Reversible potential-induced switching of alkyl chain aggregation in octyl-triazatriangulenium adlayers on Au(111)

In situ scanning tunneling microscopy and cyclic voltammetry studies of self-assembled octyl-triazatriangulenium monolayers on Au(111) electrode surfaces in 0.1 M HClO4 reveal a complex surface phase behavior, involving two fast, highly reversible transitions between different ordered adlayer phases: With decreasing potential, the preadsorbed (√19 × √19)R23.4° adlayer first is converted into a (7√3 × 7√3) and then into a (2√3 × 2√3)R30° phase, corresponding to a stepwise increase in the local packing density of the molecules. The (7√3 × 7√3) → (2√3 × 2√3)R30° transition is accompanied by a reorientation of the peripheral octyl chains from a more planar to a close-packed vertical arrangement. This reversible potential-induced switching between a homogeneous adlayer of small vertical extension and a Au surface partially covered by islands of a compact hydrocarbon layer is attributed to changes in the adsorbate charge state and associated changes in the intermolecular interactions.