Stable Π-Extended Thio[7]helicene-Based Diradical with Predominant Through-Space Spin-Spin Coupling

In this work, a novel pi-extended thio[7]helicene scaffold was synthesized, where the alpha-position of the thiophene unit could be functionalized with bulky phenoxy radicals after considerable synthetic attempts. This open-shell helical diradical, ET7H-R, possesses high stability in the air, nontrivial pi conjugation, persistent chirality, and a high diradical character (y(0) of 0.998). The key feature is a predominant through-space spin-spin coupling (TSC) between two radicals at the helical terminals. Variable-temperature continuous-wave electron spin resonance (cw-ESR) and superconducting quantum interference device (SQUID) magnetometry in the solid state reveal a singlet ground state with a nearly degenerate triplet state of ET7H-R. These results highlight the significance of a stable helical diradicaloid as a promising platform for investigating intramolecular TSCs.