Axial coordination changes the morphology of porphyrin assemblies in an organogel system

The gelation properties of a zinc porphyrin bearing peripheral urea groups (1 x Zn) were evaluated in the absence and the presence of several diamines. In aromatic solvents such as benzene, toluene and p-xylene, 1 x Zn only provided the precipitate. In contrast, 1 x Zn with 0.5 and 1.0 equiv. of piperazine formed gels, and the gel with 0.5 equiv. of piperazine showed a unique physical property called 'thixotropy'. On the other hand, upon addition of similar diamines such as DABCO, ethylenediamine and N,N'-dimethylethylenediamine, 1 x Zn did not gelate these solvents. When the critical gelation concentration was plotted against the ratio of piperazine versus 1 x Zn, it afforded a minimum breakpoint at 0.5 equiv. and the critical concentration increased with further increase in the fraction of piperazine, indicating that the stable gel is formed from the 1 x Zn + piperazine 2:1 complex and the subsequent transformation to the 1:1 complex rather destabilizes the gel. Very interestingly, it was clearly shown by SEM and TEM observations that such structural changes of the unit complex induced by the ratio of piperazine versus 1 x Zn can lead to gradual morphological transitions: that is, spherical structure at 0 equiv., 1-D fibrous structure at 0.5 equiv. and 2-D sheet-like structure at 1.0 equiv. In addition, UV-VIS spectra revealed that 1 x Zn itself adopts a J-aggregation mode, whereas 1 x Zn + piperazine 2:1 and 1:1 complexes adopt an H-like aggregation mode. On the other hand, upon addition of 0.5 equiv. of other diamines, 1 x Zn + diamine complexes result in different morphologies other than the 1-D fibrous structure. To explore a reasonable rationale for these results, we conducted computational studies. As a result, we found that the complex symmetry of the unit complex plays an important role in determining the final ordered structure.