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702

Microporous Conjugated Poly(thienylene arylene)Networks

By Johannes Schmidt, Jens Weber, Jan Dirk Epping, Markus Antonietti, and

Arne Thomas*

[*] Dr. A. Thomas, J. Schmidt, Dr. J. Weber, Prof. M. AntoniettiDepartment of Colloid ChemistryMax-Planck-Institute of Colloids and InterfacesResearch Campus Golm, 14424 Potsdam (Germany)E-mail: arne.thomas@mpikg.mpg.de

Dr. J. D. EppingDepartment of Chemistry, Sekr. C2Technische Universitat BerlinStrasse des 17. Juni 135, 10623 Berlin (Germany)

DOI: 10.1002/adma.200802692

� 2009 WILEY-VCH Verlag Gm

Microporous materials, which are materials with small pores(<2 nm) and very high surface areas, have found widespreadapplications in diverse technological areas, such as gas storage,separation, or catalysis. Zeolites or activated carbons are thetraditional choice for these purposes, but more recently metal–organic frameworks also showed promising properties to comple-ment carbon and silica in certain fields of application. This is dueto the valuable combination of high surface area and low density,with control over the chemical and physical properties of the porewalls. While metal–organic frameworks combine tunable poro-sities with organic functionalities, their coordination-bond-basedarchitecture severely limits the conditions under which thesematerials can be applied. This deficit should be possible toovercome with the use of covalently bonded analogues, which canenhance the range of applications to more realistic conditions(e.g., heat, water, acid/base, or oxygen), where most coordinationbonds start to dissolve. Thus, it is not surprising that severalgroups recently attempted to prepare new porous materials on acovalently bonded, purely organic base.

So far, three main strategies can be distinguished for thepreparation of microporous polymer networks. First, additionalcross-linking of swollen polystyrene[1–4] and, more recently, poly-aniline networks[5] generate so-called hyper-cross-linked poly-mers (HCP), with surface areas up to 2000m2g�1. Condensationreactions of arylates or aromatic benzylchlorides can also be usedfor the synthesis of HCPs.[1,6–8] Using a reversible chemistry, thatis, thermodynamic control, to form the regular, structure-generating motif of a polymer scaffold yielded another class ofcovalently bonded microporous materials: 2D and 3D crystallineorganic frameworks (COFs), with very high surface areas andwell-controllable pore sizes, were prepared via trimerization ofboronic acids.[9] Very recently, similar materials were preparedusing a trimerization reaction of dinitrile compounds, formingcovalent triazine-based frameworks (CTFs).[10,11] A third type ofmicroporous organic materials are polymers of intrinsic micro-porosity (PIM), introduced by Budd, McKeown and co-workers.[12–14] Here, the insertion of bulky structural motifs

into rigid polymer chains can create polymers with high freevolume, finally yielding observable microporosity. It was shownthat the introduction of stiff 90 8 kinks, based on spiromolecules,in every repeating unit of a ladder-like poly(dibenzodioxane)could prevent efficient packing of the polymer chains, resulting inmicroporous materials. This versatile approach allows the syn-thesis of soluble cross-linked polymers, and could be expanded toother polymer classes, such as polyimides.[15,16] Recently, Cooperet al. synthesized conjugated microporous polymer networks,namely poly(arylene ethynylene)s (PAEs) and poly(phenylenebutadiynylene)s (PPBs), using transition-metal-catalyzed couplingreactions.[17,18] Futhermore, for alkyne-linked microporous poly-mers, it was shown that pore properties are almost independent ofmonomer-to-solvent ratio, suggesting that the porosity is indeedmostly defined by the polymer backbone structure, although notruling out solvent-templating effects.[19] A comparable approachwas used by us in order to obtain conjugated microporousspirobifluorene-p-phenylene networks.[20]

Herein, we describe the convenient and simple synthesisof a new class of microporous conjugated polymers, namelypoly(arylene thienylene)s, based on thiophene linkages. Besidethe usual applications known for high surface area materials, likefor example gas storage or catalyst support, these polymers mightopen new possibilities for advanced applications in the field oforganic electronics or optoelectronic compounds. Conjugatedpolymers in general, and polythiophenes in particular, have foundwidespread use in organic–electronic devices, such as organiclight-emitting diodes (OLEDs), organic field-effect transitors(OFETs), and organic solar cells. Polythiophene networks havebeen described as 3D organic semiconductors, enabling fullyisotropic electronic properties.[21] Microporosity added to suchsystems would add typical interface properties, such asinterpenetration with a second materials, the ability of chargeseparation, and sensing properties via interactions with theelectron pool. In this communication, we describe a convenientsynthesis of such polythiophene networks by oxidative polymer-ization of specially designed precursor molecules, namely2,20,7,70-tetrakis(2-thienyl)-9,90-spirobifluorene and 1,3, 5-tris(2-thienyl)benzene (Scheme 1).

Monomer 1 was synthesized via a Suzuki coupling reactionof thienyl boronic acid on tetrabrominated spirobifluorene.[22]

Monomer 2 was synthesized via trimerization reaction of

2-acetylthiophene.[23] Both monomers or related structures have

been used before as starting compounds (molecular tectons) for

the generation of 3D conjugated architectures, such as

star-shaped oligothiophenes.[22–24]

Oxidative polymerization of the monomers is expected to yieldC–C coupling of the thiophene moieties in the 5-position,

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Scheme 1. Structure of monomers used for the generation of micro-porous polythiophene networks.

Scheme 2. Modeled structure of a network unit from poly-2,20,7,70-tetra-thienyl-9,90-spirobifluorene.

forming bithiophene-linkers. These bithiophenes are rather stiffand almost linear, and direct polymerization leads to definedbridges between the structure-directing crossing points. From atopological point of view, it is important to start with definedcross-linking points, which are subsequently polymerized. Thatway, a stiff network made up of well-defined, contorted molecularmotifs is obtained.

The oxidative polymerization of the monomers was carried outusing stoichiometric amounts of FeCl3 dissolved in acetonitrile asthe reaction medium. The reaction sets in immediately, and theprecipitation of a yellow solid is observed for both monomers.The almost quantitative yields indicate a high degree of poly-merization. Washing and drying results in complete removal ofthe inorganic metal salts, as confirmed by energy-dispersive X-ray(EDX) measurements (Fig. S1 in the Supporting Information).

Both polymer networks exhibit as-synthesized high surfaceareas and microporosity. The nitrogen sorption isotherms inFigure 1 reveal surface areas of 577m2 g�1 and 1060m2 g�1, andcomparably high pore volumes of 0.61 cm3 g�1 and 0.71 cm3 g�1

at relative pressure of p/p0¼ 0.9, respectively (Table 1). Notably,these high values were reached without addition of a selectedporogen, showing that this microporosity is indeed intrinsic tothe structure of the polymer scaffold. As spirobifluorene and1,3,5-substituted benzene are bulky and rigid structural units,their incorporation into polymer networks is expected to result innondense packing and a large amount of free volume, heredetectable as accessible microporosity.

Figure 1. Nitrogen sorption isotherms for microporous polythiophenenetworks 1 and 2.

Adv. Mater. 2009, 21, 702–705 � 2009 WILEY-VCH Verlag Gm

Average pore diameters of 1.6 nm were calculated from thenitrogen sorption isotherms for both polymers (Fig. S2 inSupporting Information). Indeed, molecular modeling experi-ments of the networks reveal comparable pore sizes, hereillustrated for the example when four spiro-bifluorene units areconnected via bithiophenes (Scheme 2), but also more complexstructures can occur via catenation or interpenetration.

However, care is advised applying evaluation methods for poresize distribution on soft, organic microporous materials. Neitherstate-of-the-art methods like nonlinear density functional theory(NLDFT) nor traditional methods like Horvarth–Kawazoe (HK)consider, for instance, swelling effects, rendering these methodsphysically untrustworthy. Indeed, both thiophene networksshowed pressure-dependent structural changes. This is evi-denced by small-angle X-ray scattering (SAXS) measurements,while comparing measurements under vacuum and in a heliumatmosphere at ambient pressure (Fig. 2). Similar findings hadalready been observed for polymers and networks of aromaticpolyimides with intrinsic microporosity.[16]

Figure 2. SAXS pattern of networks 1 and 2, under helium and vacuum.

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Figure 3. Transmission electron microscopy image of polythiophene net-work 2, with incorporated palladium nanoclusters.

Table 1. Surface area and total pore volume of polythiophene networks.

Network SBET [m2 g�1] Vtot [cm3 g�1][a]

1 577 0.609

2 1060 0.709

[a] Determined at p/p0¼ 0.90.

704

The polythiophene networks are amorphous, as confirmed bywide-angle X-ray scattering (WAXS) measurements. This is inagreement with other polymer networks showing intrinsicmicroporosity, synthesized under polymerization conditions thatare kinetically controlled.

The incorporation of intact thiophene and spirobifluorene orbenzene units, respectively, into the networks was shown by13C{1H} solid-state cross-polarization magic-angle spinning (CP/MAS) NMR (Fig. S3) measurements, and additionally confirmedby quantitative 13C single-pulse MAS measurements (not shownhere).

The microporous networks formed from polythiophenescontain large amounts of thiol functionalities, as confirmed byelemental analyses of the networks (Table S1). As the thiol groupsare part of the organic bridges, it can be assumed that thesefunctionalities are highly accessible for molecules or ions enteringthe porous system. Thus, besides using the electronic properties ofpolythiophenes, interesting applications, such as sorption andstorage materials and catalyst supports, can also be assumed.

A first proof of principle of the latter application was shown bythe incorporation of nanostructured palladium into the organicpolymer network. This was achieved by simple impregnation ofthe polythiophene network (2) with a palladium-salt solution.Metal loadings of 13 and 15 wt% were detected by inductivelycoupled plasma (ICP) and EDX, respectively, and the nitrogensorption showed a loss of porosity of 300m2 g�1. Reduction of thepalladium ions yielded very small, monodisperse palladiumclusters, with an average diameter of 1.5 nm and homogeneouslydispersed in the polymer matrix (Fig. 3 and in SupportingInformation Fig. S4). No larger aggregates were detectedthroughout the sample, showing the strict confinement of thenanoclusters by the pores of the microporous polymer network.

An initial catalytic test showed a promising activity of theinfiltrated porous network. After 2 h of hydrogenation, completeconversion of diphenylacetylene to 1,2-diphenylethane wasachieved. As an indirect result, this demonstrates that thepalladium clusters are accessible in the porous network. Theseproperties, along with the strong ligation from the support due tothe heteroatoms incorporated, make these networks interestingnot only as catalyst-support materials, but also for electrocatalysisand sensor materials.

To summarize, conjugated microporous poly(thienylenearylene) networks have been synthesized using oxidativepolymerization of multiple thiophene-functionalized structure-directing motifs. Reaction with FeCl3 can be carried out in veryshort reaction times and with high yields, rendering the poly-merization very simple and convenient. The resulting powdersexhibit very high surface areas of 577 and 1060m2 g�1,respectively. These scaffolds are a new class of microporouspolymers, but also an interface rich, conjugated-polymerarchitecture, highly wanted for organic devices such as sensors

� 2009 WILEY-VCH Verlag Gm

and solar cells. Using such polymers for organic–electronicdevices, the generation of interpenetrating networks withcontrolled domain structures by simply filling the pores of theconjugated network can be envisaged. Further work will conse-quently be devoted to the fabrication of such devices. Since it wasshown that such covalent microporous organic networks containlarge amounts of sulfur-based ligands in the pore walls, these canbe effectively used as binding sites for metals, as catalyticallyactive nanoparticles.

Experimental

Methods: The NMR measurements in solution were conducted using aBruker DPX-400 spectrometer operating at 400.1MHz. The solid-state NMR13C{1H} CP/MASmeasurements were carried out using a Bruker Avance 400spectrometer operating at 100.6MHz for 13C using a Bruker 4mm double-resonance probe-head operating at a spinning rate of 10 kHz. Nitrogensorption experiments and micropore analysis were conducted at 77K usingan Autosorb-1 from Quantachrome Instruments. Before sorption measure-ments, the samples were degassed in vacuum overnight at 100 8C.

SAXS was performed using a Kratky camera from Anton Paar Instru-ments (wavelength 0.154 nm). The catalysis products were analyzed by gaschromatography-mass spectrometry (GC-MS, Agilent Technologies,GC6890N, MS5975).

The amount of palladium was determined by ICP optical emissionspectrometry (OES) (Optima 2100 DC). Before measurements, the samplewas dissolved in nitric acid and perchloric acid (ratio 3:1) under microwaveirradiation, and the clear solution was diluted with water.

Synthesis of 1,3,5-Tris(2-thienyl)benzene: 2-Acetylthiophene (37mmol)was dissolved in an anhydrous mixture of ethanol-toluene (1:1; 10mL).Tetrachlorosilane (185mmol) was added to the solution, and the mixturewas stirred for 24 h under nitrogen at room temperature. The reactionmixture was poured into water and extracted with dichloromethane threetimes. The combined organic layers were dried over sodium sulphate, andthe solvent was removed under reduced pressure. The brown residue waspurified by column chromatography with light petroleum/dichloromethane(1:1) as eluent. The product was obtained as a yellow solid, with 50% yield.1H NMR (400MHz, CDCl3, d): 7.82–7.86 (4H, m), 7.65–7.68 (4H, m),7.13–7.18 (8H, m), 6.99 (4H, d, J¼ 1.6Hz), 6.94 (4H, t, J¼ 4.5Hz); 13CNMR (100MHz, CDCl3, d): 143.4, 135.6, 128.0, 125.3, 123.8, 122.7.

Synthesis of 2,2(,7,7(-Tetrakis(2-thienyl)-9,9(-spirobifluorene: To a degas-sed solution of Pd(PPh3) and 2,20,7,70-Tetrabromo-9,90- spirobifluorene intetrahydrofuran (THF), 2-Acetylthiophene and a saturated solution ofK2CO3 were added and refluxed for 4 h. The reaction mixture was pouredinto a saturated solution of ammonium chloride and extracted with ethyl

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acetate three times. The combined organic layers were washed with brineand dried over NaSO4, and then removed under reduced pressure. Theremaining oil was purified by column chromatography with n-hexane/chloroform (25:1) as eluent, to obtain yellow crystals as pure product, at80% yield.1H NMR (400MHz, CDCl3, d): 7.73 (3H, s), 7.40 (3H, dd,J¼ 1.0Hz, J¼ 3.6Hz), 7.34 (3H, dd, J¼ 1.0Hz, J¼ 5.0Hz), 7.11 (3H, dd.J¼ 3.6Hz, J¼ 5.0Hz); 13C NMR (100MHz, CDCl3, d): 149.3, 144.0, 140.6,134.2, 127.8, 126.0, 124.6, 123.3, 121.4, 120.5.

General Procedure of Polymerization: FeCl3 was stirred in anhydrousacetonitrile, and the solution was cooled to �20 8C under argonatmosphere. 2,20,7,70-tetrakis(2-thienyl)-9,90-spirobifluorene or 1,3,5-tris(2-thienyl)benzene dissolved in acetonitrile was added, and the precipitatewas filtered and washed with methanol, HCl(aq) and THF.

Generation of Palladium Nanoclusters in the Polymer Networks: Palladiumchloride (PdCl2) was dissolved in ethanol at 50 8C in an ultrasonic bath. Thepolymer network was impregnated with this saturated solution three timeswith repeated drying under vacuum. The palladium chloride was reduced at80 8C under hydrogen atmosphere.

Hydrogenation of Diphenylacetylene: 1mmol of diphenylacetylene wasdissolved in dry methanol under nitrogen atmosphere. 5mg of the catalystwere added, and the flask was purged with hydrogen. The mixture wasstirred at 40 8C and 1.01� 105 Pa of hydrogen for 2 h.

Acknowledgements

The Max Planck Society is acknowledged for support within the frameworkof the project house ENERCHEM. The Deutsche Forschungsgemeinschaftis acknowledged for support within the framework of the excellence clusterUNICAT. Supporting Information is available online from Wiley Inter-Science or from the author.

Received: September 11, 2008

Revised: October 15, 2008

Published online: December 4, 2008

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