Solubility modulation of polyfluorene emitters by thermally induced (retro)-diels-alder cross-linking of cyclopentadienyl substituents

Article abstract of DOI:10.1021/acs.chemmater.8b01703

For cost-efficient organic electronic devices, the consecutive deposition of active layers by solution-based processes is a key benefit. We report a synthetic approach enabling solubility reduction of bis(cyclopentadienyl)- substituted polyfluorenes as emissive layers in organic light-emitting diodes (OLEDs). Thermally induced retro-Diels-Alder reaction liberates free cyclopentadiene as "protecting group" and pending cyclopentadienyl units, which crosslink the polymer strands upon cooling via [4+2] cycloadditions. The activation temperature is tuned in the range of 180-250°C through alkyl, alkoxy, or ester linkages. Ultimately, macrocyclic self-protected bis(cyclopentadienylene) moieties avoid extrusion of volatile cyclopentadiene during activation. The solvent resistance of the emissive layers after cross-linking is examined by absorption spectroscopy and white light scanning interferometry. The influence of the desolubilization procedure on the performance of solution-processed OLEDs is investigated.

Full text of DOI:10.1021/acs.chemmater.8b01703

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Article
Solubility Modulation of Polyfluorene Emitters by Thermally Induced
(Retro)-Diels-Alder-Crosslinking of Cyclopentadienyl Substituents
Lukas Ahrens, Stefan Schlisske, Karl-Philipp Strunk, Felix Hinkel, Christian Melzer, Uwe H. F.
Bunz, Uli Lemmer, Gerardo Hernandez-Sosa, Daniel Jänsch, Jan Freudenberg, and Klaus Müllen
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01703 • Publication Date (Web): 04 Jun 2018
Downloaded from http://pubs.acs.org on June 4, 2018
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Solubility Modulation of Polyfluorene Emitters by Thermally
of
Induced
Cyclopentadienyl Substituents
(Retro)-Diels-Alder-Crosslinking
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Lukas Ahrensa,b, Stefan Schlisskea,c, Karl-Philipp Strunkd, Felix Hinkelb,d, Christian Melzerd,
Uwe H. F. Bunzb,d, Uli Lemmerc,e, Gerardo Hernandez-Sosaa,c, Daniel Jänscha,b, Jan
Freudenberga,b, and Klaus Müllena,f*
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a InnovationLab, Speyerer Straße 4, 69115 Heidelberg, Germany
b
Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer
Feld 270, 69120 Heidelberg, Germany
c
Light Technology Institute, Karlsruhe Institute of Technology, Engesserstr. 13, 76131
Karlsruhe, Germany
d
Centre for Advanced Materials, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer
Feld 225, 69120 Heidelberg, Germany
e
Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-von-
Helmholtz-Platz 1, 76344 Eggenstein Leopoldshafen, Germany
f Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
*corresponding author: muellen@mpip-mainz.mpg.de
Abstract
For cost-efficient organic electronic devices the consecutive deposition of active layers by solution-
based processes is a key merit. We report a synthetic approach enabling solubility reduction of
bis(cyclopentadienyl)-substituted polyfluorenes as emissive layers in OLEDs. Thermally induced retro-
Diels-Alder reaction liberates free cyclopentadiene as “protecting group” and pending
cyclopentadienyl units, which crosslink the polymer strands upon cooling via [4+2] cycloadditions.
The activation temperature is tuned in the range of 180 °C to 250 °C through alkyl, alkoxy or ester
linkages. Ultimately, macrocyclic self-protected bis(cyclopentadienylene) moieties avoid extrusion of
volatile cyclopentadiene (Cp) during activation. The solvent resistance of the emissive layers after
crosslinking is examined by absorption spectroscopy and white-light scanning interferometry. The
influence of the desolubilization procedure on the performance of solution-processed OLEDs is
investigated.
Introduction
Both organic light-emitting diodes (OLEDs) and organic solar cells (OPVs) rely upon multi-layer
architectures.1,2 Sequential deposition of different layers by vacuum techniques offers defined
interfaces, but is restricted to small organic molecules or inorganics3 and not applicable to polymers.
The latter exhibit superior properties for future applications, e.g. in terms of morphological
robustness, flexibility4,5,6,7 and self-healing ability,8,9 but have to be deposited from solution.10
Polymers and small molecules can be cast into films by high-throughput printing technologies,
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advantageous for large area fabrication at low production costs.3,11 Thus, the development of
protocols for solution-based consecutive layer deposition is vital. Each material must exhibit
resistance to the subsequently used solvent while retaining or improving device characteristics.
Strategies involve orthogonal solvents,12 or transformation of soluble precursors into immobilized
active layers by external stimuli such as (UV-)light, heat or treatment with additives. Solubility is
reduced either by cleavage of solubilizing side-chains,1 as demonstrated by our group13 and
others14,15 on polyfluorenes13 and polythiophenes,14 or by formation of insoluble covalently
crosslinked networks (Figure 1).1,2,16
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Figure 1: Concepts for solution processing of consecutive layers (selection). a) orthogonal coating
utilizing solubility/insolubility in solvents of varying polarity, b) thermal/photochemical removal of
side-chains, c) photochemical/oxidative crosslinking, d) additive-free, thermal crosslinking without
byproducts (this work).
Removal of solubilizing groups (Figure 1b) necessitates the cleavage products either to leave the thin
film as volatiles, possibly changing the film morphology, or to be tolerated under device operation. In
contrast, crosslinking is a more (atom-)economical approach: prominent examples include
dimerization of cinnamic esters17 induced by UV-irradiation. The radiation dose employed is crucial,
as adverse photochemical processes can occur upon over-exposure which compromise device
performance.18,19 Other protocols involve additives, i.e. diiodooctane as cross-linking reagent for
amines,20 in-situ generated nitrenes by UV-irradiation,21,22 or (photo)acids inducing cationic ring-
opening polymerization (CROP) of oxetanes.23 Thermal, additive-free crosslinking of polymers for
organic electronics remains challenging, as the utilized functional groups should not decrease the
material’s performance.24 Benzocyclobutene-derived monomers applied as additives for
polyethylene undergo conversion to ortho-xylylenes at 200 °C, which irreversibly dimerize to inert
dibenzocyclooctane linkers without noticeable byproducts.25 Desolublization, however, required
rather long annealing periods of 4 h. Additionally, benzocyclobutadiene-derived building blocks are
scarce and expensive due to their reactive nature,26 and are not universally applicable if additional
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olefinic bonds are present in the active material. Reaction kinetics of less reactive and more selective
Diels-Alder partners (vide supra) can be gained from dynamic covalent chemistry,27,28 well-explored
for self-healing polymers.8,9,29,30 The activation barrier of the [4+2] cycloaddition is tuned by changing
the electron demand of dienophile and diene components, allowing for adjustment of crosslinking
temperatures and annealing times.
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Scheme 1: Schematic illustration of the crosslinking process of different polymer strands through
Diels-Alder reaction of cyclopentadienyl groups. Top left: Retro-Diels-Alder reaction liberating
protective cyclopentadiene as volatile by-product to furnish cyclopentadienyl groups for
dimerization. Top right: Cleavage of a macrocyclic self-protected bis(cyclopentadienylene) via retro-
Diels-Alder reaction to avoid formation of volatile side-products. X denotes the variable linkers (-CH2-
, -O-, -COO-).
In this contribution, we establish cyclopentadienyl groups as crosslinking moieties in organic
electronics, at the same time choosing easily accessible polyfluorenes (PFs) as common emitters in
OLEDs. Bis(cyclopentadienyl)-substituted fluorene monomers were synthesized and copolymerized
with conventional ethylhexyl-substituted fluorene monomers in different ratios through statistic
Yamamoto coupling. A thermal stimulus applied to the polyfluorenes leads to retro-Diels-Alder
reactions, liberating cyclopentadiene used as a “protecting group” for the cyclopentadienyl-
functionalized polymer strands (Scheme 1). The pending cyclopentadienyl units undergo dimerization
or trimerization via [4+2] cycloadditions upon cooling, causing the desired network formation and
solubility decrease. This process tolerates various functional groups. The cleavage temperature of the
retro-Diels-Alder reaction is determined by the electron demand of the bis(cyclopentadienyl)s and
tuned through linkage via C-C-bond, ether, or ester group. Self-protected macrocyclic
bis(cyclopentadienylene) units avoid the necessity of sacrificial protecting units, leading to purely
thermal, additive-free and comparably fast modulation of solubility without extrusion of any by-
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products (Scheme 1, top right and Figure 1d). We examine the solvent resistance of thin films after
their annealing at elevated temperature to evaluate the degree of crosslinking. The self-protected
polyfluorene exhibits superior performance in solution-processed proof-of-concept OLEDs in
comparison to their Cp-protected counterparts. In contrast to other successful and benign
immobilization strategies, e.g. CROP employing oxetanes, additional post-treatment steps like
washing are precluded, rendering our process compatible with roll-to-roll techniques.16
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Experimental
Synthesis
All reactions requiring exclusion of oxygen and moisture were carried out in dried glassware under a
dry and oxygen free nitrogen or argon atmosphere using Schlenk techniques. Commercially available
monomer building blocks for Yamamoto polymerization were purified before usage applying column
chromatography and the procedure described by Meijer et al.31
2,7-Dibromo-9,9-bis(6-bromohexyl)-9H-fluorene (2):32 2,7-Dibromo-9H-fluorene 1 (2.50 g, 7.72
mmol, 1.00 eq.) and tetrabutylammonium iodide (291 mg, 772 µmol, 98 wt%, 0.10 eq.) were
suspended in 40 mL of aqueous 50 wt% NaOH solution. Applying three freeze-pump-thaw-cycles the
suspension was degassed. Thereafter 1,6-dibromohexane (8.57 mL, 13.6 g, 54.0 mmol, 97 wt%, 7.00
eq.) was added and the resulting mixture was stirred at 70 °C for 4.5 h. The aqueous suspension was
extracted with chloroform, the combined organic layers were washed three times with deionized
water and dried over MgSO4. The remaining 1,6-dibromohexane was distilled off at 70 °C at 1 mbar
and purified by flash column chromatography (SiO2, PE : EA 9 : 1). 2 was isolated as a colorless solid
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(3.61 g, 5.55 mmol, 72.0 %). H-NMR (301 MHz, CDCl3): δ 0.52 - 0.66 (m, 4 H), 1.01 - 1.15 (m, 4 H),
1.15 - 1.27 (m, 4 H), 1.61 - 1.73 (m, 4 H), 1.88 - 1.97 (m, 4 H), 3.30 (t, J = 6.8 Hz, 4 H), 7.42 - 7.49 (m, 4
H), 7.50 - 7.55 (m, 2 H).
2,7-Dibromo-9,9-bis(1-(3a,4,7,7a-tetrahydro-1H-4,7-methanoinden-3-yl)-1l5,6l3-hexyl)-9H-fluorene
and its stereoisomers (F-C-Cp2): Sodium cyclopenta-2,4-dien-1-ide (271 mg, 3.08 mmol, 2.00 eq.) was
suspended in 40 mL freshly degassed, anhydrous THF and 5.00 mL of freshly distilled
cyclopentadiene. Then 2,7-dibromo-9,9-bis(6-bromohexyl)-9H-fluorene 2 (1.00 g, 1.54 mmol, 1.00
eq.) was added slowly at 0 °C and the resulting mixture was stirred at room temperature for 6 h and
thereafter at 50 °C for 120 h. The mixture was filtrated over Celite, the solvent was removed under
reduced pressure and the crude product was absorbed on Celite. After flash column chromatography
(SiO2, PE) a crude mixture of F-C-Cp2 was isolated as a slightly yellow oil (1.04 g, 1.38 mmol, 90.0 %).
To eliminate not fully reacted sideproducts crude F-C-Cp2 (650 mg, 864 µmol, 1.00 eq.) was dissolved
in 10mL anhydrous DCM in a heatgun dried Schlenk tube under an atmosphere of nitrogen. To
scavenge unreacted cyclopentadienyl groups, 4-phenyl-3H-1,2,4-triazole-3,5(4H)-dione was added at
-78 °C and the resulting mixture was stirred at -78 °C for 30 min. Then the reaction mixture was
allowed to warm to room temperature and was stirred for another 1 h. The solvent was removed
under reduced pressure and the crude product was absorbed on Celite. After flash column
chromatography (SiO2, PE), gel permeation chromatography (toluene) and high pressure liquid
chromatography (PE, 5 mLmin-1) F-C-Cp2 was isolated as a highly viscous, colorless oil (578 mg, 768
µmol, 80.1 %). 1H-NMR (500 MHz, CD2Cl2): δ 0.49 - 0.66 (m, 4 H), 0.92 - 2.23 (m, 30 H), 2.51 - 3.47 (m,
8 H), 4.95 - 5.08 (m, 1 H), 5.34 - 5.43 (m, 2 H), 5.66 - 6.43 (m, 3 H), 7.41 - 7.48 (m, 4 H), 7.49 - 7.55 (m,
2 H). 13C-NMR (126 MHz, CDCl3): δ 23.7, 23.7, 26.6, 27.0, 27.4, 27.6, 27.7, 29.0, 29.3, 29.4, 29.8, 29.9,
29.9, 30.1, 30.3, 31.1, 31.6, 32.6, 33.2, 33.8, 34.3, 34.8, 35.0, 37.1, 40.3, 41.3, 41.5, 41.7, 43.6, 44.5,
45.5, 45.8, 46.2, 46.3, 46.4, 49.2, 50.1, 50.2, 53.8, 54.4, 54.7, 55.8, 56.3, 57.1, 59.6, 121.3, 121.6,
123.5, 124.4, 125.1, 126.3, 130.2, 131.4, 131.8, 132.0, 132.3, 132.3, 133.2, 135.5, 136.4, 139.2, 139.4,
145.4, 146.6, 151.2, 152.6. IR (cm-1): 3042, 2925, 2853, 1448, 1416, 1339, 1248, 1130, 1061, 876, 807,
743, 722, 687, 667. HRMS (DART+): calc. for C45H5379Br2 [M+H]+: 751.2514, found: 751.2563.
+
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3a,4,7,7a-Tetrahydro-1H-4,7-methanoinden-1-ol (4):33 Monopotassium phosphate (780 mg, 5.67
mmol, 99 wt%, 0.15 eq.) was suspended in 9 mL 1,4-dioxane and 1 mL deionized water. Freshly
distilled cyclopentadiene 3 (5.10 mL, 5.00 g, 37.8 mmol, 1.00 eq.) was added and the mixture was
heated to 90 °C. Thereafter selenium dioxide (2.14 g, 18.9 mmol, 98 wt%, 0.50 eq.) were added in
portions over a period of 45 min. Stirring was continued at 90 °C for 1 h and then at 105 °C for 3 h.
The mixture was allowed to cool to room temperature, filtrated and the residue washed twice with
diethyl ether. The organic layer was washed twice with 20mL saturated NaHCO3-solution each, 20mL
brine and was dried over MgSO4. The solvent was removed under reduced pressure and the crude
product was absorbed on Celite. After flash column chromatography (SiO2, PE : EA 10 : 1) 4 was
isolated as a yellow solid (3.02 g, 20.4 mmol, 53.9 %). 1H-NMR (500 MHz, CD2Cl2): δ 1.38 - 1.42 (m, 2
H), 2.48 - 2.52 (m, 1 H), 2.76 - 2.80 (m, 1 H), 3.02 - 3.05 (m, 1 H), 3.34 - 3.39 (m, 1 H), 4.00 - 4.05 (m,
1H), 5.57 (td, J = 5.7 Hz, J = 1.9 Hz, 1 H), 5.75 - 5.77 (m, 1 H), 5.83 (dd, J = 5.7 Hz, J = 3.1 Hz, 1 H), 5.93
(dd, J = 5.7 Hz, J = 3.0 Hz, 1 H).
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1-((6-Iodohexyl)oxy)-3a,4,7,7a-tetrahydro-1H-4,7-methanoindene (5): Sodium hydride (425 mg,
10.6 mmol, 60wt% in mineral oil, 1.05 eq.) was suspended in 10 mL anhydrous DMF. 4 (1.50 g, 10.1
mmol, 1.00 eq.) in 10 mL anhydrous DMF was added dropwise over a period of 20 min at 0 °C and
then the reaction mixture was stirred at 0 °C for 1 h. Thereafter 1,6-diiodohexane (2.58 mL, 5.29 g,
15.2 mmol, 97 wt%, 1.50 eq.) in 5 mL anhydrous DMF were added at 0 °C and the resulting mixture
was stirred at 0 °C for 30 min. Stirring was continued at room temperature for 24 h. The mixture was
quenched with 30 mL ice water and then extracted twice with diethyl ether. The combined organic
layers were washed with deionized water and were dried over MgSO4. The solvent was removed
under reduced pressure and the crude product was absorbed on Celite. After flash column
chromatography (SiO2, PE : EA 100 : 1) 5 was isolated as a slightly yellow oil (820 mg, 2.29 mmol, 22.6
%). 1H-NMR (500 MHz, CDCl3): δ 1.36 - 1.44 (m, 5 H), 1.54 - 1.59 (m, 3 H), 1.79 - 1.87 (m, 2 H), 2.58 -
2.62 (m, 1 H), 2.77 - 2.80 (m, 1 H), 2.97 - 3.01 (m, 1 H), 3.19 (t, J = 7.1 Hz, 2 H), 3.34 - 3.40 (m, 2 H),
3.45 - 3.50 (m, 1 H), 3.75 - 3.78 (m, 1 H), 5.58 (td, J = 5.7 Hz, J = 1.8 Hz, 1 H), 5.80 - 5.82 (m, 1 H), 5.86
(dd, J = 5.8 Hz, J = 2.9 Hz, 1 H), 5.95 (dd, J = 5.8Hz, J = 3.0 Hz, 1 H). 13C-NMR (126 MHz, CDCl3): δ 8.0,
26.1, 30.7, 31.1, 34.2, 45.3, 46.0, 50.8, 52.1, 55.5, 69.0, 87.3, 133.0, 133.2, 136.2, 139.0. IR (cm-1):
3054, 2957, 2929, 2854, 1726, 1452, 1427, 1361, 1339, 1204, 1108, 1084, 909, 848, 807, 770, 728.
HRMS (DART+): calc. for C16H24IO+ [M+H]+ 359.0866, found: 359.0871.
2,7-Dibromo-9,9-bis(6-((3a,4,7,7a-tetrahydro-1H-4,7-methanoinden-1-yl)oxy)hexyl)-9H-fluorene (F-
O-Cp2): 2,7-Dibromo-9H-fluorene (355 mg, 1.06 mmol, 97 wt%, 1.00 eq.) and 5 (800 mg, 2.23 mmol,
2.10 eq.) were dissolved in 30 mL anhydrous THF. Applying three freeze-thaw-cycles the solution was
degassed. Thereafter potassium tert-butoxide (274 mg, 2.39 mmol, 98 wt%, 2.25 eq.) in 50 mL freshly
degassed, anhydrous THF were added over a period of 1 h and the resulting mixture was stirred at
room temperature for 48 h. The solvent was removed under reduced pressure and the crude product
redissolved in 40 mL DCM. The organic layer was washed three times with deionized water, dried
over MgSO4. The solvent was removed under reduced pressure and the crude product was absorbed
on Celite. After flash column chromatography (SiO2, PE : EA 100 : 1) F-O-Cp2 was isolated as a highly
1
viscous, colorless oil (534 mg, 681 µmol, 64.0 %). H-NMR (500 MHz, CDCl3): δ 0.53 - 0.63 (m, 4 H),
1.03 - 1.13 (m, 8 H), 1.35 - 1.41 (m, 6 H), 1.53 - 1.57 (m, 2 H), 1.87 - 1.94 (m, 4 H), 2.52 - 2.56 (m, 2 H),
2.74 - 2.78 (m, 2 H), 2.93 - 2.96 (m, 2 H), 3.22 - 3.28 (m, 2 H), 3.31 - 3.38 (m, 4 H), 3.68 - 3.72 (m, 2 H),
5.58 (td, J = 5.7 Hz, J = 1.8 Hz, 2 H), 5.77 (d, J = 5.4 Hz, 2 H), 5.84 (dd, J = 5.7 Hz, J = 3.0 Hz, 2 H), 5.92
(dd, J = 5.7 Hz, J = 3.0 Hz, 2 H), 7.41 - 7.46 (m, 4 H), 7.51 (d, J = 8.0 Hz, 2 H). 13C-NMR (126 MHz,
CDCl3): δ 24.4, 26.7, 30.5, 30.7, 40.9, 45.2, 45.9, 50.7, 52.1, 55.4, 56.4, 69.1, 87.2, 121.9, 122.2, 126.8,
130.9, 133.0, 133.1, 136.2, 138.8, 139.8, 151.1. IR (cm-1): 3050, 2928, 2855, 1449, 1415, 1361, 1339,
1251, 1108, 1081, 1060, 1003, 909, 848, 807, 770, 724, 666, 432. HRMS (DART+): calc. for
+
C45H5679Br2NO2 [M+NH4]+: 800.2672, found: 800.2691.
General procedure for nucleophilic substitution: Under a nitrogen atmosphere, 2,7-dibromo-9H-
fluorene (1.00 eq.) and tetrabutylammonium iodide (0.10 eq.) were suspended in an aqueous 50 wt%
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NaOH solution and toluene. Applying three freeze-thaw-cycles the suspension was degassed.
Thereafter the bromide (2.50 – 2.60 eq.) was added and the resulting mixture was stirred at 110 °C
for 18 h. The reaction mixture was diluted with water and extracted with water/EA. The combined
organic layers were dried over MgSO4 and the solvent was evaporated before further purification
steps were carried out.
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2,2'-(((2,7-Dibromo-9H-fluorene-9,9-diyl)bis(ethane-2,1-diyl))bis(oxy))bis(tetrahydro-2H-pyran)
(6a):34 According to the general procedure, 2,7-dibromo-9H-fluorene 1 (2.38 g, 7.33 mmol, 1.00 eq.),
tetrabutylammonium iodide (276 mg, 733 µmol, 98 wt%, 0.10 eq.) and 2-(2-bromoethoxy)
tetrahydro-2H-pyran (3.00 mL, 4.15 g, 19.1 mmol, 96 wt%, 2.60 eq.) were reacted in 20 mL of
aqueous 50 wt% NaOH and 20 mL toluene. The crude product was purified by column
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chromatography (SiO2, PE : EA 10 : 1) to yield 6a as a colorless solid (4.10 g, 7.06 mmol, 96.4 %). H-
NMR (301 MHz, CDCl3): δ 1.25 - 1.65 (m, 12H), 2.36 (t, J = 6.9 Hz, 4 H), 2.67 - 2.77 (m, 2 H), 3.09 - 3.20
(m, 2 H), 3.21 - 3.30 (m, 2 H), 3.43 - 3.52 (m, 2 H), 4.09 - 4.14 (m, 2 H), 7.42 - 7.47 (m, 2 H), 7.48 - 7.52
(m, 2 H), 7.53 - 7.57 (m, 2 H).
2,2'-(((2,7-Dibromo-9H-fluorene-9,9-diyl)bis(hexane-2,1-diyl))bis(oxy))bis(tetrahydro-2H-pyran)
(6b):35 According to the general procedure, 2,7-dibromo-9H-fluorene 1 (2.44 g, 7.54 mmol, 1.00 eq.),
tetrabutylammonium iodide (284 mg, 754 µmol, 98 wt%, 0.10 eq.) and 2-((6-
bromohexyl)oxy)tetrahydro-2H-pyran (4.14 mL, 5.00 g, 18.9 mmol, 2.50 eq.) were reacted in 20 mL
of aqueous 30 wt% NaOH and 30 mL toluene. The crude product was purified by column
1
chromatography (SiO2, PE : EA 20 : 1) to yield 6b as a colorless solid (4.97 g, 7.18 mmol, 95.2 %). H-
NMR (301 MHz, CDCl3): δ 0.52 - 0.67 (m, 4 H), 1.03 - 1.19 (m, 8 H), 1.35 - 1.45 (m, 4 H), 1.46 - 1.59 (m,
8 H), 1.69 (td, J = 12.4 Hz, J = 3.0 Hz, 2 H), 1.73 - 1.84 (m, 2 H), 1.86 - 1.96 (m, 4 H), 3.27 (td, J = 9.7 Hz,
J = 6.6Hz, 2 H), 3.41 - 3.51 (m, 2 H), 3.62 (td, J = 9.5 Hz, J = 6.9 Hz, 2 H), 3.76 - 3.87 (m, 2 H), 4.48 - 4.54
(m, 2 H), 7.41 - 7.47 (m, 4 H), 7.49 - 7.54 (m, 2 H).
General procedure for deprotection: The THP-protected alcohol (1.00 eq.) was suspended in 40 mL
ethanol, 10 mL of hydrochloric acid (10% HCl in water, v/v) were added and the resulting mixture was
stirred at 100 °C for 1 h. The organic solvent was removed in vacuo and the residue extracted with
water/chloroform. The combined organic layers were washed with concentrated NaHCO3 solution,
dried over MgSO4 and the solvent was evaporated to yield the corresponding alcohol.
2,2'-(2,7-Dibromo-9H-fluorene-9,9-diyl)bis(ethan-1-ol) (7a):34 According to the general procedure,
6b (4.10 g, 7.06 mmol, 1.00 eq.) was reacted in 40 mL ethanol and 10 mL of hydrochloric acid (10%
HCl in water, v/v). 7a was isolated as a colorless solid (2.53 g, 6.15 mmol, 87.1 %). 1H-NMR (301 MHz,
CDCl3): δ 0.89 (bs, 2 H), 2.34 (t, J = 7.4 Hz, 4 H), 3.03 (t, J = 7.4 Hz, 4 H), 7.47 - 7.52 (m, 2 H), 7.52 - 7.59
(m, 4 H).
2,2'-(2,7-Dibromo-9H-fluorene-9,9-diyl)bis(hexan-1-ol) (7b):35 According to the general procedure,
6b (4.97 g, 7.18 mmol, 1.00 eq.) was reacted in 40 mL ethanol and 10 mL of hydrochloric acid (10%
HCl in water, v/v). 7b was isolated as a slightly yellow solid (3.70 g, 7.06 mmol, 98.2 %). 1H-NMR (301
MHz, CDCl3): δ 0.52 - 0.67 (m, 4 H), 1.03 - 1.20 (m, 8 H), 1.33 - 1.44 (m, 4 H), 1.88 - 1.96 (m, 4 H), 3.53
(t, J = 6.5 Hz, 4 H), 7.42 - 7.48 (m, 4 H), 7.49 - 7.54 (m, 2 H).
3a,4,7,7a-Tetrahydro-1H-4,7-methanoindene-2,5-dicarboxylic acid (8):30 Freshly prepared sodium
cyclopenta-2,4-dien-1-ide (11.5 g, 14.5 mmol, 1.00 eq.) was transferred to a beaker containing a
slurry of dry ice and 300 mL anhydrous THF and was stirred at -78 °C for 2 h before the mixture was
allowed to slowly warm to room temperature over night. The residue was dissolved in 400 mL
deionized water and then extracted three times with DCM. The crude product was precipitated by
acidifying the solution with hydrochloric acid (10% HCl in water, v/v), filtrated and washed three
times with deionized water. The crude product was recrystallized from methanol and dried in vacuo
1
to yield 8 as a colourless solid (9.60 g, 42.8 mmol, 32.8 %). H-NMR (301 MHz, DMSO-d6): δ 1.36 (d, J
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= 8.0 Hz, 1 H), 1.54 (td, J = 8.4 Hz, J = 1.5 Hz, 1 H), 1.84 - 1.95 (m, 1 H), 2.30 (tdd, J = 17.2 Hz, J = 10.2
Hz, J = 2.1 Hz, 1 H), 2.82 - 2.94 (m, 1 H), 3.07 - 3.13 (m, 1 H), 3.15 - 3.19 (m, 1 H), 3.41 - 3.53 (m, 1 H),
6.39 (d, J = 2.1 Hz, 1 H), 6.72 (d, J = 3.2Hz, 1 H), 12.11 (bs, 2 H).
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9
3a,4,7,7a-Tetrahydro-1H-4,7-methanoindene-2,5-dicarbonyl dichloride (9):30 8 (1.35 g, 6.14 mmol,
1.00 eq.) was suspended in 20 mL oxalyl chloride. The resulting mixture was stirred at 55 °C for 36 h
until clear solution was obtained and formation of CO2 had ceased. Thereafter it was allowed to cool
to room temperature and the solvent was removed under reduced pressure. The crude product was
used without further purification.
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General procedure for macrocycle formation: Under an argon atmosphere, pyridine (5.00 eq.) was
dissolved in 60 mL anhydrous THF. Thereafter a solution of the diol (1.00 eq.) in 100 mL anhydrous
THF and a solution of the carbonyl chloride (1.00 eq.) in 100mL anhydrous THF were added at
approximately equal rates at room temperature over a period of 5 h. The solvent was removed under
reduced pressure, the residue was dissolved in 100 mL of hydrochloric acid (10% HCl in water, v/v)
and extracted three times with DCM. The combined organic layers were dried over MgSO4 and the
solvent was evaporated before further purification steps were carried out.30
CF-COO-Cp2a: According to the general procedure, pyridine (3.97 mL, 3.90 g, 49.3 mmol, 5.00 eq.), 7a
(4.06 g, 9.86 mmol, 1.00 eq.) and 9 (2.54 g, 9.86 mmol, 1.00 eq.) were reacted in 260 mL of
anhydrous THF. The crude product was purified by column chromatography (SiO2, PE : EA 20 : 1) to
1
yield CF-COO-Cp2a as a colorless solid (379 mg, 636 µmol, 6.5 %). MP: 173 °C. H-NMR (500 MHz,
CDCl3): δ 1.40 (d, J = 8.5 Hz, 1 H), 1.65 (td, J = 8.7 Hz, J = 1.6 Hz, 1 H), 1.85 (ddd, J = 15.3 Hz, J = 6.5 Hz,
J = 3.2 Hz, 1 H), 2.01 (ddd, J = 14.9 Hz, J = 8.9 Hz, J = 3.5 Hz, 1 H), 2.24 - 2.35 (m, 2 H), 2.55 (tdd, J =
17.7 Hz, J = 10.5 Hz, J = 2.3 Hz, 1 H), 2.64 (ddd, J = 14.3 Hz, J = 8.7 Hz, J = 5.4 Hz, 1 H), 2.85 - 2.91 (m, 1
H), 2.97 - 3.04 (m, 1H), 3.24 - 3.28 (m, 1 H), 3.48 - 3.52 (m, 1 H), 3.55 - 3.61 (m, 1 H), 3.95 - 4.00 (m, 1
H), 4.16 - 4.23 (m, 1 H), 4.68 - 4.74 (m, 1 H), 6.70 (d, J = 2.0 Hz, 1 H), 6.82 (d, J = 2.9 Hz, 1 H), 7.46 (d, J
= 1.5 Hz, 1 H), 7.52 (dd, J = 8.1 Hz, J = 1.4 Hz, 2 H), 7.56 (dd, J = 8.1 Hz, J = 5.7 Hz, 2 H), 7.62 (d, J = 1.4
13
Hz, 1 H). C-NMR (126 MHz, CDCl3): δ 33.3, 37.6, 37.8, 41.0, 46.8, 47.9, 48.6, 52.6, 53.6, 53.8, 61.4,
63.1, 121.7, 121.7, 121.7, 122.1, 127.3, 127.9, 131.3, 131.3, 137.7, 138.5, 138.5, 138.7, 144.6, 148.7,
149.8, 151.6, 166.0, 166.4. IR (cm-1): 2980, 2916, 2861, 1722, 1693, 1625, 1588, 1447, 1380, 1263,
+
1213, 1158, 1076, 894, 804, 770,733, 670, 442. HRMS (DART+): calc. for C29H2879Br2NO4 [M+NH4]+:
612.0380, found: 612.0372.
CF-COO-Cp2b: According to the general procedure, pyridine (2.84 mL, 2.79 g, 35.3 mmol, 5.00 eq.), 7b
(3.70 g, 7.06 mmol, 1.00 eq.) and 9 (1.81 g, 7.06 mmol, 1.00 eq.) were reacted in 260 mL of
anhydrous THF. The crude product was purified by column chromatography (SiO2, gradient PE : EA
1
20 : 1 to 10 : 1) to yield CF-COO-Cp2b as a colorless solid (103 mg, 145 µmol, 2.1 %). H-NMR (500
MHz, CDCl3): δ 1.25 - 1.82 (m, 22H), 2.03 - 2.16 (m, 1 H), 2.59 (tdd, J = 17.7 Hz, J = 10.5 Hz, J = 1.9 Hz,
1 H), 3.06 (ddd, J = 14.8 Hz, J = 8.9 Hz, J = 4.2 Hz, 1 H), 3.14 - 3.20 (m, 1 H), 3.37 - 3.43 (m, 1 H), 3.48 -
3.57 (m, 1 H), 3.94 - 4.04 (m, 1 H), 4.06 - 4.31 (m, 3 H), 6.59 (d, J = 2.0 Hz, 1 H), 6.95 (d, J = 3.2 Hz, 1
H), 7.46 (dd, J = 8.1 Hz, J = 1.7 Hz, 2 H), 7.50 - 7.57 (m, 4 H). 13C-NMR (126 MHz, CDCl3): δ 23.5, 23.7,
24.8, 25.0, 28.2, 28.3, 28.5, 28.7, 33.3, 36.7, 37.5, 41.6, 46.8, 47.5, 51.7, 54.5, 54.6, 64.3, 64.7, 64.7,
64.3, 121.3, 121.4, 127.3, 127.3, 130.4, 138.2, 138.6, 138.6, 139.8, 142.9, 147.2, 153.6, 153.7, 165.3,
165.3. IR (cm-1): 2933, 2855, 1705, 1450, 1394, 1348, 1268, 1232, 1156, 1089, 1004, 905, 809, 763,
+
729. HRMS (DART+): calc. for C37H4479Br2NO4 [M+NH4]+: 724.1632, found: 724.1644.
General procedure for Yamamoto polymerization: Under a nitrogen atmosphere in a glovebox were
suspended bis(1,5-cyclooctadiene)nickel(0) (2.25 eq.), 2,2'-bipyridine (2.25 eq.) and (1Z,5Z)-
cycloocta-1,5-diene (2.25 eq.) in anhydrous DMF and stirred at 60 °C for 20 min. Thereafter 2,7-
dibromo-9,9-bis(2-ethylhexyl)-9H-fluorene (0.80 - 1.00 eq.), the dibrominated crosslinker monomer
(0.00 - 0.20 eq.) and 4-bromo-N,N-di-p-tolylaniline (1.5 mol%) in anhydrous THF (final solvent ratio:
DMF:THF 1:3, v/v) were added. The resulting reaction mixture was stirred at 60 °C for 36 h. The crude
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polymer was precipitated in an excess of a mixture of methanol/hydrochloric acid (2:1, v/v, 3 molL-1).
Crosslinker monomers prone to decomposition in acidic media were precipitated in an excess of a
mixture of methanol/hydrochloric acid (2:1, v/v, 1 molL-1) and the resting time in the acidic non-
solvent mixture was minimized. The precipitate was collected by filtration, filtered through a PTFE
filter (0.45 µm, THF or chloroform) and then purified by preparative gel permeation chromatography
(toluene) to yield the desired statistical copolymer.
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Optical and Photoluminescence Spectroscopy
Absorption spectra were recorded on a Jasco UV-Vis V-660 or Jasco UV-Vis V-670. Fluorescence
spectra were recorded on a Jasco FP-6500. Quantum yields were determined by an Ulbricht sphere
(6 inch) using a PTI QuantaMaster 40 equipped with a Hamamatsu R928P photomultiplier. For all
measurements in THF the solvent was purchased from Sigma Aldrich in the quality CHROMASOLV
PLUS (purity ≥ 99.9 %). For films the substance was dissolved in chloroform (1 mg/mL). The solution
was spin-coated with a Spin 150 from S.P.S. with a rotational speed of 1000 rpm for 10 s followed by
3000 rpm for 5 s.
OLED Fabrication and Characterization
Glass/ITO substrates (Kintec) were cleaned in an ultrasonic bath for 10 min in acetone and
isopropanol consecutively and treated with O2 plasma for 5 min. For a regular device architecture, a
PEDOT:PSS solution (Heraeus P VP AI 4083) was filtered through a 0.45 µm PVDF filter, spin-coated in
ambient conditions with ω = 3800 rpm and a = 800 rpm s-1 for 45 s and afterwards annealed on a
hotplate for 20 min at 135 °C. The film thickness averages at 25 nm. The further film preparation was
performed under nitrogen atmosphere. The PF derivatives were separately dissolved in toluene
(VWR >99.8%) with a concentration of 10 mg ml-1. The solutions were spin-coated with ω = 1250 rpm
and a = 1000 rpm s-1 for 60 s. The films were either annealed at 80 °C for 5 min (dried) or at 200 (PF-
EtHex) 180 (PF-COO-CP 10%), 200 (PF-C-CP 10%) or 250 °C (PF-O-CP 10%) for 30 min to obtain a film
thickness of 60 nm. TPBi (Ossila), LiF and Al (45 nm / 1 nm / 100 nm) were thermally evaporated at a
pressure of 10-6 mbar yielding an active area of 24 mm2. The OLEDs were characterized under
nitrogen atmosphere with a calibrated Botest LIV functionality test system. The electroluminescence
was examined with an OceanOptics 2000+ spectrometer. The film thicknesses were determined with
a Vecco Dektak 150 profilometer.
Results and Discussion
We first synthesized two bis(cyclopentadienyl)-substituted fluorene monomers F-C-Cp2 and F-O-Cp2
with covalently bound cyclopentadienyl groups masked as their Diels-Alder adducts with
cyclopentadiene, via alkyl or alkoxy linkers, respectively (Scheme 2). Nucleophilic substitution of 2,7-
dibromofluorene (1) with 1,6-dibromohexane yielded bis(bromohexyl)fluorene 2. Addition of sodium
cyclopentadienide in THF in the presence of an excess of Cp furnished crude F-C-Cp2 as a mixture of
several stereoisomers in 90% yield. Traces of monomers with unprotected diene functionalities were
trapped with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) and the resulting polar cycloadducts were
removed via column chromatography on silica. GPC analysis (see Supporting Information) confirmed
the monomeric nature of F-C-Cp2. F-O-Cp2 was obtained after dimerization and subsequent Riley
oxidation of cyclopentadiene 3 to vinyl alcohol 4, which in turn was reacted with 1,6-diiodohexane to
obtain vinylether 5 in 23% yield. Final nucleophilic substitution with dibromofluorene 1 furnished F-
O-Cp2 in 64% yield. To avoid liberation of Cp from the target polymers during thermal activation,
possibly compromising electronic properties and morphology of the emissive layer, we aimed for
synthesis of cyclic and thus self-protected CF-C-Cp2. Reaction of 2 with sodium cyclopentadienide in
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THF in high dilution (150
µ
M) was not successful, as only oligomeric species or unreacted starting
material were observed. After a controlled retro-Diels-Alder reaction of highly diluted F-C-Cp2 in
1,2,5- trichlorobenzene at 200 °C and subsequent intramolecular Diels-Alder reaction upon cooling to
50 °C, only traces of CF-C-Cp2 were detected (Scheme 2).
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Scheme 2: Synthesis of crosslinkable fluorene monomers F-C-Cp2 (as mixture of endo/exo and
constitutional isomers) and F-O-Cp2.
To circumvent the need for an intramolecular Diels-Alder reaction, Thiele’s acid 830,32 was utilized as a
readily available Cp dimer. The conversion of 8 into the corresponding acid chloride 932 and
subsequent alcoholysis with fluorenediol 7a, after reaction of 1 with THP-protected 6-bromoethanol,
afforded cyclic CF-COO-Cp2a (Scheme 3). CF-COO-Cp2a was obtained as the isomerically pure endo-
product as confirmed via the X-ray single crystal structure (Figure 2). The hexamethylene-based
macrocycle CF-COO-Cp2b was prepared from 7b and 9 by the same synthetic protocol. These
intramolecular macrocyclizations only occurred in low yields due to competing polymerization
processes despite working at low concentrations (7% and 2% for CF-COO-Cp2a and CF-COO-Cp2b,
respectively). However, as the crosslinkable monomers are added into the statistic polymerization
with only a share of 5% to 20%, the amount of CF-COO-Cp2a, obtained on a 400 mg scale, was
sufficient for polymer syntheses.
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O
O
n
n
O
n O
HO
n OH
5
6
7
8
Br
Br
Br
Br
Bu4NI, NaOH
HCl
1
+
n
O
O
Br
toluene/H2O,
110 °C, 18 h
EtOH/H2O,
100 °C, 1 h
6a
: n=2: 96%
6b: n=6: 95%
7a
: n=2: 87%
7b: n=6: 98%
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O
O
O
O
COOH
COCl
i) Na
ii) CO2
iii) HCl
7a or 7b,
pyridine
(COCl)2
quant.
3
n-1
THF,
rt, 5 h
n-1
rt-160°C, 6 h
33%
Br
Br
8
9
COOH
COCl
CF-COO-Cp a
2
: n=2, 6.5%
: n=6, 2.1%
CF-COO-Cp b
2
Scheme 3: Synthesis of macrocyclic monomers CF-COO-Cp2a and CF-COO-Cp2b for crosslinking.
Figure 2: ORTEP representation of CF-COO-Cp2a.
We investigated the retro-Diels-Alder reactions of the monomers by thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC, see Supporting Information). For F-C-Cp2 and F-O-
Cp2 cleavage temperatures were estimated to be 200 °C and 250 °C, respectively. The measured
mass loss is in the range of the theoretically expected one with two Cp units per monomer – in a
further cooling/heating cycle no mass loss was observed. The DSC trace of macrocyclic monomer CF-
COO-Cp2a revealed a transition at 180 °C, attributed to the retro-Diels-Alder reaction of the Cp dimer
to its open form without any loss of mass (TGA), supporting the validity of our concept. To
demonstrate monomer crosslinking, we heated all monomers (F-C-Cp2, F-O-Cp2 and CF-COO-Cp2a) to
their respective retro-Diels-Alder temperature for a period of 5 to 60 minutes under inert
atmosphere. Subsequent analysis by gel permeation chromatography (GPC) suggested the formation
of oligomeric Diels-Alder-adducts after only five minutes of heating (see Supporting Information).
Polymerization under standard Yamamoto conditions requires reaction temperatures between room
temperature and 120 °C.36 Control experiments conducted in the aforementioned manner show that
our monomers are inert towards retro-Diels-Alder reactions up to 80 °C (see Supporting
Information). Thus, monomers F-C-Cp2, F-O-Cp2 and CF-COO-Cp2a were subjected to statistic co-
polymerization with 1,6-dibromo-9,9-(2-ethylhexyl)fluorene (10) by Yamamoto coupling at 60 °C,
using 5, 10 or 20% of the crosslinkable dibromofluorenes to yield polyfluorenes PF-C-Cp2-x, PF-O-Cp2-
x and PF-COO-Cp2-x (x denotes the molar percentage of crosslinkable monomer).37 As a reference,
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the homopolymer of 10 (PF-EtHex) was prepared under the same conditions (Scheme 3). The
polymers were end-capped with 4-bromo-N,N-di-para-tolylaniline, increasing stability and efficiency
of polyfluorenes in OLEDs without affecting electronic properties of the backbone.38 All polymers
were obtained in yields between 37% and 82% by precipitation from a mixture of methanol/3 M
hydrochloric acid (2:1, v/v) and subsequent purification by preparative size exclusion
chromatography. They were fully characterized via analytical gel permeation chromatography versus
polystyrene standards in THF, TGA/DSC and optical spectroscopy, both in solution and thin films
(Table 1, Table 2, Figure 3 and Supporting Information).
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Scheme 4: Nickel-mediated statistic Yamamoto polymerization yielding crosslinkable polyfluorenes
using 5%, 10% and 20% of crosslinkable monomers.
Table 1: Yield, molecular weights and thermal properties of polyfluorenes prepared by statistic
Yamamoto coupling.
b
yield
(%) (104 g/mol)
Mn
n+ma
Mw
(104 g/mol)
10.4
Ð
Tonset
c
ꢀ
mtheo ꢀ
mc
(°C)
(%) (%)
0.0 0.0
1.7 1.3
3.2 2.4
6.2 3.7
1.7 1.2
3.2 2.6
6.1 3.5
0.0 0.0
0.0 0.0
0.0 0.0
d
PF-EtHex
PF-C-Cp2-5%
PF-C-Cp2-10%
PF-C-Cp2-20%
PF-O-Cp2-5%
PF-O-Cp2-10%
PF-O-Cp2-20%
PF-COO-Cp2-5%
66
67
63
37
63
65
56
81
4.7
3.7
4.1
1.7
2.4
2.0
1.4
6.0
4.6
4.2
121
92
2.2
-
10.2
2.8 205
6.0 202
5.1 200
1.9 240
1.8 225
100
40
20.8
10.2
61
4.7
49
3.6
32
2.1
1.5 225
d
152
117
105
22.9
3.8
3.8
6.5
-
-
-
d
d
PF-COO-Cp2-10% 82
17.6
PF-COO-Cp2-20% 76
27.1
a
b
Number of repeat units calculated from Mn. Onset temperature of retro-Diels-Alder reaction
c
determined via TGA (scanning rate 10 K/min). (Theoretical/observed) mass loss after retro-Diels-
Alder reaction and complete Cp expulsion. d No mass loss detected.
Table 2: Optical properties of polymers in THF and thin films at room temperature.
in solution
thin film before
ꢀ
Ta
thin film after ꢀ
Tb
λmax, abs λmax, em
Φ
λmax, abs λmax, em
Φ
λmax, abs λmax, em
Φ
(nm)
377
379
(nm)
414
414
(%)
(nm)
380
383
(nm)
422
422
(%)
(nm)
379
383
(nm)
422
423
(%)
17.4
13.3
PF-EtHex
80.1
83.3
22.8
20.9
PF-C-Cp2-5%
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PF-C-Cp2-10%
PF-C-Cp2-20%
PF-O-Cp2-5%
385
383
384
383
380
385
385
386
415
415
415
415
415
415
415
415
80.2
81.4
86.9
85.1
86.0
84.7
85.2
78.9
385
384
385
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26.7
22.5
25.9
23.6
32.8
25.9
25.7
23.6
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21.4
14.5
12.3
11.3
11.8
31.0
25.4
25.4
PF-O-Cp2-10%
PF-O-Cp2-20%
PF-COO-Cp2-5%
PF-COO-Cp2-10%
PF-COO-Cp2-20%
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a Before application of a thermal stimulus. b After application of a thermal stimulus ꢀT, where ꢀT
exceeds the determined retro-Diels-Alter temperature.
The number of repeat units of all polyfluorenes well exceeded the effective conjugation length (n >
12).39 With increasing amount of crosslinkable units, dispersity increases deviating from the value
expected for polycondensations (Ð = 2). These characteristics are due to minor dimeric impurities
(<1%) in the starting materials, resulting in decreased solubility and lower yields of the polymers
containing 20% of crosslinker. The DSC peaks attributed to the retro-Diels-Alder reaction in addition
to the weight loss observed in the mass trace (see Supporting Information) are in agreement with the
cleavage temperatures of the monomers. The absorption spectra measured from solutions in THF
exhibit absorption maxima at λmax,em = 377 - 386 nm typical of polyfluorenes (see Table 2, Figure 3 and
Supporting Information). The fluorescence spectra reveal emission maxima at λmax,em = 414 - 415 nm
with fluorescence quantum yields in the range of Φ = 79-87%. The emission spectra of thin films do
not exhibit bands at lower energies attributed to fluorenone units. Even after thermal treatment at
250 °C, necessary for the activation of PF-O-Cp2, the solid-state emission spectrum does not show
any sign of keto-defect formation.40,41
Figure 3: Representative absorption and fluorescence spectra of PF-COO-Cp2-10% in solution (THF)
and thin films on glass before and after annealing at 180 °C.
Effective crosslinking was demonstrated by rinsing drop-cast polymer films with toluene as a good
solvent before and after thermal annealing at 200 °C for 30 min (see Supporting Information). For
process optimization, 50-70 nm thick polymer films containing 10 mol% of crosslinker were spun-cast
from polymer solutions (8 mg/mL) in toluene. These films were annealed thermally (180 °C, 200 °C
and 250 °C for PF-COO-Cp2, PF-C-Cp2 and PF-C-Cp2, respectively) for different time periods and stored
over night at room temperature. After treatment with 150 µL toluene for 5 seconds before spinning
off the solvent, the absorption spectra were recorded. The changes in absorption were quantified
and compared to those of the samples before thermal treatment (see Figure 4 and Supporting
Information).
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Figure 4: Differential absorption spectra of reference polymer PF-EtHex and PF-COO-Cp2-10% before
and after annealing at 180 °C for a given amount of time. Smaller changes correspond to a higher
degree of desolubilization and crosslinking.
For non-crosslinkable PF-EtHex, absorbance was reduced by ca. 60%-70%, independent of the
annealing time. Heating films of PF-O-Cp2-10%, PF-C-Cp2-10% and PF-COO-Cp2-10% resulted in
complete desolubilization: After 30 minutes at the respective cleavage temperatures, a change of
only max. 3% in absorbance was observed for all polymers – even 10 min sufficed to form insoluble
networks. Films of higher crosslinking content were even more solvent resistant, whereas a lower
crosslinking content of 5% appeared to be insufficient. Additionally, since the above method is
susceptible to potential errors from film inhomogenieties, we investigated crosslinking with white
light interferometry, visualizing changes of film thickness in the nanometer range. Droplets of various
solvents were placed on 50-70 nm thin polymer films by inkjet-printing before and after thermal
treatment for 30 min (Figure 5, chlorobenzene as solvent). Changes in height profiles are summarized
in the Supporting Information. Both methods showed that untreated polymer layers were re-
dissolved by chlorobenzene, whereas annealed films exhibited excellent solvent resistivity. Utilizing
10% and 20% of crosslinker, only a small coffee ring effect42 as a result of swelling was
observed.43,44,45 PF-C-Cp2 (20% crosslinker content) were an exception in our series: A small amount
of already crosslinked moieties within the monomer (vide supra) might explain solvent persistence
already after drying at room temperature. Solvent resistance was not limited to chlorobenzene as
demonstrated for non-crosslinkable PF-EtHex and PF-COO-CP2-10% for five additional organic
solvents utilizing the same experimental setup (see Supporting Information). The shorter connection
of the Cp to the polymer backbone of PF-COO-CP2-10% in terms of linker length (4 atoms vs 6 or 7 in
PF-C-Cp2-10% or PF-O-Cp2-10%, respectively) and the resulting decreased flexibility do not prevent
desolublization for the crosslinker ratios and conditions investigated.
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Figure 5: Surface profiles of chlorobenzene droplets on thin films before and after thermal
treatment. a) PF-EtHex, b) PF-C-Cp2-5%, c) PF-C-Cp2-10%, d) PF-C-Cp2-20%, e) PF-O-Cp2-5%, f) PF-O-
Cp2-10%, g) PF-O-Cp2-20%, h) PF-COO-Cp2-5%, i) PF-COO-Cp2-10% j) PF-COO-Cp2-20%.
Solution processed proof-of-concept OLEDs were fabricated to evaluate the influence of the
crosslinked side chains. Pristine PF-EtHex was chosen as the reference and compared to
polyfluorenes with 10% crosslinker, representing optimum desolubilization as previously discussed.
Device performances of OLEDs with active layers treated at the respective crosslinking procedure
(see Experimental) were evaluated against dried layers (i.e. 80 °C, 5 min). Figure 6 displays the LIV-
characteristics of the devices showing turn-on voltages < 7 V and luminance up to 104 cd m-2. The
OLEDs’ figures of merit are summarized in Table 3. As a note, all the dried (80 °C) samples revealed
comparable device performances (see Supporting Information).
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Figure 6: LIV-characteristics of the pixel closest to average of at least 4 samples of each polymer (a),
electroluminescence spectra (b) and micrographs of the light emission (c-f) of annealed polyfluorene
layers.
Table 3: LIV parameters from measurements of dried and annealed OLEDs.
PF-COO-Cp2-10%
PF-C-Cp2-10%,
PF-O-Cp2-10%
PF-EtHex
Dried Annealed Dried Annealed Dried Annealed Dried Annealed
Luminance[a] 2972.1 5041.9 ± 1382.7 2715.8 ± 4572.9 1350.6 ± 1316.5 12071.2 ±
[cd/m2]
±
1061.1
± 59.8 940.9
±
198.0
±
3516.9
1980.3
478.2
2.8 ± 1.5 3.7
227.5
0.5 ± 0.1 3.5
1.9
Efficiency[b]
[cd/A]
5.1
0.3
±
4.6 ± 0.2
3.9
0.2
±
±
±
5.1 ± 0.6
0.5
Efficacy
[lm/W]
Turn-On-
Voltage[c] [V] 0.2
2.6
0.4
5.4
±
±
2.0 ± 0.1
5.6 ± 0.0
1.5
0.1
7.1
0.1
±
±
1.2 ± 0.7 1.6
±
±
0.2 ± 0.0 1.5
±
±
2.4 ± 0.2
5.9 ± 1.5
0.2
6.9 ± 0.8 6.5
0.2
0.9
6.7 ± 0.0 6.9
0.4
[a] Maximum luminance. [b] Maximum value (not a defined luminance). [c] @ 1 cd.
In terms of performance, a clear trend is observed among the crosslinkable polyfluorenes: self-
protected PF-COO-Cp2-10% exhibits the best behavior in terms of luminance, efficiency, efficacy as
well as turn-on voltage after high-temperature annealing. Compared to the dried devices, an increase
in luminance with a slight reduction in efficacy was observed for the crosslinked PF-COO-CP2-10%
(180 °C) and PF-C-Cp2-10% (200 °C). The higher crosslinking temperature of 250 °C required for
activation of PF-O-Cp2-10% lead to a decrease in device efficiency down to 0.5 cd A-1 emphasizing the
need of lowering annealing temperatures. The best performing crosslinked devices were prepared on
the basis of PF-COO-CP2-10% achieving a luminance of 0.5x104 cd m-2, current (power) efficiency of
4.6 cd A-1 (2.0 lm W-1) and a turn-on voltage of 5.6 V. These might compete with the reference device
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after further optimization, which exhibits the highest efficiency and efficacy after a thermal
annealing (200°C, 30 min) with 5.1 cd A-1 and a maximum luminance of ~1.2x104 cd m-2. To gain an
explanation for the decrease in device performance, morphological changes in the active layers were
investigated. Comparing the surface topologies of dried and thermally crosslinked films by AFM
measurements (see Supporting Information) indicated that thermal treatment lead to morphological
changes resulting in holes which were also observed in the treated reference sample PF-EtHex.
Nevertheless, short-contacts did not occur in any device. We assume that the cavities do not extend
through the entire layer and that they are fully covered by TPBi. Figure 6b presents the typical
electroluminescence-spectra (EL) and micrographs of devices under operation for the annealed
devices. The EL spectra correspond to the photoluminescence spectra presented in Figure 3. The
integrity of the emission properties is preserved after desolubilization.
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As PF-COO-Cp2-10% exhibits superior device performance compared to PF-C-Cp2-10%, which has only
been treated at slightly higher temperatures also employed for annealing PF-EtHex, we conclude that
extrusion of Cp during desolubilization has to be avoided. Incorporation of macrocyclic CF-COO-Cp2a
or CF-COO-Cp2b into other functional polyfluorenes or synthesis of structurally related macrocyclic
monomers seems promising to facilitate production of purely thermally and by-product free
crosslinkable polymeric layers for organic electronics. These could be directly employed after a short
heat stimulus without the need of further steps, like annealing or washing, and are thus compatible
with roll-to-roll processes.
Conclusion
Thermally induced solubility modulation based on reversible Diels-Alder reactions of
cyclopentadienes attached to polyfluorenes was utilized to form solvent-resistant emissive layers in
OLEDs. The semi-conducting polymers were synthesized via statistic nickel-mediated Yamamoto
polymerization of conventional ethylhexyl-substituted fluorene monomers with crosslinkable
bis(cyclopentadienyl)-functionalized bis(bromo)fluorenes in different ratios (5%, 10% and 20% of
crosslinker). These bis(cyclopentadiene)s, incorporated into the polymer side-chains through
different linkages (C-C bonds, ether or ester groups), are thermally activated by retro-Diels-Alder
reaction in the temperature range of 180 °C – 250 °C, controllable via their electron demand. The
generated pending cyclopentadienyl units crosslink by additive-free [4+2] cycloadditions upon
cooling with high functional group tolerance. Two polyfluorenes bearing macrocyclic self-protected
bis(cyclopentadienylene) units were synthesized to avoid extrusion of Cp as sacrificial protecting
groups during network formation. The solvent resistance of the resulting PF layers was examined by
absorption spectroscopy and white-light scanning interferometry – a comparably short heat stimulus
of 10 to 30 min at the respective activation temperatures is sufficient to render these films insoluble.
Among the crosslinkable PFs applied in proof-of-concept OLED devices, the macrocyclic self-
protected bis(cyclopentadienylene) demonstrated superior performances over their externally Cp-
protected counterparts. Thus, the necessity of by-product free immobilization processes was
emphasized. In the future, we will extend our concept of self-protected macrocyclic systems to
higher performing polymeric and monomeric semiconductors for fully printed organic electronic
devices. The relatively low activation temperature additionally allows application of our process to a
variety of flexible plastic substrates. The reversibility of the dimer formation offers further
opportunities to increase stability of layers at their interfaces or for patterning.
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Supporting Information Description
1
General remarks, synthetic details and compound characterization, H, 13C NMR and GPC spectra,
TGA/DSC measurements, UV-Vis and PL spectra, further information on crosslinking tests, single
crystal structure analysis and AFM measurements of films.
7
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Acknowledgements
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The authors acknowledge financial support of the Federal Ministry for Education and Research
(BMBF) through Grant FKZ: 13N13695 and FKZ 13N13691. We thank Dr. Frank Rominger for single
crystal structure analysis of our macrocycle.
TOC
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