(BO)2-Doped Tetrathia[7]helicene: A Configurationally Stable Blue Emitter

Helicenes combine two central themes in chemistry: extended π-conjugation and chirality. Heteroatom doping preserves both characteristics and allows to modulate the electronic structure of a helicene. Herein, we report the (BO)2-doped tetrathia[7]helicene 1, which was prepared from 2-methoxy-3,3'-bithiophene via four steps. 1 is formally derived by substituting two (Mes)B-O moieties in place of (H)C=C(H) fragments in two benzene rings of parent tetrathia[7]helicene. X-ray crystallography revealed a dihedral angle of 50.26(9)° between the two terminal thiophene rings. (P)-/(M)-1 enantiomers were separated by chiral-HPLC and are configurationally stable at room temperature. The experimentally determined enantiomerization barrier of 27.4 ± 0.1 kcal mol-1 is lower than that of tetrathia[7]helicene (39.4 ± 0.1 kcal mol-1). Circular dichroism spectra of (P)- and (M)-1 show a perfect mirror-image relationship. 1 is a blue emitter (λem = 411 nm) with a photoluminescence quantum efficiency of ΦPL = 6% (cf. tetrathia[7]helicene: λem ≈ 405 nm, ΦPL = 5%).