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 pi-pi stacking in the 9-benzyl-substituted triptycene system. A new series of model compounds (3a-i) capable of revealing quantitatively pi-pi 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 pi-pi stacking interactions was observed with the methyl group in the ortho position comparing to that in the para position. No difference in pi-pi stacking interactions was observed when the other aromatic ring was a pentafluorobenzoate group. (C) 2009 Elsevier Ltd. All rights reserved.