Synthesis and characterization of conjugated mono- and dithiol oligomers and characterization of their self-assembled monolayers

The characterization of self-assembled monolayer molecular arrays, which serve as active layers in electronic devices, is an important step toward understanding molecular-scale electronics. To correlate the properties of self-assembled monolayers with their molecular structures, a series of pi-conjugated mono-and dithiols and aromatic dithiols with an oxygen or sulfur atom between two aromatic units have been designed and synthesized. Their optical properties were determined by UV-vis spectroscopy, Their self-assembled monolayer films on gold surfaces were characterized by cyclic voltammetry, grazing incidence Fourier transform infrared spectroscopy (GI-FTIR), and contact angle and ellipsometry measurements. Increasing the chain length from two to four phenyl rings showed a more than linear increase of the intensity of the aromatic C=C ring stretch modes in GI-FTIR, indicating that the longer p-phenylene system is oriented toward the surface normal. Similar to oligophenylenes, when the number of repeat units for oligothiophene is increased, a more than linear increase of the intensity of the C=C stretch and C-H bend modes implies that the longest oligothiophenedithiol molecule is oriented close to the surface normal. Ellipsometry showed a smaller deviation from the calculated monolayer thickness with increasing number of thiophene or phenylene rings, corroborating the GI-FTIR data. We conclude that the conjugated phenylene-and thiophene-based dithiols demonstrate a less tilted molecular orientation with respect to the surface normal with increasing chain length.