Conformational subtlety in large polyphenylene molecules

1,3-Bis(nonaphenyl-3-biphenylyl)benzene (3), a large polyphenylene molecule (C138H94), was prepared by the Diels–Alder addition of 2 equiv. of 1-(pentaphenylphenyl)-2-phenylacetylene to 1,3-bis(3-oxo-2,4,5-triphenylcyclopenta-1,4-dienyl)benzene, and its X-ray structure was determined. The experimental structures of decaphenylbiphenyl (4) and 3 were compared with calculated structures of these molecules obtained by using methods ranging from molecular mechanics to hybrid density functional theory. Surprisingly, none of the computational methods examined correctly predicted the experimentally observed conformations of both 3 and 4.