Conjugated Polyfluorene/Polyaniline Block Copolymers

Cristopher Schmitt, Heinz-Georg Nothofer, AurØlie Falcou, Ullrich Scherf* a

Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany

IntroductionConjugated polymers are widely used as electronicallyactive materials, e.g., as emitters in organic materials-based light emitting diodes (OLEDs) or polymerlasers.[1, 2] For these applications, active polymers aremostly used as single-component materials, isotropicfilms or layers of uniform morphology. For some otherapplications, especially the use of conjugated materials inphotovoltaic devices (solar cells, photodetectors) alsocomplex materials have become more and more attrac-tive.[3, 4] The efficiency of such devices is often limitedbecause of the short exciton diffusion range in conjugatedpolymers in relation to their optical absorption depth.Alternative structures in which a heterojunction is distrib-uted throughout the film have become increasinglyimportant, e.g., conjugated polymers doped with fullereneacceptors or fullerene/polymer multilayer systems.[5, 6]

However, only light absorbed close to the heterojunctionbetween the components results in charge generation.The width of this active region is equated to the exciton

diffusion range of photogenerated charge carriers (in con-jugated polymers typically 10–20 nm[7, 8] depending onthe morphology and purity of the materials). Novel sys-tems, which are able to undergo internal microphase-separation, came in the focus of interest, since suchmeso- and nanoscopically structured systems could allowfor an effective charge separation (and transport) of opti-cally generated electron/hole pairs.

Two different approaches to the control of such struc-tures are possible. First, the use of two immiscible poly-mers can lead to the generation of complex, orderedphase-separated structures[9] The length scale of phaseseparation is often in the “meso” length regime (10–100 nm), and is convenient but relatively less controlled.Second, in conjugated-conjugated block copolymers thescale length of phase separation and the electronic natureof the phases is expected to be directly related to thechemical structure and the block lengths of both blocks.It should be possible to fine-tune the scale length of phaseseparation to be in the range of the diffusion lengths ofthe optically formed excitons. That would guarantee suf-ficient charge carrier mobility of holes and electrons,respectively, in the corresponding tailor-made phases.Conjugated-conjugated block copolymers represent anovel, up to now unknown class of electronically active

Communication: Novel, soluble poly(9,9-dialkylfluor-ene)/poly(2-alkylaniline) block copolymers were synthe-sized following a three-step synthetic procedure: (i) aryl-aryl coupling of alkylated dibromofluorenes and fluorenediboronic esters according to Suzuki in the presence of 4-nitrobromobenzene as a monofunctional end-cappingreagent, (ii) reduction of nitrophenyl to aminophenylfunctions with H2/Pd/C and (iii) subsequent oxidative con-densation with 2-undecylaniline. The colored conjugated-conjugated block copolymers were characterized bymeans of GPC, NMR and UV-Vis spectroscopy.

Macromol. Rapid Commun. 2001, 22, No. 8 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1336/2001/0805–0624$17.50+.50/0

AFM of a spin-coated film of PF/PANI block copolymer 3(film thickness ca. 100 nm) after thermal treatment (1 h,173 8C).

a Present adress of U.S.: Universität Potsdam, Institut für Phy-sikalische und Theoretische Chemie, Lehrstuhl für Poly-merchemie, Karl-Liebknecht-Str. 24–25, Haus 25, D-14476Golm, GermanyE-mail: scherf@rz.uni-potsdam.de

624 Macromol. Rapid Commun. 2001, 22, 624–628

Conjugated Polyfluorene/Polyaniline Block Copolymers 625

polymers. Especially, such ones composed of two conju-gated blocks of different oxidation and reduction poten-tials are very attractive. If they form suited microphase-separated structures, efficient charge separation canoccur, followed by an efficient transport of the chargecarriers through the phases to the corresponding electro-des. In this communication, we describe the first syn-thesis of poly(9,9-dialkylfluorene)/poly(2-alkylaniline)(PF/PANI) block copolymers following a three-step syn-thetic procedure. The products are of special interest assystems containing two blocks of different reduction andoxidation energies.

Results and DiscussionUp to now, conjugated block copolymers have not beendescribed. Our novel approach toward polyfluorene/poly-aniline (PF/PANI) block copolymers involves the reac-tion of aminophenyl end-functionalized PF prepolymerswith an alkylated aniline monomer (2-undecylaniline) inthe key reaction step.

Amino end-functionalized polyfluorenes are accessibleby (i) co-condensation (according to Suzuki) of 9,9-bis(2-ethylhexyl)-2,7-dibromofluorene, 9,9-bis(2-ethylhexyl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)fluor-ene, and 4-nitrobromobenzene as a monofunctional chainterminating agent (end-capping reagent), and (ii) subse-quent reduction of the terminal nitro groups with H2/Pd/C(Scheme 1). The molecular weight of the polyfluoreneprepolymer can be controlled by the feed ratio of bifunc-tional fluorene derivatives/4-nitrobromobenzene. Increas-ing the amount of monofunctional end-capping reagent inthe reaction mixture, the molecular weight of the conden-sation product decreases. This indicates that the end-cap-pers are indeed chemically attached to the prepolymerchains, which can be further proven by means of1H NMR analysis. The 1H NMR spectrum of nitrophenyl-terminated PF prepolymer 1 displays characteristic sig-nals of the protons in ortho-position relative to the 1NO2

group at d = 8.25 (doublet), well-separated from the sig-

nals of all other protons (d = 7.8–7.1). In relation to themolecular weight of prepolymers 1, integration of thissignal allows to estimate the concentration of terminalnitrophenyl units. The estimated concentrations of endgroups are (i) well below the monomer feed ratios whichmirrors different reactivities of the bi- and monofunc-tional monomers in the reductive coupling reaction, and(ii) correspond to ca. 1–1.2 nitrophenyl end groups perprepolymer 1 chain indicating that chain termination viadebromination/deboronation is competing with theattachment of the nitrophenyl end-capper. A representa-tive example: feed ratio of chain forming/chain terminat-ing monomers 10:1; M

—n = 7 700 (Dp = 19), integrated

ratio of protons in ortho-postion to the nitro group/allother protons: 1 :407.5, which leads to a value of ca. 1.07nitrophenyl groups/prepolymer 1 chain.

MALDI-TOF MS analysis of 1 revealed the presenceof H-[PF]x-R as well as Br-[PF]x-R oligomers. A quantita-tive analysis of these MS data is, however, not possible.

The subsequent reduction of the terminal nitro toamino functions was carried out with H2/Pd/C. The com-pleteness of reduction was controlled by means of1H NMR analysis. The signal at d = 8.25 (doublet) is lostcompletely, whereas new broad signals at ca. d = 7.0–6.5are formed. The molecular weight of aminophenyl termi-nated PF prepolymer 2 remains nearly unchanged (in ourexample: M

—n = 7 700; Dp = 20); no decomposition of the

prepolymer chains occurs during reduction (Scheme 2).The final reaction step then involves the oxidative cou-

pling of NH2-terminated prepolymer 2 with 2-undecylani-line (reagents: (NH4)2S2O8/H+). The “AB”-type characterof the aniline monomer guarantees the exclusive forma-tion of di- (and tri-) block PF/PANI copolymers 3 duringthis final reaction step without crosslinking, branching ormultiblock formation (Scheme 3).

The undecyl-substituted aniline monomer was chosenbecause of the improved solubility of the correspondingPANI building block.[10] In contrast to insoluble unsubsti-tuted polyaniline, poly(2-undecylaniline) homopolymeris slightly soluble in some polar organic solvents (e.g.THF), but insoluble in toluene. Therefore, homocouplingproduct poly(2-undecylaniline), which is formed asunwanted side-product, remains as a toluene-insolubleresidue after an extraction of the crude reaction mixturewith toluene. Extracted and re-precipitated PF/PANIblock copolymer 3 was characterized by means of GPC,

Scheme 1. Synthesis of nitrophenyl-terminated PF prepolymer1 (R: 2-ethylhexyl).

Scheme 2. Synthesis of aminophenyl-terminated PF prepoly-mer 2 (R: 2-ethylhexyl).

626 C. Schmitt, H.-G. Nothofer, A. Falcou, U. Scherf

MALDI-TOF MS and UV-Vis spectroscopy (Figure 1).1H NMR analysis displays, unfortunately, no well-resolved signals for the PANI building block. The13C NMR spectrum exhibits four new signals at d = 129.1,127.4, 120.6 and 99.8 in the aromatic region, belongingto the poly(2-undecylaniline) block, and five additional

signals in the aliphatic region (d = 40.5, 32.1, 30.3, 29.5,24.2). The UV-Vis spectrum indicates the presence ofpoly(2-undecylaniline) blocks, for which a red-shifted,low intensity absorption band centered at ca. 541 nm ischaracteristic (kmax (PF homopolymer) = 385 nm,[11] kmax

(poly(2-undecylaniline) homopolymer) L 550 nm[10]).The observed absorbance of the PANI block correspondsto a degree of polymerization of the poly(2-undecylani-line) blocks of ca. 6–7 (y), based on the absorbances ofhomopolymers and the molecular weights/degrees ofend-functionalization of prepolymers 1 and 2. Based onthe degree of polymerization of the PF block (x; ca. 19), amolar ratio x/y of ca. 3 results, corresponding to a weightratio of ca. 5.

Molecular weight M—

n of block copolymer 3 wasincreased compared with prepolymer 2 (in our example:M—

n (2) = 7 700; M—

n (3) = 14 400). However, the peakmolecular weight M

—p was only slightly increased (2:

18 900, 3: 22 200), in accordance with the low absolutecontent of 2-undecylaniline moieties, due to the lowdegree of polymerization of the PANI building blocks.The more pronounced increase of M

—n should be due to a

partial fractionation during the work-up of 3 (severalwashings with acetone). Indeed, acetone washings con-tained some low-molecular weight material (M

—n = 1 000–

2 000). An independent proof that the PANI blocks arereally covalently attached to the PF blocks came fromGPC analysis with simultaneous UV-Vis detection at theabsorption maxima of the PF (380 nm) as well as thePANI block (550 nm), and RI detection (control experi-ment; Table 1). Especially, identical peak molecularweights M

—p indicate that both absorbing species originate

from macromolecules with identical molecular weights.Considering the low molecular weight of poly(2-undecyl-aniline) homopolymer, which is formed as a side-product(M

—n L 1.300), these results are a strong proof of the pres-

ence of expected block copolymers 3.The thermotropic LC properties of the corresponding

poly[9,9-bis(2-ethylhexyl)fluorene] homopolymer[11] are

Scheme 3. Synthesis of poly(9,9-dialkylfluorene)/poly(2-alkylaniline) (PF/PANI) block copolymer 3 (R: 2-ethylhexyl;R9: undecyl).

Figure 1. (a) UV-Vis spectrum of block copolymer 3 and (b)UV-Vis spectra of the corresponding homopolymers for compar-ison.

Table 1. Molecular weights of PF prepolymer 2 and PF/PANIblock copolymer 3 (GPC, PS calibration) determined by meansof RI and UV-Vis detection at 380 and 550 nm, respectively.

Prepolymer 2 Block copolymer 3

RI detection UV detection(380 nm)

RI detection UV detection(380 nm)

Vis detection(550 nm)

M—

n 8 500 7 700 7 100a) 14 400 10 500M—

w 21 500 21 400 75 200 62 400 D100 000b)

M—

p 18 900 18 900 22 500 22 200 23 700

a) Two low molecular weight peaks around M—

= 1000 (impuri-ties), which are not present by means of UV-Vis detection at380 and 550 nm.

b) The low intensity of the signal does not allow an exact esti-mation of M

—w.

Conjugated Polyfluorene/Polyaniline Block Copolymers 627

preserved in PF/PANI block copolymer 3. Above ca.1508C, 3 displays a birefringent fluid phase, i.e. liquidcrystallinity (polarizing microscopy). A PF homopolymerof comparable chain length exhibits a slightly loweredtransition temperature (ca. 1358C). Phase transition canbe further characterized by DSC (endothermal signal dur-ing heating).

In summary, our novel method toward the synthesis ofPF/PANI block copolymers represents a simple andstraightforward synthetic procedure, and also enables thesynthesis of gram amounts, because separation from thePANI homopolymer (formed as a side-product in the finalreaction step) is possible by simply extracting withtoluene. Alternative methods to prepare conjugated blockcopolymers (coupling of preformed building blocks)would involve more extensive multi-step procedures.

Next step will be a detailed characterization of thesolid state morphology and the mesoscopic structure of 3by polarization microscopy, AFM and electron micro-scopy with respect to the relative and absolute lengths ofthe PF and PANI blocks, and the processing conditions(film formation, thermal treatment). First tapping modeAFM investigations of block copolymer 3 are depicted inFigure 2: (a) spin-coated film of PF/PANI block copoly-mer 3, (b) this film after thermal treatment (1 h, 1738C).Clearly, the thermal treatment leads to a nanoscopicallystructured morphology of the film indicating phaseseparation in the “meso” length regime (ca. 100 nm).

Experimental Part

General

All reactions were carried out under an argon atmosphere.Solvents were used in commercial quality (HPLC grade).The following compounds were synthesized according to theliterature: 2-undecylaniline,[10] 9,9-bis(2-ethylhexyl)-2,7-di-bromofluorene,[12] 9,9-bis(2-ethylhexyl)-2,7-bis(4,4,5,5-tetra-methyl-1,3,2-dioxaborolan-2-yl)fluorene[13] (using 2-ethyl-hexyl instead of octyl substituents). All other chemicals arecommercially available.

1H and 13C NMR data were obtained on either a BrukerAMX 250 or a Bruker AMX 500 spectrometer. UV-Vis spec-tra were recorded on a Perkin-Elmer Lambda 9 spectrometer(solutions in CH2Cl2). For gel permeation chromatographicanalysis (GPC), PS-columns (three columns, 10 mm gel,pore widths 500, 104, and 105 �) were utilized connectedwith UV-Vis detection. All GPC analyses were performedusing solutions of the polymers in THF (concentration of thepolymer: 2 g/L). Calibration was based on polystyrene stan-dards with narrow molecular weight distributions.

4-Nitrophenyl-Endcapped Poly[9,9-bis(2-ethylhexyl)fluorene] (1)

To a stirred solution of 1.48 g (3 mmol) 9,9-bis(2-ethyl-hexyl)-2,7-dibromofluorene, 1.93 g (3 mmol) 9,9-bis(2-ethylhexyl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)fluorene, and 121 mg (0.6 mmol) 4-nitrobromobenzenein 30 ml of toluene, 3 g Na2CO3 in 15 ml of H2O were addedand heated to 1208C under an argon atmosphere. Then,90 mg tetrakis(triphenylphosphino)palladium(0) and 40 mgof Aliquat 336 were added, and the mixture was refluxed forthree days. After cooling to room temperature, 50 ml ofCH2Cl2 were added, the organic layer was isolated andwashed with H2O, 2 n aqueous HCl, and again H2O. Theorganic layer was dried over MgSO4, concentrated, and thepolymer was precipitated into methanol (1 :10). Yield:1.77 g (76%); M

—n = 7 700; M

—w = 22 700; M

—w/M

—n = 2.9 (GPC).

1H NMR (250 MHz, CD2Cl2): d = 8.25, 7.89–7.25 (6 H),2.35–1.90 (4 H), 1.10–0.45 (30 H).

13C NMR (75 MHz, C2D2Cl4, 353 K): d = 151.5, 140.6,140.4, 126.4, 123.3, 120.0, 55.4, 45.1, 35.3, 34.6, 28.8, 27.6,23.0, 14.2, 10.7.

H–(C29H40)n –C6H4NO2 ((388.63)n(123.11)): Calcd. for n =19 C 89.29, H 10.11, Br 0.19; Found: C 88.4, H 10.0,Br a 0.4.

4-Aminophenyl-Endcapped Poly[9,9-bis(2-ethylhexyl)fluorene] (2)

770 mg (2 mmol) 1 were dissolved in 50 ml of dry THF, and200 mg of Pd/C were added. The mixture was stirred vigor-ously under 60 bar of hydrogen at 508C for 24 h. The solu-tion was filtered through a funnel consisting of a small layerof Celite 545, a layer of silica gel and a thin layer of sand.The organic layer was concentrated, and the polymer wasprecipitated into methanol (1 :10). Yield: 640 mg (83%); M

—

n = 7 700; M—

w = 21 400; M—

w/M—

n = 2.9 (GPC).

Figure 2. AFM of a spin-coated film of PF/PANI block copo-lymer 3 (film thickness ca. 100 nm) (a) after spin coating and(b) after thermal treatment (1 h, 173 8C).

628 C. Schmitt, H.-G. Nothofer, A. Falcou, U. Scherf

1H NMR (250 MHz, C2D2Cl4): d = 7.89–7.25 (6 H), 7.0–6.5, 2.35–1.90 (4 H), 1.10-0.45 (30 H).

13C NMR (75 MHz, C2D2Cl4, 353 K): d = 151.5, 140.6,140.4, 126.4, 123.3, 120.0, 55.4, 45.1, 35.3, 34.6, 28.8, 27.6,23.0, 14.2, 10.7.

Poly[9,9-bis(2-ethylhexyl)fluorene]-block-poly(2-undecyl-aniline) (3)

388 mg (1 mmol) 2 were dissolved in 10 ml of dry toluene,and 247 mg (1 mmol) 2-undecyaniline in 1.6 ml of THF,0.07 ml of methanesulfonic acid in 1.6 ml of H2O, and 1.14mg (NH4)2S2O8 in 3 ml of H2O were added via syringe. Themixture was stirred vigorously for 3 d at room temperatureunder an argon atmosphere. After adding 40 ml of toluene,the organic layer was isolated and washed with brine, satu-rated aqueous sodium carbonate solution, and again brine.The organic layer was then dried over MgSO4, concentrated,and the polymer was precipitated into methanol (1 :10). Thesolid was transferred into a soxhlet apparatus and first extra-ced with acetone for 12 h and then with toluene for addi-tional 12 h. The toluene extract was concentrated, and finallythe copolymer was re-precipitated into methanol (1 :10).Yield: 270 mg (59% based on M

—p); M

—n = 14 400; M

—w =

62 400; M—

w/M—

n = 4.3 (GPC).1H NMR (250 MHz, CD2Cl2): d = 7.9–7.2, 2.4–1.6, 1.3–

0.5.13C NMR (75 MHz, CD2Cl2): d = 152.0, 151.5, 140.6,

140.4, 129.1, 127.4, 126.4, 123.3, 121.9, 120.6, 120.0, 99.8,55.4, 45.1, 40.5, 35.3, 34.6, 32.1, 30.3, 29.5, 28.8, 27.6, 24.2,23.0, 14.2, 10.7.

(C29H40)x (C17H27N)y ((388.63)x(245.40)y): Calcd. for x =19, y = 6 C 88.71, H 10.35, N 0.93; Found C 86.9, H 10.4,N 0.9.

Acknowledgement: Generous support of these investigationsby Prof. Dr. Klaus Müllen, MPI-P Mainz, is gratefully acknowl-edged. One of us (C.S.) would like to thank the StiftungVolkswa-genwerk for a grant.

Received: November 9, 2000Revised: December 21, 2000

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