Chirality and conformational changes in 4,5-diaryltriphenylenes

Line-shape analysis of temperature dependent NMR spectra of several substituted 4,5-diphenyl-triphenylenes has been performed to determine the free energy of activation for rotation (ΔG_rot^*) of the phenyl groups. The rotational barrier (ΔG_rot^*) depends on the presence and position of substituents on the phenyl groups; it is the largest in compounds with ortho-substituents. The independent determined free energy of activation of racemization (ΔG_rac^*) is about equal to ΔG_rot^* in 4-phenyl-5-(3,5-dimethylphenyl)triphenylene, but in 4,5-bis-(3,5- dimethylphenyl) triphenylene ΔG_rac^* is much larger than ΔG_rot^*. It is concluded that the racemization does not occur via a process in which the phenyl groups remain parallel but via a molecular movement in which the phenyl groups turn around each other like cog wheels.