Synthesis and properties of polyacetylenes with directly attached bis(4-alkoxyphenyl)terephthalate mesogens as pendants

Synthesis and Properties of Polyacetylenes withDirectly Attached Bis(4-alkoxyphenyl)terephthalateMesogens as Pendants

LIE CHEN, YIWANG CHEN, DAIJUN ZHA, YAN YANG

School of Materials Science and Engineering, Nanchang University, Nanjing East Road 235, 330047 Nanchang, China

Received 22 October 2005; accepted 31 January 2006DOI: 10.1002/pola.21363Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Liquid-crystalline, monosubstituted polyacetylenes containing lateralpendants of bis(4-alkoxyphenyl)terephthalate with no flexible spacers and alkoxy tails{RO, where R is CH3 [P(1)] or C6H13 [P(6)]} were synthesized, and the effects of thebackbone structure and alkoxy tails on the properties of the polymers were investi-gated. The polymerizations of acetylene monomers were carried out with chloronorbor-nadiene rhodium(I) dimer as a 1,2-insertion catalyst in toluene. The structures andproperties of the monosubstituted polyacetylenes were characterized and evaluatedwith nuclear magnetic resonance, infrared spectroscopy, thermogravimetry, differentialscanning calorimetry, polarized optical microscopy, ultraviolet spectroscopy, and photo-luminescence analyses. The molecular weights of the polymers were measured by gelpermeation chromatography. The polymer with long tails (p-hexyloxy), that is, P(6),formed a smectic mesophase upon heating above the melting temperature, but the otherone with short tails (p-methoxy), that is, P(1), could not exhibit liquid crystallinity atelevated temperatures. The steric effect of bulky, liquid-crystalline mesogens and adirect connection with the main chain prevented the planar conformation of the polyenebackbone and, therefore, led to the lower absorption and emission wavelength of thepolymers. VVC 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2499–2509, 2006

Keywords: conjugated polymers; liquid crystallinity; phase behavior; polyacetylenes;rhodium catalysts

INTRODUCTION

The introduction of orientable, pendant, mesogenicmoieties onto a polyacetylene backbone mightincrease the conjugation in the main chain andyield polymers with permanent coupling betweenthe electroactive properties and order. Recently,the use of a rigid polymer backbone, such as a poly-ene chain, to prepare side-chain, liquid-crystallinepolymers has been considered with growing inter-est by various research groups.1–6 Conjugated poly-mers with liquid-crystalline groups in their side

chains are currently drawing interest from theviewpoint of multifunctional electrical and opticalmaterials.7–11 Polyacetylene is an archetypal conju-gated macromolecule, and its functionalization hasattracted much synthetic effort over the past de-cades.12–18 A variety of polyacetylenes containingliquid-crystalline mesogens and light-emittingchromophores have been prepared.19–25 The mono-substituted polyacetylenes show multifaceted me-sophases, with their transition temperatures tuna-ble by their molecular structures.26 The mesogenicorientations of the polymers and their moleculararrangements can be readily manipulated by suchexternal stimuli as mechanical forces and electricalfields.27,28 With the introduction of liquid-crystal-line groups into polyacetylene, it is expected that

Correspondence to: Y. Chen (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 2499–2509 (2006)VVC 2006 Wiley Periodicals, Inc.

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the orientation of the liquid-crystalline side chainsmay enhance the alignment of the main chain viathe spontaneous orientation of the liquid-crystallinegroups. The spontaneous orientation and externallyforced alignment of the liquid-crystalline sidechains enable us to control the electrical and opticalproperties as well as their anisotropies.29–32

A class of mesogen-jacketed liquid-crystal poly-mers (MJLCPs) has been synthesized, in whichthe mesogenic units are attached laterally to themain chain without spacers or with only veryshort flexible spacers. A jacket is formed aroundthe chain backbone by the mesogenic unitsbecause of their high population around the back-bone and because they are both bulky and stiff.The jacket, in turn, forces the main chain toextend and to show properties of rigid or semi-rigid polymers.33,34 All MJLCPs have been basedon flexible polymer backbones, such as polysilox-ane and polyvinyl. Rigid polymer backbones, suchas polynorborane and polyacetylene, are believedto play a destructive role in mesophase formation.In this study, polyacetylene was used as the poly-mer backbone. The mesogenic units were at-tached laterally to the polyacetylene via a directconnection. The effect of the polymer backbone onthe mesomorphic properties of the obtained poly-mers is discussed.

EXPERIMENTAL

Materials

2-Bromoterephthalic acid, 4-n-hexyloxyphenol, tri-methylsilylacetylene, tetrabutylammonium fluo-ride trihydrate, hexachloroethane, bis(triphenyl-phosphine)palladium(II) chloride, and chloronor-bornadiene rhodium(I) dimer were purchased fromAcros and used as received without any further pu-rification. Tetrahydrofuran (THF) and toluenewere dried over sodium, and dioxane and triethyl-amine (Et3N) were dried over calcium hydride andthen distilled under nitrogen. Other chemicalswere obtained from Shanghai Reagent Co., Ltd.,and used as received.

Techniques

The infrared (IR) spectra were measured on aShimadzu IRPrestige-21 Fourier transform infra-red (FTIR) spectrophotometer with the KBrmethod. The proton nuclear magnetic resonance(1H NMR) spectra were recorded on a BrukerARX 400 NMR spectrometer with chloroform-d asthe solvent and tetramethylsilane (d ¼ 0) as the

internal reference. The ultraviolet–visible spectraof the samples were obtained with a Hitachi UV-2300 spectrophotometer. The photoluminescence(PL) of the polymers was measured on a Shi-madzu RF-5301PC spectrofluorophotometer. Gelpermeation chromatography was run on a Waters1515 high-performance-liquid-chromatography in-strument equipped with a 2414 differential refrac-tometer and a set of Styragel mixed-C columns(Styragel Waters HR4E and HR5E) to separatemolecular weights ranging from 102 to 106. Theoven temperature was set at 40 8C. THF was usedas the eluent, and the flow rate was 1 mL/min.Monodispersed polystyrene standards (AldrichChemical Co.) were used to generate the calibra-tion curve. Thermogravimetry was performedunder nitrogen with a PerkinElmer TGA 7 at aheating rate of 20 8C/min and with a sample size of8–10 mg. Phase-transition temperatures weredetermined with a PerkinElmer DSC 7 differentialscanning calorimeter with a constant heating/cool-ing rate of 10 8C/min. Texture observations weremade with a Nikon E600POL polarizing opticalmicroscope equipped with an Instec HS 400 heat-ing and cooling stage. The X-ray diffraction (XRD)study of the samples was carried out on a Shi-madzu XRD-6000 X-ray diffractometer operatingat 30 kV and 20 mAwith a copper target (k ¼ 1.54A) and at a scanning rate of 18/min.

Synthesis of the Monomers

The synthesis and structures of the monomersare outlined in Scheme 1. All the reactions andmanipulations were carried out under a nitrogenatmosphere.

1,4-Bis[(p-hexyloxyphenyl)oxycarbony]-2-bromobenzene

2-Bromoterephthalic acid (4.0 mmol, 1.1 g) andtriphenylphosphine(8.4 mmol, 2.2 g) were dis-solved in 10 mL of dried pyridine to obtain solu-tion A. 4-n-Hexyloxyphenol (8.4 mmol, 1.63 g)and hexachloroethane (8.8 mmol, 2.1 g) were dis-solved in 10 mL of dried pyridine to obtain solu-tion B. B was slowly added to A. The mixture wasstirred at 60 8C for 10 h. After the mixture hadcooled to room temperature, it was poured intoa methanol/water solution to precipitate theproduct. The precipitate was collected and purifiedby recrystallization from ethanol (yield ¼ 69%, mp¼ 84–85 8C). The characterization data of this com-pound were as follows.

2500 CHEN ET AL.

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

IR (KBr, m, cm�1): 1720 (C¼¼O). 1H NMR (400MHz, CDCl3, d, ppm): 8.52 (s, 1H, o-bromoben-zene-H), 8.23 (d, 1H, bromobenzene-H), 8.04 (d,1H, bromobenzene-H), 7.18 (d, 2H, phenyl-2H),7.13 (d, 2H, phenyl-2H), 6.93 (d, 4H, phenyl-4H),3.97 [t, 4H, 2(��OCH2)], 1.79 [p, 4H, 2(OCH2CH2)],1.47 [p, 4H, 2(OCH2CH2CH2)], 1.35 (m, 8H, 4CH2),0.92 (t, 6H, 2CH3).

1,4-Bis[(p-methoxylphenyl)oxycarbony]-2-bro-mobenzene was obtained similarly.

Yield: 70%. mp: 111–113 8C. IR (KBr, m, cm�1):1729 (C¼¼O). 1H NMR (400 MHz, CDCl3, d,ppm): 8.52 (s, 1H, o-bromobenzene-H), 8.23 (d,1H, bromobenzene-H), 8.05 (d, 1H, bromoben-zene-H), 7.20 (d, 2H, phenyl-2H), 7.15 (d, 2H,phenyl-2H), 6.95 (d, 4H, phenyl-4H), 3.84 [s, 6H,2(��OCH3)].

1,4-Bis[(p-methoxylphenyl)oxycarbony]-2-acet-ylenyl Benzene [M(1)] and 1,4-Bis[(p-hexyloxy-phenyl)oxycarbony]-2-acetylenyl Benzene [M(6)]

The obtained compound (16.5 mmol), 0.33 mmol (63mg) of copper (I) iodide, and 0.16 mmol (116 mg) ofdichlorobis(triphenylphosphine) palladium(II) were

dissolved in 50 mL of dry toluene, and then to themixture were added 3 mL of dry diisopropylamineand 3 mL (21.2 mmol) of trimethylsilylactylenedropwise. The mixture was stirred at 70 8C for 2days under nitrogen. Afterward, the diisopropyl-amine was evaporated in vacuo, and the toluenesuspension was filtered through a pad of silica gel.The solvent was evaporated, and the residue wasdissolved in 100 mL of THF. Then, 16.5 mmol(4.31 g) of tetrabutylammonium fluoride trihy-drate was added, and the reaction was stirred for1 h at room temperature. Afterward, 200 mL ofwater and 100 mL of diethyl ether were added,and the organic layer was separated out and driedover magnesium sulfate, filtered, and evaporated.The residue was purified by column chromatogra-phy with silica gel as the stationary phase andhexane/dichloromethane (v/v 2/3) as the eluent.The yield was 50–53%.

M(1). mp: 151–152 8C. IR (KBr, m, cm�1): 3253(C�C��H), 2110 (C�C), 1731(C¼¼O). 1H NMR(400 MHz, CDCl3, d, ppm): 8.48 (s, 1H, o-bromo-benzene-H), 8.24 (d, 2H, bromobenzene-2H), 7.19(dd, 4H, phenyl-4H), 6.95 (d, 4H, phenyl-4H), 3.84

Scheme 1. Synthesis of P(1) and P(6).

SYNTHESIS AND PROPERTIES OF POLYACETYLENES 2501


(s, 6H, 2OCH3), 3.49 (s, 1H, C�CH). ELEM. ANAL.Calcd. for C24H18O6: C, 71.64%; H, 4.51%; O,23.86%. Found: C, 71.23%; H, 4.67%; O, 23.56%.Mass spectrometry:m/z ¼ 402.

M(6). mp: 110–112 8C. IR (KBr, m, cm�1): 3253(C�C��H), 2110 (C�C), 1728(C¼¼O). 1H NMR(400 MHz, CDCl3, d, ppm): 8.48 (s, 1H, o-bromo-benzene-H), 8.24 (d, 2H, bromobenzene-2H), 7.18(dd, 4H, phenyl-4H), 6.94 (d, 4H, phenyl-4H), 3.97[t, 4H, 2(��OCH2)], 3.49 (s, 1H, C�CH), 1.79 [p,4H, 2(OCH2CH2)], 1.48 [p, 4H, 2(OCH2CH2CH2)],1.36 (m, 8H, 4CH2), 0.92 (t, 6H, 2CH3). ELEM.ANAL. Calcd. for C29H28O6: C, 75.25%; H, 7.06%;O, 17.69%. Found: C, 75.43%; H, 6.87%; O,17.66%. Mass spectrometry:m/z ¼ 542.

Polymerization of the Monomers

All the polymerization reactions and manipula-tions were carried out under a nitrogen atmos-phere, except for the purification of the polymers,which was done in an open atmosphere. A typicalexperimental procedure for the polymerizationwas as follows.

Into a baked 20-mL Schlenk tube was added0.8 mmol (433.6 mg) of M(6). The tube was evac-uated in vacuo and then flushed with dry nitro-gen three times. Freshly distilled toluene (2 mL)was injected into the tube to dissolve the mono-mer. The catalyst solution was prepared inanother tube by the dissolution of 0.08 mmol(36.88 mg) of chloronorbornadiene rhodium(I)dimer in 2 mL of toluene. The two tubes wereaged at 60 8C for 15 min, and the monomer solu-tion was transferred to the catalyst solution witha hypodermic syringe. The reaction mixture wasstirred at room temperature under nitrogen for24 h. The solution was then cooled to room tem-perature, diluted with 3 mL of chloroform, andadded dropwise to 500 mL of methanol under stir-ring. The precipitate was allowed to stand over-night and was then filtered. Polymer P(6) waswashed with methanol and dried in a vacuumoven to a constant weight. The yield was 48%.

P(1)

Red-brown solid. IR (KBr, m, cm�1): 3060(C¼¼C��H), 1731 (C¼¼O). 1H NMR (400 MHz,CDCl3, d, ppm): 8.00–8.11 (m, 3H, benzene-3H),6.51–7.07 (m, 9H, phenyl-8H and olefin H), 3.56–3.82 [m, 6H, �2(OCH3)]. ELEM. ANAL. Calcd. for

C24H18O6: C, 71.64%; H, 4.51%; O, 23.86%.Found: C, 70.97%; H, 4.77%; O, 22.68%.

P(6)

Red-brown solid. IR (KBr, m, cm�1): 3060(C¼¼C��H), 1731 (C¼¼O). 1H NMR (400 MHz,CDCl3, d, ppm): 8.02–8.23 (m, 3H, benzene-3H),6.54–7.05 (m, 9H, phenyl-8H and olefin H), 3.67–3.87 [m, 4H of �2(OCH2)], 0.85–1.71 [m, 22H,�2(C5H11]. ELEM. ANAL. Calcd. for C29H28O6: C,75.25%; H, 7.06%; O, 17.69%. Found: C, 75.75%;H, 6.57%; O, 17.07%.

RESULTS AND DISCUSSION

Synthesis of the Monomers

The synthetic routes of the monomers are shown inScheme 1. The acetylene monomers were synthe-sized through a three-step reaction route. First, 1,4-bis[(p-alkyloxylphenyl)oxycarbony]-2-bromobenzenewas prepared by the esterification of 2-bromotereph-thalic acid and p-alkoxyphenol in high yields (70%).Then, trimethylsilylacetylene was introduced intothe aromatic ring. The monomers were obtained bydesilylation with tetrabutylammonium fluoride tri-hydrate. The structures of all the intermediates andthe final monomers were confirmed by FTIR (Fig. 1)and 1H NMR (Fig. 2). The resonance absorptionpeaks of C��H of acetylene and C�C in FTIR spec-tra were observed around 3253 and 2110 cm�1,respectively. The structures were verified by 1H

Figure 1. IR spectra of the monomers and their poly-mers.

2502 CHEN ET AL.


NMR spectroscopy as well. It is noteworthy that theabsorption peak at 3.49 in both of the NMR spectraof the monomers is attributable to the chemical shiftof the proton in C�CH. All the intermediates and

monomers were characterized by standard spectro-scopic methods, from which satisfactory analysisdata were obtained (see the Experimental sectionfor details).

Figure 2. 1H NMR spectra of chloroform-d solutions of (a) M(1), (b) M(6), (c) P(1), and(d) P(6).



Figure 2. (Continued from the previous page)

2504 CHEN ET AL.


Synthesis of the Polymers

Fe(acac)3–AlEt3, a Ziegler–Natta catalyst, andMoCl5–Ph4Sn, a metathesis catalyst, can poly-merize monosubstituted acetylene derivatives. Inparticular, very high molecular weights can beobtained by the Fe catalyst in the polymerizationof phenylacetylene or 4-phenyl-1-butyne.35 How-ever, these systems are not valid for polymeriza-tions of monomers with ester groups becausemetal ions react with the ester group in the mono-mer, Thus, we adopted a rhodium complex cata-lyst, [Rh(NBD)Cl]2 (where NBD is 2,5-norborna-diene), for the polymerization of the acetylenemonomers with a liquid-crystalline substituentbecause the Rh complex catalyst has no poisoninginteraction with the polar ester group.36 The Rh-catalyzed polymerization of acetylene monomersproceeds via an insertion mechanism. Polymer-izations of the acetylene monomers [M(1) andM(6)] were carried out in toluene at 60 8C for24 h. After the polymerization reaction, lower mo-lecular weight fractions were removed by wash-ing with methanol followed by filtration to yield ared-brown solid. All the synthesized polymerswere fusible and soluble in common organic sol-vents such as THF and chloroform. The polymeri-zation of the acetylene monomers by the Rh com-plex catalyst gave polymers [P(1) and P(6)] withmoderate molecular weights [weight-average mo-lecular weight (Mw) ¼ 16,200 and number-aver-age molecular weight (Mn) ¼ 9500 for P(1) andMw ¼ 18,100 andMn ¼ 11,300 for P(6)].

Structural Characterization

The polymeric products were characterized byspectroscopic methods, and all the polymers gavesatisfactory data corresponding to their expectedmolecular structures (see the Experimental sec-tion for details). The monomers showed a weakband at 2110 cm�1 associated with the C�Cstretch, which completely disappeared in thespectra of their polymers (Fig. 1). The absorptionband of HC� stretching at 3253 cm�1 in themonomers was converted to the HC¼¼ band at3060 cm�1 in the polymeric products when themonomers were polymerized with the Rh com-plex catalyst. The strong C¼¼O band at 1731cm�1, however, experienced little change, andthis proved that the acetylene polymerizationwas harmless to the carbonyl-functional group.In the 1H NMR (400 MHz) spectra, no signalscharacteristic of the acetylenic moiety in the

monomers [e.g., the singlet signal at 3.49 ppmoriginating from acetylene hydrogen (HC�)]appeared. These results indicate that an openingof the acetylenic carbon in the monomers cata-lyzed by the Rh complex catalyst gives a linearpolymer with a conjugated double bond. Accord-ing to the results of the 1H NMR spectra, P(1)and P(6) had peaks around 6 ppm characteristicof a proton attached to an olefin proton signal(Fig. 2). The NMR spectra were obscured by theacid-accelerated cyclization along the polyenebackbone, which depleted the cis-polyene reso-nance to form a new resonance associated with1,3-cyclohexadiene moieties.37–42

Thermal Stability and Liquid Crystallinity

Before investigating the mesomorphic propertiesof the polymers, we first examined their thermalstability. Poly(1-hexyne), a poly(1-alkyne), is sounstable that it starts to lose its weight when it isheated to a temperature as low as�150 8C.26 Poly-mers P(1) and P(6), however, lost almost noweight when they were heated to 350 8C in thethermogravimetric analysis. Clearly, the incorpo-ration of the stable aromatic ester pendants intothe polyacetylene structure dramatically en-hanced the thermal stability of the polymers be-cause of the protective jacket effect of the meso-genic groups43 and an additional contribution bythe phenyl group directly attached to the polyenebackbone. Even though results have shown thatthe thermal treatment of polyarylacetylene deriv-atives induces thermally induced structuralchanges in bulk and solution,44–46 thermogravi-metric analysis shows that the two polymersunderwent no weight loss before 350 8C.

Acetylene monomers M(1) and M(6) and polymerP(6) exhibited optical anisotropy when heated orcooled, but P(1) did not; this suggests that the acety-lene monomers and P(6) were liquid-crystalline butP(1) was not. The zero spacer between the meso-genic pendant and the polyene backbone limited themesogens to undergo thermal transitions in a rela-tively independent fashion. The designated polyace-tylenes, P(1) and P(6), resembled the structure of re-ported jacket liquid-crystalline polymers in whichthe polyacetylene backbone is replaced by the polyo-lefin or polysiloxane.47 The polymer with a long tailin the mesogenic pendant, P(6), readily formed thesmectic A (SmA) mesophase enantiotropically. Simi-larly, the short tail in the mesogenic pendant of P(1)prevented P(1) from showing any mesomorphism. Itwas expected that a liquid-crystalline monomer



would result in a polymer with at least one liquid-crystalline phase;48 however, the bulk and rigid sidechain demolished the expectation.

Figure 3 shows microphotographs of the meso-morphic textures of M(1), M(6), and P(6) taken

under a polarized optical microscope. M(1) andM(6) formed anisotropic melts with a birefringenttexture, which indicated thermotropic liquid-crys-talline behavior of these monomers. The droplettextures indicated that the mesophasic nature ofthe two acetylene monomers was a nematic phase.The polymer with a long tail in the mesogen [P(6)],however, readily formed the focal conic texture ofthe SmA phase. The dynamic process of the tex-ture formation revealed that many batonnet-likestructures emerged from the dark background andgrew to bigger domains when P(6) was cooled fromits isotropic melt. However, the exact nature of themesophase was difficult to identify. We repeatedlytried to grow the liquid crystals with care butfailed to obtain any readily identifiable character-istic textures. With the aid of XRD measurements,the texture was identified to be associated with anSmA phase (discussed later).

To learn more about the thermal transitions ofthe monomers and P(6), we measured their ther-mograms under nitrogen on a differential scan-ning calorimeter. As can be seen in Figure 4, P(6)showed two transition peaks (i 246 SmA 194 k/g)in the first cooling cycle and two transitions (k/g209 SmA 256 i) in the second heating run. Thewide mesomorphic temperature range [�52 8C (i �k/g) or �47 8C (k/g � i)] was in agreement with thegood packing order of its mesogens revealed bypolarized optical microscopy observation. Thelower melting temperature and wider mesomor-phic temperature range of M(6) (k 109 N 144 i), incomparison with those of M(1) (k 117 N 141 i),observed in the second heating run indicated thatthe long tail in the mesogen of the monomersplayed an important role in the packing order andstability of their liquid-crystalline phase. We car-ried out powder XRD experiments to gain more in-formation concerning the molecular arrange-ments, modes of packing, and types of order in themesophases of the polyacetylene liquid crystals.The XRD patterns were obtained from the meso-genic polyacetylene quenched with liquid nitrogenfrom its liquid-crystalline states, whereas the mes-ophases in the liquid-crystalline states at the giventemperature were frozen by the rapid quenchingwith liquid nitrogen. The diffractogram of P(6)showed a sharp reflection and a broad halo at 2h¼ 2.758 and 2h ¼ 20.858, respectively, from whichd-spacings of 32.10 and 4.26 A were derived. Thed-spacing derived from the low-angle peak wasclose to the calculated mesogenic length for therepeat unit of P(6) at its most extended conforma-tion (32.97 A), thus proving the monolayer nature

Figure 3. Polarized optical micrographs of melts ofM(1) at 151 8C (top), M(6) at 129 8C (middle), and P(6)at 209 8C (bottom).


2506 CHEN ET AL.

of the SmA phase. The existence of long-rangepositional order in the polymer again rules out thepossibility of a nematic classification.

Electronic Absorption and PL

The ultraviolet (UV) and PL spectra of P(1) andP(6) in chloroform and solid thin films are given inFigure 5. The monomers and polymers did notabsorb photons at wavelengths longer than 300nm. The absorptions of the monomers and poly-mers were assignable to the p–p* bands of themesogenic pendants because the monomers andpolymers had similar absorption wavelengths. Thepolyene backbone absorptions of the polymerswere, however, too weak to be observed. The low

absorptivity of the polymer main chain may havebeen due to the steric effect of the bulky mesogensdirectly attached to the polyene backbone, whichtwisted the double bonds and reduced the effectiveconjugation length along the main chain. Theground-state electronic transitions of the polymerswere not affected by the tail length of the mesogen.The absorption spectra of P(1) and P(6) were differ-ent from those of ortho-substituted polyarylacety-lenes, in which very long wavelength absorptionshave been observed.38 This is due to the less stericeffect of ortho-substituted polyarylacetylenes.

A polymer with both liquid-crystalline and light-emitting properties may find unique technologicalapplications,49 and we thus investigated the fluo-rescence properties of the polymers. When the poly-

Figure 4. Differential scanning calorimetry thermo-grams of the monomers recorded in the second heatingrun (top) and mesomorphic polyacetylene P(6) recordedunder a nitrogen atmosphere during the first coolingand second heating scans at a rate of 10 8C/min (bottom).

Figure 5. UV spectra of chloroform solutions of themonomers and their polymers (top) and PL spectra ofCHCl3 solutions of the polymers and their solid thinfilms (bottom). The concentration was 0.125 mM. Theexcitation wavelength was 300 nm.



mers were photoexcited at 300 nm, they emitted astrong UV light of 373 nm in chloroform (Fig. 5).This UV emission was not obviously from the PL ofthe polyene backbone. Instead, the UV emission ofthe polymers may have originated from their meso-genic pendants. The results are in good agreementwith the ground-state electronic transition, whichindicates nonplanar conformation of the polyenebackbone due to the steric effect of bulky, liquid-crystalline mesogens. Most linear conjugated poly-mers emit intensely in solution but become weakemitters when fabricated into films.50,51 This ismainly caused by strong interchain interactions inthe solid state. The UV and PL spectra of films ofP(1) and P(6) were, however, nearly identical tothose of their solutions, and this was suggestive oflittle or no excimer absorption and emission. Thelong alkyl and bulky mesogens of the polymers mayhave hampered the stacking of the polyacetylenechains, enabling P(1) and P(6) to emit even whenfabricated into thin films. We tried to measure theirquantum efficiency (FF), but no reliable resultscould be obtained.

CONCLUSIONS

Polyacetylenes withmesogens as pendants directlyattached to the backbone were successfully synthe-sized via rhodium complex catalyzed polymeriza-tion, in which the mesogens were hung parallel tothe main chain with no spacer in the close neigh-borhood of the double bonds. This structural differ-ence made the new polymers behave quite differ-ently from the liquid-crystalline polyacetyleneswith long spacers. The polymer with a long tail inthe mesogens, P(6), exhibited thermotropic smec-ticity enantiotropically. The steric effect of thebulky mesogens twisted the double bonds of poly-ene and thus prevented the ground-state elec-tronic transition and excitation of the backboneupon UV irradiation. The studies of the structure–property relationship encourage us to design jacketliquid-crystalline polyacetylenes with spacers be-tween the mesogens and polyene backbone toreduce the electronic interactions and steric hin-drance of side and main chains, hence enabling effi-cient light emission.

This project was sponsored by the Scientific ResearchFoundation for Returned Overseas Chinese Scholars,the State Education Ministry, and the Natural ScienceFoundation of Jiangxi Province (no. 520044). Theauthors thank the Analytic Centre of Nanchang Uni-versity for providing analytic finance.

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