Conjugated compounds for molecular electronics: from synthesis to conductance studies.

Abstract

This project concerns the synthesis and characterization of linear conjugated compounds, and assessment of their electrical performance. To that end, several oligo(phenylene-ethynylene)s (OPEs) were prepared and the available synthetic procedures reviewed and optimized. Monolayers of these compounds were transferred onto solid substrates by means of the Langmuir-Blodgett technique, and their physical and electrical properties evaluated by Atomic Force Microscopy (AFM) and Scanning Tunnelling Microscopy (STM).
Two novel approaches to the formation of the top electrode on sandwich like metal|molecule|metal devices, based on the in-situ decomposition of gold complexes, are detailed. The synthetic methodologies appropriate for the preparation of gold ethynyl complexes Au(C≡CR)(L) (R = aryl, L= PPh3 and CNR) that are critical to the thermal and photochemical decomposition protocol are described.
A convenient synthesis of bis(ethynyl) complexes trans-Ru(CCR)2(dppe)2 and unsymmetrically substituted derivatives trans-Ru(CCR1)(CCR2)(dppe)2 is described. A detailed structural and spectroelectrochemical study of the complexes, supported by TD-DFT calculations, demonstrated the relationships between the underlying electronic transitions that are responsible for the NIR absorption band shape and the relative orientations of the metal fragment and arylethynyl moieties in the oxidized species.
Single molecule STM conductance studies were performed on a series of OPE and polyyne derivatives devoting special attention to the role of the molecular linker in the overall performance of the molecular junction. In that regard, a novel molecular linker CCSiMe3 was introduced and its electrical and mechanical properties benchmarked against literature known molecular anchoring groups i.e. NH2 and SH. Transistor-like behaviour was observed in preliminary electrochemical STM studies performed on complex Ru(CCC6H4CCSiMe3)2(dppe)2. Finally, the statistical treatment of the conductance (STM) and force (AFM) measurements and the Visual Basic (VBA) codes designed to analyse the large datasets collected are included.