Effects of hindered internal rotation on packing and conductance of self-assembled monolayers

Self-assembled monolayers of 4-thiobiphenyl and 2-thiophenanthrene molecules on Au{111} were probed using scanning tunneling microscopy (STM) to investigate their assembled order, to measure the conductance through them, and to determine what molecular features are necessary for molecules to exhibit conductance switching. The 4-thiobiphenyl molecules assemble into two distinct packing structures, whereas no order is observed for 2-thiophenanthrene. Both molecules show rectifying behavior, but although 4-thiobiphenyl is more conductive at positive sample biases, 2-thiophenanthrene is more conductive at negative sample bias. Individual molecules of each type were also inserted into alkanethiol SAMs and tracked over several hours to investigate the conductance states of each molecule. Both molecules exhibit two conductance states, indicating that internal ring rotation is not required for conductance switching.