Abstract
In order to reveal the influence of the guest molecular structure, the interactions between 4-sulfonatocalix[n]arene (SCXn) cavitands (n = 4 or 6) and two series of quinolinium derivatives were studied in neutral aqueous solutions at 298 K. For this, the long alkyl chain of the quinoliniums was attached either to the heterocyclic nitrogen (CmC1OQ+ m = 10, 12, or 14) or to the oxygen located in position 6 of the aromatic system (C1CmOQ+ m = 8, 10, or 12). All the quinolinium derivatives self-assembled with SCXn into nanoparticles (NP), whose size, zeta potential and composition were determined over a large molar mixing ratio range. Isothermal titration calorimetry showed that host-guest binding assisted the formation of negatively charged NPs in exothermic processes. The enthalpy gain in these associations significantly increased with the lengthening of the 1-alkyl group but was insensitive to the size of the SCXn macrocycle. The morphology of NPs was studied by cryo-TEM method. CmC1OQ+ organization with SCXn led to spherical NPs without regular inner structure. In contrast, C1CmOQ+-SCXn nanoaggregates usually had various shapes and the original morphologies exhibited lamellar domains with ~3 nm layer thickness. The different orientation of CmC1OQ+ and C1CmOQ+ in the cavitand was proposed to rationalize the morphological alterations.
Highlights
Host–guest binding of simple molecular building blocks is a powerful tool to mediate the formation of a wide variety of supramolecular architectures [1]
Before the study of the association with SCXn, the enthalpy change upon dilution of 1-alkyl-6-methoxy-quinolium aqueous solutions was examined by isothermal titration calorimetry (ITC)
When the total degree of substitution (TDS) of the cyclodextrin cavitand was N5, barrel-like assemblies with columnar inverse hexagonal structure were produced, whereas spherical NPs with a multibilayer structure were created at TDS b 5
Summary
Host–guest binding of simple molecular building blocks is a powerful tool to mediate the formation of a wide variety of supramolecular architectures [1]. The most frequently used hosts are cyclodextrins, which are valuable constituents of nanoparticle-based drug carrier systems [2] They can modulate the self-organization of amphiphiles, and their complexes with surfactants can produce various types of aggregates [3,4,5]. 4-Sulfonatocalix[n]arenes (SCXn) can be more advantageously applied as hosts than cyclodextrins in the construction of self-assembled systems This is related to the polyanionic character, pH-sensitivity, larger solubility and conformational flexibility of the SCXn macrocycles. The structural motifs of SCXn complexes have been intensively examined in the solid state [22,23], but the relationship between the molecular structure of SCXn-containing supramolecular amphiphiles and the properties of their nanoparticles produced in solution is still not fully understood It cannot be predicted how the characteristics of the constituents influence the size, shape and inner structure of the nanospecies created by spontaneous self-organization in water
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