The new article of Sollogoub’s group explores a method for selectively modifying cyclodextrin molecules, that allows hydrophobic domains to be directed in a geometrically controlled manner. This controlled orientation ultimately gives access to an original hierarchical assembly in the solid state. This assembly spans over three levels of hierarchy which are governed by synergistic host-guest inclusions, directed hydrophobic effect and hydrogen bonding. This combination of interactions precisely positioned in space through regioselective functionalization of a cyclodextrin creates a porous organic architecture.
Figure 1. The structure (left, 1) and the X-ray structure (right, view from (a) the side and (b) the top) of the cyclodextrin derivative.
The solid-state structure of monomer 1 (Fig. 1) provides a striking example of the influence of the functionalization degree of CDs towards the complexity of hierarchical supramolecular self-assembly. This behaviour, made possible through the precise introduction of functional groups, is due to both hydrophobic and hydrogen-bonding encoding at the molecular scale (Fig. 2 a,b). Indeed, the hydrophobic interactions are encoded in the two phenyl rings and the cavity of the CD. The axial complementarity between phenyl-A and the cavity brings axial directionality and leads to the first level of hierarchy that is supramolecular polymerization. The phenyl-D brings lateral directionality and leads to the second level of hierarchy forming hexameric columns of supramolecular polymers. These two levels are cooperatively assisted by specific hydrogen bonding networks involving water relays. Furthermore, the glucose units act in pairs to encode for specific part of the solid state-assembly. Hence, units E and D are mainly involved in the inter-strand interactions within the hexameric supramolecular polymers (2nd level of hierarchy). Units A and F act as connectors between edges of the hexameric columns (3rd level of hierarchy) and units B and C provide interactions within the hydrophilic pores hosting the residual water molecules. The expression of all the encoded interactions affords the unprecedented three-level hierarchical porous assembly (Fig. 2 c).
Figure 2. Summary of the hierarchical supramolecular assembly:(a) roles of the phenyl rings in the hydrophobic interactions; (b) roles of the glucose units in the hydrogen bonding networks; (c) summary of the different hierarchical levels.
In summary, this research demonstrates that precise functionalization of a macrocyclic structure allows the control of the architecture of an assembly with three levels of hierarchy. It has been shown that mono-functional CDs gave simple assemblies, while difunctionalized ones afforded hierarchy. Now, this hierarchy was tuned, brought to a higher level of complexity, and even produced pores.
Colesnic D, Hernando PJ, Chamoreau LM, Bouteiller L, Ménand M, Sollogoub M. (2023) Precisely Designed Difunctionalized Cyclodextrin Produces a Solid-State Organic Porous Hierarchical Supramolecular Assembly. Chemistry 29(35), e202300150. doi: 10.1002/chem.202300150