Effect of N-Alkyl Substituents on the Hierarchical Self-Assembly of β-Cyclodextrin-Linked Pyrene-Pyromellitic Diimide Charge-Transfer Complexes.

Abstract

Hierarchical self-assemblies of β-cyclodextrin-linked pyrene and N-alkyl derivatives of pyromellitic diimides are studied in detail. The charge-transfer interaction between pyrene and pyromellitic diimide is augmented by β-cyclodextrin-pyromellitic diimide binding interactions in these cases. When the alkyl group is adamantyl, a 1:1 complex was formed with a very high association constant (Ka =1.82×106 m-1 ). Here, the charge-transfer interaction is reinforced by inclusion binding of the adamantyl group in the β-cyclodextrin cavity leading to the formation of 2D sheets, which undergo twisting to give twisted fibres. When the alkyl group is tert-butyl, a 1:2 complex was formed with a high association constant (Ka =2.91×104 m-1 ). A detailed analysis showed that the tert-butyl pyromellitic diimide undergoes both inclusion and rim-binding interactions with the β-cyclodextrin. The charge-transfer complex further self-assembled into chiral nanostructures as evident from SEM, TEM and AFM analysis. In the case of N-methyl-substituted pyromellitic diimide, the interaction with β-cyclodextrin-linked pyrene was only through rim binding, which resulted in the formation of a weak charge-transfer complex with Ka =4.2×103 m-1 . Formation of a hierarchical assembly was not observed in this case. A rational mechanism for the self-assembly, which relies on the strength of the cyclodextrin-pyromellitic diimide complexation is presented.