Theoretical CD spectrum evaluation of the indolylfulgide molecules by using semi-empirical molecular orbital calculations

A fulgide is a photochromic molecule which can not be isomerized by a thermal reaction. The CD spectra of the optically resolved helically chiral indolylfulgide (99% enantiomer excess), whose absolute configurations have been determined by an X-ray crystallographic analysis of their binaphthyl derivative, were estimated theoretically within the framework of semi-empirical molecular-orbital calculations. The optical rotatory strength of the indolylfulgide was calculated based on the Rosenfeld equation by using the optimized structures, conformational energies, and molecular orbitals. The estimated CD spectra reproduced characteristic features, such as the peak shape and sign of the Cotton effect, quite well, though the accuracy of the calculated wavelengths for the individual CD peaks' maxima was fair.