Geminal Imidazolium Salts: A new Class of Gelators

Abstract

The gelling behavior of some geminal diimidazolium salts was investigated in solvents differing in polarity and hydrogen bond donor ability. The used salts, namely the 3,3′-di-n-decyl-1,1′(1,4- phenylenedimethylene)diimidazolium dibromide [p-Xyl-(decim)2][Br]2 (1), the 3,3′-di-n-dodecyl-1,1′(1,4-phenylenedimethylene)diimidazolium dibromide [p-Xyl-(dodecim)2][Br]2 (2), and the 3,3′-di-n-dodecyl- 1,1′(1,4-phenylenedimethylene)diimidazolium ditetrafluoroborate [p- Xyl-(dodecim)2][BF4]2 (3), differ in the alkyl chain length and in the anion properties, such as size, shape, and coordination ability. In all cases in which gelation process was observed, the obtained gels were characterized by gel melting temperature determination, resonance light scattering, and UV−vis measurements. On the whole, the investigation allowed to get information about both the thermodynamic stability and the features of the aggregates characterizing the soft materials at the equilibrium. Data collected by us point out that the used organic salts are able to behave as both hydro- and organogelators. In particular, bromide salts formed hydrogels in the presence of α-cyclodextrin allowing to hypothesize that the gelation process is favored by the formation of supramolecular assemblies. To verify this hypothesis, 1D and 2D 1H NMR measurements were carried out. Both the alkyl chain length and the anion ability to reticulate the three-dimensional network proved to be determinant factors in affecting the gelation process as well as the features of the gel phases. Finally, with the future aim to use the obtained gels as reaction media, the effect of a guest molecule such as the UV−vis active probe Nile Red was studied.