Substituent effects on helical structures and chiroptical properties of fused anthracenes with bulky phenyl groups

In order to investigate substituent effects on the structures and properties of the helically fused anthracene system, various bulky phenyl groups were introduced at the sterically crowded positions. Three derivatives with mesityl (Mes), 2,4,6-triisopropylphenyl (Tip), and pentafluorophenyl (C6F5) groups were synthesized by Pt-catalyzed cycloisomerization of the corresponding alkyne precursors. The X-ray crystallographic analyses revealed that the helical structures were significantly expanded along the helical axis in the order of steric size of the substituents, C6F5 < Mes < Tip. The structural features are discussed on the basis of intramolecular noncovalent interactions such as CH···π and π···π interactions. Enantiomers of the Tip and C6F5 derivatives, resolved by chiral HPLC, showed very intense bands in circular dichroism (CD) and circularly polarized luminescence (CPL) bands. In particular, the luminescence dissymmetry factor |glum| values of these enantiomers (ca. 0.012) were large for simple organic molecules. Regardless of the large structural changes, the substituent effects on the optical and chiroptical properties were rather small. This structure–property relationship is discussed with the aid of DFT calculations.