Thienylene-Based Diradicaloids: Electrochromic Ring-Closing of Dithienylethene-Based Bis-Amidinium Cation

Due to the lower aromaticity of thiophene compared to benzene, thiophene-based diradicaloid compounds prefer larger weightage of their quinoid form (closed-shell electronic configuration) than their benzoid form (open-shell electronic configuration). At the same time, the lower aromaticity of thiophene facilitates its participation in electrocyclic reactions and as a result dithienylethene-derivatives represent a classical example of photochromism. Here a modular approach for the synthesis of crystalline thienylene-, bithienylene-, and terthienylene-based diradicaloids is reported with acyclic diaminocarbene-scaffold as end-caps. The influence of the π-conjugated spacer is reflected in their triplet population at room temperature: thienylene- and bithienylene-bridged diradicaloids are EPR inactive whereas the terthienylene-bridged one exhibits an EPR signal, the intensity of which increases with rising temperature. A dithienylethene-based photochromic bis-amidinium cation was also designed and synthesized to obtain a related thienylene-based diradicaloid. Under two-electron reduction the crystalline diradicaloid is formed with a concurrent reductive ring-closing of the dithienylethene-core. Moreover, a two-electron oxidation of this dithienylethene-based diradicaloid leads to the closed-form of the dithienylethene-based photochromic bis-amidinium cation. Overall, this demonstrates the structurally characterized electrochromic (reductive as well as electron-mediated) ring-closing of a dithienylethene-based photochromic π-non-conjugated bis-amidinium cation to the diradicaloid and π-conjugated bis-amidinium cation, establishing an alternative stimulus in place of UV light.