A novel type of optically active helical liquid crystalline polymers: Synthesis and characterization of poly(p-phenylene)s containing terphenyl mesogen with different terminal groups

A Novel Type of Optically Active Helical LiquidCrystalline Polymers: Synthesis and Characterization ofPoly(p-phenylene)s Containing Terphenyl Mesogen withDifferent Terminal Groups

LIE CHEN,1 YIWANG CHEN,1 KAI YAO,1 WEIHUA ZHOU,1 FAN LI,1 LIPING CHEN,2 RONGRONG HU,3

BEN ZHONG TANG3

1Institute of Polymers/Department of Chemistry, Nanchang University, Xuefu Road 999, Nanchang 330031, China

2Department of Chemical Engineering, East China Jiaotong University, Nanchang 330013, China

3Department of Chemistry, The Hong Kong University of Science and Technology (HKUST), Clear Water Bay,Kowloon, Hong Kong

Received 6 April 2009; accepted 14 May 2009DOI: 10.1002/pola.23526Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Series of poly(p-phenylene)s (PPPs) containing terphenyl mesogenicpendants with cyano and methoxy terminal groups by flexible ACOO(CH2)6OAbridge [P(CN) and P(OCH3)] are synthesized through Yamamoto polycondensationwith Ni-based complex catalysts. The effects of the structural variation on their prop-erties, especially their mesomorphism, ultraviolet–visible (UV), and photolumines-cence behaviors, are studied. All of the polymers are stable, losing little of theirweights when heated to �340 �C. The polymers show good solubility and can be dis-solved in common solvents. P(CN) with cyano terminal group shows enantiotropicSmAd phase with bilayer packing arrangement, while P(OCH3) with methoxy termi-nal group readily forms nematic and SmAd phase when heated and cooled. Photoexci-tation of their solutions induces strong blue light emission. Compared withP(OCH3), the light-emitting bands of polymer P(CN) is slightly redshifted to 428nm and the emission intensity of P(CN) is much stronger, due to the existence of do-nor–acceptor pairs. More interestingly, both of the polymers exhibit obvious Cottoneffect on the CD spectra, resulting from the predominant screw sense of the back-bone. This indicates that the bulky mesogenic pendant orientating around the back-bone will force the main chain with helical conformation in the long region due tosteric crowdedness. VVC 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4723–

4735, 2009

Keywords: conjugated polymers; helical structure; liquid-crystalline polymers(LCP); luminescence; photoluminescence; poly(p-phenylene); terpenyl

INTRODUCTION

Liquid crystalline conjugated polymers (LCCP)are of both theoretical and practical interestbecause they combine the anisotropic properties

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 4723–4735 (2009)VVC 2009 Wiley Periodicals, Inc.

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

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of liquid crystals with the conjugated polymericproperties and have potential application andstimulate technological innovations in the devel-opment of novel electronic and photonic devicessuch as liquid crystal displays (LCDs), light-emitting diodes (LEDs), photovoltaic cells, filmtransistors, and plastic lasers.1,2 Particularly, con-jugated polymers with liquid crystalline (LC)groups in their side chains (SCLCCP) have beenconsidered with growing interest by variousresearch groups,3,4 because its electrical and opti-cal properties are expected to be controlled usingmolecular orientation of LC side chain. In thistype of polymers, the main chain can be alignedby virtue of spontaneous orientation of the LCside chain, and macroscopic alignment of LCdomains is also achieved by an external forcesuch as shear stress, electric or magnetic field.5

Attracted by the application perspective,recently, a variety of SCLCCP based on differentconjugated main chain have been prepared, suchas polyacetylenes, polythiophenes, and polyphe-nylenes, which can be endowed with such func-tional properties as mesomorphism, lumines-cence, photoconductivity, gas permeability, chainhelicity.6–19 Tang and coworkers have synthesizeda series of polyacetylenes bearing light-emittingchromophore with different functional bridgesand spacer length, such as {A[CH¼¼C(CmH2m þ 1)-OCO-biphenyl-OCO(CH2)10CH3]A},20 {A[ArC¼¼C(CH2)mO-biphenyl-O(CH2)6CH3]A}21 and {A[ArC¼¼C(CH2)mO-Naphthyl]A},22 which found that thelight-emitting chromophore endows the polymerwith high luminescence, and the longer spacersfavor the strong light emitting as well as betterpacking arrangement of the mesogens. Akagiand coworkers also synthesized a group of ferro-electric LC polyacetylenes/or polythiophenes/or polyphenylenes through substitution of fluo-rine-containing chiral LC groups into sidechains.16,23,24 The polymers exhibited quickresponse to electric field. Masuda and coworkersrecently synthesized novel polyacetylenes carry-ing cholesteryl moieties and reported that mostof the polymers showed LC properties but alsoexhibits helical conformation.25

Poly(p-phenylene)s (PPPs) are good blue light-emitting polymers, which are particularly inter-esting because strong blue emitters are still rareand blue light can act as ‘‘color converters’’ toreach the need of full-color display.26 Unfortu-nately, PPPs are insoluble in many organic sol-vents, which limit their applications. Therefore,attachment of substitutent to the backbone has

been accessible to synthesize the soluble and proc-essable PPPs. If the substituent is a LC group,their useful electrical and optical properties areexpected to be controllable via the molecular ori-entation of LC side chain.

Backbone conformation, side-chain ordering,and the resulting morphology affect various prop-erties of the polymeric materials. In our previousstudy, we find that the terphenyl mesogen pend-ants endow the polymer with good mesomorphismand high luminescence, besides, the energy canbe transferred from mesogen to main chain, favor-ing the fluorescence efficiency.27,28 The terphenylis not only a light-emitting chromophore but alsoa mesogenic core.29,30 Thus, if introducing the ter-phenyl mesogen pendants onto the high electoop-tically active PPP backbone might likely lead toobtained polymer with some charming properties.On one hand, the orientation of the mesogenpendants will endow the polymers with liquidcrystallinity and high luminescence. On the otherhand, because of the stereoeffect, the bulky meso-gens probably orientate around the main chainand induce the backbone with chiral screw sensein the long region, which is likely to open a newfascinating pathway to create novel opticallyactive SCLCCP without introducing any chiralcenter. To enrich the field of LCCPs containingterphenyl mesogen and to gain more informationon their structure property relationships, weattempt to synthesize a group of PPPs containingterphenyl mesogenic pendants with different ter-minal groups (ACN and AOCH3). In view of thefact that the short spacer may hamper the pack-ing arrangement of the mesogens, the flexibleACOO(CH2)6OA spacers with a moderate lengthare thus inserted between the rigid backbone andmesogen. The effects of structural variations onthe LC behaviors and the optical properties of thepolymers are studied, and the secondary struc-tures of polymers have also been investigated.

EXPERIMENTAL

Materials

Trimethyl borate, n-butyllithium, 2,5-dibromoto-luene, 6-bromo-1-hexanol, trimethyl borate, 4-(4-bromophenyl)phenol, 1,3-dicyclohexylcarbodiimide(DCC), 4-bromobenzonitrile, and 4-(dimethyl-amino)pyridine (DMAP) and tetrakis(triphenyl-phosphine)palladium were purchased from AlfaAesar and used as received without any furtherpurification. Tetrahydrofuran (THF) was dried

4724 CHEN ET AL.

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

over sodium. 4-Methoxybromobenzene and otherchemicals were obtained from Shanghai ReagentCo., Ltd., and used as received.

Techniques

The nuclear magnetic resonance (NMR) spectrawere collected on a Bruker ARX 400 NMR spec-trometer with deuterated chloroform or THF orDMSO as the solvent and with tetramethylsilane(d ¼ 0) as the internal standard. The infrared (IR)spectra were recorded on a Shimadzu IRPrestige-21 Fourier transform infrared (FTIR) spectropho-tometer by dropcasting sample solution on KBrsubstrates. The ultraviolet–visible (UV) spectra ofthe samples were recorded on a Hitachi UV-2300spectrophotometer. Fluorescence measurementfor photoluminescence (PL) of the polymers wascarried out on a Shimadzu RF-5301 PC spectro-fluorophotometer, with a xenon lamp as the lightsource. The gel permeation chromatography(GPC), so-called size-exclusion chromatography(SEC) analysis, was conducted with a BreezeWaters system equipped with a Rheodyne injector,a 1515 Isocratic pump, and a Waters 2414 differ-ential refractometer, using polystyrene as thestandard and THF as the eluant at a flow rate of1.0 mL/min and 40 �C through a Styragel columnset, Styragel HT3 and HT4 (19 mm � 300 mm,103 þ 104 A) to separate molecular weight (MW)ranging from 102 to 106. Thermogravimetric anal-ysis (TGA) was performed on a PerkinElmer TGA7 for thermogravimetry at a heating rate of 20 �C/min under nitrogen, with a sample size of 8–10 mg. Differential scanning calorimetry (DSC)was used to determine phase-transition tempera-tures on a Perkin-Elmer DSC 7 with a constantheating/cooling rate of 10 �C/min. Texture obser-vations by polarizing optical microscopy (POM)were made with a Nikon E600POL POMequipped with an Instec HS 400 heating and cool-ing stage. The X-ray diffraction (XRD) study ofthe samples was carried out on a Bruker D8Focus X-ray diffractometer operating at 30 kVand 20 mA with a copper target (k ¼ 1.54 A) andat a scanning rate of 1�/min. CD spectra wererecorded with a JASCO 810 apparatus.

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.

2,5-Dibromo Benzoic Acid (1)

2,5-Dibromotoluene 8.0 g (32.0 mmol) and 20 gKMnO4 was dissolved in 150 mL of pyridine and250 mL of water, the mixture was heated at refluxfor 2 h. Then, 40 g KMnO4 was added for severaltimes and kept refluxing overnight. The MnO2

precipitate was filtered hot and washed with boil-ing water. The filtrate was concentrated and theproduct was precipitated by addition of HCl. Theacid product was dried overnight at 80 �C in avacuum oven. 86% yield. IR (KBr, cm�1): 3439,1713, 1583, 1472, 1250, 584, 492.

4-Cyanobenzeneboronic Acid [2(CN)]

A solution of n-butyllithium (30 mL, 2.87 M inhexane, 0.086 mol) was added dropwise to astirred, cooled (�110 �C) solution of 4-bromoben-zonitrile (15 g, 0.082 mol) in dry THF (180 mL)under dry nitrogen. The solution was stirred atbelow �100 �C for 1 h, and a solution of trimethylborate 20.8 mL in dry THF (60 mL) was added atbelow �100 �C. The solution was allowed to warmto room temperature overnight. Ten percent hy-drochloric acid was added, and the solution wasstirred for 1 h at room temperature. The productwas extracted into ether, and the organic layerwas washed with water and dried with MgSO4.The solvent was removed in vacuo, and the crudeproduct was dissolved in THF and precipitatedwith n-hexane to give a yellow solid with an yieldof 70%. 2(OCH3), white solid, the synthetic routeis the same to 2(CN).

4-Hydroxy-40-cyanoterphenyl [3(CN)]

Under a dry nitrogen atmosphere, a solution of2.00 g of 4-cyanobenzeneboronic acid (13.6 mmol)in 10 mL of ethanol was added to a solution of2.75 g of 4-(40-bromophenyl)phenol (97%, 11.02mmol) and 0.42 g of tetrakis(triphenylphosphine)-palladium(0) (99%, 0.36 mmol) in 20 mL of ben-zene and 20 mL of aqueous Na2CO3 (2 M). Thereaction was conducted under reflux overnight.The reaction mixture was then shaken with ethylacetate and the insoluble parts were filtered off.The organic layer was dried with anhydrousMgSO4, and the solvent was removed by evapora-tion in vacuo. The crude product was recrystal-lized from acetone to provide a yellow crystal,65% yield. 3(OCH3), white crystal, the syntheticroute is the same to 3(CN).

HELICAL LIQUID CRYSTALLINE POLYMERS 4725


3(CN), IR (KBr, cm�1): 2215 (CBN), 3351(AOH). 1H NMR (ppm, CDCl3): 7.73–7.65 (m, aro-matic, 8H), 7.53 (d, aromatic, 2H), 6.93 (d, aro-matic, 2H ortho to hydroxyl), 4.91 (s, 1H, AOH).

3(OCH3), colorless crystal, IR (KBr, cm�1):3393, 2956, 2835, 1608, 1491, 1251, 1031, 815,644. 1H NMR (ppm, CDCl3): 7.60–7.51 (m, aro-matic, 8H), 7.00–6.99 (d, aromatic, 2H ortho tohydroxyl), 6.90, 6.93 (d, aromatic, 2H ortho toOCH3), 5.52 (s, 1H, AOH).

4-(6-Hydroxyhexyloxy)-40-cyanoterphenyl [4(CN)]

Mixing 20 mmol of [3(CN)], 24 mmol of 6-bromo-1-hexanol, 40 mmol of K2CO3, and 4.01 mmol ofKI in 200 mL of DMF, the reaction mixture wasrefluxed at 80 �C for 24 h. It was then cooled, andthe solvent was removed by evaporation in vacuo.The residue was recrystallized from absolute

ethanol to give a light yellow solid in 73% yield.4(OCH3), light yellow solid, the synthetic route isthe same to 4(CN).

4(CN), IR (KBr, cm�1): 3416, 3033, 2935, 2852,2228, 1600, 1491, 1258, 1047, 812. 1H NMR (ppm,THF-d): 7.64–7.77 (m, aromatic, 8H), 7.58–7.62(d, aromatic, 2H), 6.87–6.89 (d, aromatic, 2H),4.55 (t, 1H, AOH), 3.89–3.92 (t, 2H, ACH2OArA),3.36–3.39 (t, ACH2O OC, 2H), 1.33–1.72 (m, 8H,A(CH2)4).

4(OCH3), IR (KBr, cm�1): 3317, 2933, 2854,1602, 1491, 1250, 1017, 806.

2,5-Dibromo-1-[(6-(4-(40-cyano)terphenyloxy)hexyloxy)carbonyl]benzene [M(CN)]

2,5-Dibromobenzoic acid (3.36 g, 12 mmol) wasadded to a mixture of 4-(6-hydroxyhexyloxy)-40-cyanoterphenyl (3.71 g, 10 mmol), DMAP 1.47 g

Scheme 1. Illustration of procedures for synthesis of P(CN) and P(OCH3).

4726 CHEN ET AL.


(12 mmol), and DCC (2.46 g, 12 mmol) in 200 mLof absolute THF and further stirred for 24 h atroom temperature under an argon atmosphere.Then the solution was filtered to remove the ureacrystals, and the solvent was removed by evapora-tion. The crude product was purified by columnchromatography (n-hexane/CHCl3 ¼ 1/4) to affordM(CN) as white powder. Yield ¼ 70%. IR (KBr,cm�1): 2935, 2863, 2218, 1705, 1604, 1491, 816,719, 524. 1H NMR (ppm, CDCl3): 7.90 (s, aro-matic, 1H), 7.75–7.66 (m, aromatic, 10H), 7.58,7.56 (d, aromatic, 1H), 7.46, 7.45 (d, aromatic,1H), 7.00–6.98 (d, aromatic, 2H ortho to AOA),4.38–4.35 (t, 2H, ACH2AOAAr), 4.04–4.01 (t, 2H,ACH2AOOCA), 1.85–1.25 (m, 8H, ACH2(CH2)4CH2A).

2,5-Dibromo-1-[(6-(4-(40-methoxy)terphenyloxy)-hexyloxy)carbonyl]benzene [M(OCH3)]

M(OCH3), white powder, was prepared by 2,5-dibromobenzoic acid and 4-(6-hydroxyhexyloxy)-40-methoxyterphenyl 4(OCH3), and the syntheticroute is the same to M(CN). Yield: 68%. IR (KBr,cm�1): 2933, 2863, 1733, 1602, 1493, 1250, 1017,806, 718, 503. 1H NMR (ppm, CDCl3): 7.90 (s, aro-matic, 1H), 7.61–7.54 (m, aromatic, 10H), 7.53,7.51 (d, aromatic, 1H), 7.46, 7.45 (d, aromatic,1H), 7.006.96 (d, aromatic, 2H ortho to AOA),4.38–4.35 (t, 2H, ACH2AOAAr), 4.04–4.00 (t, 2H,ACH2AOOCA), 3.80 (s, 3H, AOCH3A), 1.87–1.25(m, 8H, ACH2(CH2)4CH2A).

Polymerization

All the polymerization reactions and manipula-tions were carried out under nitrogen usingSchlenk techniques in a vacuum line system or inan inert-atmosphere glovebox (Vacuum Atmos-pheres), except for the purification of the poly-mers, which was done in an open atmosphere. Atypical experimental procedure for the polymer-ization of P(OCH3) is given later.

A 50-mL three-necked round-bottom flaskequipped with condenser, rubber septum, nitro-gen inlet–outlet and magnetic stirrer was chargedunder nitrogen with 0.568 g (1.0 mmol)M(OCH3), 0.08 g (0.0304 mmol) PPh3, 2.024 g(30.96 mmol) Zn, 0.0076 g (0.05 mmol) bpy,0.0064 g (0.05 mmol) NiCl2, and 5 mL of drydimethyl acetamide (DMAC). The reaction wasperformed at 85 �C, under nitrogen. The mixturewas stirred for 24 h. Then, the polymer was pre-cipitated in excess methanol/HCl mixture, filtered

and dried. The polymer was then redissolved inTHF and precipitated in methanol. A gray solidwas obtained.

P(CN), gray solid: IR (KBr, cm�1): 2933, 2854,2218, 1713, 1592, 1481, 806. 1H NMR (ppm, THF-d): 8.00–7.90 (m, aromatic, 1H), 7.79–7.47 (m, aro-matic, 12H), 6.88–6.86 (d, aromatic, 2H ortho toAOA), 4.30–4.25 (t, 2H, ACH2AOAAr), 3.92–3.91(t, 2H, ACH2AOOCA), 1.71–1.18 (m, 8H,ACH2(CH2)4CH2A). Mn ¼ 21,979; Mw/Mn ¼ 1.74.

P(OCH3), gray solid: IR (KBr, cm�1): 2954,2863, 1733, 1613, 1511, 1259, 1038, 825. 1H NMR(ppm, DMSO-d6): 7.99 (s, aromatic, 1H), 7.75–7.52 (m, aromatic, 12H), 7.02 (d, aromatic, 2Hortho to AOA), 4.29 (t, 2H, ACH2AOAAr), 4.02 (t,2H, ACH2AOOCA), 3.80 (s, 3H, AOCH3A), 1.77–1.14 (m, 8H, ACH2(CH2)4CH2A). Mn ¼ 15,136;Mw/Mn ¼ 2.11.

RESULTS AND DISCUSSION

Synthesis of the Monomers

The synthetic routes of the monomers are shownin Scheme 1. The compound of 3(R) was synthe-sized through Suzuki reaction between the com-pound of 2(R) and 4-(4-bromophenyl) phenolusing tetrakis(triphenylphosphine)palladium(0)as the catalyst. The compound of 4(R) was pre-pared by 6-bromo-1-hexanol with 4-hydroxy-40-cyanoterphenyl and 4-hydroxy-40-methoxyter-phenyl via etherification, respectively. Finally, themonomers were synthesized through esterifica-tion reaction route in the presence of 1,3-DCCand DMAP. The reactions went smoothly, and theproducts were isolated in high yields near 70% af-ter purifications by silica gel chromatography fol-lowed by recrystallization. All the intermediateand final products were thoroughly purified andfully characterized, and satisfactory analysis datawere obtained (detailed spectroscopic data for thekey intermediates and for all the monomers beinggiven in the ‘‘Experimental’’ section).

Synthesis of the Polymers

Many functionalized regioregular conjugatedpolymers show fascinating properties such ashigh conductivity, mobility, chemosensitivity, liq-uid crystallinity, or chirality.31 Nickel-complexpolycondensation (Yamamoto reaction) is the mostpopular methodology for the direct synthesis ofregioregular PPPs, due largely to their preserva-tion of regiochemistry and nearly quantitative



yields. Furthermore, it also can tolerate the func-tional groups and offers the polymers with goodsolubility and excellent processing. Taking theadvantages into consideration, we chose Ni-basedcomplex as the catalyst to carry out the polymer-ization in the DMAC solution via dehalogenativepolycondensation. Both of the polymers synthe-sized are fusible and soluble in common organicsolvents including THF, DMAC, DMF, and soforth. The chemical structures of the polymers areconfirmed by FTIR and 1H NMR.

Structural Characterization

All the purified monomers and polymerizationproducts gave satisfactory spectroscopic data cor-responding to their expected molecular structures(see ‘‘Experimental’’ section for details). A typicalexample of the IR spectrum of P(OCH3) is shownin Figure 1. For comparison, the spectrum of itsmonomer M(OCH3) is also given. The C¼¼O andArAO stretching vibrations of M(OCH3) arelocated at 1733 and 1250 cm�1, respectively. TheCAH stretching and bending vibrations ofA(CH2)6A are obviously observed at 2933–2863and 718 cm�1. The CABr bending vibration at�500 cm�1 is also a distinct peak in the both spec-tra of the monomers. Besides, sharp absorptionfrom CBN stretching can be observed at 2218cm�1 in the M(CN) spectrum. The spectra of poly-mers are similar to their corresponding mono-mers, except the CABr bending vibrations absorp-tion bands of monomers disappearing completelyin the spectra of polymers, which indicates thatthe Ni-based complex catalyst is effective for poly-merization of PPPs, and the monomers have beenconverted into polymeric products successfully byYamamoto dehalogenative polycondensation. The1H NMR spectra further confirm the structures ofpolymers and monomers. The aromatic H reso-nance peaks are located at the region of low mag-netic field, while the aliphatic H resonance peaksremain at higher magnetic field. The resonancepeaks in the 1H NMR spectra of polymers arebroader than that of monomers, which meansthat the monomers are polymerized successfully.Except the peaks of solvent and water remainedin the spectrum, no unexpected signals areobserved in the spectrum of the polymer, and allthe resonance peaks can be assigned to appropri-ate protons as marked in Figure 2. The 1H NMRspectra as well as the IR analyses confirm thatthe CABr bond has been consumed by the poly-merization reaction and that the molecular struc-

tures of the polymeric products are indeed poly-mers.

Thermal Stability and Liquid Crystallinity

All our polymers exhibit good thermal stability.As shown in Figure 3, P(OCH3) and P(CN)decompose at a temperature as high as �340 �Cin the TGA, regardless of terminal groups. Thus,the incorporation of the rigid terphenyl onto mainchain can enhance thermal stability of these poly-mers. The backbone of the polymers may havebeen surrounded by a rigid jacket formed by theside chains, shielding the polymer main chainsfrom thermal attack.

All the monomers are white crystals at roomtemperature and exhibit liquid crystallinity atelevated temperatures. The thermal transitionbehaviors of the monomers and polymers areexamined by DSC and polarized optical micro-scopy (POM). The POM textures of the monomersare displayed in Figure 4. M(CN) and M(OCH3)form anisotropic melts with a birefringent texturewhen cooled from their isotropic state, which indi-cated the thermotropic LC behavior of thesemonomers. The focal conic fan texture indicatesthat the mesophasic natures of monomers aresmectic phases. Reheating the monomers regener-ates the birefringent textures, that is, the meso-morphism is enantiotropic. The polymers P(CN)and P(OCH3) readily exhibit bright colorfulmesogenic textures, which does not disappearuntil they enter into anisotropic melts, because

Figure 1. FTIR spectra of the monomer M(OCH3)and polymer P(OCH3). [Color figure can be viewedin the online issue, which is available at www.interscience.wiley.com.]

4728 CHEN ET AL.


the ACOO(CH2)6OA flexible spacers between theterphenyl mesogenic pendants and the poly-(p-phenyl)s backbone favor the mesogens toundergo thermal transitions in a relatively inde-pendent fashion. With the aid of XRD measure-ments, the mesomorphic texture of P(CN) isSmAd (discussed later). Different from the P(CN),P(OCH3) possesses two different mesophase. Itforms a nematic phase with a typical droplet tex-ture first, before the emergence of the SmAd

phase with fan texture from its anisotropic melts.Reheating P(OCH3) regenerates the SmAd andnematic phase textures in sequence. Those alsohave been observed by Lam and Tang that thepolymer with polar cyano tails undergoes anenantiotropic smectic transition, but its counter-part with less polar methoxy tails goes throughenantiotropic smectic phases and nematic transi-tions.32

The thermal transitions of P(CN) andP(OCH3) and their corresponding monomers are

Figure 2. 1H NMR spectra of monomers and polymers.

Figure 3. TGA thermograms of polymers undernitrogen at a heating rate of 20 �C/min. [Color figurecan be viewed in the online issue, which is availableat www.interscience.wiley.com.]



shown in Figure 5. In the case of monomerM(OCH3), upon cooling from its isotropic melt, aSmA phase with a focal-conic fan texture isobserved at 190.5 �C, Finally, monomer M(OCH3)changes into crystal at 144.8 �C. The DSC ther-mogram recorded in the second heating scan alsoexhibits two peaks at 149.2 and 195.5 �C, a focal-

conic fan texture corresponding to a smectic Aphase emerges. With comparison to M(OCH3),M(CN) enters into SmAd phase at lower tempera-ture, in agreement with our previous observationthat ACN terminal group endows the polymerwith the lower melting temperature than AOCH3

terminal group.33 The DSC thermogram of

Figure 4. Mesomorphic textures observed on cooling: (a) M(OCH3) to 190 �C, (b)M(CN) to 116 �C, (c) P(CN) to 220 �C, (d) P(OCH3) to 240 �C, and (e) P(OCH3) to271 �C from their isotropic melts.

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M(CN) shows two transition peaks at 90.9 and130.4 �C in the second heating cycle, associatedwith the k-SmAd and SmAd-i transitions, respec-tively. Its corresponding SmAd-k transition isdetected at 86.5 �C. The i-SmAd transition is notshown in the cooling scan, probably due to thefast cooling rate, but can be noticed at 122 �C byPOM. In agreement with POM, on heating ofP(OCH3) at a rate of 10 �C/min�1, all P(OCH3)possess three peaks at 238.0, 276.8, and 284.3 �Cin the second heating scan. It enters the SmAd

mesophase from its solid state at 238.0 �C. Themesophase is stable in a temperature range above38.8 �C before P(OCH3) melts into its nematicphase. The mesophase in this temperature range

(276.8–284.3 �C) is identified to be nematic phase.Because of the high degree of polymerization(DP ¼ 31) and the high isotropization temperature(Ti), the type of nematic phase may correspond toWarner’s NII phase, where the nematic director isdetermined by the backbone.17,34 Associated withits monomer, P(CN) shows the lower tempera-tures of the g-SmAd and SmAd-i transitions at185.8 and 220.3 �C than P(OCH3), respectively.The first cooling curves of the polymers are similarand there exist only broad exothermic bump peaksat 233.0 and 212.4 �C. The SmAd phases are thusenantiotropic. From the DSC curves, no clear glasstransition is observed, because of excellent crystal-lization property of the polymers, which is alsoidentified by XRD (discussed later).

We carried out XRD experiments to gain moreinformation on the mesomophic structures of thepolymers and on the molecular packing arrange-ments within the mesomorphic phases. WAXDpatterns of all the monomers and polymers wereobtained at room temperature after the sampleshad been quenched from LC states with liquidnitrogen. The diffractogram of a powder samplecan be generally divided into low-angle Braggreflections corresponding to the layer spacing ofmolecular orientational order and the high-anglepeaks associated with the liquid-like intermeso-genic organization within the layers. The appear-ance of a broad or sharp peak serves as a qualita-tive indication of the degree of order.35 The WAXDpattern of M(OCH3) shows a sharp reflection at2y ¼ 2.93� (d-spacing d ¼ 30.17 A) (shown in Fig.6). The calculated length of one repeat unit ofM(OCH3) is 29.51 A, which confirms the SmA na-ture of the mesophase and suggests that themesogens are packed in a monolayer structure.The XRD diffractogram of M(CN) displays Braggreflections at low and high angles. The sharpreflection at the low angle (2h ¼ 2.2�) correspondsto a layer spacing of 40.08 A, which is longer thanthe molecular length (28.40 A). Because the d1/lratio is �1.40, the bilayer structure is thus ofSmAd type, in which the cycanoterphenyl meso-gen are interdigitated in an antiparallel fashion.Similar to its monomer, P(CN) also exhibits asmectic phase with good packing arrangement.The sharp reflection at 2y ¼ 2.05� corresponds toa layer spacing of 42.50 A, which is considerablyin excess of the molecular length of the repeatunit of P(CN) in its most extended conformation(l ¼ 27.14 A), giving a d/L ratio of �1.50, a value(about 1.4) often found in the SmAd LC mole-cules containing cyano terminal groups.36 The

Figure 5. DSC thermograms of the monomers andpolymers recorded under nitrogen during (a) the sec-ond heating and (b) first cooling scans at a scan rateof 10 �C/min.



mesophase of the polymer thus may also consistof a bilayer structure, with the mesogens arrang-ing in an antiparallel overlapping interdigitatedmanner, Because the [(cyanoterphenylyl)oxy]mesogen groups of the polymer are polarized tominimize the electronic repulsive interactions,partial interdigitation of the polarized mesogensis required.37 P(OCH3) shows not only the peakin the high angle region but also Bragg reflectionsat middle and low angles. P(OCH3) shows asharp reflection at a low angle of 2y ¼ 2.01�, corre-sponding to a layer spacing of 44.14 A, which is in

considerable excess of the molecular length of therepeat unit of P(OCH3) in its most extended con-formation (l ¼ 28.23 A). Different from the mono-layer arrangement of its monomer, a SmAd phasewith bilayer structure may form in the polymerdue to giving a d/L ratio of �1.5 (schematicallyshown in Fig. 7). The peak located at 2y ¼ 19.84�

(d ¼ 4.46 A) corresponds to the interside chaindistance, that is, the distance between the meso-genic core of the side chains. The mesogens of thepolymers are packing so well that the high-ordersecondary reflection at a middle angle at 5–6� isreadily detected by the diffractometer, from whicha d spacing of about 15.00–16.00 A, which is asso-ciated to the distance of the terphenyl mesogen.Because of excellent crystallization property ofthe polymers, it is difficult to freeze the samplecompletely in the LC phase, and it always par-tially contains crystalline state, even quenching itas soon as possible. Thus, the XRDs of both poly-mers show some crystalline peaks from 2y ¼ 20–30�.

Electronic Absorption and PL

The UV and PL spectra of M(OCH3), M(CN),P(OCH3), and P(CN) in THF are given in Fig-ures 8 and 9, respectively. With the existence ofthe terphenyl chromophore, M(OCH3) andM(CN) can absorb strongly UV light at 311 and322 nm, respectively. The absorptions of themonomers are assignable to the p-p* bands of theterphenyl mesogenic pendants. P(OCH3) andP(CN) have the similar absorption bands at about310 nm to the monomers, indicating that the

Figure 6. X-ray diffraction patterns of the mono-mers and polymers quenched from their liquid crys-talline states. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

Figure 7. Proposed bilayer packing arrangement of P(OCH3) within the SmAd

layer with the smectogens interdigitating in antiparallel fashion.

4732 CHEN ET AL.


strong absorptions originate from the mesogenicpendants. Different from M(OCH3) and M(CN),both of the polymers absorb the UV light wellextending to beyond 500 nm. The absorption inthe long-wavelength visible spectral region isthus obviously from the PPP backbone of the poly-mers. Absorption also can be understood by thestructural difference of the two mesogenic chro-mophores. In fact, the terphenyl chromophore ofP(CN) is well polarized by the push–pull interac-tion of the electron-donating AOA and the elec-tron-accepting cyano terminal group. On theother hand, in P(OCH3), both the AOA andmethoxy groups are electron donating, which stillpolarizes the terphenyl chromophore but to alesser extent,4 thus, the absorption of P(CN) isabout 5 nm redshifted with higher intensity thanthat of P(OCH3).

Since P(OCH3)and P(CN) are PPP derivativesbearing chromophoric pendant groups, it is of in-terest to check the effects of the structural varia-bles on their luminescence behaviors. Owing tothe chromophoric property of the terphenyl meso-gens, when the polymers were photoexcited at320 nm, the two strong light-emitting bands at388 and 408 nm observed for a THF solution ofP(OCH3) are assigned to the emitting center ofthe terphenyl mesogenic core and that of the con-jugated main chain, respectively. On the otherhand, energy transfer from terphenyl mesogeniccore to the backbone and the segregation of thebackbone by the flexible spacers also favor stron-ger emission in the PL of PPPs. Compared with

P(OCH3), the light-emitting bands of polymerP(CN) is slightly redshifted to 428 nm, and theemission intensity of P(CN) is much stronger(shown in Fig. 9). It is in agreement with Lamand Tang’s observations that the polymers withdonor–acceptor pairs luminescent more stronglythan those without such push–pull pairs.32 Thephotographs of blue fluorescence of the polymersin THF solution, excited by irradiation of UV lightof 320 nm, are shown in Figure 10, and pure THFis also shown in the same figure for comparison.

Secondary Structure of the Polymers

Synthetic helical polymers with p-conjugationalong the main chains is under hot pursuits inrecent years because of the challenge they offer inpolymer chemistry as well as their wide practicaland potential applications, such as optical polariz-ing films, chiral stationary phases, asymmetricelectrodes, anisotropic molecular wires, and fluo-rescent chemosensors.38 Generally, helical conju-gated polymers are obtained by the introductionof chiral substituents, polymerization using a chi-ral catalyst system, or preparation in a chiral LCsolvent, a new method reported by Goto.39

As we all know, in LCCP, the main chain canbe aligned and become more coplanar by virtue ofspontaneous orientation of the LC side chain. Butif the bulky mesogens as pendants are linked tothe main chain, reducing the repellent from stericcrowdedness, the mesogenic pendants may likelyorientating around their backbone and force the

Figure 8. UV spectra of the monomers and poly-mers in THF solutions (0.05 mM). [Color figure canbe viewed in the online issue, which is available atwww.interscience.wiley.com.]

Figure 9. Photoluminescence spectra of polymers inTHF solutions (0.05 mM). Excitation wavelength: 320nm. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com.]



backbone with the spiral direction in the longregion. Thus, CD spectroscopic analysis of the pol-ymers is conducted to check our assumption. Fig-ure 11 depicts the CD spectra of P(OCH3) andP(CN) measured in THF. P(OCH3) shows strongCotton effects at 278, 283, 289, 293, and 315 nm,while P(CN) exhibits strong Cotton effects at 284,293, and 313 nm, which are located in their UVabsorption region. This suggests that P(OCH3)and P(CN) take a regulated higher-order struc-ture: a helix with an excess of preferred handed-ness. There are no chiral groups or center in thestructures of the polymers; thus, the helical con-

formation with a preferred screw sense shouldresult from the PPP main chain. To further sup-port the conclusion, the molecular geometries ofpolymers are simulated by computer, and that ofP(CN) is shown in Figure 12, where the obvioushelical conformation of the main chain becomes afavorable arrangement.

CONCLUSIONS

In this work, we have synthesized new terphenylchromophore-containing PPPs with different ter-minal groups and the thermal, mesogenic, andluminescent properties of PPPs are investigated.The polymers are prepared successfully by Ni-based catalysts via Yamamoto polycondensation.All the polymers are thermally very stable,thanks to the protective jacket effect contributedby their mesogenic pendants. With the favor offlexible spacers, the polymers readily form LCmesophase. PPPs with CN terminal group displayenantiotropic SmAd with a bilayer packingarrangement, whereas the one with AOCH3 ter-minal substituent exhibits enantiotropic nematic-ity and smecticity (SmAd) with a bilayer arrange-ment when cooled and heated. Both of the poly-mers induced strong blue light emission byphotoexcitation. The photoelectricity is influencedby the electron-withdrawing nature of the termi-nal substituent and the existence of donor–acceptor pairs, thus, P(CN) possesses a strongerelectronic absorption and PL than the P(OCH3)with AOCH3 terminal substituent. A significantand interesting finding also can be observed inthis type of polymers: the bulky terphenyl meso-gen exerts great influence on the conformation ofthe skeleton. The strong Cotton effects in the CDspectra indicate that the backbones of the poly-mers are induced to form a preferred screw sensein the long region by the mesogen orientatingaround them. It is probably a good beginning toobtain novel optically active helical LCCPs with-out introducing any chiral groups. This encour-aged us to extend our investigations to find out

Figure 10. The photo graphs of blue fluorescence ofthe polymers in THF solution: (a) P(CN), (b)P(OCH3), and (c) pure THF, excited by irradiation ofUV light of 320 nm. [Color figure can be viewed inthe online issue, which is available at www.interscience.wiley.com.]

Figure 11. The CD spectra of P(OCH3) and P(CN)measured in THF (c ¼ 0.25 mM). [Color figure can beviewed in the online issue, which is available atwww.interscience.wiley.com.]

Figure 12. Computer-generated representation ofspiral direction of P(CN) with about 30 repeat units.[Color figure can be viewed in the online issue, whichis available at www.interscience.wiley.com.]

4734 CHEN ET AL.


the relationship between the structures and prop-erties.

Financial support for this work was provided by theNational Natural Science Foundation of China(50773029), the Natural Science Foundation of JiangxiProvince (2007GZC1727 and 2008GQH0046), JiangxiProvincial Department of Education, the Program forNew Century Excellent Talents in University (NCET-06-0574), and Program for Innovative Research Team ofNanchangUniversity, Program for Innovative ResearchTeam in University of Jiangxi Province, and Programfor Changjiang Scholars and Innovative Research TeaminUniversity (IRT0730).

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Documents

A novel type of optically active helical liquid crystalline polymers: Synthesis and characterization of poly(p-phenylene)s containing terphenyl mesogen with different terminal groups