Supramolecular Chirality Using Both Cooperative and Isodesmic Self-Assembly: hierarchical growth through competition

A novel set of chiral self-assembling discotic molecules has been prepared based on two previously studied self-assembling C3-discotic systems, namely alkyl substituted benzene-1,3,5-tricarboxamides (alkyl-BTAs) and 3,3'-bis(acylamino)-2,2'-bipyridine substituted BTAs (BiPy-BTAs). By replacing one of the three bipyridyl side chains by an alkyl chain we have prepared a hybrid system that shows properties of both parent compounds. Due to the different packing parameters and mechanisms of self-assembly of the parent BTAs, one might expect a lack of ordering in such a hybrid system. However, we observe columnar mesophases and unique self-assembly behavior in solution. We propose that the solution self-assembly passes through three stages. First, disordered aggregates are formed that evolve into ordered aggregates via an isodesmic process. Finally, these ordered aggregates rearrange in a cooperative fashion into a higher ordered structure. Through the interplay between pp interactions and H-bonding, the hybrid molecules combine the cooperative self-assembly behavior of alkyl-BTAs with the isodesmic behavior of BiPy-BTA discotics. This work shows that merging two systems with seemingly incompatible requirements can lead to new and complex self-assembly behavior.