Helical Dinaphthothiophenes Reveal the Limits of BN‐Based Optical Tuning

The formal substitution of carbon–carbon (CC) units by boron–nitrogen (BN) units is an established strategy to improve the optical properties of polycyclic aromatic hydrocarbons while retaining their geometries. We report an example of such a substitution in a 7‐thia[5]helicene, which led to almost the same geometry. Although the absorption and fluorescence spectra are alike, BN substitution did not improve the fluorescence quantum yield, which is atypical. Both compounds mostly undergo nonradiative decay pathways, which can be attributed to their helical structure and the heavy‐atom‐effect of the sulfur atom. Compared to the sulfur‐free [5]helicene, racemization occurs very fast at ambient temperature. The molecules demonstrate the limits of BN substitution in improving the optical properties of (thia)helicenes.