Communication
Efficient Separation of Conjugated Polymers Using a Water Soluble Glycoprotein Matrix: From Fluorescence Materials to Light Emitting Devicesa Netta Hendler, Jurjen Wildeman, Elad. D. Mentovich, Tobias Schnitzler, Bogdan Belgorodsky, Deepak K. Prusty, Dolev Rimmerman, Andreas Herrmann,* Shachar Richter*
Optically active bio-composite blends of conjugated polymers or oligomers are fabricated by complexing them with bovine submaxilliary mucin (BSM) protein. The BSM matrix is exploited to host hydrophobic extended conjugated p-systems and to prevent undesirable aggregation and render such materials water soluble. This method allows tuning the emission color of solutions and films from the basic colors to the technologically challenging white emission. Furthermore, electrically driven light emitting biological devices are prepared and operated.
1. Introduction N. Hendler, E. D. Mentovich,[+] Dr. B. Belgorodsky, Prof. S. Richter School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 69998, Israel E-mail: [email protected] N. Hendler, E. D. Mentovich, D. Rimmerman, Prof. S. Richter University Center for Nanoscience and Nanotechnology, Tel Aviv University, Ramat Aviv, Tel Aviv 69998, Israel J. Wildeman, Dr. T. Schnitzler, Dr. D. K. Prusty, Prof. A. Herrmann Department of Polymer Chemistry, University of Groningen, Zernike Institute for Advanced Materials, Nijenborgh 4, 9747, AG Groningen, The Netherlands E-mail: [email protected] [+] Present address: Mellanox Technologies, P.O. Box 586, Beit Mellanox, Yokneam 2069200, Israel a Supporting Information is available from the Wiley Online Library or from the author.
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Conjugated polymers (CPs) and oligomers are an established class of functional materials.[1,2] They are characterized by a conjugated polymer backbone with a delocalized electronic structure. The combination of semi-conducting and light-harvesting properties makes them valuable building blocks in organic electronic devices, such as field effect transistors, photovoltaic cells, and light emitting diodes (LED) as well as in chemical and biological sensors.[3–7] The most important CP scaffolds are derived from poly(fluorene) (PF),[8–10] poly(thiophene) (PT),[11] poly(p-phenyleneethinylene) (PPE),[12] and poly(p-phenylenevinylene) (PPV).[13] The success of these materials originated from their favorable chemical and physical properties, their straightforward synthesis and ease of functionalization. The electronic and optical properties
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DOI: 10.1002/mabi.201300329
Efficient Separation of Conjugated Polymers Using a Water Soluble Glycoprotein Matrix . . . www.mbs-journal.de
of CPs are mainly determined by their HOMO–LUMO levels,[14] their molecular conformation of the conjugated backbone and their supramolecular assembly.[15] The control of the HOMO–LUMO levels of CPs is in particular important since the band-gap does not only determine the color of emission but also the charge injection efficiency and the charge carrier mobility in optoelectronic devices. Band gap engineering starts from appropriate selection of the conjugated polymer backbone and is further tuned by donor/acceptor substitution, copolymerization or variation of the conjugation length. Evenly important is controlling the aggregation behavior of CPs, which has been extensively studied to optimize the processing conditions in organic electronic device fabrication.[16,17] In this context, efficient separation of CPs is critical for the construction of multi-component active layers. When CPs are arranged in close proximity, non-radiative processes often occur, which has been recognized as a problem particularly in white lightemitting devices.[5] Superstructure formation has a strong influence on the optical and electronic properties of CPs and was therefore controlled by the introduction of polar- (e.g., oligoethylene glycol) or non-polar substituents (e.g., branched alkyl- or alkoxy chains) that are able to adjust solubility and aggregation in solvents of different polarities.[18,19] In recent times, water as a solvent for the application and processing of CPs gained more and more importance.[20] This development can be explained on the one hand by the utilization of CPs in bio-assays to achieve compatibility with important biological analytes like nucleic acids and proteins.[21,22] On the other hand, the manufacturing of organic electronic devices, in which large scale production facilities are established, calls for environmentally friendly procedures avoiding the extensive use of organic solvents. In this regard, new water-soluble CP materials and water-based fabrication processes are required. Several examples of water soluble CPs, mainly carrying ionic groups, have been reported,[23] however, these structures still tend to aggregate due to the presence of the extended conjugated polymer backbones.[24] Here we present an appealing alternative strategy for the aqueous processing of CPs. It is proposed to utilize bovine submaxillary mucin (BSM) proteins, a class of saliva proteins that can accommodate CPs with their hydrophobic interior to efficiently separate and encapsulate several types of water insoluble CPs and oligomers (Figure 1). By this encapsulation process a complete isolation of the extended p-conjugated systems is achieved and aggregation of the backbones prevented in solution and in the solid state. Moreover, these CP–protein hybrids are processable from aqueous solutions yielding highly photoluminescent films or even
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functioning light-emitting diodes when incorporated as active layer. Furthermore, we successfully demonstrate color-tuning of the emission by blending BSMs containing different CP guests.
2. Experimental Section 2.1. Single Color Complex A solid polymer (2 mg) was added to 1 ml solution of BSM protein (10 mg mL 1) in sodium phosphate buffer (2 mM, pH 7.2) at room temperature. The mixture was stirred at 500–700 rpm for 96 h and was then filtered through a 0.45 mm filter.
2.2. Two Color Complexes A two-component BSM–CP solution was prepared by mixing the two single-color complex solutions (i.e., blue and green or yellow) in equal ratios.
2.3. Three Color Complexes White color solution was prepared by mixing three single-color complexes with approximate ratio of 1:1:2 yellow-green-blue, respectively.
2.4. Film Preparation Films were prepared by drop-casting of the respective solution on a quartz surface followed by drying on a hot plate at 40 8C.
2.5. Device Preparation The BIODE device was prepared in a single layer configuration using indium tin oxide (ITO) on glass as the hole injection electrode. The BSM–CP complex solution (2 mg mL 1) was drop-cast and heated to 40 8C untill dry. As the electron injection electrode, a 30 nm aluminum and 70 nm gold layer were deposited by a cold evaporation process, in which the sample was cooled by liquidnitrogen cold trap, which was in contact with BSM–CP complex layer.
2.6. Electroluminescence (EL) Measurements The light emitting performance of the BIODE was measured using an Agilent 4155C Semiconductor analyzer coupled to a Cary Eclipse spectrofluorometer.
3. Results and Discussion Previously, we have shown that BSM can dissolve and efficiently separate small hydrophobic dyes in aqueous
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Figure 1. a) Schematic of BSM structure of mucin oligomers (protein-core, green; oligosaccharide, yellow; disulfide-bonds, red); b) synthesis of PF-TPA 1; c) synthesis of OPV75. d) Structure of OPV17 6. R represents 2-ethyl-hexyl chains.
solution.[25] Key for this success was the amphiphilic structure of BSM. This proteoglycan macromolecule is composed of a protein backbone and moderately branched carbohydrate side chains containing 5–15 monomer units. To adopt a ‘‘bottle-brush’’ architecture the oligosaccharide units are grafted to serine and threonine residues of the protein core by O-glycosidic bonds. Several monomeric BSM units (MW 170 kDa) are connected via disulfide-rich domains and form a branched structure (Figure 1a).[26] Here we generalize and extend the approach of exploiting the amphiphilic nature of BSM to act as host matrix for extended hydrophobic CPs and oligomers and their utilization as photoactive films as well as active materials within organic electronic devices. For this task, three CPs have been synthesized and used as fillers in BSM. Each conjugated material was designed to emit in a specified wavelengths regime while its length allows efficient charge transfer in the layer. PF-TPA 1 is a blue emitting conjugated polymer that consists of an alternating polyfluorene-bis(triphenylamine) backbone.
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The polymer was synthesized by Pd-catalyzed Suzuki coupling of a bis-boronicester-fluorene 2 and brominefunctionalized bis(triphenylamine) (Figure 1b). To increase the solubility of the polymer in organic solvents, a mixture of 3 and its alkoxy-functionalized analog 4 were applied in 1:7 ratio.[27] OPV7 5 and OPV17 6 are monodisperse p-phenylenevinylene oligomers that emit in the green and yellow-green regime, respectively. 5 was built up in a stepwise synthesis by consecutive Heck- and Horner– Wadsworth–Emmons olefinations (Figure 1c).[28,29] The higher homologue 6 was fabricated in a similar approach using the same synthetic tools yielding a soluble monodisperse oligomer with 17 ethylene bridged phenyl rings (see Supporting Information for details). Each CP was incorporated into the BSM matrix in aqueous solution resulting in BSM–CP complexes (BSM– CP). This incorporation process allowed the efficient separation of various types of CPs and thus controlling the emission color of the BSM–CPs mixtures. The effect
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Efficient Separation of Conjugated Polymers Using a Water Soluble Glycoprotein Matrix . . . www.mbs-journal.de
Figure 2. Fluorescence spectra of various CP mixtures (excitation wavelength: 380 nm). a) BSM-PF-TPA and BSM–OPV7 mixture in buffer solution, emission maxima at 438, 526, and 562 nm (solid curve); PF-TPA and OPV7 mixture in toluene (5 mg ml 1), emission maxima at 425 and 502 nm with a shoulder at 535 nm (dashed curve). b) BSM-PF-TPA and BSM–OPV17 mixture, emission maxima at 437, 546, and 575 nm (solid curve); PF-TPA and OPV17 mixture in toluene (5 mg mL 1), emission maxima at 428 and 506 nm (dashed curve).
of complex formation of CPs with BSM in respect to the optical behavior of CPs was studied by florescence spectroscopy (Figure 2). Here, the emission behavior of a mixture of two CPs in organic solvent (toluene) and a mixture of two BSM–CP complexes in aqueous buffer were compared. The fluorescence spectra of PF-TPA and OPV7 (1:1) in BSM in buffer (solid curve) and in toluene (dashed curve) are shown in Figure 2a. It can be seen that in case of the BSM–CP complexes three distinct emission peaks are detected, while only two peaks were resolved for the BSM–free polymer mixture. Additionally, the emission intensity of PF-TPA in toluene is reduced while that of OPV7 is increased compared to the emission bands of the conjugated structures complexed with BSM. This emission behavior indicates that a nonradiative energy transfer process takes place in the organic solution and results in a blue-green instead of white emission that is achieved in the case of the BSM–CP mixture. A larger difference of the emission spectra of the BSM-complexed and free CPs is observed when investigating a mixture of PF-TPA and OPV17 (1:1) (Figure 2b). While only one distinct emission peak is detected for the protein-free CP mixture in organic solution (Figure 2b, dashed curve), three clear emission peaks are visible for BSM–CP complexes (Figure 2b, solid curve) resulting in white emission. Comparison of the two spectra suggests that when complexed in BSM both CPs emit independently, while without BSM aggregation of the two CPs occurs and the emission of OPV17 is probably strongly quenched by the triphenylamine units of the PF-TPA. These results indicate that a two-component based white emission material can be easily produced using the BSM–CP complexes containing blue (PF-TPA) and green (OPV7) or yellow (OPV17), respectively. These optical features can be explained by the separation ability of the
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BSM matrix for CPs preventing unwanted energy transfer processes and minimizing CP aggregation due to BSM–CP complex formation. Next, solid films of free CP species as well as the conjugated oligomers and polymers complexed with BSM were investigated. Therefore, the various materials were spread on quartz substrates using a previously established procedure.[25] As a result, large-scale films were obtained. Figure 3 shows fluorescence optical images and the corresponding CIE 1931 chromaticity diagrams of the various films. The color coordinates were calculated from the photographs of the films under 365 nm excitation. Figure 3a shows the emission colors of the single components cast from toluene. The emission coordinates correspond to blue for PF-TPA (0.17, 0.13), green for OPV7 (0.35, 0.5) and yellow for OPV17 (0.43, 0.51). For the binary and ternary mixtures of the BSM-free conjugated materials, the emission colors were the following: (0.33, 0.46) for PF-TPA and OPV7 (ratio 1:1), (0.41, 0.49) for PF-TPA and OPV17 (ratio 1:1) and (0.33, 0.44) for PF-TPA, OPV7, and OPV17 (ratio: 2:1:1, respectively) (Figure 3b). The data clearly indicate that mixing the CPs in toluene always results in a green-yellow emission and no emission coordinates corresponding to the white region could be obtained. After complexation of the CPs with BSM in an aqueous buffer system similar emission colors for the corresponding films were observed as when the pristine materials were casted from toluene solution (Figure 3c). The emission color coordinates of each CP inside BSM were: (0.16, 0.13) for PF-TPA, (0.36, 0.52) for OPV7, and (0.4, 0.5) for OPV17. When individual BSM–CP complexes were mixed and transformed into films, however, clear differences of the emission properties were obtained compared to the BSM-free films of mixtures (Figure 3d). The emission colors of films of binary and
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Figure 3. Left: Optical images of films of a,c) individual CPs and b,d) mixtures either without BSM (a and b, cast from toleuene solution) or encapsulated by BSM (c and d, cast from aqueous solution) excited by UV light (365 nm) and the corresponding CIE 1931 chromaticity diagrams (middle: without BSM and right with BSM): a) 1. PF-TPA, 2. OPV7, 3. OPV17; b) 1. PF-TPA–OPV7, 2. PF-TPA–OPV17, 3. PF-TPA–OPV7–OPV17; BSM–CPs: c) 1. BSM–PF-TPA, 2. BSM–OPV7, 3. BSM–OPV17; d) 1. BSM–PF-TPA–OPV7, 2. BSM–PF-TPA–OPV17, 3. BSM–PF-TPA–OPV7–OPV17.
ternary mixtures of BSM–CPs exhibit a blue shift as detailed: (0.21, 0.23) for PF-TPA and OPV7 (ratio: 1:1), (0.33, 0.36) for PF-TPA and OPV17 (ratio: 1:1) and (0.31, 0.35) for PF-TPA, OPV7, and OPV17 (ratio: 2:1:1, respectively). From this emission behavior it can be concluded that the BSM matrix preserves the separation between the polymer and oligomers suppressing energy transfer processes to occur. This efficient separation allows the formation of white emitting films (Figure 3d). For the individual fluorescence spectra of the BSM–CP and CP films, see Supporting Information. Since the films are optically active, we attempted to use them as emissive materials in electrically driven biomolecule light emitting diodes (BIODES).[30,31] Figure 4a details a scheme of the BSM–CP BIODE and additional reference devices that were fabricated. The devices were composed of a drop-cast solution film of the active bioorganic hybrid materials sandwiched between ITO as a cathode and an evaporated aluminum/gold anode. Figure 4b (black curve) shows the electro-luminescence spectrum measured for a BSM–CP device, and its characteristics upon voltage cut (Figure 4c). A broad continuous emission was detected (400–700 nm centered at 530 nm). This wide range gives rise to a white emission BIODE. For comparison, a reference device was prepared and measured. Here a drop-cast film of PF-TPA and OPV7 from organic solution (toluene) was deposited and served as the active layer (Figure 4b, red curve). It can be clearly seen that in the latter case only a blue-green emission spectrum was obtained exhibiting much lower output power compared to the BIODEdevice. The abovementioned experiments indicate that one indeed can fabricate an efficient white BIODE using only a two-component BSM–CP matrix employing
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Figure 4. a) Schematics of the BIODE. b) EL spectra (smoothed). Black curve: BSM-PF-TPA mixed with BSM–OPV7 as active layer (operation voltage 7 V); red curve (magnified): mixture of PF-TPA and OPV7 cast from toluene (operation voltage 20 V). c) EL plot for an active layer composed of BSM-PF-TPA and BSM–OPV7 measured at 7 V including a voltage cut.
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Efficient Separation of Conjugated Polymers Using a Water Soluble Glycoprotein Matrix . . . www.mbs-journal.de
aqueous solution processing.[25] We note that the EL and PL peak correlate well while the structure of the spectra is different. This difference was also observed in the case of multi-emission layers (EML) WOLEDs and was attributed to different non-radiative energy transfer processes taken place in EL and PL.[32] A study which focuses on understanding this phenomenon in our devices is currently taken place.
4. Conclusion The use of biomolecules as active components in organic photonic devices was suggested previously.[33] DNA was successfully applied in organic light-emitting devices (OLEDs) to improve electron injection and as hole-blocking layer.[34,35] Proteins as other class of important biomaterials were employed in OLEDs as well, coining the term BIODE. With such a device being composed of an ITO/cytochrome c/Al sandwich structure EL could be generated that resulted from the iron containing heme structure.[31] In another approach, amyloid fibrils were coated with CPs and acted as active layer in a BIODE. Due to improved carrier injection, the external quantum efficiency of the device was one order of magnitude higher compared with a pristine polymer layer device.[36–38] However, for CP-protein hybrid formation and device fabrication mixtures of organic solvent and water were employed. Furthermore, to achieve efficient attachment of the CP to the protein scaffold chemical modification of the CP was required and no color tuning was reported. In the CP–protein hybrid approach reported herein these shortcomings were successfully overcome. Complexation of CPs with BSM and the active layer fabrication were exclusively carried out with water as solvent. No additional functionalities needed to be incorporated into the important standard PF and PPV polymer systems and the BSM–CP complexes were simply mixed to achieve variation of the emission color. Compared with our previous work where small organic dyes were incorporated in a protein matrix,[25] we significantly improved the scope of BSM complexation to hydrophobic high molecular weight optically active materials and demonstrated successful function of BSM–CP films as active layer in BIODEs compared to passive coatings. The spectroscopic properties of the host–guest systems were characterized in aqueous solution as well as in the solid state and future work will be devoted to studying the structure of the protein–CP complexes in more detail. With this approach, it was possible to process high molecular weight hydrophobic organic materials in a ‘‘green’’ aqueous based way. The resulting large area films were still optically active and emitted light under photon and voltage excitation. Color tuning was an easy matter by
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mixing complexes with different optical active guest molecules. Due to the fact that the protein matrix suppresses undesired energy transfer processes white light emitting films and BIODES were accessible. By employing BSM in host–guest approaches, hydrophobic materials become processable in an easy and environmental friendly way that can broaden their applicability in material sciences and in large-scale production of electronic devices. It might also allow a facile layer-bylayer processing with non-water soluble compounds due to complementary solubility.
Acknowledgements: S.R. would like to thank Prof. G. L. Frey, Department of Materials Engineering, Technion – Israel Institute of Technology, Haifa, for her fruitful help in EL measurements. This research was supported by the James Frank foundation and the Tashtiot funds. A.H. greatfully acknowledges financial support from the EU (ERC starting grant), the Netherlands Organization for Scientific Research (NWO-Vici, NWO-Echo) and the Zernike Institute for Advanced Materials.
Received: July 17, 2013; Revised: September 29, 2013; Published online: November 8, 2013; DOI: 10.1002/mabi.201300329 Keywords: bioelectronics; composite materials; electroluminescence; nanomaterials; proteins
[1] A. J. Heeger, Angew. Chem. Int. Ed. 2001, 40, 2591. [2] A. J. Heeger, Chem. Soc. Rev. 2010, 39, 2354. [3] U. Scherf, S. Allard, M. Forster, B. Souharce, H. Thiem, Angew. Chem. Int. Ed. 2008, 47, 4070. [4] C. S. Hsu, Y. J. Cheng, S. H. Yang, Chem. Rev. 2009, 109, 5868. [5] A. C. Grimsdale, K. L. Chan, R. E. Martin, P. G. Jokisz, A. B. Holmes, Chem. Rev. 2009, 109, 897. [6] T. M. Swager, S. W. Thomas, G. D. Joly, Chem. Rev. 2007, 107, 1339. [7] S. Wang, X. L. Feng, L. B. Liu, D. B. Zhu, Chem. Soc. Rev. 2010, 39, 2411. [8] W. Huang, Q. Zhao, S. J. Liu, Macromol. Chem. Phys. 2009, 210, 1580. [9] C. J. Kudla, N. Koenen, W. Pisula, U. Scherf, Macromolecules 2009, 42, 3483. [10] S. Masuo, H. Yoshikawa, H. G. Nothofer, A. C. Grimsdale, € llen, H. J. Masuhara, Phys. Chem. B 2005, 109, U. Scherf, K. Mu 6917. [11] I. F. Perepichka, D. F. Perepichka, H. Meng, F. Wudl, Adv. Mater. 2005, 17, 2281. [12] U. H. F. Bunz, Chem. Rev. 2000, 100, 1605. [13] A. C. Fou, O. Onitsuka, M. Ferreira, M. F. Rubner, B. R. Hsieh, J. Appl. Phys. 1996, 79, 7501. [14] R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Bredas, M. Logdlund, W. R. Salaneck, Nature 1999, 397, 121. [15] J. Kim, Pure Appl. Chem. 2002, 74, 2031. [16] T. Q. Nguyen, V. Doan, B. J. Schwartz, J. Chem. Phys. 1999, 110, 4068.
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[17] J. Kim, B. G. Kim, E. J. Jeong, H. J. Park, D. Bilby, L. Guo, J. ACS Appl. Mater. Interfaces 2011, 3, 674. [18] G. C. Bazan, B. S. Gaylord, S. J. Wang, A. J. Heeger, J. Am. Chem. Soc. 2001, 123, 6417. [19] K. Mullen, H. N. Tsao, Chem. Soc. Rev. 2010, 39, 2372. [20] D. A. Bernards, G. G. Malliaras, R. M. Owens, G. C. Bazan, S. Wang, in: Inorganic Semiconductors in Sensor Applications, Springer, Berlin, Heidelberg 2008, p 107. [21] Q. L. Fan, X. F. Liu, W. Huang, Biosens. Bioelectron. 2011, 26, 2154. [22] S. Wang, F. D. Feng, L. B. Liu, Q. Yang, Macromol. Rapid Commun. 2010, 31, 1405. [23] F. Huang, H. B. Wu, Y. Cao, Chem. Soc. Rev. 2010, 39, 2500. [24] D. L. Wang, D. Moses, G. C. Bazan, A. J. Heeger, Chem. Phys. Lett. 2001, 348, 411. [25] N. Hendler, B. Belgorodsky, E. Mentovich, M. Gozin, S. Richter, Adv. Mater. 2011, 23, 4261. [26] G. J. Strous, J. Dekker, Crit. Rev. Biochem. Mol. Biol. 1992, 27, 57.
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[27] P. W. M. Blom, N. I. Craciun, J. Wildeman, J. Phys. Chem. C 2010, 114, 10559. [28] T. Maddux, W. J. Li, L. P. Yu, J Am. Chem. Soc. 1997, 119, 844. [29] H. Jian, J. M. Tour, J. Org. Chem. 2005, 70, 3396. [30] J. S. Brooks, Chem. Soc. Rev. 2010, 39, 2667. [31] H. Tajima, K. Shimatani, T. Komino, S. Ikeda, M. Matsuda, Y. Ando, H. Akiyama, Colloids Surf. A 2006, 284, 61. [32] S. Chen, Q. Wu, M. Kong, X. Zhao, Z. Yu, P. Jia, W. Huanf, J. Mater. Chem. C 2013, 1, 3508. [33] A. J. Steckl, Nat. Photonics 2007, 1, 3. [34] P. Zalar, D. Kamkar, R. Naik, F. Ouchen, J. G. Grote, G. C. Bazan, T. Q. Nguyen, J. Am. Chem. Soc. 2011, 133, 11010. [35] J. Steckl, J. A. Hagen, W. Li, J. G. Grote, Appl. Phys. Lett. 2006, 88, 171109. [36] M. Hamedi, A. Herland, R. H. Karlsson, O. Inganas, Nano Lett. 2008, 8, 1736. [37] H. Tanaka, A. Herland, L. J. Lindgren, T. Tsutsui, M. R. Andersson, O. Inganas, Nano Lett. 2008, 8, 2858. [38] A. Rizzo, N. Solin, L. J. Lindgren, M. R. Andersson, O. Inganas, Nano Lett. 2010, 10, 2225.
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Efficient Separation of Conjugated Polymers Using a Water Soluble Glycoprotein Matrix: From Fluorescence Materials to Light Emitting Devicesa Netta Hendler, Jurjen Wildeman, Elad. D. Mentovich, Tobias Schnitzler, Bogdan Belgorodsky, Deepak K. Prusty, Dolev Rimmerman, Andreas Herrmann,* Shachar Richter*
Optically active bio-composite blends of conjugated polymers or oligomers are fabricated by complexing them with bovine submaxilliary mucin (BSM) protein. The BSM matrix is exploited to host hydrophobic extended conjugated p-systems and to prevent undesirable aggregation and render such materials water soluble. This method allows tuning the emission color of solutions and films from the basic colors to the technologically challenging white emission. Furthermore, electrically driven light emitting biological devices are prepared and operated.
1. Introduction N. Hendler, E. D. Mentovich,[+] Dr. B. Belgorodsky, Prof. S. Richter School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 69998, Israel E-mail: [email protected] N. Hendler, E. D. Mentovich, D. Rimmerman, Prof. S. Richter University Center for Nanoscience and Nanotechnology, Tel Aviv University, Ramat Aviv, Tel Aviv 69998, Israel J. Wildeman, Dr. T. Schnitzler, Dr. D. K. Prusty, Prof. A. Herrmann Department of Polymer Chemistry, University of Groningen, Zernike Institute for Advanced Materials, Nijenborgh 4, 9747, AG Groningen, The Netherlands E-mail: [email protected] [+] Present address: Mellanox Technologies, P.O. Box 586, Beit Mellanox, Yokneam 2069200, Israel a Supporting Information is available from the Wiley Online Library or from the author.
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Conjugated polymers (CPs) and oligomers are an established class of functional materials.[1,2] They are characterized by a conjugated polymer backbone with a delocalized electronic structure. The combination of semi-conducting and light-harvesting properties makes them valuable building blocks in organic electronic devices, such as field effect transistors, photovoltaic cells, and light emitting diodes (LED) as well as in chemical and biological sensors.[3–7] The most important CP scaffolds are derived from poly(fluorene) (PF),[8–10] poly(thiophene) (PT),[11] poly(p-phenyleneethinylene) (PPE),[12] and poly(p-phenylenevinylene) (PPV).[13] The success of these materials originated from their favorable chemical and physical properties, their straightforward synthesis and ease of functionalization. The electronic and optical properties
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DOI: 10.1002/mabi.201300329
Efficient Separation of Conjugated Polymers Using a Water Soluble Glycoprotein Matrix . . . www.mbs-journal.de
of CPs are mainly determined by their HOMO–LUMO levels,[14] their molecular conformation of the conjugated backbone and their supramolecular assembly.[15] The control of the HOMO–LUMO levels of CPs is in particular important since the band-gap does not only determine the color of emission but also the charge injection efficiency and the charge carrier mobility in optoelectronic devices. Band gap engineering starts from appropriate selection of the conjugated polymer backbone and is further tuned by donor/acceptor substitution, copolymerization or variation of the conjugation length. Evenly important is controlling the aggregation behavior of CPs, which has been extensively studied to optimize the processing conditions in organic electronic device fabrication.[16,17] In this context, efficient separation of CPs is critical for the construction of multi-component active layers. When CPs are arranged in close proximity, non-radiative processes often occur, which has been recognized as a problem particularly in white lightemitting devices.[5] Superstructure formation has a strong influence on the optical and electronic properties of CPs and was therefore controlled by the introduction of polar- (e.g., oligoethylene glycol) or non-polar substituents (e.g., branched alkyl- or alkoxy chains) that are able to adjust solubility and aggregation in solvents of different polarities.[18,19] In recent times, water as a solvent for the application and processing of CPs gained more and more importance.[20] This development can be explained on the one hand by the utilization of CPs in bio-assays to achieve compatibility with important biological analytes like nucleic acids and proteins.[21,22] On the other hand, the manufacturing of organic electronic devices, in which large scale production facilities are established, calls for environmentally friendly procedures avoiding the extensive use of organic solvents. In this regard, new water-soluble CP materials and water-based fabrication processes are required. Several examples of water soluble CPs, mainly carrying ionic groups, have been reported,[23] however, these structures still tend to aggregate due to the presence of the extended conjugated polymer backbones.[24] Here we present an appealing alternative strategy for the aqueous processing of CPs. It is proposed to utilize bovine submaxillary mucin (BSM) proteins, a class of saliva proteins that can accommodate CPs with their hydrophobic interior to efficiently separate and encapsulate several types of water insoluble CPs and oligomers (Figure 1). By this encapsulation process a complete isolation of the extended p-conjugated systems is achieved and aggregation of the backbones prevented in solution and in the solid state. Moreover, these CP–protein hybrids are processable from aqueous solutions yielding highly photoluminescent films or even
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functioning light-emitting diodes when incorporated as active layer. Furthermore, we successfully demonstrate color-tuning of the emission by blending BSMs containing different CP guests.
2. Experimental Section 2.1. Single Color Complex A solid polymer (2 mg) was added to 1 ml solution of BSM protein (10 mg mL 1) in sodium phosphate buffer (2 mM, pH 7.2) at room temperature. The mixture was stirred at 500–700 rpm for 96 h and was then filtered through a 0.45 mm filter.
2.2. Two Color Complexes A two-component BSM–CP solution was prepared by mixing the two single-color complex solutions (i.e., blue and green or yellow) in equal ratios.
2.3. Three Color Complexes White color solution was prepared by mixing three single-color complexes with approximate ratio of 1:1:2 yellow-green-blue, respectively.
2.4. Film Preparation Films were prepared by drop-casting of the respective solution on a quartz surface followed by drying on a hot plate at 40 8C.
2.5. Device Preparation The BIODE device was prepared in a single layer configuration using indium tin oxide (ITO) on glass as the hole injection electrode. The BSM–CP complex solution (2 mg mL 1) was drop-cast and heated to 40 8C untill dry. As the electron injection electrode, a 30 nm aluminum and 70 nm gold layer were deposited by a cold evaporation process, in which the sample was cooled by liquidnitrogen cold trap, which was in contact with BSM–CP complex layer.
2.6. Electroluminescence (EL) Measurements The light emitting performance of the BIODE was measured using an Agilent 4155C Semiconductor analyzer coupled to a Cary Eclipse spectrofluorometer.
3. Results and Discussion Previously, we have shown that BSM can dissolve and efficiently separate small hydrophobic dyes in aqueous
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Figure 1. a) Schematic of BSM structure of mucin oligomers (protein-core, green; oligosaccharide, yellow; disulfide-bonds, red); b) synthesis of PF-TPA 1; c) synthesis of OPV75. d) Structure of OPV17 6. R represents 2-ethyl-hexyl chains.
solution.[25] Key for this success was the amphiphilic structure of BSM. This proteoglycan macromolecule is composed of a protein backbone and moderately branched carbohydrate side chains containing 5–15 monomer units. To adopt a ‘‘bottle-brush’’ architecture the oligosaccharide units are grafted to serine and threonine residues of the protein core by O-glycosidic bonds. Several monomeric BSM units (MW 170 kDa) are connected via disulfide-rich domains and form a branched structure (Figure 1a).[26] Here we generalize and extend the approach of exploiting the amphiphilic nature of BSM to act as host matrix for extended hydrophobic CPs and oligomers and their utilization as photoactive films as well as active materials within organic electronic devices. For this task, three CPs have been synthesized and used as fillers in BSM. Each conjugated material was designed to emit in a specified wavelengths regime while its length allows efficient charge transfer in the layer. PF-TPA 1 is a blue emitting conjugated polymer that consists of an alternating polyfluorene-bis(triphenylamine) backbone.
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The polymer was synthesized by Pd-catalyzed Suzuki coupling of a bis-boronicester-fluorene 2 and brominefunctionalized bis(triphenylamine) (Figure 1b). To increase the solubility of the polymer in organic solvents, a mixture of 3 and its alkoxy-functionalized analog 4 were applied in 1:7 ratio.[27] OPV7 5 and OPV17 6 are monodisperse p-phenylenevinylene oligomers that emit in the green and yellow-green regime, respectively. 5 was built up in a stepwise synthesis by consecutive Heck- and Horner– Wadsworth–Emmons olefinations (Figure 1c).[28,29] The higher homologue 6 was fabricated in a similar approach using the same synthetic tools yielding a soluble monodisperse oligomer with 17 ethylene bridged phenyl rings (see Supporting Information for details). Each CP was incorporated into the BSM matrix in aqueous solution resulting in BSM–CP complexes (BSM– CP). This incorporation process allowed the efficient separation of various types of CPs and thus controlling the emission color of the BSM–CPs mixtures. The effect
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Figure 2. Fluorescence spectra of various CP mixtures (excitation wavelength: 380 nm). a) BSM-PF-TPA and BSM–OPV7 mixture in buffer solution, emission maxima at 438, 526, and 562 nm (solid curve); PF-TPA and OPV7 mixture in toluene (5 mg ml 1), emission maxima at 425 and 502 nm with a shoulder at 535 nm (dashed curve). b) BSM-PF-TPA and BSM–OPV17 mixture, emission maxima at 437, 546, and 575 nm (solid curve); PF-TPA and OPV17 mixture in toluene (5 mg mL 1), emission maxima at 428 and 506 nm (dashed curve).
of complex formation of CPs with BSM in respect to the optical behavior of CPs was studied by florescence spectroscopy (Figure 2). Here, the emission behavior of a mixture of two CPs in organic solvent (toluene) and a mixture of two BSM–CP complexes in aqueous buffer were compared. The fluorescence spectra of PF-TPA and OPV7 (1:1) in BSM in buffer (solid curve) and in toluene (dashed curve) are shown in Figure 2a. It can be seen that in case of the BSM–CP complexes three distinct emission peaks are detected, while only two peaks were resolved for the BSM–free polymer mixture. Additionally, the emission intensity of PF-TPA in toluene is reduced while that of OPV7 is increased compared to the emission bands of the conjugated structures complexed with BSM. This emission behavior indicates that a nonradiative energy transfer process takes place in the organic solution and results in a blue-green instead of white emission that is achieved in the case of the BSM–CP mixture. A larger difference of the emission spectra of the BSM-complexed and free CPs is observed when investigating a mixture of PF-TPA and OPV17 (1:1) (Figure 2b). While only one distinct emission peak is detected for the protein-free CP mixture in organic solution (Figure 2b, dashed curve), three clear emission peaks are visible for BSM–CP complexes (Figure 2b, solid curve) resulting in white emission. Comparison of the two spectra suggests that when complexed in BSM both CPs emit independently, while without BSM aggregation of the two CPs occurs and the emission of OPV17 is probably strongly quenched by the triphenylamine units of the PF-TPA. These results indicate that a two-component based white emission material can be easily produced using the BSM–CP complexes containing blue (PF-TPA) and green (OPV7) or yellow (OPV17), respectively. These optical features can be explained by the separation ability of the
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BSM matrix for CPs preventing unwanted energy transfer processes and minimizing CP aggregation due to BSM–CP complex formation. Next, solid films of free CP species as well as the conjugated oligomers and polymers complexed with BSM were investigated. Therefore, the various materials were spread on quartz substrates using a previously established procedure.[25] As a result, large-scale films were obtained. Figure 3 shows fluorescence optical images and the corresponding CIE 1931 chromaticity diagrams of the various films. The color coordinates were calculated from the photographs of the films under 365 nm excitation. Figure 3a shows the emission colors of the single components cast from toluene. The emission coordinates correspond to blue for PF-TPA (0.17, 0.13), green for OPV7 (0.35, 0.5) and yellow for OPV17 (0.43, 0.51). For the binary and ternary mixtures of the BSM-free conjugated materials, the emission colors were the following: (0.33, 0.46) for PF-TPA and OPV7 (ratio 1:1), (0.41, 0.49) for PF-TPA and OPV17 (ratio 1:1) and (0.33, 0.44) for PF-TPA, OPV7, and OPV17 (ratio: 2:1:1, respectively) (Figure 3b). The data clearly indicate that mixing the CPs in toluene always results in a green-yellow emission and no emission coordinates corresponding to the white region could be obtained. After complexation of the CPs with BSM in an aqueous buffer system similar emission colors for the corresponding films were observed as when the pristine materials were casted from toluene solution (Figure 3c). The emission color coordinates of each CP inside BSM were: (0.16, 0.13) for PF-TPA, (0.36, 0.52) for OPV7, and (0.4, 0.5) for OPV17. When individual BSM–CP complexes were mixed and transformed into films, however, clear differences of the emission properties were obtained compared to the BSM-free films of mixtures (Figure 3d). The emission colors of films of binary and
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Figure 3. Left: Optical images of films of a,c) individual CPs and b,d) mixtures either without BSM (a and b, cast from toleuene solution) or encapsulated by BSM (c and d, cast from aqueous solution) excited by UV light (365 nm) and the corresponding CIE 1931 chromaticity diagrams (middle: without BSM and right with BSM): a) 1. PF-TPA, 2. OPV7, 3. OPV17; b) 1. PF-TPA–OPV7, 2. PF-TPA–OPV17, 3. PF-TPA–OPV7–OPV17; BSM–CPs: c) 1. BSM–PF-TPA, 2. BSM–OPV7, 3. BSM–OPV17; d) 1. BSM–PF-TPA–OPV7, 2. BSM–PF-TPA–OPV17, 3. BSM–PF-TPA–OPV7–OPV17.
ternary mixtures of BSM–CPs exhibit a blue shift as detailed: (0.21, 0.23) for PF-TPA and OPV7 (ratio: 1:1), (0.33, 0.36) for PF-TPA and OPV17 (ratio: 1:1) and (0.31, 0.35) for PF-TPA, OPV7, and OPV17 (ratio: 2:1:1, respectively). From this emission behavior it can be concluded that the BSM matrix preserves the separation between the polymer and oligomers suppressing energy transfer processes to occur. This efficient separation allows the formation of white emitting films (Figure 3d). For the individual fluorescence spectra of the BSM–CP and CP films, see Supporting Information. Since the films are optically active, we attempted to use them as emissive materials in electrically driven biomolecule light emitting diodes (BIODES).[30,31] Figure 4a details a scheme of the BSM–CP BIODE and additional reference devices that were fabricated. The devices were composed of a drop-cast solution film of the active bioorganic hybrid materials sandwiched between ITO as a cathode and an evaporated aluminum/gold anode. Figure 4b (black curve) shows the electro-luminescence spectrum measured for a BSM–CP device, and its characteristics upon voltage cut (Figure 4c). A broad continuous emission was detected (400–700 nm centered at 530 nm). This wide range gives rise to a white emission BIODE. For comparison, a reference device was prepared and measured. Here a drop-cast film of PF-TPA and OPV7 from organic solution (toluene) was deposited and served as the active layer (Figure 4b, red curve). It can be clearly seen that in the latter case only a blue-green emission spectrum was obtained exhibiting much lower output power compared to the BIODEdevice. The abovementioned experiments indicate that one indeed can fabricate an efficient white BIODE using only a two-component BSM–CP matrix employing
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Figure 4. a) Schematics of the BIODE. b) EL spectra (smoothed). Black curve: BSM-PF-TPA mixed with BSM–OPV7 as active layer (operation voltage 7 V); red curve (magnified): mixture of PF-TPA and OPV7 cast from toluene (operation voltage 20 V). c) EL plot for an active layer composed of BSM-PF-TPA and BSM–OPV7 measured at 7 V including a voltage cut.
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aqueous solution processing.[25] We note that the EL and PL peak correlate well while the structure of the spectra is different. This difference was also observed in the case of multi-emission layers (EML) WOLEDs and was attributed to different non-radiative energy transfer processes taken place in EL and PL.[32] A study which focuses on understanding this phenomenon in our devices is currently taken place.
4. Conclusion The use of biomolecules as active components in organic photonic devices was suggested previously.[33] DNA was successfully applied in organic light-emitting devices (OLEDs) to improve electron injection and as hole-blocking layer.[34,35] Proteins as other class of important biomaterials were employed in OLEDs as well, coining the term BIODE. With such a device being composed of an ITO/cytochrome c/Al sandwich structure EL could be generated that resulted from the iron containing heme structure.[31] In another approach, amyloid fibrils were coated with CPs and acted as active layer in a BIODE. Due to improved carrier injection, the external quantum efficiency of the device was one order of magnitude higher compared with a pristine polymer layer device.[36–38] However, for CP-protein hybrid formation and device fabrication mixtures of organic solvent and water were employed. Furthermore, to achieve efficient attachment of the CP to the protein scaffold chemical modification of the CP was required and no color tuning was reported. In the CP–protein hybrid approach reported herein these shortcomings were successfully overcome. Complexation of CPs with BSM and the active layer fabrication were exclusively carried out with water as solvent. No additional functionalities needed to be incorporated into the important standard PF and PPV polymer systems and the BSM–CP complexes were simply mixed to achieve variation of the emission color. Compared with our previous work where small organic dyes were incorporated in a protein matrix,[25] we significantly improved the scope of BSM complexation to hydrophobic high molecular weight optically active materials and demonstrated successful function of BSM–CP films as active layer in BIODEs compared to passive coatings. The spectroscopic properties of the host–guest systems were characterized in aqueous solution as well as in the solid state and future work will be devoted to studying the structure of the protein–CP complexes in more detail. With this approach, it was possible to process high molecular weight hydrophobic organic materials in a ‘‘green’’ aqueous based way. The resulting large area films were still optically active and emitted light under photon and voltage excitation. Color tuning was an easy matter by
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mixing complexes with different optical active guest molecules. Due to the fact that the protein matrix suppresses undesired energy transfer processes white light emitting films and BIODES were accessible. By employing BSM in host–guest approaches, hydrophobic materials become processable in an easy and environmental friendly way that can broaden their applicability in material sciences and in large-scale production of electronic devices. It might also allow a facile layer-bylayer processing with non-water soluble compounds due to complementary solubility.
Acknowledgements: S.R. would like to thank Prof. G. L. Frey, Department of Materials Engineering, Technion – Israel Institute of Technology, Haifa, for her fruitful help in EL measurements. This research was supported by the James Frank foundation and the Tashtiot funds. A.H. greatfully acknowledges financial support from the EU (ERC starting grant), the Netherlands Organization for Scientific Research (NWO-Vici, NWO-Echo) and the Zernike Institute for Advanced Materials.
Received: July 17, 2013; Revised: September 29, 2013; Published online: November 8, 2013; DOI: 10.1002/mabi.201300329 Keywords: bioelectronics; composite materials; electroluminescence; nanomaterials; proteins
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