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

Helicenes combine two central themes in chemistry: extended pi-conjugation and chirality. Hetero-atom doping preserves both characteristics and allows modulation of 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 in 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 the parent tetrathia[7]helicene. X-ray crystallography revealed a dihedral angle of 50.26(9)degrees between the two terminal thiophene rings. The (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)). The circular dichroism spectra of (P)- and (M)-1 show a perfect mirror-image relationship. 1 is a blue emitter (lambda(em)=411 nm) with a photoluminescence quantum efficiency of phi(PL)=6 % (cf. tetrathia[7]helicene: lambda(em)approximate to 405 nm, phi(PL)=5 %).