Self-assembly of amphiphilic aryl-squaramides in water driven by dipolar π–π interactions

Abstract

Squaramides are versatile compounds with a great capacity to interact via non-covalent interactions and therefore of interest for the development of supramolecular systems and functional materials. In the present work, a new series of aryl-squaramide amphiphiles (1–5) were prepared to form supramolecular polymers in water. Interestingly, only compounds 1 and 2 that contain electron-deficient aryl groups are capable of forming hydrogels (∼10⁻² M) upon treatment with a base (NaOH or PBS). The aggregation behaviour of 1 and 2 was studied by static light scattering, UV-Vis, ¹H NMR, FT-IR, and atomic force microscopy, and it was found that these compounds aggregate forming well-defined 1D nanofibers below the critical gelation concentration (<10⁻³ M). Moreover, the combination of these experiments with 1D and 2D NMR studies and theoretical calculations revealed that 1 and 2 self-assemble via an unprecedented interaction motif showing dipolar π–π interactions between the squaramide rings and the 4-nitrophenyl or 3,5-bis(trifluoromethyl)phenyl rings of 1 and 2, respectively. Such kinds of assemblies are stabilized by the compensation of the dipole moments of the stacked molecules. This interaction mode contrasts with those typically driving squaramide-based assemblies based on either hydrogen bonds or antiparallel stacking. We believe that this interaction motif is of interest for the design and development of new squaramide nanomaterials with free hydrogen bonding groups, which might be useful in drug delivery applications.