(BO)2-doped tetrathia[7]helicenes: synthesis and property-change induced by “BO bond inversion”

Helical distortion of polyaromatic hydrocarbons gives rise to a special class of pi-conjugated systems, namely helicenes. Owing to their configurational stability and easily tunable optoelectronic properties (via heteroatom-doping), helicenes have recently come to the fore as building blocks for applications in materials science (CP-OLEDs, chiroptical switches). In this context, boron-doped helicenes are particularly promising. Herein, we report the synthesis of the new (BO)2-doped tetrathia[7]helicene 2, derived by the formal inversion of (Mes)B-O moieties in the (previously reported) isomer 1. Theoretical characterization of 2, and comparison with 1, revealed that the inversion of the BO vectors promotes the extension of the LUMO via the central thiophene-benzene-thiophene fragment (and not via the terminal thiophene rings, as in 1), resulting in a considerable lowering of the LUMO energy (ELUMO(2) = -2.22 eV vs. ELUMO(1) = -1.65 eV). Spectroscopic studies revealed that the "BO bond inversion" also contributes to the narrowing of the energy gap (Eoptg (2) = 2.90 eV vs. Eoptg (1) = 3.16 eV) and causes a significant red-shift of the absorption/emission bands (approximate to 40 nm). Interestingly, besides low fluorescence quantum yield (Phi PL(2) = 7%), 2 shows detectable circularly polarized luminescence (glum = 0.8 x 10-3) and pronounced phosphorescence at low temperature (77 K). (P)-/(M)-enantiomers of 2 were successfully separated by CSP-UHPLC and proved to be stable (Delta G double dagger enant = 29.4 +/- 0.1 kcal mol-1 at 353 K). Racemization studies combined with theoretical calculations confirmed that BO-doping is an extremely perturbative tool for tuning the mechanical rigidity of tetrathia[7]helicenes (Delta G double dagger enant (2) is 10 kcal mol-1 lower than Delta G double dagger enant (7TH)).