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

    Year: 2017

    Journal: J. Mater. Chem. C, Volume 5, DEC 7, page 11717–11723

    Authors: Osorio, Henrry M.; Martin, Santiago; Milan, David C.; Gonzalez-Orive, Alejandro; Gluyas, Josef B. G.; Higgins, Simon J.; Low, Paul J.; Nichols, Richard J.; Cea, Pilar

    Organizations: Ministerio de Economia y Competitividad from Spain; fondos FEDER [MAT2016-78257-R]; DGA/fondos FEDER (construyendo Europa desde Aragon) [E-54]; University of Zaragoza [JIUZ-2016-CIE-04]; EPSRC [EP/M029522/1]; Australian Research Council [FT120100073, DP140100855]

    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) (G = 1.25 X10(-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) X 10(-5) G(0), break-off distance (1.56 +/- 0.09) nm). The second, lower conductance junction (G = (0.84 +/- 0.09) X 10(-5) G(0)) 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 airwater interface can be transferred onto a gold-on-glass substrate by the LangmuirBlodgett (LB) technique to give well-ordered, compact films with surface coverage Gamma = 2.0 X 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) X 10(-5) G(0), break-off distance (1.56 +/- 0.09) nm) due to the restricted range of molecular conformations in the tightly packed, well-ordered LB films.
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