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Versatile synthesis of various conjugated aromatic homo- and copolymers
Synthetic Metals 122 (2001) 1±5
Versatile synthesis of various conjugated aromatic homo- and copolymers Rainer E. Martina, Florence Genestea, Beng Sim Chuaha, Cedric Fischmeistera, Yugang Maa, Andrew B. Holmesa,*, Robert Riehnb, Franco Caciallib, Richard H. Friendb a
Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK b Cavendish Laboratory, Department of Physics, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK
Abstract In recent years, variously substituted derivatives of poly(1,4-phenylene vinylene)s have emerged as ef®cient candidates for the emissive layer in polymer light emitting diodes. The synthetic routes for these polymers divide between precursor routes and those leading to fully conjugated solvent-processible polymers. The Gilch dehydrohalogenation polycondensation has largely been used for the latter class. In this presentation, we describe a novel family of 2,3-disubstituted aromatic precursors, derived from catechol, and we report their ef®cient polymerisation as homo- and copolymers with, for example, silyl-substituted derivatives to give materials which are highly ¯uorescent and serve as interesting materials in polymer LEDs. # 2001 Published by Elsevier Science B.V. Keywords: Gilch polymerisation; Conjugated polymers; Light emitting diodes
1. Introduction The research devoted to the synthesis of p-conjugated polymers has seen a notable increase over recent years [1±6]. The synthetic ¯exibility, potential ease of processing and the possibility of tailoring characteristic properties to attain a desired function makes semiconducting organic polymers attractive candidates for future optoelectronic devices such as light-emitting diodes (LEDs). Among the numerous conjugated polymeric structures that have been investigated for use as the emissive layer in LEDs, poly(arylene vinylene)s (PPVs) have been identi®ed as a particularly interesting class of polymers owing to the large synthetic versatility they offer. In particular, the synthesis of PPV copolymers has received considerable attention as various polymer parameters such as emission maxima, solubility, redox or ®lm forming properties can be manipulated to a signi®cant extent [7]. Recently, we have been interested in the investigation of PPVs bearing dialkoxy substituents at the 2,3-positions of the benzene ring, which led to the synthesis and characterisation of poly(2,3-dibutoxy-1,4-phenylene vinylene) (DBPPV, 1 [8]). DB-PPV showed a notable blue-shift of the longest wavelength absorption lmax accompanied by a
*
Corresponding author. Tel.: 44-1223-334370; fax: 44-1223-334866. E-mail address: [email protected] (A.B. Holmes). 0379-6779/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 1 3 1 8 - 7
considerable increase in the solid-state photoluminescence (PL) ef®ciency compared with classical 2,5-substituted PPV derivatives. For instance, solid ®lms of polymer 1 revealed a PL ef®ciency of 40%, whereas for the widely investigated poly[(2-methoxy-5-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) values between 15±20% have been reported [9,10]. A second class of conjugated materials exhibiting extremely high PL ef®ciencies of up 60% in the solid-state are PPVs containing silyl side-chains like poly(2-dimethyloctylsilyl-1,4-phenylene vinylene) (DMOS-PPV, 2) [11,12] and poly[bis(2,5-dimethyl-octylsilyl)-1,4-phenylene vinylene] (BDMOS-PPV, 3) [13]. However, devices fabricated from these two polymers suffered from rather high turn-on voltages of about 15 V.
2. Discussion We now wish to report a new and versatile synthetic route for the preparation of 2,3-dialkoxy-1,4-bis(benzylbromide) monomers, the synthesis of poly[2,3-bis(2-ethylhexyloxy)1,4-phenylene vinylene)] (BEH-PPV, 4) and the merger of the two important classes of PPVs affording the statistical copolymers poly[(2-dimethyloctylsilyl-1,4-phenylene viny-
2
R.E. Martin et al. / Synthetic Metals 122 (2001) 1±5
lene)-co-(2,3-di-butoxy-1,4-phenylene vinylene)] (DMOSco-DB-PPV, 5) and poly{[2,5-bis(dimethyloctylsilyl)-1,4phenylene vinylene]-co-(2,3-dibutoxy-1,4-phenylene vinylene)} (BDMOS-co-DB-PPV, 6), respectively. As expected, the new copolymers combine the intrinsically high EL ef®ciency present in the corresponding homopolymers with the considerably reduced turn-on voltages of 2,3-dialkoxysubstituted PPVs.
The new, improved synthetic route for 2,3-dialkoxy bis(bromomethyl)-benzenes has been applied to the synthesis of BEH-PPVand is illustrated in Scheme 1. Mannich reaction of catechol 7 with the in situ generated imine from formaldehyde and morpholine afforded diamine 8 in 56% yield [14,15]. Alkylation with 2-ethylhexylbromide in EtOH,
followed by acetylation gave compounds 9 and 10 in 93 and 65% yield, respectively. Saponi®cation and reaction of the bis(hydroxy)benzyl alcohol 11 with CBr4 and PPh3 in THF [16,17] furnished bis(bromomethyl)benzene 12 in excellent yield (98%). The Gilch polycondensation polymerisation of monomer 12 was performed in carefully degassed THF using excess KOt-Bu to ensure high degree of conversion. BEH-PPV was obtained as yellow ®bres after three repetitive precipitations from concentrated THF solutions into methanol in a fairly good yield of 51%. Interestingly, the molecular weight of Mn 51 000 and M w 400 000 determined by gel-permeation chromatography (GPC, calibrated with polystyrene standards) was found to be relatively low compared with the previously reported DB-PPV [8], which might be a direct consequence of the bulkier 2-ethylhexyl side-chains compared to the relatively short and sterically less demanding n-butoxy groups. UV±VIS measurements in CHCl3 at r.t. (Fig. 1) revealed for polymer 4, a longest-wavelength absorption maximum of lmax 446 nm with a clearly visible shoulder around 460 nm [DB-PPV: lmax (CHCl3 454 nm] [8]. Measurements on spin-coated ®lms gave a slightly hypsochromically shifted value of lmax 443 nm, accompanied by a more pronounced bathochromic shift of the shoulder to about 470 nm and a signi®cant overall broadening of the entire absorption band. The onset of absorption of BEH-PPV was 528 nm, corresponding to a solid-state band gap of Eopt 2:35 eV. The PL maximum of thin-®lms of polymer
Scheme 1. Synthesis of BEH-PPV.
R.E. Martin et al. / Synthetic Metals 122 (2001) 1±5
3
Fig. 1. Normalised optical absorption, PL and EL spectra of BEH-PPV measured as thin-film.
Fig. 2. Normalised optical absorption, PL and EL spectra of polymer 6 measured as thin-films.
4, appeared at lem 513 nm with a PL ef®ciency of 28%, which lies below the 40% reported for DB-PPV [8]. Thermal gravimetric analysis (TGA) of BEH-PPV revealed a 5% weight loss at 3598C. Cyclic voltammetry (CV) measurements (CH3CN, Ag/AgCl, reference Fc/Fc) on spin-coated ®lms of polymer 4, showed an onset of oxidation at about Eox;onset 1:0 V followed by a ®rst non-reversible oxidation event at Eox1 1:1 V (scan rate 10 mV/s). A second, quasireversible oxidation occurred at Eox2 1:3. In the cathodic scan, which was run at 100 mV/s, the onset of reduction was observed around Ered;onset ÿ1:6 V and a quasi-reversible reduction step took place at Ered ÿ1:8 V. From these redox potentials, the HOMO and LUMO energy levels of BBH-PPV were estimated to lie at ÿ5.4 and ÿ2.8 eV, respectively [18]. Double-layer PLEDs of the con®guration ITO/PEDOT (80 nm)/BEH-PPV (100 nm)/Ca emitted light with an emission maximum at lmax 505 nm and showed a turn-on voltage of 6.5 V [threshold 0:01 cd/m2, ITO: indium tin oxide, PEDOT: poly(3,4-ethylenedioxythiophene)]. The EL ef®ciency was 0.13 cd/A with a maximum luminance of 86 cd/m2 at 12 V. The synthesis of the two statistical copolymers DMOSco-DB-PPV 5 and BDMOS-co-DB-PPV 6 from the 2,3dibutoxy monomer 13, which was prepared according to the synthetic route outlined in Scheme 1, and the corresponding silylated bis(dihalo)benzyl derivatives 14 [12] and 15 [13] following a Gilch dehydrohalogenation route is given in Scheme 2.
In both polymerisations, a 1:1 feed ratio of the respective monomers 13/14 and 13/15 was employed. Both polymers are bright yellow solids, showing good solubilities in aprotic solvents like THF, toluene or CH2Cl2. The actual incorporation ratio (n:m) according to 1H-NMR side-chain analysis was n:m 3:4 for polymer 5 and n:m 4:3 in the case of 6. Molecular weight determination via GPC revealed for DMOS-co-DB-PPV, a number-average molecular weight of Mn 290 000. For BDMOS-co-DB-PPV a slightly lower value of Mn 180 000 was observed. UV±VIS absorption measurements performed in CHCl3 at r.t. showed for polymers 5 and 6 longest-wavelength absorption maxima at lmax 440 and 442 nm, respectively (Fig. 2). Interestingly, DMOS-co-DB-PPV displayed in the solid-state a slightly bathochromically shifted value of lmax 448 nm, whereas for BDMOS-co-DB-PPV the absorption is blue shifted (lmax 440 nm). The redox properties of BDMOS-coDB-PPV 4 were also measured on a thin polymer ®lm coated on Pt disk electrodes in MeCN as described above. They revealed an onset of oxidation at ca. 1.2 V followed by three subsequent non-reversible oxidations at 1.38, 1.63 and 1.80 V. The cathodic sweep showed onset of reduction at ca. ÿ1.6 Vand a quasi-reversible reduction step at ÿ1.83 V. The electrochemically measured band gap was 2.82 V, corresponding well with the optically measured HOMO-LUMO gap of 2.82 eV. Combining this information led to an estimate of the HOMO at 5.6 eV and the LUMO at 2.8 eV. The solid-state PL ef®ciencies of polymers 5 and 6 were 35 and 28%, respectively, and both polymers emitted light in
Scheme 2. Synthesis of PPV copolymers 5 and 6.
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R.E. Martin et al. / Synthetic Metals 122 (2001) 1±5
the blue±green region of the visible spectrum with longestwavelength emission maxima at lmax 548 nm (2.26 eV) and lmax 544 nm (2.28 eV), respectively. TGA measurements for DMOS-co-DB-PPV and BDMOS-co-DB-PPV showed a 5% weight loss at about 370 and 3208C, respectively. For BDMOS-co-DB-PPV DSC displayed an exothermic phase transition at about 1708C. Double-layer devices employing PEDOT as a hole transporting material and Al cathodes gave for DMOS-co-DB-PPV a turn-on voltage of 2.0±2.4 V with EL ef®ciencies of 0.05 cd/A and a maximum luminance of 36 cd/m2 at 11 V. Interestingly, LEDs fabricated from polymer 5 only showed EL emission with Ca cathodes if these were evaporated at pressures of some 1 mbar, but little or no EL was observed for devices fabricated at vacuum deposition pressures less than 3 10ÿ6 mbar. Substantially better performance was observed for devices made with BDMOS-co-DB-PPV and Ca cathodes. EL ef®ciencies were up to 0.72 cd/A with a maximum luminance of 1384 cd/m2 at 12 V and turn-on voltages of 4.0 V for a device having an active layer of 80 nm thickness. 3. Conclusion A new and versatile synthetic route for the preparation of 2,3-dialkoxy-1,4-bis(bromomethyl) monomers was developed. Both statistical copolymers, DMOS-co-DB-PPV and BDMOS-co-DB-PPV, respectively, showed a tuning of the linear optical properties with respect to the corresponding homopolymers. Devices with signi®cantly improved performance (both ef®ciency and luminance) were obtained using BDMOS-co-DB-PPV 6 as the emissive layer compared with devices made from DMOS-co-DB-PPV 5. 4. Experimental 4.1. Synthesis of polymer 4 To a solution of the monomer 12 (0.43 g, 0.83 mmol, 1 eq.) in dry and carefully degassed THF (25 ml) was added a solution of KOt-Bu (0.65 g, 5.78 mmol, 7.0 eq.) in THF (60 ml) within 30 min at 258C under an atmosphere of N2. After the reaction had been stirred for 18 h, the highly viscous and strongly green±yellow ¯uorescing solution was poured carefully into methanol (750 ml) and the precipitate was collected. The polymer was redissolved in THF (40 ml) precipitated from methanol (1200 ml) and the procedure was repeated using 10 ml THF and 750 ml methanol, yielding the polymer 4 in 51% as bright yellow ®bres (0.15 g) which turned deep yellow to orange upon being dried under high vacuum; nmax (CHCl3): 3004, 2964, 2929, 2877, 1601, 1461, 1436, 1382, 1285, 1196, 1052, 807 cmÿ1; lmax (CHCl3): 446 nm; lmax (®lm): 443 nm; GPC (CHCl3): Mw: 400 000, Mn: 51 000, Mw/Mn: 7.9; dH (500 MHz;
CDCl3): 7.51±7.44 (m, ArH), 3.91 (br s, OCH2), 1.82 (br s, CH), 1.55 (br s, CH2), 1.37 (br s, CH2), 0.98 (br s, CH3), 0.91 (br s, CH3); TGA: 3598C (5% weight loss); DSC: 2308C (decomp.). 4.2. Synthesis of copolymer 6 To a solution of the monomer 13 (0.14 g, 0.35 mmol, 1 eq.) and the monomer 15 (0.21 g, 0.35 mmol, 1 eq.) in dry and carefully degassed THF (25 ml) was added a solution of KOt-Bu (0.55 g, 4.89 mmol, 14.0 eq.) in THF (25 ml) within 30 min at 258C under an atmosphere of N2. After the reaction mixture had been stirred for 18 h, the highly viscous and strongly green±yellow ¯uorescing solution was poured carefully into a mixture of methanol/water (2:1, 500 ml) and the precipitate was collected. The resulting polymer was carefully washed with methanol (500 ml) to afford the copolymer 6 in 43% as bright yellow ®bres (0.10 g); nmax (CHCl3): 3041, 3016, 1603, 780 cmÿ1; lmax (CHCl3): 442 nm; lmax (®lm): 440 nm; GPC (CHCl3) Mw: 1 000 000, Mn: 180 000, Mw/Mn: 5.5; dH (500 MHz; CDCl3): 7.49±7.31 (br s, ArH), 3.89 (br s, OCH2), 1.80± 1.23 (m, alkyl), 0.98±0.84 (m, alkyl); TGA: 3188C (5% weight loss); DSC: 1708C (phase transition). Acknowledgements We thank the Engineering and Physical Sciences Research Council (UK) and the EPSRC Mass Spectrometry Service (Swansea), the Swiss National Science Foundation, Churchill College, Cambridge (Fellowship to REM), the Royal Society (University Research Fellowship to FC; BP China Fellowship to YM), the Commission of the European Union (support under the TMR network `SELOA', MarieCurie Fellowship (FG), Brite-Euram contract BRPR-CT970469 `OSCA'), and Cambridge Display Technology (CDT) for ®nancial support. References [1] F. Cacialli, Curr. Opin. Coll. Int. Sci. 4 (1999) 159. [2] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. Engl. 37 (1998) 402. [3] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. dos Santos, J.L. BreÂdas, M. LoÈgdlund, W.R. Salaneck, Nature 397 (1999) 121. [4] J.L. Segura, Acta Polym. 49 (1998) 319. [5] Special Issues on Molecular Materials for Electronic and Optoelectronic Devices, J. Mater. Chem. 9 (1999) 1853. [6] Special Issues on Molecular Materials for Electronic and Optoelectronic Devices, Acc. Chem. Res. 32 (1999) 191. [7] H. Spreitzer, H. Becker, E. Kluge, W. Kreuder, H. Schenk, R. Demandt, H. Schoo, Adv. Mater. 10 (1998) 1340. [8] B.S. Chuah, F. Cacialli, J.E. Davies, N. Feeder, R.H. Friend, A.B. Holmes, E.A. Marseglia, S.C. Moratti, J.-L. BreÂdas, D.A. dos Santos, Mater. Res. Soc. Symp. Proc. 488 (1998) 87. [9] D. Braun, A.J. Heeger, Appl. Phys. Lett. 58 (1991) 1982.
R.E. Martin et al. / Synthetic Metals 122 (2001) 1±5 [10] N.C. Greenham, I.D.W. Samuel, G.R. Hayes, R.T. Philips, Y.A.R.R. Kessener, S.C. Moratti, A.B. Holmes, R.H. Friend, Chem. Phys. Lett. 241 (1995) 89. [11] D.H. Hwang, S.T. Kim, H.K. Shim, A.B. Holmes, S.C. Moratti, R.H. Friend, Chem. Commun. (1996) 2241. [12] S.T. Kim, D.H. Hwang, X.-C. Li, J. GruÈner, R.H. Friend, A.B. Holmes, H.K. Shim, Adv. Mater. 8 (1996) 979. [13] H.-Y. Chu, D.H. Hwang, L.M. Do, J.-H. Jang, H.K. Shim, A.B. Holmes, T. Zyung, Synth. Metals 101 (1999) 216.
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[14] R.C. Helgeson, T.L. Tarnowski, J.M. Timko, D.J. Cram, J. Am. Chem. Soc. 99 (1977) 6411. [15] D.L. Fields, J.B. Miller, D.D. Reynolds, J. Org. Chem. 29 (1964) 2640. [16] C.J. Hawker, J.M.J. FreÂchet, J. Chem. Soc., Chem. Commun. (1990) 1010. [17] C.J. Hawker, J.M.J. FreÂchet, J. Am. Chem. Soc. 112 (1990) 7638. [18] J. Pommerehne, H. Vestweber, W. Guss, R.F. Mahrt, H. BaÈssler, M. Porsch, J. Daub, Adv. Mater. 7 (1995) 551.
Versatile synthesis of various conjugated aromatic homo- and copolymers Rainer E. Martina, Florence Genestea, Beng Sim Chuaha, Cedric Fischmeistera, Yugang Maa, Andrew B. Holmesa,*, Robert Riehnb, Franco Caciallib, Richard H. Friendb a
Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK b Cavendish Laboratory, Department of Physics, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK
Abstract In recent years, variously substituted derivatives of poly(1,4-phenylene vinylene)s have emerged as ef®cient candidates for the emissive layer in polymer light emitting diodes. The synthetic routes for these polymers divide between precursor routes and those leading to fully conjugated solvent-processible polymers. The Gilch dehydrohalogenation polycondensation has largely been used for the latter class. In this presentation, we describe a novel family of 2,3-disubstituted aromatic precursors, derived from catechol, and we report their ef®cient polymerisation as homo- and copolymers with, for example, silyl-substituted derivatives to give materials which are highly ¯uorescent and serve as interesting materials in polymer LEDs. # 2001 Published by Elsevier Science B.V. Keywords: Gilch polymerisation; Conjugated polymers; Light emitting diodes
1. Introduction The research devoted to the synthesis of p-conjugated polymers has seen a notable increase over recent years [1±6]. The synthetic ¯exibility, potential ease of processing and the possibility of tailoring characteristic properties to attain a desired function makes semiconducting organic polymers attractive candidates for future optoelectronic devices such as light-emitting diodes (LEDs). Among the numerous conjugated polymeric structures that have been investigated for use as the emissive layer in LEDs, poly(arylene vinylene)s (PPVs) have been identi®ed as a particularly interesting class of polymers owing to the large synthetic versatility they offer. In particular, the synthesis of PPV copolymers has received considerable attention as various polymer parameters such as emission maxima, solubility, redox or ®lm forming properties can be manipulated to a signi®cant extent [7]. Recently, we have been interested in the investigation of PPVs bearing dialkoxy substituents at the 2,3-positions of the benzene ring, which led to the synthesis and characterisation of poly(2,3-dibutoxy-1,4-phenylene vinylene) (DBPPV, 1 [8]). DB-PPV showed a notable blue-shift of the longest wavelength absorption lmax accompanied by a
*
Corresponding author. Tel.: 44-1223-334370; fax: 44-1223-334866. E-mail address: [email protected] (A.B. Holmes). 0379-6779/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 1 3 1 8 - 7
considerable increase in the solid-state photoluminescence (PL) ef®ciency compared with classical 2,5-substituted PPV derivatives. For instance, solid ®lms of polymer 1 revealed a PL ef®ciency of 40%, whereas for the widely investigated poly[(2-methoxy-5-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) values between 15±20% have been reported [9,10]. A second class of conjugated materials exhibiting extremely high PL ef®ciencies of up 60% in the solid-state are PPVs containing silyl side-chains like poly(2-dimethyloctylsilyl-1,4-phenylene vinylene) (DMOS-PPV, 2) [11,12] and poly[bis(2,5-dimethyl-octylsilyl)-1,4-phenylene vinylene] (BDMOS-PPV, 3) [13]. However, devices fabricated from these two polymers suffered from rather high turn-on voltages of about 15 V.
2. Discussion We now wish to report a new and versatile synthetic route for the preparation of 2,3-dialkoxy-1,4-bis(benzylbromide) monomers, the synthesis of poly[2,3-bis(2-ethylhexyloxy)1,4-phenylene vinylene)] (BEH-PPV, 4) and the merger of the two important classes of PPVs affording the statistical copolymers poly[(2-dimethyloctylsilyl-1,4-phenylene viny-
2
R.E. Martin et al. / Synthetic Metals 122 (2001) 1±5
lene)-co-(2,3-di-butoxy-1,4-phenylene vinylene)] (DMOSco-DB-PPV, 5) and poly{[2,5-bis(dimethyloctylsilyl)-1,4phenylene vinylene]-co-(2,3-dibutoxy-1,4-phenylene vinylene)} (BDMOS-co-DB-PPV, 6), respectively. As expected, the new copolymers combine the intrinsically high EL ef®ciency present in the corresponding homopolymers with the considerably reduced turn-on voltages of 2,3-dialkoxysubstituted PPVs.
The new, improved synthetic route for 2,3-dialkoxy bis(bromomethyl)-benzenes has been applied to the synthesis of BEH-PPVand is illustrated in Scheme 1. Mannich reaction of catechol 7 with the in situ generated imine from formaldehyde and morpholine afforded diamine 8 in 56% yield [14,15]. Alkylation with 2-ethylhexylbromide in EtOH,
followed by acetylation gave compounds 9 and 10 in 93 and 65% yield, respectively. Saponi®cation and reaction of the bis(hydroxy)benzyl alcohol 11 with CBr4 and PPh3 in THF [16,17] furnished bis(bromomethyl)benzene 12 in excellent yield (98%). The Gilch polycondensation polymerisation of monomer 12 was performed in carefully degassed THF using excess KOt-Bu to ensure high degree of conversion. BEH-PPV was obtained as yellow ®bres after three repetitive precipitations from concentrated THF solutions into methanol in a fairly good yield of 51%. Interestingly, the molecular weight of Mn 51 000 and M w 400 000 determined by gel-permeation chromatography (GPC, calibrated with polystyrene standards) was found to be relatively low compared with the previously reported DB-PPV [8], which might be a direct consequence of the bulkier 2-ethylhexyl side-chains compared to the relatively short and sterically less demanding n-butoxy groups. UV±VIS measurements in CHCl3 at r.t. (Fig. 1) revealed for polymer 4, a longest-wavelength absorption maximum of lmax 446 nm with a clearly visible shoulder around 460 nm [DB-PPV: lmax (CHCl3 454 nm] [8]. Measurements on spin-coated ®lms gave a slightly hypsochromically shifted value of lmax 443 nm, accompanied by a more pronounced bathochromic shift of the shoulder to about 470 nm and a signi®cant overall broadening of the entire absorption band. The onset of absorption of BEH-PPV was 528 nm, corresponding to a solid-state band gap of Eopt 2:35 eV. The PL maximum of thin-®lms of polymer
Scheme 1. Synthesis of BEH-PPV.
R.E. Martin et al. / Synthetic Metals 122 (2001) 1±5
3
Fig. 1. Normalised optical absorption, PL and EL spectra of BEH-PPV measured as thin-film.
Fig. 2. Normalised optical absorption, PL and EL spectra of polymer 6 measured as thin-films.
4, appeared at lem 513 nm with a PL ef®ciency of 28%, which lies below the 40% reported for DB-PPV [8]. Thermal gravimetric analysis (TGA) of BEH-PPV revealed a 5% weight loss at 3598C. Cyclic voltammetry (CV) measurements (CH3CN, Ag/AgCl, reference Fc/Fc) on spin-coated ®lms of polymer 4, showed an onset of oxidation at about Eox;onset 1:0 V followed by a ®rst non-reversible oxidation event at Eox1 1:1 V (scan rate 10 mV/s). A second, quasireversible oxidation occurred at Eox2 1:3. In the cathodic scan, which was run at 100 mV/s, the onset of reduction was observed around Ered;onset ÿ1:6 V and a quasi-reversible reduction step took place at Ered ÿ1:8 V. From these redox potentials, the HOMO and LUMO energy levels of BBH-PPV were estimated to lie at ÿ5.4 and ÿ2.8 eV, respectively [18]. Double-layer PLEDs of the con®guration ITO/PEDOT (80 nm)/BEH-PPV (100 nm)/Ca emitted light with an emission maximum at lmax 505 nm and showed a turn-on voltage of 6.5 V [threshold 0:01 cd/m2, ITO: indium tin oxide, PEDOT: poly(3,4-ethylenedioxythiophene)]. The EL ef®ciency was 0.13 cd/A with a maximum luminance of 86 cd/m2 at 12 V. The synthesis of the two statistical copolymers DMOSco-DB-PPV 5 and BDMOS-co-DB-PPV 6 from the 2,3dibutoxy monomer 13, which was prepared according to the synthetic route outlined in Scheme 1, and the corresponding silylated bis(dihalo)benzyl derivatives 14 [12] and 15 [13] following a Gilch dehydrohalogenation route is given in Scheme 2.
In both polymerisations, a 1:1 feed ratio of the respective monomers 13/14 and 13/15 was employed. Both polymers are bright yellow solids, showing good solubilities in aprotic solvents like THF, toluene or CH2Cl2. The actual incorporation ratio (n:m) according to 1H-NMR side-chain analysis was n:m 3:4 for polymer 5 and n:m 4:3 in the case of 6. Molecular weight determination via GPC revealed for DMOS-co-DB-PPV, a number-average molecular weight of Mn 290 000. For BDMOS-co-DB-PPV a slightly lower value of Mn 180 000 was observed. UV±VIS absorption measurements performed in CHCl3 at r.t. showed for polymers 5 and 6 longest-wavelength absorption maxima at lmax 440 and 442 nm, respectively (Fig. 2). Interestingly, DMOS-co-DB-PPV displayed in the solid-state a slightly bathochromically shifted value of lmax 448 nm, whereas for BDMOS-co-DB-PPV the absorption is blue shifted (lmax 440 nm). The redox properties of BDMOS-coDB-PPV 4 were also measured on a thin polymer ®lm coated on Pt disk electrodes in MeCN as described above. They revealed an onset of oxidation at ca. 1.2 V followed by three subsequent non-reversible oxidations at 1.38, 1.63 and 1.80 V. The cathodic sweep showed onset of reduction at ca. ÿ1.6 Vand a quasi-reversible reduction step at ÿ1.83 V. The electrochemically measured band gap was 2.82 V, corresponding well with the optically measured HOMO-LUMO gap of 2.82 eV. Combining this information led to an estimate of the HOMO at 5.6 eV and the LUMO at 2.8 eV. The solid-state PL ef®ciencies of polymers 5 and 6 were 35 and 28%, respectively, and both polymers emitted light in
Scheme 2. Synthesis of PPV copolymers 5 and 6.
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the blue±green region of the visible spectrum with longestwavelength emission maxima at lmax 548 nm (2.26 eV) and lmax 544 nm (2.28 eV), respectively. TGA measurements for DMOS-co-DB-PPV and BDMOS-co-DB-PPV showed a 5% weight loss at about 370 and 3208C, respectively. For BDMOS-co-DB-PPV DSC displayed an exothermic phase transition at about 1708C. Double-layer devices employing PEDOT as a hole transporting material and Al cathodes gave for DMOS-co-DB-PPV a turn-on voltage of 2.0±2.4 V with EL ef®ciencies of 0.05 cd/A and a maximum luminance of 36 cd/m2 at 11 V. Interestingly, LEDs fabricated from polymer 5 only showed EL emission with Ca cathodes if these were evaporated at pressures of some 1 mbar, but little or no EL was observed for devices fabricated at vacuum deposition pressures less than 3 10ÿ6 mbar. Substantially better performance was observed for devices made with BDMOS-co-DB-PPV and Ca cathodes. EL ef®ciencies were up to 0.72 cd/A with a maximum luminance of 1384 cd/m2 at 12 V and turn-on voltages of 4.0 V for a device having an active layer of 80 nm thickness. 3. Conclusion A new and versatile synthetic route for the preparation of 2,3-dialkoxy-1,4-bis(bromomethyl) monomers was developed. Both statistical copolymers, DMOS-co-DB-PPV and BDMOS-co-DB-PPV, respectively, showed a tuning of the linear optical properties with respect to the corresponding homopolymers. Devices with signi®cantly improved performance (both ef®ciency and luminance) were obtained using BDMOS-co-DB-PPV 6 as the emissive layer compared with devices made from DMOS-co-DB-PPV 5. 4. Experimental 4.1. Synthesis of polymer 4 To a solution of the monomer 12 (0.43 g, 0.83 mmol, 1 eq.) in dry and carefully degassed THF (25 ml) was added a solution of KOt-Bu (0.65 g, 5.78 mmol, 7.0 eq.) in THF (60 ml) within 30 min at 258C under an atmosphere of N2. After the reaction had been stirred for 18 h, the highly viscous and strongly green±yellow ¯uorescing solution was poured carefully into methanol (750 ml) and the precipitate was collected. The polymer was redissolved in THF (40 ml) precipitated from methanol (1200 ml) and the procedure was repeated using 10 ml THF and 750 ml methanol, yielding the polymer 4 in 51% as bright yellow ®bres (0.15 g) which turned deep yellow to orange upon being dried under high vacuum; nmax (CHCl3): 3004, 2964, 2929, 2877, 1601, 1461, 1436, 1382, 1285, 1196, 1052, 807 cmÿ1; lmax (CHCl3): 446 nm; lmax (®lm): 443 nm; GPC (CHCl3): Mw: 400 000, Mn: 51 000, Mw/Mn: 7.9; dH (500 MHz;
CDCl3): 7.51±7.44 (m, ArH), 3.91 (br s, OCH2), 1.82 (br s, CH), 1.55 (br s, CH2), 1.37 (br s, CH2), 0.98 (br s, CH3), 0.91 (br s, CH3); TGA: 3598C (5% weight loss); DSC: 2308C (decomp.). 4.2. Synthesis of copolymer 6 To a solution of the monomer 13 (0.14 g, 0.35 mmol, 1 eq.) and the monomer 15 (0.21 g, 0.35 mmol, 1 eq.) in dry and carefully degassed THF (25 ml) was added a solution of KOt-Bu (0.55 g, 4.89 mmol, 14.0 eq.) in THF (25 ml) within 30 min at 258C under an atmosphere of N2. After the reaction mixture had been stirred for 18 h, the highly viscous and strongly green±yellow ¯uorescing solution was poured carefully into a mixture of methanol/water (2:1, 500 ml) and the precipitate was collected. The resulting polymer was carefully washed with methanol (500 ml) to afford the copolymer 6 in 43% as bright yellow ®bres (0.10 g); nmax (CHCl3): 3041, 3016, 1603, 780 cmÿ1; lmax (CHCl3): 442 nm; lmax (®lm): 440 nm; GPC (CHCl3) Mw: 1 000 000, Mn: 180 000, Mw/Mn: 5.5; dH (500 MHz; CDCl3): 7.49±7.31 (br s, ArH), 3.89 (br s, OCH2), 1.80± 1.23 (m, alkyl), 0.98±0.84 (m, alkyl); TGA: 3188C (5% weight loss); DSC: 1708C (phase transition). Acknowledgements We thank the Engineering and Physical Sciences Research Council (UK) and the EPSRC Mass Spectrometry Service (Swansea), the Swiss National Science Foundation, Churchill College, Cambridge (Fellowship to REM), the Royal Society (University Research Fellowship to FC; BP China Fellowship to YM), the Commission of the European Union (support under the TMR network `SELOA', MarieCurie Fellowship (FG), Brite-Euram contract BRPR-CT970469 `OSCA'), and Cambridge Display Technology (CDT) for ®nancial support. References [1] F. Cacialli, Curr. Opin. Coll. Int. Sci. 4 (1999) 159. [2] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. Engl. 37 (1998) 402. [3] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. dos Santos, J.L. BreÂdas, M. LoÈgdlund, W.R. Salaneck, Nature 397 (1999) 121. [4] J.L. Segura, Acta Polym. 49 (1998) 319. [5] Special Issues on Molecular Materials for Electronic and Optoelectronic Devices, J. Mater. Chem. 9 (1999) 1853. [6] Special Issues on Molecular Materials for Electronic and Optoelectronic Devices, Acc. Chem. Res. 32 (1999) 191. [7] H. Spreitzer, H. Becker, E. Kluge, W. Kreuder, H. Schenk, R. Demandt, H. Schoo, Adv. Mater. 10 (1998) 1340. [8] B.S. Chuah, F. Cacialli, J.E. Davies, N. Feeder, R.H. Friend, A.B. Holmes, E.A. Marseglia, S.C. Moratti, J.-L. BreÂdas, D.A. dos Santos, Mater. Res. Soc. Symp. Proc. 488 (1998) 87. [9] D. Braun, A.J. Heeger, Appl. Phys. Lett. 58 (1991) 1982.
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