Diarylethene Self-Assembled Monolayers: Cocrystallization and Mixing-Induced Cooperativity Highlighted by Scanning Tunneling Microscopy at the Liquid/Solid Interface.

Stimulus control over 2D multicomponent molecular ordering on surfaces is a key technique for realizing advanced materials with stimuli-responsive surface properties. The formation of 2D molecular ordering along with photoisomerization was monitored by scanning tunneling microscopy at the octanoic acid/highly oriented pyrolytic graphite interface for a synthesized amide-containing diarylethene, which underwent photoisomerization between the open- and closed-ring isomers and also a side-reaction to give the annulated isomer. The nucleation (Kn) and elongation (Ke) equilibrium constants were determined by analysis of the concentration dependence of the surface coverage by using a cooperative model at the liquid/solid interface. It was found that the annulated isomer has a 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 into the formation of molecular ordering composed of the open-ring isomer. A quantitative analysis of the formation of ordering by using the cooperative model has provided a new view of the formation of 2D multicomponent molecular ordering.