Chemistry Reference
In-Depth Information
we have already discussed can also be switched under the appropriate conditions,
including Sauvage's transition metal-based MIMs and Leigh's benzylic amide
MIMs. Reviews on the subject of mechanically interlocked molecular machines
are abundant in the literature [
218
-
222
].
We are unlikely to appreciate a car that does not run or a computer that does not
compute. Likewise, molecular switches that bump and tumble randomly in solution
cannot yield a functional effect; they must be organized in bulk or at interfaces to
produce a coherent and cooperative effect of technological relevance [
223
-
225
].
Only then might it be possible to dub them machines in the true sense of the word.
Thus, the most beautiful molecular machines are those [
222
] that we put to work for
us. Here we provide several examples.
Would not it be amazing if the tiny nanomotions of molecular machines could be
cooperatively scaled up to do work on macroscopic objects? Toward that end, it has
been demonstrated [
226
,
227
] that a monolayer of “molecular muscles” - palin-
dromic bistable [3]rotaxanes (Fig.
31a
) immobilized on the surface of a thin
a
S
S
S
S
O
O
O
O
O
O
+
+
N
N
O
O
+
+
N
N
S
S
S
S
O
O
O
O
O
O
O
OO
OOO
S
S
S
S
N
N
+
+
N
N
+
+
c
Detector
b
Pot
entio
stat
Ag / AgCl
Laser
Extended
Pt
Si
Au
Oxidation
Reduction
Si
-
Δ
Z
Au
0.1 M NaClO
4
Contracted
Fig. 31 Mechanical actuation of a gold-coated microcantilever by “molecular muscles” [
227
].
(a) Structural formula of a palindromic, bistable [3]rotaxane with gold-binding dithiolane groups
attached to the cyclophanes. (b) Reversible bending up and down of a cantilever by actuation of
a monolayer (~8 billion molecules) of the rotaxanes on its surface. The gold surface bends when
the rotaxanes “contract” under the influence of an electrochemical oxidation that causes the
cyclophanes to shuttle inward from the periphery of the molecule. (c) Electrochemical cell
(Ag/AgCl, Pt, and the cantilever are the reference, counter, and working electrodes, respectively)
and combined AFM device used to measure the bending by detecting a laser beam reflected off of
the cantilever's surface
