Electronic properties of p-xylylene and p-phenylene chains subjected to finite bias voltages: a new highly conducting oligophenyl structure.

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.