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Heat-Set Gel-like Networks of Lipophilic Co(II) Triazole Complexes in Organic Media and Their Thermochromic Structural Transitions
journal contribution
posted on 2004-02-25, 00:00 authored by Keita Kuroiwa, Tomoko Shibata, Akihiko Takada, Norio Nemoto, Nobuo KimizukaA 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 (Td) 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 Td 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 (Oh) 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 °C, at which temperature they gave comparable values. On the other
hand, G‘ exceeds G‘‘ both in the gel-like phase (temperature above 27 °C) and in the solution phase
(temperature below 25 °C). These observations indicate that Td complexes are present as low-molecular
weight species around at 25−27 °C. They are self-assembled to polymeric Td complexes by heating and
form gel-like networks. Upon cooling the solution below 25 °C, Td complexes are converted to Oh 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.