Charge and spin transport in single and packed ruthenium-terpyridine molecular devices: Insight from first-principles calculations

Probing the effect of nitro-substituents in the modulation of LUMO energies for directional electron transport through 4d6 ruthenium(II)-based metallosurfactants

Electron-withdrawing nitro-substituents were installed onto terpyridine- and phenanthroline-based metallosurfactants with 4d6 ruthenium(II), which were deposited as Langmuir-Blodgett monolayers aiming to study the feasibility of charge transport in Au|LB|Au junctions. The nitro groups are intended to modulate the energy of the frontier molecular orbitals to near to, or match that of Fermi levels in the gold electrodes. A series of heteroleptic metallosurfactants [RuII(C18OPh-terpy)(X-terpy)](PF6)2 and [RuII(C18OPh-terpy)(X-phen)Cl]PF6 were synthesized, where C18OPh-terpy is the 4'-[4-(octadecyloxy)phenyl]-2,2':6',2''-terpyridine amphiphile common to all species, X-terpy is a terpyridine with-H (1) or-phenyl-NO2 (2) and X-phen is a phenanthroline with-H (3) or-NO2 (4) groups. These metallosurfactants were characterized by experimental and computational methods, and the presence of nitro groups affect more affordable reductions at less negative potentials, as well as slightly more positive oxidations, these changes are less pronounced in species 2 than in 4. Species 1 and 2 showed limited Pockels-Langmuir and Langmuir-Blodgett film formation with lower collapse pressure of 27 mN m-1. In contrast, metallosurfactants 3 and 4 showed enhanced hydrophilicity indicated by higher collapse pressures of ca. 36 mN m-1. The LB monolayers of 3 and 4 were deposited on gold electrodes to form Au|LB|Au junctions and electron transport was measured as I/V curves. The NO2-bearing species 4 showed asymmetric curves associated with directional electron transport with amplitudes up to -2.0 nA and rectification ratios from 5 to 26 between -1 to +1 V and from 3 to 14 between -3 to +3 V.

Charge transport pathways in metal porphyrin as interplay between long and short range scattering processes

We have investigated the ballistic transport for Mn, Fe-porphyrin molecules in contact with Au(111) electrodes by using density functional theory. We show that the information resulted from the projected density of electronic states does not provide a complete picture of the transport mechanism. Instead, we propose a methodology based on the concept of the orbital projected transmissions for selected groups of atoms. We have found that the transmission channels for occupied states can be assigned to short range scattering processes at gold-molecule interface, while for the states above Fermi level the transmission takes place via long range scattering processes. The interplay between these two conduction mechanisms is responsible for the transport path in metal-porphyrin; our model is in qualitative agreement with the existing experimental data on transport pathway on single porphyrin molecules.

Translation of metal-phthalocyanines adsorbed on Au(111): from van der Waals interaction to strong electronic correlation

Using first-principles calculations, we investigate the binding energy for six transition metal - phthalocyanine molecules adsorbed on Au(111). We focus on the effect of translation on molecule - surface physical properties; van der Waals interactions as well as the strong correlation in d orbitals of transition metals are taken into account in all calculations. We found that dispersion interaction and charge transfer have the dominant role in the molecule-surface interaction, while the interaction between the transition metal and gold has a rather indirect influence over the physics of the molecule-surface system. A detailed analysis of the physical properties of the adsorbates at different geometric configurations allows us to propose qualitative models to account for all values of interface dipole charge transfer and magnetic moment of metal-phthalocyanines adsorbed on Au(111).