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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.