LAUSR

Symmetry and pattern are valuable forms of beauty that can be cherished on both the macroscopic and molecular levels. Scientists have always appreciated the intimate connections between symmetry and molecular structures, which is reflected in their design of highly symmetrical compounds. Traditionally, the same story goes with macrocycles, a class of supramolecular host molecules [1]. This class of molecules attracted significant attention because of their synthetic accessibility, host-guest chemistry, defined structures, range of available functionalization methods, and tunable noncovalent interactions. Pillarenes are one of the recent members of macrocycle family with symmetrical structures [2,3]. Recently, Prof. Ying-Wei Yang from Jilin University [4] broke the above tradition and made an important progress in pillarene chemistry by inducing desymmetrization to the macrocycle. In stereochemical terms, desymmetrization is the modification of a molecule that results in the loss of one or more symmetry elements. Formally, such conversions required the loss of an improper axis of rotation such as mirror plane, center of inversion, or rotation-reflection axis. Specifically, the Yang’s group succeeded to induce chiral planes to pillarenes by removing the two hydroxy/alkoxy functionalities from two of the six phenylene units in an opposite, parallel position (Figure 1). In general, conventional pillarenes possess “pillar” like self-assembly in their single crystal states. However, the new functional modification to the pillar[6]arene core results in substantial increase in flexible rotation that allows the phenylene rings to adopt a favorable “leaning” conformation which leads to “leaning pillar[6]arenes”. They were able to crystalize different derivatives and study their self-assembly in crystal state. In the observed leaning state, the dihedral angles between the phenylene rings and the plane of methylene bridges were decreased. Interestingly, both unsubstituted phenylene rings tilted to the same direction, leading to a leaning conformation of the entire macrocycle and resulting in a significant change of the symmetry to Ci, which is in drastic contrast to the perpendicular pillar structure and D6h symmetry of per-functionalized version. The entire pillar exhibits an approximately 25° lean because of the non-perpendicular dihedral angle of the individual phenylene rings, in contrast to the 0° lean of conventional pillar[6]arenes (Figure 2). The leaning conformation and desymmetrization effect render four chiral planes to modified pillar[6]arenes. This approach turned out to be very useful for the synthesis of cyclo-oligomeric quinone via direct oxidation of newly modified pillar[6]arenes, which was otherwise proven to be a tough task in case of traditional pillar[6]arene because of two-dimensional sheet formation during oxidation. The very first crystal structure of cyclic[6]quinone shows layer-bylayer packing modes forming elongated cavity channels stabilized by multiple C–H···O interactions. The single crystal self-assembly of newly synthesized leaning pillar[6] arenes confirms the necessity of twelve hydroxy groups with multiple hydrogen-bonding interactions for maintaining the pillar structure in traditional pillar[6]arenes.