Thermal-responsive ground-state spin switching in novel butterfly-shaped overcrowded ethylene featuring a benzodithiophene core

A novel class of butterfly-shaped overcrowded ethylenes (OCEs), featuring a heterocyclic benzodithiophene (BDT) core, has been synthesized and investigated. X-ray crystallography analysis revealed that the BDT core exhibits an almost planar geometry, distinct from the well-studied AQD-based OCEs with a polycyclic aromatic hydrocarbon core. Under thermal stimuli, open-shell species of the BDT-based overcrowded ethylene (BD) were accessible in solution, as confirmed by variable-temperature ¹H NMR and ESR spectroscopy. Theoretical studies propose that the ground-state spin switching behavior of BD is primarily attributed to the rotation of the terminal trifluoromethylbenzene (TFB) groups. Open-shell species could be generated when the dihedral angles between the benzodithiophene core and terminal TFB groups twisted to approximately 40°, without requiring an orthogonal conformation through rotation.