Mechano-switchable, luminescent gels derived from salts of a long-chained, fatty-acid gelator

Abstract

Stimulus-responsive molecular gel systems, based on metal salts of a luminescent gelator, 9,10-dioxooctadecanoic acid (DODA), are reported. These salts are structurally the simplest metallo-gelators of which we are aware that exhibit controllable mechano-responsive and luminescent properties. Aggregation is more favored by the metal salts than for DODA itself. However, gelation ability differs dramatically depending on the metal ion: whereas the salts with zinc(ii) and calcium(ii) are inefficient gelators, those with nickel(ii) and copper(ii) can gelate various aromatic liquids, alkanes, and long-chained alcohols. Unlike the DODA gels, no aggregation-induced shift in the positions of the emission spectra of the metal salts could be observed as the sols were transformed to their gel phases. Gels of both nickel(ii) and copper(ii) salts in benzonitrile are among the few known examples with crystalline networks and exhibiting thixotropic behavior. However, there are significant differences in their abilities to recover the initial viscoelastic properties. Structural data for the solid and gel states lead us to conclude that differences among the gelating abilities can be attributed principally to the specific nature of interactions of the salts at their head groups. They appear to control the mechanical and emissive properties of the gels as well as whether the initial aggregation of the salts in the sol phases will support the growth of 1D objects that are capable of maintaining strong contacts, leading to 3D networks and gel formation. Overall, the results provide a facile strategy for the design of luminescent materials with controllable mechano-responsiveness by modifying the metal ions within fibrillar assemblies.

Gelating abilities of metal salts of ricinelaidic acid. Structural and rheological considerations

“…As discussed earlier, both D‐REA‐Ca and D‐REA‐Na can form thixotropic gels in nitrobenzene and/or water with rapid recovery of viscoelasticity after the cessation of destructive strain. We hypothesize that the 3D SAFiN s break into 1D fibers or bundles of them, but not revert to 0D objects (such as isolated molecules or small aggregates that exist in the sol phase) . Some of the disconnected 1D fibers then rejoin once the destructive strain ceases.…”

“…Because the longest d ‐spacing is only slightly shorter than 2 extended molecules, the packing within the SAFiN s of D‐REA‐Ca appear to be non‐interdigitated (or very slightly interdigitated), bilayered, and lamellar. The longest d ‐spacing of D‐REA‐Na , 46.7 Å, is again shorter than the sum of the lengths of 2 extended molecules; its molecular packing within the SAFiN s of D‐REA‐Na seems to be more interdigitated than that of D‐REA‐Ca or involves tilted D‐REA chains …”

“…Some of the disconnected 1D fibers then rejoin once the destructive strain ceases. The longer recovery time and lower degree of recovery of D‐REA‐Ca gels than of D‐REA‐Na gels suggest a lower ability of D‐REA‐Ca to reform inter‐fiber interactions; [12, 21b, 22] the reorganization of the D‐REA‐Ca objects is more demanding entropically.…”

“…Clearly, there are subtle factors that influence differently the manners in which the sols of the salts of D‐REA and DODA aggregate into crystalline objects. In both systems, we hypothesize that the fibers in the thixotropic samples are retained to a large degree when placed under destructive strain and reestablish connections to reform SAFiN s after the cessation of destructive strain …”

“…However, there are no studies we have been able to find for the gelating ability of metal salts of D‐REA . This research complements our recent investigation of the gelating abilities of metal salts of another long‐chained, substituted alkanoic acid, 9,10‐dioxo‐octadecanoic acid ( DODA ) . Our results demonstrate that the nature of the interactions of the metal ions with the carboxylate groups of D‐REA or DODA plays a key role in determining whether a gel will be formed and the structural viscoelastic properties of those gels.…”