The ring-opening/closing reaction between spiropyran (SP) and merocyanine (MC) is a prototypical thermally and optically induced reversible reaction. However, MC molecules in solution are thermodynamically unstable at room temperature, thus returning to the parent closed form on short time scales. Here, we report a contrary behavior of a submonolayer of these molecules adsorbed on a Au(111) surface. At 300 K, a thermally-induced ring-opening reaction takes place on the gold surface, converting the initial highly ordered SP islands into MC-dimer chains. We find that the thermally induced ring-opening reaction has a similar activation barrier as in solution. However, on the metal surface the MC structures turn out to be the most stable phase. Based on the experimentally determined molecular structure of each molecular phase, we ascribe the suppression of the back reaction to a stabilization of the planar MC form on the metal surface as a consequence of its conjugated structure and large electric dipole moment. The metal surface thus plays a crucial role in the ring-opening reaction and can be used to alter the stability of the two isomers.
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