Heat-Set Gel-like Networks of Lipophilic Co(II) Triazole Complexes in Organic Media and Their Thermochromic Structural Transitions

A novel class of thermally responsive supramolecular assemblies is formed from the lipophilic cobalt(II) complexes of 4-alkylated 1,2,4-triazoles. When an ether linkage is introduced in the alkylchain moiety, a blue gel-like phase is formed in chloroform, even at very low concentration (ca. 0.01 wt %, at room temperature). The blue color is accompanied by a structured absorption around 580−730 nm, which is characteristic of cobalt (II) in the tetrahedral (<i>T</i><i><sub>d</sub></i>) coordination. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) of the gel-like phase confirms the formation of networks of fibrous nanoassemblies with widths of 5−30 nm. The observed widths are larger than a molecular length of the triazole ligand (ca. 2.2 nm) and they are consisted of aggregates of <i>T</i><i><sub>d</sub></i> coordination polymers. Very interestingly, the blue gel-like phase turned into a solution by cooling below 25 °C. A pale pink solution is obtained at 0 °C, indicating the formation of octahedral (<i>O</i><i><sub>h</sub></i>) complexes. The observed thermochromic transition is totally reversible. The formation of gel-like networks <i>by</i> <i>heating</i> is contrary to the conventional organogels, which dissolve upon heating. Temperature dependence of the storage and loss moduli (<i>G</i><i>‘</i> and <i>G</i><i>‘</i><i>‘</i><i>)</i> shows minima around at 27 °C, at which temperature they gave comparable values. On the other hand, <i>G</i><i>‘</i> exceeds <i>G</i><i>‘</i><i>‘</i> both in the gel-like phase (temperature above 27 °C) and in the solution phase (temperature below 25 °C). These observations indicate that <i>T</i><i><sub>d</sub></i> complexes are present as low-molecular weight species around at 25−27 °C. They are self-assembled to polymeric <i>T</i><i><sub>d</sub></i> complexes by heating and form gel-like networks. Upon cooling the solution below 25 °C, <i>T</i><i><sub>d</sub></i> complexes are converted to <i>O</i><i><sub>h</sub></i> complexes and they also self-assemble into oligomeric or polymeric species at lower temperatures. The observed unique thermochromic transition (pink solution → blue gel-like phase) is accompanied by an exothermic peak in differential scanning calorimetry (DSC), and is shown to be an enthalpy-driven process. The lipophilic modification of one-dimensional coordination systems provides unique solution properties and it would be widely applicable to the design of thermoresponsive, self-assembling molecular wires.