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New CF3-substituted PPV-type oligomers and polymers for use as hole blocking layers in LEDs
ELSEVIER
Synthetic Metals 84 (1997) 293-294
New CF3-substituted PPV-type oligomers and polymers for use as hole blocking layers in LEDs A. Luxa, A.B. Holme~~?~, R. Cervinia,
J.E. Daviesa, S.C. Morattib , J. Gri.inerc, F. Caciallic,
R.H. FriendC
aDepartment of Chemistry, University Chemical Laboratory University of Ccnnbridge, Lemfield Rwd, Cambridge CB2 IEW, U.K. bMelville Laboratory for Polymer Synthesis, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3R.4, U.K. ‘Department of Physics, Cavendish Laborafory Madingley Rod Cambridge CB2 OHE, U.K.
Abstract New CF+ubstituted PPV derivatives have been synthesised by Wittig-Horner polycondensation. To obtain a better understanding of the relationship between absorption and luminescence properties and structure a single crystal X-ray analysis of a model oligomer has been performed. The effect of electron withdrawing trifluoromethyl groups at vinylidene linkages in PPVs on absorption, luminescence, hole blocking and electron injecting properties has been investigated. Keywords:
1.
Wittig-Horner
reaction,
electron withdrawing
groups, hole blocking
layer, light emitting
diodes
Introduction
The discovery of semiconducting polymers which emit light as a result of double charge injection has stimulated extensive research [l, 21. Our current research interest is focused on the design of materials which operate in devices with improved efficiencies. Calculations have shown that the introduction of an electron withdrawing substituent onto the vinyl group of poly(p-phenylenevinylene) (PPV) would lower HOMO and LUMO energies of the compounds, thus permitting the construction of a device utilizing a higher work function metal [3]. The use of a high electron affinity PPV derivative containing electron withdrawing cyan0 groups as an emissive layer with PPV as hole transport layer and aluminium as cathode produced a highly efficient (4%) device [4]. We now report the synthesis of new PPV derivatives bearing electron withdrawing CFg-groups at the vinylidene unit and show that this modification of structure provides a new class of light emitting, hole blocking materials possessing a highly improved photooxidation stability. 2. Results
and
discussion
A facile synthetic pathway leading to PPV derivatives incorporating monosubstituted vinylidene linkages is given by Wittig-Horner polycondensation of aromatic diketones and diphosphonates and was reported first by HGrhold et ai. [5, 61 for the synthesis of phenylsubstituted PPVs. Following this procedure, new CF3-substituted polymers 3 were obtained by condensation of 2,5-dialkoxy-p-xylylene-bis(diethylphosphonate) 2 with appropriate bis(perfluoromethyl)ketones 1 in refluxing toluene under argon using ‘BuOK as base, according to Scheme 1. The assigned structures of new polymers 3 were established by NMR, IR and UV-VIS spectroscopy, GPC and microanalysis data. Both polymers were isolated as yellow, solution processible powders with good film-forming properties. Although polymer 3a exhibits a fully conjugated n-electron system along the polymer backbone there is no significant red shift in the absorption maximum (Table 1) in comparison to polymer 3b in which modified distyrylbenzene chromo03796779/97/317.00 Q 1997 Elsevier Science S.k All rights reserved PII s0379-6779(96)04012-x
3a
X=
-Q-
Scheme 1 phores are linked by flexible, nonconjugated spacers. This can be explained using the results of a single crystal X-ray analysis (Fig. 1) of model oligomer 4a, also synthesised by Wittig-Horner reaction according to Scheme 2. These show that the CFg-substituents not only force the lateral phenyl groups into a cis relationship but also cause them to be considerably twisted out of plane by an angle of 84’.
* -
‘0uOK toluen., reflux
4a R=H
4b
R = OCeH,,
4
Scheme 2
Fig. 1. Single crystal X-ray structure of oligomer
4a
294
A. Lux et al. /SynrheticMetals 84 (1997) 293-294
Table 1 Optical and Redox Properties of Polymers 3 and Oligomers 4 Compound &ax. PL (rim) QPL (%) Eox (v) hmax (rim) in CHC13 film film Polymer 3a 382 382 580 21 1.4 (h-rev) Polymer 375 y” Oligomer 3b4a 370 300 282 Oligomer 4b 366 370 465 acalculated from the onset of absorption (film)
11 -b 21
Hence, there can only be a small contribution of the lateral phenyl rings to the x-conjugation which would, assuming that the distyrylbenzene units in the polymers possess a similar geometry, explain the small difference of the absorption maxima of both polymers. To evaluate the influence of the CFs-substituents on donoracceptor properties of the materials cyclic voltammetry was performed on cast films. Oxidation potentials are significantly higher than PPV (0.85V) [7] implying that within a solid state device polymers 3 should possess good hole blocking properties. Reduction peaks of polymers 3a and 3b were observed at -1.3 and -1.6 V respectively. Hence, 3a is even easier to reduce than CN-PPV (-1.6V) [7], whereas 3b exhibits a similar reduction behavior. New polymers 3 therefore possess a considerable electron affinity and should be very suitable materials for electron transport applications. Preliminary investigations incorporating polymer 3a as electron transport material in PPV-based LEDs have been carried out showing that the internal device efficiency can be 5
Field
(mV/cm)
Fig. 2. EL vs applied electric field for a single layer device (ITOIPPVIAl) and a two layer device (ITOlPPVMaIAl) increased by two orders of magnitude in comparison to a single layer device (Fig. 2), owing to the hole blocking and electron transporting properties of the CFX-substituted polymer. Fig. 3 shows the time denendence of PL intensitv of a film of polymer 3 under constant UV irradiation in comparison to
2 0.6 op E , 0.4 i: 0.2
c
Eg,el.chem. (eV)
-1.3 @rev)
3.0
2.7
1.4 -b (h-rev) -1.6 !iyw) 1.1 (rev) -2.0 (irrev) bno measurement on account
3.0 yi” 3.8 3.1 3.1 of bad film quality
PPV. As can be seen, the nifluoromethyl groups not only increase the electron affinities of the PPV derivatives but also appear to enhance their stability against photooxidation remarkably. 3.
Experimental
Characterisation of polymer 3a: IH-NMR (200 MHz, CDC13) s 7.52 (IH, m, vinyl), 7.35 (2H, s, arom.), 6.18 (IH, m, arom.), 3.20 (m, 2H, OCHz), 1.5-0.7 (m, 15H, alkyl); IR (KBr) vmax/cm-l 2927 (s), 2857 (m), 1718 (m, C=O endgroup), 1495 (m), 1428 (m), 1298 (s), 1219 (s), 1146 (s), 1111 (s), 1025 (m), 926 (w), 862 (w), 703 (w); GPC (polystyrene standards, CHC13) Mn = 3,100; Mw = 3,900; PDI = 1.3, [Found: C, 68.4; H, 7.3. C34H~2F602requires C, 68.4; H, 7.1%]. Polymer 3b: ‘H-NMR (250 MHz, CDC13) 6 7.41 (s, lH, vinyl) 7.18 (d, 2H, arom.), 6.89 (d, 2H, arom.), 3.95 (m, OCH2 main chain), 3.28 (m, 2H, OCH2 side chain), 1.9-0.7 (m, alkyl); IR (KBr) vmax /cm-’ 2929 (s), 2857 (m), 1638 (w), 1608 (s), 1512 (s), 1423 (m), 1298 (m), 1248 (s), 1214 (s), 1171 (s), 1113 (s), 1027 (m), 793 (s); GPC (polystyrene standards, CHC13) Mn = 7,400; Mw = 11,100; PDI = 1.5. [Found C, 70.0; H, 7.4. C46HsgFeG4requires C, 70.0; H, 7.4%]. Cyclic voltammetry was conducted on spin-cast films in CHsCN (containing O.lM t-Bu4NCFsS03) in a three-electrode cell incorporating a Pt working electrode, a Ag electrode (Fc/Fc+ = 0.42V) and a Pt gauze counter electrode.
5
10
We thank the EPSRC, the European Commission (ESPRIT Project No. 8013 ‘LEDFOS’; Brite-EURAM Contract ‘POLYLED’, BRE2-CT93-0592) and the British Council (ARC and UK-Israel Science and Technology Research Fund) for financial support. 5.
111 PI c31 [41
[51 PPV
I.
0
Eg, opt. (eV
Acknowledgements
0
2l.O E0.6 .a
%t?dW)
[cl I
15
20
25
30
Time (min)
Fig. 3. Tie dependence of PL-intensity of polymer 4 and PPV under UV irradiation (excit. 351/364 nm, 30 mW/cm’)
171
References J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns and A.B. Holmes, Nature, 347 (1991) 539. G. Gustaffson, Y. Cao, G.M. Treaty, F. Klavetter, N. Colaneri and A.J. Heeger, Nature, 357 (1992) 477. J.L. Bredas and A.J. Heeger, C/rem. Phys. Z-&t. , 217 (1994) 507. NC. Greenham, SC. Moratti, D.D.C. Bradley, R.H. Friend and A.B. Holmes, Nature, 365 (1993) 628. H.-H. H&hold and S. SchBn, DD-Pat. 84272 (1971); Chem. Abst. 78 (1971) 12516. W. Kreuder, D. Lupo, J. Salbeck, H. Schenk, T. Stehlin, H.-H. H&hold, A. Lux, A. Teuschel, M. Wieduwilt,WO-Pat..96110617 (1996); Chem. Abstr. 124 (1996) 345038~. X.-C. Li, A. Kraft, R. Cervini, G.C.W. Spencer, F. Cacialli, R. H. Friend, J. Grtiner, A.B. Holmes, J.C. De Mello, S.C. Moratti, MRS Symp. Proc, 413 (1996) 13.
Synthetic Metals 84 (1997) 293-294
New CF3-substituted PPV-type oligomers and polymers for use as hole blocking layers in LEDs A. Luxa, A.B. Holme~~?~, R. Cervinia,
J.E. Daviesa, S.C. Morattib , J. Gri.inerc, F. Caciallic,
R.H. FriendC
aDepartment of Chemistry, University Chemical Laboratory University of Ccnnbridge, Lemfield Rwd, Cambridge CB2 IEW, U.K. bMelville Laboratory for Polymer Synthesis, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3R.4, U.K. ‘Department of Physics, Cavendish Laborafory Madingley Rod Cambridge CB2 OHE, U.K.
Abstract New CF+ubstituted PPV derivatives have been synthesised by Wittig-Horner polycondensation. To obtain a better understanding of the relationship between absorption and luminescence properties and structure a single crystal X-ray analysis of a model oligomer has been performed. The effect of electron withdrawing trifluoromethyl groups at vinylidene linkages in PPVs on absorption, luminescence, hole blocking and electron injecting properties has been investigated. Keywords:
1.
Wittig-Horner
reaction,
electron withdrawing
groups, hole blocking
layer, light emitting
diodes
Introduction
The discovery of semiconducting polymers which emit light as a result of double charge injection has stimulated extensive research [l, 21. Our current research interest is focused on the design of materials which operate in devices with improved efficiencies. Calculations have shown that the introduction of an electron withdrawing substituent onto the vinyl group of poly(p-phenylenevinylene) (PPV) would lower HOMO and LUMO energies of the compounds, thus permitting the construction of a device utilizing a higher work function metal [3]. The use of a high electron affinity PPV derivative containing electron withdrawing cyan0 groups as an emissive layer with PPV as hole transport layer and aluminium as cathode produced a highly efficient (4%) device [4]. We now report the synthesis of new PPV derivatives bearing electron withdrawing CFg-groups at the vinylidene unit and show that this modification of structure provides a new class of light emitting, hole blocking materials possessing a highly improved photooxidation stability. 2. Results
and
discussion
A facile synthetic pathway leading to PPV derivatives incorporating monosubstituted vinylidene linkages is given by Wittig-Horner polycondensation of aromatic diketones and diphosphonates and was reported first by HGrhold et ai. [5, 61 for the synthesis of phenylsubstituted PPVs. Following this procedure, new CF3-substituted polymers 3 were obtained by condensation of 2,5-dialkoxy-p-xylylene-bis(diethylphosphonate) 2 with appropriate bis(perfluoromethyl)ketones 1 in refluxing toluene under argon using ‘BuOK as base, according to Scheme 1. The assigned structures of new polymers 3 were established by NMR, IR and UV-VIS spectroscopy, GPC and microanalysis data. Both polymers were isolated as yellow, solution processible powders with good film-forming properties. Although polymer 3a exhibits a fully conjugated n-electron system along the polymer backbone there is no significant red shift in the absorption maximum (Table 1) in comparison to polymer 3b in which modified distyrylbenzene chromo03796779/97/317.00 Q 1997 Elsevier Science S.k All rights reserved PII s0379-6779(96)04012-x
3a
X=
-Q-
Scheme 1 phores are linked by flexible, nonconjugated spacers. This can be explained using the results of a single crystal X-ray analysis (Fig. 1) of model oligomer 4a, also synthesised by Wittig-Horner reaction according to Scheme 2. These show that the CFg-substituents not only force the lateral phenyl groups into a cis relationship but also cause them to be considerably twisted out of plane by an angle of 84’.
* -
‘0uOK toluen., reflux
4a R=H
4b
R = OCeH,,
4
Scheme 2
Fig. 1. Single crystal X-ray structure of oligomer
4a
294
A. Lux et al. /SynrheticMetals 84 (1997) 293-294
Table 1 Optical and Redox Properties of Polymers 3 and Oligomers 4 Compound &ax. PL (rim) QPL (%) Eox (v) hmax (rim) in CHC13 film film Polymer 3a 382 382 580 21 1.4 (h-rev) Polymer 375 y” Oligomer 3b4a 370 300 282 Oligomer 4b 366 370 465 acalculated from the onset of absorption (film)
11 -b 21
Hence, there can only be a small contribution of the lateral phenyl rings to the x-conjugation which would, assuming that the distyrylbenzene units in the polymers possess a similar geometry, explain the small difference of the absorption maxima of both polymers. To evaluate the influence of the CFs-substituents on donoracceptor properties of the materials cyclic voltammetry was performed on cast films. Oxidation potentials are significantly higher than PPV (0.85V) [7] implying that within a solid state device polymers 3 should possess good hole blocking properties. Reduction peaks of polymers 3a and 3b were observed at -1.3 and -1.6 V respectively. Hence, 3a is even easier to reduce than CN-PPV (-1.6V) [7], whereas 3b exhibits a similar reduction behavior. New polymers 3 therefore possess a considerable electron affinity and should be very suitable materials for electron transport applications. Preliminary investigations incorporating polymer 3a as electron transport material in PPV-based LEDs have been carried out showing that the internal device efficiency can be 5
Field
(mV/cm)
Fig. 2. EL vs applied electric field for a single layer device (ITOIPPVIAl) and a two layer device (ITOlPPVMaIAl) increased by two orders of magnitude in comparison to a single layer device (Fig. 2), owing to the hole blocking and electron transporting properties of the CFX-substituted polymer. Fig. 3 shows the time denendence of PL intensitv of a film of polymer 3 under constant UV irradiation in comparison to
2 0.6 op E , 0.4 i: 0.2
c
Eg,el.chem. (eV)
-1.3 @rev)
3.0
2.7
1.4 -b (h-rev) -1.6 !iyw) 1.1 (rev) -2.0 (irrev) bno measurement on account
3.0 yi” 3.8 3.1 3.1 of bad film quality
PPV. As can be seen, the nifluoromethyl groups not only increase the electron affinities of the PPV derivatives but also appear to enhance their stability against photooxidation remarkably. 3.
Experimental
Characterisation of polymer 3a: IH-NMR (200 MHz, CDC13) s 7.52 (IH, m, vinyl), 7.35 (2H, s, arom.), 6.18 (IH, m, arom.), 3.20 (m, 2H, OCHz), 1.5-0.7 (m, 15H, alkyl); IR (KBr) vmax/cm-l 2927 (s), 2857 (m), 1718 (m, C=O endgroup), 1495 (m), 1428 (m), 1298 (s), 1219 (s), 1146 (s), 1111 (s), 1025 (m), 926 (w), 862 (w), 703 (w); GPC (polystyrene standards, CHC13) Mn = 3,100; Mw = 3,900; PDI = 1.3, [Found: C, 68.4; H, 7.3. C34H~2F602requires C, 68.4; H, 7.1%]. Polymer 3b: ‘H-NMR (250 MHz, CDC13) 6 7.41 (s, lH, vinyl) 7.18 (d, 2H, arom.), 6.89 (d, 2H, arom.), 3.95 (m, OCH2 main chain), 3.28 (m, 2H, OCH2 side chain), 1.9-0.7 (m, alkyl); IR (KBr) vmax /cm-’ 2929 (s), 2857 (m), 1638 (w), 1608 (s), 1512 (s), 1423 (m), 1298 (m), 1248 (s), 1214 (s), 1171 (s), 1113 (s), 1027 (m), 793 (s); GPC (polystyrene standards, CHC13) Mn = 7,400; Mw = 11,100; PDI = 1.5. [Found C, 70.0; H, 7.4. C46HsgFeG4requires C, 70.0; H, 7.4%]. Cyclic voltammetry was conducted on spin-cast films in CHsCN (containing O.lM t-Bu4NCFsS03) in a three-electrode cell incorporating a Pt working electrode, a Ag electrode (Fc/Fc+ = 0.42V) and a Pt gauze counter electrode.
5
10
We thank the EPSRC, the European Commission (ESPRIT Project No. 8013 ‘LEDFOS’; Brite-EURAM Contract ‘POLYLED’, BRE2-CT93-0592) and the British Council (ARC and UK-Israel Science and Technology Research Fund) for financial support. 5.
111 PI c31 [41
[51 PPV
I.
0
Eg, opt. (eV
Acknowledgements
0
2l.O E0.6 .a
%t?dW)
[cl I
15
20
25
30
Time (min)
Fig. 3. Tie dependence of PL-intensity of polymer 4 and PPV under UV irradiation (excit. 351/364 nm, 30 mW/cm’)
171
References J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns and A.B. Holmes, Nature, 347 (1991) 539. G. Gustaffson, Y. Cao, G.M. Treaty, F. Klavetter, N. Colaneri and A.J. Heeger, Nature, 357 (1992) 477. J.L. Bredas and A.J. Heeger, C/rem. Phys. Z-&t. , 217 (1994) 507. NC. Greenham, SC. Moratti, D.D.C. Bradley, R.H. Friend and A.B. Holmes, Nature, 365 (1993) 628. H.-H. H&hold and S. SchBn, DD-Pat. 84272 (1971); Chem. Abst. 78 (1971) 12516. W. Kreuder, D. Lupo, J. Salbeck, H. Schenk, T. Stehlin, H.-H. H&hold, A. Lux, A. Teuschel, M. Wieduwilt,WO-Pat..96110617 (1996); Chem. Abstr. 124 (1996) 345038~. X.-C. Li, A. Kraft, R. Cervini, G.C.W. Spencer, F. Cacialli, R. H. Friend, J. Grtiner, A.B. Holmes, J.C. De Mello, S.C. Moratti, MRS Symp. Proc, 413 (1996) 13.