An Organometallic Super‐Gelator with Multiple‐Stimulus Responsive Properties

Stimulus-responsive gels have recently attracted widespread attention as new functional materials for potential applications in sensors, actuators, shape memories, drug delivery devices, and displays. One of the promising properties that organogels based on low molecular mass organic gelators (LMOGs) can offer is their reversible sol–gel phase transition as a result of external stimuli. As far as we know, redoxresponsive organogels from LMOGs, however, are limited. Shinkai and coworkers reported the first example of organogels of this kind, which contains a redox-active Cu/ Cu center. Besides, they also synthesized a series of quater-, quinque-, and sexithiophene derivatives bearing two cholesteryl moieties at the a-position. It was found that a sol–gel phase transition can be implemented by addition of oxidizing and reducing reagents. Zhu and colleagues prepared an electro-active LMOG containing a tetrathiafulvalene (TTF) entity. The gel formation can be tuned by means of oxidation/ reduction of the TTF group chemically or electrochemically. Although these gel systems are redox responsive, their properties, such as mechanical strength, flexibility, and sensitivity to external stimulus, are far from those required for practical uses. Therefore, creating instant, reversible, redox-responsive, and mechanically flexible organogels still remains a challenge. As a remarkable organometallic compound, ferrocene (Fc) contains an oxidizable metal ion, Fe, and is a nonpolar compound in the neutral state, and thereby it dissolves readily in hydrocarbon solvents. This property, however, can be easily reversed by simple oxidation of the central ion. Our interest in stimulus-responsive supramolecular gel systems led us to consider the compound as a neutral–cation redox pair that may be employed to tune the gelling ability of a gelator containing it. Actually, the same idea has been adopted by a number of groups for studies of switchable complexation and molecular aggregation in micelles and vesicles. However, all com-

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