Enhanced Optical Properties of Azaborole Helicenes by Lateral and Helical Extension

The synthesis and characterization of laterally extended azabora[5]-, [6]- and -[7]helicenes, assembled from N-heteroaromatic and dibenzo[g,p]chrysene building blocks is described. Formally, the pi-conjugated systems of the pristine azaborole helicenes were enlarged with a phenanthrene unit leading to compounds with large Stokes shifts, significantly enhanced luminescence quantum yields (Phi) and dissymmetry factors (g(lum)). The beneficial effect on optical properties was also observed for helical elongation. The combined contributions of lateral and helical extensions resulted in a compound showing green emission with Phi of 0.31 and vertical bar g(lum)vertical bar of 2.2x 10(-3), highest within the series of pi-extended azaborahelicenes and superior to emission intensity and chiroptical response of its non-extended congener. This study shows that helical and lateral extensions of pi-conjugated systems are viable strategies to improve features of azaborole helicenes. In addition, single crystal X-ray analysis of configurationally stable [6]- and -[7]helicenes was used to provide insight into their packing arrangements.