Anchor Effect on Pedal Motion Observed in Crystal Phase of an Azobenzene Derivative

Abstract

Some molecules having a molecular skeleton similar to that of stilbenes and azobenzenes show orientational disorder in the crystals due to pedal motion. Heretofore, the orientational disorder through pedal motion has been observed for the compounds containing only two aromatic rings in the absence of bulky substituent groups. Here we report that the pedal motion can be detected even in the presence of a bulky substituent group to which orientational disorder becomes invisible as a result of anchor effect arising from phenoxyphtalonitrile group. X-ray crystallographic analysis of the compound, C23H18N4O, reveals the existence of partially overlapped two pedal conformers. The compound crystallizes in the monoclinic space group P2(1)/c with a = 12.9429(11) a"<<, b = 8.5075(5) a"<<, c = 21.063(2) a"<< and beta = 123.155(6)A degrees. Major pedal conformer is stabilized by weak C-H center dot center dot center dot O type hydrogen bond and C-H center dot center dot center dot pi type edge-to-face interactions in solid state. Quantum chemical calculations at B3LYP/6-311G+(d,p) level suggest that the stabilization of the compound decreases with increasing deviation from the planar geometry of trans-azobenzene fragment. Molecular and crystal structure of 4-[2-Methyl-4-(4-ethylphenyldiazenyl)]phenoxyphtalonitrile, C23H18N4O, indicate the existence of partially overlapped two pedal conformers in which orientational disorder becomes invisible as a result of anchor effect arising from phenoxyphtalonitrile group.