Scanning tunnelling microscopy analysis of octameric o-phenylenes on Au(111)

Abstract

Oligomeric o-phenylenes are an emerging class of helical molecules that exhibit complex conformational behaviour, which is determined by their terminal groups and the solvent environment. Using scanning tunnelling microscopy (STM), we investigated the conformational behaviour of octameric o-phenylenes (OP₈s) featuring Br or NO₂ terminal groups (OP₈Br and OP₈NO₂). These o-phenylene oligomers are known to adopt a well-defined helical conformation in the solid state, but undergo a rapid helical inversion in solution at ambient temperature. We show that STM microscopy allows the direct observation of perfectly and partially folded helical conformers of OP₈Br and OP₈NO₂, both of which are simultaneously present in their respective molecular films on Au(111). Possible electronic interactions between the Au(111) surface and the exposed π-system of the OP₈ derivatives might result in a thermodynamic bias able to stabilize the metastable, partially folded conformation. Recent studies have revealed that the helical inversion rate of OP₈NO₂ in solution decreases substantially upon oxidation to the corresponding radical cation (OP₈NO₂˙⁺). The OP₈NO₂ and OP₈NO₂˙⁺ film morphologies on Au(111) observed by STM differed substantially with respect to each other, most likely reflecting differences arising from the conformational states of OP₈NO₂ and OP₈NO₂˙⁺ at the molecular level, as well as improved charge-transport properties of the oxidized state.