Self-assembled monolayers composed of aromatic thiols on gold: Structural characterization and thermal stability in solution

Self-assembled monolayers (SAMs) derived from 1,2-bis(mercaptomethyl)-4,5-dialkylbenzene (1), 1-mercaptomethyl-3,4-dialkylbenzene (2),1-mercaptomethyl-4-alkylbenzene (3),1-mercapto-4-alkylbenzene (4), and 4-mercaptomethyl-4'-alkoxybiphenyl (5) were prepared by adsorption from solution onto evaporated gold. The SAMs were characterized by contact angle goniometry, optical ellipsometry, and polarization modulation infrared reflection absorption spectroscopy. Comparison of SAMs generated from 1 and 2 to well-known SAMs generated from normal alkanethiols (6) revealed that the former SAMs are densely packed and highly oriented. The alkyl chains of the SAMs derived from 1 and 2 are, however, less crystalline (i.e., less conformationally ordered) than those derived from 6. The adsorption of compounds 3, 4, and 5 onto gold yielded highly crystalline SAMs. Solution-phase thermal desorption of SAMs 1-6 at temperatures ranging from 60 to 110 degreesC was monitored using ex situ ellipsometry. The desorption profiles of SAMs derived from 1-6 revealed two distinct kinetic regimes: a fast initial desorption followed by a substantially slower desorption. The rates of film desorption in both regimes were observed to increase with increasing temperature. Comparison of the desorption profiles of SAMs 1-6 showed that the structure of the adsorbate strongly influenced the rate of desorption; the differences were more apparent during the advanced stages of desorption. These studies provided a framework for evaluating the structural features and the mechanistic pathways that dictate the thermal stability of SAMs on gold.