Stereochemistry of bistricyclic aromatic enes and related polycyclic systems.

Bistricyclic aromatic enes (BAEs) and related polycyclic systems are a class of molecular materials that display a rich variety of conformations, dynamic stereochemistry and switchable chirality, color, and spectroscopic properties. This is due to the a subtle interplay of the inherent preference for planarity of aromatic systems and the competing necessity of non-planarity due to intramolecular overcrowding in the fjord regions built into the general molecular structure of BAEs. The conformational, dynamic, and spectroscopic properties may be designed and fine-tuned, e.g., by variation of the bridging groups X and Y, the overcrowding in the fjord regions, extensions of the aromatic system, or other modifications of the general BAE structure, based on the fundamental understanding of the structure-property relationships (SPR). The present review provides an analysis of the conformational spaces and the dynamic stereochemistry of overcrowded bistricyclic aromatic enes applying fundamental symmetry considerations. The symmetry analysis presented here allows deeper insight into the conformations, chirality, and the mechanisms of the dynamic stereochemistry, and will be instrumental in future computational studies.