Aqueous self-assembly of an electroluminescent double-helical metallopolymer

Article abstract of DOI:10.1021/ja308055s

A new type of water-soluble copper-containing polymer has been synthesized using the technique of subcomponent self-assembly. Copper(I)-directed imine bond formation between triethylene glycol functionalized 1,2-phenylenediamine and 2,9-diformylphenanthroline subcomponents resulted in the formation of a chain in which two conjugated helical ligand strands wrap around a linear array of metal ions. Characterization data from a variety of analytical methods are consistent with our formulation of this material. After purification by dialysis, the polymer was shown to possess several properties of conceptual and practical interest. (1) Individual double-helical strands appear to further aggregate through entanglement of their side chains to form well-defined superstructures such as nanoscale bow ties and macrocycles, which can be imaged on a surface. (2) The material's copper(I) ions underwent reversible electrochemical oxidation in solution, whereas analogous model compounds were observed to decompose upon oxidation: the polymer's greater length appeared to stabilize oxidized states through delocalization or entrapment. (3) Photophysical measurements reveal this material to be photo- and electroluminescent. It has been successfully used for the fabrication of electroluminescent devices and shows a weak emission of white-blue light with CIE coordinates of (0.337, 0.359). This study further demonstrates the utility of the technique of subcomponent self-assembly for the straightforward generation of materials with useful properties.

Full text of DOI:10.1021/ja308055s

Article
pubs.acs.org/JACS
Aqueous Self-Assembly of an Electroluminescent Double-Helical
Metallopolymer
Xavier de Hatten,† Demet Asil,‡ Richard H. Friend,*,‡ and Jonathan R. Nitschke*,†
†Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
‡Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
S
* Supporting Information
ABSTRACT: A new type of water-soluble copper-containing
polymer has been synthesized using the technique of
subcomponent self-assembly. Copper(I)-directed imine bond
formation between triethylene glycol functionalized 1,2-
phenylenediamine and 2,9-diformylphenanthroline subcompo-
nents resulted in the formation of a chain in which two
conjugated helical ligand strands wrap around a linear array of
metal ions. Characterization data from a variety of analytical
methods are consistent with our formulation of this material.
After purification by dialysis, the polymer was shown to
possess several properties of conceptual and practical interest.
(1) Individual double-helical strands appear to further
aggregate through entanglement of their side chains to form
well-defined superstructures such as nanoscale bow ties and macrocycles, which can be imaged on a surface. (2) The material’s
copper(I) ions underwent reversible electrochemical oxidation in solution, whereas analogous model compounds were observed
to decompose upon oxidation: the polymer’s greater length appeared to stabilize oxidized states through delocalization or
entrapment. (3) Photophysical measurements reveal this material to be photo- and electroluminescent. It has been successfully
used for the fabrication of electroluminescent devices and shows a weak emission of white-blue light with CIE coordinates of
(0.337, 0.359). This study further demonstrates the utility of the technique of subcomponent self-assembly for the
straightforward generation of materials with useful properties.
effort away from designing molecules and toward the design of
chemical systems6,7 that are capable of self-assembling in such a
way as to express the desired materials properties.8−11 Here we
report on the application of this concept to the preparation of a
new kind of conjugated polyelectrolyte LEC.
INTRODUCTION
■
Light-emitting electrochemical cells (LECs) are the simplest
variety of electroluminescent device,1 consisting of a thin layer
of conjugated polymer sandwiched between two electrodes.
The polymer layer contains an ionically conductive species that
allows formation of a light-emitting p−i−n junction.2,3 Unlike
nonionic polymer light-emitting diodes, LECs do not require
careful matching of electrode work functions to the band
structure of the conjugated polymer,4 allowing electrodes to be
chosen from a wide range of conductive materials. Conjugated
polyelectrolytes5 can be used to construct LECs, bringing
further advantages: such polymers can be easier to process,
being soluble in polar solvents such as water, and their use
precludes phase separation between the polymer and ion
conductor during LEC operation.
The development of conjugated polyelectrolyte LECs
represents a shift of complexity away from device fabrication
and into the domain of chemical synthesis, allowing the deep
well of synthesis knowledge to be drawn upon and applied to
the creation of materials and devices that solve a clear and
present problem: the generation of new, energy-efficient
lighting and display technologies.
The polymer we report herein contains complexed transition
metal ions,12 the incorporation of which is beneficial for the
operation of LEDs13−15 and LECs16 because metal centers can
augment spin−orbit coupling to allow radiative emission from
the spin-triplet excitons formed by electron−hole capture, a
process which has allowed devices to reach high efficiencies.
Though platinum17 and iridium18 complexes have been used to
make efficient light-emitting materials, it is desirable to avoid
the use of such scarce and expensive metals; copper, as used
here, has thus been investigated in emissive materials.19−21
Our polymer has also been designed to be water-soluble,
enabling the dynamic covalent polymerization process22 to
occur under ambient conditions in water,23 without requiring a
catalyst and with no undesirable side products. The double
helical polymer consists of a linear array of CuI metal ions
resembling a molecular wire.24−32
The use of chemical self-assembly as a synthetic technique
can further simplify materials preparation by shifting intellectual
Received: August 13, 2012
© XXXX American Chemical Society
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reinforced by the preference of copper(I) to adopt a tetrahedral
coordination geometry42 (Figure 1).
RESULTS AND DISCUSSION
Synthesis. Compounds A and B (Scheme 1) were prepared
by following a modified literature protocol.33 Compound C,
■
a
Scheme 1. Preparation of Water-Soluble Monomer C
Figure 1. Mismatch between subcomponent geometries favors helical
polymers over macrocycles.
a
(a) H2SO4, HNO3, 100 °C, 12 h;33 (b) NaH, (2-(2-methoxyethoxy)-
In order to ensure solubility of the double-helical polymeric
products, the 1,4-phenylenediamine subcomponent C was
synthesized to incorporate two triethylene glycol chains, as
shown in Scheme 1. Water-soluble diamine C was characterized
ethoxy)methanol, DME, rt, 12 h; (c) Pd/C, H2, MeOH, rt, 12 h.
(4,5-bis(2-(2-(2-methoxy ethoxy)ethoxy)ethoxy)benzene-1,2-
diamine), was prepared through catalytic hydrogenation of B.
The polymerization technique chosen for this study was
metal-templated imine34,35 polycondensation. In order to
validate this method in the context of double-helical metal-
lopolymers and to provide spectroscopic points of comparison,
model compounds 1 and 2 were prepared as shown in Scheme
2. We have previously reported a tetracopper helicate
1
by H and 13C NMR, ESI-MS, and elemental analysis (see
Supporting Information).
Model compounds 1 and 2 (Scheme 2) were synthesized and
characterized in order to serve as discrete spectroscopic points
of reference for polymer 3 (Scheme 3). Under the same
Scheme 3. Preparation of Conjugated Metal−Organic
a
Polymer 3
Scheme 2. Preparation of Discrete Model Compounds 1 and
a
2 via Subcomponent Self-Assembly
a
Degassed D2O, 12 h, 80 °C.
conditions and at the same concentrations used for the
preparation of 3, model compounds 1 and 2 were observed to
form cleanly, suggesting that this reaction is suitable for
polymer preparation. The model compounds were charac-
terized by 1H and 13C NMR, ESI-MS, elemental analysis, cyclic
voltammetry (CV), and UV−vis spectroscopy (see Supporting
Information).
Polymer 3 (Scheme 3) was prepared through the aqueous
reaction of equimolar amounts of 2,9-diformyl-1,10-phenan-
throline and diamine C with copper(I).
a
D2O or CD3CN, 65 °C, 12 h.
structurally similar to 2, containing four closely spaced, linearly
arrayed CuI ions.36 Prior studies revealed that copper-imine
subcomponent self-assembly is suitable for the generation of
polymers37 including those with photoluminescent proper-
ties;38 in order to seek new materials with useful photophysical
properties, we thus sought to extend the use of subcomponent
self-assembly39 to the synthesis of double-helical water-soluble
metal-containing polymers.
Polymer chain growth thus involves stepwise dynamic-
covalent40 imine bond formation between phenylene-1,2-
diamine and 2,9-diformyl-1,10-phenanthroline subcomponents
around copper(I) template ions. Both diamine and dialdehyde
are preorganized for the formation of double-helical polymer
strands as opposed to macrocycles,41 a tendency which is
Characterization of 3 was carried out by NMR, elemental
analysis, DLS, SEM, AFM, CV, ESI-MS, UV−vis, and
photoluminescence measurements, as detailed below. All results
were consistent with the formation of a polymeric double
helical structure.43
Upon formation of polymer 3, 1H NMR signals were
observed to broaden and the end-group signals (amine and
aldehyde) disappeared in contrast with spectra of model
compounds 1 and 2, which displayed well-defined NMR
spectra. These spectra (Figure 2) are presented in an
acetonitrile solution because the peaks of 1 and 2 are sharpest
in this solvent; 1H NMR spectra of 3 in acetonitrile (Figure 2)
and water (Figure S4) are both broad over the same spectral
range.
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of discrete analogs.36 The modeled wire-like structure can be
regarded as a linear array of CuI centers surrounded by a pair of
helical conjugated π-systems, surrounded in turn by a sheath of
electrically insulating TEG chains.
Polymer Characterization. The size distribution of the
polymer chains in solution was measured by dynamic light
scattering (DLS), which has proven useful in the study of rigid-
rod polyelectrolytes in water,45 and in the dry state by SEM and
AFM images (images were obtained from a spin-cast of 0.5−5
wt % solutions of the polymer mixture on a silicon surface).
Both the particle size analysis performed by DLS in solution
and the SEM and AFM images, recorded on a dried layer, are
consistent with two regimes of particle sizes. The smallest-
diameter DLS signal (Figure 4) is assigned to the individual
1
Figure 2. H NMR spectra (CD3CN solution) for 1, 2, and 3.
The broad spectra of 3 were consistent with the formation of
longer, rigid chains. The disappearance of the NMR signals
from the terminal amine and aldehyde functionalities showed
that they represent less than 5% of the overall signal, based
upon the detection limit of the NMR, a result consistent with a
polymer chain longer than 10 units on average, where each
monomer unit contains two CuI centers (Scheme 3). After the
polymerization step the mixture was purified by standard
membrane dialysis against a large volume of water using
SnakeSkin filtration with a membrane cutoff of 7.0 kDa, so as to
remove the smaller oligomers and unreacted material. The mass
removed by washing, as measured following the evaporation of
the washing solution, was observed to correspond to less than
0.1 wt % of the overall mass.
Molecular Modeling. In order to visualize the three-
dimensional structure of a polymer strand, molecular mechanics
calculations were performed using the enhanced MM2 force
field of CAChe.44 The minimized structure of an 8-mer
(containing 16 Cu atoms) is shown in Figure 3. The energy-
minimized structure incorporates a linear array of copper(I)
ions, and the helically twisted ligand strands do not appear
more distorted than has been observed in the crystal structures
Figure 4. DLS measurement of size distribution by intensity of a 2 w%
solution of purified 3 in degassed water.
polymer chains. Their diameter in the Stokes−Einstein
approximation ranged from 3 to 11 nm. According to the
MM2 model, this corresponds to a mixture of single chains of 5
to 18 dicopper monomer units. The second signal detected by
DLS, centered on 270 nm with a larger polydispersity, was
assigned to larger aggregates. The intensity of this signal is
consistent with the aggregates representing less than 0.05% of
the total mass of the sample in solution.
Larger aggregates were also observed by AFM (Figure S5)
and SEM (Figure 5). As shown in Figure 5, these micrometer-
scale structures resemble bow ties or twisted loops. According
to the intensity of the scattering, the concentration of
aggregates is approximately 6 orders of magnitude larger on
the surface layer than in solution, which indicates that the
aggregation process was highly concentration dependent.
Figure 3. CAChe44 MM2 minimized structure of a 8-mer (carbon
atoms are represented in gray, oxygen in red, nitrogen in blue, and
copper in orange; hydrogen atoms have been omitted for clarity).
Figure 5. Scanning electron micrographs of unpurified polymer 3 spin-
coated onto a silicon surface from a 1 wt % solution in water.
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We consider the bowtie-like structures observed by SEM to
be the result of nonspecific aggregations between polymer
chains, possibly driven by the entanglement of the TEG side
chains. Indeed, the aggregation of lipophilic structures bearing
pendant ethylene glycol chains has been reported via dipole−
dipole interactions on surfaces and in solution.46−48 Our
inference is supported by the observation that the concen-
tration of aggregates in the solid state is larger than in solution
by roughly 6 orders of magnitude: with fewer water molecules
hydrating the side chains, the attractive interactions between
oligo(ethylene glycol) moieties are expected to be enhanced.
Upon dilution or addition of LiCl to the polymer solution, the
apparent concentration of the aggregated structures was
observed to decrease both in solution and in the solid state.
In the case of dilute polymer (0.2 w%) in a 1 M LiCl solution,
the aggregates were no longer observed by DLS or by surface
imaging. Lithium cations are known to have a strong affinity for
oligo(ethylene glycol) chains;49,50 Coulombic repulsion
between these cations is expected to prevent the polymer
chains to which they are bound from interacting and
aggregation.
Within aggregations, we hypothesize that individual polymer
chains pack parallel to each other so as to maximize contacts
between the TEG side chains. The twisted loops of Figure 5
may thus consist of circular bundles of polymer chains, in which
each chain’s long axis is parallel to the tangent of the cyclic
loop. The relatively large radius of curvature, with respect to the
length of an individual polymer chain, would allow the rigid
polymer helices to avoid bending, and the crossover point at
the center of the ‘figure eight’ would allow for further stabilizing
interactions between TEG chains of parallel polymer chains
from opposite sides of a circular aggregate.
UV−vis spectra were taken of 1, 2, and 3 in order to probe
their electronic structures. These spectra are shown in Figure
6a. The UV regions (up to 380 nm) are characterized by ligand-
centered (LC) bands, associated with the π−π* transitions of
conjugated aromatic systems. The bands in the visible range are
weaker than those in the UV; the former are assigned to metal-
to-ligand charge transfer (MLCT) transitions.51,52 The
similarities between the MLCT absorption features of 2 and
3 suggest that the ground state symmetries and electronic
configurations of both entities are similar. The slight difference
in UV range is attributed to the extension of the aromatic π-
system in 3.
Optical band gaps were calculated by determining the low-
energy absorption onset values. A band gap of 2.49 eV was
measured for 1, whereas band gaps of 2.19 and 2.14 eV were
observed for 2 and 3 respectively. The closeness of the energy
gap values of 2 and 3 suggest that linear extension does not
greatly affect the band gap for oligo-helicates of this type, in
keeping with previous theoretical predictions;36 longer
polymeric chains appear thus unlikely to display much lower
band gaps than that of 3. We posit this limited degree of
decrease in band gap to be a result of the helical twisting of the
polymer strands around the linear array of copper(I) template
ions, which limits the degree of conjugation.
Compounds 1, 2 and 3 exhibited weak photoluminescence
(PL) in dichloromethane upon excitation into the MLCT band
region (λex = 407 nm), as shown in Figure 6b (see Figure S6 for
the PL spectra in water). The emission band is broad (450−
750 nm) and exhibits a λmax at 557 nm for 1 and 561 nm for 2
and 3. Upon absorption of a photon, an electron is promoted
into one of the ligands, forming a photo-oxidized metal
Figure 6. (a) UV−vis absorbance. (b) PL behavior of 1, 2, and 3 in
CH2Cl2 (λex = 407 nm).
center.53 Previous work36,54 suggests that this oxidized state is
delocalized across multiple metal centers, as the proximity of
the linearly arrayed copper(I) ions results in orbital overlap and
a HOMO spanning multiple copper centers. The ground-state
geometry of an oxidized CuI4 helicate was calculated to be
compressed along the helical axis with respect to the ground
state of the nonoxidized parent.36 In analogy to the case for
bisphenanthroline CuI complexes,52,55−57 which are known to
undergo changes in ligand geometry upon MLCT excitation,
we therefore infer that PL originates from the MLCT state and
these compounds undergo structural changes during and
following excitation, as evidenced by the large Stokes shift
between absorption and PL spectra. The low PL intensity of
compounds in both film and solution is attributed either to a
slow radiative decay rate or to a fast nonradiative decay channel.
Attempts were made to measure the radiative decay rate using
time-correlated single photon counting (TCSPC), but it was
found that the decay was faster than the detection limit (150
ps) of this technique.
Electrochemistry. Cyclic voltammetry (CV) studies were
carried out on 1, 2, and 3 at room temperature in nitrogen-
purged 50:50 (v/v) CH2Cl2/EtOH solutions. Redox processes
are reported versus the Ag/AgCl gel reference electrode. None
of the complexes showed a reduction couple within the solvent
window; therefore only oxidation properties were studied. As
shown in Figure 7, polymer 3 was observed to be the most
electrochemically stable species, showing a quasi-reversible
metal-centered redox process with the half-wave potential
(E1/2) at 0.52 V. The oxidation peak at 0.59 V was assigned to
metal-centered oxidation, while the return peak at 0.45 V was
attributed to the corresponding reduction.58 The absence of
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irreversible oxidation during the first oxidation cycle; the
appearance of additional CV features upon cycling is consistent
with chemical rearrangement and decomposition occurring
following oxidation, as has been observed in related
structures.42
The highest occupied molecular orbital (HOMO) levels
were estimated with reference to the energy level of Fc and
using the oxidation onset of compounds via eq 1:
EHOMO = −[Eox−onset + 4.8 eV]
(1)
The HOMO level of Fc was taken as 4.8 eV below vacuum59
and E1/2Fc/Fc+ to be 450 mV vs Ag/AgCl. The HOMO level of 3
was thus calculated to be −5.23 eV below vacuum level, and the
corresponding lowest occupied molecular orbital level
(LUMO) was calculated by subtracting the optical band gap
(2.14 eV, as noted above) from the HOMO level obtained
through CV. The LUMO level of 3 calculated by this method
was −3.09 eV. As reported recently,60 HOMO−LUMO levels
estimated by this method do not give an exact value for the
electron affinity but, nevertheless, help to make a comparison
between different polymers and allow qualitative trends to be
established.
Device Fabrication. LECs are composed of a luminescent
semiconductor material in an ionic environment.1,19 Lumines-
cent materials studied include conjugated polymers in
combination with inorganic salts and ionic transition-metal
complexes (iTMCs). Due to their poor emission properties,
unsubstituted bisphenanthroline CuI complexes have not been
extensively studied to date.61 Homoleptic substituted bi-
sphenanthroline CuI complexes have attracted more attention
due to their better luminescence properties, but their
electroluminescence (ECL) properties have not been reported
yet. Heteroleptic complexes involving bulky phosphane ligands,
on the other hand, have received a great deal of attention due
to their better luminescence efficiencies, and several of them
have been employed in iTMC based LECs.20,62−64 Owing to
the π-acidity of the phosphane ligands, nonradiative deactiva-
tions are decreased; the MLCT excited states are found at a
higher energy level compared to homoleptic bis-
(phenanthroline) CuI complexes.65 Additionally, the excited
state distortion is sterically hindered due to the bulkiness of the
phosphane ligands. However, the main drawback of LECs
containing heteroleptic CuI complexes is their short lifetime
due to the chemical instability of the complexes during device
operation.20,65 Investigations of homoleptic CuI complexes
represent an alternative approach, complementary to current
avenues of investigation for new luminescent materials.
We present herein the first transition-metal-containing
conjugated-polymer-based LEC (iTMP LEC). In polymer 3,
the ionic matrix was made indivisible from the main chain of
the polymer, in order to speed up the diffusion of the emitting
material to the electrodes and potentially improve the quantum
efficiency of the LEC. The devices, as shown in Figure 8, were
prepared by spin coating 3 from CH2Cl2 (20 mg/mL), yielding
a final thickness of 100 nm on an ITO substrate, with no
addition of ionic matrix. In solution and solid state, 3 was
observed to be air-stable. Nevertheless, in order to prevent
oxidation and contamination, the devices were fabricated in a
glovebox.
Figure 7. Cyclic voltammograms of 1, 2, and 3 in 0.1 M TBAPF6
CH2Cl2/EtOH 50:50 (v/v) at 100 mV s−1.
additional peaks at higher potential and the reversibility of the
redox cycle suggest that the polymeric architecture of 3 lends
stability against decomposition following oxidation of a portion
of the copper(I) centers of the chain.
The CVs of 1 and 2, in contrast, showed a quasi-reversible
redox process with a gradual decrease of current and
disappearance of the corresponding reduction peak, showing
that the complex was not stable upon successive cycling. The
redox process showed anodic to cathodic peak current ratios
(ia/ic) of 1.81 and 0.76 for 1 and 3 respectively, indicating
quasi-reversible systems, and peak-to-peak separations (ΔEpp)
of 0.15 and 0.14 V respectively. To provide a point of
comparison with a reversible redox process, ferrocene+/0
(Fc+/0) exhibited a ΔEpp of 0.502 V and ia/ic = 1.32 under
the same experimental conditions. Complex 2 displayed an
The devices thus prepared showed weak electroluminescence
upon applying a bias of 15 to 25 V, as depicted in Figure S7.
Turn-on voltage and the luminescence were found to be
dependent on the thickness of the active layer. Heteroleptic CuI
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chain ends, which may be terminated by either aldehydes or
amines and which may feature two-, three-, or four-coordinate
copper(I) ions or metal-free sites. Chemically different emitting
centers are known to contribute to emission out of proportion
to their relative abundance via energy transfer following
excitation in an ECL device,71 which could result in the
observed broad-spectrum emission in the present case.
We believe that the emission efficiency was weak due to the
following three features of the molecular structure of the
polymer. First, the film morphology was not optimal, as it was
observed from AFM studies that the polymer forms aggregates
in acetone and water which might quench the PL through
formation of interchain excited state species that relaxed
nonradiatively. Second, the TEG solubilizing chains on the
polymer act as insulators and hinder the mobility of holes and
electrons, increasing resistivity and explaining why high voltage
is required to turn the device on. Third, the double-helical
shape of the polymer results in a nonplanar π-system, which in
turn reduces conjugation efficiency. Efforts to mitigate these
factors will be the focus of our design efforts for the next
generation of iTMP LEC materials.
Figure 8. Device architecture on a glass substrate patterned by
transparent ITO as the anode, 3 as the emissive layer, and Al as
cathode.
complexes in similar device architectures have been shown to
require similar turn-on voltages (18 V in ref 20 and 25 V for 40
s in ref 64). In contrast to LEDs, the emission in LECs can only
occur after ionic double layers have been built up at the
electrode interfaces.66 Upon application of a constant bias, the
current density and brightness of the device increase with time
due to the motion and redistribution of ions. Therefore LECs
do not respond to applied bias instantaneously. Response
times, i.e. the time to reach the maximal brightness, are known
to range from several minutes to hours.66 It is known from
extensive work on LECs based on ionic complexes of
ruthenium and iridium complexes that iTMC LECs exhibit
particularly long response times owing to the relatively low ion
mobility.67,68 Long response times and prebiasing were also
observed for iTMC LECs employing heteroleptic CuI
complexes. However in our case the device switched on within
seconds of applying the bias voltage, and without any need for
prebiasing.
CONCLUSIONS
■
We have designed, synthesized, and fully characterized a
multifunctional polymer using the technique of subcomponent
self-assembly. The polymerization process is environmentally
friendly as it occurs in water without the help of a catalyst. A
simple SnakeSkin filtration from water is the only purification
step required. The polymer thus obtained shows superstructure
formation in the solid state and in solution via entanglement of
its peripheral TEG chains. The polymer shows a quasi-
reversible CV signal, consistent with cooperative stabilization of
the partially oxidized extended metal ion chain. Promising
photophysical properties were observed for the novel ionic
transition metal containing a double helical polymer system,
and the fabrication of the first white light emitting iTMP LEC is
reported. Future work will focus upon investigation of
structure−property relationships relevant to the optimization
of the electrical conductivity and light-emission properties of
this new class of materials.
The ECL spectrum is shown for polymer 3 in Figure 9. A
broad spectrum was observed, peaking at 500 and 650 nm,
EXPERIMENTAL SECTION
Reagents and solvents were purchased from Sigma-Aldrich and used
■
without further purification unless otherwise stated. Cu-
72
(CH3CN)4BF4 and 4,5-bis(2-(2-(2-methoxyethoxy)ethoxy)thoxy)-
benzene-1,2-diamine were synthesized according to modified literature
protocols (described below).33,73 Polymer 3 was purified using
SnakeSkin filtration with a cutoff of 3.5 or 7 kDa. DMSO-d6 was
1
purchased from Eurisotop and used without further purification. H
Figure 9. Absorption in CH2Cl2 (black), PL in CH2Cl2 (blue), PL of
thin film (red), and ECL from the device ITO/3 (100 nm)/Al
(dashed-black) of compound 3. Inset: picture of the white-blue light
emitting pixel from the device ITO/3 (100 nm)/Al.
and 13C NMR spectra were recorded on a Bruker Avance 400 or 500
spectrometer. Chemical shifts are reported in ppm downfield from the
residual solvent peak for H and 13C, or from the methyl signal of
1
t
added BuOH (at 1.24 ppm or 70.36 ppm, respectively) in D2O.
Electronic absorbance spectra were measured in D2O or chloroform
with a Cary 100 spectrometer. Dynamic Light Scattering (DLS)
measurements were recorded at 20 °C in water on a Malvern Zetasizer
Nano Z. In a typical experiment 20 mg of the purified polymer were
dissolved in 10 mL of degassed water. The measurements were
performed at 20 °C, and 16 measurement cycles were repeated 3
times. The polydispersity obtained was 0.65. MALDI-TOF mass
spectra provided by the EPSRC National MS Service Centre at
Swansea were run on an Applied Biosystems 4700 Proteomics
instrument; the method used was Positive Reflector Mode (PRM), the
sample was scanned from 500 to 5000 m/z, and the laser intensity was
set at 4230. Low-resolution electrospray ionization mass spectra (ESI-
which gives a white-blue color with CIE coordinates (0.337,
0.359) (see Supporting Information, Figure S8). The ECL
spectrum appears shifted and broadened compared to PL
spectra. The precise physical underpinnings of this phenom-
enon, which has been observed in other systems ranging from
nanoparticles69,70 to organic polymers,71 are still under
investigation. In the present system, the broadening and
shifting of ECL may be due to the polydisperse size distribution
of individual polymer chains, as reflected in DLS measurements
(Figure 4), as well as the presence of different functionalities at
F
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MS) were obtained on a Micromass Quattro LC, infused from a
Harvard Syringe Pump at a rate of 10 μL/min.
(2-(2-methoxyethoxy)ethoxy)methanol (A) (22 mmol, 3.01 mL)
was diluted in dry DME (dimethoxyethane, 100 mL). The flask was
then purged with nitrogen and held under positive nitrogen pressure.
NaH (60% dispersion in mineral oil) (2.5 equiv, 25 mmol, 600 mg)
was added to the flask under a continuous nitrogen stream. The
resulting mixture was subsequently stirred at room temperature until
gas evolution stopped (approximately 1 h). 1,2-Difluoro-4,5-
dinitrobenzene (1 equiv, 10 mmol, 2.04 g) was then added in
portions (over 30 min) with a continuous nitrogen stream. The flask
was then closed, and the resulting slurry was stirred at room
temperature overnight under a positive nitrogen pressure. The flask
was then placed in an ice bath, and the reaction was quenched by slow
addition of cold distilled water (1 mL). The resulting mixture was
filtered through Celite, and the Celite was washed with 50 mL of
dichloromethane. The solvent were subsequently removed by rotary
evaporation. The residue was further purified by column chromatog-
raphy on silica gel with dichloromethane/methanol (99/1 to 98/2) as
the eluent. After evaporation of the solvent, 1,2-bis(2-(2-(2-
methoxyethoxy)ethoxy)ethoxy)-4,5-dinitrobenzene (B) was recov-
The model of 3 shown in Figure 3 was generated through energy
minimization (augmented MM2 force field, 300 cycles) using the
CAChe44 software package. Atomic Force Microscopy (AFM)
measurements were performed on a Dimension 2100 (Veeco
Instruments, Santa Barbara, USA) in tapping mode. Phase and height
images were taken at speeds of 0.5−1 lines/s using gold-coated silicon
tips with spring constants of ∼20−40 N/m. Scanning Electron
Microscopy (SEM) was carried out on a LEO Ultra 55 SEM with a
field emission source. Accelerating voltages of 3−5 keV were used. The
samples were sufficiently conducting to allow for successful imaging
without requiring additional coating.
The electrochemical properties of compounds were determined by
CV. Experiments were carried out in a three-electrode electrochemical
cell at room temperature under N2. A CH2Cl2−EtOH 50:50 (v/v)
solution containing a tetrabutylammonium hexafluorophosphate
(TBAPF6) (0.1 M) supporting electrolyte was used as the working
solution, and a Pt disk, as the working electrode. Pt wire and Ag/AgCl
electrodes were used as counter and reference electrodes, respectively.
Absorption measurements, on film and in solution, were performed
on a diode array Hewlett−Packard 8453 UV−vis spectrophotometer
with a spectral range of 190−1100 nm and an approximately 1 nm
resolution. Data were normalized, and background correction was
done by a blank spectrosil. The solutions for UV−vis absorption
spectroscopy were prepared by diluting solutions from stock solutions
(5 mg/mL of solvent). The dilutions were 1/16, 1/10, and 1/12 in
CH2Cl2 and 1/12, 1/10, and 1/10 in water for 1, 2, and 3, respectively.
PL measurements in solution and on film were done by a pulsed
407 nm laser excitation source, a 10 MHz diode laser (PicoQuant
LDH400), and the luminescence was detected using a microchannel-
plate photomultiplier (Hamamatsu Photonics) coupled to a
monochromator using a 500 mm spectrograph (SpectraPro2500i,
Princeton Instruments) combined with a charge-coupled device
(CCD) camera (PIXIS 100-F, Princeton Instruments).
Device Preparation. iTMP LECs were fabricated on indium−tin
oxide (ITO) patterned glass substrates. Substrates were cleaned in an
ultrasonic bath with acetone and propan-2-ol (IPA) for 15 min each,
and an oxygen plasma treatment was performed for 10 min at 250 RF
shortly before spin coating. The emissive layer was spin coated in the
glovebox with a thickness in the range of 100 to 200 nm. The final step
for device fabrication was the deposition of the top electrode and its
encapsulation. An aluminum (Al; 100 nm) cathode was fabricated by
thermal evaporation under vacuum (10−6 mbar) onto the device
utilizing a shadow mask. The intersection of the ITO and the metal
electrodes gives an active device area of 4.5 mm2. The EL spectra were
recorded using a multimode optical fiber (diameter = 600 μm)
attached to an intensity-calibrated Ocean Optics USB2000 spec-
trometer while applying a constant voltage from a Keithley 2400
source meter.
1
ered (1.1 g, 22%). H NMR (400 MHz, CDCl3): δ = 7.46 (2H, s),
4.29 (2H, t, J = 4.7 Hz), 3.89 (4H, t, J = 4.6 Hz), 3.71 (4H, t, J = 4.8
Hz), 3.63 (8H, m), 3.52 (4H, t, J = 4.5 Hz), 3.52 (6H, s).
Synthesis of 4,5-Bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-
benzene-1,2-diamine (C). In a round-bottom flask equipped with a
large magnetic stir bar, 1,2-bis(2-(2-(2-methoxyethoxy)ethoxy)-
ethoxy)-4,5-dinitrobenzene (B) (350 mg, 0.71 mmol) was dissolved
in methanol (35 mL). A catalytic amount of 10% Pd/C (35 mg) was
added to the solution. The mixture was subsequently placed under a
positive pressure of hydrogen using a balloon and needle. The reaction
was left to stir at room temperature overnight, resulting in the
complete conversion of the nitro groups to amines. The reaction
mixture was then filtered through Celite, and the Celite was washed
with methanol (20 mL). The solvents were then removed by rotary
evaporation to yield the pure 4,5-bis(2-(2-(2-methoxyethoxy)ethoxy)-
ethoxy)benzene-1,2-diamine (C) as a dark green oil (Quant.). The
1
product was used without further purification for the next step. H
NMR (400 MHz, D2O): δ = 7.46 (2H, s), 4.29 (2H, t, J = 4.7 Hz),
3.89 (4H, t, J = 4.6 Hz), 3.71 (4H, t, J = 4.8 Hz), 3.63 (8H, m), 3.52
(4H, t, J = 4.5 Hz), 3.52 (6H, s). 13C NMR (500 MHz, CD3CN):
150.4, 145.5, 118.4, 71.5, 70.2, 70.0, 69.9, 69.3, 69.1, 57.9. ESI-MS: m/
z: 433.5 ([M + H]+).
Synthesis of Polymer 3. In a typical experiment the crude 4,5-
bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)benzene-1,2-diamine
(C) (320 mg 0.69 mmol) was dissolved in freshly degassed D2O (7.0
mL) and placed in a 50 mL Schlenk flask equipped with a magnetic stir
bar under a nitrogen atmosphere. To this solution, Cu(CH3CN)4BF4
(216.6 mg, 0.69 mmol) and 2,9-diformyl-1,10-phenanthroline (162.8
mg, 0.69 mmol) were added under a continuous nitrogen stream. The
resulting mixture was stirred in an oil bath at 80 °C overnight. Solvents
were then removed under reduced pressure, and the residue dissolved
in acetone (5 mL). The subsequent addition of pentane (50 mL)
resulted in the separation of a brownish oil. Collection of the oil and
subsequent removal of volatiles under reduced pressure afforded the
desired polymer 3 in 81% yield. 1H NMR (400 MHz, D2O): δ = 9.0−
6.0 ppm (broad aromatic signals), 4.4−2.5 ppm (broad TEG signals).
No signals were observed in the 13C NMR. ESI-MS: m/z: 671.74
([2Phe + 2Dia + Cu + H]2+), 878.35 ([2Phe + 3Dia + Cu + H]2+),
931.27 ([2Phe + Dia + Cu]+), 988.38 ([3Phe + 3Dia + Cu + H]2+),
1123.42 ([Phe + 2Dia + Cu]+. Elemental analysis calcd for the
polymeric species (C34H40BCuF4N4O8)n·3nH2O: C, 48.78; H, 5.54; N,
6.67. Found: C, 48.56; H, 4.84; N, 6.74.
Synthesis of Model Compound 2. In a Schlenk flask equipped
with a magnetic stir bar, freshly prepared 4,5-bis(2-(2-(2-
methoxyethoxy)ethoxy)ethoxy)benzene-1,2-diamine (C) (43.3 mg,
0.1 mmol), Cu(CH3CN)4BF4 (62.8 mg, 0.2 mmol), 2,9-diformyl-1,10-
phenanthroline (47.2 mg, 0.2 mmol), and 2-aminoethanol (12.2 mg,
12.05 μL, 0.2 mmol) were dissolved in degassed CD3CN (2.0 mL).
The flask was placed under a nitrogen atmosphere by three vacuum/
nitrogen cycles. The resulting mixture was stirred in an oil bath at 65
°C overnight. The reaction was subsequently cooled to room
Synthesis of 1,2-Difluoro-4,5-dinitrobenzene (A). Following a
modified literature procedure,33,73 a 500 mL two-necked round-
bottom flask equipped with a reflux condenser, a large magnetic stir
bar, and a Suba seal was placed in an ice bath (0 °C). 1,2-
Difluorobenzene (12.6 g, 110 mmol) was charged into the flask,
followed by the careful addition of 40 mL of H2SO4 (95%) and 100
mL of fuming HNO3. The resulting mixture was stirred at room
temperature for another 2 h. The flask was then placed in an oil bath
and slowly heated to 100 °C over a period of 2 h. The reaction mixture
was then stirred at 100 °C for 12 h. The flask was subsequently cooled
to room temperature, and the content was poured slowly with
constant stirring over 500 mL of crushed ice. The white needle-like
solid product was then collected by suction filtration and washed with
500 mL of distilled water. The white powder was then thoroughly
dried under vacuum (12 h at room temperature) to yield 8.7 g (38%)
1
of 1,2-difluoro-4,5-dinitrobenzene. H NMR (400 MHz, CDCl3): δ
= 9.18 (2H, t, J = 7.5 Hz).
Synthesis of 1,2-Bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-
4,5-dinitrobenzene (B). Into a two-necked 250 mL round-bottom
flask equipped with a large magnetic stir bar, a Suba seal, and a gas tap,
G
dx.doi.org/10.1021/ja308055s | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
temperature and 20 mL of diethyl ether were added to the reaction
mixture, resulting in the formation of a brown precipitate. The
supernatant was decanted, and the solid was thoroughly dried under
vacuum to afford 2 as a dark brown solid (86%). 1H NMR (400 MHz,
CD3CN): δ = 8.82−8.73 (8H, m), 8.60 (4H, s), 8.41 (8H, d, J = 7.9
Hz), 8.36 (4H, s), 8.22−8.15 (8H, m), 8.06 (4H, d, J = 7.9 Hz), 7.99
(4H, d, J = 8.9 Hz), 7.91 (4H, d, J = 8.0 Hz), 7.82 (4H, d, J = 8.7 Hz),
7.25 (4H, s), 5.28 (4H, s), 8.36 (4H, s), 3.9−3.20 (60H, bm), 2.88
(4H, t, J = 10.2 Hz), 2.79 (4H, m), 1.53 (4H, d, J = 11.9 Hz). 13C
NMR (500 MHz, CD3CN): δ = 163.2, 163.4, 153.8, 150.5, 149.0,
147.7, 142.3, 142.30, 141.5, 141.5, 138.8, 138.4, 137.6, 133.5, 132.8,
132.4, 129.3, 128.7, 127.0, 104.1, 72.6, 71.6, 71.4, 71.1, 70.1, 69.8, 66.2,
61.4, 61.0, 60.9, 58.9, 58.8, 15.5. ESI-MS: m/z: 541.23 ([5]4+), 750.63
([5 + BF4]3+), 1168.91 ([M + 2BF4]2+). Elemental analysis calcd for
C104H116B4Cu4F16N16O20·9H2O: C, 46.72; H, 5.05; N, 8.38. Found: C,
46.65; H, 4.66; N, 8.96.
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Synthesis of the Model Compound 1. In a Schlenk flask
equipped with a magnetic stir bar, freshly prepared 4,5-bis(2-(2-(2-
methoxyethoxy)ethoxy)ethoxy)benzene-1,2-diamine (C) (43.3 mg,
0.1 mmol), Cu(CH3CN)4BF4 (31.4 mg, 0.1 mmol), and 2-
formylpyridine (21.4 mg, 0.1 mmol) were dissolved in degassed
CD3CN (2.0 mL). The flask was placed under a nitrogen atmosphere
by three vacuum/nitrogen cycles. The resulting mixture was stirred in
an oil bath at 65 °C overnight. The reaction was subsequently cooled
to room temperature and diethyl ether (20 mL) was added to the
reaction mixture, resulting in the formation of a brown precipitate. The
supernatant was decanted, and the solid was thoroughly dried under
vacuum to afford (1) as a dark brown solid (73%). NMR (400 MHz,
CD3CN): δ = 8.06 (4H, t, J = 6.7 Hz), 7.99 (4H, s), 7.97 (4H, bd),
7.63 (4H, d, J = 6.8 Hz), 7.53 (4H, bt), 6.93 (4H, s), 4.24 (8H, s), 3.86
(8H, s), 3.67 (8H, s), 3.60−3.42 (24H, s), 3.29 (12H, s). 13C NMR
(500 MHz, CD3CN): δ = 155.4, 150.9, 149.2, 138.9, 135.8, 129.7,
129.0, 107.6, 72.5, 71.4, 71.1, 70.9, 70.0, 69.9, 58.8. ESI-MS: m/z:
673.38 (62+), 1435.60 ([6 + BF4]+). Elemental analysis calcd for
C64H84B2Cu2F8N8O16·3H2O: C, 48.77; H, 5.76; N, 7.11. Found: C,
48.64; H, 5.45; N, 7.02.
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ASSOCIATED CONTENT
* Supporting Information
■
S
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2010, 43, 6923−6935.
Experimental data, including NMR spectra, AFM, UV−vis, PL,
and EL are available in the Supporting Information. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
rhf10@cam.ac.uk; jrn34@cam.ac.uk
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. Army Research Office, the
Marie Curie Intra-European Fellowship Scheme of the 7th EU
Framework Program (X.H.), the European Research Council
and the Council of Higher Education in Turkey, and Middle
East Technical University (D.A.). Mass spectra were provided
by the EPSRC mass spectrometry service (Swansea). We thank
Rachel O'Reilly for helpful advice and Shane Heffernan for
carrying out preliminary investigations into the electrical
conductivity of 3.
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