A study on conformational changes by electron charges in viologen single molecules by using STM

The topography of self-assembled viologen derivatives (VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH) molecules on an octanethiol (C8) self-assembled monolayer (SAM) modified gold surface was measured using ultrahigh-vacuum scanning tunneling microscopy (UHV-STM). We demonstrate here a novel matrix SAM appropriate for isolation of the viologen molecules. The C8 was used for a matrix SAM, in which the VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH were inserted at molecular lattice defects. The isolated single molecules of viologen derivatives inserted in the matrix SAM were observed as protrusions in STM topography using a constant current mode. We measured the topographic heights (VC8SH: 1.53 nm, VC10SH: 2.01 nm, HSC8VC8SH: 2.71 nm, and HSC10VC10SH: 3.3 nm) of the molecular protrusions using STM. Also, changes in the central axis of viologen molecules were observed as VC8SH (0.5–0.73 nm), VC10SH (0.4–0.74 nm), HSC8VC8SH (0.67–0.84 nm), and HSC10VC10SH (0.67–0.99 nm), respectively.