Bidirectional Ring-Opening and Ring-Closing of Cationic 1,2-Dithienylcyclopentene Molecular Switches Triggered with Light or Electricity†

Either light or electricity can be used to trigger the reversible cyclization reactions of three bis(N-methylpyridinium)dithienylethene derivatives that differ from each other by the presence of either thiophene rings or methyl groups at the two carbon atoms of the photoresponsive hexatriene system involved in forming the new C–C bond. All three derivatives undergo ring-closing isomerization reactions when irradiated with UV light (365 nm) or when electrochemically reduced (–1.0 V). All three derivatives can also be ring-opened by irradiating them with visible light (> 490 nm) or by electrochemically oxidizing them (+1.5 V). The presence of additional thiophene rings attached to the two C2 ring positions of the dithienylethene (DTE) backbone enhances the electrochromic behavior, while methyl groups in these positions results in improved photochromic performance. The nature of these groups also greatly affects the thermal properties of the compounds in their ground states. Replacing each methyl group at the C2 ring positions with a thiophene ring systematically lowers the activation energy of spontaneous ring-opening by 8 kJ mol–1, which correlates with the enhanced efficiency of the oxidative ring-opening reactions and with the limited photochromism of the thiophene-functionalized derivative.