Synthesis and Exciton Optical Activity of D3-Cryptophanes

Article abstract of DOI:10.1021/ja00255a035

Six optically active D₃-cryptophanes (1-6) have been synthesized and thei absolute configurations established unambiguously.In order to explain their circular dichroism (CD) spectra, a simple theoretical model, based on the Kuhn-Kirkwood excition mechanism, has been developed for the coupling of six equivalent oscillators in a D₃ molecular point group.The model predicts for each B_2u and B_1u benzene transition the presence of three CD components, one (A₂) being polarized along the C₃ axis and two (E) in the plane containing the C₂ axes.The two E components have opposite signs and different intensities, the stronger having sign opposite to the A₂ transition.The splitting between the E states in each band system is dependent on the twist angle (2γ) between the upper and the lower caps of the molecule.For 2γ = 60 deg (staggered conformation), this splitting is estimated to be very small, compared to the corresponding E - A₂ energy difference, and the E components are never resolved in the theoretical plotted spectra for any realistic bandwith.For 2γ = 0 deg (eclipsed conformation), the calculated splitting of the E states is large enough to become apparent in plotted spectra for the B_1u system.When a staggered conformation is assumed, the experimental CD spectra of cryptophanes 1-6 have been, in most cases, satisfactorily interpreted in terms of the model, with the polarization direction of the transitions set in accordance to the spectroscopic moment theory by using the rules previouly established for C₃-cyclotriveratrylenes.In some cases, the energy sequence of the E states for the B_2u system is not agreement with the predictions, probably due to the failure of the point-dipole approximation to determine the exact sequence of almost degenerate states.The results of this study point out the importance of the coupled-oscillator model in explaining the CD spectra of complex molecules containing more than three oscillators.