Synthesis and Characterization of Conjugated Mono- and Dithiol Oligomers and Characterization of their Self-Assembled Monolayers

The characterization of self-assembled monolayer molecular arrays as the active layer in electronic devices is an important step towards understanding molecular-scale electronics. In order to correlate the properties of the self-assembled monolayer to the molecular structure, a series of pi-conjugated mono- and dithiols, and alpha-pi-conjugated aromatic dithiols with an oxygen or sulfur atom between two aromatic units have been designed and synthesized. Their self-assembled monolayer films on gold surfaces were chaaracterized by cyclic voltametry, grazing incidence FTIR (GI-FTIR), contact angle and ellipsometry measurements. Based on the combined data from GI-FTIR, ellipsometry and contact angle measurements we concluded that the conjugated phenylene- and thiophene-based dithiols demonstrate a less tilted molecular orientation with respect to the surface normal with increasing chain length of conjugated dithiol molecules. Increasing the chain length from two to four phenyl rings, showed a more than linear increase of the aromatic C-C stretch modes in GI-FITR, indicating that the longer p-phenylene system is oriented towards the surface normal. By increasing the number of repeat units for oligothiophene, an almost complete reduction of the C-C stretch mode was observed, implying that the longest oligothiophene dithiol molecule is tilted very close to the surface normal. Ellipsometry showed a smaller deviation from the calculated monolayer thickness with increasing number of thiophene- or phenylene rings. This corroborates the GI-FTIR data and suggests a smaller tilt angle for the longer dithiol oligomers.