Removal of Conflicting Molecular Symmetries Restores a Hexagonal Array of Six-Fold Phenyl Embraces in a bis(Trityl)-Containing Compound. I. Crystals of 1,1,1,6,6,6-Hexaphenyl-2,4-hexadiyne

ABSTRACT: Previously reported structures of 1,4-bis(3,3,3-triphenylpropynyl)benzene (1) showed molecules adopting a Ci point group in the triclinic space group P1h. Reasoning that a symmetry conflict between the 3-fold symmetric triphenylmethyl groups and the 2-fold symmetric phenyl group may be responsible for the low symmetry crystal structure, it was speculated that the removal of the central benzene ring might lead to crystal symmetries where the molecular 3-fold axes might be elicited. To test that, we analyzed the crystallization and packing of 1,1,1,6,6,6-hexaphenyl-2,4-hexadiyne (2). Satisfyingly, compound 2 was found to crystallize in the rhombohedral space group R3h with three different molecular structures, one displaying a chiral C3 point group and the others a meso conformation with an S6 point group. In efforts to engineer molecular motions in crystalline solids for potential materials application,1 we have recently investigated various compounds with a tendency to adopt packing arrangements that permit the rotation of a central group. Several structures consisting of trityl2 or triptycyl3 groups attached to 1,4-diethynylphenylenes have been analyzed, and some of their crystals have been found to display 2-fold phenylene rotation with rate constants a