Organogelating and narcissistic self-sorting behaviour of non-preorganized oligoamides†
Several series of structurally flexible, non-preorganized oligoamides with carboxylic acid, hydride, N-propargyl amide and N-propyl amide end groups were synthesized and characterized. They were found to be strong organogelators for aromatic solvents. Both the number and density of the amide units, and the nature of the end group were found to influence the gel strength as well as the kinetics of gel formation. Oligomers with a higher number and density of amide units were found to form stronger gels. Rather unexpectedly, a slight change of the end group could significantly affect the enthalpy and entropy of formation of the resulting self-assembly. Oligomers with the N-propargyl amide end group were found to be kinetically trapped in the melted state, which devoid them of forming crystalline zone upon cooling. On the other hand, the corresponding saturated N-propyl amide analog did not show the same effect. Upon mixing of these oligomers, narcissistic self-sorting could be observed in both the wet gel and xerogel states if (i) the difference in the number of amide units is greater than one or (ii) the amide unit densities are different. A semi-quantitative assessment of the extent of self-sorting could be achieved by constructing the corresponding phase diagram of the mixed gel systems. It is concluded that narcissistic self-sorting does not just prevail in mixtures making up of structurally different molecules, but appears to be fairly general even in mixtures of structurally similar molecular homologs.