pH control of conductance in a pyrazolyl Langmuir–Blodgett monolayer

Abstract

In this contribution, pyrazole is identified as an excellent anchoring group capable of forming well-ordered Langmuir–Blodgett (LB) monolayers onto gold substrates. In contrast, the formation of self-assembled (SA) monolayers is prevented due to the extraction of gold atoms from the surface upon the incubation process required for the formation of these films. The electronic coupling strength of pyrazole–gold junctions is explored through evaluation of the electrical properties of LB films incorporating a double pyrazole terminated molecular wire, namely 1,4-bis(1H-pyrazol-4-ylethynyl)benzene, 1; these electrical properties are compared with those of molecules having a similar chemical structure, geometry and length. The conductance value for LB films of 1 is high when the monolayer is transferred from a Langmuir film prepared onto a basic subphase, 1.19 × 10⁻⁴G₀, as it is more than four times larger than the conductance of the same compound incorporated on a LB film prepared from a water subphase, 0.27 × 10⁻⁴G₀. These results are interpreted in terms of the deprotonation of the pyrazole group in a basic media that leads to the formation of the pyrazolate (deprotonated pyrazole) moiety. This results in a more efficient electronic coupling with the gold tip of the scanning tunnelling microscope used for film characterization.