Rational Design of Dithienylethenes with Photoelectrochromic Properties

1,2-Dithienylethene compounds (DTEs) reversibly interconvert between two isomeric forms, referred to as the colourless (ring−open isomer) and coloured (ring−closed isomer). This interconversion can be achieved through photochemical, electrochemical and thermal means. The photochemical toggling of DTEs is well characterized in the literature. However, their electrochromic behaviour is seldom reported and almost all known examples degrade through electropolymerisation. The fully electroactive DTEs are unstable in the coloured form at room temperature and undergo ring−opening in the dark. Depending on the application, DTEs may require a “single time use” (biological application / imaging) or must undergo extensive cycling (ophtalmics, smart windows). The goal of this work was to find the structural demands that allow integration of three desirable features, photo-, electrochemical and thermal stability. The first part of this thesis addresses the synthetic manipulation of the DTE core capable of catalytic oxidative ring−opening reactions. Key functional groups were appended in the α−positions of the electroactive DTE framework. These groups were carefully chosen in the context of thermal stability of the coloured−form and the mitigation of the undesired electropolymerisation. These substituents range from electron donating (through induction) methyl and t-butyl, to electron withdrawing fluorine atoms and electron rich thiophene rings. It was found that the DTEs studied presented an additive driving force, π-π stacking, for the undesired thermal ring−opening in the dark, a first observation among these type of compounds. The second part of the thesis is concerned with the mechanism of the ring−closing reaction triggered by the reduction of the uncoloured form. A new hypothesis regarding the mechanism of the process is presented. The preliminary studies suggest that the intermediate for the ring−closing reaction is the doubly reduced form of the ring−open isomer rather than the mono-reduced form as previously thought.