Antonietti, Markus

Tauer, Klaus

Vilela, Filipe

Vobecka, Zuzana

Zhang, Kai

English

Kai Zhang

Zuzana Vobecka

Klaus Tauer

Markus Antonietti

Filipe Vilela

Crossref

Chemical Communications

π-Conjugated polyHIPEs as highly efficient and reusable heterogeneous photosensitizers

Heriot Watt Pure

11158 Chem. Commun., 2013, 49, 11158--11160 This journal is c The Royal Society of Chemistry 2013Cite this: Chem. Commun.,2013,49, 11158p-Conjugated polyHIPEs as highly eﬃcient andreusable heterogeneous photosensitizers†Kai Zhang, Zuzana Vobecka, Klaus Tauer, Markus Antonietti and Filipe Vilela*p-Conjugated microporous polyHIPEs are synthesized via Suzukicross-coupling. The polymeric backbone consists of electron donorand acceptor moieties with suitable energetic characteristics topromote the eﬃcient photosensitizing ability for the activation ofsinglet oxygen, showing extraordinarily high stability. The reactioncan be carried out in a continuous flow reaction set-up, realizingcomplete conversion of a-terpinene into ascaridole.Polymerized high internal phase emulsions (polyHIPEs) are arelatively new class of porous lightweight organic materials with anappealing prospect to be used as catalysts,1 supports in organicsynthesis2 or as filtration and separation media.3,4 In principle,a HIPE is formed by adding a disperse droplet phase (usuallyaqueous) to themechanically agitated continuous phase containingall monomeric reactants, reagents and a surfactant. The continuousphase is then polymerized (e.g. via UV, thermal, redox or catalyticcuring) and, after the removal of the droplets, a monolithic materialwith well-defined interconnected pores is achieved.5–8Combination of this large scale porosity with the very highspecific surface area of a p-conjugated microporous polymer(CMP) or framework oﬀers the opportunity to syntheticallytailor the physical behavior and electronic properties of thepolymer according to a targeted application.9 We will illustratethat conjugated polyHIPEs are prospective candidates for hetero-geneous photocatalysis due to the possibility of tuning theirmorphology and tailoring the energy levels and absorptionwavelengths in the visible light region. Moreover, polyHIPEsare cross-linked, insoluble in common solvents, have optimizedtransport and flow properties, and therefore can be easilyengulfed by the reaction mixture, ideally in a continuous flowand without the need for catalyst separation after reaction.From our previous findings,10,11 a well-defined combination ofa weak electron donor and a strong electron acceptor may lead toan eﬀective exciton localization, slow radiative recombination ofthe generated charge pair, but also encourage a high yield ofintersystem crossing to the triplet state of the excited polymerframework. Additionally, if the absolute positions of HOMO andLUMO are chosen to enable the suﬃcient oxidation stability of theframework as well as reactivity, this can result in an appropriateenergetic state for photocatalytic generation of singlet oxygen.Unlike most natural compounds, molecular oxygen has tripletmultiplicity in the ground state and an excited singlet state lying95 kJ mol1 (corresponding to a 1250 nm photon) higher.11 Themost common and widely used generation of singlet oxygen isbased on photosensitizers.12,13 This simple method requiresonly an oxygen source, light of an appropriate wavelength and aphotosensitizer possessing an energetically suitable excitedtriplet state. By absorption of light, the photosensitizer excitesto its singlet state (S) followed through subsequent intersystemcrossing by population of a triplet state (T). The excited sensitizercan transfer energy and spin to the ground state triplet oxygen,thus forming the excited singlet oxygen.12,13 Singlet oxygen has alifetime of less than 1 s in solution and can be rapidly deactivatedback to the ground state via physical or chemical quenching.Given its strong electrophilic and oxidizing character, it hasbeen widely used as a versatile synthetic reagent in range ofapplications including organic synthesis,14 environmental watertreatment15 or photodynamic therapy of cancer.16Most commonly used photosensitizers include dyes such asrose bengal, eosin, methylene blue, or biocompatible porphyrinor phthalocyanine derivatives.13 These materials are usually usedin a homogeneous manner, which creates an additional issue ofsubsequent isolation from the system or even proliferation inbiological systems. Compared to this conventional homogenousapproach, the use of heterogeneous photosensitizers in principalwould oﬀer several advantages in practical applications. Theycan be easily isolated, recovered and recycled when applied,for instance, in water purification systems. Moreover, due totheir insolubility, they can be used in a variety of solventsand subsequently filtered oﬀ from the reaction mixture, thusestablishing an unparalleled versatility and simplicity in termsof chemical synthesis and purification procedures.13Max Planck Institute of Colloids and Interfaces, Am Mu¨hlenberg 1, D-14424 Potsdam,Germany. E-mail: filipe.vilela@mpikg.mpg.de; Tel: +49 331-567-9508† Electronic supplementary information (ESI) available: Synthetic routes, elementalanalysis, FTIR spectra, CDMAS 13C NMR in solid state, TGA analysis of B-FL3, B-BT-FL2and B-BT2-FL, experimental details of the singlet oxygen generation set-up, and the1H NMR spectrum and UV absorption of BT–FL. See DOI: 10.1039/c3cc45597aReceived 23rd July 2013,Accepted 8th August 2013DOI: 10.1039/c3cc45597awww.rsc.org/chemcommChemCommCOMMUNICATIONPublished on 08 August 2013. Downloaded by Heriot Watt University on 30/01/2014 13:34:51. View Article OnlineView Journal  | View IssueThis journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 11158--11160 11159In this work, we will employ an electron system consisting of astrong electron-withdrawing moiety benzothiadiazole (BT) andweak electron-donating moieties, here benzene (B) and fluorene(FL), within a conjugated backbone. Based on its small band gap,high absorption coeﬃcient and suitable chemical stability, benzo-thiadiazole has been utilized in optoelectronic materials such aslow-band-gap polymers,17–20 non-fullerene acceptors,21 or n-typefield effect transistors.22 Benzene can be easily functionalized indifferent positions by a wide range of substituents, and many ofthe building blocks are commercially available at low cost.This layout of photocatalytic activity of the novel fully conju-gated porous polymers is then coupled to HIPE polymerization(CMP-HIPEs) for themicroscale design. The synthesis was carriedout via palladium-catalyzed Suzuki cross-coupling, where 1,3,5-tribromobenzene (B) was used as a cross-linker reacting withdiﬀerent ratios of boronic ester derivatives of benzothiadiazole(BT) and fluorene (FL). The resulting polymers are expected tohave statistically random distributions of BT and FL linkers dueto the unequal reactivities of these moieties. From the analysisprovided, the resulting polymers are structurally in good agree-ment with the ratio of the starting materials. The exact proce-dures of the synthesis and characterization of the polymers aredescribed in the ESI† (Scheme S1, Fig. S1–S6 and Tables S1–S3).It is noted that to produce a stable emulsion resulting inmonolithic CMP-HIPEs, alkylated monomers such as the FL unitemployed need to be present. Attempts to produce the B-BT3HIPE in the absence of FL units were not successful.The SEM images and optical photographs of the resultingpolymer scaﬀolds are presented in Fig. 1, proving that highlymacroporous monolithic structures were formed.This is in agreement with the BET results, which show surfaceareas of 35–50 m2 g1, which are within the range of previouslyreported polyHIPE materials.2,5 In addition, the pore distributionanalysis confirms the presence of micropores within the structuresproving the hierarchical morphology of the scaffold (ESI,† Fig. S6).Moreover, owing to the highly cross-linked and rigid aromatic char-acter of the polymeric backbone, very stablemonolithic architecturesare achieved. These monoliths are, after drying, in all cases in therange of 1 cm height, with densities ranging from 50 to 80mg cm3.Fluorescence spectra were recorded in the solid state using aquartz sample chamber (ESI,† Fig. S1). The emission of thepolymers containing the BT unit (B-BT-FL2 and B-BT2-FL) isstrongly red-shifted due to its strong electron withdrawingcharacter, which results in a lower LUMO level and narrowerband gaps. The same trend is also observed in UV reflectionspectra (ESI,† Fig. S2), also recorded in the solid state.Two diﬀerent methods were used to investigate the feasibilityof heterogeneous photocatalysis to be executed by the CMPHIPEs. The first technique was based on a classical, stirredreaction set-up. Small pieces of the porous CMP-HIPEs togetherwith a solution of a-terpinene were placed in a reaction vessel,which was irradiated with blue light at 420 nm, while oxygen gaswas bubbled through it (reaction condition: 5 mg of CMP-HIPEs,10 mL of 0.1 M a-terpinene solution in CHCl3, oxygen flow rate =10 mLmin1). The well-known transformation of a-terpinene viain situ generated singlet oxygen yields ascaridole.Reaction progress was followed by 1H NMR, and the con-version calculated from the ratio of peaks assigned to startingmaterial and product (ESI,† Fig. S8).10 50% conversion was observedafter 6 h when B-FL3 was used. In comparison, complete con-versions were achieved by using B-BT-FL2 and B-BT2-FL after 60and 30 min, respectively (Fig. 2).This observation suggests that incorporation of the strongelectron accepting BT unit into the polymer backbone favoursthe triplet state under excitation, which is essential for thephotosensitized generation of singlet oxygen. The fact thatB-BT2-FL facilitated faster reaction could be interpreted as astronger inter-crossing process due to higher BT content withinthe conjugated system resulting in a more stable triplet state ofthe charge pair under excitation. However, further photophysicalmeasurements are required to confirm this hypothesis. The secondmethod utilized a reaction in a continuous flow system as depictedin Fig. 3a (and ESI,† Fig. S7). The continuous flow set-up oﬀersseveral advantages over conventional reaction methods, whichinclude the opportunity to scale-up via an increase of the reactiontime rather than the size of the reaction vessel, or the possibility ofprecise control of the reaction parameters (temperature, contacttime, mass transfer or mixing).14 In the particular case of hetero-geneous photocatalysis, light can more eﬃciently penetratethrough thin tubing rather than bulk solution in a flask, givingrise to more eﬃcient singlet oxygen generation. Moreover, thepolymeric catalyst can be permanently placed in one part of thewhole system (e.g. fritted glass column) whilst the solutionflows through it; this can dramatically ease up isolation steps.The best performing polymer from the previous experiments(B-BT2-FL) was used. Small pieces of the B-BT2-FLmonolith (10mg,)Fig. 1 Idealized structures and SEM images of (a) B-FL3, (b) B-BT-FL2 (c) B-BT2-FL,scale bar = 1 mm. Insets – photographs of corresponding CMP-HIPE monoliths.Fig. 2 Conversion of ascaridole from the oxidation of a-terpinene using theCMP-HIPEs as heterogeneous photocatalysts.Communication ChemCommPublished on 08 August 2013. Downloaded by Heriot Watt University on 30/01/2014 13:34:51. View Article Online11160 Chem. Commun., 2013, 49, 11158--11160 This journal is c The Royal Society of Chemistry 2013were placed into a photoreactor consisting of a glass column(r = 0.35 cm, L = 7 cm) and FEP (fluorinated ethylene propylene)tubing. A solution of a-terpinene (0.1 M in 10 mL CHCl3) wasmixed with oxygen and pumped through the reactor which wasirradiated with blue light at 420 nm (see ESI,† Fig. S7). Fullconversion to ascaridole was observed.To demonstrate the lifetime or reusability of the CMP-HIPEs,10 repeating experiments were carried out using the same batchof B-BT2-FL at constant flow rates of oxygen (5 mL min1) anda-terpinene solution (20 mL h1). The perpetuated quantitativeconversion of ascaridole in all experiments proves the excep-tional photoactivity and photostability of B-BT2-FL (ESI,† Fig. S8).The reaction kinetics were investigated by varying the flowrates of the a-terpinene solution, whilst keeping the oxygen flowconstant (5 mL min1). Fig. 3b indicates that complete conver-sion to ascaridole is observed only up to rates of 30 mL h1,up to which a suﬃcient contact time for the singlet oxygen–dienereaction was provided. Increasing the flow rate of the a-terpinenesolution decreases the conversion to 52% of product at 80mL h1.This translates into an overall reaction time, under the given set ofconditions, of about 2 minutes.To determine the amount of generated singlet oxygen usingB-BT2-FL as the photosensitizer, a concentrated a-terpinenesolution (0.33 M and 20 mL h1) was injected into the glasscolumn along with oxygen (5 mLmin1). In this case, a conversionof 23% of ascaridole was detected and therefore the calculatedminimum amount of produced singlet oxygen by 1 mg of B-BT2-FLequals 0.0025 mmol min1.As a control experiment, a linear BT–FL alternating polymer(BT–FL)23 was synthesized (see ESI† for experimental details,Fig. S10–S12) and tested as a homogeneous photocatalyst. Com-plete conversion of ascaridole was determined after 20 minutesusing the bulk reaction set up. This supports a molecular layoutconsisting of BT and FL units satisfying the required parametersfor stability and efficient triplet transfer. In addition, a directcomparison of the molecular homogeneous catalyst with theinsoluble microporous CMP-HIPEs architecture indeed showsthat heterogenization comes with practically no loss in reactionspeed, but enables the reaction to proceed under heterogeneousconditions and to be used in flow reactors, without the need forcatalyst separation.In conclusion, a series of fully p-conjugated microporouspolymers with polyHIPEs macrostructure were prepared viaSuzuki cross-coupling. This allows for p-conjugated micro-porous materials to be combined with a favorable macroarchitecture in order to create functional materials for a varietyof applications. To the best of our knowledge this is the firsttime that fully conjugated polyHIPEs are reported, and that aSuzuki coupling could be performed under such conditions.Diﬀerent combinations of electron donor and acceptor moietieswere incorporated into the polymer backbones in order to inves-tigate their influence on photocatalytic activity for singlet oxygengeneration, as monitored by an in situ reaction with a-terpinene.The pure electron donor system (B-FL3) showed low photocatalyticactivity, whereas incorporation of strong electron acceptor unitsinto the polymeric chain (B-BT-FL2 and B-BT2-FL) demonstrateddramatic improvement of the photocatalytic activities. B-BT2-FLin particular proved to be a stable, reusable and eﬃcient photo-catalyst, facilitating full conversion of ascaridole in a continuousflow reaction set-up with overall contact timesas low as 2 min.CMP-HIPEs, in our opinion, represent a new promising family ofrecyclable heterogeneous photocatalysts, which will significantlysimplify the handling and purification of sensitive reactions.Notes and references1 W. Li, W. Zhang, X. Dong, L. Yan, R. Qi, W. Wang, Z. Xie and X. Jing,J. Mater. Chem., 2012, 22, 17445.2 P. Krajnc, N. Leber, J. F. Brown and N. R. Cameron, React. Funct.Polym., 2006, 66, 81.3 Z. Bhumgara, Filtr. Sep., 1995, 32, 245.4 E. H. Mert, H. Yıldırım, A. T. U¨zu¨mcu¨ and H. Kavas, React. Funct.Polym., 2013, 73, 175.5 N. Brun, S. Ungureanu, H. Deleuze and R. Backov, Chem. Soc. Rev.,2011, 40, 771.6 H. Tai, A. Sergienko and M. Silverstein, Polym. Eng. Sci., 2001,41, 1540.7 S. D. Kimmins and N. R. Cameron, Adv. Funct. Mater., 2011, 21, 211.8 N. R. Cameron, Polymer, 2005, 46, 1439.9 F. Vilela, K. Zhang and M. Antonietti, Energy Environ. Sci., 2012,5, 7819.10 K. Zhang, D. Kopetzki, P. Seeberger, M. Antonietti and F. Vilela,Angew. Chem., Int. Ed., 2013, 52, 1432.11 H. Urakami, K. Zhang and F. Vilela, Chem. Commun., 2013, 49, 2353.12 C. Schweitzer and R. Schmidt, Chem. Rev., 2003, 103, 1685.13 M. C. DeRosa and R. J. Crutchley, Coord. Chem. Rev., 2002,233–234, 351.14 F. Le´vesque and P. H. Seeberger, Angew. Chem., Int. Ed., 2012, 51, 1706.15 S. Canonica and P. G. Tratnyek, Environ. Toxicol. Chem., 2003, 22, 1743.16 A. Juarranz, P. Jae´n, F. Sanz-Rodrı´guez, J. Cuevas and S. Gonza´lez,Clin. Transl. Oncol., 2008, 10, 148.17 S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J. S. Moon,D. Moses, M. Leclerc, K. Lee and A. J. Heeger, Nat. Photonics,2009, 3, 297.18 R. C. Coﬃn, J. Peet, J. Rogers and G. C. Bazan, Nat. Chem., 2009,1, 657.19 F. Huang, K. S. Chen, H. L. Yip, S. K. Hau, O. Acton, Y. Zhang,J. D. Luo and A. K. Y. Jen, J. Am. Chem. Soc., 2009, 131, 13886.20 L. J. Huo, J. H. Hou, S. Q. Zhang, H. Y. Chen and Y. Yang, Angew.Chem., Int. Ed., 2010, 49, 1500.21 P. E. Schwenn, K. Gui, A. M. Nardes, K. B. Krueger, K. H. Lee,K. Mutkins, H. Rubinstein-Dunlop, P. E. Shaw, N. Kopidakis,P. L. Burn and P. Meredith, Adv. Energy Mater., 2011, 1, 73.22 K. Mutkins, K. Gui, M. Aljada, P. E. Schwenn, E. B. Namdas,P. L. Burn and P. Meredith, Appl. Phys. Lett., 2011, 98, 153301.23 F. Lemasson, N. Berton, J. Tittmann, F. Hennrich, M. M. Kappes andM. Mayor, Macromolecules, 2011, 45, 713.Fig. 3 (a) Schematic presentation of the continuous flow set-up for photo-oxidation of a-terpinene; (b) ascaridole conversions at diﬀerent flow rates ofa-terpinene solution.ChemComm CommunicationPublished on 08 August 2013. Downloaded by Heriot Watt University on 30/01/2014 13:34:51. View Article Online

Zhang, K.

Vobecka, Z.

Tauer, K.

Antonietti, M.

Vilela, F.

MPG.PuRe

    Heriot-Watt University Research Gateway                                                          Heriot-Watt University-Conjugated polyHIPEs as highly efficient and reusable heterogeneousphotosensitizersZhang, Kai; Vobecka, Zuzana; Tauer, Klaus; Antonietti, Markus; Vilela, FilipePublished in:Chemical CommunicationsDOI:10.1039/c3cc45597aPublication date:2013Link to publication in Heriot-Watt Research GatewayCitation for published version (APA):Zhang, K., Vobecka, Z., Tauer, K., Antonietti, M., & Vilela, F. (2013). -Conjugated polyHIPEs as highly efficientand reusable heterogeneous photosensitizers. Chemical Communications, 49, 11158-11160.10.1039/c3cc45597a11158 Chem. Commun., 2013, 49, 11158--11160 This journal is c The Royal Society of Chemistry 2013Cite this: Chem. Commun.,2013,49, 11158p-Conjugated polyHIPEs as highly eﬃcient andreusable heterogeneous photosensitizers†Kai Zhang, Zuzana Vobecka, Klaus Tauer, Markus Antonietti and Filipe Vilela*p-Conjugated microporous polyHIPEs are synthesized via Suzukicross-coupling. The polymeric backbone consists of electron donorand acceptor moieties with suitable energetic characteristics topromote the eﬃcient photosensitizing ability for the activation ofsinglet oxygen, showing extraordinarily high stability.The reactioncan be carried out in a continuous flow reaction set-up, realizingcomplete conversion of a-terpinene into ascaridole.Polymerized high internal phase emulsions (polyHIPEs) are arelatively new class of porous lightweight organic materials with anappealing prospect to be used as catalysts,1 supports in organicsynthesis2 or as filtration and separation media.3,4 In principle,a HIPE is formed by adding a disperse droplet phase (usuallyaqueous) to the mechanically agitated continuous phase containingall monomeric reactants, reagents and a surfactant. The continuousphase is then polymerized (e.g. via U V ,t h e r m a l ,r e d o xo rc a t a l y t i ccuring) and, after the removal of the droplets, a monolithic materialwith well-defined interconnected pores is achieved.5–8Combination of this large scale porosity with the very highspecific surface area of a p-conjugated microporous polymer(CMP) or framework oﬀers the opportunity to syntheticallytailor the physical behavior and electronic properties of thepolymer according to a targeted application.9 We will illustratethat conjugated polyHIPEs are prospective candidates for hetero-geneous photocatalysis due to the possibility of tuning theirmorphology and tailoring the energy levels and absorptionwavelengths in the visible light region. Moreover, polyHIPEsare cross-linked, insoluble in common solvents, have optimizedtransport and flow properties, and therefore can be easilyengulfed by the reaction mixture, ideally in a continuous flowand without the need for catalyst separation after reaction.From our previous findings,10,11 a well-defined combination ofa weak electron donor and a strong electron acceptor may lead toan eﬀective exciton localization, slow radiative recombination ofthe generated charge pair, but also encourage a high yield ofintersystem crossing to the triplet state of the excited polymerframework. Additionally, if the absolute positions of HOMO andLUMO are chosen to enable the suﬃcient oxidation stability of theframework as well as reactivity, this can result in an appropriateenergetic state for photocatalytic generation of singlet oxygen.Unlike most natural compounds, molecular oxygen has tripletmultiplicity in the ground state and an excited singlet state lying95 kJ mol1 (corresponding to a 1250 nm photon) higher.11 Themost common and widely used generation of singlet oxygen isbased on photosensitizers.12,13 This simple method requiresonly an oxygen source, light of an appropriate wavelength and aphotosensitizer possessing an energetically suitable excitedtriplet state. By absorption of light, the photosensitizer excitesto its singlet state (S) followed through subsequent intersystemcrossing by population of a triplet state (T). The excited sensitizercan transfer energy and spin to the ground state triplet oxygen,thus forming the excited singlet oxygen.12,13 Singlet oxygen has alifetime of less than 1 s in solution and can be rapidly deactivatedback to the ground state via physical or chemical quenching.Given its strong electrophilic and oxidizing character, it hasbeen widely used as a versatile synthetic reagent in range ofapplications including organic synthesis,14 environmental watertreatment15 or photodynamic therapy of cancer.16Most commonly used photosensitizers include dyes such asrose bengal, eosin, methylene blue, or biocompatible porphyrinor phthalocyanine derivatives.13 These materials are usually usedin a homogeneous manner, which creates an additional issue ofsubsequent isolation from the system or even proliferation inbiological systems. Compared to this conventional homogenousapproach, the use of heterogeneous photosensitizers in principalwould oﬀer several advantages in practical applications. Theycan be easily isolated, recovered and recycled when applied,for instance, in water purification systems. Moreover, due totheir insolubility, they can be used in a variety of solventsand subsequently filtered oﬀ from the reaction mixture, thusestablishing an unparalleled versatility and simplicity in termsof chemical synthesis and purification procedures.13Max Planck Institute of Colloids and Interfaces, Am Mu ¨hlenberg 1, D-14424 Potsdam,Germany. E-mail: filipe.vilela@mpikg.mpg.de; Tel: +49 331-567-9508† Electronic supplementary information (ESI) available: Synthetic routes, elementalanalysis, FTIR spectra, CD MAS13C NMR in solid state, TGA analysis of B-FL3, B-BT-FL2and B-BT2-FL, experimental details of the singlet oxygen generation set-up, and the1H NMR spectrum and UV absorption of BT–FL. See DOI: 10.1039/c3cc45597aReceived 23rd July 2013,Accepted 8th August 2013DOI: 10.1039/c3cc45597awww.rsc.org/chemcommChemCommCOMMUNICATIONPublished on 08 August 2013. Downloaded by Heriot Watt University on 30/01/2014 13:34:51. View Article OnlineView Journal | View IssueThis journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 11158--11160 11159In this work, we will employ an electron system consisting of astrong electron-withdrawing moiety benzothiadiazole (BT) andweak electron-donating moieties, here benzene (B) and fluorene(FL), within a conjugated backbone. Based on its small band gap,high absorption coeﬃcient and suitable chemical stability, benzo-thiadiazole has been utilized in optoelectronic materials such aslow-band-gap polymers,17–20 non-fullerene acceptors,21 or n-typefield effect transistors.22 Benzene can be easily functionalized indifferent positions by a wide range of substituents, and many ofthe building blocks are commercially available at low cost.This layout of photocatalytic activity of the novel fully conju-gated porous polymers is then coupled to HIPE polymerization(CMP-HIPEs) for themicroscale design. The synthesiswas carriedout via palladium-catalyzed Suzuki cross-coupling, where 1,3,5-tribromobenzene (B) was used as a cross-linker reacting withdiﬀerent ratios of boronic ester derivatives of benzothiadiazole(BT) and fluorene (FL). The resulting polymers are expected tohave statistically random distributions of BT and FL linkers dueto the unequal reactivities of these moieties. From the analysisprovided, the resulting polymers are structurally in good agree-ment with the ratio of the starting materials. The exact proce-dures of the synthesis and characterization of the polymers aredescribed in the ESI† (Scheme S1, Fig. S1–S6 and Tables S1–S3).It is noted that to produce a stable emulsion resulting inmonolithic CMP-HIPEs, alkylated monomers such as the FL unitemployed need to be present. Attempts to produce the B-BT3HIPE in the absence of FL units were not successful.The SEM images and optical photographs of the resultingpolymer scaﬀolds are presented in Fig. 1, proving that highlymacroporous monolithic structures were formed.This is in agreement with the BET results, which show surfaceareas of 35–50 m2 g1, which are within the range of previouslyreported polyHIPE materials.2,5 In addition, the pore distributionanalysis confirms the presence of micropores within the structuresproving the hierarchical morphology of the scaffold (ESI,† Fig. S6).Moreover, owing to the highly cross-linked and rigid aromatic char-acter of the polymeric backbone, very stable monolithic architecturesa r ea c h i e v e d .T h e s em o n o l i t h sa r e ,after drying, in all cases in therange of 1 cm height, with densities ranging from 50 to 80 mg cm3.Fluorescence spectra were recorded in the solid state using aquartz sample chamber (ESI,† Fig. S1). The emission of thepolymers containing the BT unit (B-BT-FL2 and B-BT2-FL)i sstrongly red-shifted due to its strong electron withdrawingcharacter, which results in a lower LUMO level and narrowerband gaps. The same trend is also observed in UV reflectionspectra (ESI,† Fig. S2), also recorded in the solid state.Two diﬀerent methods were used to investigate the feasibilityof heterogeneous photocatalysis to be executed by the CMPHIPEs. The first technique was based on a classical, stirredreaction set-up. Small pieces of the porous CMP-HIPEs togetherwith a solution of a-terpinene were placed in a reaction vessel,which was irradiated with blue light at 420 nm, while oxygen gaswas bubbled through it (reaction condition: 5 mg of CMP-HIPEs,10 mL of 0.1 M a-terpinene solution in CHCl3,o x y g e nf l o wr a t e=10 mL min1). The well-known transformation of a-terpinene viain situ generated singlet oxygen yields ascaridole.Reaction progress was followed by1H NMR, and the con-version calculated from the ratio of peaks assigned to startingmaterial and product (ESI,† Fig. S8).10 50% conversion was observedafter 6 h when B-FL3 was used. In comparison, complete con-versions were achieved by using B-BT-FL2 and B-BT2-FL after 60and 30 min, respectively (Fig. 2).This observation suggests that incorporation of the strongelectron accepting BT unit into the polymer backbone favoursthe triplet state under excitation, which is essential for thephotosensitized generation of singlet oxygen. The fact thatB-BT2-FL facilitated faster reaction could be interpreted as astronger inter-crossing process due to higher BT content withinthe conjugated system resulting in a more stable triplet state ofthe charge pair under excitation. However, further photophysicalmeasurements are required to confirm this hypothesis. The secondmethod utilized a reaction in a continuous flow system as depictedin Fig. 3a (and ESI,† Fig. S7). The continuous flow set-up oﬀersseveral advantages over conventional reaction methods, whichinclude the opportunity to scale-up via an increase of the reactiontime rather than the size of the reaction vessel, or the possibility ofprecise control of the reaction parameters (temperature, contacttime, mass transfer or mixing).14 In the particular case of hetero-geneous photocatalysis, light can more eﬃciently penetratethrough thin tubing rather than bulk solution in a flask, givingrise to more eﬃcient singlet oxygen generation. Moreover, thepolymeric catalyst can be permanently placed in one part of thewhole system (e.g. fritted glass column) whilst the solutionflows through it; this can dramatically ease up isolation steps.The best performing polymer from the previous experiments(B-BT2-FL) was used. Small pieces of the B-BT2-FL monolith (10 mg,)Fig. 1 Idealized structures and SEM images of (a) B-FL3,( b )B-BT-FL2 (c) B-BT2-FL,scale bar = 1 mm. Insets – photographs of corresponding CMP-HIPE monoliths.Fig. 2 Conversion of ascaridole from the oxidation of a-terpinene using theCMP-HIPEs as heterogeneous photocatalysts.Communication ChemCommPublished on 08 August 2013. Downloaded by Heriot Watt University on 30/01/2014 13:34:51. View Article Online11160 Chem. Commun., 2013, 49, 11158--11160 This journal is c The Royal Society of Chemistry 2013were placed into a photoreactor consisting of a glass column(r = 0.35 cm, L = 7 cm) and FEP (fluorinated ethylene propylene)tubing. A solution of a-terpinene (0.1 M in 10 mL CHCl3) wasmixed with oxygen and pumped through the reactor which wasirradiated with blue light at 420 nm (see ESI,† Fig. S7). Fullconversion to ascaridole was observed.To demonstrate the lifetime or reusability of the CMP-HIPEs,10 repeating experiments were carried out using the same batchof B-BT2-FL at constant flow rates of oxygen (5 mL min1)a n da-terpinene solution (20 mL h1). The perpetuated quantitativeconversion of ascaridole in all experiments proves the excep-tional photoactivity and photostability of B-BT2-FL (ESI,† Fig. S8).The reaction kinetics were investigated by varying the flowrates of the a-terpinene solution, whilst keeping the oxygen flowconstant (5 mL min1). Fig. 3b indicates that complete conver-sion to ascaridole is observed only up to rates of 30 mL h1,up to which a suﬃcient contact timef o rt h es i n g l e to x y g e n – d i e n ereaction was provided. Incre a s i n gt h ef l o wr a t eo ft h ea-terpinenesolution decreases the conversion to 52% of product at 80 mL h1.This translates into an overall reaction time, under the given set ofconditions, of about 2 minutes.To determine the amount of generated singlet oxygen usingB-BT2-FL as the photosensitizer, a concentrated a-terpinenesolution (0.33 M and 20 mL h1) was injected into the glasscolumn along with oxygen (5 mL min1). In this case, a conversionof 23% of ascaridole was detected and therefore the calculatedminimum amount of produced singlet oxygen by 1 mg of B-BT2-FLequals 0.0025 mmol min1.As a control experiment, a linear BT–FL alternating polymer(BT–FL)23 was synthesized (see ESI† for experimental details,Fig. S10–S12) and tested as a homogeneous photocatalyst. Com-plete conversion of ascaridole was determined after 20 minutesusing the bulk reaction set up. This supports a molecular layoutconsisting of BT and FL units satisfying the required parametersfor stability and efficient triplet transfer. In addition, a directcomparison of the molecular homogeneous catalyst with theinsoluble microporous CMP-HIPEs architecture indeed showsthat heterogenization comes with practically no loss in reactionspeed, but enables the reaction to proceed under heterogeneousconditions and to be used in flow reactors, without the need forcatalyst separation.In conclusion, a series of fully p-conjugated microporouspolymers with polyHIPEs macrostructure were prepared viaSuzuki cross-coupling. This allows for p-conjugated micro-porous materials to be combined with a favorable macroarchitecture in order to create functional materials for a varietyof applications. To the best of our knowledge this is the firsttime that fully conjugated polyHIPEs are reported, and that aSuzuki coupling could be performed under such conditions.Diﬀerent combinations of electron donor and acceptor moietieswere incorporated into the polymer backbones in order to inves-tigate their influence on photocatalytic activity for singlet oxygengeneration, as monitored by an in situ reaction with a-terpinene.The pure electron donor system (B-FL3) showed low photocatalyticactivity, whereas incorporation of strong electron acceptor unitsinto the polymeric chain (B-BT-FL2 and B-BT2-FL)d e m o n s t r a t e ddramatic improvement of the photocatalytic activities. B-BT2-FLin particular proved to be a stable, reusable and eﬃcient photo-catalyst, facilitating full conversion of ascaridole in a continuousflow reaction set-up with overall contact timesas low as 2 min.CMP-HIPEs, in our opinion, represent a new promising family ofrecyclable heterogeneous photocatalysts, which will significantlysimplify the handling and purification of sensitive reactions.Notes and references1 W. Li, W. Zhang, X. Dong, L. Yan, R. Qi, W. Wang, Z. Xie and X. Jing,J. Mater. Chem., 2012, 22, 17445.2 P. Krajnc, N. Leber, J. F. Brown and N. R. Cameron, React. Funct.Polym., 2006, 66, 81.3 Z. Bhumgara, Filtr. Sep., 1995, 32, 245.4 E. H. Mert, H. Yıldırım, A. T. U ¨zu ¨mcu ¨ and H. Kavas, React. Funct.Polym., 2013, 73, 175.5 N. Brun, S. Ungureanu, H. Deleuze and R. Backov, Chem. Soc. Rev.,2011, 40, 771.6 H. Tai, A. Sergienko and M. Silverstein, Polym. Eng. Sci., 2001,41, 1540.7 S. D. Kimmins and N. R. Cameron, Adv. Funct. Mater., 2011, 21, 211.8 N. R. Cameron, Polymer, 2005, 46, 1439.9 F. Vilela, K. Zhang and M. Antonietti, Energy Environ. Sci., 2012,5, 7819.10 K. Zhang, D. Kopetzki, P. Seeberger, M. Antonietti and F. Vilela,Angew. Chem., Int. Ed., 2013, 52, 1432.11 H. Urakami, K. Zhang and F. Vilela, Chem. Commun., 2013, 49, 2353.12 C. Schweitzer and R. Schmidt, Chem. Rev., 2003, 103, 1685.13 M. C. DeRosa and R. J. Crutchley, Coord. Chem. Rev., 2002,233–234, 351.14 F. Le ´vesque and P. H. Seeberger, Angew.Chem., Int. Ed., 2012, 51, 1706.15 S. Canonica and P. G. Tratnyek, Environ. Toxicol. Chem., 2003, 22, 1743.16 A. Juarranz, P. Jae ´n, F. Sanz-Rodrı ´guez, J. Cuevas and S. Gonza ´lez,Clin. Transl. Oncol., 2008, 10, 148.17 S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J. S. Moon,D. Moses, M. Leclerc, K. Lee and A. J. Heeger, Nat. Photonics,2009, 3, 297.18 R. C. Coﬃn, J. Peet, J. Rogers and G. C. Bazan, Nat. Chem., 2009,1, 657.19 F. Huang, K. S. Chen, H. L. Yip, S. K. Hau, O. Acton, Y. Zhang,J. D. Luo and A. K. Y. Jen, J. Am. Chem. Soc., 2009, 131, 13886.20 L. J. Huo, J. H. Hou, S. Q. Zhang, H. Y. Chen and Y. Yang, Angew.Chem., Int. Ed., 2010, 49, 1500.21 P. E. Schwenn, K. Gui, A. M. Nardes, K. B. Krueger, K. H. Lee,K. Mutkins, H. Rubinstein-Dunlop, P. E. Shaw, N. Kopidakis,P. L. Burn and P. Meredith, Adv. Energy Mater., 2011, 1, 73.22 K. Mutkins, K. Gui, M. Aljada, P. E. Schwenn, E. B. Namdas,P. L. Burn and P. Meredith, Appl. Phys. Lett., 2011, 98, 153301.23 F. Lemasson, N. Berton, J. Tittmann, F. Hennrich, M. M. Kappes andM. Mayor, Macromolecules, 2011, 45, 713.Fig. 3 (a) Schematic presentation of the continuous flow set-up for photo-oxidation of a-terpinene; (b) ascaridole conversions at diﬀerent flow rates ofa-terpinene solution.ChemComm CommunicationPublished on 08 August 2013. Downloaded by Heriot Watt University on 30/01/2014 13:34:51. View Article Online

π-Conjugated polyHIPEs as highly efficient and reusable heterogeneous photosensitizers

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