Reversing the Thermal Stability of a Molecular Switch on a Gold Surface: Ring-Opening Reaction of Nitrospiropyran

Abstract

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 and thus return to the parent closed form on short time scales. Here we report 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 have found that the thermally induced ring-opening reaction has an activation barrier similar to that in solution. However, on the metal surface, the MC structures turn out to be the most stable phase. On the basis of 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.

Identification of Molecular‐Adsorption Geometries and Intermolecular Hydrogen‐Bonding Configurations by In Situ STM Manipulation

“…Surprisingly, by in situ STM manipulation we could controllably switch the different hydrogen-bonding configurations within these three structural motifs at room temperature. Since these self-assembled dim and bright nanostructures are clearly different from the structure before annealing, and since, according to the previous studies, [9,20] the on-surface ring-opening reaction of molecules of this type is completely triggered as soon as the anneal temperature reaches 330 K, we believe that both the dim and the bright structures are formed by the open-form molecules. This method may also be extended to other surface supramolecular systems to supplement qualitative understanding of intermolecular interactions on the basis of STM results.…”

“…0.1 monolayer), two kinds of distinct self-assembled nanostructures (dim molecular stripes and bright molecular clusters) were revealed by STM (Figure 1 b). Since these self-assembled dim and bright nanostructures are clearly different from the structure before annealing, and since, according to the previous studies, [9,20] the on-surface ring-opening reaction of molecules of this type is completely triggered as soon as the anneal temperature reaches 330 K, we believe that both the dim and the bright structures are formed by the open-form molecules. As shown in the STM image, the dim stripes consist of dimers as the elementary structural motif, and the bright clusters are mainly composed of monomers, dimers, and tetramers (the corresponding closeup STM images are shown in Figure 2).…”

Switching Properties of a Spiropyran–Cucurbit[7]uril Supramolecular Assembly: Usefulness of the Anchor Approach

“…Based on these data it becomes clear that the described molecular design based on an anchor approach constitutes a viable strategy for extraordinary merocyanine stabilization. [32,64,71] This becomes even more evident when considering the highly desirable conservation of the photochromic properties as will be discussed in the following section.…”

Responsive Cell–Material Interfaces

“…For reversible materials, in principle two scenarios are possible: a sustained response wherein the surface change only persists for the duration of the presence of the stimulus (e.g., pH or temperature) and a permanent response that persists even after the stimulus (e.g., enzymes and electrochemical potentials) has been removed. These differences may not always be strictly applicable as some materials such as photoresponsive molecules (e.g., azobenzene and spiropyran) may undergo spontaneous transitions back to the thermodynamically stable state over a prolonged period of time even without any additional stimulation [101,102].…”