Molecular-mechanical switch-based solid-state electrochromic devices

Article abstract of DOI:10.1002/anie.200461723

You only need eyes to appreciate the color change that occurs in a polymer matrix when the bistable rotaxane shown is switched between its ground-state (green) and metastable-state (red) co-conformers. Not only is an electrochromic device within reach, bu

Full text of DOI:10.1002/anie.200461723

Communications
Molecular Switches
these devices could be controlled over a dynamic range of 103
to 104 s. The fundamental properties of these devices were
quantified through time- and temperature-dependent cyclic
voltammetry (CV) measurements. In this way, the kinetic
parameters (DGꢀ, DHꢀ, DSꢀ, and Ea) of the rate-limiting step
in the switching cycle of the device could be evaluated for
several different molecular switches.
Molecular-Mechanical Switch-Based Solid-State
Electrochromic Devices**
David W. Steuerman, Hsian-Rong Tseng,
Andrea J. Peters, Amar H. Flood, Jan O. Jeppesen,
Kent A. Nielsen, J. Fraser Stoddart,*and
James R. Heath*
Four bistable molecular-mechanical systems[2,3a,17,18]—two
[2]catenanes C14+ and C24+ and two [2]rotaxanes R14+ and
R24+—along with appropriate control compounds, were
investigated (Figure 1) for electrochromic device applica-
The dynamics of electrochemically driven, bistable molecular
mechanical switches—such as certain nondegenerate, two-
station, donor–acceptor [2]catenanes and [2]rotaxanes—have
been the subject of numerous experimental investigations[1–3]
in the solution phase, in which the general mechanistic details
of the redox-activated switching processes are becoming
increasingly well understood.[4] These molecular machines
may have many technological applications,[5] although few are
likely to be liquid-solution-phase based.[6–9] Thus, significant
effort has been directed towards understanding and exploit-
ing the bistability of [2]catenanes and [2]rotaxanes in other
environments, including both Langmuir–Blodgett (LB)[10,11]
and self-assembled monolayers (SAMs),[12] and in solid-state
molecular-switch tunnel junctions (MSTJs).[13–16]
Herein we explore, at a fundamental level, the solid-state
application of electrochromic devices by taking advantage of
the colorimetric changes that accompany the electrochemi-
cally driven switching of certain bistable [2]catenanes and
[2]rotaxanes. The molecular switches were immobilized
within a solid-state polymer electrolyte, and a microfabri-
cated, planar, three-terminal equivalent of a standard electro-
chemical cell was used for electrical addressing. The polymer
environment significantly slows down certain steps within the
molecular-mechanical switching cycle, but the overall mech-
anism remains unchanged from that observed in other
environments. We also find that by varying the molecular
structure of the switch, the colorimetric retentions times of
Figure 1. The structural formulas of the two bistable [2]catenanes C14+
and C24+ and the two bistable [2]rotaxanes R14+ and R24+ investigated
within solid-state polymer electrolyte environments. The use of color—
namely, red, blue, and green—for certain units and components
relates to the graphical representations shown in Figure 2.
[*] Dr. D. W. Steuerman, Dr. H.-R. Tseng, Dr. A. J. Peters, Dr. A. H. Flood,
Dr. J. O. Jeppesen, K. A. Nielsen, Prof. J. F. Stoddart
The California NanoSystems Institute, and
Department of Chemistry and Biochemistry
University of California, Los Angeles
tions. Most of these compounds have been studied[2,3,19] in the
liquid-solution phase by using 1H NMR spectroscopy and
temperature-dependent electrochemistry. In addition, the
switching cycle of a SAM[12] formed from a [2]rotaxane that
is closely related to R14+ was previously reported. Herein, we
present a description of the general switching cycle mecha-
nism that is applicable to all four molecules, followed by a
brief description of how certain details of this cycle vary
between the four molecules.
405 Hilgard Avenue, Los Angeles, CA 90095-1569 (USA)
Fax: (+1)310-206-1843
E-mail: stoddart@chem.ucla.edu
Prof. J. R. Heath
Division of Chemistry and Chemical Engineering, MC 127-72
California Institute of Technology
1200 East California Boulevard, Pasadena, CA 91125 (USA)
Fax: (+1)626-395-2355
E-mail: heath@caltech.edu
The switching cycle mechanism for the bistable [2]cate-
nanes and [2]rotaxanes is illustrated in Figure 2. There is
always an equilibrium (K(D/T)4+) established between the
cyclobis(paraquat-p-phenylene) (CBPQT4+) ring encircling
the tetrathiafulvalene (TTF) site (labeled as the ground-state
co-conformer or GSCC) and the dioxynaphthalene (DNP)
site (labeled as the metastable-state co-conformer or
MSCC).[20] These labels reflect the fact that the equilibrium
is most commonly shifted toward the CBPQT4+ ring encir-
cling the TTF site. The first oxidation state of the GSCC
[**] This research was funded by the Office of Naval Research, the
National Science Foundation, and the Moletronics Program of the
Defense Advanced Research Projects Agency, the Microelectronics
Advanced Research Corporation, its Focus Centers on Functional
Engineered NanoArchitectonics and Advanced Materials and
Devices, and the Center for Nanoscale Innovation for Defense.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200461723
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Angewandte
Chemie
MSCC to the GSCC. There is a long history, dating back to
the early work of Kramers,[22] of investigations into molecular-
mechanical processes within high-viscosity solvents,[23,24]
although those studies have typically focused on low-ampli-
tude molecular motions with energy barriers of only a few
kcalmolꢀ1. Based on the assumption that the large-amplitude,
high-energy-barrier motions in our systems follow similar
trends, the expectation is that the rate constant for the
recovery of the equilibrium distribution will scale as k = hꢀg (h
is the solvent viscosity and g has a value between 0.5 and
0.75).[25]
The molecular switches investigated herein exhibit sig-
nificant differences from one another that should be reflected
in the switching characteristics of the electrochromic devices.
R14+ and R24+ are characterized[3] by very different equili-
brium dynamics in solution. R14+ contains a simple TTF unit,
and the room-temperature affinity of the CBPQT4+ ring for
this TTF unit over the DNP ring system is 10:1. R24+ does not
have a simple TTF unit and the CBPQT4+ ring has an equal
probability for being located on the “quasi-TTF” or the DNP
at 298 K, although this equilibrium is temperature-dependent.
As chemical constitution impacts the dynamic equilibrium
between translational isomers[26] in solution for R14+ and
R24+, we expect a similar situation to pertain within the solid-
state polymer environment.
Figure 2. The analogous switching cycles for a) the bistable catenanes
and b) the bistable rotaxanes, starting from the centrally located
ground-state co-conformers. The green and red sites on the ring and
dumbbell components refer to the TTF- and DNP-recognition units,
respectively. When the TTFunit is oxidized, it is drawn as a highlighted
green unit carrying yellow ends. The CBPQT4+ ring is modeled as the
blue ring encircling the green site with positive charges indicated as
white spots. The metastable co-conformers are isomeric (and isoelec-
tronic) with the appropriate ground-state co-conformers.
C14+ and C24+, on the other hand, are characterized[2,16] by
similar equilibrium dynamics. For both structures, the ground-
state equilibrium strongly favors the CBPQT4+ ring encircling
the TTF unit. However, the molecular-mechanical motion in
C24+, with its two bulky diazapyrenium units, is significantly
more sterically hindered than that in C14+, and this constitu-
tional feature is expected, in any environment, to slow the
recovery of C24+ from the MSCC to the GSCC substantially
relative to that of C14+.
corresponds to removal of an electron from the TTF site, and
is accompanied by a rapidly driven shuttling (Coulombic
repulsion) of the CBPQT4+ ring to the DNP site with a rate
described by the constant, kT+!D. Upon reduction of the TTF
site back to its charge-neutral state, the MSCC is formed. The
MSCC and GSCC are readily distinguished from each other
by using CV because the oxidation potential of the (bare)
TTF unit on the MSCC is approximately 200 mV less[12]
positive than that for the GSCC (vs. a Pt reference elec-
trode)—a result that is valid for both the bistable [2]catenanes
and the bistable [2]rotaxanes. The return of the MSCC to the
GSCC, which is really a recovery of the equilibrium
distribution described by K(D/T)4+, is thermally activated. This
recovery is typically too fast to observe at room temperature
in the solution phase.
Figure 3 shows two successive CVs collected from a solid-
state polymer device containing R14+ as well as measurements
CV measurements on the bistable [2]rotaxane SAMs[12]
revealed that the rate of recovery of the equilibrium
distribution was strongly dependent[21] on the physical envi-
ronment—the rate was much slower for the SAM environ-
ment than for the solution phase. For the SAM, it was also
demonstrated that the GSCC/MSCC equilibrium distribution
may be recovered at least 1000 times faster through the
pathway that accesses the two-electron reduction of the
CBPQT4+ ring to its bis-radical cation, CBPQT2C2+
(Figure 2).[12] This pathway is precisely that proposed origi-
nally as the mechanism for “opening” a MSTJ device.[13]
The solid-state polymer environment, which can be
considered as a highly viscous solvent, should also affect the
rate of recovery of the equilibrium distribution from the
Figure 3. The first and second CVs (298 K, scan rate = 150 mVsꢀ1) of
the R14+-containing solid-state polymer electrolyte device. The initial
scan (gray trace) reveals a weak feature at 115 mV and two oxidation
peaks (partially resolved) at 315 and 465 mV. For the second scan
(black trace) virtually all the signal corresponding to the feature at
315 mV has shifted to 115 mV. The features at 115 and 315 mV were
assigned as the first oxidation state of the metastable and ground-
state co-conformers, respectively. A control device that contains only
the dumbbell component of R14+ reveals two well-separated oxidation
peaks at 55 and 355 mV that do not change between the first and
second scans.
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from a control device containing the dumbbell component of
R14+. This bistable [2]rotaxane exhibits an equilibrium
GSCC/MSCC population of about 10:1 under ambient
conditions, similar to that observed in solution at 298 K.
The first CV trace reflects this distribution—three oxidation
features are recorded at 115, 315, and 465 mV. The relatively
+
weak feature at 115 mV corresponds to the TTF!TTFC
oxidation of the low-abundance MSCC. The feature at
315 mV arises from the TTF!TTFC oxidation of the
+
GSCC, and the feature at 465 mV is the second oxidation
(TTFC+!TTF2+) of R14+. This second oxidation step is
independent of the co-conformer as the CBPQT4+ ring
encircles the DNP site once the TTF site is singly oxidized.
For the second scan, nearly all the integrated current under
the 315 mV feature has moved to 115 mV. This assignment is
consistent with the control device measurements, which
reveal two well-separated oxidation peaks at 55 and 355 mV.
The hysteretic response in the successive CV curves of
R14+ reflects the slow recovery of the ground-state equili-
brium distribution (described by K(D/T)4+), and is similar to that
observed for the bistable [2]rotaxane SAMs.[12] By varying the
time between the first and second CV scans as well as the
temperature of the experiment, the various kinetic parame-
ters for relaxation from the MSCC to the GSCC were
determined. Figure 4a shows a scan-rate series of (second)
CVs. Figure 4b displays a graph of the scan-rate series
condensed into a single plot that exhibits an exponential
decay. In this case, we have taken into account the rates based
on the integrated current of the MSCC oxidation peak at
115 mV relative to the total current of the first-oxidation
peaks of the MSCC and GSCC. For all data sets, at least one
point was taken at scan rates that were much slower than the
measured relaxation times—the
Figure 4. Experimental data utilized to extract the kinetic parameters
of the metastable!ground-state relaxation process of R14+ within the
solid-state polymer electrolyte environment. a) Representative (second
scan) CVs recorded at 296 K at various scan rates. b) The population
ratios of the metastable state and the relaxation times fitted to a first-
order decay model. c) The temperature dependence of the relaxation
kinetics expressed as an Eyring plot.
Dt = 100 s in Figure 4b, for exam-
Table 1: The 1/e decay times (t298) and free energies of activation (DGꢀ298) obtained by using cyclic
voltammetry together with the thermodynamic data, obtained after an analysis of variable-temperature
cyclic voltammetry studies using the Eyring (DHꢀ, DSꢀ) and the Arrhenius (Eaꢀ) relationships for a series
of bistable rotaxanes and catenanes in solution and in a solid-state polymer electrolyte.[a]
ple—and that point was fixed in the
exponential decay fit. Figure 4c is
an Eyring plot of several such
series, each collected at a different
temperature. From this plot, we
can extract the various kinetic
parameters that describe the recov-
ery of the ground-state equilibrium
distribution. Those parameters,
along with the analogous measure-
ments for C14+, C24+, and R24+, are
summarized in Table 1. The data
for C24+ were only collected under
ambient conditions, and so only t298
and DGꢀ298 are reported.
Compound t298
k298
DGꢀ298
DHꢀ
DSꢀ
Eaꢀ
[s]
[sꢀ1
]
[kcalmolꢀ1
]
[kcalmolꢀ1
]
[calmolꢀ1 Kꢀ1
]
[kcalmolꢀ1
]
[2]R14+[b]
[2]R24+
[2]C14+
[2]C24+
3.50(ꢁ0.02)
!1
0.6
800
0.286(ꢁ0.002) 18.1(ꢁ0.2) 19.5(ꢁ0.2)
@1
4.7(ꢁ1.4) 19.6(ꢁ0.2)
15.8(ꢁ0.4) 9.1(ꢁ0.4) ꢀ22(ꢁ3)
9.7(ꢁ0.5)
1.7
17.0(ꢁ0.4) 14.8(ꢁ0.4) ꢀ7.5(ꢁ2.5) 15.4(ꢁ0.4)
0.00125
21.0
–
–
–
[a] Solid-state polymer data obtained from samples mixed within a polymer matrix (MeCN/PPMA/PC/
LiClO4 (w/w/w/w)=70:7:20:3) at a Pt electrode over a range of temperatures R14+ (290–320 K). [b] Self-
assembled monolayers of the analogue to R14+, bearing a disulfide tether, was characterized by variable-
temperature CV to obtain thermodynamic data: k293 =0.39 sꢀ1, DG2ꢀ98 =18.0 kcalmolꢀ1, DHꢀ =17.6ꢁ
3.0 kcalmolꢀ1, DSꢀ =ꢀ1.4ꢁ10 calmolꢀ1 Kꢀ1, Ea =17.7ꢁ2.8 kcalmolꢀ1
.
The polymer utilized in this
case has a viscosity approximately 104 times greater than
that of MeCN, which is the commonly used solvent in the
investigation of the electrochemical properties of these
molecules in solution. Thus, the implication is that the
relaxation rates in solution should be between 102 and 103
times faster than those observed. Thus, under cryogenic
conditions, it should be possible to observe the MSCC!
GSCC relaxation in the liquid-solution phase, and we have
now confirmed that this prediction does, indeed, hold true. [19]
The viscosity of the polymer matrix changes by approx-
imately a factor of two over the temperature range of these
investigations (280–320 K). This trend can certainly impact
the measured thermodynamic parameters—most notably
DSꢀ. However, t298, k298, DG2ꢀ98, and Ea are less sensitive and
so it is in the case of these parameters that we have the most
confidence. In reference to these values, we find that the
MSCC!GSCC relaxation rates, within the polymer environ-
ment, are under significant chemical control. In fact, they
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varied by more than 1000 times in moving from R24+ to C24+.
This result directly translates into chemical control over
colorimetric retention times in these electrochromic devices.
In Figure 5, we present a demonstration of such a device
this relaxation rate by more than 1000 times. This variation
has interesting implications for controlling the colorimetric
retention times (and hence power efficiencies) in solid-state
electrochromic devices,[27] and we have demonstrated such a
device. Finally, these results suggest that electrochemical
measurements[19] on these same molecules in cryogenic
solutions should go far toward quantifying how the kinetic
parameters that describe the switching cycle are impacted by
the physical environment of the molecular switches.
Experimental Section
A three-terminal, microfabricated device, operated as a function of
voltage scan rate and temperature, was employed to interrogate the
kinetics of electrochemically-induced molecular mechanical switch-
ing within a solid-state polymer. The polymer electrolyte matrix was a
standard solid-state electrolyte prepared from a solution in MeCN
that contained polymethylmethacrylate (PMMA, Mw = 300000),
propylene carbonate, (PC)—as a plasticizer—and LiClO4 (MeCN/
PMMA/PC/LiClO4 (w/w/w/w) = 70:7:20:3). These materials were
handled only in flame-dried glassware under an inert atmosphere of
Ar. The LiClO4 was purified by fusion under vacuum. The MeCN was
dried either by passage through steel columns containing activated
alumina under Ar using a solvent purification system (Anhydrous
Engineering) or by distillation over CaH2. Finally, the MeCN was
deoxygenated prior to use through three freeze/pump/thaw cycles.
The solid-state electrolyte devices were fabricated with tradi-
tional CV experiments in mind. A planar three-electrode—working,
counter, reference—configuration was implemented, and each elec-
trode consisted of 10 nm of Ti covered with 50 nm of Pt. The
electrodes were patterned by using standard lithographic and metal
evaporation procedures onto
a glass substrate. The fabricated
substrates were placed in a glove box under a N2 environment. The
electrochemically active species were then combined with the
electrolyte and drop cast onto the three electrodes. These devices
were allowed to stand for 20 min to facilitate drying, and then were
pumped for a further 20 min to a final pressure of 100 mTorr. The
working devices were firm to the touch, with a consistency similar to
that of a stiff rubber band. The viscosity of similarly prepared polymer
electrolytes has been reported,[28] and, at 298 K, it is more than 104
times larger than that for the solvent MeCN.
CV measurements were performed in a sealed container on an
EG&G VersaStat II Potentiostat with the aid of three electrical
feedthroughs and a thermocouple fed back to a hot-plate for thermal
control. The key experimental variables were scan rate and temper-
Figure 5. The electrochromic response of the solid-state polymer devi-
ces. The working (W), counter (C), and reference (R) electrodes are
indicated. a) The device with the working electrode grounded. The
green color correlates with the color of the ground-state co-conformer
of R14+, with the CBPQT4+ ring encircling the TTFsite. b) After a +1 V
oxidizing potential has been applied to the working electrode, the
color change to red/purple correlates with the translational isomeriza-
tion having occurred to the metastable state. c) Several seconds after
the working electrode has been returned to 0 bias and R14+ has relaxed
back to the ground-state co-conformer.
ature and—for
a given device—these parameters were varied
randomly to avoid the influence of systematic errors.
Received: August 19, 2004
containing R14+ that was cycled from the green to the red
state, and then back to the green state again. This electro-
chromic device[27] contained substantially higher concentra-
tions of R14+ than did other devices discussed herein, but was,
in all other ways, identical to them.
Keywords: catenanes · electrochromic devices ·
.
metastable compounds · polymers · rotaxanes
In summary, we find that a single mechanistic picture can
be applied to describe the electrochemically driven switching
cycle of bistable [2]catenanes and [2]rotaxanes in solid-state
polymer electrolytes. This mechanistic picture is qualitatively
identical to that initially proposed to describe the behavior of
a MSTJ device,[13] and is also analogous to that observed for
the switching cycle of a [2]rotaxane SAM.[12] We find that the
MSCC!GSCC relaxation rate is strongly dependent upon
both physical environment and molecular structure. In
particular, by varying the molecular structure we could vary
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designate the different three-dimensional spatial arrangements
of the components of mechanically interlocked molecular
systems. Also, in identifying the ON and OFF states of the
bistable [2]catenane employed in the first MSTJ crossbar
device,[16] we employed the term “co-conformer” to the two
states. See, especially Figure 1in reference [16]. In other words,
in proposing our original mechanism, we appreciated correctly
that the ON and OFF states are both isomeric and, of course,
isoelectronic.
[21] SAMs of copper catenates have been prepared; however, the
electrochemically-triggered circumrotations are either signifi-
cantly slower than the corresponding motions in solution or
completely frozen out; see: L. Raehm, J.-M. Kern, J.-P. Sauvage,
C. Hamann, S. Palacin, J.-P. Bourgoin, Chem. Eur. J. 2002, 8,
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Scientific, Hackensack, NJ, 1993, p. 163.
[8] For
a discussion of the current/voltage characteristics of
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also the Special Issue covering the Third International Meeting
on Electrochromics (IME-3) in Electrochimica Acta 1999,
44(18), 2969 – 3258; c) Electrochromic devices have been devel-
oped out of a variety of different materials; for examples of
molecular systems, see: R. J. Mortimer, Electrochim. Acta 1999,
44, 2971– 2981, and, for an example in which an electrochromic
molecule that has been attached to a solid support, see: R.
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Langmuir–Blodgett monolayers of redox-switchable [2]cate-
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[9] Recently, it was demonstrated that an array of microcantilever
beams, when coated with a SAM of palindromic, bistable
[3]rotaxane molecules, undergoes controllable and reversible
bending when it is exposed to chemical oxidants and reductants;
see: T. J. Huang, B. Brough, C.-M. Ho, Y. Liu, A. H. Flood, P. A.
Bonvallet, H.-R. Tseng, J. F. Stoddart, M. Baller, S. Magonov,
Appl. Phys. Lett. 2004, 85, in press.
[10] I. C. Lee, C. W. Frank, T. Yamamoto, H.-R. Tseng, A. H. Flood,
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[11] For evidence that redox-controllable molecular shuttles, in the
form of amphiphilic, bistable [2]rotaxanes, are mechanically
switchable with chemical reagents in closely packed Langmuir
films, and in Langmuir–Blodgett bilayers mounted on solid
substrates, see: T. J. Huang, H.-R. Tseng, J. Sha, W. Lu, B.
6490 ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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Angewandte
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2001, 46, 2145 – 2150; d) for the use of polymer-gel electrolytes in
electrochromic devices, see: J. Vondrꢁk, M. Sedlarikova, J.
Reiter, T. Hodal, Electrochim. Acta 1999, 44, 3067 – 3073.
[28] M. Deepa, N. Sharma, S. A. Agnihory, S. Singh, T. Lal, R.
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Angew. Chem. Int. Ed. 2004, 43, 6486 –6491
www.angewandte.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6491