of 9
A solid-state switch containing an electrochemically switchable bistable
poly[
n
]rotaxane
Wenyu Zhang,
a
Erica DeIonno,
*
b
William R. Dichtel,
ac
Lei Fang,
d
Ali Trabolsi,
d
John-Carl Olsen,
d
Diego Ben

ıtez,
a
James R. Heath
*
c
and J. Fraser Stoddart
*
d
Received 14th July 2010, Accepted 3rd October 2010
DOI: 10.1039/c0jm02269a
Electrochemically switchable bistable main-chain poly[
n
]rotaxanes have been synthesised using
a threading-followed-by-stoppering approach and were incorporated into solid-state, molecular switch
tunnel junction devices. In contrast to single-station poly[
n
]rotaxanes of similar structure, the bistable
polymers do not fold into compact conformations held together by donor–acceptor interactions
between alternating stacked
p
-electron rich and
p
-electron deficient aromatic systems. Films of the
poly[
n
]rotaxane were incorporated into the devices by spin-coating, and their thickness was easily
controlled. The switching functionality was characterised both (1) in solution by cyclic voltammetry
and (2) in devices containing either two metal electrodes or one metal and one silicon electrode. Devices
with one silicon electrode displayed hysteretic responses with applied voltage, allowing the devices to be
switched between two conductance states, whereas devices containing two metal electrodes did not
exhibit switching behaviour. The electrochemically switchable bistable poly[
n
]rotaxanes offer
significant advantages in synthetic efficiency and ease of device fabrication as compared to bistable
small-molecule [2]rotaxanes.
Introduction
The field of molecular electronics
1
has continued to grow over the
past several years, despite significant challenges to the realisation
of reliable molecular electronic circuits. One particular focus in
this area has been on molecular switches
2
where the electronic
states of the molecules can be controlled externally. The use of
molecular switches is both promising and challenging. They are
attractive components in devices on account of both their scal-
ability and the potential to tune their functionality at a molecular
level. In this way, the exact desired switch may be realised. A
major difficulty, however, lies in coupling the molecules and
electrodes in a molecular device. It turns out that this step greatly
complicates matters, because the molecular device functionality
is dependent on the system
, not just the molecule.
1
Over time,
researchers have seen both extrinsic
3
switching mechanisms, in
which the molecules are passive components, and more inter-
estingly, intrinsic mechanisms, in which the switching is based on
a molecular property, such as a conformational change. Exam-
ples of intrinsic switching mechanisms include redox switching,
4
configurational isomerisation,
5
and tautomerisation
6
of certain
molecules, as well as the ones based on the translational iso-
merisation of mechanically interlocked molecules (MIMs). The
controllable relative motions of the components of MIMs,
7
such
as bi- or multistable
8
[2]catenanes and [2]rotaxanes, give rise to
changes in conductivity within molecular electronic devices
(MEDs), and thus have been used as the active component in
a variety of circuits, including isolated junctions
9
and memo-
ries,
10
and switching between high and low conductance states of
the devices has been correlated to the translational isomerisation
of the molecules.
The electromechanical switching of both bistable donor–
acceptor [2]rotaxanes and [2]catenanes has been investigated in
a variety of environments,
8
e
,11
including solution, flat surfaces,
11
polymer matrices, pendant side chains on polymers,
12
metal
nanoparticle
surfaces,
13
mesoporous
silicon
nanoparticle
surfaces,
14
and tunnel junctions.
15
The switching based on rela-
tive molecular motions of surface-immobilised MIMs has
recently been visualised
16
by scanning tunneling microscopy.
Two bottlenecks, however, have limited the further development
of MEDs based on MIMs—the multistep syntheses used to
prepare amphiphilic derivatives and the non-scalable process of
forming densely packed Langmuir–Blodgett (LB) films of the
molecules during device fabrication. When organic monolayers
are used in MEDs, defect-free LB films prevent the device from
shorting and protect the molecular switching elements from
degradation
9
d
during the deposition of the top electrode. Herein,
we investigate the ability of electrochemically switchable bistable
poly[
n
]rotaxanes to address both of these challenges—they are
available through a short, efficient synthetic pathway and can be
readily incorporated into MEDs as films prepared by spin-
coating.
In the past two decades, although mechanically bonded
macromolecules
17
such as main-chain
18
and pendant
19
poly-
[
n
]catenanes have proven to possess extraordinary mechanical
and dynamic properties, the synthetic availability of these
a
Department of Chemistry and Biochemistry, University of California, Los
Angeles, 405 Hilgard Ave., Los Angeles, CA, 90095, USA
b
The Aerospace Corporation, 2350 E. El Segundo Blvd., El Segundo, CA,
90245, USA
c
Division of Chemistry and Chemical Engineering, California Institute of
Technology, 1200 East California Boulevard, Pasadena, CA, 91125, USA
d
Department of Chemistry, Northwestern University, 2145 Sheridan Road,
Evanston, IL, 60208, USA
† This paper is part of a
Journal of Materials Chemistry
themed issue in
celebration of the 70th birthday of Professor Fred Wudl.
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systems has limited their materials applications. Specifically,
incorporating a large number of cyclobis(paraquat-
p
-phenylene)
(CBPQT
4+
)
20
rings, a key component of the switchable MIMs,
into polymeric systems was impractical, as it was limited by the
moderate efficiency of the template-directed ‘‘clipping’’ process
used for mechanical bond formation. Two complementary
synthetic advances
21
have recently overcome this problem: the
development of kinetically controlled stoppering and macro-
cyclisation reactions that occur under mild conditions that do
not disrupt host–guest binding
22
and the formation
23
of
mechanical bonds under thermodynamic control through the
reversible nucleophilic opening of the CBPQT
4+
ring. Donor–
acceptor poly[
n
]rotaxanes were synthesised by threading
p
-
electron-deficient CBPQT
4+
rings onto a polymer ‘‘thread’’
containing
p
-electron-rich 1,5-dioxynaphthalene (DNP) moie-
ties in the backbone followed by attaching bulky stoppers to the
end groups. These poly[
n
]rotaxanes exhibited a folded structure
stabilised by secondary interactions between non-encircled DNP
and the outside of the CBPQT
4+
ring. Based on our successful
attempts in the highly efficient syntheses of foldable poly-
[
n
]rotaxanes
24
and side-chain poly[2]catenanes
12
using dynamic-
controlled reactions, we prepared a main-chain bistable poly-
[
n
]rotaxane
1
$
4nPF
6
(Fig. 1), where the switchable components
are incorporated directly into the polymer chain and thus
achieved a material with a high density of functionality. The
switching properties of the poly[
n
]rotaxane were characterised in
solution before it was employed as the active component in
a solid-state switch.
Polymeric materials for solid-state switches have a distinct
advantage over small molecules in terms of device fabrication.
Thin polymer films can be deposited using a spin-coater, spray-
coater or ink-jet printing techniques.
25
The thickness of the films
can be easily tuned. Polymeric materials can also be printed on
flexible substrates using low cost printing techniques,
26
in order
to incorporate these materials into MEDs. Many MEDs are
based on small molecules and use either self-assembled mono-
layers (SAMs) or LB films. In the case of bistable [2]rotaxane
molecules, the SAMs are not dense enough to be used in a solid-
state device, because the creation of the top contact typically
results in a shorted device. Therefore, to form an ordered, dense
molecular film, a LB trough must be used. The films are of very
high quality and the packing can be precisely controlled, but the
process is low-throughput and requires the use of amphiphilic
molecules. Thus far, solid-state devices incorporating [2]rotax-
anes and [2]catenanes have been highly ordered, with the func-
tional groups well aligned with respect to the electrodes. The
polymer systems fabricated by spin-coating, spray-coating, or
printing techniques will result in thin films in which the molecular
alignment is somewhat random. In this work, we demonstrate
that (i) main-chain bistable poly[
n
]rotaxanes with high density of
functionality can be synthesised with relative ease, (ii) the
switching process of the poly[
n
]rotaxane can be realised both in
solution and in the solid state, and (iii) ordered films are not
required to access the functionality of the polymer switch, given
the appropriate choice of electrode materials.
Fig. 1 shows the molecular structure of poly[
n
]rotaxane
1
$
4nPF
6
and its switching mechanism. The linear polymer
precursor
3
contains two
p
-electron donating aromatic moieties,
a tetrathiafulvalene (TTF) unit (green) and a DNP unit (red).
MIMs with these moieties have been shown to exhibit hysteretic
behaviour, or switching, in a number of different environments,
including in solution and in the solid state. In such bistable
rotaxanes, there is always an equilibrium between the tetraca-
tionic CBPQT
4+
ring
27
(blue) encircling the TTF unit (ground
Fig. 1
The ground-state co-conformer (GSCC), oxidised form, and the metastable-state co-conformer (MSCC) of the bistable poly[
n
]rotaxane
1
$
4nPF
6
.
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state co-conformer, GSCC) and the DNP unit (metastable state
co-conformer, MSCC). The first oxidation of the TTF moiety
forms a radical cation (TTF
/
TTF

+
) and creates a strong
repulsion of the CBPQT
4+
ring, driving it to the DNP site.
Reduction of the cationic species back to neutral forms the
MSCC, which re-equilibrates to the equilibrium mixture of
GSCC and MSCC. The difference between GSCC and MSCC
can be monitored using cyclic voltammetry (CV) because the first
oxidation voltage of the TTF units in the MSCC is distinctly
lower
11
b
than that in the GSCC because of the absence of the
encircling CBPQT
4+
on TTF moieties in the MSCC.
Results and discussion
1. Synthesis of the polymer precursors
The TTF-containing monomer precursor
2
,
28
the DNP-
containing monomer
4
,
24
the stopper precursor
6
,
24
and the
cyclophane
CBPQT
$
4PF
6
29
were synthesised (Scheme 1)
according to literature procedures. Monomer
3
was synthesised
by reacting the TTF-containing precursor
2
and propargyl
bromide in THF with a yield of 85%.
2. Synthesis of the polymer dumbbell
The polymerisation was carried out (Scheme 1) by using
copper(I)-catalysed azide-alkyne cycloaddition
30
(CuAAC) in
dimethyl formamide (DMF). The TTF-dialkyne
3
and a diazide-
bearing DNP derivative
4
underwent step-growth polymerisa-
tion
31
to form the linear polymer dumbbell
5
with alternating
TTF-DNP units. This structural feature is essential as it guar-
antees that each cyclophane acts as a switching component and
therefore the polymer has the highest possible density of func-
tional groups. The ratio of the two monomers was varied
(Table 1) to control the polymer molecular weight distribution.
The crude products were then precipitated into an aqueous
EDTA solution to remove the catalysts, followed by redissolving
in CH
2
Cl
2
and precipitating into Me
2
CO to remove unreacted
monomers and oligomeric side-products.
3. Synthesis of the poly[
n
]rotaxane
The polymer dumbbell
5
and 1.5 equivalents of
CBPQT
$
4PF
6
were dissolved in DMF, forming a dark-green solution, indi-
cating (Scheme 1) the threading of the cyclophane onto the
polymer dumbbell. The mixture was stirred for 24 h to ensure the
complete threading before it was treated with the alkyne-bearing
stopper precursor
6
under CuAAC conditions. The resulting
poly[
n
]rotaxane
1
$
4nPF
6
was filtered through DNP-functional-
ised PS-coDVB cross-linked resin
24
to remove the free
CBPQT
$
4PF
6
. It was then dissolved in DMF, precipitated in
THF to remove the unreacted stopper precursor
6
, followed by
redissolving and precipitating in EDTA solution to remove the
catalysts. The stoppered polymer dumbbell
7
was synthesised in
a similar way.
Scheme 1
Synthesis of the poly[
n
]rotaxane
1
$
4nPF
6
and the polymer dumbbell
7
.
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4. Polymer molecular weight analyses
Gel permeation chromatography (GPC), coupled with light-
scattering, was employed to analyse the size and molecular
weight distributions of the polymer dumbbell
5
. Table 1 shows
that the molecular weight distributions (
M
w
¼
8–94 kDa) were
well-controlled by varying the ratio of the two starting mono-
mers, and that the polydispersity indices (PDI
¼
1.40–1.47) were
in the normal range for step-growth polymer.
Noticeably, the GPC trace of poly[
n
]rotaxane
1
$
4nPF
6
showed
decreased retention volume compared to the corresponding
polymer dumbbell
5
(Fig. 2), a phenomenon contrary to the
previously studied
24
foldable all-DNP poly[
n
]rotaxanes, where
the addition of the CBPQT
4+
ring caused the polymer to fold and
led to an increased retention volume. We speculate that this new
situation arises because
1
$
4nPF
6
lacks the secondary non-
covalent interactions which are essential for the formation of the
folded structure. Instead, the polymer remains in its random coil
morphology upon threading of the CBPQT
4+
rings, and the
hydrodynamic volume of the polymer chains increases accord-
ingly, causing a decrease of retention volume in GPC. Such
characteristics are optimal for solid-state switch applications
because drastic changes in morphology could compromise the
mechanical strength and integrity of the material. Further
evidence
for
the
absence
of
folding
in
the
bistable
Fig. 2
GPC traces of the polymer dumbbell
5a
(red) and the corre-
sponding poly[
n
]rotaxane
1a
$
4nPF
6
(green). The retention volume of the
latter (28.9) is smaller than the former (29.5), indicating the increase of
particle size upon the formation of the poly[
n
]rotaxane.
Table 1
Gel permeation chromatography (GPC) of polymer dumbbell
5(a–c)
. The number-average molecular weights (
M
n
)of
5(a–c)
were
determined by multi-angle light scattering.
Polymer
sample
N
TTF
:
N
DNP
Retention volume
(mL)
Polydispersity
index
M
n
(kDa)
5a
0.800
29.5
1.40
8
5b
0.900
28.9
1.47
25
5c
1.000
28.0
1.45
94
Fig. 3
1
HNMR spectra of (a) the polymer dumbbell
5a
, (b) the poly[
n
]rotaxane
1a
$
4nPF
6
, and (c) the cyclophane
CBPQT
$
4PF
6
, in DMF-
d
7
, at 298 K.
Upon the formation of the poly[
n
]rotaxane, chemical shifts of CBPQT
4+
protons show noticeable upfield changes, while the TTF aromatic proton
separates into several signals from various co-conformers.
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poly[
n
]rotaxanes was obtained from NMR spectroscopic studies.
The shoulder peak in the trace of
1a
$
4PF
6
indicates the existence
of a certain amount of unbound
CBPQT
$
4PF
6
. Since
CBPQT
$
4PF
6
is a small molecule exerting marginal light
scattering response, the presence of such a peak should not affect
the molecular weight measurement of the macromolecular
species during GPC.
The molecular weight of
1
$
4nPF
6
could not be measured by
the static light-scattering technique directly because the charge-
transfer absorption band between the TTF units and the
CBPQT
4+
rings overlaps with the laser wavelength (690 nm) used
for the molecular weight-measuring method. Instead, the
M
n
of
1
$
4nPF
6
was measured by using nuclear magnetic resonance
(NMR) end-group analysis.
5. NMR spectroscopic analyses
The
1
H NMR spectrum (Fig. 3) of
1
$
4nPF
6
provides clear
evidence for the formation of the interlocked structure. The
downfield shift of the CBPQT
4+
proton signals are a typical
phenomena for its binding with electron-rich moieties.
32
The
protons on the DNP units retain their ‘‘unbound’’ values, sug-
gesting that there is no secondary binding between them and the
CBPQT
4+
ring and therefore no folded substructures are formed
during the ring-threading process. Calculations using the inte-
gration of the signals from the polymer dumbbell (DNP and
TTF) and those from the CBPQT
4+
ring indicated that 90%, 84%,
and 72% of the TTF moieties were encircled by the CBPQT
4+
cyclophane in
1a
$
4nPF
6
,
1b
$
4nPF
6
, and
1c
$
4nPF
6
. We believe
that such a decrease in coverage is caused by the higher energy
barrier
33
it is necessary to overcome in moving the threaded
CBPQT
4+
rings along longer polymer threads (
M
5a
<
M
5b
<
M
5c
).
6. Cyclic voltammetry studies
The electrochemically induced switching phenomenon of the
poly[
n
]rotaxane was firstly studied in solution.Fig. 4a shows two
successive CV cycles of the poly[
n
]rotaxane
1
$
4nPF
6
solution in
MeCN compared to that of the control polymer dumbbell
7
in
CH
2
Cl
2
under identical conditions. The two oxidation features
recorded at +433 mV and +621 mV correspond to the TTF
/
TTF

+
oxidation of the MSCC and GSCC, respectively. The
TTF unit inside the CBPQT
4+
ring has a significantly higher
oxidation voltage. The stronger peak at +820 mV is assigned to
the second oxidation (TTF

+
/
TTF
2+
), which is independent of
the co-conformer as the CBPQT
4+
ring encircles DNP upon first
oxidation. In the second scan, the height of the +621 mV peak is
reduced—with a concurrent increase in the height of the peak at
+433 mV—on account of the fact that some of the MSCCs have
not relaxed to the ground state during the scan. Following
electromechanical actuation, recovery of the MSCC/GSCC
equilibrium is an activated process. A control experiment
(Fig. 4a, bottom) using the polymeric dumbbell
7
showed two
oxidation peaks at +118 mV and +594 mV which do not change
between the first and second scans.
The kinetics of relaxation of the MSCC back to the GSCC in
1
$
4nPF
6
can be quantified by variable scan rate CV and fitting
(Fig. 4b) a first-order exponential decay model to the population
ratios of the metastable state and the relaxation times. The role of
the physical environment on molecular electromechanical
switching in solutions,
34
on surfaces,
11
b
,35
in polymer gels
15,8
f
and
self-assembled monolayers,
8
e
,9
a
as well as in molecular switch
tunnel junctions (MSTJs),
36
has already been probed in consid-
erable detail. For
1
$
4nPF
6
, the free energy of activation (
D
G
)
was found to be 17.1(

0.9) kcal mol

1
in MeCN. This value is

2 kcal mol

1
higher than that observed for previously studied
bistable [2]rotaxanes in solution, and thus, once again, indicates
that the switching behaviour of a bistable [2]rotaxane is strongly
influenced by the nature of the environment. This observation,
however, is contrary to its switching behaviour
12
of a TTF-
CBPQT
4+
-based pendant poly[2]catenane, the kinetics of which
are not affected by the colloidal nature of the nanostructure. In
this case, the polymeric material itself acts as a matrix that is
similar to previously investigated solid-state junctions, such as
polymer gels and SAMs, where an increased barrier to switching
back to the GSCC was also observed.
Fig. 4
(a) Up: The first (black trace) and second (grey trace) CVs (298 K/
scan rate
¼
200 mV s

1
/
vs
Ag/AgCl) of the bistable poly[
n
]rotaxane
1
$
4nPF
6
in MeCN (298 K, [TTF]
¼
1 mM, 0.1 M tetrabutylammonium
hexakisfluorophosphate as electrolyte). (b) Fitted first order exponential
decay profiles of the relaxation of the MSCC to the GSCC and time
constants (
s
) obtained from the CV data, measured for the solution at
room temperature at different scan rates (100–800 mV s

1
).
Fig. 5
High resolution scanning transmission electron microscopy of
a Si/polymer/metal device cross-section, illustrating the sandwich struc-
ture of a polymer switch device.
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7. Solid-state switch
Solid-state switches were fabricated using both the bistable
poly[
n
]rotaxane
1
$
4PF
6
and the polymer dumbbell
7
, with no
switching functionality, as a control. Bottom electrodes were
either metallic (M) or doped poly-Si (Si). The top electrodes in all
cases were metal (M). The polymer films were spin-coated on the
electrodes and tuned to approximately 10 nm. Devices were
evaluated using both a typical current–voltage (
I
V
) scan and
a remnant scan.
37
The remnant measurement reveals the
threshold voltages for turning the device
on
and
off
and the
magnitude of the
on
/
off
ratio as well as eliminating the capaci-
tance response. One of the silicon devices, which was cross-
sectioned using a focused ion beam and imaged using high
resolution scanning transmission electron microscopy (HR-
STEM), is shown in Fig. 5. The cross section reveals a smooth
and continuous 8 nm thick polymer layer.
In the case of the devices with the poly-Si bottom electrodes,
the bistable poly[
n
]rotaxane
1
$
4nPF
6
(BP) and the dumbbell
polymer
7
(DP) behaved very differently. The
I
V
and remnant
38
characteristics from Si/Polymer/M devices are shown in Fig. 6.
Most significantly, the bistable poly[
n
]rotaxane
1
$
4nPF
6
device
showed stable hysteresis in the remnant measurement (green
trace in Fig. 6b). The current started out low (
off
state) and as the
read bias was swept to +1.5 V, the device began to increase in
current up to +2.3 V. This higher current state (
on
state) persisted
until

0.8 V, when the device returned to the low current state
(
off
). The
on
:
off
ratio of this particular device was

7. The
polymer dumbbell
7
has no hysteresis in the remnant measure-
ment (red trace in Fig. 6b) and the magnitude of the current is
greatly reduced compared to that observed for the bistable
poly[
n
]rotaxane devices.
The bistable poly[
n
]rotaxane devices could be cycled between
the
on
and
off
states up to 20 times. The current level, however,
along with the magnitude of the
on
:
off
ratio, decreased with
repeated cycling, which most likely indicates degradation of the
polymer films with applied voltage. We believe that the solution
to improving device robustness may be found in metal organic
frameworks (MOFs) which incorporate MIMs.
39
In contrast to
polymer films, where the polymer backbone is stressed and
degraded when subjected to repeated dynamics—
i.e.
, switch-
ing—MOFs incorporating MIMs will yield rigid structures that
are intrinsically robust.
Fig. 6
(a)
I
V
and (b) remnant characteristics of Si/polymer/Ti/Al devices. The green traces correspond to the switching polymer devices, and the
control devices are shown in red. The bistable poly[
n
]rotaxane
1
$
4nPF
6
(green) shows hysteresis in the remnant scan and an
on
:
off
ratio of

7. The
polymer dumbbell
7
(red) does not switch. The arrows on the remnant plot (right) indicate the direction of the scan.
Fig. 7
(a)
I
V
and (b) remnant characteristics of Au/polymer/Ti/Au devices. The green traces correspond to the bistable poly[
n
]rotaxane
1
$
4nPF
6
devices, while the polymer dumbbell
7
devices are shown in red. Similar hysteresis is observed in the
I
V
plots for both the bistable poly[
n
]rotaxane and
the polymer dumbbell devices. The remnant measurement shows no stable hysteresis for either type of device.
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A second set of devices was fabricated using metallic bottom
electrodes. The behaviour of these devices was very different
from the poly-Si devices. Both the bistable poly[
n
]rotaxane
1
$
4nPF
6
devices and the dumbbell polymer
7
devices showed
(Fig. 7a) similar
I
V
characteristics and current levels. In the Si/
Polymer/M devices, the current magnitude was very different
depending on which polymer was present in the device. For
a device where the behaviour is dominated by the molecular
properties and not the interfacial ones, a change in the molecular
structure should yield a change in the
I
V
characteristics. The
remnant characteristics from the M/BP/M and M/DP/M devices
are shown in Fig. 7b. Although there was some hysteresis present
in the
I
V
scan, the remnant characteristics of these devices did
not show consistent hysteresis and the devices—both with the
bistable poly[
n
]rotaxane
1
$
4PF
6
and the dumbbell polymer
7
did not exhibit stable switching behaviour. This molecule-inde-
pendent type of behaviour has been noted previously in devices
with metal electrodes
40
and other studies have shown that the
molecule/electrode interactions in some cases can dominate the
transport characteristics of devices.
41
Conclusions
Using highly efficient synthetic methods, we have prepared
a main-chain bistable poly[
n
]rotaxane
1
$
4nPF
6
with well-
controlled molecular structure and a high density of functional
groups along the polymer chain. GPC studies have shown that
the molecular weight of the polymer can be readily controlled by
varying the stoichiometry of the starting monomers, and that the
polymer retains its random coil nature even after the mechani-
cally interlocked structures have been formed. Cyclic voltam-
metry and
1
H NMR spectroscopic studies have provided
evidence for switching in solution, a process which is similar to
previously investigated small-molecule analogues. Variable
scanning rate CV was also used to measure the barrier of
relaxation from the MSCC to the GSCC. The
D
G
value of the
switching of
1
$
nPF
6
was determined to be 17.1(

0.9) kcal mol

1
in MeCN, a value similar to that of previously studied materials,
based on polymer gels or SAMs. The bistable poly[
n
]rotaxane
1
$
4nPF
6
was incorporated into two different types of devices
using spin coating, which resulted in disordered thin films of
polymer. The polymer dumbbell
7
was used as a control. In
devices based on a metal/polymer/metal architecture, the poly-
mer dumbbell
7
and the bistable poly[
n
]rotaxane
1
$
4nPF
6
showed very similar
I
V
characteristics and no stable remnant
signature. However, when the bottom electrode material was
changed to polysilicon, the two polymers exhibits sharply
differing behaviours. The bistable poly[
n
]rotaxane
1
$
4nPF
6
showed hysteresis and bistability in the solid-state—
i.e.
, the
ability to switch between the
on
and
off
states—whereas the
control polymer dumbbell
7
showed no hysteresis. This investi-
gation reinforces the importance of choosing the appropriate
electrode materials when fabricating MEDs, as well as showing
that disordered films exhibit the same switching functionality as
related devices incorporating well-ordered and aligned LB films
of bistable [2]rotaxanes or bistable [2]catenanes. The polymer-
based form allows access to a variety of high-throughput pro-
cessing techniques, including spin-coating, spray-coating and
ink-jet printing.
Experimental section
Methods and materials
Chemicals were purchased from commercial suppliers and were
used as received. Size exclusion chromatography was performed
on an Agilent 1100 Series liquid chromatography system with
two ViscoGELTM columns. Multiangle light scattering was
measured with Wyatt DAWN Heleos II spectrometer, while
refractive index measurements were recorded with a Wyatt
Optilab rEX spectrometer. Number-average molecular weights
(
M
n
), weight-average molecular weights (
M
w
) and poly-
dispersities (PDI
¼
M
w
/
M
n
) were determined relative to linear
polystyrene (GPC
PSt
).
1
H and
13
C NMR spectra were recorded
on a Bruker DRX500 (500 MHz) or AV600 (600 MHz) spec-
trometer, with the residual solvent resonance as the internal
standard. Electrochemistry was performed using a Gamry
Reference
600
potentiostat/galvanostat/ZRA.
Fast
atom
bombardment mass spectra (FAB) were obtained on a JEOL
JMS-600H high resolution mass spectrometer equipped with
a FAB probe.
Synthesis of the TTF monomer
NaOH (1.09 g, 27.2 mmol) was added to a solution of
2
(0.60 g,
1.36 mmol) in THF (15 mL). A solution of propargyl bromide in
PhMe (w/w 80%, 1.21 mL, 8.17 mmol) was then added to the
suspension. The heterogeneous mixture was heated at 60

C for
3 h and was then filtered through Celite. The solvent was evap-
orated under reduced pressure. The resulting residue was dis-
solved in CH
2
Cl
2
and washed with brine (2

50 mL).
Evaporation of the solvent afforded a brown solid. Chroma-
tography (SiO
2
,1:4Et
2
O/CH
2
Cl
2
) provided
3
as a red-brown
solid (0.59 g, 79%).
1
H NMR (500 MHz, CD
2
Cl
2
):
d
¼
6.23 (s,
2H, TTF–
H
), 4.27 (s, 4H, TTF–C
H
2
), 4.16 (d, 4H,
4
J
¼
2 Hz,
OC
H
2
C
^
CH), 3.53–3.67 (m, 16H, C
H
2
O), 2.46 (t, 2H,
4
J
¼
2
Hz, OCH
2
C
^
C
H
);
13
C NMR (125 MHz, CD
2
Cl
2
):
d
¼
134.5,
134.4, 116.2, 116.1, 79.7, 74.0, 70.3, 70.2, 69.3, 69.0, 68.0, 58.1.
MS (FAB): Calcd for C
22
H
28
O
6
S
4
m
/
z
¼
516.077. Found
m
/
z
¼
516.075.
General polymerisation procedure
Monomers
3
(150 mg, 0.29 mmol) and
4
(
5a
,
3
:
4
¼
0.80 : 1.0;
5b
,
3
:
4
¼
0.90 : 1.0;
5c
,
3
:
4
¼
1.0 : 1.0), ascorbic acid (10.2 mg, 0.058
mmol) and CuSO
4
$
5H
2
O (7.2 mg, 0.029 mmol) were dissolved in
DMF (2 mL). The mixture was stirred under Ar for 24 h. In the
case of
5c
, another 1 mg of
4
was added to the solution, along
with ascorbic acid (0.05 mg) and CuSO
4
$
5H
2
O (0.03 mg), after
which the reaction mixture was stirred for a further 3 h. The
mixture was then added dropwise to an aqueous EDTA solution
(0.1 M, 5 mL), forming a yellow precipitate. The precipitate was
collected by centrifugation before being washed with H
2
O(2

5 mL) and dried under high vacuum. The resulting solid was
dissolved in CH
2
Cl
2
(0.5 mL) and precipitated into Me
2
CO
(5 mL) to give the product. The solid was once again recovered
by centrifugation and finally dried under high vacuum.
1
H NMR
spectra of
5a–c
are almost identical except for some marginal
integral differences. A representative
1
H NMR data for
5a
is
provided here:
1
H NMR (500 MHz, CD
2
Cl
2
):
d
¼
8.09 (b, 2H,
This journal is
ª
The Royal Society of Chemistry 2011
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, 2011,
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Triazole-
H
), 7.76 (b, 4H, Ar-
H
2/6
), 7.40 (b, 4H, Ar-
H
3/7
), 6.90 (b,
4H, Ar-
H
4/8
), 4.67–3.52 (m, 32H, C
H
2
O), 3.48 (b, 4H, C
H
2
-Tri-
azole).
Synthesis of the poly[
n
]rotaxane
A solution of
CBPQT
$
4PF
6
(48.1 mg, 0.0437 mmol) and the
polymer thread
5
(29.1 mg,
N
DNP
¼
0.0728 mmol) in DMF
(2 mL) was stirred at room temperature for 24 h. A purple
solution, indicative of threading, resulted. Stock solutions con-
taining the stopper precursor
4
(1.6 mg, 7
m
mol), ascorbic acid
(0.1 mg, 0.7
m
mol) and CuSO
4
$
5H
2
O (0.1 mg, 0.4
m
mol), were
then added to the solution. The reaction mixture was stirred
under Ar at room temperature overnight. Resin-supported DNP
was added to the solution for 10 min before being removed by
filtration. This step was repeated until the resin ceased turning
red. The solution was then added to an aqueous EDTA solution
(0.1 M, 5 mL), forming a green precipitate. The precipitate was
recovered by centrifugation, washed with H
2
O(2

5 mL) and
dried under high vacuum for 2 h. The solid was dissolved in
a minimum amount of DMF and precipitated when added to
THF (5 mL). The resulting poly[
n
]rotaxane was once again
recovered by centrifugation and dried under vacuum.
Solid-state device fabrication
Solid-state switches were fabricated by spinning a thin film (

10
nm) of polymer, either the bistable poly[
n
]rotaxane
1
$
4nPF
6
or
the polymer dumbbell
7
as the control, on 5 micron-wide bottom
electrodes followed by electron beam deposition of 5 micron-
wide metallic top electrodes through a shadow mask. Bottom
electrodes were gold or
n
-type polysilicon. The bistable poly-
rotaxane and polymer dumbbell were diluted in solvent—MeCN
and CH
2
Cl
2
, respectively—then spun at 4000 rpm for 45 s. The
dilutions were adjusted to yield films of approximately 10 nm, as
measured by Dektak profilometry. The films were then baked at
100

C for 5 min.
The metallic electrodes were fabricated on a silicon substrate
coated with 5000

A thermal oxide using photolithography and
metal evaporation of 10 nm titanium and 50 nm gold, followed
by lift-off in Me
2
CO. The electrodes were cleaned with an oxygen
plasma before deposition of the polymer layer. The silicon elec-
trodes were fabricated from doped
n
-type polysilicon using
photolithography, followed by reactive ion etching to transfer
the electrode pattern into the 60 nm thick polysilicon layer.
Before use, the electrodes were rinsed with Me
2
CO, and the
photoresist was stripped in J.T. Baker Aleg-355 at 80

C for
30 min. The electrodes were then thoroughly rinsed with 18 M
U
water and baked at 150

C for 5 min. The top electrodes were
either 10 nm titanium and 40 nm gold (for devices with gold
bottom electrodes) or 10 nm Ti and 200 nm Al (for poly-Si
bottom electrodes). The electrical characteristics of the devices
were evaluated using a Keithley 236 SMU, controlled by Lab-
View software for current–voltage (
I
V
) and remnant charac-
teristics.
Acknowledgements
This work was supported under the Semiconductor Research
Corporation (SRC) through its focus centers on Functional
Engineered NanoArchitectonics (FENA) and Materials, Struc-
tures, and Devices (MSD), the MolApps Program funded by the
Defence Advanced Research Projects Agency (DARPA), and
The Aerospace Corporation’s Independent Research and
Development Program. The authors gratefully acknowledge
Brendan Foran of The Aerospace Corporation for the trans-
mission electron microscopy.
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