In molecular junctions increasing the length of the molecule is associated with a higher tunneling barrier, causing transport properties such as conductance to decay exponentially. However, when polycyclic aromatic hydrocarbons (PAHs) are linked to the electrodes with the terminal thiol groups arranged in cis-position, this trend is not applicable anymore and conductance increases with distance. We used first-principles density functional theory (DFT) combined with the nonequilibrium Green’s function (NEGF) formalism to calculate the conductance of a series of polycyclic aromatic hydrocarbons (PAHs) in which the first aromatic ring was benzene-based, pyridine-based or quinone-based. Our results indicate that molecules anchored in trans-position follow the expected exponential decay (Fig. 1a) whereas the molecules anchored in cis-position have a maximum conductance when 6 rings aromatic rings are included in the structure (Fig. 1b) We rationalized this by analyzing the orbital energies (Fig. 1c) and transmission functions of the systems (Fig 1d). Our results indicate that increasing the number of aromatic rings causes that the transmission peak moves closer to the Fermi energy of the electrode and the tunneling barrier competes with the HOMO energy shift caused by more aromatic rings. These results show how the anchoring configuration affect significantly the transmission properties of PAHs. 

Non-exponential length dependence behavior of molecular conductance in polycyclic aromatic hydrocarbons

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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