Article pubs.acs.org/Macromolecules
Photo-Cross-Linkable Polymeric Optoelectronics Based on the [2 + 2] Cycloaddition Reaction of Cinnamic Acid Korwin M. Schelkle,†,# Markus Bender,† Sebastian Beck,‡,# Krischan F. Jeltsch,∥,# Sebastian Stolz,#,% Johannes Zimmermann,‡,# R. Thomas Weitz,∥,#,& Annemarie Pucci,‡,# Klaus Müllen,⊥,# Manuel Hamburger,†,# and Uwe H. F. Bunz*,†,§ †
Organisch-Chemisches Institut, ‡Kirchhoff Institut für Physik, and §Centre of Advanced Materials, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany ∥ BASF SE, 67056 Ludwigshafen, Germany ⊥ Max-Planck Institut für Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany # InnovationLab GmbH, Speyerer Str. 4, 69115 Heidelberg, Germany % Lichttechnisches Institut, Karlsruher Institut für Technologie, Engesser Straße 13, 76131 Karlsruhe, Germany & Physics of Nanostructures, Faculty of Physics, Ludwig-Maximilians-Universität München, Amalienstraße 54, 80799 München, Germany S Supporting Information *
ABSTRACT: We report the synthesis of cinnamic acid-functionalized conjugated polymers, which are cross-linked via [2 + 2] cycloaddition by UV illumination, reducing their solubility. The cross-linking reaction was investigated by a combination of FTIR and optical spectroscopy, and an optimum condition for the solubility modulation of thin films, a major challenge in the solution-phase fabrication of layered optoelectronic devices, was reached. As proof of concept, OLEDs were fabricated, using these conjugated polymers as emissive layers.
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acids.16 Additionally, the optical properties of cinnamates are easily tailored to external demands by attaching electron donating or withdrawing groups to the chromophore. Cinnamates are successfully used as photo-cross-linkable moieties with side-chain polymers in hole transport materials.17,18 Cross-linking of cinnamates is limited to side-chain polymers. Our recent approach to achieve light-induced solubility modulation in thin films was based on coumarine formation from o-hydroxycinnamate-functionalized polyfluorene.19 One major drawback within this concept is the unclear role of remaining coumarine after cross-linking. In this contribution we report photo-cross-linkable conjugated polymers based on the [2 + 2] cycloaddition of attached cinnamate without the formation of byproducts and their proof-of-concept application as emitting layer in OLEDs.
INTRODUCTION Since their first discovery by Tang and van Slyke,1 organic lightemitting diodes (OLEDs) have attracted interest because of their potentially low cost fabrication, energy efficiency, and semitransparency.2−6 While physical vapor deposition is the method of choice for complex multilayer OLED architectures, the evaporation process only works for small molecules. Transferring physical vapor deposition techniques into rapid and low-cost reel-to-reel processes remains a challenge. Solution-based approaches circumvent these drawbacks and allow large area device fabrication. Additional challenges arise for solution processing when preparation of multilayer architectures is demanded. One key requirement is the application of subsequent layers without dissolution of previously deposited ones. To meet this requirement, different approaches have been developed: the “orthogonal solvent” approach,7 transformation of soluble precursors after deposition,8 and solubility modulation by photo-cross-linking9−11 or thermal cross-linking reactions.12,13 For photo-cross-linking, cinnamates have gained attention, as they readily undergo [2 + 2] cycloaddition when exposed to UV irradiation.14,15 In contrast to other photo-cross-linking protocols, cinnamates do not require additives like photo© XXXX American Chemical Society
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RESULTS AND DISCUSSION From a cinnamate-functionalized building block 1, the polyfluorene 5 and a donor−acceptor type fluorene−benzoReceived: November 5, 2015 Revised: January 30, 2016
A
DOI: 10.1021/acs.macromol.5b02407 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. Suzuki Polymerization of Functionalized Building Block 1 into Polyfluorene 5 and Fluorene−Benzothiadiazol Copolymer 6a
a
For more details see Supporting Information.
thiadiazole copolymer 6 were prepared (Scheme 1). Numberaverage molecular weights (by gel permeation chromatography vs polystyrene standards after precipitation from acetone) were 21 kg/mol (PDI = 2.5) for 5 and 17 kg/mol (PDI = 2.2) for 6. Monomers 1, 3, and 4 were employed in the ratio of 1:1:2 in the polymerization reaction. The ratio of the different fluorene−benzothiadiazole building blocks (x to y) in 6 was determined by 1H NMR spectroscopy to be around 1. Thin films of monomer 1 were spin-coated on glass slides, and the cross-linking reaction was tested. The progress of the reaction was monitored by optical spectroscopy. After 30 min irradiation at λ = 365 nm, a decrease of the absorbance at λ = 370 nm indicated successful [2 + 2] cycloaddition (for spectra and details on the irradiation setup, see Supporting Information).20 We redissolved the thin films after irradiation, and UPLC/MS analysis showed formation of the cycloaddition product (calcd [M+] 1306.97; found 1306.45, rt = 6.21 min; see Supporting Information). Dialkylamino substitution of the cinnamates shifts the absorption of the polymers beyond 350 nm to minimize photochemical degradation during cross-linking. To study the cross-linking reaction, thin films of polymers 5 and 6 were prepared via spin-coating. Remarkably, for 5, cross-linking after irradiation at λ = 365 nm under standard as well as inert conditions occurred even in the presence of the strongly absorbing polyfluorene backbone, its absorption overlapping with that of the cinnamic acid chromophore at around λ = 370 nm.21 Layer thickness variations due to cross-linking and the following rinsing step were monitored by UV−vis ellipsometry. The progress of the cross-linking reaction was monitored by a continuous decrease of the absorbance at λ = 370 nm (see Figure 1a). In parallel, thin film photoluminescence (PL) slightly increased during the progress of the reaction, most likely due to morphological fixation and thus reduction of the degree of freedom for nonradiative relaxation (see Supporting Information for doseresolved emission spectra of 1). The cross-linking of 6 was monitored by a decline in absorbance at λ = 370 nm, while the absorbance of the fluorene−benzothiadiazole backbone remained unchanged (see Figure 1b). Cross-linking of 6 appeared to be nonproductive under an inert atmosphere. No changes in absorption spectra occurred, and the resulting films were completely soluble in
Figure 1. (a) Dose-dependent relative thin film absorption spectra of 5 for irradiation at λ = 365 nm on glass substrates, irradiated under inert conditions. (b) Dose-dependent relative thin film absorption spectra for 6 for irradiation at λ = 365 nm on glass substrates, irradiated under standard conditions.
organic solvents. However, it did occur smoothly when films were irradiated in air. In contrast to 5, a significant decrease in the emission properties was observed during irradiation (see Supporting Information for dose-dependent emission spectra for 6). This observation was attributed to photochemical material degradation of 6 in the presence of oxygen. To demonstrate macroscopic insolubility, the irradiated films were immersed in organic solvents, including THF, toluene, dichloromethane, ethyl acetate, and chloroform, and the thin film absorption and emission properties were analyzed again. After irradiation for 30 min (720 J/cm2), unchanged absorption of the polymer films was recorded after rinsing, while the emission intensity had decreased slightly. Rinsing nonirradiated films of 5 and 6 resulted in their dissolution. To gain a more detailed insight into the cross-linking reaction and its effects on solubility control and material degradation, thin films of both polymers were analyzed by FTIR spectroscopy. As seen from the dose-dependent FTIR spectra in Figure 2a for 5, irradiation above 120 J/cm2 causes material degradation. The band attributed to the backbone at 1458 cm−1 changes, whereas the progress of the cross-linking reaction is monitored by the disappearance of the vinylic vibration mode (1522 cm−1) and the appearance of an additional shoulder in the ester vibration mode at 1730 cm−1. B
DOI: 10.1021/acs.macromol.5b02407 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. Dose-dependent relative IR transmission spectra and corresponding relative transmission spectra of thin films of 5 (a, b) and 6 (c, d) on silicon before UV (black), after UV (96 J/cm2, red), and rinsing (green). Backbone attributable band at 1458 cm−1, vinylic vibration mode at 1522 cm−1, and ester vibration mode at 1730 cm−1. Corresponding layer thicknesses plotted over the irradiation doses (see Supporting Information). Thin films of 5 were irradiated under an inert atmosphere and 6 under standard conditions.
Combination of dose-dependent irradiation with subsequent THF rinsing reveals that doses of 72−120 J/cm2 are sufficient to render the film insoluble, accompanied by only minute changes in the spectra (Figure 2b). For 6, similar dose-resolved FTIR spectra are shown in Figure 2c. An identical trend is observed, but 6 already degrades at doses above 36 J/cm2. Effective cross-linking occurs at doses between 9 and 36 J/cm2, as displayed by the disappearance of the vinylic vibration mode (1522 cm−1) and a shift of the ester vibration mode (1705 cm−1). Samples irradiated with 18−72 J/cm2 remained insoluble in THF with unchanged FTIR spectra after rinsing (Figure 2d). Measured layer thickness plotted over the irradiated dose is displayed in the Supporting Information. By combining these results, an optimal illumination dose was identified; the film is insoluble but not damaged by irradiation. The relative change in peak intensity of the vinylic vibration mode at 1522 cm−1 was used to estimate the degree of crosslinking. Therefore, the peak areas before and after the crosslinking reaction were determined by fitting the corresponding absorption band using a Voigt profile (convolution of a Lorentzian and Gaussian line shape) and were divided by each other for normalization. The observed change in the normalized peak areas with increasing illumination dose is a direct measure of the extent of cross-linking in the layers. Our evaluations reveal that approximately 50% of polymer 5 and 40% of polymer 6 are cross-linked at the respective optimal illumination dose of 96 and 18 J/cm−1 (see Supporting Information). In conclusion, all results obtained by FTIR spectroscopy for polymers 5 and 6 are summarized in Table 1. Intrigued by the observation for 6 to cross-link only in the presence of oxygen, we reasoned about the role of oxygen and the excited states involved during the course of reaction. The dimerization of the cinnamate can either follow a singlet or triplet mediated mechanism.22 Based on our observations that cross-linking takes place under standard conditions for both polymers and monomer, a triplet-mediated mechanism can be excluded, since it would be quenched in the presence of oxygen, a potent triplet quencher. Assuming a singlet-mediated mechanism for the [2 + 2] cycloaddition, we thought of reaction pathways involving the conjugated backbone of the
Table 1. Conclusions Derived from FTIR Spectroscopy Experiments monomer 1 cross-linking under inert conditions cross-linking under standard conditions illumination dose range for crosslinking [J/cm−2] optimal illumination dose [J/cm−2] material degradation dose [J/cm−2] extent of cross-linkinga [%] layer thicknessb [nm] pristine cross-linked washed
yes yes
polymer 5
polymer 6
yes yes
no yes
72−120
9−36
96 ≥120 50
18 ≥36 40
88 84 69
90 89 77
a
Extent of cross-linking at optimal illumination dose. bLayer thickness at optimal illumination dose.
polymer. Presuming the role of the conjugated backbone is restricted to photosensitize23 the [2 + 2] cycloaddition through singlet energy transfer, the ongoing cross-linking reaction for 5 is reasonable. However, monomer 1 also cross-links after irradiation at 365 nm, but within these conditions the fluorene core is not able to photosensitize the [2 + 2] cycloaddition, since it shows no absorbance which overlaps with the cinnamic acid moiety. As a consequence, photosensitizing by the conjugated backbone of the polymers is indeed possible and might enhance the cross-linking reaction, but it is not crucial for the [2 + 2] cycloaddition to take place. If photosensitizing would be crucial for the [2 + 2] cycloaddition to proceed, crosslinking of monomer 1 would not have been observed. We thought of triplet states within the reaction pathway that prevent cross-linking under inert conditions, but allowing the reaction to proceed if suitable quenchers are present. The incorporation of heavy sulfur atoms in the backbone of fluorene polymers is known to foster intersystem crossing into a triplet state after excitation.24 In the case of polymer 6 this is presumably followed by energy transfer toward the cinnamate moiety revealing a triplet state that suppresses the singletmediated [2 + 2] cycloaddition. As a consequence, the presence of oxygen or other triplet quenchers is mandatory to quench C
DOI: 10.1021/acs.macromol.5b02407 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 2. Device Data for OLEDs Prepared with 5 and 6 as EML device
EML polyfluorene
1 2 3 4 5 6
5 5 6 6 6 6
EML treatment
Lmaxa [cd/m2]
VLmaxb [V]
CEmaxc [cd/A]
VCEmaxd [V]
UV (argon)
87 87 570
11.0 11.0 11.1
0.045 0.045 0.12
8.4 8.4 10.4
240 273
13.1 12.9
0.08 0.10
12.0 11.8
UV (air) COT UV (COT)
CE100e [cd/A]
V100f [V]
Vong [V]
0.09
9.3
6.8 7.0 7.3
0.07 0.10
11.8 11.2
10.0 8.7
a
Maximum luminance. bVoltage at maximum luminance. cMaximum current efficiency. dVoltage at maximum current efficiency. eCurrent efficiency at 100 cd/m2. fVoltage at 100 cd/m2. gTurn-on voltage.
this state, allowing the cross-linking to proceed. To test our hypothesis, we irradiated thin films of 6 in the presence of other triplet quenchers like cyclooctatetraene (COT) in an inert atmosphere. FTIR analysis shows a decrease of the vinylic bands at 1522 cm−1 and a broadening of the ester mode at 1705 cm−1, which indicates successful cross-linking when working under an atmosphere of COT (see Supporting Information). However, the resulting films did not show complete resistance to organic solvents within a COT atmosphere; therefore, longer irradiation times are needed. As proof of principle, we employed 5 and 6 as emissive layers in solution-processed OLEDs. Simple stack architecture OLEDs consisting of indium tin oxide (ITO)/poly(3,4ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS)/ emission-layer (EML) /Ca/Al were fabricated. Prior to deposition of the cathode materials, the solution-processed EML was irradiated with the optimum dose derived from our previous experiments. In parallel, OLEDs with nonirradiated emission layers were fabricated to assess the effect of irradiation on the device performance. OLED data for all the devices are summarized in Table 2 and Figure 3. OLEDs with nonirradiated emission layers displayed full functionality with current efficiencies of about 0.05 cd/A for devices based on 5 (device 1, Table 2) and 0.1 cd/A for devices based on 6 (device 3, Table 2) and a luminance of 87 and 570 cd/m2, respectively. Devices based on 6, which were crosslinked in air (device 4, Table 2), did not display electroluminescence at all, a testament to material degradation by oxygen. Owing to the need of triplet quenchers for the crosslinking mechanism, and as a less-detrimental alternative to oxygen, COT was used as a solvent-vapor atmosphere to crosslink 6 (devices 5 and 6, Table 2). The maximum luminance of the nonirradiated COT-treated OLEDs was reduced with respect to that of the pristine sample (device 3); however, an increased brightness of 273 cd/m2 and a turn-on voltage reduced by 1.3 V were obtained after the cross-linking of 6 (device 6) compared to that of the nonirradiated COT-treated sample (device 5). Note that the COT atmosphere is expected to influence the film morphology and may act as an additional contamination in the film. Additional research on low boiling triplet quenchers is required. Remarkably, for device 2 based on polyfluorene 5 we did not observe any negative influence of cross-linking on the device performance compared to that of the non-cross-linked device 1. Relevant parameters were not affected by cross-linking under inert conditions with constant luminance of 87 cd/m2 and a turn-on voltage of 7.0 V. This result is rather promising, showing full compatibility of lighttriggered solubility removal in the presence of a conjugated polymer without detrimental losses on device performance.
Figure 3. Top row: luminance−voltage (■), current density−voltage (□) (a) and efficiency−luminance plots (b) of OLEDs with 5 as emitter (black: EML UV irradiated under inert atmosphere; red: nonirradiated EML. Bottom row: luminance−voltage (■), current density−voltage (□) (c) and efficiency−luminance plots (d) of OLEDs with 6 as emitter (black: EML UV irradiated under a COT atmosphere; red: EML annealed under COT atmosphere without irradiation; green: nonirradiated EML).
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CONCLUSIONS
In summary, photoinduced [2 + 2] cycloaddition of cinnamates cross-links conjugated polymers. The effect of cross-linking on material solubility was investigated by a combination of optical and FTIR spectroscopies. Direct photo-cross-linking of solution-processed multilayer optoelectronic devices is feasible, demonstrated for modified emissive polymers used in OLED stacks. Upon irradiation and solubility modulation of thin films of 5 adverse effects on device performance were not observed. These results bear potential to exploit liquid phase processing of multiple layers by a simple protocol. The limited efficiency of the devices in this study is attributed to a simple stack geometry and sample irradiation in a non-clean-room standard glovebox. Further work will be guided toward the development of multilayered, solution-processed OLEDs, applying our proposed concept for solubility modulation as well as by investigation of the exact role of the triplet quenchers. D
DOI: 10.1021/acs.macromol.5b02407 Macromolecules XXXX, XXX, XXX−XXX
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02407. Experimental details (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Fax +49(6221)548401; e-mail
[email protected]. de (U.H.F.B.). Author Contributions
K.M.S. and M.B. contributed equally. Funding
This work was financially supported within the leading-edge cluster “Forum Organic Electronics” as part of the High-Tech Strategy for Germany of the Federal Ministry of Education and Research (FKZ 13N11701). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS K.M.S. thanks the Chemical Industry Fund of the German Chemical Industry Association (VCI) for a scholarship. ABBREVIATIONS FTIR, Fourier transform infrared; COT, cyclooctatetraene; OLED, organic light-emitting diode; EML, emission layer; PEDOT:PSS, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate; HPLC, high pressure liquid chromatography; MS, mass spectrometry; THF, tetrahydrofuran; PDI, polydispersity index.
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DOI: 10.1021/acs.macromol.5b02407 Macromolecules XXXX, XXX, XXX−XXX
