The xylylene chains (pX2 following the nomenclature employed in Ref. [28] but C1 in this work) are constructed with the central backbone alone. As mentioned in the introduction section, these oligophenyl chains were already considered in a previous work and will serve as reference structure for this work.…”

“…Up to now, such connections were limited to a qualitative framework, using in most cases arguments based on the chemical graph theory [26,27]. For instance, based on the analysis of the molecular electric response in terms of electron charge polarization, energetic stabilization and electron delocalization, some of us recently predicted the high electric conductance of a new oligophenyl chain [28]. The outstanding conductance of this chain was rationalized in terms of energetically accessible polarized Kekulé structures.…”

“…In metallic wires and the oligophenyl chains studied in Ref. [28], it was shown that only one of these channels (the one with the largest eigenvalue) accounts for an effective electron transport between the contacts with the electrodes. The corresponding eigenvectors provided almost the whole magnitude of the total electron promotion.…”