Temperature- and voltage-induced ligand rearrangement of a dynamic electroluminescent metallopolymer

Article abstract of DOI:10.1002/anie.201404186

A dynamic-covalent metal-containing polymer was synthesized by the condensation of linear diamine and dialdehyde subcomponents around copper(I) templates in the presence of bidentate phosphine ligands. In solution, the red polymers undergo a sol-gel transition upon heating to form a yellow gel, a process that can be either reversible or irreversible depending on the solvent used. When fabricated into a light-emitting electrochemical cell (LEC), the polymer emits infrared light at low voltage. As the voltage is increased, a blue shift in the emission wavelength is observed until yellow light is emitted, a process which is gradually reversed over time upon lowering the voltage. The mechanism underlying these apparently disparate responses is deduced to be due to loss of the copper phosphine complex from the polymer.

Full text of DOI:10.1002/anie.201404186

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Communications
DOI: 10.1002/anie.201404186
Metallopolymers
Temperature- and Voltage-Induced Ligand Rearrangement of
a Dynamic Electroluminescent Metallopolymer**
Demet Asil, Jonathan A. Foster, Asit Patra, Xavier de Hatten, Jesffls del Barrio,
Oren A. Scherman, Jonathan R. Nitschke,* and Richard H. Friend*
Abstract: A dynamic-covalent metal-containing polymer was
synthesized by the condensation of linear diamine and
dialdehyde subcomponents around copper(I) templates in the
presence of bidentate phosphine ligands. In solution, the red
polymers undergo a sol–gel transition upon heating to form
a yellow gel, a process that can be either reversible or
irreversible depending on the solvent used. When fabricated
into a light-emitting electrochemical cell (LEC), the polymer
emits infrared light at low voltage. As the voltage is increased,
a blue shift in the emission wavelength is observed until yellow
light is emitted, a process which is gradually reversed over time
upon lowering the voltage. The mechanism underlying these
apparently disparate responses is deduced to be due to loss of
the copper phosphine complex from the polymer.
Furthermore, the polymer was electroluminescent and
could be fabricated into light emitting electrochemical cells
(LECs), the simplest class of light-emitting devices formed by
sandwiching a conjugated polymer containing ionically con-
ductive species between two electrodes.[7] The devices showed
voltage-dependent electroluminescence (EL), undergoing
a shift in the wavelength of emission that reverses over
time. Herein we report the synthesis of metallopolymer 1 and
investigate the structural transformation responsible for gel
formation as well as the chemical and physical reasons behind
the color shift observed in the LEC devices.
Polymer 1 was produced by stirring 6,6’-diformyl-3,3’-
bipyridine, benzidine, copper(I) tetrafluoroborate and bis[2-
(diphenylphosphino)phenyl]ether (POP) in dichloroethane at
508C for 3 h (Scheme 1). The heteroleptic metallopolymer
C
onstitutionally dynamic polymers[1] can reconfigure their
structures and properties in response to stimuli in ways that
conventional polymers cannot.[2] We report the self-assembly
of a dynamic-covalent metallopolymer,[3] which undergoes
a reversible structural rearrangement in response to changes
in both temperature in solution and electric field in the solid
state, transforming its mechanical and photophysical proper-
ties. The polymer was formed using the approach of
subcomponent self-assembly,[4] in which the condensation of
aromatic amine and pyridine aldehyde subcomponents is
driven by the preferential coordination of the resulting imines
to metal ions. In solution, the polymer showed both thermo-
chromism[5] and “heat-set” gel formation[3c,6] which can be
either reversible or irreversible depending on the solvent
used.
Scheme 1. Synthesis of polymer 1 from subcomponents.
1
was characterized by H, 13C and 31P NMR spectroscopy.[3c,8]
Diffusion-ordered NMR spectroscopy (DOSY) studies of 1 in
dimethyl sulfoxide (DMSO) showed that its diffusion coef-
ficient decreased upon increasing the sample concentration
(see Supporting Information, SI, Figure S5). This effect is
attributed to the dynamic-covalent nature of the links
between the individual repeating units of this dynamic
polymer.[9] DOSY studies of the discrete dimeric complex 2,
which was formed using 2-formylpyridine in place of 6,6’-
diformyl-3,3’-bipyridine (SI Section S1.3) and is a model for
the repeating unit of 1, indicated that the hydrodynamic
radius of the polymer is ca. 3.3 times that of 2 at a concen-
tration of 10.0 gLꢀ1 in DMSO (SI Section S2.1.2). Compar-
ison of the residual amine peaks with the newly formed imine
peaks indicates a degree of polymerization of approximately
seven repeating units (SI Section S2.1.3), which is consistent
with the DOSY results and indicates that 1 has an average
molecular weight of 12 kDa. The size distribution of 1 was
measured by dynamic light scattering (DLS) in DMSO at
a concentration of 10.0 gLꢀ1, and showed species with sizes in
the 10–20 nm range (SI Section 2.2). Larger species were
[*] D. Asil,[+] Prof. R. H. Friend
Department of Physics, University of Cambridge
JJ Thomson Avenue, Cambridge, CB3 OHE (UK)
E-mail: rhf10@cam.ac.uk
Dr. J. A. Foster,[+] Dr. A. Patra,[$] Dr. X. de Hatten, Dr. J. del Barrio,
Dr. O. A. Scherman, Prof. J. R. Nitschke
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge, CB2 1EW (UK)
E-mail: jrn34@cam.ac.uk
[$] Current address: Organic & Hybrid Solar Cell, Physics of Energy
Harvesting, CSIR-National Physical Laboratory
Dr. K. S. Krishnan Marg, New Delhi-110012 (India)
[+] These authors contributed equally to this work.
[**] This work was supported by the European Research Council, the
Middle East Technical University & Turkish Council of Higher
Education, and the Schlumberger-Faculty for the Future Fellowship
program (D.A.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404186.
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observed in the 50–500 nm range and are inferred to be
polymer aggregates;[3] these aggregates were also observed by
multi-angle static light scattering (SLS) (SI Figure S11).
Polymer 1 dissolved in dimethyl formamide (DMF) to
give a deep red solution. The strong absorption peaks
observed in the UV/Vis spectra of 1 up to 400 nm were
attributed to the p-p* transitions of the polymer backbone
(N^N) and POP ligands, whereas absorption beyond 400 nm
was attributed to the metal-to-ligand charge transfer
(MLCT)[16–18] (Figure 1). The polymer has a photolumines-
several months. Scanning electron microscopy images of the
gels showed the formation of fibrous aggregates ca. 10 nm in
width and hundreds of micrometers in length (SI Figure S19).
The sol–gel transition was followed in a 1% [D6]DMSO
solution by variable temperature NMR (VT-NMR) spectros-
copy in order to elucidate the molecular basis for gelation (SI
1
Section 2.4). The broad H NMR spectrum of the polymer
disappeared upon heating to 908C due to gel formation[10] and
was replaced by sharp peaks corresponding to those of
a [Cu(POP)]+ complex. Comparison with the integrated
intensities of peaks from an added adamantane reference
indicated total displacement of the [Cu(POP)]+ complex from
the polymer upon gel formation. Minimal change was
observed in the NMR spectra of the DMSO sample after
standing at 258C for three days. The transition was further
investigated by 31P NMR and mass spectrometry, which
provided data consistent with the liberation of [Cu(POP)]+
upon heating. Similar behavior was observed to occur in DMF
by NMR spectroscopy (SI Figure S15). Temperatures greater
than 1408C were required to induce gel formation, and the
samples had to be heated outside of the spectrometer and
measured rapidly for [Cu(POP)]+ formation to be observed.
After leaving the sample to cool to room temperature for
30 min the spectrum had reverted to that of the polymer
solution, and after 24 h it matched that of the sample before
heating. Repeated heating and cooling of the sample resulted
in the gradual build-up of an insoluble yellow precipitate.
The formation of polymer 1 was driven by the tendency of
copper(I) to form heteroleptic complexes of the type
CuIP2N2.[8,16] However, heating this polymer in high boiling,
coordinating solvents such as DMSO and DMF results in
Figure 1. UV/Vis absorption (black) and photoluminescence (red) of
the polymer 1 in solution and photoluminescence spectrum of the
polymer in the gel state (orange). The blue line shows the photo-
luminescence of the isolated gel matrix. Inset: Photograph showing
reversible formation of a yellow gel upon heating a red polymer
solution of 1 and the reversible gel forming upon heating.
ꢀ
breaking of the weaker Cu N bonds and release of [Cu-
(POP)]+ into the solution leaving behind the imine-containing
polymer backbone [Eq. (1)].
cence (PL) maximum at 780 nm, also attributed to the MLCT
state. Heating a solution of the polymer (1 wt.%) to temper-
atures above 1608C resulted in its transformation to an
opaque yellow state which did not flow when its container was
inverted, indicating gel formation (Figure 1-inset). The PL
spectrum of the gel showed an emission maximum at 580 nm,
significantly blue-shifted compared to the polymer solution.
The color change, shift in photoluminescence, and gel
formation reversed upon cooling the sample to room temper-
ature. This type of reversible gel formation upon temperature
rise[4,8–10] is the opposite behavior to that observed for most
gelators, which undergo a sol–gel transition upon cooling. We
had previously reported a similar effect in a series of related
polymers, however, the high temperature of gel formation and
poor solubility of the polymers inhibited further study or
device fabrication.[3c]
Heat-set gel formation was also observed to occur in
DMSO at lower temperatures than in DMF, allowing the
process to be monitored using rheometry. When the sample
was heated to 1408C, the elastic modulus (G’) became an
order of magnitude greater than the viscous modulus (G’’)
and behaved linearly over a wide range of frequencies,
indicating gel formation (SI Section S3.1). The DMSO gels
remained stable in the rheometer upon cooling to room
temperature and were observed to be stable in a vial over
½CuðN^NÞðPOPÞꢁþ Ð ½ðN^NÞꢁn þ n ½CuðPOPÞꢁþ
ð1Þ
n
Without the bulky phosphine groups and cationic charge
of the copper, the polymers are inferred to aggregate into the
fibrous structure of the gel matrix observed by scanning
electron microscopy (SEM; Figure S19). In DMF, cooling of
the solution appears to allow the [Cu(POP)]+ groups to
reattach to the vacant pyridyl imine sites, restoring the
polymers and dissolving the gel. In DMSO, we infer that the
lower solubility of the polymers results in phase separation
due to greater aggregation, rendering the process irreversible.
This mechanism is different than the one that we proposed to
explain the heat-set gelation in a related system,[3c] in that we
do not propose here that CuI remains in the polymer.
In order to characterize the PL observed at high temper-
atures, the [Cu(POP)]+ complex and metal-free polymer
backbone [(N^N)] were isolated and their photolumines-
cence spectra were measured separately. Compound 3, [Cu-
(POP)(CH3CN)2]BF4, was synthesized according to a modi-
fied literature procedure (SI Section S1.4) and was found to
show no luminescence. The demetallated polymer backbone
was isolated by forming a gel (1.5 wt.% 1 in DMSO), then
washing the gel repeatedly with DMSO until no [Cu(POP)]+
was detected in the washings by NMR. The spectrum provides
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a good match with the PL of the gel formed in solution
(Figure 1), from which we infer that the organic polymer
backbone is responsible for the PL emission observed.
When polymer 1 was deposited as a thin film on
Spectrosil, neither thermochromism nor PL shift was
observed upon thermal annealing. Differential scanning
calorimetry (DSC) measurements of the solid polymer
showed no evidence of ligand rearrangement or other
transitions taking place up to 2508C. These results indicated
that the transition observed in solution did not take place in
the solid state due to heating. Cyclic voltammetry studies of
polymer films, formed through drop casting 1 onto Pt
electrodes, were undertaken. The voltammogram showed
a peak at 1.2 V during the first cycle which was attributed to
the oxidation of CuI–CuII (SI Figure S20). This peak was
diminished in subsequent cycles and the film changed color
from red to yellow. This electrochromic behavior was
attributed to the loss of the copper ions from the polymer
backbone and indicates that an applied potential can bring
about a transition similar to that observed in solution with
a temperature change.
LEC devices fabricated with polymer 1 showed a similar
color transition in electroluminescence as the PL transition
observed in the solution phase. The LEC devices were
fabricated by spin coating polymer 1 from DCM/DMF
mixture onto PEDOT:PSS-coated ITO before thermally
depositing an aluminum cathode on top. The polymer
showed electroluminescence with an onset voltage of approx-
imately 4 V and luminescence up to 3 Cdmꢀ2. The I-V-L
spectrum for the device is shown in the SI Figure S21. At low
voltage (Vapp < 6 V) the devices emitted light in the IR region
of the spectrum. However, as the voltage was increased,
a blue shift in emission was observed, reaching a lmax of
580 nm at 20 V. The color shift upon increased voltage could
clearly be seen in the photographs taken from the pixels in
operation (inset in Figure 2a).
The electroluminescence spectra of the devices operating
at a low bias correspond well with the photoluminescence
observed for the polymers in solution at low temperature
(Figure 2a). Notably, the electroluminescence spectra of the
polymers at high bias correspond well with the PL spectra of
the gel. This indicates that a similar transition may be taking
place in the devices with the application of voltage as is
observed in solution at high temperatures.
Figure 2. a) Photoluminescence spectrum of the polymer solution in
DMF (1 wt.%) (red), corresponding gel (orange), and electrolumines-
cence spectrum of the device at low bias (shaded red) and high bias
(shaded orange). Arrows indicate the corresponding axis for PL and
EL. Inset: series of pictures taken of the pixel at increasing voltages.
b) Schematic energy level diagram showing the dominant transition
giving rise to electroluminescence emission for the device at low and
high bias.
p-p* transition decreases as the metallopolymer loses its
metallic character under strong bias (Figure 2b). Therefore
the final emission originates from the ligand-centered p-p*
transitions and is in between the HOMO and the new LUMO
states formed in the absence of copper.
In order to understand the driving force behind the
transformation in the solid state, experiments were under-
taken to measure the effect of temperature on the device.
Current densities of up to 1 Acmꢀ2 were reached at bias
voltages above 4 V. This caused a temperature rise in the
device, measured at 748C on the ITO side of the device and
estimated to reach up to 868C inside the device based on heat
transfer calculations (SI Section S6). DSC measurements of
the bulk polymer indicate that no transitions take place up to
this temperature. Moreover, devices which had been ther-
mally annealed at 1608C immediately before testing were
found to behave identically to devices that had not been pre-
heated and started emitting in the IR region when a low bias
was applied.
Remarkably, the EL shift in the LEC devices appears to
be quasi-reversible (Figure 3). The time required for recovery
from yellow emission to IR depended on the device history.
When the applied voltage stayed at moderate levels (< 8 V),
pixels regained their red emission rapidly when the voltage
was lowered. However, once the voltage was increased to
As described previously in the literature, the onset of light
emission in this type of LEC device occurs after the formation
of double layers at the electrode interfaces.[7a,11] Under the
ꢀ
influence of the external bias, the BF4 ions start to migrate
toward the ITO electrode while positively charged polymer
accumulates at the negative electrode. The electrolumines-
cence spectrum that consists of mixed MLCT and ligand-
centered p-p* transitions, blue shifts under strong bias
(Figure 2). As the applied bias is increased, the [Cu(POP)]+
ions become detached from the polymer and drift toward the
cathode. The loss of nonluminescent [Cu(POP)]+ complexes
from the polymer backbone is inferred to result in a new
higher energy emission peak comparable to the PL spectrum
of the metal free N^N ligand (gel). Therefore we conclude
that the ratio of the MLCT transition to the ligand-centered
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Figure 3. EL spectra showing the emission of the ionic transition metal
complex (iTMC) LEC devices at different voltages and over time. The
initial bias was increased from 8 V (red) up to 24 V (orange) until the
yellow emission was persistent. The dotted red line shows the EL
spectrum taken after five days.
24 V, and the devices emitted yellow light, re-biasing the
device to low voltages did not initially result in IR emission.
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emission band had shifted to longer wavelength and red
emission was restored. This is thought to be due to the
[Cu(POP)]+ complexes re-coordinating to the vacant pyridyl-
imine coordination sites, regenerating the metallopolymer, as
we propose to explain reversible gel formation in DMF
solution. Some irreversible reorganization of the polymer
following the application of high bias is likely to account for
the lower emission intensity observed after five days.
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Received: April 10, 2014
Published online: June 24, 2014
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Keywords: copper · gels · luminescence · polymers ·
self-assembly
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