Self-assembled Monolayers of Rigid Mercaptobiphenyls

Another, more direct approach to overcome the problem of surface reconstruction, is the variation of the mesogenic unit of the surface active molecule. Instead of using flexible n-alkyl moieties with high conformational freedom, SAMs formed from molecules featuring a rigid mesogen in which conformational disorder has been eliminated as completely as possible should be suitable for the preparation 

Several groups investigated aromatic thiol systems for the formation of rigid, conjugated SAM systems [86-94] and it soon became clear that among the discussed systems, mercaptobiphenyls and 4 -substituted-4-mercaptobiphenyls are promising candidates and currently the most intensively studied thiol-based SAM systems. 

Proposed structure for the low coverage striped phase of MMB (Reprinted with permission from [4] F. Schreiber Prog. Suif. Sci. 

151-256 Copyright 2000 Elsevier Science), d) Proposed structure of 4-chloro-4 -mercaptobiphenyl SAM (Reprinted with permission from [10] J. F. Kang, A. Ulman, S. Liao, et al., Langmuir 2001, 77, 95—106. Copyright 2001 American Chemical Society). 

In conclusion, the overall picture arises that a biphenyl is a suitable if not ideal mesogen for the formation of highly ordered SAM systems if compared with ah-phatic thiols. This is also reflected by the significantly higher thermal stabihty (melting temperature, Tm 140°C for MMB as compared with Tm 100°C for n-decanethiol) of mercaptobiphenyl SAMs [95]. 

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