Thermally and electrochemically controllable self-complexing molecular switches

Article abstract of DOI:10.1021/ja048164t

Self-complexing molecular systems are obtained when an arm component (a π-donor) is covalently linked to a preformed macrocycle (a π-acceptor). The resulting self-complexing compounds are not only attractive in relation to their topology, but also for the

Full text of DOI:10.1021/ja048164t

Published on Web 07/07/2004
Thermally and Electrochemically Controllable Self-Complexing Molecular
Switches
Yi Liu, Amar H. Flood, and J. Fraser Stoddart*
California Nanosystems Institute and Department of Chemistry and Biochemistry, UniVersity of California,
Los Angeles, 405 Hilgard AVenue, Los Angeles, California 90095-1569
Received March 30, 2004; E-mail: stoddart@chem.ucla.edu
Self-complexing molecules1 represent an interesting class of
compounds in the field of molecular recognition. Their molecular
structures often have an arm component covalently linked to a
macrocycle with sufficient flexibility such that the arms can be
included inside the macrocycle’s cavity by virtue of stabilizing
noncovalent bonding interactions. Such compounds are attractive
not only on account of their topology, but also because of their
potential to undergo reversible movement of the arm into and out
of the macrocycle’s cavity in response to a particular stimulus,
rendering them viable candidates for constructing nanoscale mo-
Figure 1. (a) Variable temperature UV/vis absorption spectra of 1‚4PF6
lecular machinery.2
recorded in MeCN (1.0 × 10-3 M) from 0 °C (pink) to 50 °C (yellow).
Several groups have developed1c-e self-complexing systems
The inset illustrates the changes in color of this same solution going from
wherein a π-donor in the form of 1,5-dioxynaphthalene (DNP) or
tetrathiafulvalene (TTF) units resides in arms that are attached
covalently to the π-accepting cyclophane, cyclobis(paraquat-p-
phenylene) (CBPQT4+).3 The π-donor is self-complexed by the
π-accepting cyclophane as a result of a combination of [π‚‚‚π]
stacking4 and [C-H‚‚‚π]5 and [C-H‚‚‚O] interactions.6 In most
of the existing systems, however, these intramolecular interactions
are either too strong or too weak, creating a situation where control
over the movement of the arm in and out of the cavity is inefficient
at best. In this communication we demonstrate that, by fine-tuning
the structures of these molecules, their self-complexing abilities
can be changed delicately and dramatically. Indeed, these in-and-
out movements can be made highly sensitive to temperature and
applied voltages, rendering the systems potential thermosensors and
electroswitches.
+22 °C down to +4 °C and then down to -40 °C. (b) Cyclic voltammo-
grams of the oxidation and reduction of 2‚4PF6 and the oxidation of the
control 3 recorded at 1000 mV s-1 in a 0.1 M TBAPF6 MeCN solution at
room temperature with a glassy carbon electrode.
diimide moiety also serves to increase the distances between the
donors and acceptors, as well as simplifying the spectroscopic
analyses. As shown in Scheme 1, an equilibrium is rapidly
established for each compound in either MeCN or Me2CO solutions.
The equilibrium between “uncomplexed” (UC) and self-complexed
(SC) conformations was measured by titration experiments employ-
1
ing UV/vis and H NMR spectroscopies. The absorption UV/vis
spectrum (Figure 1a) of 1‚4PF6 recorded in MeCN shows a broad
charge-transfer (CT) band in the visible region (λmax ) 521 nm), a
feature which is characteristic of donor-acceptor interactions
involving DNP/CBPQT4+. Linear correlations (see Supporting
Information) between absorption and concentration over the range
3.6 × 10-3 to 2.8 × 10-5 M are observed, suggesting the presence
of a unimolecular equilibrium and the absence of higher order
equilibria. The 1H NMR spectra of 1‚4PF6 recorded in CD3CN (298
K) and CD3COCD3 (243 K) show no concentration dependence
from 3.11 × 10-2 down to 9.70 × 10-4 M. It is unlikely, therefore,
that supramolecular oligomers are formed to any significant extent
under the conditions examined. The same conclusion was reached
for 2‚4PF6.
Compounds 1‚4PF6 and 2‚4PF6 contain (Scheme 1) DNP and
TTF units, respectively, in their arms, which are attached by a spacer
to a diimide fused symmetrically onto the CBPQT4+ ring. This
Scheme 1. Equilibria between the “Uncomplexed” (UC) and
Self-Complexed (SC) Conformations of 1‚4PF6 and 2‚4PF6 in
Solution
As evidenced by variable temperature UV/vis spectroscopy,
equilibrium 1 (see Scheme 1) between UC-1‚4PF6 and SC-1‚4PF6
in MeCN solution is highly sensitive to changes in temperature.
Since the UC conformation is colorless and the SC one is purple,
a thermoreversible color change is observed as a result of these
thermally induced conformational changes in 14+. Figure 1a shows
that, as the temperature is increased from 0 to +50 °C, the intensity
of the CT band centered on λmax ) 521 nm decreases, in keeping
with equilibrium 1 shifting more toward UC-1‚4PF6. The inset
illustrates visually the color changes; namely, the purple color
becomes more intense as the temperature of the MeCN solution is
lowered from +22 through +4 to -40 °C. The temperature-
dependent color change is completely reversible and can be recycled
many times. In contrast, the TTF-bearing 2‚4PF6 displays very small
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10.1021/ja048164t CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
temperature-dependent spectroscopic change in the temperature
range +10 to +50 °C with an intense green color7 persisting
throughout.8
In conclusion, we have demonstrated the operation of simple
molecular switches, based on self-complexing donor-acceptor pairs.
The switching can be effected thermally (14+) as an equilibrium
event and electrically (24+) as a reaction process. In addition to the
obvious applications 14+ and 24+ could find as thermo- and
electroswitches, the thermochromism associated with equilibrium
1 could lead to compounds such as 1‚4PF6 having futures as
imaging and sensing materials.
Although temperature is more or less ineffective in controlling
the movement of the TTF arm in SC-24+, both chemical and
electrochemical methods work well. The reason is that molecular
recognition between a TTF donor and a CBPQT4+ acceptor can be
turned “off” by the oxidation of the TTF unit to a charged speciess
either TTF•+ or TTF2+sand “on” by their reduction back to the
neutral form. The chemical redox cycle has been monitored (Figure
2) by 1H NMR spectroscopy at 253 K in CD3COCD3 solution. The
TTF unit in SC-2‚4PF6 is observed (Figure 2a) to be a 2:3 mixture
(δ ) 6.0-7.0 ppm) of cis/trans-isomers. The 1H NMR spectrum is
characteristic of a conformation for SC-24+ with average Cs
symmetry. No resonances are detected for UC-2‚4PF6. Upon
addition of 2 equiv of tris(p-bromophenyl)aminium hexachloroan-
timonate9 into the CD3COCD3 solution of 2‚4PF6, a much simpler
spectrum (Figure 2b) is observed, indicating that, upon oxidation,
the TTF2+ dication no longer resides inside the CBPQT4+ cavity
and its protons resonate at δ ) 9.83 ppm. The remainder of the
spectrum is commensurate with the UC-26+ conformation having
average C2V symmetry. When Zn dust is added to the NMR tube,
the original spectrum is regenerated (Figure 2c), indicating a return
to the SC-24+ conformation with the neutral TTF unit back inside
the cavity of the CBPQT4+ ring.
Acknowledgment. This work was supported by the Defense
Advanced Research Projects Agency (DARPA). Some of the
compound characterizations were supported by the National Science
Foundation under equipment Grant No. CHE-9974928 and CHE-
0092036.
Supporting Information Available: Experimental details and
spectroscopic data of 1‚4PF6 and 2‚4PF6. This material is available
free of charge via the Internet at http://pubs.acs.org.
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