Determination of Photoswitching Dynamics through Chiral Mapping of Single Molecules Using a Scanning Tunneling Microscope

Single-molecule-resolved scanning tunneling microscopy of tetra-tert-butyl azobenzene (TTB-AB) molecules adsorbed onto Au(111) reveals chirality selection rules in their photoswitching behavior. This observation is enabled by the fact that trans-TTB-AB molecules self-assemble into homochiral domains. Cis-TTB-AB molecules produced via photoisomerization are found in two distinct conformations with final state chirality determined by the initial trans isomer chirality. Based on these observations and ab initio calculations, we propose a new inversion-based dynamical photoswitching mechanism for azobenzene molecules at a surface.

Figure 1

(a) Image of trans-TTB-AB island composed of two mirror-image chiral domains (domain boundary indicated by dashed white line). Trans-TTB-AB single-molecule structural models are superimposed on right-handed and left-handed configurations. Red crosses indicate locations of TB lobe maxima. (b) Average locations of trans-TTB-AB TB lobes averaged over 394 left-handed domain molecules (left plot) and 125 right-handed domain molecules (right plot). Left-handed and right-handed domain trans-TTB-AB molecules form parallelograms that are angled by 2° from the horizontal and vertical.

Figure 2

(a) High-resolution STM image shows structure of two distinct types of photoswitched cis-TTB-AB isomers: a left-handed Type 1 cis and a right-handed Type 2 cis, contained within dashed white boxes. (b) Ab initio DFT simulated STM image of the two cis-TTB-AB types surrounded by trans-TTB-AB molecules, with structural models shown of trans-TTB-AB as well as cis type 1 and cis type 2 molecules used to calculate simulated STM image.

Figure 3

STM images showing different chiral states of single cis-TTB-AB molecules (within white boxes): (a) cis type 1, left-handed (within right-handed trans domain), (b) cis type 1, right-handed (within left-handed trans domain), (c) cis type 2, left-handed (within left-handed trans domain), and (d) cis type 2, right-handed (within right-handed trans domain). Location of single bright lobe indicated by green cross and the locations of three dim peripheral lobes indicated by white crosses. Dotted lines are guides to the eye.

Figure 4

Proposed photoisomerization dynamical pathway for TTB-AB on Au(111). A photoexcited right-handed trans-TTB-AB molecule (top center, TB legs numbered) begins the isomerization process with an in-plane inversion of the left phenyl ring (middle center). With continuing inversion, steric repulsion due to contact between the phenyl rings forces either: left phenyl to twist out of plane, lifting TB #1 and producing right-handed type 2 cis-TTB-AB (bottom left), or right phenyl to twist out of plane, lifting TB #4 and producing left-handed type 1 cis-TTB-AB (bottom right).