synthesis and properties of soluble, strongly fluorescent poly(m-phenylene)s with pendant...

Macromol. Chem. Phys. 2001, 202, 2367–2376 2367

Synthesis and Properties of Soluble, StronglyFluorescent Poly(m-Phenylene)s with Pendant Stilbene-Based Chromophores

John A. Mikroyannidis

Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece

Introductionp-Conjugated polymers have attracted a major interest ofresearchers due to their electrical and optical properties.[1]

In particular, poly(p-phenylenevinylene)s (PPVs) displaygood electrical as well as photoluminescence[2] and arewidely used for fabrication of light emitting diodes(LEDs).[2–4] Since the first report on electroluminescencefrom PPV,[2 a] a considerable progress has been made onthe development of polymeric LEDs and especially ontheir color tuning,[5] device efficiency,[6] and stability.[7]

Numerous PPV polymers have been recently synthesizedfor applying them in LEDs. Their synthesis has been car-ried out either by the method of polymer precursor or bypolycondensation via a Witting or Heck reaction.

On the other hand, significant attention has been direct-ed toward the synthesis of poly(p-phenylene)s (PPPs) andtheir derivatives. Their good thermal and chemical stabi-lity,[8] electrical conductivity following doping,[9] and

optical properties[10] make these rigid-rod polymersattractive candidates for scientific and industrial applica-tions. The halogenative polycondensation of various diha-loaromatic compounds using nickel complex is useful forpreparing p-conjugated polymers and particularly PPPs.

Generally, the main objective of the present investiga-tion was the synthesis through pyrylium salts of a newclass of soluble, strongly fluorescent polymers using asimple and convenient method. The properties of thesepolymers were investigated and correlated with their che-mical structure. Our interest was focused on their opticalproperties both in solution and in solid state. The depend-ence of the photoluminescence maximum and the quan-tum yield from the polymer chemical structure was inves-tigated in detail. The present polymers are potential can-didates for optical applications.

More particularly, in this contribution, we describe thesynthesis and characterization of a new class of process-

Full Paper: The nickel-catalyzed dehalogenative cou-pling of substituted m-dichlorobenzenes afforded a newseries of poly(m-phenylene)s with pendant 2,6-bisstil-benyl-N-alkylpyridinium tetrafluoroborate groups. Char-acterization of polymers was accomplished by viscosime-try, GPC, FT-IR, NMR, X-ray, differential scanningcalorimetry, thermomechanical analysis, UV-vis, andluminescence spectroscopy. The polymers were amor-phous and showed an excellent solubility being readilysoluble at room temperature in THF, chloroform, andchlorobenzene. Their Tg values ranged from 96 to 1488C.The polymers with pendant stilbene-based chromophoreswere strongly fluorescent both in solution and in solidstate. Their photoluminescence (PL) spectra showed max-ima at 391–410 nm and 445–532 nm in solution and inthin film, respectively. The structure of the alkyl attachedto the pyridinium nitrogen influenced remarkably the PLquantum yield of polymers. The most efficient fluorescentpolymers obtained were those bearing long chain aliphaticgroup on this nitrogen. Their PL quantum yield in THFsolution was up to 0.65.

Macromol. Chem. Phys. 2001, 202, No. 11 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1352/2001/1107–2367$17.50+.50/0

2368 J. A. Mikroyannidis

able poly(m-phenylene)s with pendant stilbene-basedchromophores. 2,4-Dichlorobenzaldehyde, a widely usedand inexpensive compound, was used as starting materialfor the preparation of polymers. It reacted with substi-tuted acetophenone in the presence of BF3 Et2O to yieldpyrylium salts.[11–13] They were converted either to thecorresponding aromatic compounds or to N-alkyl pyridi-nium salts.[14] The monomer thus taken, was a substitutedderivative of m-dichlorobenzene that was polymerized bythe nickel catalyzed coupling.

The present polymers behaved like intrinsically photo-luminescent materials, that carried two stilbene-basedchromophores attached as side groups to each monomericunit. Most polymers were ionic with pendant of 2,6-bis-stilbenyl-N-alkylpyridinium tetrafluoroborate groups. Inorder to enhance the solubility of the resulting polymers,the quaternization of the pyridinium nitrogen was carriedout by reacting the pyrylium salt with a long chain alipha-tic amine. In addition, cyclohexylamine and aniline wereused for quaternization to compare the polymer proper-ties. It is well known that the quaternization of the ringnitrogen lowers the energy levels of HOMO and LUMO,thus affecting the optical properties of polymers.[15, 16]

Experimental Part

Characterization Methods

Melting temperatures were determined on an electrothermalmelting point apparatus IA6304 and are uncorrected. IRspectra were recorded on a Perkin-Elmer 16PC FT-IR spec-trometer with KBr pellets. The 1H NMR (400 MHz) and 13CNMR (100 MHz) spectra were obtained using a Bruker spec-trometer with DMSO-d6 or CDCl3 as solvent. Chemical shifts(d values) are given in parts per million with tetramethylsi-lane as an internal standard. UV-vis spectra were recordedon a Varian Cary 1E spectrometer. The emission spectrawere obtained with a Perkin Elmer LS55B luminescencespectrometer. DSC and TGA were performed on a DuPont990 thermal analyzer system. The DSC thermograms wereobtained at a heating rate of 108C/min in N2 atmosphere at aflow rate of 60 cm3/min. Dynamic TGA measurements weremade at a heating rate of 208C/min in atmospheres of N2 orair at a flow rate of 60 cm3/min. Thermomechanical analysis(TMA) was recorded on a DuPont 943 TMA using a loadedpenetration probe at a scan rate of 108C/min in N2 with aflow rate of 60 cm3/min. The TMA experiments were con-ducted at least in duplicate to assure the accuracy of theresults. The TMA specimens were pellets of 8 mm diameterand 2 mm thickness prepared by pressing powder of polymerfor 3 min under 5–7 kpsi at ambient temperature. The inher-ent viscosities of polymers were determined for solutions of0.5 g/100 mL in CHCl3 at 308C using an Ubbelohde sus-pended level viscometer. Elemental analyses were carriedout with a Hewlett-Packard model 185 analyzer. The wide-angle X-ray diffraction patterns were obtained for powderspecimens on a X-ray PW-1840 Philips diffractometer. GPCanalysis was conducted with an apparatus equipped with a

2410 differential refractometer as detector (Waters Associ-ate), and Styragel HR columns using polystyrene as standardand THF as eluent.

To measure the fluorescence quantum yields, a degassedsolution of polymer in THF was prepared. The concentrationwas adjusted so that the absorbance of the solution would belower than 0.1. The exciting wavelength was 320 nm and asolution in 1n H2SO4 of quinine sulfate, which has a quantumyield of 0.546, was used as standard.

Reagents and Solvents

4-Acetylstilbene was synthesized according to a reportedmethod from a palladium catalyzed Heck coupling betweenstyrene and 49-bromoacetophenone with palladium (II) acet-ate and tris(2-tolyl)phosphane as catalyst and cocatalyst,respectively, in triethylamine.[17] It was recrystallized fromethanol 95%. 2,4-Dichlorobenzaldehyde and 49-phenylaceto-phenone were recrystallized from ethanol 95%. Hexadecyl-amine was recrystallized from benzene.

Preparation of Monomers

2,6-Bis(4-biphenylyl)-4-(2,4-dichlorophenyl)pyryliumtetrafluoroborate (1a)

A flask was charged with a mixture of 2,4-dichlorobenzalde-hyde (1.00 g, 5.70 mmol), 49-phenylacetophenone (2.21 g,1.40 mmol), and 1,2-dichloroethane (25 mL). Boron trifluor-ide etherate (1.8 mL, 14.25 mmol) was added portion wise tothe stirred mixture at room temperature, and it was refluxedfor 5 h under N2. The reaction mixture was concentratedunder reduced pressure, and ether was added to the concen-trate. The dark red precipitate was filtered, washed withether, then with water, and dried to afford 1a. It was recrys-tallized from a mixture of THF/ether (1 :1 vol.-%) (1.10 g,yield 31%, m.p. 253–2558C).

IR (KBr): 1620, 1602, 1486 (aromatic and pyrylium struc-ture), 1084 cm–1 (BF4

–).1H NMR (DMSO-d6): d = 8.74 (s, 2H, aromatic meta to

O+), 7.84–7.29 (m, 21H, other aromatic).

2,6-Bisstilbenyl-4-(2,4-dichlorophenyl)pyryliumtetrafluoroborate (1b)

Compound 1b was prepared as a dark purple solid by react-ing 2,4-dichlorobenzaldehyde (0.88 g, 5.00 mmol) with 4-acetylstilbene (2.22 g, 10.00 mol) and boron trifluorideetherate (1.6 mL, 12.50 mmol) according to the procedureapplied for 1a. It was recrystallized from a mixture of 1,4-dioxane/ether (1 :3 vol.-%) (2.64 g, yield 79%, m.p. 167–1698C).

IR (KBr): 1618, 1598, 1474 (aromatic and pyrylium struc-ture), 1080 (BF4

–); 962 cm–1 (HC2CH trans).1H NMR (CDCl3): d = 8.17–8.07 (m, 2H, aromatic meta

to O+), 7.62–6.88 (m, 21H, aromatic and 4H, HC2CH).

1,3-Bis(4-biphenylyl)-5-(2,4-dichlorophenyl)benzene (2a)

A mixture of 1a (3.52 g, 5.70 mmol), fused CH3COONa(0.94 g, 11.40 mmol) and acetic anhydride (11 mL) wasrefluxed overnight. It was concentrated under reduced pres-

Synthesis and Properties of Soluble, Strongly Fluorescent ... 2369

sure and the concentrate was poured into water and stirred atroom temperature. The red-brown precipitate was filtered,washed with water, and dried to afford 2a. It was recrystal-lized from a mixture of chloroform/ether (1 :2 vol.-%)(2.76 g, yield 92%, m.p. 108–1108C).

IR (KBr): 3026, 1602, 1486, 1050, 696 cm–1 (aromatic).1H NMR (CDCl3): d = 7.68–7.28 (m, aromatic).C32H24Cl2 (479.4): Calcd. C 80.17, H 5.04; Found C 79.65,

H 5.12.

1,3-Bisstilbenyl-5-(2,4-dichlorophenyl)benzene (2b)

In a method similar to 2a, compound 2b was prepared as abrown solid from 1b (2.60 g, 3.88 mmol), fused CH3COONa(0.64 g, 7.76 mmol) and acetic anhydride (8 mL). It wasrecrystallized from a mixture of 1,4-dioxane/water (2 :1 vol.-%) (2.11 g, yield 94%, m.p. 149–1518C).

IR (KBr): 3024, 1600, 1470, 1048, 698 (aromatic), 960cm–1 (HC2CH trans).

1H NMR (DMSO-d6): d = 7.71–7.29 (m, 24H, aromaticand 4H, HC2CH).

C40H28Cl2 (579.5): Calcd. C 82.89, H 4.87; Found C 82.16,H 4.95.

2,6-Bisstilbenyl-4-(2,4-dichlorophenyl)-N-butylpyridiniumtetrafluoroborate (4a)

A mixture of 1b (2.54 g, 3.79 mmol), butylamine (0.27 g,3.79 mmol) and DMAc (15 mL) was heated at 1408C for15 h under N2. The solution was subsequently concentratedunder reduced pressure, and water was added to the concen-trate. The yellow-brown precipitate was filtered, washedwith water, and dried to afford 4a. It was recrystallized froma mixture of chloroform/ether (1 :1 vol.-%) (2.20 g, yield99%, m.p. 133–1358C).

IR (KBr): 2926, 2854 (CH3 and CH2 stretching), 1680,1600, 1472 (aromatic and pyridinium structure), 1050(BF4

–), 962 cm–1 (HC2CH trans).1H NMR (CDCl3): d = 8.22 (s, 2H, aromatic meta to N+),

7.57–7.23 (m, 21H, other aromatic and 4H, HC2CH), 4.35(m, 2H, N+CH2), 1.50 (m, 4H, N+CH2(CH2)2), 0.95 (m, 3H,N+(CH2)3CH3).

C43H36Cl2NBF4 (724.5): Calcd. C 71.29, H 5.01, N 1.93;Found C 70.83, H 5.12, N 1.85.

2,6-Bisstilbenyl-4-(2,4-dichlorophenyl)-N-hexadecyl-pyridinium tetrafluoroborate (4b)

Compound 4b was prepared as a yellow-brown solid fromthe reaction of 1b (1.90 g, 2.84 mmol) with hexadecylamine(0.68 g, 2.84 mmol) in DMAc (15 mL) according to themethod applied for 4a. It was recrystallized from a mixtureof chloroform/ether (1 :1 vol.-%) (2.46 g, yield 97%, m.p.141–1438C).

IR (KBr): 2924, 2852 (CH3 and CH2 stretching), 1639,1600, 1470 (aromatic and pyridinium structure), 1084(BF4

–), 962 cm–1 (HC2CH trans).1H NMR (CDCl3): d = 8.23 (s, 2H, aromatic meta to N+),

7.66–7.22 (m, 21H, aromatic and 4H, HC2CH), 4.33 (m,2H, N+CH2), 1.28 (m, 28H, N+CH2(CH2)14), 0.91 (s, 3H,N+(CH2)15CH3).


2,6-Bisstilbenyl-4-(2,4-dichlorophenyl)-N-phenylpyridiniumtetrafluoroborate (4c)

Compound 4c was prepared as a yellow-brown solid fromthe reaction of 1b (2.00 g, 2.98 mmol) with aniline (0.28 g,2.98 mmol) in DMAc (15 mL) according to the methodapplied for 4a. It was recrystallized from a mixture ofchloroform/ether (1:1 vol.-%) (2.20 g, yield 99%, m.p.133–1358C).

IR (KBr): 3026, 1676, 1600, 1542, 1496 (aromatic andpyridinium structure); 1084 (BF4

–), 962 cm–1 (HC2CHtrans).

1H NMR (CDCl3): d = 7.96 (s, 2H, aromatic meta to N+),7.54–7.10 (m, 26H, other aromatic and 4H, HC2CH).


2,6-Bisstilbenyl-4-(2,4-dichlorophenyl)-N-cyclohexyl-pyridinium tetrafluoroborate (4d)

Compound 4d was prepared as a yellow solid from the reac-tion of 1b (2.70 g, 4.03 mmol) with cyclohexylamine(0.40 g, 4.03 mmol) in MDAc (12 mL) according to themethod applied for 4a. It was recrystallized from a mixtureof chloroform/ether (1 :2 vol.-%) (2.80 g, yield 93%, m.p.140–1428C).

IR (KBr): 2926, 2850 (CH2 stretching), 3026, 1680,1651, 1540, 1512, 1470 (aromatic and pyridinium struc-ture), 1104 (BF4

–), 962 cm–1 (HC2CH trans).1H NMR (CDCl3): d = 8.24–8.22 (d, 2H, aromatic meta

to N+), 7.68–7.14 (m, 21H, other aromatic and 4H,HC2CH), 4.30 (m, 1H, aliphatic N+CH), 1.75–1.05 (m,10H, other aliphatic).


Preparation of Polymers

The synthesis of 3b is given as a typical example for the pre-paration of polymers. A flask was charged with a mixture oftriphenylphosphine (0.40 g, 3.04 mmol), 2,29-bipyridine(0.12 g, 0.76 mmol), NiCl2 (0.10 g, 0.76 mmol), Zn powder(2.28 g, 34.91 mmol) and DMAc (40 mL). Compound 2b(4.40 g, 7.59 mmol) was added to the mixture, and it wasstirred and refluxed under N2 for 24 h. The mixture was sub-sequently concentrated under reduced pressure, and watercontaining hydrochloric acid 5% by volume was added tothe residue. After stirring for 1 h the precipitate was filtered,washed with water, and dried. Purification of the crude prod-uct was accomplished by the following procedure. The solidwas dissolved in boiling 1,4-dioxane (50 mL) and the solu-tion obtained was centrifuged to remove inorganic materials.The solution was subsequently concentrated under reducedpressure and the residue was triturated by ether. The solidwas filtered, washed thoroughly with ether and dried toafford 3b (3.55 g, yield 92%).

The same method was applied for the preparation and pur-ification of other polymers. The reaction yields, the inherent


viscosities, the number-average molecular weights, and theelemental analyses for all polymers are listed in Table 1.

Results and Discussion

Synthesis of Monomers and Polymers

Scheme 1 outlines the synthesis of substituted m-di-chlorobenzenes and their polymerization by Ni(0) cata-lyzed dehalogenative coupling. Specifically, 2,4-di-chlorobenzaldehyde reacted with 49-phenylacetophenoneor 4-acetylstilbene in the presence of boron trifluorideetherate to yield pyrylium salts 1. The reaction of oneequivalent of substituted benzaldehydes with two equiva-lents of substituted acetophenones mediated by boron tri-fluoride etherate in benzene or other solvents has beenreported for the synthesis of a large number of 2,4,6-triar-ylpyrylium salts.[18] The pyrylium salts 1 reacted withacetic anhydride/sodium acetate to afford compounds 2.It is well known that the reaction of 2,4,6-triarylpyryliumsalts with phenylacetic acid anhydrides, generated in situfrom phenylacetic acid sodium salt and excess of aceticanhydride, affords 1,3,5-triarylbenzene.[19] Finally, thenickel catalyzed polymerization of compounds 2 yieldedsubstituted poly(m-phenylene)s 3. The method utilized anickel coupling system that had resulted from a catalyticamount of nickel chloride and triphenylphosphine with anexcess of reduced metal like zinc.[20] The use of 2,29-bipyridine accelerated the coupling reaction.[20 In addi-tion, 2,29-bipyridine suppressed effectively the principalside reaction of phenyl transfer from triphenylphosphine,which would limit the polymer molecular weight.[20]

The preparation of poly(m-phenylene)s carrying lateralsubstituents of 2,6-bisstilbenylpyridinium salt is outlinedin Scheme 2. More particularly, pyrylium salt 1b reactedwith an equimolar amount of butylamine, hexadecyl-

amine, aniline or cyclohexylamine to yield the corre-sponding pyridinium salts 4. Since the latter containedsubstituted m-dichlorobenzene ring, their coupling by themethod described above, afforded polymers 5.

The polymerization reactions were proceeded in homo-genous state and no precipitation of the resulting poly-mers was observed. The polymers were purified from theinorganic materials, that were used as coupling reagents,by dissolving them in boiling p-dioxane and centrifuga-tion. The solution of polymer was subsequently concen-trated and the solid thus obtained was thoroughly tritu-rated by ether to remove triphenylphosphine and 2,29-bipyridine. The yields of the preparation reactions werehigh (75–98%) for all polymers and their inherent viscos-ities ranged from 0.16 to 0.58 dL/g (Table 1). In addition,the number-average molecular weights (M

—n) of polymers

were determined by gel permeation chromatography(GPC) using the polystyrene as the standards to be 9000–

Table 1. Yields, inherent viscosities, number-average weights, and elemental analyses of polymers.

Polymer Yield%

ginhaÞ

dl=gM—

nb) M

—w/M

—n Empirical formula Elemental analyses

C H N

3a 87 0.28 21000 1.8 (C36H24)n (456.6)n Calcd.Found

94.7094.32

5.305.45

3b 92 0.16 11000 1.7 (C40H28)n (508.17)n Calcd.Found

94.4594.07

5.555.48

5a 98 0.25 17000 1.5 (C43H36NBF4)n (653.6)n Calcd.Found

79.0278.86

5.555.63

2.142.08

5b 90 0.14 9000 1.6 (C55H60NBF4)n (821.9)n Calcd.Found

80.3779.96

7.367.42

1.701.73

5c 75 0.58 31000 2.1 (C45H32NBF4)n (673.6)n Calcd.Found

80.2479.87

4.794.95

2.081.97

5d 96 0.43 27000 1.9 (C45H38NBF4)n (679.6)n Calcd.Found

79.5379.15

5.645.78

2.061.95

a) Inherent viscosity in CHCl3 (0.5 g/dL) at 258C.b) GPC values based on polystyrene standards in THF.

Scheme 1.


31000 with a polydispersity index of 1.5–2.1 (see Table1).

Characterization of Polymers

The polymers were confirmed to be the correspondingsubstituted poly(m-phenylene)s by means of elementalanalyses (Table 1) as well as by FT-IR and NMR spectro-scopies. Figure 1 presents typical FT-IR spectra of 3aand 5a. The wholly aromatic polymer 3a showed absorp-tions at 3056, 3028, 1602, 1406, 1178, 1120, and 756cm–1 assigned to the aromatic structure. Polymer 5a dis-played characteristic absorption bands at 3026, 1180,1118, 752 (aromatic), 1680, 1602, 1472 (aromatic andpyridinium structure), 2926, 2850 (CH3 and CH2 stretch-ing), 1050 (BF4

–) and 962 cm–1 (HC2CH trans). Note thatthe IR spectra of polymers did not essentially differ fromthose of the corresponding monomers, since only theabsorptions associated with the C1Cl bond disappearedfrom the down-field spectrum region (near 730–720 and660–650 cm–1).

The most conclusive spectra evidence for the proposedpolymer structures was provided by 1H and 13C NMRspectroscopies. The 1H NMR spectrum of 3a in CDCl3

solution showed only aromatic protons located at 7.60–7.15d. The 1H NMR spectrum of 5a in CDCl3 solutiondisplayed peaks at 8.23 (s, 2H, aromatic meta to N+),7.70–7.23 (m, 21H, other aromatic and 4H, HC2CH);4.32 (m, 2H, N+CH2), 1.30 (m, 4H, N+CH2(CH2)2), 0.95(m, 3H, N+(CH2)3CH3).

Figure 2 presents the 13C NMR spectrum of 5b togetherwith assignments of the observed resonances. This spec-

trum indicated a significant degree of polymerization asevidenced by the peak at 141 ppm, which is typical of thequaternary carbons in the poly(m-phenylene)s backbone.The 13C NMR spectrum of this compound as well as of

Scheme 2.

Figure 1. FT-IR spectra of polymers 3a (top) and 5a (bottom).


the other synthesized polymers are complicated probablydue to different regiochemical regularities in the place-ment of the lateral 2,6-bisstilbenyl-N-alkylpyridinium tet-rafluoroborate groups along the polymer backbone. Spe-cifically, these groups can have head-to-tail and head-to-head structures.[21] It is known that the head-to-head link-ages increase barriers to rotation between the adjacentphenyl rings and exhibit steric inhibition of resonance,which is manifested in their UV-vis and NMR spectra.[21]

The wide-angle X-ray diffractograms of polymersobtained at room temperature with as prepared powderspecimens revealed their generally amorphous character.The diffuse intensity X-ray scattering curves indicated arandom structure in the solid state. The rigid backbone ofpoly(m-phenylene)s containing a bulky side group atortho position in each repeat unit that increased their non-planar conformation and caused a loose chain packingshould be responsible for this behavior. As was men-tioned above, the ortho-substituted poly(m-phenylene)scan have head-to-head and head-to-tail linkages.Obviously, the former linkages give a more random struc-ture to the polymers. The amorphous nature of polymersreflects to their enhanced solubility and it is in agreementwith the lack of melting, as was shown by DSC (seebelow).

All polymers showed an excellent solubility beingreadily soluble at room temperature in THF, chloroform,1,2-dichloroethane, and chlorobenzene (Table 2). In addi-tion, they dissolved at ambient temperature or upon heat-ing in toluene and cyclohexanone. Polymer 5b with sideN-hexadecylpyridinium groups was the most solublepolymer synthesized, since it dissolved even in hot car-bon tetrachloride.

The polymers exhibited no evidence of crystallinity byDSC. The glass transition temperatures (Tg) were deter-

Figure 2. 13C NMR spectrum of polymer 5b in CDCl3 solu-tion.

Figure 3. DSC thermograms (first heating; solid line) as wellas TMA traces (second heating; dashed line) of polymers 3a,5a, 5b, and 5c. Conditions: N2 flow, 60 cm3/min; heating rate,10 8C/min.

Table 2. Solubilities of Polymers. Solubility: ++, soluble at room temperature; +, soluble in hot solvent; +–, partially soluble; –,insoluble.

Polymer Solventsa)

THF CHCl3 CCl4 DCE C6H5Cl Toluene Cyclohexanone

3a ++ ++ +– ++ ++ ++ ++3b ++ ++ – ++ ++ + +5a ++ ++ – ++ ++ + ++5b ++ ++ + ++ ++ ++ ++5c ++ ++ +– ++ ++ ++ ++5d ++ ++ – ++ ++ + +

a) THF, tetrahydrofuran; DCE, 1,2-dichloroethane.


mined by means of DSC and they were obtained from thecenter of the DSC step drop. In addition, the Tg values ofpolymers were determined from the TMA method using aloaded penetration probe. The Tg is defined by the firstinflection point in the TMA curve and it was obtainedfrom the onset temperature of this transition. Figure 3presents typical DSC and TMA traces of polymers. TheTg values for all polymers, which were determined bothby the DSC and TMA method, are listed in Table 3. TheTg’s ranged from 96 to 1488C. Polymer 3b displayed sig-nificantly higher Tg than 3a, and this supports the greaterchain stiffness attained by replacement of biphenylylwith stilbenyl group. In addition, all polymers with pen-dant stilbene-based chromophores (3b and 5a–d) exhib-ited comparable Tg values. Note that relatively high Tg’sare required for many applications such as in light-emit-ting diodes.[22]

The thermal stability of polymers was evaluated byTGA. The temperatures (DT) at which weight loss of 1, 5and 50% were obtained in both N2 and air as well as theanaerobic char yield (Yc) at 8008C for all polymers arelisted in Table 4. No weight loss of polymers wasobserved up to 220–2988C, and their anaerobic Yc at8008C ranged from 41 to 62%. The wholly aromaticpolymer 3a was more thermally stable than 3b, becauseit showed higher TD and Yc values. Comparison of poly-

mers 5 with pyridinium structure showed that 5c startedto loose weight at higher temperature than 5a, 5b, and5d due to the presence of the thermally sensitive aliphaticsegments in the latter.

Optical Properties of Polymers

All polymers consisted of poly(m-phenylene) backboneand most of them carried pendant stilbene-based chromo-phores. Therefore, the investigation of their optical prop-erties both in solution and solid state is of interest. Figure4 depicts typical UV-vis spectra of polymers in diluteTHF solution. They showed absorption maxima approxi-mately at 260 and 320 nm assigned to the aromatic struc-ture and the p-p* transition of stilbene moieties. The opti-cal energy gaps Eg of polymers were calculated from theonset wavelength of their UV-vis absorption spectra inTHF solution. The Eg values were 3.55 eV for 3a and3.11–3.27 eV for other polymers.

The polymers were strongly fluorescent and their PLspectra in solution and in thin film are presented in Figure5 and 6. The films were prepared by spin coating onquartz plates from the solution of polymers. The PLcurves were obtained by excitation at 320 nm, which isthe maximum of the absorption band. The wavelengths atpeak maxima and the corresponding energies are listed in

Table 3. Glass transition temperatures, fluorescence wavelength maxima in solution and in thin film, and fluorescence quantumyields in solution of polymers.

Polymer 3a 3b 5a 5b 5c 5d

Tg from DSC in8C 96 146 137 136 120 130Tg from TMA in 8C 100 148 140 136 108 132kf, max in solutiona) in nm 368 391, 410 395 395 404 401Ef, max in solutiona) in eV 3.37 3.17, 3.03 3.14 3.14 3.07 3.09kf, max in thin filma) in nm 459 475 458, 514 461, 532 445, 523 465Ef, max in thin filma) in eV 2.70 2.61 2.71, 2.42 2.69, 2.33 2.79, 2.37 2.67Uf in solution 0.12 0.26 0.65 0.59 0.21 0.27

a) Underlined numerical values denote absolute maxima. kf, max represents the fluorescence maxima, Ef, max the corresponding ener-gies, and Uf the fluorescence quantum yields.

Table 4. Thermal stabilities of polymers.

Polymer In N2 In Air

DT1aÞ

�CDT5

aÞ

�CDT50

aÞ

�CYcbÞ

%

DT1aÞ

�CDT5

aÞ

�CDT50

aÞ

�C

3a 286 333 62 283 329 5493b 248 311 51 240 303 5035a 267 340 61 256 325 5605b 260 333 59 252 310 5565c 298 351 58 286 374 6115d 229 298 705 41 220 294 530

a) DT1, DT5, DT50: Temperatures at which weight loss of 1%, 5%, and 50%, respectively, was obtained.b) Char yield at 800 8C.


Table 3. These maxima were not influenced upon theexcitation wavelength.

The PL spectra in solution showed maximum at368 nm for 3a and 391–410 nm for 3b and 5a–d. Thesespectra were located in the violet-blue range. A remark-able red-shift was observed, when the pendant biphenylylgroups were replaced with stilbenyl. It is noteworthy thatall polymers carrying stilbenyl side groups displayedsimilar PL maxima regardless of their particular chemicalstructure. The spectra of these polymers were not relatedto that of trans-stilbene, because the maximum of the lat-ter appears around 355 nm. It should be emphasized thatall polymers exhibited almost the same PL maximumwith that of the corresponding parent dichloro-monomers,and this indicates that there was not essential conjugationbetween the adjacent repeat units in polymer due to thedisruption of planarity.

In addition, the influence of the solution concentrationon the emission maximum was studied. More particu-larly, the PL spectra of typical polymers 3a and 3b wererecorded for their solutions in THF of various polymerconcentrations. It was shown that the PL curve becamegradually broader and its maximum was slightly red-shifted with increasing the solution concentration. Such abehavior suggests that in solution there were not impor-tant interactions between the pendant chromophoricgroups. Finally, the effect of the solvent nature on theemission maximum was examined. Thus, the PL spectraof 5a and 5b were obtained in various solvents with dif-ferent polarity (THF, acetonitrile, chloroform, toluene),but their maxima were almost identical.

The PL spectra of polymers exhibited a significantbroadening and bathochromic shift upon going from solu-tion to the solid state. The PL curves in solid stateextended at the violet-green range and their maximaappeared at 445–532 nm. It has been well establishedthat in solid state considerable stacking interactions existand aggregates are formed.[23, 24] Specifically, it has beenreported that the spectral maximum of poly(stilbenyl-p-methoxystyrene), a light emitting polymer with pendantchromophoric groups, appeared at 450 nm was attributedto a fluorescent dimer, formed through the interaction ofadjacent pendant groups, and that no monomeric emis-sion existed in the solid state.[25]

The fluorescence efficiency of polymers was estimatedby measuring[26] their quantum yields (U) in THF solution(Table 3) relative to quinene sulfate (U = 0.546). Polymer3a that lacked stilbenyl groups showed a relatively low U

value of 0.12. In contrast, the other polymers with pend-ant stilbene-based chromophores displayed remarkablyhigher U (0.21–0.65). Generally, the quaternization ofthe ring nitrogen with flexible, long chain aliphatic seg-ments increased remarkably the U value. The most effi-cient fluorescent polymers taken, were 5a and 5b with N-

Figure 4. UV-vis spectra of polymers 3a, 5a, and 5b in THFsolution.

Figure 5. PL spectra for all polymers in THF solution (excitationat 320 nm).

Figure 6. PL spectra for all polymers in thin film (excitation at320 nm).


butylpyridinium and N-hexadecylpyridinium structure,respectively, the U values of which were 0.65 and 0.59.Since U was of the order 5a A 5b, it seems that U wasincreased with decreasing the chain length of the alkylgroup. The quaternization with the cyclohexyl ring didnot improve fluorescence, because the U values werealmost same for 3b and 5d. Finally, a negative result wasobtained by quaternization with the compact phenyl ring(U of 3b A U of 5c).

ConclusionsA new series of poly(m-phenylene)s bearing pendantchromophoric groups were successfully synthesizedthrough pyrylium salts by the nickel catalyzed couplingof substituted m-dichlorobenzenes. The most attractivepolymers obtained were those with pendant 2,6-bisstilbe-nyl-N-alkylpyridinium tetrafluoroborate groups. Due totheir asymmetric structure, the nonplanar conformation,and the possibility to obtain head-to-head and head-to-taillinkages, all polymers were practically amorphous anddissolved in common organic solvents. The solubility ofpolymers was increased with increasing the chain lengthof the alkyl attached to the pyridinium nitrogen. Theyshowed Tgs at 96–1488C and were stable up to 220–2988C with anaerobic char yield of 41–62% at 8008C.The polymers with pendant stilbene-based chromophoresdisplayed strong violet-blue and violet-green fluores-cence in solution and in solid state, respectively. Thefluorescence quantum yield of the polymers with side2,6-bisstilbenyl-N-alkylpyridinium tetrafluoroborategroups was significantly influenced upon the structure ofthe alkyl group. When the alkyl was butyl or hexadecyl,the quantum yield of polymers in THF solution was 0.65and 0.59, respectively.

Acknowledgement: Financial support for this work from theGreek General Secretariat of Research and Technology and theEuropean Social Fund is gratefully acknowledged (ResearchProgram PENNED ’99, code: 99ED 405).

Received: September 25, 2000Revised: January 15, 2001

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synthesis and properties of soluble, strongly fluorescent poly(m-phenylene)s with pendant...

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