Remarkable Stability of High Energy Conformers in Self-Assembled Monolayers of a Bistable Electro- and Photoswitchable Overcrowded Alkene

Although bistability of molecular switches in solution is well established, achieving highly robust bistable molecular switching in self-assembled monolayers remains a challenge. Such systems are highly attractive as components in organic electronics and molecular-based photo and electrochromic devices. Here we report a remarkably robust surface confined bisthiaxanthylidene redox switch that shows excellent bistability, manifested in reversible changes in spectroscopic and electrochemical properties and in physical properties such as water contact angle changes (ca. 30° difference in water contact angle between the two redox states of a bisthiaxanthylidene self-assembled monolayer). The effect of surface immobilization of bis-thiaxanthylidene on its photochromic, thermal and electrochemical properties is described. Surface immobilization is achieved by incorporating thiol- and alkylsiloxy-terminated “legs” on one of the tricyclic aromatic units. The molecular switch in its neutral and dicationic state, generated by bulk electrolysis, was characterized in solution, in the solid state and on surfaces, by UV/vis absorption, Fourier transform infrared, X-ray photoelectron, and Raman spectroscopies and by cyclic voltammetry. In solution, the redox switching to the dicationic state is achieved by oxidation at 1.2 V versus SCE. Reduction of the dication at <0.4 V results in initial formation of a highly unstable twisted conformation that reverts via a <i>syn</i>-folded conformational state to the most stable (<i>anti</i>-folded) conformer. Although the <i>syn</i>-folded state can be obtained by UV irradiation at <0 °C, the twisted conformation is not observable in solution, even at 200 K. Remarkably, in monolayers on electrodes this highly unstable form (which is generated by reduction of the dicationic state) is relatively stable even for several minutes. This stability is ascribed to the formation of densely packed monolayers in which the intermolecular interactions provide for a substantial barrier to thermal interconversion of the various conformational states.