Introduction of polar or nonpolar groups at the hydroquinone units can lead to the destruction of the columnar structure of Pillar[5]arenes

Pillar[5]arenes, a type of novel macrocycles containing di-substituted hydroquinone units linked by methylene bridges in para-positions, have attracted extensive attention in supramolecular chemistry as interesting candidates to be used in the preparation of host-guest complexes. Functionalization by means of rim substitution and sustaining an ordered substituent arrangement on both sides of the rim is important for the development of new pillararene-based materials. In order to achieve this, the rim inversion process of rotating the hydroquinone units through the pillar[5]arenes has to be controlled. In this context we have studied the effect of different types of hydroquinone substituents on the rotational energy profile using density functional theory combined with the hybrid M06-2X functional. The influence of polar ([sbnd]CH₂F, [sbnd]CH₂Cl, [sbnd]CH₂OH, [sbnd]CH₂SH, [sbnd]CH₂NH₂) and nonpolar alkyl ([sbnd]CH₃, [sbnd]CH₂CH₃, [sbnd]CH₂CH₂CH₃, [sbnd]CH(CH₃)₂ and [sbnd]CH₂CH₂CH₂CH₃) substituents on the on the energy barriers of the rotation mechanism, and different local minima was investigated. The stabilization of the intermediate structures by non-covalent van der Waals and interactions and also by hydrogen bonds constitute a major factor affecting barrier heights. In case of polar substituents, the largest barriers were found for [sbnd]CH₂OH and [sbnd]CH₃ substitutions and the lowest ones for [sbnd]CH₂SH and [sbnd]CH₂NH₂. For the alkyl series, the barrier decreased significantly up to propyl due to increasing stabilizing dispersion interactions while it increased again for n-butyl since the chain did not fit in well the cavity to rotate through.