Highly Luminescent Encapsulated Narrow Bandgap Polymers Based on Diketopyrrolopyrrole

Article abstract of DOI:10.1021/jacs.7b13447

We present the synthesis and characterization of a series of encapsulated diketopyrrolopyrrole red-emitting conjugated polymers. The novel materials display extremely high fluorescence quantum yields in both solution (>70%) and thin film (>20%). Both the absorption and emission spectra show clearer, more defined features compared to their naked counterparts demonstrating the suppression of inter and intramolecular aggregation. We find that the encapsulation results in decreased energetic disorder and a dramatic increase in backbone colinearity as evidenced by scanning tunnelling microscopy. This study paves the way for diketopyrrolopyrrole to be used in emissive solid state applications and demonstrates a novel method to reduce structural disorder in conjugated polymers.

Full text of DOI:10.1021/jacs.7b13447

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Communication
Highly Luminescent Encapsulated Narrow
Bandgap Polymers Based on Diketopyrrolopyrrole
Anastasia Leventis, Jeroen Royakkers, Alexandros G. Rapidis, Niall Goodeal, Merina K. Corpinot,
Jarvist M. Frost, Dejan-Krešimir Bu#ar, Matthew Oliver Blunt, Franco Cacialli, and Hugo Bronstein
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13447 • Publication Date (Web): 16 Jan 2018
Downloaded from http://pubs.acs.org on January 16, 2018
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Highly Luminescent Encapsulated Narrow Bandgap Polymers Based
on Diketopyrrolopyrrole
Anastasia Leventis1‡, Jeroen Royakkers1‡, Alexandros Rapidis2, Niall Goodeal1, Merina Corpinot3, Jar-
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vist M. Frost4, Dejan-Krešimir Bučar3, Matthew Oliver Blunt3, Franco Cacialli2, Hugo Bronstein1*
1 Department of Chemistry & Physics, University of Cambridge, Lensfield Rd, Cambridge CB2 1EW
2 Department of Physics and Astronomy and LCN, University College London, Gower Street, London WC1E 6BT, UK
3 Department of Chemistry, University College London, 20 Gordon St., London, WC1H 0AJ, UK
4 Department of Materials, Imperial College London, Exhibition Road, London, SW7 2AZ, UK
Supporting Information Placeholder
which were threaded through non-covalently bound cyclodextrins,
to form conjugated polyrotaxanes.10 Alternatively, a covalently
bound approach can be taken; in 2010, Sugiyasu et al. reported a
polythiophene whose conjugated backbone was sheathed within its
own cyclic side chains.11 It was suggested that the macrocycle
which was bound to adjacent monomers prevented the rotational
motion of the conjugated polymer backbone, giving rise to the ob-
served long conjugation lengths. More recently in 2013, Pan et al.
synthesized a red-emitting IMW which achieved a rare and prom-
ising solid-state ΦF of 13%.12 To our knowledge, the work exhib-
ited by Sugiyasu and Pan represent the only two examples in the
literature of red-emitting IMWs; unusual given how it could poten-
tially overcome the solid-state quenching commonly observed in
red-emitting conjugated polymers.
ABSTRACT:
We present the synthesis and characterisation of a series of encap-
sulated diketopyrrolopyrrole red-emitting conjugated polymers.
The novel materials display extremely high fluorescence quantum
yields in both solution (>70%) and thin film (>20%). Both the ab-
sorption and emission spectra show clearer, more defined features
compared to their naked counterparts demonstrating the suppres-
sion of inter and intra-molecular aggregation. We find that the en-
capsulation results in decreased energetic disorder and a dramatic
increase in backbone co-linearity as evidenced by STM. This study
paves the way for DPP to be used in emissive solid state applica-
tions and demonstrates a novel method to reduce structural disorder
in conjugated polymers.
Diketopyrrolopyrrole (DPP) is one of the most commonly used mo-
tifs in conjugated polymer synthesis.13-14 The electron deficient and
planar lactam core of DPP as well as its chemical stability and fac-
ile ability to be modified has resulted in its use in many applica-
tions, from dyes in automotive paints, to organic solar cell and tran-
sistor devices.13 DPP has also been used extensively in solution as
a fluorescencent dye due to its high quantum yield and photostabil-
ity.15 However, due to its planarity, DPP has a strong propensity to
π-π stack and aggregate, which becomes even more prominent in
the solid state. Hence the development of DPP polymers for solid-
state emissive devices has been limited to host-guest type ap-
proaches.16-17
Conjugated polymers are an attractive class of material for optical
and optoelectronic applications owing to their tuneable properties
through variation of chemical structure, solution processability and
semi-conductivity. In condensed phases, emissive conjugated pol-
ymers are particularly sought after as they can be utilised in a wide
range of applications, from organic light-emitting diodes, biomed-
ical imaging, light emitting transistors and wavelength-tuneable la-
sers. It is generally understood that emission in conjugated materi-
als arises from the radiative decay of singlet excitons. However,
competing non-radiative mechanisms such as energy migra-
tion/transfer, charge separation and inter-chain processes such as
excimer formation, present means of deactivation, thus the fluores-
cence quantum yield (ΦF) is usually reduced.1 This is particularly
evident at high concentrations (i.e. in the solid state), due to the
more facile formation of aggregates which can result in the for-
mation of non-emissive charge transfer states.2 In the red region of
the electromagnetic spectrum conjugated polymer emission is often
heavily compromised, primarily due to solid state packing effects
resulting from the increased planarity required to achieve narrow
optical energy gaps.3 The energy-gap law further predicts reduced
emissive quantum yields in the red region although the extent to
which this plays a role is not known4-6. Therefore the development
of efficient red-emitting conjugated polymers remains a key chal-
lenge. To overcome these issues, electronic cross-communication
between adjacent polymer chains can be prevented using insulating
layers that surround and isolate the π-conjugated polymer back-
bone, generating insulated molecular wires (IMWs).7-9 Anderson et
al. pioneered the synthesis of luminescent conjugated polymers
Herein, we report the synthesis of novel encapsulated IMWs based
on DPP. Using a covalent approach,7 we have sheathed the DPP
core using its own side chains to minimize the effects of π-π stack-
ing interactions between adjacent polymer backbones. This allows
our polymers to be highly emissive, even in the solid state and we
describe how this encapsulation affects the structure–property rela-
tionships of these IMWs. The synthesis of the novel encapsulated
monomers (4a and 4b) are presented in Scheme 1a with detailed
synthetic procedures in the SI (S1).
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Scheme 1: a) Synthesis of encapsulated monomers (4a) and Table 1: Physical properties of novel polymers
Page 2 of 5
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(4b); b) Encapsulated polymers c) Naked polymers
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Polymer
E-DPPF
E-DPPP
E-DPPT
N-DPPF
N-DPPP
N-DPPT
Mna (kDa)
28.7
Mwa (kDa)
65.8
PDIa
2.29
1.32
1.21
2.03
2.89
1.56
20.2
26.7
19.7
23.8
3.8
7.7
16.9
47.4
41.8
65.1
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aDetermined by GPC (CB) against PS standards;
Interestingly, despite multiple attempts it was not possible to suc-
cessfully synthesize N-DPPF in reasonable molecular weight de-
spite it having the largest amount of alkyl chain density. Due to its
unsuitably low molecular weight N-DPPF was discounted from
further study. With the exception of N-DPPF all polymers were
synthesized in reasonable molecular weights and polydispersities
(Table 1) and displayed good solubility in chlorinated solvents.
Figure 2 shows the solution and thin film absorption and emission
spectra of the encapsulated (E) and naked (N) polymers synthe-
sised. The absorption and emission profiles of all polymers show a
bathochromic shift from solution to thin film (Table 2), attributed
to planarization that arises in the solid state. For E-DPPF the solu-
tion absorption and emission profiles show an absorption maxima
at ~ 528 nm and a slight shoulder which we attribute to vibronic
coupling at shorter wavelengths (~510 nm). The emission in solu-
tion clearly shows the vibronic fine structure with the emission
maximum from the 0-0 transition at 571 nm. Going from solution
to thin film, the polymer appears to retain some of its vibronic
structure in the absorption spectrum. The vibrational fine structure
in emission can be observed with the 0-1 transition being increased
in intensity. Encapsulated phenyl derivative E-DPPP presents a
similar set of observations. In solution, the absorption has a maxi-
mum at ~532 nm with a shoulder at ~500 nm. The emission again
shows clear fine structure with a maximum at ~572 nm. In compar-
ison the naked polymer N-DPPP shows very different features. The
absorption in solution displays a much shallower onset and a blue-
shifted absorption maximum with no evidence of any fine structure.
The emission spectrum of N-DPPP is very broad and red-shifted,
although there appears to be some evidence of the 0-0 emissive
transition. In the solid state the E-DPPP absorption spectrum dis-
plays a slightly more prominent shoulder than in solution, and the
emission spectrum shows fine structured emission with a red-
shifted maximum corresponding to the 0-1 transition. In contrast,
N-DPPP shows a broad featureless emission spectrum with no fine
structure reminiscent of excimeric or other aggregate emission. The
encapsulated thiophene containing polymer E-DPPT has a red-
shifted absorption maximum (581 nm) due to the enhanced planar-
ity with a shoulder at ~550 nm which is enhanced in the solid state.
The emission of E-DPPT shows narrow fine-structured emission
in both solution and solid state with a solution maximum at 621 nm.
Its naked equivalent N-DPPT shows a more featureless, blue-
shifted absorption in both solution and solid state and a signifi-
cantly red-shifted, broad featureless emission. The steeper, more
well-defined absorption spectra of the encapsulated polymers and
the sharper emission peaks suggests a lower degree of energetic
disorder than their naked counterparts.18-19
The structure of the encapsulated monomer was determined by X-
ray crystallographic analysis (single crystal grown from chloro-
form) which confirmed sheathing of the DPP core (Figure 1). The
encapsulated monomer (4a) was then copolymerised with a fluo-
rene derivative in a Suzuki cross-coupling reaction to form the
novel encapsulated polymer E-DPPF. We sought to reduce the dis-
tance between the encapsulated monomers thereby increasing the
density of encapsulation within our polymers. This led us to look
at phenyl and thiophene as alternatives to fluorene.
Figure 1: Crystal structure of encapsulated DPP monomer (4a)
Polymer E-DPPP was synthesised by coupling 1,4-benzenedibo-
ronic acid bis(pinacol)ester with our encapsulated monomer (4b) in
a Suzuki cross-coupling reaction and polymer E-DPPT was syn-
thesised from a Stille cross-coupling reaction of (4b) with 2,5-
bis(trimethylstannyl) thiophene. In addition to the encapsulated
polymers (Scheme 1b) the naked counterparts N-DPPF, N-DPPP
and N-DPPT were also synthesised for comparative purposes
(Scheme 1c).
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DPPF
0-0
DPPP
DPPT
0-0
1.2
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
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0.6
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0.0
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1.0
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0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
3
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E
N
E
N
E
N
E
N
E
E
0-0
1.0
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0-1
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0.0
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800
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800
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Thin Film
DPPF
DPPT
DPPP
0-1
1.2
1.0
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0.6
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E
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N
E
N
0-0
0-1
0-0
N
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1.0
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0-0
0-1
0.0
400
600
800
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600
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800
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Figure 2: Normalised solution (CB ~4 µg/mL) and thin film (spin-coated from CB 10 mg/mL) absorption and emission spectra of encapsu-
lated (E) polymers EDPPF, EDPPP and EDPPT, and naked (N) polymers NDPPP and NDPPT.
Sugiyasu et al. reported a similar effect which was attributed to
their macrocycle enforcing rigidity across two adjacent thio-
phenes.11 However, in our case the macrocycle does not span adja-
cent units so this is not possible. Such structural rigidification is
typically only observed on going from solution to solid state (where
stacking effects introduce greater ordering), thus our reduced rela-
tive disorder in solution is highly unusual. The red-shifted broad
emission in solution from the naked polymers indicates that con-
siderable intra-molecular aggregation is occurring, which has
clearly been completely suppressed through encapsulation. It is
also evident that the aggregate (either inter or intra) emission ob-
served in the solid state for the naked polymers is largely sup-
pressed through our encapsulation strategy.
This interpretation is strongly supported by the photoluminescence
quantum yields (PLQY). In solution, the encapsulated polymers
display extremely high fluorescence quantum yields (>70%) with a
particularly remarkable ΦF 95% for E-DPPF. In contrast, the naked
polymers show significant PL quenching (ΦF ~20%) further sug-
gesting that intra-molecular aggregation is occurring. In the solid
state, the emission of the naked polymers is further quenched to
<10% PLQY. Most importantly, the encapsulated polymers retain
impressive PLQY in the solid state (~20%). E-DPPT displays a
thin film fluorescent quantum yield of ~28% which is amongst the
highest value reported for a neat conjugated polymer in this wave-
length region. We note that these findings are in direct contradic-
tion to the recent study which suggests that polymers with higher
energetic disorder should have higher emissive quantum yields.20
We suggest that the introduction of the encapsulating macrocycle
prevents the formation of non-radiative quenching sites which was
the origin of the authors’ observations. The emissive quantum
yields demonstrate our encapsulation strategy has been highly ef-
fective, resulting in the first examples of highly fluorescent DPP
polymers in the solid state potentially allowing for their use in sen-
sors, LEDs and visible light communications applications.21 Proof
of concept OLEDs were fabricated (SI7) and the encapsulated pol-
ymers outperformed their naked analogues demonstrating the en-
hancement of improved optoelectronic function.
Table 2: Solution and thin film emission maxima and fluo-
rescence quantum yields (ΦF) of all novel polymers
c
c
Polymer
Emis-
sion
Emis-
sion
ΦF (Solution)
ΦF (Film)
(%)
(%)
max
max
soln.a
(nm)
filmb
(nm)
E-DPPF
E-DPPP
E-DPPT
N-DPPF
N-DPPP
N-DPPT
571
572
621
-
589
641
641
-
94.8 ± 1.0 19.8 ± 1.1
74.0 ± 1.2 22.6 ± 1.1
73.6 ± 1.2 27.8 ± 1.3
In order to understand the unusual apparent decrease in energetic
disorder the DPPP and DPPT polymers were imaged using scan-
ning tunnelling microscopy (STM). Figure 3 displays the encapsu-
lated polymers alongside their naked equivalents. All four poly-
mers display lamellar type packing into nanostructured domains.
However, there is a stark difference between the encapsulated and
naked polymers. E-DPPP and E-DPPT show extremely low defect
domains where there are virtually no twisted or bent polymer
-
-
629
686
644
703
b
19.0 ± 3.3 8.0 ± 0.3
18.2 ± 1.3 5.6 ± 0.5
aCB solution (~4 µg/mL); Spin-coated from CB (10 mg/mL);
cMeasured using an integrating sphere.
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strands. E-DPPT in particular shows effectively defect free poly-
Supporting Information
mer domains where each strand is in a completely linear confor-
mation. In contrast N-DPPP and N-DPPT show typical coiled and
worm-like nanostructuring, characteristic of other polymers im-
aged by STM with numerous ‘hairpin’ like conformational de-
fects.22
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The Supporting Information is available free of charge on the ACS
Publications
website.
Synthesis and characterization of polymers, crystallographic data
and additional theoretical calculations.
E-DPPP
E-DPPT
Accession Codes
8
9
CCDC 1565490 and 1565494 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
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40nm
AUTHOR INFORMATION
Corresponding Author
N-DPPP
N-DPPT
hab60@cam.ac.uk
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interests.
40nm
40nm
ACKNOWLEDGMENT
This work was funded by EU project 679789 – 455 CONTREX.
This work was also supported by the EU H2020 ETN
SYNCHRONICS under grant agreement 643238. FC is a Royal So-
ciety Wolfson Research Merit Award holder.
Figure 3. STM images of conjugated polymers (E-DPPP: sample
bias (Vs) of -0.50 V with respect to the tip and a tunnelling current
of (It) of 5.0 pA; E-DPPT: Vs = -0.55 V, It = 5.0 pA; N-DPPP: Vs
= -0.85 V, It = 7.5 pA; N-DPPT: Vs = -0.70 V, It = 7.0 pA.)
REFERENCES
Given the similarity of the solution and thin film UV-Vis absorp-
tion spectra we believe that these encapsulated polymers therefore
have a general tendency to adopt highly linear conformations in
both solution and solid state. We believe that this increased order-
ing is from the 3-dimensional steric shielding effect from the en-
capsulating unit which prevents bending of the polymer backbone.
Disorder-free charge transport in conjugated polymers has recently
been observed but the synthetic guidelines for reducing energetic
disorder are not clear.23 Thus polymer encapsulation offers a novel
method for increasing backbone linearity in both solution and solid
state.
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In conclusion, we have synthesized a series of encapsulated conju-
gated diketopyrrolopyrrole polymers. These materials display ex-
tremely high fluorescence quantum yields in both solution and thin
film allowing this ubiquitous chromophore to be used, for the first
time, as an emissive material in the solid state. The encapsulated
polymers display much sharper emissive spectral features than their
naked counterparts demonstrating that both intra and intermolecu-
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linearity resulting in sharper absorption features in both solution
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