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

Abstract

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 [Ru^II(C₁₈OPh-terpy)(X-terpy)](PF₆)₂ and [Ru^II(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.