Perfectly regioregular electroactive polyolefins: Impact of inter-chromophore distance on PLED EQE

Article abstract of DOI:10.1021/ma202409k

Acyclic diene metathesis polymerization (ADMET) was used to synthesize a series of perfectly regioregular polyolefins, in which the number of backbone atoms between pendant terfluorene groups was precisely controlled at 8, 14, or 20 carbons. Analogous random copolymers containing identical chromophore densities were also synthesized to study the impact of regioregularity on the performance of this class of materials in polymer light emitting diodes (PLEDs). Additionally, the backbone alkene remnants of ADMET were saturated to generate materials with somewhat different ordering. These saturated derivatives led to improvements in PLED external quantum efficiencies (EQEs) over their unsaturated analogues in most cases, with a large improvement in one material. Charge mobility, as manifested in current density during PLED characterization, and relative solid-state fluorescence quantum yield (Φ_F) also exhibit reasonable dependencies, with longer distances between electroactive groups yielding lower PLED current densities and higher Φ_F. Regioregularity has the opposite effect, giving rise to higher current densities and lower Φ_F as compared to regiorandom analogues.

Full text of DOI:10.1021/ma202409k

Article
pubs.acs.org/Macromolecules
Perfectly Regioregular Electroactive Polyolefins: Impact of Inter-
Chromophore Distance on PLED EQE
Brian S. Aitken, Patrick M. Wieruszewski, Kenneth R. Graham, John R. Reynolds,*
and Kenneth B. Wagener*
The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, Center for Macromolecular Science and
Engineering, University of Florida, Gainesville, Florida 32611-7200, United States
S
* Supporting Information
ABSTRACT: Acyclic diene metathesis polymerization (ADMET) was used to synthesize a series of perfectly regioregular
polyolefins, in which the number of backbone atoms between pendant terfluorene groups was precisely controlled at 8, 14, or 20
carbons. Analogous random copolymers containing identical chromophore densities were also synthesized to study the impact of
regioregularity on the performance of this class of materials in polymer light emitting diodes (PLEDs). Additionally, the
backbone alkene remnants of ADMET were saturated to generate materials with somewhat different ordering. These saturated
derivatives led to improvements in PLED external quantum efficiencies (EQEs) over their unsaturated analogues in most cases,
with a large improvement in one material. Charge mobility, as manifested in current density during PLED characterization, and
relative solid-state fluorescence quantum yield (ΦF) also exhibit reasonable dependencies, with longer distances between
electroactive groups yielding lower PLED current densities and higher ΦF. Regioregularity has the opposite effect, giving rise to
higher current densities and lower ΦF as compared to regiorandom analogues.
excitons, high fluorescence quantum yields (ΦF) are required as
well. However, attaining high performance in both parameters
for one material is quite challenging. Highly ordered organic
semiconductors tend to exhibit increased charge mobility over
their amorphous counterparts,27,32−38 but ordering may
simultaneously reduce ΦF by enhancing quenching mechanisms
via increased energy migration and formation of non emissive
complexes.39−47
INTRODUCTION
■
As global economic development occurs, the demand for cheap,
energy-efficient displays and lighting continues to increase,1−3
placing higher demands on our understanding of design
parameters for fabrication of high performance polymer light
emitting diodes (PLEDs). These devices can be processed from
solution by, for example, ink jet printing or spin coating, as
opposed to the costly processes requiring high temperature,
ultra clean conditions and high vacuum needed in inorganic
LED and small molecule OLED construction. Thus, PLEDs
afford a low cost option for the fabrication of large and or
flexible displays such as television screens, computer monitors,
and large emissive lighting surfaces.4−10
However, even state-of-the-art PLEDs currently suffer from
lower performance compared to their inorganic and molecular
counterparts.3,9−24 Figure 1 shows that the design of efficient
PLEDs necessitates a balance of hole and electron mobilities,
which must also be sufficiently high, to ensure that charge
recombination occurs near the center of the emissive layer of
the device.22,25−31 For those based on emission from singlet
Donor−donor energy migration (DDEM, also referred to as
homo resonance energy transfer), a process closely related to
Forster resonance energy transfer (FRET) but between
̈
chemically identical species,48−57 enhances quenching in
fluorescent materials by allowing excited state energy to
migrate via adjacent identical fluorophores to either dark
complexes (static quenching) or to fluorophores which have a
greater propensity to undergo dynamic quenching as a result of
Received: October 30, 2011
Revised: November 30, 2011
Published: January 6, 2012
© 2012 American Chemical Society
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Figure 1. Schematic of the various processes leading to quenching or light emission that may occur in a PLED.
their microenvironments;58−62 the latter is referred to as partial
donor−donor energy migration (PDDEM).63−68 The rate of
resonance energy transfer (RET) is dependent on, among other
variables, the distance between energy donor and energy
acceptor to the inverse sixth power.66,69,70 Therefore, increasing
the intermolecular distance between the fluorophores of
interest is advantageous if one wishes to reduce the effects of
RET. Particularly, in PLEDs and OLEDs, unintentional RET is
disadvantageous as it reduces the device’s external quantum
efficiency (EQE) by enhancing the effects of both self-
quenching and quenching due to impurities such as residual
transition metal catalysts. However, the immediate solution of
increasing the distance between chromophores may be
deleterious to device performance, since charge mobility will
simultaneously drop. The obvious question is “how distant
should chromophores be to limit quenching while, still allowing
for sufficiently high charge mobility for exciton generation?”
One might attempt to approach the problem theoretically,
however calculation of the resulting changes in DDEM,
PDDEM, and FRET rates upon variation of the aforemen-
tioned distance, and ultimately the corresponding changes in
ΦF, requires knowledge of the geometric attitude of the donor
with respect to the acceptor for calculation of the orientational
the current level of theory, and charge injection rates remain an
additional theoretical problem.72 Therefore, an empirical
structure−property study to assess the impact of chromo-
phore−chromophore distance variation on PLED performance
is necessary.
To determine the effects of chromophore separation on
device parameters, we have prepared a set of perfectly
regioregular electroactive polyolefins with carefully controlled
average chromophore−chromophore distances. As shown in
Figure 2A, regioregular electroactive polyolefins comprise a
factor (κ2) in the Forster treatment.55−57,66−71 This parameter
̈
is extremely difficult or even impossible to predict for materials
of the general type presented here, particularly in the solid
state, where orientational randomization is quite slow and
precludes the oft used approximation of κ2 = 2/3, which is valid
for small molecules in solution.55−57,66−68,70,71 Furthermore,
estimation of the variation in charge mobility, which will
invariably result from such structural changes, is impossible at
Figure 2. Architecture of polymeric electroptic materials.
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Scheme 1. Synthetic Approach to Regioregular Electroactive Polyolefins and Two Regiorandom Analogues
limited to differential scanning calorimetry and gel permeation
chromatography of the polymers. Complete characterization of all
materials, including NMR, mass spectrometric data, and elemental
analysis is now fully divulged in the Supporting Information for this
article.
class of materials in which the active pendant group occurs at
exactly defined backbone run length intervals (spacer
lengths).73,74 These regioregular materials are analogous to
previously reported vinyl type or regiorandom polymers
containing pendant electroactive moieties (Figure 2B).75−93
Similar to main chain π-functional materials (Figure 2C),94−98
pendant π-functional polymers offer the opportunity for
inclusion of various electroactive units, which may serve their
purposes cooperatively (e.g., various chromophores for white
emission, emissive units and charge transporters, etc.)84,92,93
and thus afford an interesting and relatively new avenue for
exploration into optoelectronic materials, as compared to
conjugated polymers (Figure 2D).
Good control over the average distance between chromo-
phores can be accomplished via acyclic diene metathesis
polymerization (ADMET) of symmetric α,ω-diene functional-
ized chromophores followed by alkene hydrogenation or, as
was demonstrated in our earlier work, by post polymerization
coupling of chromophore fragments to regioregular borylated
polyolefins.73,74 In the work reported here, terfluorenylidenes
were placed pendant to fully linear polyethylene at exactly
defined spacer lengths via the former method. We have found
that polymers with longer spacer lengths exhibit higher PLED
performance, despite yielding reduced current densities at
identical device driving voltages. Furthermore, analogous
regiorandom polymers exhibit enhanced relative solid-state
ΦF, presumably due to relatively reduced ordering and
therefore less quenching. However, they also exhibit somewhat
lower charge mobilities, as manifested in reduced PLED current
densities, also resulting from their disorder. The concepts
demonstrated here may also prove useful in the design of
polymers for other ΦF dependent devices such as scintillation
detectors99−101 and luminescent solar concentrators.102−106
Device Fabrication and Characterization. PLEDs were
fabricated with a device architecture of glass/ITO/PEDOT:PSS/
polymer/LiF/Al. Prepatterned indium tin oxide (ITO) coated glass
(25 mm ×25 mm) was used with either a 12 or 20 Ω/□ resistance.
ITO-coated glass was cleaned by sequential sonication in sodium
dodecyl sulfate solution, 18.2 MΩ MiliQ water, acetone, and 2-
propanol. Immediately following 2-propanol sonication, substrates
were blown dry with nitrogen and exposed to oxygen plasma for 20
min. Under a particle free hood, substrates were spun cast with
PEDOT:PSS (Baytron P VP Al 4083 which was filtered through 0.45
μm nylon filters) at 5000 rpm for 45 s, then annealed for 20 min at
130 °C in an Ar filled glovebox with H2O and O2 concentrations of
less than 0.1 ppm. In the glovebox, active layer solutions were
prepared at 15 mg/mL concentrations in anhydrous and deoxygenated
chlorobenzene and spun cast onto the PEDOT:PSS coated substrates
for 60 s at 1000 rpm. Cathodes consisting of 1 nm LiF and 100 nm Al
were thermally deposited on the substrates at a pressure of 1 × 10−6
mbar through shadowmasks defining 8 pixels/substrate, each with an
area of 0.071 cm2. Electroluminescence spectra were collected using an
ISA SPEX Triax 180 spectrograph, maintained at ∼140 K, with a
Keithley 2400 sourcemeter driving the device at a constant current.
Radiant emittance and current density data were acquired using a
custom written LabVIEW program coupled with a calibrated UDT
Instruments silicon diode and a Keithley 2400 sourcemeter.
Luminance data were collected using a Konica CS-100 minolta
chromameter with the device at a constant voltage bias.
RESULTS AND DISCUSSION
■
Synthesis and Physical Characterization. Perfectly
regioregular polymers of the general architecture shown in
Figure 2A may be synthesized in at least one of two ways. In
our first report, we described a general method which allows for
Suzuki coupling of aryl groups to regioregular borylated
fluorene functionalized polyolefins.74 However, this method
was limited to polymers with no more than one electroactive
group on every 21st polymer backbone carbon. This limitation
was the result of solubility issues stemming from ionization of
the polymer during the Suzuki coupling step. As shown in
Scheme 1, this problem has been alleviated by reversing the
EXPERIMENTAL SECTION
■
Materials and Instrumentation. All materials were purchased
from Aldrich and used as received, unless noted otherwise. Absorption
spectra were measured with a PerkinElmer Lambda 25 UV−vis
spectrometer. Photoluminescence spectra were obtained using a
Horiba Jobin Yvon Fluorolog-3 fluorimeter.
Synthesis. Synthesis of these materials was briefly described
elsewhere;73 however, due to length constraints, characterization was
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order of synthesis from post polymerization modification for
installation of the chromophore to polymerization of fully
functionalized monomers. The latter method allows for
increased chromophore density, and materials containing
chromophores on exactly every 21st, 15th, and ninth carbon
have been prepared (though presumably, these numbers could
be raised or lowered further via known synthetic proto-
cols107−109). Furthermore, the choice of terfluorenylidenes as
the active species allows for hydrogenation of the residual
backbone olefin after ADMET, thus providing a handle on
material morphology, which is known to dramatically change
upon olefin hydrogenation in similar polymers.110 The
regioregularity of these polymers, as opposed to the
regiorandom materials produced in various copolymerizations,
and the distribution of chromophore densities over a
reasonable range, provides good control over the average
distance between electroactive groups in a set of materials.
Their interchromophore distances as well as any potential
aggregates’ sizes and shapes should exhibit greater homogeneity
(reduced polydispersity in distance/size/shape) than in
analogous random copolymers, as previously observed in
other systems containing interactive pendant groups.111,112
Although the synthesis of the polymers in this work was
reported previously,73 the design rationale and issues
encountered during their preparation were not provided in
detail, thus a more thorough discussion can be found in the
Supporting Information.
Characterization of the polymers by gel permeation
chromatography (GPC), differential scanning calorimetry
(DSC), polarized optical microscopy, and atomic force
microscopy (AFM) was carried out. The GPC and DSC data,
were reported previously73 and can also be found in the
Supporting Information. Molecular weights were high in all
cases (Mn = 21−65 kDa), although P-3,3-TF (21 kDa) and P-
3,3-TF-S (22 kDa) exhibited somewhat lower Mn and PDI due
to a cyclization issue also discussed in the Supporting
Information. No melting transitions were detected during
DSC experiments up to 200 °C at various scan rates ranging
from 1 to 20 °C/min, and no birefringence was observed under
polarized light microscopy. To further probe morphology, AFM
measurements were carried out on spun cast polymer films;
however, we again observed no unique features in any polymer.
Instead, AFM indicated only very flat surfaces with root mean
squared surface roughnesses around 0.3 nm (see Supporting
Information for micrographs).
Figure 3. Solid-state absorbance and front-face fluorescence spectra
(corrected for optical density at excitation = 350 nm). Error bars are
95% confidence intervals for the corrected fluorescence intensity at
λmax. All data points used to create the plots were averaged over four
trials.
decreased energy migration and quenching. Furthermore, as
evidence against the possibility of method error or false results
due to various levels of residual transition metal catalyst
contamination, we carried out similar comparative fluorescence
measurements in chloroform solution; all polymers exhibited
identical fluorescence intensities and therefore identical ΦF (see
Figure 4).
UV−Vis Characterization. To assess whether or not
control over ΦF in this system was achieved, thin films with
nearly identical optical densities ( 5%) were prepared by spin
coating onto glass substrates from dilute solutions of each
polymer in chloroform. Using identical instrument parameters,
the front face fluorescence spectra of each film was then
measured and corrected for the small changes in optical density
of each sample (see Figure 3). Each measurement was carried
out in duplicate after rotating the substrate 90°, and two films
were prepared for each polymer (total of four measurements
each).
Except for P-9,9-TF-S, it is evident that the relative ΦF
increases as the spacer between each fluorophore increases in
length, and that regiorandom analogues also exhibit increased
ΦF. Both increases in ΦF are presumably due to reduced π−π
interaction, which is known to systematically vary as a function
of copolymer composition in regiorandom pendant π-func-
tional materials;87−90 reduced π−π interactions lead to
Figure 4. Chloroform solution absorbance and fluorescence spectra
(corrected for optical density at excitation =350 nm) for all polymers.
Careful examination of the data will show that P-9,9-TF-S is
the only polymer which does not fit the aforementioned spacer
length trend; indeed, it has the lowest ΦF of the backbone-
saturated polymers. This is perhaps the result of a unique
morphology relative to the other materials. However, as
previously discussed, we have been unable to directly observe
any morphological features by AFM or optical microscopy in
any of these materials.
PLED Fabrication and Characterization. Having dem-
onstrated systematic control of solid-state ΦF through polymer
backbone run length variation, we were motivated to prepare
relatively simple PLEDs using a Glass/ITO/PEDOT:PSS/
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ActiveLayer/LiF/Al architecture. Electroluminescence spectra,
radiant emittance, current density, and luminance data were
collected for each device, and EQEs were calculated.
As shown in Figure 5, all devices exhibited similar but subtly
different electroluminescence spectra. This is to be expected
Figure 7. Luminance and current density vs driving voltage for devices
fabricated from saturated polymers.
Figure 5. Normalized electroluminescent spectra of saturated
polymers.
since each polymer contains identical chromophores but
differing degrees of interaction between them. On the other
hand, device performance varies greatly. Figures 6 and 7 show
Figure 8. Average EQEs measured from eight pixels per polymer
(error bars represent a full standard deviation).
lower current density. One might attribute this difference to
reduced charge mobility resulting from reduced ordering in the
regiorandom polymer; however, P-R21-TF-S and P-9,9-TF-S
devices show nearly identical current densities, although the
latter is somewhat special in its characteristics, as will be
discussed below.
In contrast with the current density and solid-state ΦF data,
which shows general spacer length effects, device luminance
output is somewhat less systematic. Instead, P-3,3-TF, P-3,3-
TF-S, P-6,6-TF, and P-6,6-TF-S show similar luminance (<35
cd/m2), P-R21-TF and P-R21-TF-S show some improvement
(max of 43, 45 cd/m2 respectively), while P-9,9-TF and P-9,9-
TF-S show far greater luminosity (max of 93, 366 cd/m2
respectively). Finally, EQEs have been calculated and are
shown in Figure 8. Again, a general trend emerges, with longer
backbone run lengths resulting in improved EQEs, thus it can
be concluded that lower chromophore densities are actually
beneficial to the efficiency of this class of materials in PLEDs.
Additionally, for all polymers except P-9,9-TF-S, device EQE
was only slightly effected by backbone saturation; however in P-
9,9-TF-S, saturation led to an over 2-fold improvement in EQE.
Much like the fluorescence and PLED luminance measure-
Figure 6. Luminance and current density vs driving voltage for devices
fabricated from unsaturated polymers.
the current density and luminance as a function of applied
voltage for devices fabricated from unsaturated and saturated
polymers, respectively. These data are from single devices;
however, device reproducibility is excellent as indicated by
averaged EQE data shown in Figure 8. Examination of the
current density plots for each device reveals an interesting
trend; as the polymer backbone run length separating pendant
chromophores increases, current density at identical driving
voltages decreases. This is presumably due to reduced charge
mobilities resulting from lower chromophore densities and
longer hopping distances. Furthermore, regiorandom P-R21-
TF, which is an analogue of P-9,9-TF, exhibits a somewhat
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ments, in which P-9,9-TF-S was an outlier, such an improve-
ment indicates again that morphological changes may have
occurred as a result of backbone saturation in this polymer and
that such changes play an important role in its performance.
Furthermore, electrode surface texturization, which will be
discussed below, provides additional evidence for morpho-
logical differences between the materials studied here.
Having demonstrated systematic control of PLED EQE via
spacer length variation, it is important to discuss two
anomalous observations: emergence of gold coloration on the
aluminum electrode surface of only P-9,9-TF-S devices,
indicative of morphological changes leading to surface
texturization, as well as temporal and spatial modulation of
PLED pixel luminosity. As expected, vacuum-deposited
aluminum electrodes for all other devices were bright silver in
color (see Figure 9). Here again, the morphological differences
device is turned off or is driven to burn out at high voltage
biases. Annealing at 80 °C (well above polymer Tgs) before
carrying out measurements makes no difference in EQE or the
occurrence of the luminosity fluctuation. We are currently
investigating the possibility of utilizing the residual backbone
alkenes in the unsaturated polymers for cross-linking, as this
would reduce the segmental motion hypothesized as the cause
of this optical phenomenon.
CONCLUSIONS
■
We have synthesized and characterized a series of six perfectly
regioregular electroactive polyolefins and two regiorandom
analogues. Differential scanning calorimetry and polarized
optical microscopy illustrate that these materials are amor-
phous. They exhibit spacer length dependent solid-state ΦF,
PLED current densities (presumably resulting from spacer
length dependent charge mobilities), and PLED EQEs. In
summary, longer spacer lengths result in higher ΦF, lower
charge mobility (as manifested in PLED current density), and
higher PLED EQE. Furthermore, by comparing the four P-9,9
and P-R21 derivatives, we found that regioregularity reduces
solid-state ΦF. Interpretation of current densities in the set of
four is less straightforward; charge mobility was improved by
regioregularity in the unsaturated pair but not in the saturated
pair (which we believe is a special case due to suspected
morphological differences in P-9,9-TF-S). While the PLED
performance of this series of polymers leaves much to be
desired, we believe the general trends which they exhibit will be
useful in the design of new electroactive materials based on
pendant π-functional architectures, and their direct application
in other ΦF dependent devices such as scintillation detectors or
luminescent solar concentrators may prove more valuable.
Figure 9. Optical microscope images of electrode surfaces on P-9,9-
TF-S and, as a representative image, P-3,3-TF-S devices.
in P-9,9-TF-S are highlighted by the occurrence of electrode
surface coloration/texturization; however, attempts to elucidate
the nature of this phenomenon via AFM of the electrode have
proven quite challenging.
The observation of spatial and temporal fluctuation of PLED
luminosity (see Figure 10) is somewhat less puzzling. Since the
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis discussion, detailed experimental data, and AFM
images. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: (J.R.R.) Reynolds@chem.ufl.edu; (K.B.W.)
Wagener@chem.ufl.edu.
ACKNOWLEDGMENTS
■
B.S.A and K.B.W gratefully acknowledge financial support from
the National Science Foundation (DMR 0703261). P.M.W.,
K.R.G., and J.R.R gratefully acknowledge financial support from
the U.S. Army (W31P4Q-08-1-0003). The views and
conclusions contained in this document are those of the
authors and should not be interpreted as representing the
official policies, either expressed or implied, of the U.S. Army,
the National Science Foundation, or the U.S. Government.
Figure 10. Sequence of time lapsed photographs of P-9,9-TF-S pixel at
a constant driving voltage of 13 V beginning at device turn on (top
left) and ending 22.5 s later (bottom right).
Tgs of these materials are relatively low (22−50 °C, see
Supporting Information), we believe the fluctuation results
from Joule heating, which has been demonstrated to raise the
temperature of PLEDs by several tens of degrees,113−115 thus
providing the polymer chains with sufficient thermal energy to
undergo long-range segmental motion. This would allow
microdomains to temporarily attain a highly emissive
morphology before temporarily changing to a darker one.
Furthermore, overall luminance increases over the first few
seconds of device operation before reaching a plateau,
indicating that Joule heating is, in this case, beneficial to device
performance. The luminosity fluctuation continues until the
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