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- Title
Controlling the structural and electrical properties of diacid oligo(phenylene ethynylene) Langmuir-Blodgett films.
- Authors
Ballesteros, Luz Marina; Martín, Santiago; Cortés, Javier; Marqués-González, Santiago; Higgins, Simon J; Nichols, Richard J; Low, Paul J; Cea, Pilar
- Abstract
The preparation, characterization and electrical properties of Langmuir-Blodgett (LB) films composed of a symmetrically substituted oligomeric phenylene ethynylene derivative, namely, 4,4'-[1,4-phenylenebis(ethyne-2,1-diyl)]dibenzoic acid (OPE2A), are described. Analysis of the surface pressure versus area per molecule isotherms and Brewster angle microscopy reveal that good-quality Langmuir (L) films can be formed both on pure water and a basic subphase. Monolayer L films were transferred onto solid substrates with a transfer ratio of unity to obtain LB films. Both L and LB films prepared on or from a pure water subphase show a red shift in the UV/Vis spectrum of about 14 nm, in contrast to L and LB films prepared from a basic subphase, which show a hypsochromic shift of 15 nm. This result, together with X-ray photoelectron spectroscopic and quartz crystal microbalance experiments, conclusively demonstrate formation of one-layer LB films in which OPE2A molecules are chemisorbed onto gold substrates and consequently -COO-Au junctions are formed. In LB films prepared on a basic subphase the other terminal acid group is also deprotonated and associates with an Na(+) counterion. In contrast, LB films prepared from a pure water subphase preserve the protonated acid group, and lateral H-bonds with neighbouring molecules give rise to a supramolecular structure. STM-based conductance studies revealed that films prepared from a basic subphase are more conductive than the analogous films prepared from pure water, and the electrical conductance of the deprotonated films also coincides more closely with single-molecule conductance measurements. This result was interpreted not only in terms of better electron transmission in -COO-Au molecular junctions, but also in terms of the presence of lateral H-bonds in the films formed from pure water, which lead to reduced conductance of the molecular junctions.
- Publication
Chemistry (Weinheim an der Bergstrasse, Germany), 2013, Vol 19, Issue 17, p5352
- ISSN
1521-3765
- Publication type
Journal Article
- DOI
10.1002/chem.201203261