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

Abstract

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 (T(d)) 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 T(d) coordination polymers. Very interestingly, the blue gel-like phase turned into a solution by cooling below 25 degrees C. A pale pink solution is obtained at 0 degrees C, indicating the formation of octahedral (O(h)) complexes. The observed thermochromic transition is totally reversible. The formation of gel-like networks by heating is contrary to the conventional organogels, which dissolve upon heating. Temperature dependence of the storage and loss moduli (G' and G") shows minima around at 27 degrees C, at which temperature they gave comparable values. On the other hand, G' exceeds G" both in the gel-like phase (temperature above 27 degrees C) and in the solution phase (temperature below 25 degrees C). These observations indicate that T(d) complexes are present as low-molecular weight species around at 25-27 degrees C. They are self-assembled to polymeric T(d) complexes by heating and form gel-like networks. Upon cooling the solution below 25 degrees C, T(d) complexes are converted to O(h) 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.