Rational Design of Highly Photoresponsive Surface-Confined Self-Assembly of Diarylethenes: Reversible Three-State Photoswitching at the Liquid/Solid Interface

Photoresponsive behavior of systems in equilibrium at the liquid/solid interface can be modulated through the design of photochromic core structures and the process of self-assembly. In this study, we investigated the effects of altering the substitution position of a thienyl group on two-dimensional (2-D) molecular ordering composed of photochromic diarylethenes (DAEs) at a liquid/highly oriented pyrolytic graphite (HOPG) interface using scanning tunneling microscopy (STM). We found that both the open- and the closed-ring isomers of a 3-thienyl-type DAE formed stripe-patterned orderings on a HOPG substrate at similar concentrations and that the process of self-assembly on a 2-D surface was highly cooperative for both isomers. In the case of 2-thienyl-type DAE, similar stripe-patterned ordering was observed only for the open-ring isomer, but no ordering was observed for the closed-ring isomer. Upon irradiation with UV and visible light, reversible three-state photoswitching over the formation/disappearance of 2-D ordering was observed using the 3-thienyl-type DAE. The instability of the ordering of the 2-thienyl-type closed-ring isomer was rationalized by the bend angle of the DAE core framework. Fundamental understanding of the relationship between the photoresponsive behavior of 2-D molecular ordering and molecular structures has been deepened through quantitative analyses of the concentration dependence of surface coverage and computational studies that included molecular mechanics/molecular dynamics (MM/MD) calculations.