Stochastic and photochromic switching of diarylethenes studied by scanning tunnelling microscopy

We investigate switching of photochromic, thiophene-substituted diarylethene switches on gold. For this, the conjugated isomer is inserted in a dodecanethiol self-assembled monolayer. This layer is subsequently scanned by tunnelling microscopy. First, we study the statistics of stochastic switching events, using two different counting methods. One method monitors reversible switching, whereas the other counts irreversible switching events. For each STM bias voltage, we observe a saturation to a dynamic equilibrium of 'on' and 'off' state molecules. This equilibrium shifts to the 'off'-side when the voltage bias is increased. Second, we separate light-induced switching from stochastic switching. In this way, we demonstrate photochromic switching of single molecules, from the conjugated to the non-conjugated form. The switching times we measure are comparable to those found for conjugated switches self-assembled on gold colloids in solution.