Experimental and theoretical investigations of circular dichroism of donor-acceptor 1,1'-binaphthyls: influence of substitution on the coupling amplitude and cotton effect of the charge-transfer band.

The electronic circular dichroism (CD) spectra of donor-acceptor binaphthyls were investigated experimentally and theoretically. The enantiomerically pure forms of 1-(2-methoxy-1-naphthyl)- and 1-(2,3-dimethoxy-1-naphthyl)-2-methylisoquinolinium tetrafluoroborates (DA and D'A) were prepared, and their UV-vis and CD spectra were compared. The donor-acceptor interaction was apparent from the absorption at longer wavelengths, whereas its strength was not very different from each other. In addition, very similar structures were obtained for the two aromatic planes in DA and D'A when the geometry was optimized by the density functional theory. The additional methoxy group in the latter spices scarcely disturbed the UV-vis spectrum but significantly affected the CD spectrum. Thus, the observed CD spectra were considerably different from each other, especially in the (1) B(b) band couplet, where the amplitude was reduced to almost one-fourth in D'A. The theoretical investigations led to the following conclusions: (1) The potential curve associated with the central C-C dihedral angle of 1,1'-binaphthyl is fairly flat at the bottom for both DA and D'A and freely rotating at an ambient temperature. The potential curve of D'A is, however, significantly different from that of DA, in which the curve is much steeper and biased to the s-cis side. As the observed CD spectrum is an ensemble of conformers of various dihedral angles, such difference in potential certainly affects the overall spectrum; (2) The additional methoxy group introduced at the 3-position effectively altered the CD spectral pattern, which was theoretically supported by the calculation at the RI-CC2/TZVPP level; (3) Consequently, the classical coupled oscillator theory, in which the angle between the transition dipole moments of two aromatic planes is solely considered, is not applicable to the quantitative evaluation of the chiroptical properties of 1,1'-binaphthyls; rather, the quantum chemical approach is preferred, permitting a direct comparison with the experiment.