Control over molecular nanostructure has become of the upmost importance in the bottom-up strategy to elaborate functionalized surfaces for electronic devices and advanced materials. In this context, the study of self-assembled monolayers (SAM) of organic building blocks on surfaces using scanning tunnelling (STM) microscopy has been the subject of intense research. The effect of electric field/pulse, concentration, temperature, or addition of guests has been extensively studied.  Light is another stimulus commonly used to achieve distant control and several advances on photoresponsive supramolecular self-assemblies at the liquid/solid interface have been reported recently.

The 2-D molecular ordering formation along with photoisomerization for the synthesized amide-containing diarylethene was monitored by STM at octanoic acid/HOPG interface. This diarylethene underwent photoisomerization between open- and closed-ring isomers and also side reaction to give annulated isomer. By the analysis of concentration dependence of surface coverage, the nucleation equilibrium constant (Kn) and the elongation equilibrium constant (Ke) were determined using cooperative model at the liquid/solid interface. It was found that the annulated isomer has very large equilibrium constant which explains the predominantly observed ordering of the annulated isomer. It was also found that the presence of the closed-ring isomer induces cooperativity in the formation of the molecular ordering of the open-ring isomer. The quantitative analysis of the ordering formation using cooperative model provided a new viewpoint toward 2-D multicomponent molecular ordering formation.