Symmetry and polar-pi effects on the dynamics of enshrouded aryl-alkyne molecular rotors

The central ring of a 1,4-bis(arylethynyl)arene (ditolan) can be viewed as a molecular rotor with an extremely low barrier to rotation in the gas phase or solution. The torsional energy profile of that ring is shown to be dependent on the relative conformation of the end capping arenes. When the capping arenes are sterically bulky m-terphenyl units, it is possible to rationalize the conformational dynamics of the central ring by a factorization analysis, involving perturbation of the basic torsional energy profile by polar-pi, and dispersion interactions between the flanking rings of the cap and the central ring of the ditolan. The symmetry of the construct can modulate the effect of these interactions. These principles apply to the design of materials in which a steric shroud excludes packing distortions.