Synthetic Metals 156 (2006) 590–596

Synthesis of polyhedral oligomeric silsesquioxane-functionalizedpolyfluorenes: Hybrid organic–inorganic �-conjugated polymers

Jonghee Lee a, Hoon-Je Cho a, Nam Sung Cho a, Do-Hoon Hwang b, Hong-Ku Shim a,∗a Center for Advanced Functional Polymers, Department of Chemistry and School of Molecular Science (BK21), Korea Advanced Institute of Science and

Technology, Dae-jon 305-701, Republic of Koreab Department of Applied Chemistry, Kumoh National Institute of Technology, Kumi 730-701, Republic of Korea

Received 9 March 2005; received in revised form 16 February 2006; accepted 20 February 2006Available online 2 May 2006

Abstract

Polyfluorenes with appended polyhedral oligomeric silsesquioxanes (POSSs) were successfully synthesized through Ni(0)-mediated polymer-ization. The resulting copolymers exhibit good solubility in common organic solvents such as chloroform, chlorobenzene, etc. and good thermalstability. The PL quantum yields of the copolymers were drastically enhanced compared to PDHF because the POSS-functionalized PFs stronglyssb©

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1

amdapafScpgftnam

0d

uppressed intermolecular aggregation and/or thermal oxidation and crosslinking. This effective dilution effect and the higher stability of the POSShowed a deep blue emission in electroluminescent devices. LED devices with these polymers showed low turn-on voltage of near 4.0 V andrightness of 260–460 cd/m2.

2006 Elsevier B.V. All rights reserved.

eywords: Polyhedral oligomeric silsesquioxane (POSS); Yamamoto coupling; Electroluminescence

. Introduction

Since the discovery by the Cambridge group of the potentialpplications of �-conjugated polymers as active light-emittingaterials for flat panel displays [1], polymeric light-emitting

iodes (PLEDs) have attracted a great deal of attention fromcademia and industry. These materials offer advantageousroperties such as good processibility, low operating voltages,nd simple color tunability compared to organic LEDs (OLEDs)abricated with low molecular weight organic materials [2,3].table, blue-emitting conjugated polymers are the most recenthallenge in creating PLED-based materials to emit all threerimary colors (RGB: red, green, blue), and among the conju-ated polymers, polyfluorenes (PFs) are promising candidatesor this application [4,5]. PFs have many advantageous proper-ies, both physical (high photoluminescent (PL) and electrolumi-escent (EL) efficiencies) and chemical (facile functionalizationt the fluorene C-9 position provides good solubility in com-on organic solvents like chloroform, toluene, and p-xylene

compared to other conjugated polymers) [3]. However, PLEDsfabricated with PF derivatives show an emission band tailing tolonger (green) wavelengths and deterioration in both color purityand stability [6,7]. The suggested reasons for the tailing emis-sions in their EL spectrum include (1) aggregation and excimerformation [5,8] and/or (2) thermal oxidation and intermolecularcrosslinking between PF chains [9]. Many types of PF deriva-tives, such as spiro-functionalized PFs [7], PFs encapsulatedwith dendrimer moieties [8], networked PFs [5c] asymmetric9,9′-substituted PFs [6d], end-capped PFs with bulky unreactivemoieties [6c], cyclodextrin-threaded PFs [10], and PFs copoly-merized with perylene dyes [11], have been demonstrated to beeffective towards suppressing excimer formation and aggrega-tion and/or thermal oxidation and intermolecular crosslinkingbetween PF chains. However, obtaining conjugated polymersthat exhibit pure blue emission remains a goal [7,8,12]. POSS-functionalized polymers are organic–inorganic hybrid materialsthat have received considerable recent attention as modifiers fornanoparticles [13,14]. The POSS unit is a cube-shaped molecule,in which an inorganic core is surrounded by eight organic groups[13,14]. The POSS moieties in linear polymeric POSS sys-

∗ Corresponding author. Tel.: +82 42 869 2827; fax: +82 42 869 2810.E-mail address: hkshim@kaist.ac.kr (H.-K. Shim).

tems contain seven organic solubilizing groups (cyclohexyl orcyclopentyl) and one functional group on the inorganic core.

379-6779/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2006.02.011

J. Lee et al. / Synthetic Metals 156 (2006) 590–596 591

POSS units retain their nanoscale dimensions in a polymermatrix [14,15]. Heeger et al. demonstrated that thermal sta-bilities and light-emitting device performances of conjugatedpolymers might be enhanced by introduction of POSS as anend-capper [16a]. Recently, Cacialli et al. illustrated that par-tially insulated conjugated polymers with cyclodextrins shouldhave enhanced luminescence efficiencies and stabilities throughreducing interactions of the polymer chains [10,17]. In thispaper, we report a novel method for preparing POSS-substitutedPFs with enhanced optical properties. We hypothesized thatsmall amounts of nanoscale POSSs attached to the C-9 posi-tion of PFs would reduce the intermolecular �–� interactions,and thus limit the aggregation and/or thermal oxidative degra-dation and crosslinking between polymer chains [7,8,10,12]. Asexpected, the PL and EL studies of POSS-functionalized PFsrevealed strongly suppressed intermolecular aggregation and/orthermal oxidation and crosslinking [16].

2. Experimental

2.1. Instrumentation

1H NMR and 13C NMR spectra were recorded on a BrukerAVANCE 300 and 400 spectrometer, respectively, with tetram-ethylsilane as an internal reference. For the NMR measure-ments, chloroform-d (CDCl ) was used as the solvent. FT-IRsewmGatwowsas1taEcFs

2

tp(1oAB

Chemicals. Sodium hydroxide was purchased from JunseiChemical Co. and all other reagents and solvents were pur-chased commercially as analytical-grade quality, and usedwithout further purification.

2.2.1. 2-[2-(2,7-Dibromo-9-hexylfluoren-9-yl)ethoxy]perhydro-2H-pyran (2)

It was synthesized according to the procedures outlined inthe literatures [18].

2.2.2. 2-(2,7-Dibromo-9-hexylfluoren-9-yl)ethan-1-ol (3)It was synthesized according to the procedures outlined in

the literatures [18].

2.2.3. 1-[2-(2,7-Dibromo-9-hexylfluoren-9-yl)ethoxy]prop-2-ene (4)

It was synthesized according to the procedures outlined inthe literatures [18].

2.2.4. 2,7-Dibromo-9,9′-dihexylfluorene (6)It was synthesized according to the procedures outlined in

the literatures [18].

2.2.5. 2,7-Dibromo-9-hexyl-9′-POSSfluorene (5)1.23 g (2.50 mmol) of allyl-functionalized fluorene com-

pound (4) and 2.61 g (2.55 mmol) of 1-(hydridodimethylsilyl-o1tTsttAichp4δ

321

231

2

05uodata

3pectra were obtained using a Nicolet Model 800 spectrom-ter with samples prepared as KBr pellets. The number- andeight-average molecular weights of the polymers were deter-ined by gel permeation chromatography (GPC) on a WatersPC-150C instrument, using THF as eluent and polystyrene

s standard. Thermogravimetric analysis (TGA) and differen-ial scanning calorimetry (DSC) measurements of the polymersere performed under a nitrogen atmosphere at a heating ratef 10 ◦C min−1 using a Dupont 9900 analyzer. UV–vis spectraere measured on a Jasco V-530 UV–vis Spectrometer and PL

pectra of the polymers were measured at room temperature onSpex Fluorolog-3 spectrofluorometer (model FL3-11) using

pin-coated films. EL spectra were obtained with a Minolta CS-000. The voltage–luminance and voltage–current characteris-ics were recorded on a current–voltage source (Keithley 238)nd a luminance detector (Minolta LS-100). Polymer films forL measurement were prepared by spin-coating solutions of theorresponding polymers (1 wt.% in chlorobenzene (Aldrich)).ilm thickness were measured with a TENCOR alpha-step 500urface profiler.

.2. Materials

2,7-Dibromofluorene, 1-bromohexane, 2-(2-bromoethoxy)etrahydro-2H-pyran, tetrabutylammonium bromide (TBAB),latinum(0) 1,3-divinyl-1,1,3,3-tetramethyldisiloxane complexPt(dvs)) solution, 1-(hydridodimethylsilyloxy)-3,5,7,9,11,13,5-heptacyclopentylpentacyclo-[9.5.1.13,9.15,15.17,13]-octasil-xane, 2,2′-dipyridyl, 1,5-cyclooctadiene, were obtained fromldrich Chemical Co. and used without further purification.is(1,5-cyclooctadienyl)nickel(0) was purchased from STREM

xy)-3,5,7,9,11,13,15-heptacyclopentyl pentacyclo-[9.5.1.13,9.5,15. 17,13]-octasiloxane were dissolved in 20 mL of anhydrousoluene, and 0.5 mL of the toluene solution of Pt(dvs) was added.he resulting solution was stirred at 60 ◦C for 24 h, and then apoonful of activated carbon was dispersed into the reaction solu-ion at room temperature for 1 h. The mixture solution was fil-ered through a medium-size glass filter packed with Celite-545.fter the mixture had been filtered, the toluene was evaporated

n vacuo and the concentrated crude product was purified byolumn chromatography using a mixture of ethyl acetate and n-exane (1/30) as the eluent, yielding 2.88 g (yield 78.5%) of theroduct as a white solid: Anal. Calcd. for C61H98Br2O14Si9: C,9.91; H, 6.73. Found: C, 50.27; H, 6.79. 1H NMR (CDCl3,ppm): 0.07 (m, 6H), 0.39 (m, 2H), 0.59 (b, 2H), 0.76 (t,

H), 0.84–1.20 (m, 13H), 1.29–1.83 (m, 58H), 1.93 (m, 2H),.28 (t, 2H), 2.73 (t, 2H), 2.97 (t, 2H), 7.39–7.56 (m, 6H),3C NMR (CDCl3, δ ppm): −0.38, 13.84, 13.88, 22.28, 22.34,2.39, 22.48, 23.23, 23.42, 27.03, 27.05, 27.33, 27.36, 29.46,1.40, 39.44, 40.48, 53.92, 66.64, 73.52, 121.07, 121.58, 126.57,30.42, 138.87, 152.00.

.2.6. PolymerizationA mixture of 0.735 g of bis(1,5-cyclooctadienyl)nickel(0),

.417 g of 2,2′-dipyridyl, 0.2 mL of 1,5-cyclooctadiene, andmL of anhydrous DMF was maintained at 80 ◦C for 30 minnder argon atmosphere. To this solution, a total of 1.8 mmolf monomers, comprising a mixture of 2,7-dibromo-9,9′-ihexylfluorene (6) and POSSs-functionalized fluorene (5) inmolar ratio of 95:5, 90:10, and 80/20, dissolved in 15 mL of

oluene was added dropwise. The resulting solution was stirredt 80 ◦C for 3 days. Then, 0.1 g of 9-bromoanthracene (the end

592 J. Lee et al. / Synthetic Metals 156 (2006) 590–596

capper) in 5 mL of anhydrous toluene was added to the polymersolution, and the resulting solution was stirred at 80 ◦C for a fur-ther 24 h. The polymer was precipitated in 400 mL of a mixtureof HCl, acetone, and methanol (vol.% of 1:1:2). The filteredcrude polymer was extracted with chloroform from NaHCO3aq. solution, washed with distilled water, and dried over anhy-drous magnesium sulfate. The organic layer was concentratedin vacuo, dissolved in chloroform, and precipitated in methanolagain. The resulting polymer was purified by Soxhlet extractionin methanol (80 ◦C, 3 days) and dried under vacuum. Finally,the dried polymer was dissolved in chloroform, precipitated inmethanol, and dried in vacuo to give 0.61 g (yield 91%) of whitepolymer. Anal. Calcd. for PF-POSS05: C, 88.59; H, 9.59. Found:C, 88.84; H, 9.75; Anal. Calcd. for PF-POSS10: C, 86.87; H,9.49. Found: C, 86.12; H, 9.38; Anal. Calcd. for PF-POSS20:C, 83.44; H, 9.27. Found: C, 82.79; H, 9.18. 1H NMR (CDCl3,δ ppm): 0.03 (m), 0.42 (m), 0.78 (t), 1.13 (m), 1.54 (m), 1.70(m), 2.11 (m), 7.44–8.01 (m). FT-IR (KBr, cm−1): 2952, 2928,2858, 1458, 1402, 1377, 1251, 1118, 885, 813, 749, 507.

3. Results and discussion

3.1. Synthesis and characterization

The molecular structures and synthetic procedures for thePpctcCCwfNiPmw

Fig. 1. (a) 1H NMR spectrum of monomer 1 in the range −1 to 9 ppm. (b) 1HNMR spectrum of polymers.

onomer synthesis.

OSS-containing PFs are shown in Schemes 1 and 2 [18]. Com-ounds 2 and 6 were synthesized by alkylation of 1 under basiconditions, and 2 was deprotected to yield 3 under acidic condi-ions. As we previously reported, 3 is a useful and easily appli-able intermediate to prepare asymmetric fluorene derivatives.ompound 4 was prepared by alkylation of 3 with allyl bromide.ompound 5, the mono-POSS-substituted fluorene monomer,as prepared using a hydrosilation reaction between POSS-

unctionalized hydrido siloxane and 4 with a Pt catalyst. Thei(0)-mediated coupling reaction was employed to copolymer-

ze 5 and 6 (2,7-dibromo-9,9′-dihexylfluorene) to produce PF-OSSs (PF-POSS05, PF-POSS10, and PF-POSS20) [18]. Theolecular structures of monomers and polymers were confirmedith 1H NMR, 13C NMR, and FT-IR spectroscopy and elemental

Scheme 1. M

J. Lee et al. / Synthetic Metals 156 (2006) 590–596 593

Scheme 2. Polymer synthesis.

analysis (Fig. 1). The results were consistent with the moleculesshown in Scheme 1. For monomers and polymers, the conven-tional proton and carbon signals of the Si–CH3 groups adjacentto POSS units, near 0 ppm, were observed in 1H NMR spectra.

As the amount of 5 in the copolymers was increased, thesignals from these groups significantly increased. For 5 and allthe polymers, a strong Si–O stretching band at 1118 cm−1 anda weak Si–C stretching band at 1251 cm−1 were observed inthe FT-IR spectra. Grill and Neumayer reported that Si–O–Sistretching bands in cage structures like POSS are observedat 1100–1150 cm−1 [19]. All the copolymers showed verygood solubility in common organic solvents such as chlo-roform, tetrahydrofuran (THF), toluene, and p-xylene with-out any gel formation, and, notably, copolymers with higherlevels of POSS showed better solubility. This good solu-bility is probably due to the introduction of soluble POSSunits, in which each POSS unit supports seven cyclopentylgroups. The copolymers easily formed films by spin-coating.The molecular weights of the polymers were determined bygel permeation chromatography (GPC) against polystyrenestandards with THF as eluent. PF-POSS05 (Mn = 27,000;Mw = 56,000; PDI = 2.1) had a higher molecular weight thanPF-POSS10 (Mn = 24,000; Mw = 53,000; PDI = 2.2) and PF-POSS20 (Mn = 19,000; Mw = 50,000; PDI = 2.6).

The thermal properties of the polymers investigated usingdifferential scanning calorimetry (DSC; heating rate of1rs

Fig. 2. UV–vis absorption and PL spectra of the polymers as spin-coated films(in p-xylene).

be 99, 104, and 105 ◦C for PF-POSS05, PF-POSS10, and PF-POSS20, respectively. The decomposition temperatures for 5%weight loss of the polymers (Td) were above 420 ◦C for all thecopolymers.

3.2. Optical properties

The UV–vis absorption and PL spectra of the PF-POSSs asthin films on quartz plates are shown in Fig. 2. The maximumabsorption and the maximum emission of the copolymers are

TP

P ield (%) Mw (×104) PDI Tg (◦C) Tdb (◦C)

P 1 5.6 2.1 99 429P 5 5.3 2.2 104 424P 3 5.0 2.6 105 420

x:y indicates the ratio of 2,7-dibromo-9,9′-dihexylfluorene to the POSS-substitutedfl

0 ◦C min−1) and thermogravimetric analysis (TGA; heatingate of 10 ◦C min−1) are summarized in Table 1. The glass tran-ition temperatures (Tg) of the copolymers were measured to

able 1hysical and optical properties of the polymers

olymer Feed ratio (x:y) Content ratioa (x:y) Y

F-POSS05 95/5 96.3/5.7 9F-POSS10 90/10 88.1/11.9 8F-POSS20 80/20 75.8/24.2 6

a Content ratios were calculated by NMR-assignment of the polymers, anduorine [18].b Temperature resulting in 5% weight loss based on initial weight.

594 J. Lee et al. / Synthetic Metals 156 (2006) 590–596

Table 2Optical properties of the polymers

Polymer λmax (UV, nm) λmax (PL, nm) λmax (PL, nm), annealed (150 ◦C)

Solution Film Solution Film

PF-POSS05 386 390 414 422 425PF-POSS10 384 389 413 421 426PF-POSS20 385 388 414 421 424

Fig. 3. Electroluminescence spectra of the polymers.

388–390 and 421–422 nm, respectively, in the solid state. Asshown in Fig. 2, the PL spectra of the solid-state copolymers withPOSSs showed no signs of aggregation or excimer formation[18]. From these results, it is likely that nanosized POSS unitsappended on PF chains suppress intermolecular aggregation, andPFs with POSSs exhibit thermally stable blue emission. Inter-estingly, upon excitation of PF-POSS05, PF-POSS10, and PF-POSS20 at 380 nm all the copolymers showed higher quantum

yields ΦFL compared to homo poly-dihexylfluorenes (PDHF;ΦFL = 0.55) in the solid state. PF-POSS05, PF-POSS10, andPF-POSS20 had ΦFL values of 0.65, 0.73, and 0.83, respectively[8b,18]. The efficient size effect or dilution effect of nanosizedPOSSs in the copolymers seem to enhance the quantum yieldsof the copolymers without any change in polymer’s main chain[16] (Table 2).

EL devices based on from the copolymers were fabricatedin an ITO/PEDOT:PSS/polymer/Ca/Al configuration [18].PEDOT was spin-coated on an ITO substrate to a film thicknessof 30 nm and baked at 150 ◦C for 30 min. Polymer filmswere formed on the PEDOT layer to a film thickness of

Fig. 5. V–L (a) and I–V (b) curves of the EL devices.

Fig. 4. CIE (1931) coordinates (x and y) of PDHF and PF-POSS.

J. Lee et al. / Synthetic Metals 156 (2006) 590–596 595

Table 3Performances of single layer EL devices of the configuration of ITO/PEDOT:PSS/polymer (PF-POSS05 or PF-POSS10 or PF-POSS20)/Ca/Al

Polymer EL λmax (nm) CIE coordinatesa (x, y) Maximum luminance (cd/m2) Threshold voltageb (V)

PF-POSS05 426, 448 (0.20, 0.17) 263 (at 10.0 V) 3.9PF-POSS10 425, 447 (0.19, 0.16) 463 (at 10.0 V) 4.0PF-POSS20 419, 443 (0.18, 0.13) 420 (at 8.5 V) 4.1

a Obtained from EL spectra at current density of 10 mA/cm2. CIE coordinates is CIE 1931 color coordinates according to the 1931 CIE convention.b Electric field where the luminance is 1 cd/m2.

80 nm. The metal layers of Ca and Al were consecutivelydeposited on top of the polymer layer. The EL spectra ofthe copolymers are shown in Fig. 3. As the mole fractionof the POSS-substituted monomer increased, the wavelengthof the maximum EL peaks of the copolymer blue-shifted.The maximum EL peaks of PF-POSS05 appeared at 426 and448 nm, of PF-POSS10 at 425 and 447 nm, and of PF-POSS20at 419 and 443 nm. The PFs containing POSSs showed noadditional long-wavelength tails, which is otherwise observedin the EL spectrum of homo-PF. As shown in Fig. 3, the ELspectra of PF-POSS05, PF-POSS10, and PF-POSS20 showedpure blue emission. Fig. 4 shows the color (CIE) coordinatesof the polymers. As the mole fraction of the POSS-substitutedmonomer in the copolymers increased, the CIE coordinatesof the polymers moved from greenish-blue to deep blue [theCIE coordinates of PDHF is (0.25, 0.29), of PF-POSS05 (0.20,0.17), of PF-POSS10 (0.19, 0.16), and of PF-POSS20 (0.18,0.13) [8b]]. PF-POSS20 especially gives a deep blue emission(Table 3).

The current density–electric field (I–V) and luminance–electric field (L–V) characteristics of the polymers are shownin Fig. 5. The current densities of all the copolymers increasedwith forward bias. The threshold voltage was almost 4.0 V for allthe POSS-containing copolymers. Maximum luminance valuesof PF-POSS05, PF-POSS10, and PF-POSS20 were 263 cd/m2

(at 10.0 V), 463 cd/m2 (at 10.0 V), and 420 cd/m2 (at 8.5 V),r

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f

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. Conclusion

We have demonstrated the synthesis of PFs in which POSSsere introduced into the C-9 position of fluorene. The PL spectraf the polymers exhibited no features characteristic of aggre-ation, excimer formation, thermal oxidative degradation, orrosslinking between polymer chains. In addition, the PL quan-um yields of the copolymers were drastically enhanced com-ared to PDHF. The EL spectra of the polymers also showedeep blue emission in EL devices. The introduction of POSS at-9 position of PFs is an efficient and simple solution method

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