Chemistry Reference
In-Depth Information
Kuroiwa et al. (2004) prepared metal-containing organogels by the addition of
either FeCl
2
or CoCl
2
to chloroform solutions of a lipophilic triazole. The resulting
linear “molecular wires” self-assemble into an interconnected, fibrous, gel-forming
microstructure. The Co
II
-based system shows an unusual thermoresponse by convert-
ing into a sol upon cooling, the opposite of normal supramolecular gel behavior.
It is proposed that this occurs through a change in the coordination behavior
of the metal ion. Alternatively, the stimuli-responsive component can be placed
on the ligand (Kume et al. 2006). For example, photoresponsive gels have been
prepared using Fe(BF
4
)
2
and an azobenzene-containing triazole in chlorocyclo-
hexane. Upon exposure to UV light the gels are converted in the sol state because
of the trans-cis photoinduced conversion of the azobenzene in the ligand.
The system reverts to the gel upon exposure to visible light and converts back to
the trans-azobenzene conformation.
7.5. CONCLUSION AND OUTLOOK
The development of MSPs has come a long way in recent years, and an array of
ligands and metal salts have been used to access a dramatic range of materials. In
addition to the structures we described in this chapter, the versatility of this approach
has allowed access to more complex architectures, such as helical polymers (Kimura
et al. 1999; Ikeda et al. 2006) and block copolymers (Nair et al. 2006). The combi-
nation of metal ion functionality, ligand tuneability, and the potential to change the
properties of both upon complexation should open the door to polymeric materials
that exhibit a unique combination of properties or even to new material properties
altogether. Thus, in addition to the metal ligand motifs that we described, there is
still plenty of room for the development and utilization of new ligands that offer
different properties (e.g., Chow et al. 2007). The variability in the dynamics of differ-
ent coordination motifs can allow the development of materials that range from being
under continuous equilibrium, and thus highly responsive to environmental stimuli,
to being irreversible on a realistic timescale and behave more akin to traditional
covalent polymers. MSPs, which exhibit rapid and controllable complexation/
decomplexation, could permit access to a new generation of electronic, sensing,
self-healing, and/or catalytic materials (Park et al. 2006). The investigation of
these polymers is still in its infancy, but their potential as functional materials
suggests an exciting future for MSPs.
REFERENCES
Abd-El-Aziz AS, Carraher Jr CE, Pittman Jr CU, Zeldin M. Macromolecules containing metal
and metal-like elements. Hoboken (NJ): Wiley InterScience; 2006. Vols. 1-7.
Bailey JA, Miskowski VM, Gray HB. Spectroscopic and structural properties of binuclear
platinum-terpyridine complexes. Inorg Chem 1993;32:369-370.
