[Paper Review] Electron transport through dipyrimidinyl-diphenyl diblock molecular wire: protonation effect

Zhenyu Li|arXiv (Cornell University)|Nov 26, 2006

Molecular Junctions and Nanostructures6 citations

TL;DR

This study investigates electron transport in a dipyrimidinyl-diphenyl (PMPH) molecular wire using first-principles NEGF-DFT calculations to explore protonation effects. Protonation enhances conductance and induces rectification, but the direction of rectification remains consistently from the dipyrimidinyl to the diphenyl side—contrary to experimental observations of rectification inversion. The results suggest the experimental inversion may not arise from intrinsic molecular protonation effects, highlighting a need for improved theoretical models beyond standard DFT and NEGF.

ABSTRACT

Recently, rectifying direction inversion has been observed in dipyrimidinyl-diphenyl (PMPH) diblock molecular wire [J. Am. Chem. Soc. (2005) 127, 10456], and a protonation mechanism was suggested to explain this interesting phenomena. In this paper, we study the protonation effect on transport properties of PMPH molecule by first principles calculations. No significant rectification is found for the pristine diblock molecular wire. Protonation leads to conductance enhancement and rectification. However, for all considered junctions with rectifying effect, the preferential current directions are samely from dipyrimidinyl side to diphenyl side. Effect of molecule-electrode anchoring geometry is studied, and it is not responsible for the discrepancy between experiment and theory.

Motivation & Objective

• To understand the microscopic origin of rectification inversion observed in experiments on dipyrimidinyl-diphenyl (PMPH) molecular wires.
• To investigate how protonation of the dipyrimidinyl moiety affects electron transport properties such as conductance and rectification.
• To evaluate whether molecular anchoring geometry (e.g., hollow, apex, cluster models) could explain discrepancies between theory and experiment.
• To determine if standard NEGF-DFT methods can reproduce the experimentally observed direction reversal of rectification.

Proposed method

• Non-equilibrium Green's function (NEGF) formalism combined with density functional theory (DFT) to calculate electronic transport through molecular junctions.
• Use of the SIESTA code for DFT calculations with DZP basis sets for all atoms except Au, which uses SZP basis sets.
• Inclusion of electrode self-energies from semi-infinite Au(111) surfaces with (3×3) unit cells, using multigrid electrostatics to handle non-periodicity in the transport direction.
• Systematic modeling of three anchoring geometries: standard hollow site, apex Au atom on surface, and Au13 cluster separated by 4 Å.
• Calculation of transmission spectra and current-voltage (I-V) characteristics under varying bias voltages to assess rectification and conductance.
• Analysis of orbital polarization and bias-dependent shifts in frontier molecular orbitals (HOMO/LUMO) relative to Fermi level to explain transport behavior.

Experimental results

Research questions

• RQ1 Does protonation of the dipyrimidinyl group in the PMPH molecular wire lead to rectification inversion as observed experimentally?
• RQ2 What is the role of molecular orbital polarization and bias-dependent energy shifts in determining rectification behavior?
• RQ3 Can different molecular anchoring geometries (hollow, apex, cluster) account for the discrepancy between theoretical predictions and experimental observations of rectification direction reversal?
• RQ4 Why does the standard NEGF-DFT approach fail to reproduce the experimentally observed rectification inversion despite protonation-induced conductance enhancement?

Key findings

• Protonation of the dipyrimidinyl group enhances conductance and induces rectification in the PMPH molecular wire, with rectification ratios reaching up to 9.2 for the diprotonated form.
• Despite protonation, all calculated junctions show rectification in the same direction—from the dipyrimidinyl side to the diphenyl side—contrary to the experimentally observed inversion.
• The anchoring geometry (hollow, apex, or cluster models) does not alter the rectification direction, indicating it is not responsible for the experimental discrepancy.
• Transmission spectra show that protonation induces strong polarization of the highest occupied molecular orbital (MPSH), especially at zero bias, leading to enhanced bias-dependent conductance and rectification.
• The cluster model shows higher conductance than the standard model due to new transmission peaks near the Fermi level, arising from the electronic structure of the Au13 cluster.
• The failure to reproduce rectification inversion suggests that either environmental effects (e.g., solvent, dodecane) or electron correlation effects beyond DFT are missing in the current theoretical framework.