Note pubs.acs.org/Macromolecules
Removal of Solubilizing Side Chains at Low Temperature: A New Route to Native Poly(thiophene) Eva Bundgaard,*,† Ole Hagemann,† Morten Bjerring,‡ Niels Chr. Nielsen,‡ Jens W. Andreasen,† Birgitta Andreasen,† and Frederik C. Krebs† †
Department of Energy Conversion and Storage, Technical University of Denmark, Frederiksborgvej 399, DK-4000 Roskilde, Denmark ‡ Centre for Insoluble Protein Structures (inSPIN), Interdisciplinary Nanoscience Centre (iNANO) and Department of Chemistry, Langelandsgade 140, Aarhus University, DK-8000 Aarhus C, Denmark S Supporting Information *
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films that are then made insoluble post-formation vis-à-vis the original precursor routes, except that the conjugated backbone is in place already during solution processing of the film. There have been two overall approaches to this, the first one being the use of functionalized side chains allowing for cross-linking postfilm formation. Side chains carrying a bromine atom, a vinyl group, an azide group, or an oxetane group can be cross-linked in the solid state through irradiation with light or through the use of a catalyst for ring-opening polymerization.10 The second approach is perhaps the most difficult but also the most elegant from a fundamental point of view and presents a method whereby the side chains are there during film formation and then subsequently removed post-film formation, yielding a conjugated polymer film without side chains, which presents the largest optical density achievable for a given thickness. There have, however, been few routes to this, and they all require a very high temperature heating step. An example is the tertiary esters11 that required ∼300 °C. This is too high for use beyond the laboratory setting and brings with it problems of thermally induced defects and undesirable side reactions. Studies aimed at lowering the temperature for side-chain removal have achieved a lowering to around 150−200 °C through systematic studies of side-chain chemistry and use of acid catalysis.12 The ultimate aim is a room-temperature process where the side-chain removal is achieved by chemical means and not through a high-temperature step. The control of a chemical reaction using temperature implies that the reaction is spontaneous at some temperature, and to ensure stability of the material in its processable form, the treatment temperature must be significantly elevated with respect to the storage temperature (ambient temperature). This led us to explore new means, and we prepared silane-substituted poly(thiophene), as shown in Scheme 1. The polydimethyloctylsilylterthiophene (PDMOST) was found to solution process very well and was also found to enable the removal through treatment with acids. We employed many agents for the removal of the silane including trifluoromethanesulfonic acid (TfOH), tetrabutylammonium-
he possibility for solution processing of conjugated polymers is undoubtedly the major reason for their tremendous success within many areas of application. Conjugated polymers are actively being employed within all areas of organic electronics and are almost exclusively applied as thin films through deposition from a dilute solution followed by drying. The most popular technique has until now been spincoating, which as a batch process enables the simple and fast formation of even films over many square centimeters.1 Alternative film-forming techniques are gradually entering the scene, enabling even larger areas to be formed, also using continuous roll-to-roll processes.1 The conjugated polymers have a rigid backbone that does not automatically grant solubility in a solvent; therefore, the successful approach has so far been to employ solubilizing side chains attached to the rigid conjugated polymer backbone. Aliphatic or ether side chains have been in almost exclusive use, whereas there are a number of reports where ionic or polar side chains have been employed. The choice of side chain has been ruled by the desired solvent for film forming. In historic terms, the early conjugated polymers were intractable materials arrived at through electrochemical or chemical polymerization of unsubstituted monomers. Those materials were difficult to process when polymerized, and the most successful approach at the time was electropolymerization and subsequent study of the material on the electrodes.2 A breakthrough was the precursor routes to conjugated polymers such as poly(acetylene) through the Durham route3 and poly(phenylenevinylene) through the Gilsch4 or the sulphinyl route.5 Later, the dithiocarbamate route followed.6 The precursor route allows for solution processing the materials where the final conjugated polymer is achieved post-processing. Both soluble and insoluble polymer materials can be processed through a precursor route, and in the mid-1990s synthetic routes to conjugated polymers with good solubility through side chains attached to the backbone were reported, and for the past two decades these have dominated the field.7 Once the film is formed, however, there is often little need for the side chains that in most instances behave passively with respect to film function (e.g., visible absorption, carrier transport), and they may even adversely affect the properties of the film by incurring poor morphological stability,8 poor photochemical stability, and so on.9 From this point of view, there has recently been a drive to develop solution proccessable routes to conjugated polymer © 2012 American Chemical Society
Received: January 11, 2012 Revised: March 16, 2012 Published: April 2, 2012 3644
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Scheme 1. Synthesis of PDMOSTa
a R = octyl. (a) (1) nBuLi, Et2O, −78 °C, (2) Cl-SiMe2Oc. (b) (1) nBuLi, THF, −78 °C, (2) CBr4. (c) (1) nBuLi, THF, −78 °C, (2) Me3SnCl. (d) (1) Pd2dba3, tri-o-tolylphosphine, toluene, 110 °C. (e) (1) NBS, K2CO3, THF. (f) (1) 2,5-di-(trimethyltin)-thiophene, Pd2dba3, tri-o-tolylphosphine, toluene, 110 °C.
fluoride (TBAF), and the acids: HBr, HCl, trichloroacitic acid (TCA), and dichloroacitic acid (DCA). TfOH, which was used in a similar procedure,12c did not show the successful control, and the cleaved polymer started to precipitate before all side chains had been cleaved, and the result was a film that was not smooth (see Supporting Information.) However, TCA was found to be most efficient in organic solvents and could be mixed into the coating solution or applied post-film formation. Subsequent drying of the film led to a pure insoluble poly(thiophene), PT film. In previous studies, it was demonstrated how P3MHOCT could be converted into PT at 300 °C.11 Solid-state NMR spectroscopy here proved to be extremely useful, and the choice of PT was particularly instructive because the soluble polymer is random in nature and the 13C CP/MAS NMR spectrum presents a complex set of signals including both signals from aliphatic groups as well as the thiophene moieties. On the contrary, native PT only presents two distinct carbon atoms from the thiophene unit. Upon successful formation of native PT, the solid-state 13C CP/MAS NMR spectrum thus becomes very simple and presents only two distinct peaks. In Figure 1, we demonstrate how native PT can be formed by solution processing PDMOST, followed by acid treatment at room temperature. Figure 1 also demonstrates the color and solubility difference between PDMOST before and after acid treatment, that is, a yellow film that is soluble in chloroform for the noncleaved polymer and a red insoluble film for the cleaved polymer. The solid-state 13C CP/MAS NMR spectrum provides unequivocal proof of the efficiency of this lowtemperature method. The 13C CP/MAS NMR spectrum was found to match the material arrived at through the hightemperature route, as shown in Figure 1. The treatment at room temperature may result in voids in the polymer film due to the removal of the side chains, which in the case of PDMOST accounts for 57% of the molecular weight and therefore a very significant part of the volume in the film. In the case of PT arrived at through the thermal route, very dense films are obtained and the question of whether a lowtemperature method such as the one we present here would yield a dense or a porous film was thus investigated. Through the use of X-ray reflectivity that probes a very large area of the sample (square centimeteres), we found that the film collapses upon the removal of the side chains, thus yielding a compact PT film at room temperature identical to the PT film arrived at through the high-temperature method. In Figure 2, X-ray reflectivity data as a function of scattering vector Qz = 4π sin θ/λ (θ is the scattering angle, λ is the X-ray wavelength), show the changes in film thickness and density when the polymer is cleaved and washed. The critical angle is directly related to the electron density and thus, knowing the
Figure 1. Removal of side chains. Top: two different routes to native polythiophene; the route reported here at room temperature and a previous reported route using high temperature.11 Middle: Demonstration of color change upon the removal of the side chains (yellow to red) and the demonstration of insolubility of the cleaved polymer film when immersed in chloroform. Bottom: 13C CP/MAS solid-state NMR spectra demonstrating the removal of the aliphatic side chains, that is, PDMOST before (red) and after (blue) acid treatment.
chemical composition, the mass density. The data show a clear increase in density of the film when washed, from 1.0 g/cm3 for PDMOST to 1.2 g/cm3 for the native PT film, and a reduction in film thickness from 127 to 49 nm. The removal of side chains corresponds to a mass loss of 57%, and with the concomitant reduction in thickness, this should result in a film density of 1.1 g/cm3 for an initial density of 1.0 g/cm3. The higher final density indicates a further densification by washing. In summary, we have demonstrated the facile removal of solubilizing side chains at room temperature using acid catalysis. This allows for traditional solution processing of the functional material into thin films, followed by insolubilization enabling subsequent solution processing. This is likely to have implications within the field of large area coating and printing of multilayer organic film on heat-sensitive substrates. 3645
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Figure 2. X-ray reflectivity of spin-coated film of acid-treated PDMOST. Black line: the reflectivity of a spin coated film of cleaved polymer; red dashed line: the reflectivity of the same film after washing with chloroform. Green dotted vertical lines: the position of the scattering vectors corresponding to the critical angle for total reflection for the film.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental synthetic details, UV−vis, TGA, DCS curves, and AFM images. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the Danish National Research Foundation (Danish-Chinese Centre for Organic-Based Photovoltaic Cells with Morphology Control) and Centre for Insoluble Protein Structures (inSPIN)). We gratefully acknowledge Martin Meedom Nielsen who wrote the code for analyzing the X-ray reflectometry data.
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