Polymorphism in biphenyl-based self-assembled monolayers of thiols

Self-assembled monolayers of omega-(4'-methylbiphenyl-4-yl) butanethiol (H3C-C6H4-C6H4-(CH2)n-SH) on Au(111) substrates were investigated with scanning tunneling microscopy and contact angle measurements. A striking polymorphism was observed upon annealing, and structural changes were paralleled by a switch in stability against exchange by other thiols from unstable to stable. The phase formed at temperatures above 413 K was characterized by a very high structural perfection over areas exceeding 105 nm2. The results suggest an additional dimension in the control of structure and properties of thiol monolayers if different factors contributing to the energetics of SAMs enter in a competing rather than a cooperative way.