Containing Carbazole and Oxadiazole Dipolar Pendent
Groups and Its Application to Electroluminescent Devices
MAO-CHUAN YUAN, PING-I SHIH, CHEN-HAN CHIEN, CHING-FONG SHUDepartment of Applied Chemistry, National Chiao Tung University, Taiwan, Republic of China
Received 17 January 2007; accepted 14 February 2007 DOI: 10.1002/pola.22048
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: We have synthesized a blue-light-emitting polyfluorene (PF) derivative (PF-CBZ-OXD) that presents bulky hole-transporting carbazole and electron-trans-porting oxadiazole pendent groups functionalized at the C-9 positions of alternating fluorene units. The results from photoluminescence and electrochemical measure-ments indicate that both the side chains and the PF main chain retain their own electronic characteristics in the copolymer. An electroluminescent device incorporat-ing this polymer as the emittincorporat-ing layer was turned on at 4.5 V; it exhibited a stable blue emission with a maximum external quantum efficiency of 1.1%. Moreover, we doped PF-CBZ-OXD and its analogue PF-TPA-OXD with a red-light-emitting irid-ium phosphor for use as components of phosphorescent red-light emitters to investi-gate the effect of the host’s HOMO energy level on the degree of charge trapping and on the electrophosphorescent efficiency. We found that spectral overlap and individual energy level matching between the host and guest were both crucial features affect-ing the performance of the electroluminescence devices. Atomic force microscopy measurements indicated that the dipolar nature of PF-CBZ-OXD, in contrast to the general nonpolarity of polydialkylfluorenes, provided a stabilizing environment that allowed homogeneous dispersion of the polar iridium triplet dopant.VVC 2007 Wiley Peri-odicals, Inc. J Polym Sci Part A: Polym Chem 45: 2925–2937, 2007
Keywords: atomic force microscopy; charge transport; conjugated polymers; light-emitting diodes
INTRODUCTION
The feasibility of utilizing low-cost solution pro-cesses for the preparation large-area display devi-ces makes polymeric materials very attractive for use as active components in such applications as flat-panel displays and solid state lighting.1 Elec-troluminescent polymers that have sufficiently large band gaps such that they emit blue light effi-ciently are of particular interest because they can
be used either as blue light sources in full-color displays or as host materials for lower-energy flu-orescent or phosphflu-orescent dyes.2–10 Because of their high photoluminescence (PL) and electrolu-minescence (EL) efficiencies and thermal stabil-ities, polyfluorenes (PFs) are very promising can-didates for use as such blue-light-emitting materi-als.11–14 In addition, the facile functionalization at the C-9 position of the fluorene unit provides the opportunity to improve both the solubility and processability of the PFs, while also offering the ability to tune their optoelectronic proper-ties.9,15–21Moreover, PFs may be readily color-tuned through chemical incorporation with low-bandgap
Correspondence to: C.-F. Shu (E-mail: [email protected]) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 2925–2937 (2007) V
comonomers or through physical doping with lower-energy fluorescent and/or phosphorescent dyes.2–10,22–38Consequently, PFs can function as both the host and the blue emitter in white-light-emitting devices.39–44
Triarylamines have emerged as an attractive class of hole injecting/transporting materials for use in organic light-emitting diodes because of their relatively low ionization potentials and high mobilities.45 Mu¨ llen and coworkers first intro-duced triphenylamino groups into PFs as side chains.15Subsequently, we reported a highly effi-cient blue-light-emitting PF derivative PF-TPA-OXD that featured both hole-transporting triphe-nylamine (TPA) units and electron-transporting oxadiazole (OXD) moieties functionalized at the C-9 positions of fluorene moieties.17 These bulky bipolar pendent groups provided two functions simultaneously: suppressing aggregate/excimer formation and improving charge injection/trans-portation. This blue fluorene-based copolymer is also a very promising host material for use in molecularly doped electrophosphorescence devices because of its wide energy gap and large number of charge-transporting pendent groups, which pro-vide improved confinement of triplet excitons in the phosphorescent dopants and more-balanced charge transportation, respectively.7
Carbazoles (CBZs) are another well-established group of hole-transporting materials46–48 They have been incorporated into the main chains, or attached to the side chains, of light-emitting poly-mers to improve their hole injection and
transpor-tation properties.34,49–53 Herein, we report the synthesis, characterization, and device perform-ance of a new statistical bipolar copolymer PF-CBZ-OXD bearing hole-transporting CBZ units together with electron-transporting OXD pendent groups at the C-9 positions of fluorene moieties. For most CBZ-substituted PFs, the pendent groups have been attached to the main chains through a long alkyl chains; PF-CBZ-OXD repre-sents the first example of a fluorene-based copoly-mer containing CBZ units functionalized directly at the C-9 positions of fluorene moieties. Moreover, we used this copolymer and PF-TPA-OXD as host materials for a red-light-emitting iridium complex, and have investigated the effect of the host’s HOMO energy level on the degree of charge trap-ping and on the electrophosphorescent efficiency. We have found that spectral overlap and individ-ual energy level matching between the host and guest are both crucial features affecting the per-formance of EL device; hole trapping in the guest and subsequent recombination with an electron injected from the metal electrode requires a suffi-ciently higher oxidation potential for the host than for the guest.
RESULTS AND DISCUSSION
Polymer Synthesis and Characterization
Scheme 1 illustrates the synthetic route followed for the preparation of the dibromo monomer 5.
The acid-promoted condensation reaction between 2,7-dibromofluorenone (1) and 9-octyl-CBZ (2) gave the CBZ/fluorene hybrid 3. This method resembles a procedure that we used recently to introduce N-phenylcarbazole groups onto the C-9 position of fluorene.54 Subsequent Friedel-Crafts acylation of 3 with acetyl chloride yielded com-pound 4, on which the acetyl groups were reduced to ethyl groups through Wolff-Kishner reduction with NH2NH2/KOH to furnish the desired di-bromo monomer 5, containing two 3-ethyl-9-octyl-carbazol-6-yl moieties substituted at the C-9 posi-tion of 2,7-dibromofluorene. Previous reports have indicated that CBZ derivatives undergo oxidative CC couplings at the 3- and 6-positions of the CBZ ring.55We hoped that the introduction of 3,6-disubstituted CBZs into the monomer 5 would block the electrochemically active sites of the CBZ ring to give the resultant polymer additional elec-trochemical stability.
As indicated in Scheme 2, the statistical PF co-polymer PF-CBZ-OXD, which possessed bulky hole-transporting CBZ and electron-transporting OXD pendent groups at the C-9 positions its fluo-rene units, was synthesized through Suzuki coupling of the dibromides 5 and 6 with the dibor-onate 7 at a mole ratio of 1.0:1.0:2.0. This copoly-merization was performed using Pd(PPh3)4as the
catalyst in a mixture of toluene and aqueous K2CO3(2.0 M) in the presence of Aliquat 336 as a phase transfer reagent. After the polymerization process was complete, the end groups of the poly-mer chains were capped through sequential reac-tions with phenylboronic acid and bromobenzene, each under reflux for 12 h. The structure of the re-sultant copolymer was confirmed by 1H and 13C NMR spectroscopy. In the13C NMR spectrum, we observe signals at d 65.8, 66.0, and 55.3 that corre-spond to the C-9 carbon atoms of the three differ-ent fluorene units in PF-CBZ-OXD. These signals are almost superimposed with the signals of the C-9 carbon atoms of the monomers 5, 6, and 7.
Because of the presence of solubilizing n-octyl chains in the CBZ pendent groups and fluorene units, PF-CBZ-OXD dissolves readily in com-mon organic solvents, such as toluene, chloro-form, dichloromethane, chlorobenzene, and THF. The molecular weight of this polymer was
deter-mined using size exclusion chromatography
(SEC; eluent: THF), calibrating against polysty-rene standards. The polymer possesses a num-ber-average molecular weight (Mn) of 1.6 3 104 g/mol, with a polydispersity index of 1.8. We investigated the thermal properties of PF-CBZ-OXD by performing differential scanning calo-rimetry (DSC) and thermogravimetric analysis
(TGA). In the DSC measurement, a distinct glass transition was observed at 1758C (see the inset of Fig. 1), which is much higher than that of poly(9,9-dioctylfluorene) (POF; Tg ¼ 67 8C).56 In addition, the value of Tg of PF-CBZ-OXD is slightly higher than that of a bipolar fluorene co-polymer PF-TPA-OXD (Tg¼ 166 8C).17It is evi-dent that the presence of rigid CBZ and oxadiazole groups at the C9 positions of the fluorene repeat-ing units provided a sterically bulky structure that enhanced the chain rigidity of PF-CBZ-OXD and restricted its segmental mobility, resulting in its much higher value of Tg. Such a high value of Tg, which could prevent morphological change and suppress the formation of aggregates and excimers upon exposure to heat, is desirable for polymers that are to be used as emissive materials for light-emitting applications. As revealed by TGA (Fig. 1), PF-CBZ-OXD exhibited excellent thermal stability; its 5 and 10% weight losses occurred at 428 and 4448C, respectively.
Photophysical Properties
We measured the optical properties of PF-CBZ-OXD both in dilute solution and in the solid state; Figure 2 presents these spectra and Table 1 summarizes the spectral data. In chloroform solution, PF-CBZ-OXD exhibits a main absorp-tion peak at 392 nm and a shorter-wavelength absorption at 300 nm. From a comparison of this spectrum with the absorption spectra of POF, 9-octylcarbazole, and
2,5-di(4-tert-butyl-phenyl)-1,3,4-oxadiazole (t-BuOXD), we ascribe the band at 392 nm to a p-p* transition derived from the conjugated PF backbone and we assign the band at 300 nm to the combined absorp-tions of the CBZ and OXD pendent groups. Upon excitation of the polymer main chain at 392 nm, the emission spectrum displays a vibronic fine structure with two sharp bands at 421 and 444 nm, which are nearly identical to the characteristic emission bands obtained from POF (kmax ¼ 418, 442 nm). This result suggests that the incorporation of bulky CBZ and OXD groups onto fluorene units via their C-9 carbon atoms does not perturb the conjugation of the main chain. Upon irradiation at 300 nm, the blue fluorescence PF-CBZ-OXD was identical to that under excitation of the POF backbone at 390 nm, we could not detect any luminescence at 360 nm arising from the side chains. The fact that we observed complete quenching of the side chain emissions-even in a very dilute
solu-Figure 1. TGA trace of PF-CBZ-OXD recorded at a heating rate 10 8C/min. Inset: DSC trace recorded at a heating rate of 208C/min.
Figure 2. UV–vis absorption and PL spectra of (a) PF-CBZ-OXD (excited at 390 nm), (b) POF (excited at 390 nm), (c) 9-octylcarbazole (excited at 333 nm), and (d) t-BuOXD (excited at 300 nm) in dilute chloro-form solutions. The PL spectrum of PF-CBZ-OXD excited at 300 nm (dashed line) is also included in (a).
tion (108 M)-indicates that efficient intrachain energy transfer occurred in the PF-CBZ-OXD copolymer as a result of good spectral overlap between the emission spectrum of the pendent groups and the absorption spectrum of the PF backbone (Fig. 2). In addition, these pendent groups and the p-conjugated backbone were con-nected through a common tetrasubstituted car-bon atom (C9); the close distance between the donor and acceptor may also account for the highly efficient energy transfer. Consequently, most excitons that formed in the CBZ and OXD side chains through direct photoexcitation were likely to migrate to the lower-energy main chain, from which emission occurred. The PL quantum yield in toluene when excited at 365 nm was 1.0, measured relative to 9,10-diphenylan-thrancene (Ff¼ 0.9) as a standard.57In compar-ison to the behavior in dilute solution, the absorption spectrum of a film that had been spin-coated onto a quartz substrate was slightly broadened, while the emission spectrum dis-played a red-shift of 7 nm with maxima at 428 and 451 nm.
Electrochemical Studies
We employed cyclic voltammetry to investigate the redox behavior of PF-CBZ-OXD and to esti-mate its HOMO and LUMO energy levels. The electrochemical processes of this polymer film coated on a glassy carbon electrode were moni-tored in a standard three-electrode electrochemi-cal cell using ferrocene as the internal standard in an electrolyte of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile at a scanning rate of 50 mV/s. On the basis of the onset potentials of the oxidation and reduction,
which were 0.79 and 2.32 V, respectively, we estimated the HOMO and LUMO energy levels
of PF-CBZ-OXD to be 5.59 and 2.48 eV,
respectively, with regard to the energy level of ferrocene (4.8 eV below vacuum). The high-lying HOMO level may originate from the electron-rich nature of the CBZ moiety; it is in agree-ment with the data reported previously for PF copolymers containing CBZ side chain (values from 5.58 to 5.61 eV).52 At the same time, we attribute the low-lying LUMO level to the elec-tron-deficient nature of the pendent OXD units; its value is similar to that reported for an OXD-substituted PF (2.47 eV).17 In comparison with
PF-TPA-OXD, for which the HOMO level
occurs at 5.30 eV, the replacement of the TPA units with CBZ moieties decreases the HOMO level of PF-CBZ-OXD markedly (to 5.59 eV). It is noteworthy that this lower-lying HOMO level significantly facilitated hole trapping at electrophosphorescent iridium dopants when the CBZ copolymer was employed as the host emit-ting material; consequently, we observed effi-cient electrophosphorescence (vide infra).
EL Properties of LED Devices
To evaluate the potential use of PF-CBZ-OXD as a blue emissive material in polymer LED applica-tions, we fabricated a blue-emitting device hav-ing the configuration ITO/PEDOT (35 nm)/PF-CBZ-OXD (50–70 nm)/TPBI (30 nm)/Mg:Ag (100 nm)/Ag (100 nm). The PEDOT layer was used as a hole injection layer to facilitate hole conduction, and also to smoothen the relatively rough ITO layer; the TPBI layer, deposited through thermal evaporation, was employed as an electron-trans-porting and hole-blocking layer. As indicated in
Table 1. Optical Properties of PF-CBZ-OXD, POF, 9-Octyl-carbazole, and t-BuOXD
Absorption, kmax(nm) PL, kmax(nm)
Fac f
Solutiona Filmb Solutiona Filmb
PF-CBZ-OXD 300, 392 306, 390 421, 444 428, 451 1.0
POF 389 390 418, 442 424, 448 0.83
9-octyl-carbazole 296, 333, 348 370, 386
t-BuOXD 294 344, 358
aIn chloroform.
bSpin-coated from their CHCl
3solutions. c
The relative quantum yield was measured with reference to 9,10-diphenylanthracene in toluene (F ¼ 0.90).
the inset of Figure 3, this device emitted a deep-blue color-with two sharp bands at 428 and 452 nm in the EL spectrum-corresponding to CIE color coordinates of (0.16, 0.06) at 7 V. These fea-tures in the EL spectrum are nearly identical, with respect to their position and shape, to those in the solid state PL spectrum mentioned above, indicating that both the PL and EL originate from the same radiative decay process of singlet excitons. Unlike devices constructed from poly-dialkylfluorenes, which exhibit an undesirable emission band between 500 and 600 nm resulting from keto defects, aggregation, and/or excimer formation,58–60 the device based on PF-CBZ-OXD exhibited a voltage-independent and stable
EL spectrum. There was no significant change in the appearance of the EL spectra upon increasing the applied voltage from 7 to 11 V (i.e., potential near to that required to achieve maximum brightness). Figure 3 displays the current–volt-age (I-V) and luminance–voltcurrent–volt-age (L-V) character-istics of the device turned on at 4.5 V (corre-sponding to 1 cd/m2); the maximum external quantum efficiency was 1.1% at 17.6 mA/cm2, which was superior to that of polydialkylfluorene and comparable to those of other PF deriva-tives.16,17,61,62 In addition, a reference device with the same device configuration using POF as the light-emitting material also exhibited inferior performance, with the maximum external quan-tum efficiency of 0.78%.63Table 2 summaries the performance of this PF-CBZ-OXD-based device; further improvements may be possible after opti-mizing the device structure.
PF-CBZ-OXD as a Host Material in a Red-Light-Emitting Electrophosphorescent Device
The bipolar PF derivative PF-TPA-OXD has been used extensively as a host material for the fabrication of electrophosphorescent polymer LEDs,5–7 mainly because of its improved hole-and electron-transporting capabilities. In this study, we employed the novel bipolar PF deriva-tive PF-CBZ-OXD, which possesses hole-trans-porting CBZ and electron-transhole-trans-porting OXD side chains, as a polymeric host doped with
the red-light-emitting phosphorescent dye
Table 2. Performances of Devices Having the Structure ITO/PEDOT/Polymer Emitting Layer/TPBI/Mg:Ag PF-CBZ-OXD PF-CBZ-OXD: Ir(FPQ)2(acac) PF-TPA-OXD: Ir(FPQ)2(acac) Turn-on voltage (V)a 4.5 5.6 4.2 Voltage (V)b 5.9 (7.8) 10.3 (13.0) 8.3 (10.9) Brightness (cd/m2)b 111 (540) 1760 (7380) 1020 (4050) Luminance efficiency (cd/A)b 0.55 (0.54) 8.8 (7.4) 5.1 (4.1) External quantum efficiency (%)b 1.00 (1.00) 8.6 (7.2) 4.8 (3.8) Maximum brightness (cd/m2) 1070 (at 11) 8630 (at 14 V) 5490 (at 13 V) Maximum luminance efficiency (cd/A) 0.58 8.8 5.2 Maximum external quantum efficiency (%) 1.1 8.6 4.8 EL maximum (nm)c 430 632 624
CIE coordinates, x and yc 0.16 and 0.06 0.67 and 0.32 0.48 and 0.23
aRecorded at 1 cd/m2.
bRecorded at 20 mA/cm2, data in parentheses were recorded at 100 mA/cm2. cRecorded at 7 V.
Figure 3. Current density–voltage–luminance char-acteristics of ITO/PEDOT/PF-CBZ-OXD/TPBI/Mg:Ag. Inset: PL spectrum and corresponding EL spectra recorded at different applied voltages.
Ir(FPQ)2(acac) to realized an efficient phosphor-escent polymer light-emitting diode (PLEDs). The device architecture was ITO/PEDOT (35 nm)/ PF-CBZ-OXD:1.1 mol % of Ir(FPQ)2(acac) (50– 70 nm)/TPBI (30 nm)/Mg:Ag (100 nm)/Ag (100 nm). To investigate the effect of the HOMO level of the host material on the device efficiency, we also fabri-cated a control device in which we replaced PF-CBZ-OXD with PF-TPA-OXD as the host material at the same doping concentration. Figure 4(a) depicts the PL and EL spectra of the PF-CBZ-OXD:Ir(FPQ)2(acac) device. The PL profile con-tains two components: the one occurring at 426 nm is a characteristic emission of the PF-CBZ-OXD host; the other at623 nm corresponds to the tri-plet emission of Ir(FPQ)2(acac).64In contrast, the corresponding EL spectrum is completely domi-nated by the dopant emission, i.e., a saturated-red triplet emission from the Ir-complex. The dramatic difference between the PL and EL spectra reveals that Fo¨rster energy transfer of an exciton from the host to the dopant does not account solely for the observed EL. Another dominant mechanism for exciting Ir(FPQ)2(acac) would be direct charge trapping at the dopant sites, followed by recombina-tion with opposite charges.7,65,66On the other hand, the PL and EL spectra of the PF-TPA-OXD:Ir(FP-Q)2(acac) blends are quite similar in terms of their
shape and position [Fig. 4(b)], indicating that energy transfer remained as the main operating mechanism in the EL process.
According to the energy level diagram (Fig. 5), the HOMO level of PF-CBZ-OXD occurs at5.6 eV, which is lower than that of PF-TPA-OXD (5.3 eV); this subtle change in the HOMO level energy has a major impact on both the current density and EL efficiency. In the case of the PF-CBZ-OXD:Ir(FPQ)2(acac) blend, the ionization potential of Ir(FPQ)2(acac) is 0.6 eV below the HOMO level of PF-CBZ-OXD; therefore, holes can be trapped efficiently at the guest and subse-quently they can recombine with opposite charges (electrons) to form excitons. Upon switching the host to PF-TPA-OXD, Ir(FPQ)2(acac) becomes a much less effective trap site for holes because of the shallow trap (depth: 0.3 eV) constructed by Ir(FPQ)2(acac) in the PF-TPA-OXD host. Once the holes are trapped at the HOMO of Ir(FP-Q)2(acac), there is a high possibility that they may detrap and hop back to the HOMOs of the TPA moieties by overcoming this smaller energy barrier; as a result, the excitons eventually reform at the host.7 Thus, most of excitons still form at the host under the influence of the electric field; subsequent energy transfer to the dopant contrib-utes to the triplet emission in a manner similar to that which we observed in the PL spectra.
Figure 6 displays the current density versus voltage (I-V) characteristics of Ir(FPQ)2 (acac)-doped devices that employ PF-CBZ-OXD and PF-TPA-OXD as host materials, respectively. It is apparent that the operating voltage of the PF-CBZ-OXD-based devices was higher than that of the PF-TPA-OXD device. This result is consistent with the charge trapping mechanism proposed previously for the PF-CBZ-OXD de-vice. Figure 7 presents the EL performances of these two devices; Table 2 summarizes the data. The maximum external quantum efficiency of PF-CBZ-OXD-blend reached as high as 8.6% (8.8 cd/A) at 23.0 mA/cm2; conversely, the PF-TPA-OXD-blend, which does not feature an effi-cient charge trapping mechanism in its EL pro-cess, exhibited a relatively poor performance.
Moreover, the PF-CBZ-OXD-based device
exhibited a satisfying saturated red emission that was located precisely at the standard red region (0.67, 0.32) of the CIE diagram for the National Television Systems Committee (NTSC) color system.
Phase separation processes in host–guest blends may affect their EL performance. To
Figure 4. PL and EL spectra of (a) PF-CBZ-OXD and (b) PF-TPA-OXD, both doped with 1.1 mol % Ir(FPQ)2(acac).
investigate the effects of the CBZ and OXD pendent groups on the degree of dispersion of the polar organometallic triplet dopant in the polymeric host, we used atomic force microscopy
(AFM) to investigate the phase morphology of
the PF-CBZ-OXD:Ir(FPQ)2(acac) blend. For
the sake of comparison, we also prepared a POF : Ir(FPQ)2(acac) blend. Figure 8 displays
Figure 7. Plots of external quantum efficiency and brightness versus current density of Ir(FPQ)2
(acac)-doped devices. Figure 6. Plots of current density versus voltage of
1.1 mol % Ir(FPQ)2(acac)-doped devices.
Figure 5. Energy diagram (in eV) for materials involved in EL devices having the configuration ITO/PEDOT/polymer/TPBI/Mg:Ag. The chemical structures of PF-TPA-OXD and Ir(FPQ)2(acac) are provided.
the surface topographies of the spin-coated films. The doped PF-CBZ-OXD film was smooth and featureless with a root-mean-square (RMS) surface roughness of 0.35 nm, revealing that the film was homogeneous with no phase separation or aggregation. In contrast, the doped POF film displayed the some hill-like structures and pos-sessed a higher surface roughness (0.55 nm), suggesting the formation of aggregates. These results indicate that the dipolar PF-CBZ-OXD, in contrast to the nonpolar POF, provides a compatible environment for the iridium complex and, consequently, it prevents the formation of aggregates.
CONCLUSIONS
We have synthesized a thermally stable blue-light-emitting polymer, PF-CBZ-OXD, which contains bulky hole-transporting CBZ and elec-tron-transporting OXD pendent groups at the C-9 positions of alternating fluorene units. A light-emitting diode device prepared using PF-CBZ-OXD as the emitting layer exhibited an ef-ficient, stable, blue-light emission; its turn-on voltage was 4.5 V and its maximum external quantum efficiency was 1.1%. To investigate the
effect of the host’s HOMO energy level on the degree of charge trapping and on the electro-phosphorescence efficiency, we doped PF-CBZ-OXD and its analogue, PF-TPA-PF-CBZ-OXD, with an iridium phosphor, Ir(FPQ)2(acac), for use as components of phosphorescent red-light emitters in PLEDs. We found that spectral overlap and individual energy level matching between the host and guest are both crucial features affect-ing the performance of these EL devices. The
HOMO energy level of PF-CBZ-OXD (5.6 eV)
was lower than that of PF-TPA-OXD (5.3 eV); this subtle change in the HOMO energy level has a major impact on both the current density and EL efficiency. Consequently, the maximum external quantum efficiency of the PF-CBZ-OXD blend reached as high as 8.6% (8.8 cd/A) at 23.0 mA/cm2; conversely, the PF-TPA-OXD blend, which did not feature an efficient charge trapping mechanism in its EL process, exhibited relatively poor performance.
EXPERIMENTAL
Materials
2,7-Dibromofluorenone (1),67 and 9-octyl-CBZ (2),55the monomers 6 and 7,16,68Ir(FPQ)2(acac),64
Figure 8. AFM images (tapping mode) of (a) POF and (b) PF-CBZ-OXD, both doped with 1.1 mol % Ir(FPQ)2(acac). [Color figure can be viewed in the online
and PF-TPA-OXD17 were prepared according to reported procedures. Solvents were dried using standard procedures. All other reagents were used as received from commercial sources, unless otherwise stated.
Characterization 1
H and 13C NMR spectra were recorded on a
Bruker-DRX 300 (300 MHz) spectrometer. Mass spectra were obtained using a JEOL JMS-HX 110 mass spectrometer. Size exclusion chromatography (SEC) was performed using a Waters chromatogra-phy unit interfaced with a Waters 410 differential refractometer; three 5-lm Waters styragel columns (3003 7.8 mm2) were connected in series in order of decreasing pore size (104, 103, and 102 A˚ ); THF was the eluent. Standard polystyrene samples were used for calibration. Differential scanning calorime-try (DSC) was performed using a SEIKO EXSTAR 6000DSC unit operated at heating and cooling rates of 20 and 40 8C/min, respectively. Samples were scanned from 30 to 2508C, cooled to 0 8C, and then scanned again from 30 to 2508C. The glass transi-tion temperatures (Tg) were determined from the second heating scans. Thermogravimetric analysis (TGA) was undertaken using a Perkin-Elmer TGA Pyris 1 instrument. The thermal stabilities of the samples were determined under a nitrogen atmos-phere by measuring their weight losses while heat-ing at a rate of 20 8C/min. UV–vis spectra were measured using an HP 8453 diode-array spectro-photometer. PL spectra were obtained using a Hita-chi F-4500 luminescence spectrometer. Cyclic vol-tammetry measurements were performed using a BAS 100 B/W electrochemical analyzer operated at a scan rate of 50 mV/s; the solvent was anhydrous acetonitrile and 0.1 M tetrabutylammonium hexa-fluorophosphate (TBAPF6) was the supporting elec-trolyte. The potentials were measured against an Ag/Ag+(0.01 M AgNO3) reference electrode; ferro-cene was the internal standard. The onset poten-tials were determined from the intersection of two tangents drawn at the rising and background cur-rents of the cyclic voltammogram. Atomic force microscopy measurements were undertaken in the tapping mode using a Digital Nanoscope IIIa instrument under ambient conditions.
2,7-Dibromo-9,9-bis(9-octylcarbazol-3-yl)-fluorene (3) Eaton’s reagent (7.7 wt % P2O5in CH3SO3H, 2.0 mL) was added to a solution of 1 (2.42 g, 7.14 mmol) and 2 (4.38 g, 15.7 mmol) in CH2Cl2 (20
mL), which was then stirred under nitrogen and heated under reflux for 2 h. The cooled mixture was diluted with CH2Cl2and washed with aque-ous sodium bicarbonate. The organic phase was dried (MgSO4) and concentrated under reduced pressure. The crude product was purified by col-umn chromatography, eluting with CH2Cl2 /hex-ane (1:8), followed by recrystallization from cyclohexane to afford 3 (2.10 g, 33.4%) as white crystals. 1H NMR (300 MHz, CDCl3): d 0.86 (t, J ¼ 6.6 Hz, 6H), 1.25–1.40 (m, 20H), 1.84–1.88 (m, 4H), 4.26 (t, J ¼ 7.4 Hz, 4H), 7.15 (td, J ¼ 7.4, 0.9 Hz, 2H), 7.29 (d, J ¼ 9.0 Hz, 2H), 7.33 (dd, J ¼ 8.7, 1.5 Hz, 2H), 7.37 (d, J ¼ 8.1 Hz, 2H), 7.43 (td, J ¼ 7.4, 0.9 Hz, 2H), 7.50 (dd, J ¼ 8.1, 1.5 Hz, 2H), 7.64–7.66 (m, 4H), 7.92–7.95 (m, 4H). 13C NMR (75 MHz, CDCl3): d 14.1, 22,6, 27.3, 29.0, 29.1, 29.3, 31.8, 43.2, 65.9, 108.6108.7, 118.7, 119.5, 120.5, 121.6, 121.8, 122.5, 122.6, 125.7, 126.0, 129.7, 130.7, 135.4, 138.0, 139.4, 140.8, 154.6. HRMS (m/z): [M + H]+ calcd. for C53H5579Br2N2, 877.2732; found, 877.2726. Anal. Calcd. for C53H54Br2N2: C, 72.43; H, 6.19; N, 3.19. Found: C, 72.44; H, 5.90; N, 3.43.
2,7-Dibromo-9,9-bis(3-acetyl-9-octylcarbazol-6-yl)-fluorene (4)
Acetyl chloride (0.60 g, 3.69 mmol) was added drop wise to a mixture of 3 (1.50 g, 1.71 mmol) and aluminum chloride (1.37 g, 10.3 mmol) in dry ben-zene (100 mL) at 258C. The reaction mixture was stirred at room temperature for 3 h, and then was poured into water (100 mL) and extracted with ethyl acetate (33 50 mL2). The combined organic layers were dried (MgSO4) and concentrated under reduced pressure. The crude product was recrystallized from ethyl acetate to afford 4 (1.14 g, 69.5%) as yellow crystals. 1H NMR (300 MHz, CDCl3): d 0.84 (t, J ¼ 6.8 Hz, 6H), 1.32–1.34 (m, 20H), 1.84–1.88 (m, 4H), 2.65 (s, 6H), 4.27 (t, J ¼ 7.2 Hz, 4H), 7.33–7.38 (m, 6H), 7.52 (dd, J ¼ 8.4, 1.5 Hz, 2H), 7.61 (d, J ¼ 1.5 Hz, 2H), 7.67 (d, J ¼ 8.1 Hz, 2H), 7.98 (d, J ¼ 0.9 Hz, 2H), 8.07 (dd, J ¼ 8.7, 1.5 Hz, 2H), 8.55 (d, J ¼ 1.2 Hz, 2H). 13C NMR (75 MHz, CDCl3): d 14.1, 22.6, 26.7, 27.3, 29.0, 29.1, 29.3, 29.7, 31.7, 43.5, 65.8, 108.3, 109.3, 119.9, 121.8, 121.9, 122.0, 122.4, 123.3, 126.6, 128.7, 129.5, 131.0, 136.7, 138.0, 140.3, 143.6, 154.1, 197.8. HRMS (m/z): [M + H]+ calcd. for C57H5979Br2N2O2, 961.2943; found, 961.2933. Anal. Calcd. for C57H58Br2N2O2: C, 71.10; H, 6.07; N, 2.91. Found: C, 70.81; H, 6.06; N, 3.30.
2,7-Dibromo-9,9-bis(3-ethyl-9-octylcarbazol-6-yl)-fluorene (5)
A mixture of 4 (1.13 g, 1.17 mmol), hydrazine (0.56 g, 17.6 mmol), potassium hydroxide (1.20 g, 21.4 mmol), and diethylene glycol (100 mL) was stirred at 190 8C for 12 h under N2. The cooled mixture was poured into water (200 mL) and extracted with ethyl acetate (3 3 50 mL2). The organic phases were combined, dried (MgSO4), and con-centrated under reduced pressure. The crude prod-uct was purified by column chromatography, elut-ing with hexane/ethyl acetate (1:10), followed by recrystallization from hexane/ethanol to afford 5 (0.50 g, 45.4%) as yellow crystals. 1H NMR (300 MHz, CDCl3): d 0.84 (t, J ¼ 6.6 Hz, 6H), 1.23–1.34 (m, 26H), 1.78–1.85 (m, 4H), 2.70–2.78 (q, J ¼ 7.5 Hz, 4H), 4.21 (t, J ¼ 7.2 Hz, 4H), 7.26–7.32 (m, 8H), 7.48 (d, J ¼ 8.3, 1.7 Hz, 2H), 7.61–7.64 (m, 4H), 7.74 (s, 2H), 7.91 (s, 2H).13C NMR (75 MHz, CDCl3): d 14.1, 16.6, 22.6, 27.3, 28.9, 29.0, 29.1, 29.4, 31.8, 43.2, 65.9, 108.5, 108.6, 119.3, 119.6, 121.5, 121.8, 122.6, 125.8, 126.0, 129.7, 130.6, 134.8, 135.2, 138.0, 139.3, 139.7, 154.7. HRMS (m/ z): [M + H]+ calcd. for C57H6379Br2N2, 933.3358; found, 933.3370. Anal. Calcd. for C57H62Br2N2: C, 73.23; H, 6.68; N, 3.00. Found: C, 73.30; H, 7.06; N, 3.41.
PF-CBZ-OXD
Aqueous potassium carbonate (2.0 M, 0.8 mL) and Aliquat 336 ( 20 mg) were added to a mix-ture of 5 (60 mg, 64 lmol), 6 (56 mg, 64 lmol), and 7 (82.5 mg, 128 lmol) in toluene (1.5 mL). The mixture was degassed and tetrakis(triphe-nylphosphine)palladium (5.0 mg, 7.0 mol %) was added in one portion under N2; the solution was then heated at 110 8C for 36 h. The end groups were capped by heating the mixture under reflux for 12 h with benzeneboronic acid (31.1 mg, 0.25 mmol) and then for 12 h with bromo-benzene (40.1 mg, 0.25 mmol). The reaction mix-ture was cooled to room temperamix-ture and pre-cipitated into a mixture of methanol and water (7:3 v/v, 100 mL). The crude polymer was col-lected, washed with excess methanol, dissolved in THF, and then reprecipitated into methanol. Finally, the polymer was washed with acetone for 48 h using a Soxhlet apparatus and then dried under vacuum to give PF-CBZ-OXD (110 mg, 75.6%). 1H NMR (300 MHz, CDCl3): d 0.68– 0.77 (m, 20H), 0.79–0.82 (m, 6H), 1.00–1.04 (m, 40H), 1.20–1.50 (s, 44H), 1.82–1.98 (m, 12H) 2.71–2.72 (m, 4H), 4.20 (br, 4H), 7.50–7.52 (m, 20H), 7.63–7.92 (m, 20H), 8.00–8.13 (m, 12H). 13 C NMR (75 MHz, CDCl3): d 13.9, 14.0, 14.1, 16.6, 22.5, 22.6, 22.7, 23.8, 27.3, 28.9, 29.1, 29.3, 29.7, 29.9, 30.3, 31.1, 31.6, 31.7, 31.8, 31.9, 35.1, 40.3, 43.2, 55.2, 55.3, 65.8, 66.0, 108.4, 119.3, 119.7, 120.4, 120.8, 121.0, 121.3, 122.5, 122.8, 124.5, 125.1, 125.5, 126.0, 126.5, 126.7, 127.1, 127.2, 127.3, 127.6, 128.3, 128.4, 128.5, 128.7, 128.9, 131.7, 131.9, 132.0, 132.2, 134.7, 136.7, 138.9, 139.2, 139.6, 140.1, 141.1, 141.8, 149.2, 150.7, 151.6, 151.8, 153.9, 155.3, 164.0, 164.7. Light-Emitting Devices
Polymer LED devices were fabricated in the con-figuration ITO/poly(styrenesulfonate)-doped poly (3,4-ethylenedioxythiophene) (PEDOT) (35 nm)/ light-emitting layer (50–70 nm)/TPBI (30 nm)/ Mg:Ag (100 nm)/Ag (100 nm). To improve hole injection and substrate smoothness, the PEDOT was spin-coated directly onto the ITO glass and dried at 80 8C for 12 h under vacuum. The light-emitting layer was spin-coated on top of the PEDOT layer, using toluene as the solvent, and then dried under vacuum for 3 h at 608C. Prior to casting the film, the polymer solution was filtered through a Teflon filter (0.45 lm). The TPBI layer, which was grown by thermal sublimation in a vacuum of 33 106torr, was used as an electron-transport layer that blocked holes and confined excitons. The cathode Mg:Ag (10:1, 100 nm) alloy was deposited onto the TPBI layer through coeva-poration of the two metals; an additional layer of Ag (100 nm) was deposited onto the alloy as a pro-tection layer. The current–voltage–luminance characteristics were measured under ambient conditions using a Keithley 2400 source meter and a Newport 1835C optical meter equipped with an 818ST silicon photodiode.
We thank the National Science Council and MOE ATU Program for financial support. Our special thanks go to C.-H. Cheng for his support during the preparation and characterization of the light-emitting devices.
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