highly fluorescent mono-substituted poly(arylacetylene)s containing tolane chromophores

Highly Fluorescent Mono-Substituted Poly(arylacetylene)sContaining Tolane Chromophores

CHIEN-CHUNG HAN, ARUMUGAM BALASUBRAMANIAN

Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan, Republic of China

Received 26 April 2008; accepted 12 May 2008DOI: 10.1002/pola.22869Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A new class of highly fluorescent mono-substituted polyarylacetylenes(P1–P3) has been successfully prepared with good yields and high molecular weightsfrom a series of phenylethynylene-substituted arylacetylenes (M1–M3), using W-based catalyst. These polymers are readily soluble in common organic solvents (e.g.,THF, toluene, and chloroform) and thermally stable up to �350 8C. Both 1H NMRand IR studies confirmed that all the monomers were selectively polymerized at theterminal triple bond. The 1H NMR results showed that both P1 and P2 have essen-tially trans-polyene backbone structures, while P3 contains significant amounts ofboth cis- and trans-structures. Coincidently, the photoluminescence (PL) intensity ofP3 is also much lower than both P1 and P2. Nevertheless, the PL intensity of P1–P3 are all much higher than that of PPA, suggesting that the introduction of phenyl-ethynylene group to the phenyl rings of PPAs have made the in general nonemissivePPAs to become highly photoluminescent. Most interestingly, although P1 did notcontain any substituent group on its tolane pendant group, it is already highly solu-ble in common organic solvent and emitting blue fluorescence (416 nm) with a rea-sonably good quantum yield (0.36). VVC 2008 Wiley Periodicals, Inc. J Polym Sci Part A:

Polym Chem 46: 5483–5498, 2008

Keywords: blue emitter; luminescence; polyacetylenes; structure-property rela-tions; synthesis

INTRODUCTION

In the past few decades conjugated polymershave been investigated intensively due to theirpromising electrical and optical properties, mak-ing them useful materials for electronic andphotonic devices such as field-effect transistorsand light emitting diodes (LEDs).1–4 Polyacety-

lene (PA), as the simplest conjugated polymer,exhibited high conductivity when doped with anappropriate dopant, for example, halogens.5

However, its application in electronic deviceswas not feasible due to its lack of solution proc-essability and air stability.6 Through the intro-duction of various groups, such as alkyl and arylgroups, to the polymer chain, the solubility andstability of polyacetylenes (PAs) could be muchimproved.6

Among the various PAs, poly(phenylacety-lene)s (PPAs) have been found to possess photo-conductivity, third-order nonlinear optical prop-erties, light emissivity, helical chirality, liquidcrystallinity, and self-organizability.7,8 Attractedby these properties and the corresponding

This article contains Supplementary Material availablevia the Internet at http://www.interscience.wiley.com/jpages/0887-624X/suppmat

Correspondence to: C. C. Han (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 5483–5498 (2008)VVC 2008 Wiley Periodicals, Inc.

5483

potential applications, many research groupsare working on the synthesis of novel substi-tuted PPAs. Regarding the LED applications, ithas been reported that unsubstituted PA emitsextremely weak photoluminescence (PL) only inthe infrared (IR) region instead of the visibleregion,9 while PPAs showed weak emission inthe visible region, as contributed by their phenylpendant groups. The weak emission nature ofPPAs is believed to arise from the fact that thepolyene backbones of PPAs are actually thequenching species, which absorb the light emit-ted from the phenyl pendant groups but re-emit-ted poorly only in the near-IR region.10 Thereplacement of hydrogen atom in the polyenebackbones of PPAs with a bulky substituent(i.e., forming disubstituted PAs) would help toreduce the coplanarity of the polyene backbones,thereby widening their bandgaps and shiftingtheir backbone emission from IR to visibleregion,10 and thus rendering the disubstitutedPAs, for example, poly(alkylphenylacetylene)sand poly(diarylacetylene)s with high fluores-cence in the visible region.11 Regarding to themono-substituted poly(alkylacetylene)s, most ofthem showed extremely weak PL properties,except for Tang’s novel poly(alkylacetylene)sthat have a biphenylyl pendant group beingattached to the polyene backbone through alkyl-carbonyloxy (��(CH2)mCOO-biphenyl) or alky-loxy carbonyl (��(CH2)mOCO-biphenyl) spacergroups. These polymers can be prepared fromtheir corresponding monomers by using W- orMo-based catalysts, and could display, in somecases, fairly high luminescence intensity.12 How-ever, when the same biphenylyl pendant groupwas attached to the more rigid PPA backbone atthe para-position of the phenyl pendant groupthrough the similar spacer groups, the corre-sponding monomers could only be polymerizedby Rh-based catalyst (but failed with W- andMo-based catalysts); the obtained polymers werefound to have unusually high cis-polyene con-tent (�90%), and somehow emitted extremelyweak PL signals.8(c),12 Although the underlyingchemical reason was not provided to account forsuch different behaviors for the same biphenylylchromophore between the PPA and poly(alkyla-cetylene) systems,11,8(c),12 it is conceivable thatthe biphenylyl chromophore situated in themore rigid PPA material matrix might haveencountered a higher surrounding steric stress,which may in turn render its excited biphenylylchromophore with a higher susceptibility in los-

ing its excitation energy through the possibletorsional rotation motion between its two phenylrings. Interestingly, Masuda et al. have foundthat if the same biphenyl moiety was built aspart of a rigid 3,6-di-tert-butylcarbazolyl pend-ant group and being attached to the para posi-tion of the phenyl ring of PPA through the N-site, the resultant polymer (i.e., a 3,6-di-tert-butylcarbazolyl-substituted PPA) can on theother hand be highly fluorescent.13 The aboveresults together imply that the selection of anappropriate chromophore that would havehigher stability (in maintaining its originalphysical form and/or chemical interaction) in itsexcited state may be the key to render a highlyrigid material system (like PPA) with high fluo-rescence.

To validate the above hypothesis, we wereinterested, in this study, in identifying a newclass of workable pendant group that is reason-ably rigid but with much less bulkiness thanMasuda’s 3,6-di-tert-butylcarbazolyl pendantgroup,13 so that the incorporated pendant groupwould cast a lesser extent of adverse effects onthe conductivity and electron mobility of the re-sultant polymers. From the literature results,we have come to know that conjugated polymerscontaining aryleneethynylene backbone struc-tures can emit strong fluorescence with highquantum yields.14 In addition to this, Pizzofer-rato et al. have reported that, owing to thecylindrically symmetrical distribution nature ofthe p-electron cloud of poly(aryleneethynylene),its conjugation interaction would be less sensi-tive to the changes in the torsional anglebetween the neighboring repeat units. Such con-formational change would be difficult to avoid, ifthe chromophore were situated within a rigidand highly crowded polymer matrix.15,16 Hence,it is expected that the incorporation of highlyfluorescent aryleneethynylene side chain moietyto the highly rigid PPA backbone might survivebetter than the biphenylyl pendant groupand thus render the weakly emissive mono-sub-stituted PPAs with an unexpected good PLproperty.

Furthermore, compared with the unsubsti-tuted PPA, the ortho-substituted PPAs in gen-eral showed enhanced solubility, better film-form-ing property,17 higher molecular weight, and nar-rower molecular weight distribution.18–22 Thus,we are also interested to examine the effect ofthe alkyl substituents present at the ortho- andmeta-positions to the polyene backbone of these

5484 HAN AND BALASUBRAMANIAN

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

novel substituted PPAs. In this article, we reportthe preparations and characterizations of poly-mers P1–P3 (Scheme 1) and compare their prop-erties with that of PPA prepared under the sameconditions.

EXPERIMENTAL

Characterization Methods

1H and 13C NMR spectra were recorded usingVarian Unity 400 MHz, Varian Unity Inova 500MHz, and Bruker Avance DMX 600 MHz NMRspectrometers. IR spectra were measured usingPerkin-Elmer 2000 Fourier Transform infrared(FTIR) spectrometer, based on pressed KBr pel-lets. Thermogravimetric analyses (TGA) wereperformed under a nitrogen atmosphere (at aflow rate of 100 mL/min) with a heating rate of10 8C/min using Sieko TG/DTA-320 instrument.Differential scanning calorimetry (DSC) analy-ses were carried out on a Sieko DSC-220Cinstrument under a nitrogen atmosphere (at aflow rate of 40 mL/min) with a scanning rate of10 8C/min (for both heating and cooling cycles).UV-visible spectra for the polymer solutions (inTHF) were recorded with Hitachi U-3501 spec-trophotometer; PL spectra for the same polymersolutions were measured with Hitachi F-4500fluorescence spectrophotometer using Xe-lamp.The molecular weight analysis for the polymerswas performed on a gel permeation chromatog-raphy (GPC) system equipped with four Watersstyragel columns (installed in a 40 8C oven) anda UV detector (set at the kmax of the analyzedpolymers) using THF as the mobile phase (at1 mL/min). The molecular weight was calibratedagainst polystyrene standards. The relative PL

quantum yields of the polymers were measuredin THF solutions, the values were calculated bycomparing with the reference standard Couma-rin-102 in ethanol (F ¼ 0.93).23

Chemicals

4-Bromoiodobenzene24 (1) and 2,5-dimethyl-1-bromobenzene25 (4) were synthesized accordingto the previously reported procedures. Phenyla-cetylene, 2-butyn-3-methyl-2-ol, 4-bromoaniline,1,4-diisopropylbenzene, p-xylene, tungsten(VI)chloride, triethylsilane, triphenyl phosphine, di-chlorobis(triphenylphosphine)palladium(II), cop-per(I) iodide, n-hexyl bromide, coumarin-102,carbon tetrachloride, triethylamine, N,N-dime-thylformamide (DMF), benzene, and methanolwere purchased from Aldrich and Acros; theywere used as received. THF and toluene weredried according to standard procedures beforethe use.

Monomer Synthesis

General Procedure for the Coupling Reactionbetween Aryl Iodide and Alkyne26

To a two neck round bottom flask was addedaryl iodide (1.0 equiv), PdCl2(PPh3)2 (0.5 mol %),and CuI (1.0 mol %) under nitrogen atmosphere.The system was air exchanged with nitrogen forthree times. Triethylamine and THF (1:1 ratio)were then added as the solvents to the abovemixture and again degassed with nitrogen fol-lowed by the addition of acetylene compound(1.1–1.5 equiv) via syringe. The contents werestirred at rt (�12–24 h) until the disappearanceof the aryl iodide as monitored by TLC. Thereaction mixture was filtered through shortsilica gel column. The filtrate was poured into alarge amount of ethyl acetate (�15 times theamount of aryl iodide used) and then washedwith water three times. The ethyl acetate layerwas dried over anhydrous MgSO4 and concen-trated in a rotovapor to obtain the crude prod-uct, which was then purified by column chroma-tography on silica gel, using hexane/ethyl ace-tate (v:v ¼ 8:2) as eluant.

General Procedure for the Coupling Reactionbetween Aryl Bromide and Alkyne27

A three-neck round bottom flask equipped witha condenser was charged with aryl bromide (1.0

Scheme 1. Chemical structures of PPA and poly-mers P1-P3.

FLUORESCENT POLY(ARYLACETYLENE)S 5485


equiv), PdCl2(PPh3)2 (1.0 mol %), CuI (1.0 mol %),and PPh3 (1.0 mol %). The system was airexchanged with nitrogen thoroughly for threetimes. To the above mixture, triethylamine andtoluene (0.5:1 ratio) were added as solvents andagain degassed with nitrogen. Finally, acetylenecompound (1.4–1.5 equiv) was added via syringeunder nitrogen atmosphere and the reactionmixture was stirred at 80 8C (�24 h) till thecomplete consumption of the aryl bromide(monitored by TLC). Then, the reaction mixturewas cooled down to rt and poured into ethyl ace-tate (�15 times the amount of aryl bromideused). The resultant solution was washed withwater three times. The ethyl acetate layer wasdried over MgSO4 and concentrated in a rotova-por, then the residue was purified by columnchromatography on silica gel using hexane/ethylacetate (v:v ¼ 8:2) as eluant to afford thedesired product.

4-(4-Bromophenyl)-2-methyl-3-butyn-2-ol (2)

Compound 2 was prepared by the general proce-dure A described above from 1-bromo-4-iodoben-zene (1) (6.60 g, 23.3 mmol) and 2-methyl-3-butyn-2-ol (2.36 g, 28.0 mmol) in the cosolventmedium of triethylamine (65 mL) and THF (65mL), using CuI (0.045 g, 0.23 mmol) andPdCl2(PPh3)2 (0.082 g, 0.117 mmol) as the cata-lyst. The reaction was proceeded at rt for 12 h.After purification by column chromatographyusing hexane/ethyl acetate (v:v ¼ 8:2) as eluant,compound 2 (5.5 g, 97%) was obtained as paleyellow solid. 1H NMR (500 MHz, CDCl3, TMS asreference): d 7.43 (d, J ¼ 8.25 Hz, 2H), 7.27 (d,J ¼ 8.25 Hz, 2H), 2.09 (s, broad, OH), 1.61 (s,6H). 13C NMR (125 MHz, CDCl3, using CDCl3 d¼ 77 ppm as reference): d 133.04 (CH), 131.47(CH), 122.43 (C), 121.64 (C), 94.85 (C), 81.07(C), 65.58 (C), 31.35 (CH3). MS (EI) m/z (rela-tive intensity): 238 (Mþ, 11), 240 (10), 225 (62),223 (58), 159 (5). Anal. Calcd for C11H11BrO: C,55.25; H, 4.64. Found: C, 55.16; H, 4.93.

2-Methyl-4-[4-(2-phenyl-1-ethynyl)phenyl]-3-butyn-2-ol (3)

Following by the above general procedure B,compound 3 was prepared from compound 2(15.00 g, 62.73 mmol) and phenylacetylene(9.61 g, 94.10 mmol) in the cosolvent medium oftriethylamine (75 mL) and toluene (150 mL),using CuI (0.120 g, 0.627 mmol), PdCl2(PPh3)2

(0.440 g, 0.627 mmol), and PPh3 (0.165 g,0.627 mmol) as the catalyst. The reaction wasproceeded at 80 8C for 24 h. Chromatographicpurification of the crude product on silica gelwith hexane/ethyl acetate (v:v ¼ 8:2) as eluantafforded 3 (14 g, 85%) as pale yellow solid. 1HNMR (500 MHz, CDCl3, TMS as reference): d7.53–7.52 (m, 2H), 7.47 (d, J ¼ 8.5 Hz, 2H), 7.39(d, J ¼ 8.5 Hz, 2H), 7.36–7.34 (m, 3H), 2.07 (s,broad, OH), 1.63 (s, 6H). 13C NMR (125 MHz,CDCl3, using CDCl3 d ¼ 77 ppm as reference): d131.59 (CH), 131.56 (CH), 131.41 (CH), 128.46(CH), 128.37 (CH), 123.12(C), 122.95 (C), 122.50(C), 95.49 (C), 91.09 (C), 88.93 (C), 81.83 (C),65.64 (C), 31.41 (CH3). MS (EI) m/z (relative in-tensity): 260 (Mþ, 92), 245 (100), 242 (6). Anal.Calcd for C19H16O: C, 87.66; H, 6.19. Found: C,87.13; H, 6.26.

1-Ethynyl-4-(2-phenyl-1-ethynyl)benzene (M1)

To a stirred solution of compound 3 (13.0 g,49.94 mmol) in benzene (260 mL) was added so-dium hydroxide (19.97 g, 499.4 mmol) and themixture was refluxed for 24 h.27 After completedeprotection (monitored by TLC), the reactionmixture was cooled down to rt and poured into300 mL of water. The benzene layer was sepa-rated, dried over anhydrous MgSO4, and concen-trated in a rotovapor to get the crude product,which was then purified by column chromatog-raphy on silica gel using hexane as eluant toafford M1 (6 g, 60%) as a white solid. 1H NMR(500 MHz, CDCl3, TMS as reference): d 7.53–7.52 (m, 2H), 7.48 (d, J ¼ 8.5 Hz, 2H), 7.46 (d, J¼ 8.5 Hz, 2H), 7.35–7.33 (m, 3H), 3.17 (s, 1H).13C NMR (125 MHz, CDCl3, using CDCl3 d ¼ 77ppm as reference): d 132.05 (CH), 131.62 (CH),131.45 (CH), 128.52 (CH), 128.39 (CH), 123.76(C), 122.90 (C), 121.83 (C), 91.35 (C), 88.81 (C),83.26 (C), 78.87 (CH). MS (EI) m/z (relative in-tensity): 202 (Mþ, 100), 177 (1). Anal. Calcd forC16H10: C, 95.02; H, 4.98. Found: C, 95.08; H,5.38.

1-Bromo-4-iodo-2,5-dimethylbenzene (5)

Compound 5 was synthesized by adopting thepreviously reported literature procedure.28 1-Bromo-2,5-dimethylbenzene (4) (26.61 g, 143.83mmol) was first dissolved in a cosolvent mediumof carbon tetrachloride (27 mL) and acetic acid(54 mL). To this solution, 30% aqueous sulfuricacid (8 mL), iodine (36.53 g, 143.83 mmol), and



iodic acid (12.65 g, 71.92 mmol) were sequen-tially added. The whole contents were stirred at60 8C for 6 h. After cooling down to rt, the reac-tion mixture was poured into a mixture of diethylether (300 mL) and water (200 mL). The etherlayer was separated and sequentially washedwith saturated Na2S2O3 (2 3 100 mL), 10% aque-ous NaOH (200 mL), and water (2 3 200 mL),then dried over MgSO4 and concentrated in arotovapor under reduced pressure. The residuewas recrystallized in methanol to get the desiredproduct 5 (36 g, 78%) as a white solid. 1H NMR(400 MHz, CDCl3, TMS as reference): d 7.64 (s,1H), 7.36 (s, 1H), 2.34 (s, 3H), 2.29 (s, 3H). 13CNMR (100 MHz, CDCl3, using CDCl3 d ¼ 77 ppmas reference): d 140.56 (C), 140.35 (CH), 137.07(C), 132.87 (CH), 124.75 (C), 99.06 (C), 27.15(CH3), 21.80 (CH3). MS (EI) m/z (relative inten-sity): 310 (Mþ, 3),312 (2), 231 (21), 185 (43), 183(27), 127 (6). Anal. Calcd for C8H8BrI: C, 30.90;H, 2.59. Found: C, 31.09; H, 2.80.

1-Bromo-2,5-dimethyl-4-(2-phenyl-1-ethynyl)benzene (6)

Compound 6 was synthesized according to thegeneral procedure A mentioned above from 1-bromo-4-iodo-2,5-dimethylbenzene (5) (5.0 g,15.67 mmol) and phenylacetylene (1.76 g, 17.24mmol) in the co-solvent medium of triethylamine(100 mL) and THF (100 mL), using CuI (0.030 g,0.156 mmol) and PdCl2(PPh3)2 (0.055 g, 0.078mmol) as the catalyst. The reaction was pro-ceeded at rt for 12 h. Column chromatographicpurification of the crude product on silica gelusing hexane as eluant afforded compound 6(4.30 g, 96%) as a light brown solid. 1H NMR(600 MHz, CDCl3, TMS as reference): d 7.52–7.50 (m, 2H), 7.39 (s, 1H), 7.34-7.32 (m, 4H),2.43 (s, 3H), 2.33 (s, 3H). 13C NMR (150 MHz,CDCl3, using CDCl3 d ¼ 77 ppm as reference): d139.14 (C), 134.97 (C), 133.53 (CH), 133.02 (CH),131.43 (CH), 128.34 (CH), 128.28 (CH), 124.79(C), 123.22 (C), 122.10 (C), 93.78 (C), 87.51 (C),22.19 (CH3), 19.92 (CH3). MS (EI) m/z (relativeintensity): 284 (Mþ, 24), 286 (18), 204 (55), 202(100), 201 (100). Anal. Calcd for C16H13Br: C,67.39; H, 4.59. Found: C, 68.50; H, 4.89.

4-[2,5-Dimethyl-4-(2-phenyl-1-ethynyl)phenyl]-2-methyl-3-butyn-2-ol (7)

Compound 7 was synthesized according to theabove general procedure B from 6 (2.00 g, 9.88

mmol) and 2-butyn-3-methyl-2-ol (1.24 g, 14.83mmol) in the cosolvent medium of triethylamine(10 mL) and toluene (20 mL), using CuI (0.0188 g,0.0988 mmol), PdCl2(PPh3)2 (0.0694 g, 0.0988mmol), and PPh3 (0.0258 g, 0.0988 mmol) as thecatalyst. The reaction was proceeded at 80 8Cfor 24 h. Column chromatographic purificationof the crude product on silica gel using hexane/ethyl acetate (v:v ¼ 8:2) as eluant yielded com-pound 7 (1.25 g, 62%) as a light brown solid. 1HNMR (600 MHz, CDCl3, TMS as reference): d7.53–7.52 (m, 2H), 7.36–7.33 (m, 4H), 7.26 (s,1H), 2.43 (s, 3H), 2.36 (s, 3H), 1.64 (s, 6H). 13CNMR (150MHz, CDCl3, using CDCl3 d ¼ 77 ppmas reference): d 137.20 (C), 137.17 (C), 132.63(CH), 132.51 (CH), 131.48 (CH), 128.35 (CH),128.28 (CH), 123.30 (C), 122.89 (C), 122.29 (C),98.90 (C) 94.36 (C), 88.13 (C), 80.92 (C), 65.76(C), 31.54 (CH3), 19.97 (CH3), 19.88 (CH3). MS(EI) m/z (relative intensity): 288 (Mþ, 66), 273(100), 270 (9). Anal. Calcd for C21H20O: C, 87.46;H, 6.99. Found: C, 85.14; H, 7.12.

1-Ethynyl-2,5-dimethyl-4-(2-phenyl-1-ethynyl)benzene (M2)

Monomer M2 was prepared via deacetonation bytreating compound 7 (1.20 g, 4.16 mmol) withexcess amount of sodium hydroxide (1.66 mg,41.6 mmol) in benzene (100 mL) at reflux tem-perature for 24 h.27 After complete deprotection(as monitored by TLC), the mixture was cooleddown to rt and poured into water (100 mL). Thebenzene layer was separated, dried over anhy-drous MgSO4 and concentrated in a rotovaporunder reduced pressure. The crude product thusobtained was purified by column chromatographyusing hexane as eluant to yield M2 (0.50 g, 52%)as a white solid. 1H NMR (600 MHz, CDCl3,TMS as reference): d 7.54–7.52 (m, 2H), 7.36–7.33 (m, 5H), 3.33 (s, 1H), 2.44 (s, 3H), 2.40 (s,3H). 13C NMR (150 MHz, CDCl3, using CDCl3 d¼ 77 ppm as reference): d 137.84 (C), 137.22 (C),133.28 (CH), 132.58 (CH), 131.52 (CH), 128.38(CH), 128.36 (CH), 123.46 (C), 123.26 (C), 121.73(C), 94.60 (C), 88.02 (C), 82.39 (C), 81.99 (CH),20.01 (CH3), 19.90 (CH3). MS (EI) m/z (relativeintensity): 230 (Mþ, 100), 215 (35), 200 (5).

1,4-Diiodo-2,5-diisopropylbenzene (8)

Compound 8 was synthesized by adopting thepreviously reported literature procedure28 fromthe diiodination of 1,4-diisopropyl benzene(5.0 g, 30.81 mmol) with 30% aqueous sulfuric



acid (2 mL), iodine (10.17 g, 40.05 mmol), andiodic acid (3.25 g, 18.49 mmol) in the cosolventmedium of carbon tetrachloride (5 mL) and aceticacid (15 mL). The reaction mixture was stirred at60 8C for 6 h. After cooling down to rt, the reac-tion mixture was poured into a mixture of diethylether (200 mL) and water (100 mL). The etherlayer was separated and washed sequentiallywith saturated Na2S2O3 (2 3 100 mL), 10% aque-ous NaOH (100 mL), and water (2 3 200 mL),then dried over MgSO4 and concentrated in arotovapor under reduced pressure to get thecrude product. Upon recrystallization of thecrude product in methanol, compound 8 (10.32 g,81%) was obtained as a white solid. 1H NMR(500 MHz, CDCl3, TMS as reference): d 7.60 (s,2H), 3.05 (septet, J ¼ 7.25 Hz, 2H), 1.20 (d, J ¼7.25 Hz, 12H). 13C NMR (125 MHz, CDCl3, usingCDCl3 d ¼ 77 ppm as reference): d 149.90 (C),136.59 (CH), 101.42 (C), 37.48 (CH), 22.97 (CH3).MS (EI) m/z (relative intensity): 414 (Mþ, 100),399 (90), 287 (6). Anal. Calcd for C12H16I2: C,34.81; H, 3.89. Found: C, 34.76; H, 4.17.

4-[4-(3-Hydroxyl-3-methyl-1-butynyl)-2,5-diisopropylphenyl]-2-methyl-3-butyn-2-ol (9)

Compound 9 was prepared according to theabove general procedure A from 1,4-diiodo-2,5-diisopropylbenzene (8) (9.70 g, 23.41 mmol) and2-butyn-3-methyl-2-ol (4.93 g, 58.61 mmol) in thecosolvent medium of triethylamine (100 mL) andTHF (100 mL), using CuI (0.0446 g, 0.234 mmol)and PdCl2(PPh3)2 (0.0821 g, 0.117 mmol) as thecatalyst. The reaction was proceeded at rt for24 h. Upon column chromatographic purificationof the crude product on silica gel using hexane/ethyl acetate (v:v ¼ 8:2) as eluant, compound 9(7.3 g, 96%) was obtained as a pale yellow solid.1H NMR (500 MHz, CDCl3, TMS as reference): d7.26 (s, 2H), 3.30 (septet, J ¼ 7.25 Hz, 2H), 2.02(s, broad, OH), 1.64 (s, 12H), 1.24 (d, J ¼ 7.25Hz, 12H). 13C NMR (125 MHz, CDCl3, usingCDCl3 d ¼ 77 ppm as reference): d 147.45 (C),128.85 (CH), 121.63 (C), 98.34 (C), 80.96 (C),65.78 (C), 31.45 (CH3), 31.04 (CH), 22.81 (CH3).MS (EI) m/z (relative intensity): 326 (Mþ, 57),311 (100), 283 (35). Anal. Calcd for C22H30O2: C,80.94; H, 9.26. Found: C, 79.78; H, 9.31.

4-(4-Ethynyl-2,5-diisopropylphenyl)-2-methyl-3-butyn-2-ol (10)

Compound 10 was prepared from compound 9(51.6 g, 158 mmol) by deacetonation with one

equiv amount of KOH (8.87 g, 158 mmol) in tolu-ene (800 mL).29 The reaction was proceeded atreflux temperature for 3 h. After cooling down tort, the reaction mixture was diluted with hexane(300 mL). The hexane layer was separated andwashed with water (2 3 200 mL), dried over an-hydrous MgSO4, and concentrated in a rotovaporunder reduced pressure. The residue obtainedwas purified by column chromatography usinghexane as eluant to get the desired product 10(23.3 g, 55%) as a purple solid. 1H NMR (600MHz, CDCl3, TMS as reference): d 7.34 (s, 1H),7.28 (s, 1H), 3.40 (septet, J ¼ 6.93 Hz, 1H), 3.309(s, 1H), 3.305 (septet, J ¼ 6.93 Hz, 1H), 1.65 (s,6H), 1.24 (d, J ¼ 6.93 Hz, 12H). 13C NMR (150MHz, CDCl3, using CDCl3 d ¼ 77 ppm as refer-ence): d 148.13 (C), 147.47 (C), 129.53 (CH),128.90 (CH), 122.20 (C), 121.01 (C), 98.54 (C),82.43 (C), 81.54 (CH), 80.84 (C), 65.79 (C), 31.44(CH3), 31.04 (CH), 30.94 (CH), 23.01 (CH3), 22.80(CH3). MS (EI) m/z (relative intensity): 268 (Mþ,76), 253 (100). Anal. Calcd for C19H24O: C, 85.03;H, 9.01. Found: C, 84.58; H, 9.10.

4-[2,5-Diisopropyl-4-(2-phenyl-1-ethynyl)phenyl]-2-methyl-3-butyn-2-ol (11)

Compound 11 was prepared according to thegeneral procedure A described above from 10(10.0 g, 37.4 mmol) and iodobenzene (7.62 g,37.4 mmol) in the cosolvent medium of triethyl-amine (100 mL) and THF (100 mL), using CuI(0.071 g, 0.373 mmol) and PdCl2(PPh3)2(0.131 g, 0.187 mmol) as the catalyst. The reac-tion was proceeded at rt for 24 h. The crudecompound was purified by column chromatogra-phy using hexane/ethyl acetate (v:v ¼ 8:2) aseluant to get the desired product 11 (11.3 g,87%) as a pale yellow solid. 1H NMR (600 MHz,CDCl3, TMS as reference): d 7.55–7.53 (m, 2H),7.39 (s, 1H), 7.36–7.34 (m, 3H), 7.31 (s, 1H),3.46 (septet, J ¼ 6.9 Hz, 1H), 3.33 (septet, J ¼6.9 Hz, 1H), 1.66 (s, 6H), 1.29 (d, J ¼ 6.9 Hz,6H), 1.27 (d, J ¼ 6.9 Hz, 6H). 13C NMR (150MHz, CDCl3, using CDCl3 d ¼ 77 ppm as refer-ence): d 147.50 (C), 131.45 (CH), 128.94 (CH),128.80 (CH), 128.36 (CH), 128.25 (CH), 123.38(C), 122.19 (C), 121.62 (C), 98.43 (C), 93.94 (C),88.28 (C), 81.05 (C), 65.80 (C), 31.46 (CH3),31.20 (CH), 31.10 (CH), 22.98 (CH3), 22.85(CH3). MS (EI) m/z (relative intensity): 344(Mþ, 100), 329 (76), 301 (33). Anal. Calcd forC25H28O: C, 87.16; H, 8.19. Found: C, 86.30; H,8.49.



1-Ethynyl-2,5-diisopropyl-4-(2-phenyl-1-ethynyl)-benzene (M3)

Monomer M3 was prepared from compound 11(11.3 g, 32.8 mmol) via deacetonation using oneequiv amount of KOH (1.84 g, 32.8 mmol) in tol-uene (300 mL).29 The reaction was proceeded atreflux temperature for 12 h. After cooling downto rt, the reaction mixture was diluted with hex-ane (300 mL). The organic layer was separatedand washed with water (2 3 200 mL), driedover anhydrous MgSO4, and concentrated in arotovapor under reduced pressure. The residuewas purified by column chromatography usinghexane as eluant to get the desired product M3(6.8 g, 72% yield) as a brown oil. 1H NMR (600MHz, CDCl3, TMS as reference): d 7.55–7.53 (m,2H), 7.41 (s, 1H), 7.39 (s, 1H), 7.36–7.34 (m,3H), 3.47 (septet, J ¼ 6.9 Hz, 1H), 3.43 (septet,J ¼ 6.9 Hz, 1H), 3.32 (s, 1H), 1.30 (d, J ¼ 6.9Hz, 6H), 1.27 (d, J ¼ 6.9 Hz, 6H). 13C NMR (150MHz, CDCl3, using CDCl3 d ¼ 77 ppm as refer-ence): d 148.19 (C), 147.51 (C), 131.47 (CH),129.64 (CH), 128.86 (CH), 128.37 (CH), 128.31(CH), 123.33 (C), 122.78 (C), 121.02 (C), 94.17(C), 88.15 (C), 82.53 (C), 81.65 (CH), 31.20 (CH),30.99 (CH), 23.22 (CH3), 23.16 (CH3), 23.03(CH3), 22.95 (CH3). MS (EI) m/z (relative inten-sity): 286 (Mþ, 100), 271 (21), 243 (83). Anal.Calcd for C22H22: C, 92.26; H, 7.74. Found: C,91.98; H, 7.78.

Polymerization of Monomers

All polymerization reactions and their workupprocedures were carried out under nitrogenexcept for the purification of the polymers,which was done in an open atmosphere. Thetypical experimental procedures for the polymer-ization of monomers M1–M3 are illustratedbelow.

Polymerization of M1

Monomer M1 (0.50 g, 2.48 mmol) was dissolvedin dry toluene (3.0 mL) inside the glovebox in aserum bottle. The catalyst solution was preparedin another bottle by adding Et3SiH (0.0058 g,0.05 mmol) in toluene (1.0 mL) to a solution ofWCl6 (0.02 g, 0.05 mmol) in toluene (1.0 mL).The catalyst solution was aged at rt for 20 min,to which the monomer solution was added usinga syringe and this mixture was stirred at rt for2 h. The polymerization was then terminated by

first diluting with toluene (5 mL), followed bydropwise addition into 300 mL of ammoniacalmethanol (methanol saturated with ammoniagas at 0 8C) to get a brown color precipitate. Theprecipitate was allowed to stand in the ammoniasolution for 2 h and then collected by centrifuga-tion and washed with copious amount of metha-nol to remove ammonia. After drying and in avacuum oven for at least 24 h till reaching aconstant weight, a brown powdery polymer P1(0.33 g, 66% yield) was obtained. IR (KBr), m(cm�1): 3054 (w), 3032 (w), 2922 (w), 2214 (w).UV (THF, Conc. 4E-5M): kmax 302 nm (0.742).Fluorescence (THF, 4E-5M): kmax 416 nm (Inten-sity ¼ 36) using 302 nm as the excitation wave-length.


Following the similar procedure used for thepolymerization of monomer M1, polymer P2(0.34 g, 68% yield) was prepared as a darkbrown color powder from monomer M2 (0.5 g,2.17 mmol), using WCl6 (0.0172 g, 0.043 mmol)and Et3SiH (0.0051 g, 0.043 mmol) as the cata-lyst in dry toluene (4.4 mL). IR (KBr), m (cm�1):3081 (w), 3044 (w), 2958 (s), 2927 (s), 2871 (s),2858 (s), 2209 (w). UV (THF, Conc. 4E-5M): kmax

291 nm (0.867) and 510 nm (0.164). Fluores-cence (THF, 4E-5M): kmax 355 nm (Intensity ¼43) using 291 nm as the excitation wavelength.


Following the similar procedure used for thepolymerization of monomer M1, polymer P3(0.20 g, 72% yield) was prepared as a darkbrown color powder from monomer M3 (0.275 g,1.01 mmol), using WCl6 (0.008 g, 0.020 mmol)and Et3SiH (0.0024 g, 0.020 mmol) as the cata-lyst in dry toluene (2.2 mL). IR (KBr), m (cm�1):3056 (w), 3023 (w), 2957 (s), 2929 (m) 2869 (m),2211 (w). UV (THF, Conc. 4E-5M): kmax 292 nm(0.830) and 544 nm (0.280). Fluorescence (THF,4E-5M): kmax 354 nm (Intensity ¼ 10) using 292nm as the excitation wavelength.

Polymerization of Phenylacetylene

Following the similar procedure used for the po-lymerization of monomer M1, polymer PPA(2.50 g, 83% yield) was prepared as a darkbrown powder from monomer phenylacetylene(3.00 g, 29.4 mmol), using WCl6 (0.23 g, 0.59



mmol) and Et3SiH (0.07 g, 0.59 mmol) as thecatalyst in dry toluene (60 mL). UV (THF, Conc.4E-5M): kmax 254 nm (0.207). Fluorescence(THF, 4E-5M): kmax 394 nm (Intensity ¼ 1)using 254 nm as the excitation wavelength.

RESULTS AND DISCUSSION

Monomer Synthesis

The synthetic schemes for monomers M1 andM3 are summarized in Schemes 2 and 4, respec-tively. Monomer M1 was prepared starting from1-bromo-4-iodobenzene (1) (Scheme 2) via theSonogashira coupling reaction to sequentiallyintroduce the 2-butyn-3-methyl-2-ol group (at rtto afford compound 2)26 and the phenylacetylenegroup (at 80 8C to afford compound 3),27 follow-ing by deacetonation with NaOH.27

Monomers M2 and M3 were modified fromM1 by introducing methyl and isopropyl groups,respectively, at both of the ortho- and meta-posi-tions of the internal phenylene ring. The 2,5-di-methyl substituted monomer M2 was synthe-sized from p-xylene according to Scheme 3 viabromination25 to afford 2-bromo-1,4-dimethyl-benzene (4), followed by iodination28 to afford 1-bromo-2,5-dimethyl-4-iodobenzene (5). Then, theSonogashira coupling reaction was utilizedagain to sequentially introduce the phenylacety-lene group (at rt to afford compound 6)26 andthe 2-butyn-3-methyl-2-ol group (at 80 8C to

afford compound 7),27 followed by deacetonationwith NaOH27 to afford M2.

Similarly, M3 was synthesized (Scheme 4)starting from 1,4-diisopropylbenzene via diiodi-nation by adopting the previously reported pro-cedure28 to yield 1,4-diiodo-2,5-diisopropylben-zene (8). The Sonogashira coupling reaction of 8with 2.5 equiv amount of 2-butyn-3-methyl-2-olat rt26 gave the both side protected compound 9,which was then selectively deprotected on oneside by refluxing with 1.0 equiv of KOH in tolu-ene29 for 3 h to form alkyne 10 in 55% yield.Again, the Sonogashira coupling of the alkyne10 with iodobenzene at rt26 afforded compound11, which was then deprotected with 1.0 equivamount KOH in toluene under reflux condition29

to afford monomer M3. All the monomers werecharacterized by NMR, IR, mass, and EA analy-ses, from which satisfactory data were obtained(see Experimental Section for details).

Polymerization

Mo-, W-, and Rh-based catalysts are the mostwidely used catalysts for the polymerization of

Scheme 2. Synthesis of Monomer M1. Reagentsand conditions: (i) HC:CC(CH3)2OH (1.2 equiv),PdCl2(PPh3)2 (0.5 mol %), CuI (1.0 mol %), triethyl-amine, THF, rt, 12 h, Yield: 97%; (ii) Phenylacetylene(1.5 equiv), PdCl2(PPh3)2 (1.0 mol %), CuI (1.0 mol %),PPh3 (1.0 mol %), triethylamine, toluene, 80 8C, 24 h,Yield: 85%; (iii) NaOH (10 equiv), benzene, reflux,24 h, Yield: 60%.

Scheme 3. Synthesis of Monomer M2. Reagentsand conditions: (i) I2 (1.0 equiv), HIO3 (0.5 equiv),aq. H2SO4 (30%), CCl4, acetic acid, 60 8C, 6 h, Yield:78%; (ii) Phenylacetylene (1.1 equiv), PdCl2(PPh3)2(0.5 mol %), CuI (1.0 mol %), triethylamine, THF, rt,12 h, Yield: 96%; (iii) HC:CC(CH3)2OH (1.5 equiv),PdCl2(PPh3)2 (1.0 mol %), CuI (1.0 mol %), PPh3 (1.0mol %), triethylamine, toluene, 80 8C, 24 h, Yield:62%; (iv) NaOH (10 equiv), benzene, reflux, 24 h,Yield: 52%.



acetylene- and phenylacetylene-based mono-mers.6–10,12 It has also been reported that WCl6with cocatalyst such as Ph4Sn or Et3SiH aresuitable for polymerizing ortho-substituted phe-nylacetylenes in toluene6(a),7,8,10,12,18 with goodyields and high molecular weights. We thustried to polymerize our monomers M1–M3(Scheme 5) using the same catalyst systems intoluene.

Our initial attempt to polymerize monomerM1 with WCl6 catalyst and Ph4Sn cocatalyst (Ta-ble 1, Entry 1) in dry toluene at rt for 24 h didgive a good yield of polymer P1; however, theobtained polymer was totally insoluble in any sol-vents. The polymer obtained in a shorter reactiontime (2 h) using the same concentrations ofmonomer and catalyst seem to dissolve better,with only �18% remaining insoluble (Table 1,Entry 2). The variation in the concentrations ofmonomer (0.5–1.0 M) and catalyst (5–20 mM) allyielded similar results, that is, containing �20%insoluble polymers. Therefore, we carried out thepolymerization reaction alternatively with WCl6catalyst and Et3SiH cocatalyst (Table 1, Entries 3and 4) in dry toluene at rt. Once again, the poly-

mer obtained from a 24 h reaction time wastotally insoluble; whereas the polymer P1 (Table1, Entry 4) obtained in a shorter time (2 h) andat a lower monomer concentration (using 0.5 Minstead of 1.0 M) was found to be totally soluble(in THF) and yet has a fairly high molecularweight (Mw ¼ 1.22 3 105).

When the same catalyst system (WCl6/Et3SiH) was applied to the polymerization ofM2, the obtained polymers in 2 h based on 1.0M and 0.75 M concentrations of M2 (Table 1,entries 5 and 6) were totally insoluble; while thepolymer P2 (Mw of 2.40 3 105) obtained at0.5 M monomer concentration (Table 1, Entry 7)was totally soluble. Similarly, totally solublepolymer P3 (Mw ¼ 4.69 3 105) can be obtainedfrom monomer M3 under the same polymeriza-tion conditions (Table 1, entry 8), while the poly-mer obtained in slightly longer time (i.e., 3 h)was found to be totally insoluble (Table 1, Entry9). Interestingly in all the cases, the polymerproducts would remain highly soluble as long asthe polymerization solutions were free of gels,before being quenched with ammoniacal metha-nol solution. Apparently, the insolubility of thepolymer products may be caused by the cross-linking reactions among the preformed polymerchains after the polymerization solution turnedinto the gel state. Under such circumstances,the diffusion rates of both monomer and propa-gating chain will be greatly hindered by thehighly viscous gel matrix. As the local concen-tration of monomer was temporarily depleted,the active growing chains would then initiatethe secondary polymerization reactions basedon the less reactive internal triple bonds on thepreformed polymer chains.

The GPC results (Fig. 1) for polymers P1–P3obtained under the same polymerization condi-tions (Table 1, entries 4, 7, and 8) indicated thatpolymer with a bulkier ortho-substituent actually

Scheme 5. Preparations of polymers P1-P3 frommonomers M1-M3.

Scheme 4. Synthesis of Monomer M3. Reagentsand conditions: (i) I2 (1.3 equiv), HIO3 (0.6 equiv), aq.H2SO4 (30%), CCl4, acetic acid, 60 8C, 6 h, Yield: 81%;(ii) HC:CC(CH3)2OH (2.5 equiv), PdCl2(PPh3)2 (0.5mol %), CuI (1.0 mol %), triethylamine, THF, rt, 24 h,Yield: 96%; (iii) KOH (1.0 equiv), toluene, reflux, 3 h,Yield: 55%; (iv) Iodobenzene (1.0 equiv), PdCl2(PPh3)2(0.5 mol %), CuI (1.0 mol %), triethylamine, THF, rt,24 h, Yield: 87%; (v) KOH (1.0 equiv), toluene, reflux,12 h, Yield: 72%.



has a higher Mw, that is, in the order of: P3 >P2 > P1. The results (Fig. 1, Table 1) also indi-cated that the polydispersity index (PDI) of poly-mers P1–P3 also seem to decrease as the size ofthe ortho-substituent increase from H (4.18, P1)through methyl (3.13, P2) to isopropyl (3.00, P3).These results are in consistent with the previousobservations that the PPAs with bulkier ortho-alkyl substituents had a higher Mw and a nar-rower PDI, which have been attributed to themuch increased crowd of the propagating speciesdue to the presence of the ortho-substituents thathelps to suppress the undesired chain transferand termination reactions.18–22

Structural Characterizations

The obtained polymers are studied with IR andNMR spectroscopies. The IR spectra of mono-

mers M1–M3 and their corresponding polymersP1–P3 are summarized in Figure 2. The puri-fied polymer samples were used for IR analyses.The results of monomers M1–M3 clearly showedthe characteristic IR peaks for the terminal andinternal acetylenic groups at �3280 cm�1 (acety-lenic C��H stretching), �2200 cm�1 (internalC:C stretching), �2100 cm�1 (terminal C:Cstretching) and the C��H bending modes for theacetylenic proton in the range of �670 to 650cm�1 (in-plane mode) �650 to 610 cm�1 (out-of-plane mode).8(c,e),30 After the polymerizations,all the characteristic peaks associated with theterminal acetylenic groups disappeared for the

Table 1. Polymerization Conditions and Results for P1–P3a

No M Cat/Co-Catb [Cat] (mM) [M] (M) Time (h) Yieldc (%) Mwd Mw/Mn

d

1 M1 WCl6/Ph4Sn 5 0.5 24 88 (88) – –2 M1 WCl6/Ph4Sn 5 0.5 2 84 (18) – –3 M1 WCl6/Et3SiH 10 1.0 24 92 (92) – –4 M1 WCl6/Et3SiH 10 0.5 2 57 (–) 1.22 3 105 4.185 M2 WCl6/Et3SiH 10 1.0 2 72 (72) – –6 M2 WCl6/Et3SiH 10 0.75 2 70 (70) – –7 M2 WCl6/Et3SiH 10 0.5 2 68 (–) 2.40 3 105 3.138 M3 WCl6-Et3SiH 10 0.5 2 72 (–) 4.69 3 105 3.009 M3 WCl6-Et3SiH 10 0.5 3 75 (75) – –

a Carried out under nitrogen in dry toluene at rt.b Catalyst/Co-Catalyst ¼ 1:1.c The values in the parentheses are the yield % for the insoluble polymer portion.d Mw weight average molecular weight; Mn, number average molecular weight; Mw and Mn were determined by GPC using

THF as eluant and calibrated against polystyrene standards.

Figure 1. GPC traces for polymers P1–P3.

Figure 2. IR spectra for monomers M1 (a), M2 (c),and M3 (e) and their corresponding polymers P1 (b),P2 (d), and P3 (f) based on pressed KBr pellets. Theenlarged IR spectra for the region of 2000–2350 cm�1

are shown in (a0–f0). The broken lines in (a0–f0) areplaced at 2217 cm�1 and 2110 cm�1 to indicate the in-ternal and terminal C:C bands, respectively.



polymers P1–P3, leaving behind only the inter-nal C:C at �2200 cm�1. The results clearlysuggest that the polymerization predominantlyoccurred at the terminal acetylenic sites of M1–M3.

Such observation is also supported by the 1HNMR spectra of the polymers P1–P3 (Fig. 3),which all clearly showed the disappearance oftheir acetylenic protons (�d 3.2–3.3). The 1HNMR spectrum of P1 [Fig. 3(a)] showed that itsphenyl protons and main chain olefinic-protonsare merged together, forming a set of continuousbroad peaks in between �d 8.2 and 6.0. Accord-ing to the previously reported results for a sim-ple PPA,31,32 we believe that the broad peak tailat the up-field side in the region of �d 6.0–7.0may be attributed to trans-olefinic protons.Thus, the 1H NMR result suggests that the poly-ene backbone of polymer P1 is trans-rich in na-ture. Polymer P2 [Fig. 3(b)] also showed a simi-lar spectral feature for its phenyl and mainchain trans-olefinic protons. The broad peakappeared between �d 2.9 and 1.4 can beassigned to the benzylic methyl groups of P2.

Similar to P1 and P2, polymer P3 [Fig. 3(c)]also showed, in addition to the peaks associatedwith its isopropyl group (�d 0.9 and 2.9), abroad peak at �d 7.0–8.2 for its phenyl protons.Interestingly, in this case, the up-field tail asso-ciated with the main chain trans-olefinic protonsat �d 6.0–7.0 is clearly resolved into two smallbroad peaks. Addition to this, it also shows asmall broad peak between �d 5.0 and 6.0, whichcan be attributed to the main chain cis-olefinicprotons, according to the previous observationsthat the cis-olefinic protons for a simple PPAappeared at �d 5.8.31,32 The 1H NMR resultsclearly suggest that the polyene backbones ofP1 and P2 have predominantly trans-struc-tures, which is in consistent with the previouslyreported results that W-based catalysts pro-duced >90% of trans-PPA, whereas Rh-basedcatalyst produced almost 100% cis-PPA.16 The1H NMR results also indicated that the trans-content of polymer P3 has been considerablylowered, probably due to the presence of steri-cally bulky isopropyl substituents that signifi-cantly destabilize the trans-structure of P3.

Thermogravimetric Analysis

The thermal properties of the polymers P1–P3were determined by thermogravimetric analysis(TGA) under nitrogen atmosphere from 30 to1000 8C as shown in Figure 4 and the detailedanalytical data are summarized in Table 2. Allthese polymers showed much higher initialweight loss temperature (Ti > 310 8C) than theunsubstituted PPA (�200 8C), the PPAs withortho-substituents like trimethylsilyl (�270 8C),

Figure 3. 1H NMR spectra of polymers (a) P1, (b)P2, and (c) P3 (in CDCl3; 500 MHz). The broken linesare placed at d 6.0 and 8.2 to show the phenyl protonsand the main chain trans-olefinic protons.

Figure 4. TGA thermograms (from 30 to 1000 8C)for polymers P1-P3 measured at a heating rate of10 8C/min under a nitrogen flow of 100 mL/min.



trifluoromethyl (�302 8C), and methyl (�240 8C)groups,21 and the 3,6-di-tert-butylcarbazolyl sub-stituted PPA (�200 8C).13 Regarding the tem-perature for 5% weight loss (T5%), P1 (491 8C) ishigher than P2 (�455 8C), which in turn ismuch higher than P3 (365 8C). Likewise, theonset decomposition temperature (Tonset) of P1–P3 showed similar reduction trend. Such reduc-tion trends in T5% and Tonset are probably in ac-cordance with the easiness in cleaving the ortho-and meta-substituents of the tolane pendantgroups in the order of ��iPr > ��CH3 > ��H,which are actually in consistent with the stabil-ity of the cleaved radicals. The total weight losesat 500 8C for P1, P2, and P3 were �6%, 12%,and 48%, respectively, which may be mainlycaused by the cleavage of the alkyl-substituents(Table 2, last column) and partly by the cleavageof their polyene backbone and the tolane pend-ant group. Interestingly, polymers P1–P3showed much lower weight loss at 500 8C thanthe unsubstituted PPA (45% weight loss) andthe PPAs with ortho-substituents like ��Me(54%), ��CF3 (65%), and ��SiMe3 (70%)groups.17 Furthermore, the glass transition tem-peratures (Tg) of our polymers P1–P3 (130–140 8C), measured with DSC under nitrogenwere found to be much higher than those ofunsubstituted PPA (72 8C) and the Tang’s novelfluorescent alkyl-substituted PAs (80–100 8C).12

The higher thermal stability of our polymersP1–P3 than the simple ortho-substituted PPAsmay be attributed to the introduction of therigid and conjugated phenylethynyl group at thepara-position to the phenyl ring of PPA back-bone.

Optical Properties

The UV-vis absorption spectra of polymers P1–P3 measured in THF solutions (4 3 10�5 M) are

shown in Figure 5 and the summarized data areshown in Table 3. Polymer P1 exhibited anabsorption maximum at �302 nm with a broadtail absorption extending up to �600 nm. Thetail absorptions at wavelength longer than �400nm may be due to the p–p* interband transitionof the polyene backbone and the main absorp-tion band at �302 nm may be attributed to thep–p* transition of the tolane pendant group.11

In contrast, polymers P2 and P3 showed twowell resolved absorption bands. The p–p* transi-tion bands of the tolane pendant groups of P2(�291 nm) and P3 (�292 nm) are blue-shiftedfrom that of P1 (�302 nm), probably due to thesteric hindrance of the alkyl groups at theortho-position which may force the tolane planesof P2 and P3 to twist more perpendicularlywith respect to the planes of their polyene back-bones and thus reduce the overall effective con-jugation extent of their tolane side groups.17 On

Table 2. The Results of Thermogravimetric Analysisa for Polymers P1–P3

Polymerso-/m-alkylsubstituent Ti

b Tonsetc T5%

dWt % lossat 500 8C

Wt fraction (%)e ofthe substituent

P1 ��H 340 458 491 5.74 –P2 ��CH3 321 438 455 12.07 13.03P3 ��iPr 311 395 365 47.89 30.03

a From 30 to 1000 8C at a heating rate of 10 8C/min under a nitrogen flow of 100 mL/min.b Temperature at which initial weight loss occurs.c Onset decomposition temperature.d Temperature at which 5% weight loss occurs.e Calculated weight fraction (%) for the substituent groups (-R) at the ortho- and meta-positions.

Figure 5. UV-vis spectra of polymers P1-P3 andPPA in THF solutions (4 3 10�5 M). The broken lineis placed at 400 nm to separate the p–p* transition ofthe tolane pendant group (below 400 nm) and the p–p* interband transition of the polyene backbone(above 400 nm).



the contrary, the twisting of the tolane sidegroups of P2 and P3 would yield more room fortheir polyene backbones to adapt a better copla-nar structure, which may account for the obser-vation that the p–p* interband transition of pol-yene backbones of P2 (�510 nm) and P3 (�544nm) are much more red-shifted from that of P1.Additional contribution may also arise from thehigher molecular weights of P2 and P3 thanP1. Similarly, the higher molecular weight of P3than P2 may also account for the more red-shifted (by �34 nm) p–p* interband transition ofP3 than that of P2. In contrast, the p–p* transi-tions for the tolane pendant groups of P2 andP3 were however found to appear at similarpositions, probably due to their similar poor con-jugation interactions with their polyene back-bones. Quite clearly, all these p–p* transitions ofthe tolane pendant group and the p–p* inter-band transitions of the polyene backbone of pol-ymers P1–P3 were found to be much red-shiftedfrom that of PPA (Table 3). The band gaps ofpolymers P1–P3 (estimated from the onset posi-tion of the p–p* interband transitions) werefound to be in the range of 1.96–2.25 eV, whichare lower than the energy gap of PPA (2.45 eV).The longer kmax and lower energy gaps of poly-mers P1–P3 than PPA might be attributed tothe introduction of conjugated phenylethynylgroup at the para-position to the phenyl ring ofPPA backbone and also possibly due to thehigher molecular weights of P1–P3 than PPA.

Figure 6 shows the PL spectra of polymersP1–P3 and PPA measured in THF solutions(4 3 10�5 M) by excitation at the kmax of the p–p* transition bands of their tolane (or phenyl)pendant groups and the summarized data areshown in Table 4, obviously P1–P3 are muchhighly fluorescent than PPA. The results in

Table 4 indicate that P1 emits blue light withkmax at �416 nm, while both P2 and P3 emitUV light (�355 and 354 nm, respectively). Theemission kmax of P2 and P3 are found to be sim-ilar to the emission kmax of their tolane pendantgroups, implying that the tolane pendant groupsare the major emitting chromophore for P2 andP3. In contrast, the emission kmax of P1 is muchred-shifted from its tolane pendant group, sug-gesting that both the tolane pendant group andthe polyene backbone are the emitting chromo-phores. The results are in consistent with theobservation that the tolane pendant groups ofP2 and P3 have rather poor conjugation interac-tion with their polyene backbones, due to the

Table 3. UV-vis Data for the THF Solutions (4 3 10�5 M) of polymersP1-P3 and PPA

Polymerso- and m-

Substituents

kmax in nm (Absorbance)a

BandGapd (eV)Band 1b Band 2c

PPA ��H 254 (0.207) >300 2.45P1 ��H 302 (0.742) >400 2.25P2 ��CH3 291 (0.867) 510 (0.164) 1.85P3 ��iPr 292 (0.830) 544 (0.280) 1.96

a The value in the parenthesis refers to the absorbance at the kmax.b p–p* transition of the tolane pendant group.c p–p* interband transition of the polyene backbone.d The band gap is estimated from the onset position of the absorption band 2.

Figure 6. Photoluminescence (PL) spectra of poly-mers P1-P3 and PPA in 4 3 10�5 M THF solution.



steric hindrance of the alkyl substituents at theortho-positions that forces their pendant groupsto twist more perpendicular with respect to theirpolyene backbones. Furthermore, both polymersP2 and P3 showed much smaller stokes shiftvalues (�62–64 nm) than that of P1 (�114 nm)and PPA (�185 nm), which are also in consist-ent with the expectation that a more perpendic-ularly oriented tolane pendant group (withrespect to the polyene backbone) would be lesslikely to deplete its excitation energy throughthe polyene backbone. The control PL study alsoindicated that the excitations at the kmax of p–p*interband transition of the polyene backbones ofpolymers P2 and P3 did not result in muchemission, probably due to the much enhancedcoplanarity of their polyene backbones. Amongthe polymers, P1 gives the highest quantumyield (F ¼ 0.36) (in comparison with the refer-ence standard Coumarin-102, F ¼ 0.93 in etha-nol)23 (Table 4), which is probably helped by theadditional emissive contribution from its less co-planar polyene backbone. The relatively loweremitting ability of P3 (F ¼ 0.06) might be dueto the unusual high cis-content nature of itspolyene backbone (different from P1 and P2)and/or due to the much higher rotational motionfreedom of its isopropyl groups. Similar phenom-enon has been previously observed for poly(o-iso-propylphenylacetylene),11(c) which showed muchpoorer fluorescence intensity than the unsubsti-tuted PPA. Regarding to the mono-substitutedpolyacetylenes, so far only the Tang’s novel pol-y(alkylacetylene)s (with a remotely attachedbiphenylyl pendant group) and Masuda’s 3,6-di-tert-butylcarbazolyl-substituted PPA have beenreported to be capable of emitting strong PL (at�380 nm and 410 nm, respectively)12,13; while

the rest of the mono-substituted PAs (andPPAs) have been reported to be totally hope-less.12 Thus we have successfully identified anadditional feasible approach to render the ingeneral nonemissive mono-substituted PAs and/or PPAs with strong PL properties. Most inter-estingly, the finding of this study also paved aviable way for the mono-substituted PPAs toachieve an even higher potential PL efficiency, ifthe simple tolane pendant group is replaced byits higher homologs, such as the oligomeric pol-y(phenylene ethynylene)s that are renownedwith their extremely high PL efficiency.14

CONCLUSIONS

A new class of highly fluorescent mono-substi-tuted poly(phenyl acetylene)s P1–P3 has beensuccessfully prepared from a series of novel ary-lacetylenes M1–M3, with various substitutedand unsubstituted tolane groups. The resultantpolymers are readily soluble in common organicsolvents (e.g., THF, toluene, and chloroform) andthermally stable up to �350 8C. Polymers P2and P3, with additional methyl and isopropylortho-substituents, indeed showed higher molec-ular weights than P1 as expected.18–22 The IRand NMR studies clearly suggested that the pol-ymerizations occurred predominantly at the ter-minal acetylene sites. The 1H NMR resultsshowed that both P1 and P2 have essentiallytrans-polyene backbone structures, whereas P3contains both significant amounts of cis- andtrans-structures. The PL study confirmed thatwe have successfully made the in general none-missive mono-substituted PPAs become highlyfluorescent simply by introducing a conjugated

Table 4. Photoluminescence (PL) Data for the THF Solutions (4 3 10�5 M) ofPolymers P1–P3 and PPAa

Polymerso-/m-alkylSubstituent

Emissionkmax

b (Intensity) FcStokes

Shift (nm)

PPA ��H 394 nm (1) – 185P1 ��H 416 nm (36) 0.36 114P2 ��CH3 355 nm (43) 0.15 64P3 ��iPr 354 nm (10) 0.06 62

a Measured by excitation at the kmax of the p-p* transition bands of their tolane pendantgroups.

b The value in the parenthesis refers to the fluorescence intensity at the kmax of the emis-sion peak.

c F ¼ quantum yield, estimated by using the reference standard coumarin-102 (F¼ 0.93)13 from 1 3 10�5 M solutions of the corresponding polymers in THF.



phenylethynylene group to the para position ofthe phenyl ring of PPA to convert the nonemis-sive phenyl ring into a highly fluorescent tolanetype of chromophore. The rigidity of the unsub-stituted tolane pendant group in P1 was foundto be already sufficient in reducing the coplanar-ity of its polyene backbone to an appropriateextent, making the polyene backbone becomepart of the effective emissive chromophore. Suchconjugation interaction between the tolanegroup and its nearby polyene segment alsorender P1 as a blue (�416 nm) emitter with areasonably high quantum yield (�0.36).Although the quantum yield of P1 is still lowerthan that of the Masuda’s novel 3,6-di-tert-butyl-carbazolyl-substituted PPA system (0.55),13

nevertheless the finding of this study provide anadditional feasible approach to make the gener-ally nonemissive PPA become highly fluores-cent. Actually, it is conceivable that when thesimple tolane group is replaced with its higherhomologs of the oligomeric poly(ethynylene ary-lene)s the quantum yields of the resultant PPAsystems would be expected to reach much higherlevels based on the previous observation thatthe optimal quantum yield of poly(arylene ethy-nylene)s can be very high.14 Because of the highrigidity of the tolane side groups, the resultantpolymers P1–P3 were also found to have muchhigher Tg (�130–140 8C) than those of PPA(72 8C) and the Tang’s novel fluorescent alkyl-substituted PAs (80–100 8C),12 which may pro-vide additional advantages to the lifetimes ofthe corresponding LEDs. The electrolumines-cence (EL) study for polymers P1–P3 is cur-rently undergoing in our laboratory and will bepublished elaborately in a separate article.

The authors thank National Science Council (NSC) ofROC for the financial support for this project.

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highly fluorescent mono-substituted poly(arylacetylene)s containing tolane chromophores

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