Relative substituent position on the strength of π–π stacking interactions

It was observed that the relative position of the arene substituents has a profound influence on the strength of π–π stacking in the 9-benzyl-substituted triptycene system. A new series of model compounds (3a–i) capable of revealing quantitatively π–π stacking interactions was studied. This series of compounds (3a–i) has an ortho-substituted methyl group in one of the two interacting arenes and the syn/anti ratios were determined and compared to a series previously studied compounds (4a–i) that have a para methyl group on the corresponding arene. A greater than 50% increase in the strength of π–π stacking interactions was observed with the methyl group in the ortho position comparing to that in the para position. No difference in π–π stacking interactions was observed when the other aromatic ring was a pentafluorobenzoate group.