Remarkable reinforcement of a supramolecular gel constructed by heteroditopic [18]crown-6-based molecular recognition

The heteroditopic crown ether-based ligand 1 containing the diazafluorenylimino group as a binding site for the Zn(2+) and Cs(+) was synthesized. The ligand 1 can be gelated in DMSO/water with and without Cs(+) in the presence of the Zn(2+) ion. Interestingly, the remarkable gelation reinforcement of gel 1 occurred with Cs(+) in the presence of Zn(2+), which is due to the formation of a sandwich complex. According to DFT calculations, one Zn(2+) is bound to two diazafluorenylimino moieties in a tetrahedral structure. In addition, one Cs(+) ion is bound to two crown-rings. The Zn(2+)-diazafluorenylimino ligand gel without Cs(+) shows a spherical structure with 250-800 nm diameter, whereas the Zn(2+)-diazafluorenylimino-based ligand gel with Cs(+) shows a fiber structure with 60-70 nm diameter and several micrometers of lengths. The rheological properties of the Zn(2+)-diazafluorenylimino-based ligand gel were strongly dependent on the presence of Cs(+).