Influence of surface coverage on the formation of 4,4′-bipyridinium (viologen) single molecular junctions

Abstract

Single-molecule conductance experiments using the STM-based I(s) method and samples of N,N′-di(4-(trimethylsilylethynyl)benzyl)-4,4′-bipyridinium bis(tetrafluoroborate) ([1](BF₄)₂) prepared on gold substrates with low-surface coverage of [1](BF₄)₂ (Γ = 1.25 × 10⁻¹¹ mol cm⁻²) give rise to molecular junctions with two distinct conductance values. From the associated break-off distances and comparison experiments with related compounds, the higher conductance junctions are attributed to molecular contacts between the molecule and the electrodes via the N,N′-dibenzyl-4,4′-bipyridinium (viologen) moiety and one trimethylsilylethynyl (TMSE) group (G = (5.4 ± 0.95) × 10⁻⁵ G₀, break-off distance (1.56 ± 0.09) nm). The second, lower conductance junction (G = (0.84 ± 0.09) × 10⁻⁵ G₀) is consistent with an extended molecular conformation between the substrate and tip contacted through the two TMSE groups giving rise to a break-off distance (1.95 ± 0.12) nm that compares well with the Si⋯Si distance (2.0 nm) in the extended molecule. Langmuir monolayers of [1](BF₄)₂ formed at the air–water interface can be transferred onto a gold-on-glass substrate by the Langmuir–Blodgett (LB) technique to give well-ordered, compact films with surface coverage Γ = 2.0 × 10⁻¹⁰ mol cm⁻². Single-molecule conductance experiments using the STM-based I(s) method reveal only the higher conductance junctions (G = (5.4 ± 0.95) × 10⁻⁵ G₀, break-off distance (1.56 ± 0.09) nm) due to the restricted range of molecular conformations in the tightly packed, well-ordered LB films.