Stereochemistry-Dependent, Mechanoresponsive Supramolecular Host Assemblies for Fullerenes: A Guest-Induced Enhancement of Thixotropy

Abstract

Self-assembly behaviors of a series of systems (G1, G2, and G3) possessing same organic building blocks based on a substituted anthracene have been investigated in decalin. G2 and G3 are dominated by head-to-tail (ht) and head-to-head (hh) type dimers of G1, respectively. G1 gives a thermoresponsive gel that behaves ideally, showing frequency-independent elastic and viscous moduli. Interestingly, G2 produces a thixotropic gel that shows the signature of structural relaxation, signifying the dynamic nature of the system. In contrast, G3 remains fluidlike. As investigated by scanning electron microscopy (SEM), in the assembly process of G2, first disklike nanoaggregates are formed, and in the second step these aggregates interact to construct the densely packed secondary assembly. A transition from secondary assembly to primary assembly under shear initiates the mechanoresponsive destruction of the gel. In the self-assembly process, G1 propagates in a one-dimensional fashion, whereas G2 and G3 can propagate in a two-dimensionional fashion. The same side orientation of the substituents in G3 facilitates the formation of a compact closed-shell-type structure, which results in the generation of isolated nanocrystals. The long-range weak interaction together with the capability of propagating in two dimensions is found to be essential for the construction of such a mechanoresponsive assembly. C(60) and C(70) could be incorporated successfully in G2 assembly to develop mechanoresponsive fullerene assemblies. The presence of fullerenes not only enhances the elastic properties of G2 but also intensifies the thixotropy. C(70) appears to be a superior guest in terms of property enhancement due to its better size fitting with the concave-shaped host.

Triggered formation of thixotropic hydrogels by balancing competitive supramolecular synthons

“…This kind of propensity to break down into smaller fragments is characteristic of thixotropic 15 behaviour and offers an explanation of the shear thinning response of the material. 54,[56][57][58] Mechanical agitation results in break-down of the fibres into individual short lengths of fibre and hence loss of network stability. On standing the fibres reassemble into a sample-spanning network.…”

Gels with sense: supramolecular materials that respond to heat, light and sound

“…[415][416][417] The magnitude of a thixotropic response may be modified with the use of additives that interact strongly with the gelator, 417 or by altering the aggregation environment or type of stress applied. In one notable study, co-gels of a pyridyl bis(urea) with dicarboxylic acids were found by Liu and Steed to reform after mechanical disruption, but become non-thixotropic upon exposure to heating, high pH or prolonged sonication.…”

Low Molecular Weight Supramolecular Gels Under Shear: Rheology as the Tool for Elucidating Structure–Function Correlation

“…In another remarkable work by the Shinkai research group, self‐assembling behaviour of a series of systems consisting of the stereoisomers of dianthracene‐based molecules (Figure ) in decalin has been explored, and their dynamic behaviours were closely monitored with the help of rheology and microscopy experiments . The dianthracene derivatives were obtained by the photoisomerization of the corresponding monomer .…”