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Curved Polycyclic Aromatic Molecules That Are π‑Isoelectronic to Hexabenzocoronene

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posted on 22.08.2012, 00:00 by Jiye Luo, Xiaomin Xu, Renxin Mao, Qian Miao
Reported here are two types of curved π-molecules that are π-isoelectronic to planar hexabenzocoronene (HBC) but are forced out of planarity either by an embedded seven-membered ring or by atom crowding at the fjord region. Embedding a heptagon in HBC leads to a novel saddle-shaped molecule 1, whose π-backbone is slightly less curved than the previously reported [7]­circulene in terms of the average Gauss curvature, but surprisingly much more rigid than [7]­circulene. Overcrowded fjord regions in novel derivatives of hexabenzoperylene (HBP) 2a,b lead to both chiral twisted and antifolded conformers. The successful synthesis of 1 and 2a,b is related to introducing alkoxyl groups to unprecedented positions of hexaphenylbenzenes. It is found that the red twisted isomer of 2b isomerizes at elevated temperature to the yellow anti-folded conformer. This finding along with the study on the thermodynamics and kinetics of the thermal isomerization has improved the early understandings on the conformation of HBP. In the crystals, 1 lacks π–π interactions between neighboring molecules, while twisted-2a exhibits both face-to-face and edge-to-face π–π interactions. Twisted-2b is found to function as a p-type semiconductor in thin film transistors, but the thin films of 1 appear insulating presumably due to lacking π–π interactions. By exploring three different types of curvatures in 1 and the two isomers of 2b, this study has revealed that the curvature of π-face plays a role in determining the frontier molecular orbital energy levels and π–π interactions and thus needs to be considered when one designs new organic semiconductors.