Circular dichroism of donor–acceptor cyclophanes: (4 R p ;12 R p )‐ and (4 S p ;12 R p )‐12,15‐dimethoxy[2.2]paracyclophane‐4,7‐dicarboxylic acid derivatives

The electronic circular dichroism (CD) spectra were recorded for three diastereomeric eclipsed–staggered pairs of charge‐transfer cyclophanes with different substituents, i.e., (4 R p ;12 R p )‐ and (4 S p ;12 R p )‐12,15‐dimethoxy[2.2]paracyclophane‐4,7‐dicarboxylic acid derivatives ( 1a ‐ c and 2a ‐ c , where a , b , and c denote methyl ester, carboxylic acid, and carboxylate, respectively). The effects of altering the donor–acceptor interaction between the π systems on the chiroptical properties were experimentally investigated. The anisotropy ( g ) factors of eclipsed species 2 were not significantly affected by the sort of substituent, while staggered 1 behaved in a significantly different way in CD spectra depending on the charge, affording contrasting CD profiles for neutral a / b versus anionic c . This study provides not only the novel insights into the planar chirality of substituted [2.2]paracyclophanes but also a basis for the potential application of such dramatic CD spectral difference between the acid–base pair to the chiroptical pH‐sensors. Chirality 2010. © 2010 Wiley‐Liss, Inc.