(564a) The Assembly of Conjugated Dithiols on Gold and Gallium Arsenide

The assembly of simple π-conjugated molecules has attracted substantial research interest because these molecules are model systems for more complex candidates for molecular electronics. Yet, the assembly of such π-conjugated molecules is not completely understood. Here, we report how the solvent from which these molecules are assembled can dramatically affect the way they organize on Au and GaAs.

The molecules of interest are n-phenyldithiols (n=3,4). As synthesized, these molecules are protected by acetyl end groups. To convert the acetyl end groups into thiols, NH₄OH is added. We investigated the assembly of these molecules from ethanol/tetrahydrofuran mixtures, in addition to the pure solvents, in the presence of dilute (<35 mM) NH₄OH.

To examine the surface coverage and the ensemble-average orientation of n-phenyldithiol assemblies on Au and GaAs, we carried out synchrotron-based near-edge x-ray absorption fine structure spectroscopy (NEXAFS) and in-house Fourier transform infrared spectroscopy (FTIR) experiments. Both carbon-edge NEXAFS spectra and FTIR indicate that the n-phenyldithiol surface coverage is independent of the assembly solvent on Au. Angle-resolved NEXAFS suggest that terphenyldithiol (n=3) and quaterphenyldithiol (n=4) are preferentially oriented on Au and the molecules are tilted on average 30±3^o away from the substrate normal independent of the assembly solvent. Our recent FTIR experiments concur with this preferentially upright orientation.

In contrast to Au, the surface coverage of n-phenyldithiols on GaAs is extremely solvent-sensitive. Specifically, the surface coverage decreases monotonically with decreasing ethanol volume fraction in the assembly solvent. At high ethanol volume fractions, the average orientation of terphenyl- and quaterphenyldithiols (n=3,4) is similar to that on Au, but at low ethanol fractions, all the assemblies are disordered.