Electrochemical Write and Read Functionality through Oxidative Dimerization of Spiropyran Self-Assembled Monolayers on Gold

In contrast to their photochromism, the electrochemistry of spiropyrans in self-assembled monolayers has attracted only modest attention in recent years. In this contribution the electrochemical oxidation of self-assembled monolayers (SAMs) of 6-nitro-BIPS spiropyran (SP) prepared on polycrystalline gold surfaces is described. The SAMs were characterized with cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), surface-enhanced Raman scattering (SERS), and UV/vis absorption spectroelectrochemistry. The electrochemical oxidation of spiropyrans in solution results in aryl C−C coupling of the indole units and thereby the formation of a symmetric spiropyran dimer. Comparison of spectroscopic data obtained for electrochemically oxidized spiropyran dimers in solution with data from monolayers confirms that a similar oxidative coupling occurs in the SAMs on gold also. The dimer formed can be oxidized electrochemically to monocationic and dicationic states and shows remarkably good stability in UHV and ambient conditions in all three redox states. In addition, the dimerized spiropyran self-assembled monolayer show photochromism, which was characterized by XPS and SERS spectroscopy.