Macromolecules 1998, 31, 1099-1103
1099
Thermally Stable Blue-Light-Emitting Copolymers of Poly(alkylfluorene) M. Kreyenschmidt, G. Klaerner, T. Fuhrer, J. Ashenhurst, S. Karg, W. D. Chen, V. Y. Lee, J. C. Scott, and R. D. Miller* IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 Received June 23, 1997; Revised Manuscript Received December 15, 1997
ABSTRACT: We have prepared a variety of high molecular weight, thermally stable, blue-light-emitting random copolymers of 9,9-di-n-hexylfluorene by nickel(0)-mediated polymerization. The copolymers are readily soluble and easily processable from organic solvents. Both the polymer and electronic properties may be tuned by selection of comonomer structure. The electronic properties also vary with composition and film morphology. A blue-light-emitting device has been prepared using ionic salts for electrochemical doping.
Introduction Partridge1
The earlier work of and a more recent report by Burroughes et al.2 have rekindled significant interest in electroluminescent polymers. Polymers offer considerable processing advantages over small molecule systems,3 particularly for large area applications, since polymer films can be generated by solution deposition techniques and, in general, have higher glass transition temperatures. While electroluminescent organic polymers provide device processing advantages, a major drawback has been operating stability. Recently, however, significant advances have also been realized in this area.3,4 Efficient, stable blue-light-emitting materials are needed both to complete the luminescence color spectrum and to serve as energy-transfer donors when used in conjunction with added fluorophores.5-7 For bluelight emitters, a number of nonconjugated excimers8-10 as well as conjugated polymer exciplexes11,12 have also been described, often used in multilayer configurations. The former may be employed either alone or in conjunction with molecular dopants designed to modify both the charge transport and luminescence properties. The development of efficient blue-light-emitting conjugated organic polymers has been hampered because of the normally small band gaps of many fully conjugated polymers. For this reason, poly(p-phenylene) (PPP)13,14 is of interest since ring twisting caused by steric interactions limits the intrinsic conjugation length.15 The insolubility of the parent polymer, however, causes processing difficulties, and polymeric precursors are often employed.14,16 The use of bulky substituents in the main chain improves solubility17,18 but exacerbates the twisting in the main chain, further limiting the conjugation lengths. Recently ladder and stepladder polymeric variants have been studied in an effort to mediate the chain twisting.19-22 Alternatively, conjugation-interrupting substituents may be employed to limit the effective conjugation lengths in the polymer backbone. Blue-light-emitting poly(p-phenylenevinylenes),23-26 poly(oxadiazoles),27 and others28 have been prepared using this technique. Polymeric derivatives of fluorene present an interesting approach to blue-light-emitting polymers. The facile functionalization at C-9 offers the prospect of controlling
both polymer solubility and potential interchain interactions in films.29-31 These materials contain a rigidly planarized biphenyl structure in the fluorene monomer unit, while the remote substitution at C-9 produces less steric interaction in the conjugated backbone itself. The latter can lead to significant twisting in PPP derivatives, since the substituents used to control solubility are ortho to the aryl chain linkage, as is the case for the monocyclic monomers.17,18 Experimental Section The key starting material, 2,7-dibromo-9,9-di-n-hexylfluorene (Br2DHF), was prepared by bromination of 9,9-di-nhexylfluorene using excess bromine in dimethylformamide in the presence of 5% Pd/C. The crude product was purified by chromatography over silica gel followed by recrystallization from petroleum ether-acetone-methanol (90%, mp 156-57 °C). The comonomer dibromides Br2DPF (mp 73 °C) and Br26F (mp 82 °C) were prepared from the corresponding commercially available diamines by diazotization (HBr-NaNO2) in the presence of Cu2Br2. The other bromides shown in Figure 1 were prepared by literature procedures.32 The homopolymer poly(9,9-di-n-hexylfluorene-2,7-diyl) and the random copolymers33 were prepared by nickel-mediated coupling of the respective bromides as described by Yamamoto et al.34 A Schlenk tube containing DMF, bis(1,5-cyclooctadienyl)nickel(0), 2,2′-bipyridyl, and 1,5-cyclooctadiene (molar ratios, 1:1:1) was heated under argon to 80 °C for 0.5 h. The comonomers dissolved in minimal, degassed toluene (molar ratio of dibromide/nickel complex ) 0.65) were added under argon to the DMF solution and the polymerization was maintained at 80 °C for 2-3 days in the dark. The polymers were precipitated by addition of the hot solution dropwise to a equivolume mixture of concentrated HCl, methanol, and acetone. The isolated polymers were then dissolved in toluene or methylene chloride and reprecipitated with methanol-acetone (1:1). This procedure was repeated several times, and the copolymers were dried at 80 °C (in a vacuum).
Results and Discussion The oxidative polymerization of fluorene itself as well as simple 9-alkyl-substituted derivatives has been reported.35,36 The polymers produced by this technique are of low molecular weight (DP ∼10),36 and it is difficult to remove all traces of the oxidant. Despite this, bluelight-emitting devices from these polymers have been described.37 We now describe the preparation of high molecular weight polymers from 2,7-dibromo-9,9-di-n-
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1100 Kreyenschmidt et al.
Macromolecules, Vol. 31, No. 4, 1998 Table 2. Copolymer Electronic Propertiesa entry
copolymer composition
1 2a 2b 2c 2d 3a 3b 4a 4b 5a 5b 6a 6b 7a 7b
DHFDHF-15-DHB DHF-25-DHB DHF-50-DHB DHF-15-DHB-Ab DHF-15-DPF DHF-25-DPF DHF-15-6F DHF-25-6F DHF-15-CST DHF-25-CST DHF-15-TST DHF-25-TST DHF-15-BB DHF-25-BB
λmax (nm) UV emission 388 376 365 350 325, 308 (sh) 378 372 378 374 382 378 390 388 374 368
420 417 417 414 375 415 415 422 422 434 432 430 432 419 418
Φfla 0.69 0.67 0.54 0.36 0.70 0.67 0.68 0.79 0.61 0.54
a Solution fluorescence quantum yields measured in methylene chloride relative to 9,10-diphenylanthracene. b 1:1 alternating copolymer generated by Suzuki coupling.
Figure 1. Aryl dibromide monomers and copolymers formed by nickel-mediated coupling. Table 1. Copolymer Polymer Propertiesa entry
copolymer composition
1 2a 2b 2c 2d 3a 3b 4a 4b 5a 5b 6a 6b 7a 7b
DHF-100 DHF-15-DHB DHF-25-DHB DHF-50-DHB DHF-50-DHB-Ad DHF-15-DPF DHF-25-DPF DHF-15-6F DHF-25-6F DHF-15-CST DHF-25-CST DHF-15-TST DHF-25-TST DHF-15-BB DHF-25-BB
DSCb TGAc yield Mwa (%) (×10-3) Mw/Mn (Tg, °C) (5%) 83 94 88 71 55 92 88 82 83 82 83 60 35 75 74
45.0 66.8 69.0 33.6 10.0 55.0 89.5 73.5 206.0 26.7 26.5 46.2 28.2 20.2 24.7
2.5 2.0 2.3 2.7 1.8 2.7 2.9 2.9 1.9 3.9 2.2 2.8 3.2 2.3 1.9
100 75 58 153 197 125 171 103 125 103 95 71 90
430 435 420 425 430 450 440 380 420 450 445 440 420 460 465
a Molecular weights determined by GPC analysis using polystyrene standards. b Glass transition temperatures as determined by DSC, 10 °C/min. c Temperature (°C) for 5% weight loss (TGA, 10 °C/min). d 1:1 alternating copolymer generated by Suzuki coupling.
hexylfluorene using zerovalent nickel. Both homo- and copolymers can be prepared in high yield using this technique. Very recently Pei and Yang38 have reported the preparation of homopolymers such as poly[9,9-bis(3,6-dioxaheptyl)fluorene-2,7-diyl] using a related procedure. In our work, we have prepared a number of dialkylfluorene-containing copolymers where the comonomer is used both to influence polymer solubility and to control the effective conjugation length either by steric interactions or by the deliberate introduction of tetrahedral conjugation interruptions. Table 1 lists the polymer properties of the representative copolymers. In general, the isolated polymer yields were good and the molecular weights high, sometimes exceptionally so. In the case of the homopolymer (DHF-
100), the degree of polymerization was ∼54 based on GPC analysis using polystyrene standards. The copolymers were soluble in common organic solvents such as THF, chloroform, xylene, chlorobenzene, etc. The copolymers derived from Br2TST were less soluble when the comonomer composition exceeded 25 mol %. The thermal stability of the homo- and copolymers was excellent with decomposition temperatures (5% weight loss measured by TGA analysis, 10 °C/min) in excess of 400 °C. The effect of copolymer composition on the polymer Tg (DSC analysis, 20 °C/min) depends on the nature of the comonomer. In general, the Tg value of the copolymers increases with increasing comonomer composition, sometimes significantly so (see entries 3-5 and 7, Table 1). The copolymers derived from Br2DHB are anomalous in this regard, since the copolymer Tg steadily decreases with increasing comonomer concentration (Table 1, entry 2). In the case of copolymers derived from Br2TST, the Tg is little affected, at least over the limited compositional range studied. In Table 1, entry 2d describes the 50/50 alternating copolymer derived from the palladium-catalyzed Suzuki polymerization39 of Br2DHF with 2,5-di-n-hexyl-1,4-bis(boronic acid). The polymer and electronic properties of this material were significantly different from those of the 1:1 statistical random copolymer (Table 1, entry 2c). In each of the copolymer examples cited, the monomer composition of the statistical, random copolymers was reflected in the respective monomer feed ratio. This analysis was performed by 1H NMR integration of the C-9 methylene signals from the dihexylfluorene unit (δ ) 1.8-2.4) relative to either the olefinic (entries 5 and 6, Table 1) or aromatic signals in the copolymers. In the case of the olefinic comonomers Br2CST and Br2TST, each is incorporated into the copolymers without significant isomerization of the double bond as determined by the 1H NMR chemical shifts of the respective vinyl protons in comparison with model compounds (δ ) 6.46.6 for cis vs 7.1-7.3 for the trans isomer). In a number of cases, the copolymer molecular weights seem to peak for comonomer compositions in the range of 25-35%. The stilbene-containing copolymers (Table 1, entries 5 and 6) appear to be somewhat anomalous in this regard. The spectroscopic properties of the homopolymer poly(9,9-dihexylfluorene-2,7-diyl) (DHF-100) and various copolymers are shown in Table 2. The solvent for the
Macromolecules, Vol. 31, No. 4, 1998
Blue-Light-Emitting Copolymers of Poly(alkylfluorene) 1101
Figure 2. Solution (CH2Cl2) UV spectra of copolymers of DHF-DPF.
solution studies was methylene chloride. The fluorescence quantum yields were determined by excitation of the copolymer at λmax and comparison with the solution emission of 9,10-diphenylanthracene (Φfl ) 0.83).40 In general, the introduction of comonomers with tetrahedral conjugation breaks leads to blue-shifts in the copolymer λmax relative to DHF-100 which increase with increasing comonomer concentration (Table 2, entries 3, 4, and 7). Figure 2 shows this behavior for various compositions of the DHF-DPF copolymer. This effect becomes quite significant for comonomer concentrations exceeding 25%. In the case of copolymers derived from Br2TST (Table 2, entries 6a,b), the absorption maxima are virtually identical with those of the homopolymer DHF-100, although the comonomer concentration range in this case is limited by solubility. The absorption spectra of the copolymer films are generally similar in shape to those in solution, although the absorption maxima are usually blue-shifted by a few nanometers. DHF-25-BB is anomalous in that the absorption spectra of thin films (
