Self-Assembly, Aggregation Mechanisms, and Morphological Properties of Asymmetric Perylene Diimide-based Supramolecular Polymers

The ability to control the self-assembly behavior of functional molecules offers pathways for optimizing their properties and structures for targeted applications. Asymmetric perylene diimide (PDI) derivatives, bearing similar oligo(ethylene glycol) (OEG) chains and increasing numbers of hydrophobic alkyl chains were used to investigate the effects of hydrophilic and hydrophobic groups on the properties of the resulting supramolecular polymers (SMPs). Tetrahydrofuran/water (THF/H₂O) mixtures were found to be an optimal system to induce the self-assembly behavior of these asymmetric PDI derivatives; the behavior of these new systems was monitored by recording ultraviolet-visible (UV/Vis) spectra and analyzing changes in absorbance intensity. Temperature-dependent UV/Vis studies yielded thermodynamic parameters for the self-assembly processes and showed a transition from an isodesmic to a cooperative assembly mechanism with an increase in the number of hydrophobic alkyl chains. Detailed transmission electron microscopy (TEM) and atomic force microscopy (AFM) investigations revealed the formation of nanofiber-based supramolecular structures that exhibit a coiling tendency during their assembly process accompanied by the increase in the number (and total volume) of hydrophobic alkyl chains. Exploring these carefully tuned asymmetric PDI systems provides new opportunities to tune not only self-assembled structures, but also mechanisms of assembly.