Exciton Cotton Effects of Benzoates in the ¹B Transition Region. Demonstration and Applications

Summary.

The benzoate ¹B region exciton Cotton effects, hitherto unexplored, were analyzed for their use in stereochemical assignments in both acyclic (conformationally flexible) and cyclic molecules. It was found that a strong, allowed ¹B_a transition, polarized longitudinally, dominates the ¹B region (185–210 nm) both in the UV and the CD spectra. The exciton Cotton effects due to this transition have the same sign (but differing magnitude) as those due to the ¹L_a (CT) band. The other component of the nearly degenerate ¹B system, i.e. ¹B_b transition, polarized orthogonally to the ¹B_a transition, gives a Cotton effect in the case of di- and poly(4-chlorobenzoate) chromophoric system, the sign of which is sensitive to the configuration of di- or polyol. In rigid 5β-steroid skeleton ¹B_b transition couplings appear responsible for strong exciton Cotton effects due to nearly parallelly oriented benzoate chromophores. Whereas ¹B_a transition excitation energy appears insensitive to the nature of a substituent in 4-position of the benzoate chromophore, substitution with a donor group (methoxy, dimethylamino) brings about a red shift of the ¹B_b band, although less pronounced than the red shift of the ¹L_a (CT) band.