Reversible Supramolecular Polymorphism in Solution and Solid Matrix by Manipulating Sidegroup Conformation.

Reversible switching between supramolecular polymorphs offers a great way to introduce stimuli-responsiveness. Supramolecular polymorphism is usually achieved through pathway complexity, or by exploiting solvent-solute interactions. But, steering a self-assembly along a specific pathway to form a kinetically-stable aggregate is not easy. Also, changing solvent to switch between polymorphs is impractical. We present a perylene bisimide molecule with a trans-azobenzene sidegroup that assembles into three supramolecular polymorphs with distinct colors, morphologies, packing and aggregation mechanism. Optical absorption and FTIR spectroscopy reveal the importance of hydrogen-bonding interaction between protic solvent and azo N that controls the planarity of the azobenzene group and influences molecular packing. This interaction can be further modulated using temperature, and solution pH to reversibly switch between the three polymorphs, in solution as well as in solid silica-gel matrix.