Thermoreversible Polymorph Transitions in Supramolecular Polymers of Hydrogen-Bonded Squaramides.

Hydrogen-bonded squaramide (SQ) supramolecular polymers exhibit uncommon thermoreversible polymorph transitions between particle- and fiber-like nanostructures. SQs 1-3, with different steric bulk, self-assemble in solution into particles (AggI) upon cooling to 298 K, and SQs 1 and 2, with only one dendronic group, show a reversible transformation into fibers (AggII) by further decreasing the temperature to 288 K. Nano-DSC and UV/Vis studies on SQ 1 reveal a concentration-dependent transition temperature and ΔH for the AggI-to-AggII conversion, while the kinetic studies on SQ 2 indicate the on-pathway nature of the polymorph transition. Spectroscopic and theoretical studies reveal that these transitions are triggered by the molecular reorganization of the SQ units changing from slipped to head-to-tail hydrogen bonding patterns. This work unveils the thermodynamic and kinetic aspects of reversible polymorph transitions that are of interest to develop stimuli-responsive systems.

Unveiling the Role of Hydrogen Bonds in Luminescent N-Annulated Perylene Liquid Crystals.

We report the liquid-crystalline (LC) and luminescent properties of a series of N-annulated perylenes (1-4) in whose molecular structures amide and ester groups alternate. We found that the LC properties of these compounds not only depend on the number of hydrogen-bonding units, but also on the relative position of the amide linkers in the molecule. The absence of amide groups in compound 1 leads to no LC properties, whereas four amide groups induce the formation of a wide temperature range columnar hexagonal phase in compound 4. Remarkably, compound 3, with two amide groups in the inner part of the structure, stabilizes the columnar LC phases better than its structural isomer 2, with the amide groups in the outer part of the molecule. Similarly, we found that only compounds 1 and 2, which have no hydrogen bonding units in the inner part of the molecule, exhibit luminescence vapochromism upon exposure to organic solvent vapors.

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

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.

Development of Plasmonic Chitosan-Squarate Hydrogels via Bioinspired Nanoparticle Growth.

We report on the bioinspired growth of gold nanoparticles (GNPs) in biocompatible hydrogels to develop plasmonic hybrid materials. The new hydrogel (CS-Sq) is prepared from chitosan and diethylsquarate and is formed via noncovalent interactions rising between the in situ formed ionic squaric acid derivatives and chitosan. Interestingly, when the hydrogel is prepared in the presence of HAuCl4, GNPs with controlled sizes between 15 and 50 nm are obtained, which are homogeneously distributed within the plasmonic hydrogels (GNPs-CS-Sq). We found that the supramolecular nature and the composition of the CS-Sq hydrogels are key for the growth process of GNPs where the squaric derivatives act as reducing agents and the chitosan hydrogel network provides nucleation points and supports the GNPs. Accordingly, the hydrogel acts as a bioinspired reactor and permits to gain certain control on the size of GNPs by adjusting the concentration of chitosan and HAuCl4. Besides the intrinsic and tunable plasmonic properties of the GNPs-CS-Sq hydrogels, it was found that the gels could be useful as heterogeneous catalysts for organic reactions. Furthermore, cell viability studies indicate that the new hydrogels exhibit suitable biocompatibility. Thus, the proposed method for obtaining GNPs-CS-Sq hydrogels has the potential for the development of a wide variety of other hybrid chitosan materials useful for catalysis, biosensing, cell culture, tissue engineering, and drug delivery applications.