Balancing hydrogen bonding and van der Waals interactions in cyclohexane-based bisamide and bisurea organogelators

Abstract

The solvent dependence of the gelation properties, the thermotropic behavior, and the melting enthalpy of a series of enantiomerically pure cyclohexane-based bisamide and bisurea compounds are reported. The two series of gelators examined are related structurally with the intermolecular interactions responsible for gelation differing in a systematic manner through varying the length of the alkyl tail and the number of hydrogen bonding units present. The gelation properties of the compounds in decalin, DMSO, and 1-propanol were studied by FTIR spectroscopy and by comparison of the thermal stability of their gels as determined by dropping ball experiments and by differential scanning calorimetry (DSC). FTIR spectroscopy, supported by the single-crystal X-ray diffraction of a3, indicates that the gelator molecules are aggregated through intermolecular hydrogen bonding in all of the solvents examined. The thermal stability of the gels in apolar and polar solvents was found to be dependent primarily on the relative strength of intermolecular hydrogen bonding and van der Waals interactions, respectively, compared with the strength of solvent-gelator interactions. The results of DSC indicated that the contribution of the difference in intergelator van der Waals interactions, compared with the gelator-solvent van der Waals and hydrogen bonding interactions, provided by the alkyl tail to the stability of the gel has a linear relationship with the number of methylene units in alkyl chains of length greater than six. In polar solvents, this contribution lies between 3.5 and 4.2 kJ mol(-1) per methylene unit, and in apolar solvents, it is 2.2 kJ mol(-1). The hydrogen bonding interactions were weaker in polar solvents and hence gelation occurred only where sufficient compensation was provided by intergelator van der Waals interactions. The results show that the direct relation of gelation strength to changes in solvent properties is not possible and more complex relationships should be considered. Furthermore, it is apparent that the development of design rules for the construction of LMWG molecules incorporating more than one anisotropic growth element must take into consideration the role of the solvent in determining which of the contributions is dominant.