A Multiswitchable Poly(terthiophene) Bearing a Spiropyran Functionality: Understanding Photo- and Electrochemical Control

Article abstract of DOI:10.1021/ja1114634

An electroactive nitrospiropyran-substituted polyterthiophene, poly(2-(3,3′′-dimethylindoline-6′-nitrobenzospiropyranyl) ethyl 4,4′′-didecyloxy-2,2′:5′,2′′- terthiophene-3′-acetate), has been synthesized for the first time. The spiropyran, incorporated into the polymer backbone by covalent attachment to the alkoxyterthiophene monomer units, leads to multiple colored states as a result of both photochemical and electrochemical isomerization of the spiropyran moiety to merocyanine forms as well as electrochemical oxidation of the polyterthiophene backbone and the merocyanine substituents. While electrochemical polymerization of the terthiophene monomer can take place without oxidation of the spiropyran, increasing the oxidation potential leads to complex electrochemistry that clearly involves this substituent. To understand this complex behavior, the first detailed electrochemical study of the oxidation of the precursor spiropyran, 1-(2-hydroxyethyl)-3,3-dimethylindoline-6′- nitrobenzospiropyran, was undertaken, showing that, in solution, an irreversible electrochemical oxidation of the spiropyran occurs leading to reversible redox behavior of at least two merocyanine isomers. With these insights, an extensive electrochemical and spectroelectrochemical study of the nitrospiropyran- substituted polyterthiophene films reveals an initial irreversible electrochemical oxidative ring-opening of the spiropyran to oxidized merocyanine. Subsequent reduction and cyclic voltammetry of the resulting nitromerocyanine-substituted polyterthiophene film gives rise to the formation of both merocyanine π-dimers or oligomers and π-radical cation dimers, between polymer chains. Although merocyanine formation is not electrochemically reversible, the spiropyran can be photochemically regenerated, through irradiation with visible light. Subsequent electrochemical oxidation of the nitrospiropyran-substituted polymer reduces the efficiency of the spiropyran to merocyanine isomerization, providing electrochemical control over the polymer properties. SEM and AFM images support the conclusion that the bulky spiropyran substituent is electrochemically isomerized to the planar merocyanine moiety, affording a smoother polymer film. The conductivity of the freestanding polymer film was found to be 0.4 S cm^-1.(Figure Presented)

Full text of DOI:10.1021/ja1114634

ARTICLE
pubs.acs.org/JACS
A Multiswitchable Poly(terthiophene) Bearing a Spiropyran
Functionality: Understanding Photo- and Electrochemical Control
Klaudia Wagner,† Robert Byrne,‡ Michele Zanoni,‡ Sanjeev Gambhir,† Lynn Dennany,† Robert Breukers,†
Michael Higgins,† Pawel Wagner,† Dermot Diamond,‡ Gordon G. Wallace,† and David L. Officer*,†
†ARC Centre of Excellence for Electromaterials Science and Intelligent Polymer Research Institute, University of Wollongong,
Northfields Avenue, Wollongong, NSW 2522, Australia
‡CLARITY Centre for Sensor Web Technologies, National Centre for Sensor Research, Dublin City University, Collins Avenue,
Glasnevin, Dublin 9, Ireland
S Supporting Information
b
ABSTRACT: An electroactive nitrospiropyran-substituted
polyterthiophene, poly(2-(3,300-dimethylindoline-60-nitroben-
zospiropyranyl)ethyl 4,400-didecyloxy-2,20:50,200-terthiophene-
30-acetate), has been synthesized for the first time. The spiro-
pyran, incorporated into the polymer backbone by covalent
attachment to the alkoxyterthiophene monomer units, leads to
multiple colored states as a result of both photochemical and
electrochemical isomerization of the spiropyran moiety to
merocyanine forms as well as electrochemical oxidation of the polyterthiophene backbone and the merocyanine substituents.
While electrochemical polymerization of the terthiophene monomer can take place without oxidation of the spiropyran, increasing
the oxidation potential leads to complex electrochemistry that clearly involves this substituent. To understand this complex
behavior, the first detailed electrochemical study of the oxidation of the precursor spiropyran, 1-(2-hydroxyethyl)-3,3-dimethylindo-
line-60-nitrobenzospiropyran, was undertaken, showing that, in solution, an irreversible electrochemical oxidation of the spiropyran
occurs leading to reversible redox behavior of at least two merocyanine isomers. With these insights, an extensive electrochemical
and spectroelectrochemical study of the nitrospiropyran-substituted polyterthiophene films reveals an initial irreversible electro-
chemical oxidative ring-opening of the spiropyran to oxidized merocyanine. Subsequent reduction and cyclic voltammetry of the
resulting nitromerocyanine-substituted polyterthiophene film gives rise to the formation of both merocyanine π-dimers or
oligomers and π-radical cation dimers, between polymer chains. Although merocyanine formation is not electrochemically
reversible, the spiropyran can be photochemically regenerated, through irradiation with visible light. Subsequent electrochemical
oxidation of the nitrospiropyran-substituted polymer reduces the efficiency of the spiropyran to merocyanine isomerization,
providing electrochemical control over the polymer properties. SEM and AFM images support the conclusion that the bulky
spiropyran substituent is electrochemically isomerized to the planar merocyanine moiety, affording a smoother polymer film. The
conductivity of the freestanding polymer film was found to be 0.4 S cmꢀ1
.
behavior. This has been achieved for photoswitchable molecules
by formation of self-assembled monolayers (SAMs)9 and bilayers10
and incorporation into polymer films11,12 and beads.8 While the
required structure for monolayer and bilayer formation does not
usually affect the function of the photomolecules, one of the
challenges in using polymers is that the monomer used to synthesize
the polymer, and the polymerization process itself, must be
compatible with the photo-/electroactive switching units. In addi-
tion, the incorporation of the photoresponsive functionality must
not affect the polymerization of the monomer. Wesenhagen et al.
recently reported the incorporation of a photo-/electrochromic
dithienylcyclopentene moiety into a polymer by oxidative electro-
polymerization of a methoxystyryl monomer attached to the
photoactive unit.13 The electroactive polymer was formed as only
1. INTRODUCTION
The immobilization of light-responsive molecules on electroac-
tive surfaces provides an exciting opportunity for the development
of smart materials and devices.1 Spiropyrans, reported for the first
time by Fischer and Hirshberg,2 are one of the most widely studied
classes of photoswitchable compounds. Irradiation of spiropyrans
(SP, Figure 1) with near-UV light3 or electro-oxidation4 induces
heterolytic cleavage of the spiro carbonꢀoxygen bond to produce
ring-opened structures (MC, Figure 1), represented as two reso-
nance forms, one merocyanine-like and the other quinoidal.
The intense absorption in the visible region of the open form
MC has led to the advanced study of spiropyrans in
photochromic,5 molecular optoelectronic6 optobioelectronic
fields,7 and chemical sensing.8
The exploitation of such light-responsive molecules in devices
typically requires immobilization on a surface through an appended
functionality that does not interfere with the light switching
Received: December 20, 2010
Published: March 18, 2011
r
2011 American Chemical Society
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potential for electrochemical control of photoresponsive func-
tionality in a conducting polymer system. In this Article, we
report the synthesis and properties of spiropyran-functionalized
poly(terthiophene)s. The spiropyran unit is covalently linked to
the monomer, which is electropolymerized. The resulting film is
electroactive and shows multiswitchable behavior. In this exciting
new material, we have electrochemical control of both the
polymer redox and the photochemical properties, as well as the
ability to convert the SP to MC form with potential and light. To
the best of our knowledge, this is the first report of the synthesis
and electrochemical control of electropolymerized conductive
polymer film bearing a spiropyran functionality.
Figure 1. Benzospiropyran (SP, left) and its stimuli-induced zwitter-
ionic and quinoidal isomers contribute to the open form (MC, right).
2. EXPERIMENTAL SECTION
a thin film, the thickness of several monolayers as the result of
limited conductivity.
Experimental details are given in the Supporting Information.
The photoswitchable moiety can be appended to the polymer
backbone or incorporated into it with quite different effects. In
the latter case, the incorporated photoswitchable unit can affect
the nature of the polymer backbone. Hugel and co-workers have
demonstrated that incorporation of azobenzene units into the
backbone of the polymer chain modifies the modulus of elasticity
of the polymer.14 The nature of the polymer backbone can also
influence the kinetics of open/closed ring isomerization of the
photoactive units, affect the stability, or even lead to enhance-
ment of photo degradation.15
While the introduction of photoswitchable molecules into
polymers has proved valuable, the use of conducting polymers
opens up a new avenue for electrochemical control of these
multifunctional materials. In this regard, poly(thiophene) deri-
vatives are of particular interest due to their stability and relative
ease of synthesis and functionalization,16 and they have been
applied in a broad range of devices, from simple solar cells17 to
complex electroluminescent devices.18 This exciting potential of
combining photoswitchability and electroactivity together was
first recognized by Areephong et al. who reported the ability to
control the electropolymerizability of an oligothiophene by the
introduction of a photoswitchable dithienylethene unit into the
monomer.19 Most recently, the first spiropyran-functionalized
polythiophene was obtained by chemical copolymerization of a
thiophene monomer with covalently attached spiropyran and
3-hexylthiophene.20 In this work, it was demonstrated that both
the fluorescence of this photoswitchable conductive copolymer
and its interactions with cyanide ions could be controlled
through SP to MC photoconversion. However, the real benefit
of combining the photoactivity and electrochemical activity of
these materials was not explored.
The introduction of large functionalities onto a polythiophene
can have a significant steric effect on the polymer backbone,
adversely affecting the optical and electronic properties of the
polymer as well as the polymer processability. The use of 4,400-
dialkoxy-3-substituted terthiophenes as a precursor for develop-
ing novel substituted poly(thiophene)s provides reduced steric
interactions between polymer backbone and substituent, allows
better control of polymer regiochemistry, and the alkoxy side
chains activate the monomer for polymerization and ensure
better polymer processability.21,22 We have been exploring
successfully the polymerization of functionalized dialkoxyterthio-
phene monomers to avoid these problems.23 Incorporation of
the spiropyran moiety into a dialkoxyterthiophene monomer
appeared to provide a good opportunity to investigate the
3. RESULTS AND DISCUSSION
3.1. Synthesis of Monomer. The dialkoxyterthiophene acetic
acid TThAA was chosen in this study as the terthiophene
monomer to functionalize with the SP group because mild
chemistry can be used to form reasonably stable ester linkages
with a readily available hydroxyl-substituted spiropyran, such as
N-hydroxyethylnitrobenzospiropyran SP1 (Scheme 1). Spiro-
pyran-substituted terthiophene monomer TThSP1 was readily
prepared from the acetic acid TThAA (Scheme 1). The pre-
cursor methyl acetate TThMA was synthesized as previously
described24 and hydrolyzed to give TThAA in quantitative yield.
Base-catalyzed condensation of SP1 with TThAA afforded
TThSP1 in 77% yield.
The structures of the three substituted terthiophenes are fully
consistent with their spectroscopic data. The 1H NMR terthio-
phene signals are as expected unperturbed by the change in ester
functionality with NMR spectral characteristics of TThSP1
simply being a superposition of the TTh and SP components.
As expected for nitrospiropyran derivatives, the UVꢀvis
absorption spectrum of TThSP1 (Figure 2, black line) shows
bands in the ultraviolet (not shown) due to the SP1 substituent.
In the visible, the terthiophene absorbance dominates the
spectrum at 350 nm, with no absorption due to the merocyanine
isomer above 500 nm. Irradiation of a dichloromethane solution
of TThSP1 with UV light leads to the formation of TThMC1
(Figure 2, violet line), as evidenced by the peak at 564 nm. This is
analogous to the photochemical behavior of SP1 that gives the
characteristic peak for a merocyanine at 560 nm (MC1, Figure 2,
red line). However, an additional band at 446 nm is also observed
in the MC1 spectrum. The origin of this peak has been explained
by McCoy et al. who reported that nitro-spiropyran derivatives in
dichloromethane isomerize to merocyanines and rapidly form
H-aggregates, which exhibit a band at 446 nm.25
It seems, therefore, that the terthiophene moiety does not
affect the photochromic properties of the spiropyran substituent
but does inhibit formation of aggregates.
3.2. Electrochemistry and Spectroelectrochemistry of
SP1. Our initial investigations into the electrochemical polym-
erization of TThSP1 revealed that the resulting polymer had a
more complex electrochemical behavior than expected from a
substituted poly(thiophene). Because this behavior clearly arose
from the presence of spiropyran, and there appeared to be a
limited knowledge of the electrochemistry of spiropyrans
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Scheme 1. Synthesis and Polymerization of TThSP1a
a The inset shows a computer-generated model of a short length of the ring-opened merocyanine polymer, poly-TThMC1.
they report oxidation voltammograms similar to ours, their focus
is largely on the reduction influence on the oxidation.
On the first, positive scan on the SP1 voltammogram
(Figure 3a), one irreversible oxidation peak at the potential
0.77 V is observed, with broad, overlapping reduction waves on
the reverse scan. It has been proposed that this first oxidation of
spiro compounds is a one-electron oxidation at the indoline
nitrogen.4 Because the oxidation is irreversible, a new species
must be formed from the oxidized spiropyran. Visual inspection
of the solution at the working electrode surface (Figure 3e)
during oxidation shows the presence of an orange-red color,
similar to that observed by Preigh et al. during electrochemical
oxidation of an hydroxyspiropyran and attributed to oxidized
merocyanine.4
When the SP1 solution is initially prepared, an equilibrium
mixture of the pale yellow SP1 and red MC1 forms is clearly
present as indicated by the violet coloration (Figure 3c). On
standing in the presence of light, this initial equilibrium changes
significantly in favor of the SP1 form, giving the pink color
(Figure 3d). Therefore, oxidation at the working electrode is
clearly driving the SP1 to the MC1 form (red color). Zhi et al.
proposed a mechanism for this type of electroisomerization.27
However, they suggested that the isomerization arises from the
reduced spiropyran, which cannot be the case here. As suggested
by Preigh and co-workers for an hydroxyl analogue,4 initial
oxidation of the indoline nitrogen to give the SPox, followed by
rearrangement, affords the MC radical cation. During the second
scan of the CV of SP1 (Figure 3a), two new broad overlapping
oxidation waves appeared with potential maxima at 0.47 and 0.57
V. Because several stereoisomers of the MC form have been
recognized,29 the most dominant of which are the TTC
(transꢀtransꢀcis) and TTT (transꢀtransꢀtrans)30 forms, it is
likely that these peaks may represent the electrochemistry of the
two TTC-MC1 and TTT-MC1 isomers (Scheme 2). These
species themselves could undergo reversible electrochemistry as
Figure 2. UVꢀvis spectra of MC1 after irradiation of SP1 with 254 nm
UV light (red line), monomer TThSP1 at equilibrium (black line) and
when irradiated with 254 nm UV light (violet line), and reduced poly-
TThSP1. The concentration of MC1 and TThSP1 is 1 ꢁ 10ꢀ4 M in
dichloromethane.
generally, we undertook an electrochemical study of SP1, which
had not been investigated before.
The electrochemistry of analogues of SP1 has been reported
previously.26ꢀ28 Zhi et al. discuss electrochemistry and spectro-
electrochemistry of N-methylnitrobenzospiropyran, specifically
investigating the reduction behavior of the molecule but provid-
ing little information about the oxidation of the spiropyran.27 We
do not observe the same electrooxidation characteristics as Zhi
and co-workers. Our results are similar to those of Campredon
et al.26 who investigated the electrochemical behavior of
naphthalene analogues of SP1, although their report includes
only a first scan of oxidation voltammograms and a brief
comment regarding the instability and possible degradation of
the formed radical cation. More recently, Jukes and co-workers28
reported the electrochemistry and spectroelectrochemistry of
1,3,3-trimethylindolino-60-nitrobenzopyrylospiran, and, while
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Figure 3. Cyclic voltammograms of SP1 (a), spectroelectrochemistry during oxidation of 2 ꢁ 10ꢀ3 M SP1 in an OTTLE cell (b), a solution of SP1 just
after preparation (c) and after 20 min exposure to visible light (d), and change in SP1 solution in electrochemical cell during applied potential 1 V to the
ITO electrode (e). SP1 concentrations in (a), (c), d), and (e) are 1 ꢁ 10ꢀ3 M in acetonitrile with 0.1 M TBAP as electrolyte.
Figure 3e) grew rapidly. Therefore, we ascribe these two
absorption bands to TTC-MC1ox and TTT-MC1ox. It is also
interesting to note the appearance of two absorption peaks at 880
and 997 nm, which rise (0.8 V maximum) and fall as the oxidation
potential is increased. Because these peaks reduce in intensity as
the amount of MC1þ increases, we propose that they arise from
the formation of MC1 radical cation dimers. These observations
provide significant insights into the electrochemistry and spec-
troelectrochemistry of poly-TThSP1, as we discuss later.
Consequently, we propose that electrochemical oxidation of
SP1 irreversibly leads to isomers of oxidized MC1, which
themselves undergo reversible electrochemistry as depicted in
Scheme 2. Although these are solution-based processes, we
anticipated that the spiropyran substituent of poly-TThSP1
would undergo similar electrochemistry given we had not
observed any spectroscopic interaction between the terthio-
phene and spiropyran groups.
Scheme 2. Postulated Mechanism for the Electrochemical
Redox Behavior of SP1
3.3. Electrochemical Polymerization of TThSP1. The elec-
trochemical deposition of TThSP1 on a platinum disk electrode
shows cyclic voltammograms (CVs) with an increase in current
with successive cycles, indicative of successful electroactive film
deposition (Scheme 1 and Figure 4a). The electropolymerization
process is carried out up to 0.8 V, with the onset of monomer
oxidation to its radical cation at 0.62 V (inset, Figure 4a) and the
formation of bluish film, typical of oxidized polythiophene. We
do not observe the characteristic red color of merocyanine at the
maximum applied potential of 0.8 V, leading us to conclude that
we do not open the spiropyran ring during electrodeposition.
The film-coated electrode was placed in monomer-free solution
for postpolymerization CV analysis. The post CVs show inter-
esting, dynamic redox behavior (Figure 4b). During the first scan
in the post CV of poly-TThSP1, three oxidation peaks are visible
at 0.12, 0.34, and 0.76 V (Figure 4b). While the origin of the peak
at 0.12 V is not clear, the 0.34 V peak can be assigned to oxidation
of the polymer backbone (poly-TThoxSP1, Scheme 3), because
cycling of the poly-TThSP1 film between ꢀ0.4 and 0.5 V shows
consistent capacitive behavior over more than 10 cycles (not
shown). A similar oxidation peak is observed for a poly-TThMA
film (Figure 4c), which is the analogous spiropyran free polymer,
indicated by the reduction peaks at 0.44 and 0.52 V. However,
there is no apparent electrochemical path back to SP1 from
either the oxidized or the reduced MC1 isomers as is indicated by
a drop in height of the SP1 oxidation peak at 0.77 V on the
second scan of the CV (Figure 3a).
To further probe this, the absorption spectral changes of SP1
oxidation processes in 0.1 M TBAP in acetronitrile were re-
corded at applied potentials from 0.6 to 0.9 V and are shown in
Figure 3b. Significant absorbance changes were observed when
the potential exceeded 0.65 V, and new absorption bands with
maxima at 492 and 523 nm (corresponding to the red solution in
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with a methyl substituent instead of the nitrobenzospiropyran. In
contrast, the sharp oxidation peak at 0.76 V on the first post CV
scan of poly-TThSP1 (Figure 4b) is completely lost during the
second scan, but appears to recover somewhat during cycling.
The position of this peak is at the same potential as the first
oxidation of the SP1 molecule (Figure 3a). It is likely that the
oxidation peak at 0.76 V for poly-TThSP1 results from loss of an
electron from the indoline nitrogen of the SP1 substituent,
leading to the oxidized MC1 substituent on the oxidized poly-
thiophene backbone (poly-TThoxMC1ox, Scheme 3).
We also observed that the choice of the switching potential
used during electropolymerization of poly-TThSP1 (Figure S1a,
c,e) influences the electrochemical characteristics of the post CV
voltammogram (Figure S1b,d,f). When the switching potential of
electropolymerization is set higher than 0.9 V (Figure S1e),
somewhat higher than the oxidation potential of spiropyran SP1
(0.77 V, Figure 3a), the sharp oxidation peak at 0.76 V on the
first post CV scan of poly-TThSP1 is no longer observed.
Clearly, from these post CVs, the oxidation of the appended
spiropyran is not reversible, and therefore it appears likely that
the substituent on poly-TThSP1 remains in the MC form
following further cycling. The CV following initial poly-TThSP1
oxidation (second scan, Figure 4b) has no peak due to SP
oxidation, but subsequent scans exhibit a peak at 0.65 V of
increasing intensity. This could arise from the oxidation of the
polymer MC stereoisomers, analogous to that postulated for SP1
(Scheme 2).
This oxidative electroisomerization of poly-TThSP1 is also
supported by the dramatic color changes observed on oxidation
of the polymer film. Polythiophene films generally exhibit two
colors, depending on the doped state. For example, the color of
the parent poly(terthiophene) in the reduced state is orange-red
and brown-black when the polymer is oxidized.31 Reduced poly-
TThSP1 film is a violet-pink color, but on oxidation to 0.5 V
(polymer backbone oxidation) the color changes to green-blue
(Scheme 3). Further oxidation to 0.8 V generates a more intense
violet-pink film as the result of the formation of the oxidized MC
substituent. This additional color change at the positive potential
occurs very quickly, over a narrow range of potentials (see movie
in the Supporting Information). This unique photochromic
behavior in the doped state was further investigated using
spectroelectrochemistry.
3.4. UVꢀVisible Spectroelectrochemistry of poly-TThSP1.
The spectral properties of poly-TThSP1 were examined across
the potential range used for the post-CV analyses (ꢀ0.35 to 0.9
V). Poly-TThSP1 exhibited typical polythiophene behavior as
the electrode potential was increased from ꢀ0.35 to 0.6 V, with
initial polaron band formation (870 nm) followed by the
development of a free carrier tail (Figure 5a). As the applied
potential increases from 0.6 V, the growth of a new absorbance at
540 nm is observed (Figure 5b), which corresponds to the initial
sharp oxidation shown in the post CVs (Figure 4b). This
absorbance is in a position similar to that observed for the
photoisomerized TThMC1 monomer (Figure 2, violet line), and
therefore we attribute it to the oxidized MC isomer of poly-
TThoxMC1ox.
Figure 4. Electrochemical deposition of TThSP1 with inset showing
the first scan (a), post CV voltammograms of poly-TThSP1 (b) and
poly-TThMA (c) on platinum disk electrodes, 10 scans at a scan rate
100 mV sꢀ1
.
Scheme 3
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rereduction (dashed line, Figure 6a), indicative of the formation
of ring-opened merocyanine isomers that do not ring close to
spiropyran on reduction. This observation is consistent with the
variable electrochemical response of poly-TThSP1 (Figure 4b).
To better understand these changes, a spectroelectrochemical
study of fresh films of both poly-TThSP1 (Figure 7) and control
sample poly-TThMA (see Supporting Information, Figure S2)
was undertaken. The aim of these measurements was to monitor
the absorbance changes during a “simulation” of the film cycling.
The absorbance spectra in the previous spectroelectrochemical
study (Figure 5) only show the initial changes that occur
following the oxidation of the spiropyran moiety. In this study,
absorbance spectra were obtained while holding the potential at
ꢀ0.1, 0.6, 0.8, and 1.0 V (Figure 7aꢀd), potentials at which new
peaks appeared in the post-CVs during film cycling (Figure 4b
and c). This was repeated six times, effectively mimicking six CV
cycles.
Again, it is clear that the spectra recorded during the first
“cycle” are different from all the others. Thus, the first spectrum
of the reduced poly-TThSP1 film (Figure 7a) is different across
the whole spectrum from the next five spectra recorded at that
potential (ꢀ0.1 V), consistent with the single wavelength
observation evident in Figure 6a. As expected up to 0.6 V, the
polythiophene backbone appears to be oxidized, as evidenced by
the complete loss of the absorbance at 500 nm as well as the rise
of the polaron band at 850 nm (Figure 7b). However, the
“second cycle” shows the rise of a new band below 500 nm,
which appears to come from the opening of the spiropyran.
The picture becomes clearer from the absorbance spectra at
0.8 V (Figure 7c) and 1.0 V (Figure 7d). The first spectrum at 0.8
V is again assigned essentially to the oxidized polymer backbone,
which now shows a significant free carrier tail above 700 nm. The
second and later spectra exhibit much more character with two
new peaks at 443 and 477 nm and broad absorbances at 870 and
995 nm. All of these bands are then either lost completely or
reduced substantially at higher oxidation potential (1.0 V,
Figure 7d), with the appearance of a new band around 540 nm
and the retention of the free carrier tail.
Figure 5. Spectroelectrochemistry of electrochemically polymerized
film of poly-TThSP1 (a and b) on ITO glass for potential ranges of
ꢀ0.35 to 0.55 V (a) and 0.6ꢀ0.9 V (b).
Therefore, the two bands at 443 and 477 nm are clearly not
due to the fully oxidized polymer with oxidized merocyanine,
which absorbs at 540 nm. A clue to the origin of these bands lies
in the simultaneous appearance of the broad near-infrared
absorbances that are almost identical to those observed in the
spectroelectrochemistry of the oxidized parent spiropyran SP1
(Figure 3b). All of these bands appear to arise from the SP
substituent because an identical spectroelectrochemical experi-
ment involving poly-TThMA (see Supporting Information,
Figure S2) does not show peaks at 443 and 477 and at 870
and 995 nm following initial polymer oxidation. Because the
merocyanine groups are fixed in space by the polymer backbone,
the formation of large aggregates is unlikely. However, interac-
tion of two merocyanines with antiparallel dipoles could arise
from merocyanines on neighboring polymer chains forming π-
dimers. The two peaks at 443 and 477 nm (Figure 7c) could
represent dimers formed from the two major merocyanine
isomers TTC and TTT (see Scheme 2).
Figure 6. Absorption spectra response at 540 nm for (a) poly-TThSP1
(black line) and poly-TThMA (gray line), when the applied potential
was changed from ꢀ0.1 to 0.9 V in 0.1 M TBAP in acetonitrile (b).
To probe the reversibility of this merocyanine formation and
oxidation, we monitored the 540 nm absorbance over the
potential range ꢀ0.1 to 0.9 V during cyclic voltammetry
(Figure 6). A freshly electrodeposited film was utilized to ensure
that the polymer was in the SP form. In addition, the same
experiment was carried out with a freshly prepared poly-
TThMA. As seen in Figure 5a, the absorbance at the potential
ꢀ0.1 V results from the polymer backbone, and, as the polymer
becomes oxidized, the absorbance decreases (Figure 6a, black
line). When the potential applied to the poly-TThSP1 film is
more positive than 0.6 V, the absorbance at 540 nm increases
rapidly as a result of MC isomer formation. In contrast, poly-
TThMA film (Figure 6a, gray line) does not show any absor-
bance changes at the positive potential range, as is expected for a
poly(thiophene).
The removal of a single electron from this type of π-dimer to
form a π-radical cation dimer might then be expected to be easier
than oxidation of a single merocyanine substituent. The forma-
tion of π-radical cation dimers in aromatic species is well
established in the literature.22,32,33 Both the spectroelectrochem-
istry of SP1 solutions (Figure 3b) and that of the poly-TThSP1
We also observed following poly-TThSP1 oxidation for the
first time (t = 0, Figure 6a), the same level of absorbance on
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Figure 7. Spectroelectrochemistry of poly-TThSP1 on ITO recorded six times at the potentials ꢀ0.1 V (a), 0.6 V (b), 0.8 V (c), and 1.0 V (d).
film (0.8 V, Figure 7c) are fully consistent with the formation of
merocyanine π-radical cation dimers. In both cases, the oxidation
of the merocyanine moiety leads to low intensity, longer
wavelength peaks between 800 and 1100 nm, that rise and fall
as the intensity of the oxidized merocyanine band around 500 nm
increases. This can be accounted for by the initial formation of π-
radical cation dimers at a low concentration of oxidized mer-
ocyanine followed by disappearance of the dimer with complete
oxidation of the merocyanine.
Scheme 4. Cartoon Representation of Redox Cycling and
Aggregation of poly-TThSP1
The presence of neutral or charged dimeric species or even
larger aggregates as a result of merocyanine formation and
oxidation accounts for the majority of changes seen in the
spectroelectrochemical experiments. To clarify this, we have
depicted the processes occurring during the first and subsequent
electrochemical cycles in cartoon fashion in Scheme 4.
The left-hand side of the scheme represents the oxidation
component of the first cycle with initial polymer oxidation,
followed by formation of oxidized spiropyran that rapidly ring
opens above 0.8 V to oxidized merocyanine. The reduction
component of the first cycle (going up the right-hand side of
Scheme 4) leads ultimately to a neutral merocyanine-substituted
polythiophene that contains both single, as well as dimeric (or
oligomeric), merocyanines. This accounts for the loss of peak
height and the peak broadening in the second and subsequent
spectra of the spectrochemical study at ꢀ0.1 V (Figure 7a). As
the polymer is oxidized and reduced, more ordering of the film
occurs as a result of strong π-radical cation dimer coupling,
leading to an overall increase in neutral dimer formation (and
hence peak broadening due to increasing absorptions around
446 nm). This also may explain the apparent increased difficulty
in reducing the polymer after the first cycle as evidenced by the
increased polaron band at 850 nm.
at 0.6 V (Figure 7b) leads to the loss of the neutral polythiophene
absorption at 552 nm, revealing broad absorptions from 400 to
550 nm that reflect the presence in the oxidized polymer film of
both free merocyanine substituents, as well as merocyanine π-
dimers and possibly higher oligomers. Further oxidation (0.8 V,
Figure 7c) now gives the characteristic bands of both π-dimers
On the second cycle, it is now the presence of merocyanine π-
dimers that dominates the electrochemistry (down the right-
hand side of Scheme 4). Oxidation of the merocyanine polymer
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To ensure that these spectral changes largely resulted from
spiropyran to merocyanine isomerization, irradiation of the
control polymer poly-TThMA was investigated. No change in
the absorption spectra of poly-TThMA was observed (see
Supporting Information, Figure S3) following exposure of poly-
mer film to UV light.
While these experiments demonstrate that the reduced poly-
TThSP1 film, whose SP substituent has never been electroche-
mically oxidized, undergoes the typical reversible SP to MC
isomerization (Figure 1), further experiments were undertaken
to determine whether this was the case for a poly-TThSP1 film in
which the isomerization of the SP moiety had occurred (through
irradiation). TTh-SP1 was electrochemically polymerized on
optically transparent ITO-coated glass by chronoamperometric
deposition at constant potential (0.8 V) in the dark. At this
potential as shown in Figure S1a, the SP substituent should not
be substantially oxidized to MC. The poly-TThSP1 film was
then exposed to UV light at 254 nm for 5 min, and the post-CVs
(from ꢀ0.4 to 0.9 V) obtained with the UV light were still applied
to the polymer film (Figure S4a). As expected, no initial
electrochemical response ascribed to the spiropyran moiety is
observed, suggesting that under these conditions the spiropyran
is all converted to merocyanine, giving poly-TThMC1. How-
ever, in contrast to the previous experiment that only involved
photoisomerisation, the poly-TThMC1 will have been electro-
chemically oxidized (both polymer backbone and MC sub-
stituent) during the post-CV cycling, giving π-cation dimers
and other aggregates. The polymer film was then irradiated with
visible light (390ꢀ750 nm) for 5 min, and the CVs again were
recorded over the same potential range while maintaining the
visible light irradiation (Figure S4b). Clearly, there is some
photochemical regeneration of spiropyran in the film, as evi-
denced by the appearance of a new peak at 0.55 V in the first CV
scan, and its subsequent loss in the second scan. The amplitude of
the peak is low, suggesting that only a fraction of the merocyanine
substituents in the film are isomerized. Given that the poly-
TThMC1 film is likely to be composed of both free and
aggregated merocyanine substituents following oxidation, spir-
opyran formation would be expected to be sterically limited. In
addition, the isomerization must be slow, certainly slower than
the time frame of the cyclic voltammetry, because almost no
spiropyran is reformed during the subsequent CV scans. This is
perhaps not surprising because the reduced merocyanine film
would be much more compact than that of the reduced spir-
opyran film, particularly with increased formation of π-dimers,
making it sterically and energetically more difficult to reform
spiropyran.
Figure 8. UVꢀvis spectra of reduced poly-TThSP1 after electroche-
mical deposition (blue line), after first irradiation with 254 nm UV light
(red line), followed by irradiation with visible light (green line), and
second irradiation with 254 nm UV light (violet line).
and π-radical cation dimers as well as both oxidized (∼540 nm)
and neutral merocyanines (∼560 nm) on the oxidized polymer
backbone. Finally, full oxidation of the merocyanine groups
above 1.0 V (Figure 7d) leads to separation of the merocyanines
with only the oxidized merocyanine band at 540 nm apparent in
the spectrum.
Given that the spiropyran cannot be regenerated electroche-
mically, we investigated the feasibility of photochemical control
of this process to achieve complete reversibility of this intriguing
redox system.
3.5. Light Control of SP1 Isomerization. The photochemical
conversion of merocyanine to spiropyran is typically an easily
identifiable process, as we observed in the study of SP1, because
the merocyanine absorption spectrum is largely well separated
from the spiropyran spectrum. However, in the case of poly-
TThMC1, the merocyanine absorption (assumed to be around
560 nm as for TThMC1) is at wavelengths similar to that of the
reduced polyterthiophene backbone (380ꢀ670 nm), making
identification of merocyanine formation difficult.To ensure the
presence of spiropyran in the polymer, a thin film (<1 μm) of
poly-TThSP1 on ITO glass was CV electrodeposited between 0
and 0.75 V (see Figure S1a), and its UVꢀvisible spectrum was
recorded (Figure 8, blue line), following washing to ensure the
absence of excess monomer from the polymerization. Irradiation
of the dried film with ultraviolet (UV) light (254 nm, 5 min)
afforded a more intense and broad UVꢀvisible band (Figure 8,
red line) shifted by 20 nm. This is consistent with the formation
of poly-TThMC1, because the intensity of the polythiophene
backbone absorption would be expected to remain constant but
the growth of a new absorption centered around 560 nm would
lead to a more intense broader band overall. A small broad
absorption at 383 nm also appears due to the MC formation.
Subsequent irradiation of the polymer film with visible light (40
min) gave a polymer absorption band (Figure 8, green line)
identical to that of poly-TThSP1, albeit with a slightly higher
intensity. A second UV light illumination returned the higher
wavelength absorption (Figure 8, violet line), again with small
intensity differences. These intensity changes may well be due to
conformational changes to the polymer backbone as a result of
the spiropyran to merocyanine isomerization.
3.6. Physical Properties of poly-TThSP1. It is well-known
that conducting polymers display actuation behavior under an
external redox potential, due to the movement of counterions
and associated solvent molecules into and out of the polymer,
which results in swelling and contraction.34 According to
our earlier spectroelectrochemical study, the initial oxidation of
the spiropyran moiety provides some merocyanine dimer or
oligomer aggregates within the polymer film. These processes
should markedly affect the morphology of poly-TThSP1, and for
this reason scanning electron micrograph (SEM) analysis of
the different redox stages of the polymer was undertaken. SEM
images were obtained for four poly-TThSP1 samples electro-
polymerized in the same way but then subjected to different
redox conditions. Three of those samples were oxidized
or reduced at constant potential for 2 min. The first image
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a terthiophene monomer modified with a nitrospiropyran sub-
stituent. The resulting polymer, poly-TThSP1, displayed a
number of different colored states on electrochemical redox
cycling with associated complex electrochemistry. To under-
stand this, we undertook a detailed electrochemical study of the
nitrospiropyran precursor SP1 because little had been reported
in the extensive spiropyran literature. We have shown that SP1
initially undergoes irreversible electrooxidative ring-opening to
give at least two oxidized merocyanine isomers, which can further
undergo reversible redox processes. This behavior is reflected in
the electrochemical processes observed for poly-TThSP1. The
polymer is initially formed with the spiropyran intact and can
undergo typical polythiophene redox cycling at potentials below
0.6 V. However, oxidation of the polymer film above 0.8 V leads
to spiropyran oxidation and irreversible isomerization to the
oxidized merocyanine. A detailed spectroelectrochemical study
of the resulting reduction and further redox cycling revealed that
the electrochemical and physical properties of the polymer are
then dominated by the presence of the merocyanine and its
ability to form π-dimers or oligomers and π-radical cation
dimers.
The photochemical behavior of poly-TThSP1 is dramatically
influenced by the electrochemistry. Typical spiropyran to
merocyanine isomerization within the polymer occurs
only if the polymer film has been exposed to oxidation potentials,
which do not oxidize the spiropyran moiety to the merocyanine
(below 0.8 V). Once the merocyanine is oxidized, the
photochemical behavior of the film is limited. Therefore, the
redox properties of poly-TThSP1 can be used to control its
photochemical behavior. As expected, given the large conforma-
tional differences between spiropyrans and merocyanines, there
are significant morphological changes in the polymer film,
although the polymer conductivity does not appear to be
dramatically affected and is similar to that of previous
polythiophenes.
Figure 9. SEM images of poly-TThSP1 after a constant potential
deposition of 0.8 V (a), oxidation at 0.5 V (b), oxidation at 0.85 V
(c), and reduction at ꢀ0.4 V (d).
in Figure 9a shows the morphology of the fresh film, obtained
after 1 min constant potential deposition at 0.8 V. The second
polymer sample was oxidized at 0.5 V (Figure 9b) from which we
expect only polymer backbone oxidation, while the third one was
further oxidized to 0.85 V (Figure 9c), leading to oxidation of the
spiropyran substituent on the polymer backbone. The last
sample was reduced with the potential ꢀ0.4 V (Figure 9d).
The morphology of the fresh poly-TThSP1 (Figure 9a) shows a
globular structure, with features approximately 1 μm in diameter
present on the polymer surface. Because the polymer backbone is
oxidized with associated perchlorate anions and the bulky
spiropyran is still present, this film would be expected to show
the most expanded morphology. The film oxidized state at 0.5 V
(Figure 9b) showed a similar morphology with fewer of the
island-particles present on the surface. This would be consistent
with the loss of some of the spiropyran groups following the
initial film preparation. Significantly, further oxidation of the
polymer to 0.85 V (Figure 9c) gave a film with an entirely
different morphology with a distinctive webbed and close-packed
structure, consistent with the formation of at least partially
oxidized planar merocyanine.
AFM images confirm the major change in polymer structure
that occurs when the poly-TThSP1 film is oxidized (Figure S5),
because the calculated surface roughness of the reduced (root
mean squared, rms = 181 ( 8.4 nm) and oxidized (127 (
5.1 nm) poly-TThSP1 suggest a more porous surface for the
reduced polymer, as would be expected for the more bulky SP
substituent.
To measure the conductivity, a free-standing film of poly-
TThSP1 was obtained after chronoamperometric deposition on
ITO-coated glass at a constant potential of 0.8 V for 1 h and
removal from the electrode. A conductivity of 0.4 S cmꢀ1 was
obtained, the same order of magnitude as that reported by
Gallazzi et al. for an analogous poly(alkoxyterthiophene) sub-
stituted with the somewhat smaller electron-withdrawing dicya-
noethenyl group.35
This unique multichromophoric and multiswitchable
polymer system provides an exciting platform for the develop-
ment of future materials that can exploit the other physical
and chemical properties of spiropyrans and merocyanines
such as variation in hydrophobicity, ion and molecular
complexation, and conformational effect leading to polymer
actuation. Studies to this end are underway in our
laboratories.
’ ASSOCIATED CONTENT
Supporting Information. Synthesis of methyl 4,400-dide-
S
b
cyloxy-2,20:50,200-terthiophene-30-acetate (TThMA), 4,400-dide-
cyloxy-2,20:50,200-terthiophene-30-acetic acid (TThAA), and
2-(3,300-dimethylindoline-60-nitrobenzospiropyranyl)ethyl 4,400-
didecyloxy-2,20:50,200-terthiophene-30-acetate (TThSP1), elec-
trochemical polymerization of TThSP1 and TThMA, UVꢀvis
spectroelectrochemistry, SEM and AFM imaging details and
AFM images, conductivity measurements, and a movie file
showing the color changes associated with the CV scan of
poly-TThSP1. This material is available free of charge via the
Internet at http://pubs.acs.org.
4. CONCLUSION
The integration of the photochromic properties of spiropyrans
with the electrical and optical properties of polythiophenes
presents exciting opportunities for a variety of applications. This
inspired us to successfully electropolymerize, for the first time,
’ AUTHOR INFORMATION
Corresponding Author
davido@uow.edu.au
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’ ACKNOWLEDGMENT
(33) van Haare, J. A. E. H.; Groenendaal, L.; Havinga, E. E.; Meijer,
E. W.; Janssen, R. A. J. Synth. Met. 1997, 85, 1091–1092.
(34) Wallace, G. G., Spinks, G. M., Kane-Maguire, L. A. P., Teasdale,
P. R., Eds. Conductive Electroactive Polymers: Intelligent Materials Systems;
CRC Press: Boca Raton, FL, 1996.
Financial support from the Australian Research Council and
funding from Science Foundation Ireland under award 07/CE/
I1147 “CLARITY: Centre for Sensor Web Technologies” are
gratefully acknowledged.
(35) Gallazzi, M. C.; Toscano, F.; Paganuzzi, D.; Bertarelli, C.;
Farina, A.; Zotti, G. Macromol. Chem. Phys. 2001, 202, 2074–2085.
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