synthesis and properties of soluble, fluorescent polyesters and polyethers with substituted...

Synthesis and Properties of Soluble, Fluorescent Polyestersand Polyethers with Substituted m-Terphenyl Segmentsin the Main Chain

JOHN A. MIKROYANNIDIS

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

Received 31 March 2000; accepted 11 April 2000

ABSTRACT: A new series of rigid polyesters and semiflexible polyethers were synthe-sized from 4,40-dihydroxy-59-phenyl or anthracenyl-m-terphenyl. The polymers werecharacterized by viscometry, Fourier transform infrared, NMR, X-ray, differentialscanning calorimetry, thermomechanical analysis, thermogravimetric analysis, ultra-violet–visible, and luminescence spectroscopy. The polyesters were amorphous,whereas some of the polyethers showed a low degree of crystallinity. All the polymersdisplayed an enhanced solubility even in 1,1,2,2-tetrachloroethane and tetrahydrofu-ran. The glass-transition temperatures were 123–146 °C for the polyesters and 45–117°C for the polyethers. The polymers were stable up to 213–340 °C and afforded anaer-obic char yields of 36–62% at 800 °C. Their optical properties were investigated both insolution and in the solid state. They showed ultraviolet fluorescence, violet-blue fluo-rescence, or both with emission maxima at 333–487 nm. The polymers with anthrace-nyl pendent groups exhibited higher fluorescence quantum yields and emission maximaredshifted compared with the corresponding polymers with phenyl pendent groups.© 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2381–2391, 2000Keywords: polyesters; polyethers; m-terphenyl units; pyrylium; semiflexible poly-mers; fluorescence; light-emitting polymers

INTRODUCTION

A number of articles have been recently publishedon the preparation and properties of luminescentmaterials. Among blue luminescent materials,anthracene is one of the most studied compounds,and its crystals show luminescence in the blueregion.1 The electroluminescent properties of thinfilms of anthracene have been investigated.2 It isknown that anthracene3 has a high quantum ef-ficiency of photoluminescence in the solid state(0.99), whereas poly(p-phenylenevinylene),4 whichis today one of the most widely used emitters,presents a quantum yield of only 0.08. However,

the high crystallinity of anthracene prevents thefabrication of stable amorphous thin films.

The interest of researchers was, therefore, fo-cused on the introduction of anthracene moietiesinto polymer structures. Such an approach con-tributes to overcoming the problem of the prepa-ration of films with good optical quality. The at-tachment of anthtracene units to polymer chainsdecreases the electron bandgap of polymers.5,6

Certain anthracene-containing polymers havebeen recently synthesized.7–13

This investigation deals with the synthesis andcharacterization of a new class of photolumines-cent polyesters and polyethers bearing substi-tuted m-terphenyl segments in the main chain.They were prepared from two bisphenols thatwere synthesized through pyrylium salts by aconvenient and inexpensive method applied in

Correspondence to: J. A. Mikroyannidis (E-mail: [email protected])Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 38, 2381–2391 (2000)© 2000 John Wiley & Sons, Inc.

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our laboratory.14–22 The polymers of this investi-gation contained phenyl or anthracenyl pendentgroups, so their properties could be compared.They are expected to present an enhanced solu-bility compared with p-terphenyl derivatives, be-cause the introduction of m-terphenyl into thepolymer chain reduces the symmetry and im-proves the solubility. In addition, the existence ofthe pendent anthracenyl groups should decreasethe electron bandgap of polymers and renderthem more efficient luminescent materials. m-Terphenyl segments have been introduced be-tween various numbers of para-connected aro-matic groups to achieve broken conjugation andcontrol the emitter length.23

EXPERIMENTAL

Characterization Methods

Melting temperatures were determined on anelectrothermal melting point apparatus (IA6304)and are uncorrected. IR spectra were recorded ona PerkinElmer 16PC Fourier transform infrared(FTIR) spectrometer with KBr pellets. The 1HNMR (400-MHz) and 13C NMR (100-MHz) spectrawere obtained with a Brucker spectrometer withDMSO-d6 or CDCl3 as the solvent. Chemicalshifts (d values) are given in parts per millionwith tetramethylsilane as an internal standard.Ultraviolet–visible (UV–vis) spectra were re-corded on a Varian Cary 1E spectrometer. Theemission spectra were obtained with aPerkinElmer LS50B luminescence spectrometer.Differential scanning calorimetry (DSC) and ther-mogravimetric analysis (TGA) were performed ona DuPont 990 thermal analyzer system. Groundpolymer samples, about 10 mg each, were exam-ined by TGA, and weight-loss comparisons weremade between comparable specimens. DSC ther-mograms were obtained at a heating rate of10 °C/min in an N2 atmosphere at a flow rate of 60cm3/min. Dynamic TGA measurements weremade at a heating rate of 20 °C/min in an atmo-spheres of N2 or air at a flow rate of 60 cm3/min.Thermomechanical analysis (TMA) was recordedon a DuPont 943 TMA with a loaded penetrationprobe at a scan rate of 10 °C/min in N2 with a flowrate of 60 cm3/min. The TMA experiments wereconducted at least in duplicate to ensure the ac-curacy of the results. The TMA specimens werepellets 8 mm in diameter and 2 mm thick pre-pared by the pressing of polymer powder for 3 min

under 5–7 kpsi at ambient temperature. The in-herent viscosities of the polymers were deter-mined for solutions of 0.5 g/100 mL in tetrahy-drofuran (THF) or dimethylformamide (DMF) at30 °C with an Ubbelohde suspended level viscom-eter. Elemental analyses were carried out with aHewlett–Packard model 185 analyzer. The wide-angle X-ray diffraction patterns were obtained forpowder specimens on an X-ray PW-1840 Philipsdiffractometer.

To measure the fluorescence quantum yields, adegassed solution of the polymer in THF wasprepared. The concentration was adjusted so thatthe absorbance of the solution would be lowerthan 0.1. The exciting wavelength was 310 nm,and a solution in 1 N H2SO4 of quinine sulfate,which has a quantum yield of 0.546, was used asthe standard.

Reagents and Solvents

Benzaldehyde was purified by distillation underreduced pressure. 9-Anthraldehyde and 49-meth-oxyacetophenone were recrystallized from etha-nol and ethyl ether/petroleum ether, respectively.Terephthaloyl chloride was recrystallized from n-hexane. Dimethylacetamide (DMAc) and 1,2-di-chloroethane were dried by distillation overCaH2. 1,6-Dibromohexane, 1,10-dibromodecane,boron trifluoride etherate, acetic anhydride, andhydrobromic acid (47–49%) were used as sup-plied.

Preparation of the Bisphenols

2,6-Bis(4-methoxyphenyl)-4-phenylpyryliumTetrafluoroborate (1a)

A flask was charged with a mixture of benzalde-hyde (1.60 g, 14.8 mmol), 49-methoxyacetophe-none (4.44 g, 29.6 mmol), and 1,2-dichloroethane(25 mL). Boron trifluoride etherate (4.60 mL,37.00 mmol) was added portionwise to the stirredmixture at room temperature, and it was refluxedfor 5 h under N2. The solution was concentratedunder reduced pressure to afford 1a as a purplesemisolid. It was washed thoroughly with waterand was subsequently dried in a vacuum oven(5.42 g; yield 5 81%).

IR (KBr) cm21: 2936 (COH stretching ofOCH3), 1622, 1602, 1488, 1460 (aromatic andpyrylium structure), 1262, 1176 (ether), 1026(BF4

2). 1H NMR (CDCl3, d): 8.68 (m, 2H, aromaticmeta to O1), 7.79–7.14 (m, 9H, other aromatic

2382 MIKROYANNIDIS

except those ortho to OCH3), 6.94 (m, 4H, aro-matic ortho to OCH3), 3.80 (s, 6H, OCH3).

2,6-Bis(4-methoxyphenyl)-4-anthracenylpyryliumTetrafluoroborate (1b)

Compound 1b was prepared by 9-anthraldehyde(1.53 g, 7.27 mmol) being reacted with 49-meth-oxyacetophenone (2.18 g, 14.54 mmol) andboron trifluoride etherate (2.30 mL, 18.17 mmol)in 1,2-dichloroethane (25 mL) according to themethod applied for 1a. The reaction mixture wasconcentrated under reduced pressure, and etherwas added to the concentrate. The purple precip-itate was filtered, washed with ether and thenwith water, and dried to afford 1b. It was recrys-tallized from a mixture of chloroform/ether (1/1v/v; 1.23 g; yield 5 30%; mp 5 156–158 °C).

IR (KBr) cm21: 2934 (COH stretching ofOCH3), 1626, 1598, 1492, 1422 (aromatic andpyrylium structure), 1254, 1174 (ether), 1026(BF4

2). 1H NMR (DMSO-d6; d): 8.60 (s, 2H, aro-matic meta to O1), 8.37 (s, 1H, aromatic of an-thracenyl ring at position 10), 7.89–7.00 (m, 16H,other aromatic), 3.77 (s, 6H, OCH3).

4,4(-Dimethoxy-5*-phenyl-m-terphenyl (2a)

A mixture of 1a (1.51 g, 3.31 mmol), fusedCH3COONa (0.54 g, 6.62 mmol), and acetic anhy-dride (7 mL) was refluxed overnight. It was sub-sequently poured into water and stirred at roomtemperature for about 24 h. The red-brown pre-cipitate was filtered, washed with water, anddried to afford 2a (0.84 g; yield 5 87%; mp5 92–94 °C).

IR (KBr) cm21: 2940, 2836 (COH stretching ofOCH3), 1602 (aromatic), 1260, 1178 (ether). 1HNMR (DMSO-d6; d): 7.78–7.37 (m, 12H, aromaticexcept those ortho to OCH3), 7.02 (m, 4H, aro-matic ortho to OCH3), 3.81 (s, 6H, OCH3).

4,4(-Dimethoxy-5*-anthracenyl-m-terphenyl (2b)

In a method similar to that used for 2a, compound2b was prepared from 1b (1.00 g, 1.80 mmol),fused CH3COONa (0.29 g, 3.60 mmol), and aceticanhydride (4 mL). It was recrystallized from amixture of chloroform/ether (1/2 v/v; mp 5 135–137 °C; yield 5 83%).

IR (KBr) cm21: 2932, 2834 (COH stretching ofOCH3), 1598 (aromatic), 1250, 1172 (ether). 1HNMR (DMSO-d6; d): 8.40 (s, 1H, aromatic of an-thracenyl ring at position 10), 7.96–7.43 (m, 15H,other aromatic except those ortho to OCH3), 6.97

(m, 4H, aromatic ortho to OCH3), 3.83 (s, 6H,OCH3).

4,4(-Dihydroxy-5*-phenyl-m-terphenyl (3a)

A mixture of 2a (2.16 g, 7.35 mmol), glacial aceticacid (15 mL), and hydrobromic acid (47–49%; 5mL) was refluxed for 24 h. It was subsequentlypoured into water, and the purple precipitate wasfiltered, washed with water, and dried to afford3a. It was recrystallized from ethanol (50%;1.78 g; yield 5 72%; mp 5 160–162 °C).

IR (KBr) cm21: 3208 (OOH stretching), 1622(aromatic), 1370 (OOH deformation), 1260, 1220,1174 (COOH stretching). 1H NMR (DMSO-d6; d):9.50 (br, 2H, OH), 7.80–7.37 (m, 12H, aromaticexcept those ortho to OH), 6.90 (m, 4H, aromaticortho to OH). 13C NMR (DMSO-d6; d): 155.06,142.30, 141.20, 133.82, 132.17, 131.78, 131.33,130.42, 129.69, 129.43, 129.10, 128.36, 127.63,126.66, 120.72, 118.67, 117.75, 116.60, 116.04.ELEM. ANAL. Calcd. for C24H18O2: C, 85.18%; H,5.36%. Found: C, 85.05%; H, 5.40%.

4,4(-Dihydroxy-5*-anthracenyl-m-terphenyl (3b)

Compound 3b was prepared as a purple solid byacid hydrolysis of 2b (2.06 g, 4.41 mmol) in amixture of glacial acetic acid (15 mL) and hydro-bromic acid (47–49%; 5 mL) according to themethod applied for 3a. It was recrystallized fromethanol (50%; 1.74 g; yield 5 90%; mp 5 190–192 °C).

IR (KBr) cm21: 3034 (OOH stretching), 1596(aromatic), 1372 (OOH deformation), 1262, 1208,1168 (COOH stretching). 1H NMR (DMSO-d6; d):8.70 (br, 2H, OH), 8.30 (s, 1H, aromatic of anthra-cenyl ring at position 10), 8.04–7.42 (m, 15H,aromatic except those ortho to OH), 6.88 (m, 4H,aromatic ortho to OH). ELEM. ANAL. Calcd. forC32H22O2: C, 87.65%; H, 5.05%. Found: C,87.36%; H, 5.12%.

Preparation of the Polymers

Preparation of Polyesters Sa and Sb

The synthesis of Sa is given as a typical examplefor the preparation of the polyesters. A flask wascharged with a solution of 3a (0.4500 g, 1.33mmol) in DMAc (15 mL). To the mixture, 0.5 mLof propylene oxide was added. Terephthaloyl chlo-ride (0.2703 g, 1.33 mmol), dissolved in DMAc (8mL), was added dropwise to the mixture at roomtemperature under N2. The mixture was subse-

FLUORESCENT POLYESTERS AND POLYETHERS 2383

quently stirred and heated at 100 °C overnight ina steam of N2. It was poured into water, and thepale-brown precipitate was filtered, washed withwater, and dried to afford Sa (0.57 g; yield5 91%).

ELEM. ANAL. Calcd. for (C32H20O4)n: C, 82.04%;H, 4.30%. Found: C, 81.45%; H, 4.36%.

Polyester Sb was similarly prepared in an 87%yield by 3b being reacted with terephthaloyl chlo-ride.


Preparation of Polyethers Ta1, Ta2, Tb1, and Tb2

The synthesis of Ta1 is given as a typical examplefor the preparation of the polyethers. A mixture of3a (0.80 g, 2.37 mmol), 1,6-dibromohexane (0.58g, 2.37 mmol), K2CO3 (0.69 g, 4.98 mmol), andDMAc (20 mL) was stirred and refluxed overnightunder N2. It was poured into water, and the pale-brown precipitate was filtered, washed with wa-ter, and dried to afford Ta1 (0.95 g; yield 5 95%).


Polyether Ta2 was similarly prepared in a 96%yield by 3a being reacted with 1,10-dibromo-decane.


Polyether Tb1 was prepared by a similarmethod in a 94% yield from the reaction of 3bwith 1,6-dibromohexane.


Polyether Tb2 was similarly prepared in a 97%yield by 3b being reacted with 1,10-dibromo-decane.


RESULTS AND DISCUSSION

Synthesis and Characterization of the Monomers

Bisphenols 3a and 3b, which are substituted de-rivatives of m-terphenyl, were synthesized ac-cording to the reaction sequence of Scheme 1.More particularly, benzaldehyde or 9-anthralde-

Scheme 1

2384 MIKROYANNIDIS

hyde reacted with 49-methoxyacetophenone in thepresence of boron trifluoride etherate to yieldpyrylium salts (1).24 The latter reacted with ace-tic anhydride/sodium acetate, affording substi-tuted dimethoxy-m-terphenyls (2). It is well es-tablished that 2,4,6-triarylpyrylium salts reactwith excess carboxylic acid anhydrides in thepresence of a condensing agent, such as sodiumacetate, leading to 1,3,5-trialylbenzene.25 Finally,the methoxy groups of 2 were hydrolyzed withhydrobromic acid to yield the corresponding bis-phenols, 3.

The monomers were characterized by elemen-tal analyses as well as 1H NMR and 13C NMRspectroscopy. Figure 1 presents a typical 13CNMR spectrum of bisphenol 3b and the assign-ments of its peaks. Most of the nuclei appear inthe region of 125–133 ppm, where almost all theanthracene carbons are situated.

An attempt was made to estimate certainstructural characteristics of the synthesized bis-phenols with a modeling system. The optimizedgeometry, as calculated by the CS Chem3D Proversion 3.2 modeling system, indicated that for 3athe structure is essentially coplanar because allthe rings of 1,3,5-triphenylbenzene form dihedralangles smaller than 2°. In contrast, the structureof 3b deviates significantly from the coplanar con-formation (Fig. 2). Specifically, the anthraceneforms with the adjacent phenyl ring a dihedralangle of about 60°. In addition, the phenyl ringsbearing the hydroxyl group form with the adja-cent phenyl a twist angle of 40°. These structuralcharacteristics of bisphenols are expected to affectsome properties of the synthesized polymers.

Synthesis and Characterization of the Polymers

A new series of polyesters and polyethers, thestructures of which are shown in Chart 1, weresynthesized from bisphenols (3). Specifically, thelatter reacted with terephthaloyl chloride via thesolution polycondensation method at a high tem-perature in the presence of propylene oxide as anacid acceptor to afford polyesters (S). Polyethers(T) were synthesized from the reactions of 3 witha,v-dibromoalkanes in the presence of K2CO3. Allpolymers remained in solution, and their isolationwas carried out by the reaction solution beingpoured into a nonsolvent. They were obtained athigh yields (87–97%), and their inherent viscosi-ties ranged from 0.22 to 0.45 dL/g (Table I).

The structures of the polymers were confirmedby elemental analyses, FTIR, 1H NMR and 13CNMR spectroscopy. The elemental analyses of thepolymers were consistent with the chemical struc-tures. Figure 3 presents typical FTIR spectra ofpolymers Sa and Tb2. Polyester Sa showed char-acteristic absorption bands at 1732 (CAOstretching); 1606 and 1500 (aromatic); and 1266,1174, and 1072 cm21 (COOOC stretching). The

Figure 2. Optimized geometry for bisphenol 3b (CSChem3D Pro Molecular Modeling System, version 3.2,CambridgeSoft Corporation, 1995).

Figure 1. 13C NMR spectrum of bisphenol 3b in aDMSO-d6 solution.


broad absorption around 3100 cm21 could be at-tributed to the terminal OOH and OCOOHgroups. Polyether Tb2 displayed absorptions at2924 and 2852 (COH stretching of aliphatic);1606, 1512, and 1440 (aromatic), and 1246 and1170 cm21 (ether). The 1H NMR spectrum of Sain a DMSO-d6 solution exhibited peaks at 8.13(m, 4H, aromatic of terephthalic acid segment),7.42–7.26 (m, 12H, other aromatic except thoseortho to O), and 6.79 (m, 4H, aromatic ortho to O).The 13C NMR spectrum of Sa showed a charac-

teristic peak at 164.90 ppm associated with theOOOCOO segment. Because the terephthalicacid moiety displayed peaks at 130–132 ppm, theother peaks of Sa were almost similar to those ofthe parent bisphenol, 3a. The 1H NMR spectrumof a typical polyether, Ta2, in a CDCl3 solutionshowed peaks at 7.77–7.37 (m, 12H, aromatic ex-cept those ortho to O), 6.94 (m, 4H, aromatic orthoto O), 3.88 (t, 4H, OCH2), and 1.75–1.34 (m, 16H,OCH2(CH2)8). Finally, Figure 4 depicts the 13CNMR spectrum of Ta2 with the most probableassignment of peaks.

Crystallinity and Solubility of the Polymers

The crystallinity of the polymers was estimatedby wide-angle X-ray diffractograms for powderspecimens at room temperature (Fig. 5). Thewholly aromatic rigid polyesters Sa and Sb weregenerally amorphous. In contrast, some of thesemiflexible polyethers showed a low degree ofcrystallinity. It is obvious that the long aliphaticspacers in the backbone of the polyethers in-creased the flexibility and allowed a higher order-ing of chains. Polyether Ta2 displayed well dis-tinguished peaks at the region of 20–70°, and itwas the most crystalline polymer obtained. Thehigher crystallinity of Ta2 was attributed to themore coplanar structure of the parent bisphenol,3a, as it was established by the modeling system(discussed previously), and the longer aliphaticmoieties in the polymer backbone that permitteda dense-packing. However, the noncoplanarstructure of the parent bisphenol, 3b, and thelarge size of the anthracenyl pendent groups arethe major reasons for the destruction of the crys-tallization tendency of polyethers Tb1 and Tb2.The corresponding rigid polyamide and poly-imides with m-terphenyls in the main chain,which were previously synthesized in our labora-tory,19 were completely amorphous because oftheir wholly aromatic structure.

Table II summarizes the qualitative solubilityof the polymers. All the polymers showed an en-hanced solubility, being readily soluble at ambi-ent temperature in polar aprotic solvents (N,N-dimethylformamide, N-methylpyrrolidone), strongacids (CCl3COOH, H2SO4), and pyridine. Theywere also soluble at room temperature or uponheating in less efficient solvents such as 1,1,2,2-tetrachloroethane and THF. As expected, thepolyesters showed lower solubility than the cor-responding polyethers. Polyester Sb exhibitedsomewhat lower solubility than Sa because of the

Chart 1

2386 MIKROYANNIDIS

presence of the more compact anthracenyl struc-ture. Polyethers Ta1 and Ta2 displayed an excel-lent solubility, being soluble at room temperaturein all the tested solvents and even in THF andchloroform. The elongation of the aliphatic spacersegment from O(CH2)6O to O(CH2)10O did notappreciably increase the polymer solubility be-cause polymers Ta1 and Ta2 displayed the samequalitative solubility. Finally, polyethers Tb1and Tb2, bearing the anthracenyl pendentgroups, were less soluble than the correspondingpolyethers, Ta1 and Ta2.

Thermal and Thermomechanical Propertiesof the Polymers

The thermal and thermomechanical properties ofthe polymers were investigated with DSC, TMA,

and TGA. No endotherms associated with meltingwere detected for the polymers by DSC upon heat-ing up to 250 °C; this indicates their amorphouscharacter. The DSC curves showed a step dropduring the first and second heatings, which couldbe assigned to the glass transition (Tg). In addi-tion, the Tg’s of the polymers were determined byTMA with a penetration probe. Figure 6 presentstypical DSC (first heating scan) and TMA (secondheating scan) traces for polymers Sa, Sb, Ta1,and Tb1. The Tg values for all the polymers arelisted in Table I, and they were obtained from theonset temperature of the TMA transition as wellas from the center of the DSC transition.

As expected, the wholly aromatic polyestersshowed significantly higher Tg’s (123–146 °C)

Table I. Yields, Inherent Viscosities, Glass-Transition Temperatures, Fluorescence Wavelength Maximain Solution and in Thin Film, and Fluorescence Quantum Yields in Solution

Polymer

Sa Sb Ta1 Ta2 Tb1 Tb2

Yield (%) 91 87 95 96 94 97ninh (dL/g) 0.35a 0.37b 0.25a 0.27a 0.45a 0.22a

Tg from DSC (°C) 146 123 60 53 117 78Tg from TMA (°C) 140 126 63 45 108 63lf,max in solution (nm)c 342 401, 420 353 333 418, 486 419, 487Ef,max in solution (eV)c 3.63 3.10, 2.96 3.52 3.73 2.97, 2.55 2.96, 2.55lf,max in thin film (nm)c 424 407 431 442 418 425, 487Ef,max in thin film (eV)c 2.93 3.05 2.88 2.81 2.97 2.92, 2.55Ff in solution 0.04 0.11 0.04 0.04 0.23 0.26

a Inherent viscosity in THF (0.5 g/dL) at 30 °C.b Inherent viscosity in DMF (0.5 g/dL) at 30 °C.c Underlined numerical values denote absolute maxima. lf,max 5 fluorescence maxima; Ef,max 5 corresponding energies; Ff

5 fluorescence quantum yields.

Figure 3. FTIR spectra of polymers Sa and Tb2.Figure 4. 13C NMR spectrum of polyether Ta2 in aCDCl3 solution.


than the semiflexible polyethers (45–117 °C). Theinfluence of the chemical structure of the pendentgroups on the Tg values is of interest. Specifically,the introduction of the anthracenyl group insteadof phenyl is expected to reduce the flexibility ofthe chains but simultaneously increase their freevolume because of the bulkier substituent. Poly-ester Sb showed a lower Tg than Sa. However, forthe polyethers, an opposite result was observedbecause the Tg values were of the order Ta1, Tb1 and Ta2 , Tb2. The introduction of theanthracenyl groups along the semiflexible back-

bone of the polyethers reduced remarkably thechain flexibility, and it was obviously the predom-inant effect. Finally, the Tg of the polyethers wasreduced with the increasing length of the ali-phatic segment in the backbone. This behaviourconforms with literature data.22 By comparingpolyester Sa with the corresponding polyamide,which was previously synthesized,19 we concludethat the latter showed a considerably higher Tg(290 °C) because of the hydrogen-bonding inter-actions.

Figure 5. Wide-angle X-ray scattering curves for thepolymers.

Table II. Solubilities of the Polymersa

Polymer

Solventsb

DMF NMP CCl3COOH H2SO4 TCE DCE CHCl3 THF Py 1,4-Dioxane

Sa 11 11 11 11 11 2 2 11 11 11Sb 11 11 11 1 1 2 2 12 11 1Ta1 11 11 11 11 11 11 11 11 11 11Ta2 11 11 11 11 11 11 11 11 11 11Tb1 11 11 11 11 1 12 12 1 11 11Tb2 11 11 11 11 1 12 12 1 11 12

a 11 5 soluble at room temperature; 1 5 soluble in a hot solvent; 12 5 partially soluble; 2 5 insoluble.b DMF 5 N,N-dimethylformamide; NMP 5 N-methylpyrrolidone; TCE 5 1,1,2,2-tetrachloroethane; DCE 5 1,2-dichloroethane;

THF 5 tetrahydrofurane; Py 5 pyridine.

Figure 6. DSC thermograms (first heating, solid line)and TMA thermograms (second heating, dashed line) ofpolymers Sa, Sb, Ta1, and Tb1 under the followingconditions: 60 cm3/min N2 flow and 10 °C/min heatingrate.

2388 MIKROYANNIDIS

The thermal stability of the polymers was eval-uated by TGA in N2 and air. The polymers did notshow weight loss up to 213–340 °C and affordedanaerobic char yields of 36–62% at 800 °C. Thepolyethers gave lower char yields than the poly-esters because of the presence of the thermallyunstable aliphatic moieties. The thermal stabilityof the polyethers increased with the reducedlength of the aliphatic spacer. Finally, the intro-duction of the anthracenyl groups along the poly-mer backbone improved thermal stability.

Optical Properties of the Polymers

Because the polymers contain oligophenyls in themain chain, they possess interesting optical prop-erties that were investigated both in solution andin the solid state. Figure 7 presents typical UV–vis spectra of polymers Sa and Tb1 in THF solu-tions. All the polymers exhibited absorptionbands at 260–400 nm due to the aromatic groups.They showed absorption maxima mainly around260 and 320 nm.

The polymers displayed ultraviolet fluores-cence, violet-blue fluorescence, or both. Figures 8and 9 present the emission spectra of the poly-mers in dilute THF solutions and in thin films,respectively. The films were prepared by spincoating on quartz plates from the solution of thepolymers in THF. An effort was made to preparethin films with dilute solutions of the polymersand relatively high spin rates. The fluorescencewavelengths at the peak maxima and the corre-sponding energies are summarized in Table I.Polyester Sa showed an emission maximum insolution at 342 nm, whereas Sb exhibited max-

ima at 401 and 420 nm. Obviously, the introduc-tion of the anthracenyl groups along the chainsinstead of phenyls decreased the electronicbandgap of the polymers5,6 and caused a remark-able redshift. In addition, Tb1 was redshiftedcompared with Ta1 and Tb2 in comparison withTa2. These redshifts ranged from 65 to 154 nm.

Polymers Sa, Ta1, and Ta2, bearing phenylpendent groups, showed considerable redshiftsgoing from solution to the solid state. This in-crease of conjugation was attributed to the exis-tence of appreciable stacking interactions in thesolid film and the formation of aggregates.26,27 Incontrast, polymers Sb, Tb1, and Tb2, containinganthracenyl pendent groups, displayed almostsimilar maxima in solution and in the solid state.

Figure 7. Absorption spectra of polymers Sa and Tb1in THF solutions.

Figure 8. Emission spectra for all the polymers inTHF solutions (excitation at 310 nm).

Figure 9. Emission spectra for all the polymers inthin films (excitation at 310 nm).


A reasonable explanation for this feature is thatthe bulky pendent anthracenyl groups and thenoncoplanar structure of the parent bisphenol 3breduced stacking interactions and caused a lessdense packing in the solid state.

The influence of the length of the aliphaticspacer on the fluorescence maxima was also stud-ied. Polyether Ta1, with 6 methylene units,showed a maximum in solution at 353 nm,whereas Ta2, with 10 methylene units, was blue-shifted at 333 nm. In contrast, as mentioned pre-viously, polyethers Tb1 and Tb2 displayed simi-lar maxima both in solution and in the solid state.

The fluorescence quantum yields (F) of thepolymers in THF solutions (Table I) were mea-sured28 relative to quinine sulfate (F 5 0.546).The experimental inaccuracies lead to an overallerror of approximately 10–15% in the final valuesof the quantum yields. The polymers bearing an-thracenyl pendent groups showed significantlyhigher quantum yields (0.11–0.26) than the cor-responding polymers with phenyl pendent groups(0.04). Thus, the introduction of the anthracenylinstead of phenyl as a pendent group increasedthe fluorescence efficiency of the polymers. How-ever, the polymers did not display high quantumyields in comparison with the quantum yield(0.99) of anthracene in the solid state.3 The non-coplanarity of the parent bisphenol 3b is probablyresponsible for this behaviour.

CONCLUSIONS

Two new bisphenols were synthesized throughpyrylium salts and used for the preparation ofrigid polyesters and semiflexible polyethers con-taining substituted m-terphenyl units in the mainchain. The polyesters were amorphous, whereaspolyether Ta2 showed a low degree of crystallin-ity. The polymers dissolved not only in polaraprotic solvents, strong acids, and pyridine butalso in 1,1,2,2-tetrachloroethane and THF. Theintroduction of pendent anthracenyl groups in-stead of phenyls reduced the Tg values of the rigidpolyesters but increased those of the semiflexiblepolyethers. In addition, the Tg’s of the polyetherswere reduced with the increasing length of thealiphatic moieties in the backbone. The whollyaromatic polyesters were more heat-resistantthan the corresponding polyethers with the ther-mally sensitive aliphatic segments. In any case,the attachment of the anthracenyl groups to themain chain improved thermal stability. The poly-

mers showed ultraviolet fluorescence, violet-bluefluorescence, or both with emission maxima at333–487 nm in solution and in the solid state. Thepolymers with anthracenyl pendent groups dis-played emission maxima redshifted and higherquantum yields than the corresponding polymerswith phenyl pendent groups. Moreover, the latterpolymers displayed emission maxima redshiftedupon going from solution to the solid state,whereas those of the former polymers remainedin the same range.

A grant from the General Secretariat of Research andTechnology in Greece is acknowledged.

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synthesis and properties of soluble, fluorescent polyesters and polyethers with substituted...

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