Reversible Photomechanical Switching of Individual Engineered Molecules at a Metallic Surface

Abstract

We have observed reversible light-induced mechanical switching for individual organic molecules bound to a metal surface. Scanning tunneling microscopy (STM) was used to image the features of individual azobenzene molecules on Au(111) before and after reversibly cycling their mechanical structure between trans and cis states using light. Azobenzene molecules were engineered to increase their surface photomechanical activity by attaching varying numbers of tert-butyl (TB) ligands ("legs") to the azobenzene phenyl rings. STM images show that increasing the number of TB legs "lifts" the azobenzene molecules from the substrate, thereby increasing molecular photomechanical activity by decreasing molecule-surface coupling.

Spatial periodicity in molecular switching

“…1c-f ), are found after adsorption on Au(111). Note that TBA 6,8,13 and also diM-TBA (4,4 0 -dimethoxy-3,3 0 ,5,5 0 -tetra-tert-butylazobenzene; see Supplementary Information) adopt just one structure, which is similar to structure I (Fig. 1c).…”

“…Switches represent a prototype of such functional molecules because they exhibit characteristic states of different physical/chemical properties, which can be addressed reversibly 3 . Recently, various switching entities have been studied and switching of single molecules on surfaces has been demonstrated [4][5][6][7][8][9][10][11][12][13] . However, for functional molecules to be used in a future device, it will be necessary to selectively address individual molecules, preferentially in an ordered pattern.…”

“…Molecular switches located on surfaces have been investigated extensively over recent years, particularly isolated molecules 4,5,12 and two-dimensional (2D) molecular aggregates [6][7][8][9][10][11]13 . These studies have mostly focused on azobenzene derivatives undergoing characteristic trans-cis isomerization triggered by light 7,8 , tunnelling electrons 4,5,13 or an electric field 6 .…”

“…These studies have mostly focused on azobenzene derivatives undergoing characteristic trans-cis isomerization triggered by light 7,8 , tunnelling electrons 4,5,13 or an electric field 6 . In some cases, multiple switching events seem to be influenced by adjacent molecules 9,10 .…”

“…In some cases, multiple switching events seem to be influenced by adjacent molecules 9,10 . The use of bulky tert-butyl side groups has proved to be useful because they weaken the interaction between the central azobenzene core with the substrate while, at the same time, allowing convenient monitoring of the switching event by scanning tunnelling microscopy (STM) 6,8,13 .…”