Circular Dichroism of (Di)methyl- and Diaza[6]helicenes. A Combined Theoretical and Experimental Study

Circular dichroism (CD) and relevant chiroptical properties of (di)methyl- and diaza[6]helicenes were investigated by the state-of-the-art approximate coupled cluster and density functional theory calculations, results of which were compared with the corresponding experimental data obtained for newly synthesized enantiopure helicenes. The theoretical calculation at the RI-CC2/TZVPP//DFT-D2-B97-D/TZVP level accurately reproduced the experimental CD spectra in both excitation energy and rotational strength. The electric and magnetic transition dipole moment vectors for the helical sense-responsive 1Bb and the substitution-sensitive 1Lb bands were compared with those for parent carbo[6]helicene, from which the effects of methyl and nitrogen introduced at different positions upon the experimental CD spectra were discussed to separately evaluate the electronic and steric consequences of the substitution to the chiroptical properties. The electronic effects of substitution on CD spectra were further investigated theoretically by employing a series of 3,3-disubstituted [6]helicenes. This first systematic investigation allows us not only to accurately reproduce the experimental CD spectra of known substituted helicenes but also to directly envisage the chiroptical properties of unknown helicenes.