Applications of Metallofoldamers - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 415

12.4 Metallofoldamers as Dynamic Materials
The above examples amply demonstrate that not only may a metal ion impact the confor-
mation of a metallofoldamer but also the conformation of the foldamer can influence its
binding with the metal ion. In addition to their applications as novel supramolecular hosts
and sensors, metallofoldamers are very promising as environmentally responsive materi-
als. Nature has long perfected its skills in creating responsive materials for highly sophis-
ticated tasks through the conformational approach. An amazing example is kinesin, a
motor protein that walks on microtubules to deliver various cargos within a cell [77].
Myosin, a related class of motor protein, is responsible for creating muscle contraction by
walking on actin filaments [77]. Chemists are nowhere close to creating such intricately
organized foldamer complexes that couple catalysis, conformational change, and binding
to achieve desired mechanical work in the nanoworld. The following examples, nonethe-
less, demonstrate that chemists have been able to generate prescribed local motions in
synthetic metallofoldamers with external stimuli.
In an effort to create motional dynamic devices (i.e., molecular or supramolecular
structures that undergo motional or mechanical changes triggered by external stimuli),
Lehn and colleagues synthesized alternatively joined pyridine-pyrimidine (py-pym) oligo-
mers ( 20 ) [78]. These oligomers represent primitive molecular actuators whose motions
are driven by ionic interactions. The work was built on the earlier observation that meta -
connected oligoheterocycles folded into helical structures due to the preferred transoid ,
transoid conformation [79-82]. When Pb(OTf) 2 was added, the helix switched into a lin-
ear strand due to the cisoid , cisoid conformation induced by metal-binding (Figure 12.8).
Figure 12.8 (a) Ionic modulation of extension/ contraction motions interconverting the heli-
cal, metal-free ligand H and the extended linear multinuclear complex L. (b, c) The transoid ,
transoid conformation of the helix and the metal-induced cisoid , cisoid conformation of the
linear strand. Reprinted with permission from [ref 78] Copyright 2002, National Academy of
Sciences, USA.