and were used as received.

2.2. Characterization

¹H and ¹³C NMR spectra were obtained on a JEOL ALPHA300 instrument at 25˚C at 300 and 75 MHz, respectively. Deuterated chloroform was used as a solvent with tetramethylsilane as an internal standard. Numberand weight-average molecular weights (M_n and M_w) and polydispersity index (PDI) were determined by gel permeation chromatography (GPC) analysis calibrated by standard polystyrene samples with a JASCO RI-2031 detector eluted with chloroform at a flow rate of 0.5 ml/min at room temperature. Differential scanning calorimetry (DSC) analysis was performed on a Rigaku DSC-8230 under nitrogen atmosphere at heating and cooling rates of 10 ˚C/min. UV-vis absorption spectra were obtained on a JASCO V-570 spectrophotometer and fluorescence spectra were obtained with JASCO FP-6500 spectrophotometer with an excitation at 380 nm. Cyclic voltammetry (CV) was conducted on a Niko Keisoku Model NPGFZ- 2501-A potentiogalvanostat. All measurements were carried out at room temperature in a typical three-electrode cell with a working electrode (glassy carbon electrode), a reference electrode (Ag/AgCl), and a counter electrode (Pt wire) at a scanning rate of 0.1 V/s. In all measurements, acetonitrile and tetrabutylammonium tetrafluorocarbonate (Bu₄NBF₄) (0.1 M) were used as a solvent and a supporting electrolyte, respectively.

2.3. Synthesis of 3,7-Dibromo-N-butylphenylphenoxazine (2)

To a 100 mL flask equipped with a stopcock and a condenser were placed N-butylphenylphenoxazine (1) (0.44 g, 1.39 mmol), chloroform (20 mL), acetic acid (20 mL), N-bromosuccinimide (NBS) (0.52 g, 2.91 mmol) in 0˚C, and the mixture was stirred for 10 h. After the reaction the solution extracted with chloroform, and organic layer washed with water and dried with MgSO₄. After chloroform was removed by a rotary evaporator, the crude product was purified by recrystallization with hexane. Light yellow solid was obtained. The yield was 0.49 g (75%). ¹H NMR (300 MHz, CDCl₃) δ (ppm): 0.95 (t, 3H), 1.4 (m, 2H), 1.70 (m, 2H), 2.75 (t, 2H), 5.68 (d, 2H), 5.75 (d, 2H), 6.79 (s, 2H), 7.16 (d, 2H), 7.38 (d, 2H). ¹³C NMR (75 MHz, CDCl₃) δ (ppm): 14.0, 22.4, 33.5, 35.4, 112.6, 114.4, 118.5, 126.2, 129.9, 131.2, 133.4, 135.4, 143.9, 144.1.

2.4. Synthesis of 3-Bromophenoxazine (3)

To a 50 mL flask equipped with a stopcock were placed phenoxazine (1.00 g, 5.50 mmol) and distilled THF (20 mL). THF solution (15 mL) of NBS (0.97 g, 5.5 mmol) was added dropwise to the mixture and stirred for 1 h. After the filtration and the evaporation of THF, the product was resolved in ethyl acetate and washed with water 3 times. After dried with MgSO₄, and solvent was removed with a rotary-evaporator. The crude product was purified by column chromatography on silica gel with hexane:ethyl acetate = 1:1 as an eluent. The yield was 0.62 g (42%). ¹H NMR (300 MHz, CDCl₃) δ (ppm): 6.66 - 6.81 (m, 7H)

2.5. Synthesis of PF-Ran-PPh (PF2, 0.2%)

To a 10 mL flask equipped with a stopcock and a condenser were placed 4 (0.499 g, 0.143 mmol), 5 (0.500 g, 0.143 mmol), 2 (0.001 g, 0.00028 mmol), Pd(PPh₃)₄ (1 mol%), K₂CO₃ (2 M, 2 mL) and toluene (3 mL) under nitrogen atmosphere. The mixture was heated up to 90˚C for 24 h. After the reaction the product was extracted by chloroform and washed with water and the organic layer was dried with MgSO₄. Solvent was removed with a rotary-evaporator. The crude product was purified by reprecipitation into methanol and Soxhlet extraction with acetone. The yield was 0.126 g (97%). PF2 with 8 mol% of 2 was synthesized with a similar manner (98%).

2.6. Synthesis of PF with Phenoxazine Moieties at Chain End (PF1)

To a 20 mL three-necked round-bottom flask equipped with a stopcock and a condenser were added 4 (0.166 g, 0.2757 mmol), 5 (0.226 g, 0.323 mmol), Pd(PPh₃)₄ (3.46 mg, 0.003 mmol, 0.5 mol%), K₂CO₃ (2 M, 2 mL), and toluene (4 mL) under nitrogen atmosphere, and the mixture was refluxed for 48 h. Then, 3 (0.078 g, 0.3 mmol) was added and the reflux was continued for 12 h. After the reaction the product was extracted with chloroform, and washed with salt NaCl and water. Then the organic layer was dried with MgSO₄. After the removal of the solvents, the product was obtained by precipitation into methanol. The yield was 0.20 g (79%). ¹H NMR (300 MHz, CDCl₃) δ (ppm): 0.83 - 0.88 (m, 3H), 1.24 - 1.28 (m, 10 H), 1.50 - 1.61 (m, 2H), 2.19 - 2.34 (m, 3H), 2.51 - 2.56 (m, 2H), 6.63 - 6.67 (m, 14H), 7.02 - 7.18 (m, 8H), 7.41 - 7.65 (m, 4H), 7.67 - 7.81 (m, 2H).

2.7. Synthesis of Block Copolymer with Phenoxazine Moiety at the Junction (PF-Ph-PTAA)

To a 10 mL two-necked round-bottom flask equipped with a stopcock and a condenser were added PF1 (0.131 g, 0.01 mmol of the terminal), 8 (0.114 g, 0.3 mmol), P(t-Bu)₃ (7.5 µL, 0.01 mmol), Pd(OAc)₂ (0.0014 g, 0.006 mmol), t-BuONa (0.032 g, 0.33 mmol), and THF (3 mL) under nitrogen atmosphere and the mixture was stirred under reflux for 24 h. After removing the solvent by a rotary evaporator, the product was obtained by precipitation into methanol and acetone. The yield was 0.191 g (60%).

2.8. Device Fabrication

Perior to preparation of device, a glass slide with indium tin oxide (ITO) patterns was washed by an alkaline cleaner under sonication and rinsed with deionized water. The substrate was subsequently washed by 2-propanol under sonication, and rinsed with clean 2-propanol, and dried with nitrogen. PEDOT:PSS with 30 nm of thickness was spin-coated on the substrate at 2500 rpm for 30 s from the dispersion in water filtered by 0.2 μm of membrane filter followed by annealing at 200˚C for 1 h. Polymer layer was laminated on PEDOT:PSS by spincoating at 500 rpm for 30 s from chlorobenzene solution (12 mg/mL) filtered by 0.45 μm of membrane filter, and annealed at 120˚C for 1 h under nitrogen atmosphere. On the polymer, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) with 50 nm thickness was deposited by thermally evaporation in carbon pot at 240˚C under vacuum with a rate of 1.5 Å/s. As a cathode, lithium fluoride with 0.5 nm of thickness followed by a Al with 100 nm of thickness was deposited on the organic layer with a rate of 0.1 Å/s and a rate of 4.5 Å/s using tantalum and tungsten boats, respectively. Finally the polymer device was set into a metal can with barium oxide as a drying agent under a nitrogen atmosphere, and passivated with an epoxy resin (XNR 5570-B1, Nagase Chemtex Corporation) irradiated by UV light.

3. Results and Discussion

3.1. Synthesis

In order to prepare the PF homopolymer with phenoxadine moieties (PF1 in Figure 1), Suzuki coupling polymerization of 4 and 5 in non-stoichiometric conditions (4/5 = 0.85/1.0) was carried out resulting in an end-functional polymer with boronic ester moieties, followed by the reaction with monobrominated phenoxazine. The successful introduction of phenoxazine moieties was confirmed by ¹H NMR spectrum. As shown in Figure 2(a), signals for phenoxazine protons appeared around 6.6 ppm. In order to obtain a target block copolymer (PF-Ph-PTAA) consisting of PF and PTAA functionalized with green emitting phenoxazine moiety at the junction point of two blocks, C-N coupling polymerization of 6 was conducted in the presence of PF1 using palladium acetate and tri(tert-butyl) phosphine ligand, and tert-butoxide as a base in THF for 24 h in the similar manner we reported [9,15]. As shown in Figure 2(b), two types of methyl signals around 0.85 ppm, and N-H signal from polymer ends (5.7 ppm) were observed indicating the introduction of TAA unit as expected.

Figure 3 shows GPC profiles for PF1 and PF-PhPTAA. GPC trace of PF-Ph-PTAA was monomodal and shifted toward the higher molecular weight direction compared with PF1 homopolymer. Both NMR and GPC results suggest the successful formation of the designed block copolymer. Random copolymers, PF2s, were also synthesized by Suzuki coupling polymeriztion of 2 and 4 with 5. Table 1 lists the characteristics of prepared polymers together with the data for PF-bPTAA, which is a previously prepared block copolymer without phenoxazine units [9,10].

Glass transition temperature of PF-Ph-PTAA was determined to be 148˚C by DSC, and this value is almost the same as that of PF-b-PTAA. All the polymers were soluble in common organic solvents and readily formed thin films by a solution processing.

3.2. Optical and Electrochemical Properties

The optical properties of prepared polymers were investigated with UV-vis absorption and photoluminescent (PL) spectroscopies. In chloroform solutions, all polymers we examined possess similar optical properties regardless of the existence of phenoxazine moiety as shown in Table 2. In a solution state, all copolymers showed λ_{abs, max} of 382 - 385 nm, and exhibited similar PL spectra in solution state, having distinct vibronic bands at about 418 and 445 nm and a shoulder around 470 nm upon excitation at 380 nm. As we previously reported [9,10], PTAA segments show negligible contri-

Table 1. Characteristics of synthesized polymers.

^a Determined by GPC; ^b determined by ¹H-NMR; ^c Ref. [10].

Table 2. Optical properties of synthesized polymers

^a Exited at 380 nm; ^b CHCl₃ solution, 10⁻² mg/mL; ^c 10⁻⁴ mg/mL.

bution to PL spectrum. It is also found that phenoxazine units existing at the junction point, or in the fluorene sequence of random copolymers also have almost no effects on the UV and PL spectra in the solution state.

Figure 4 shows UV-vis (a) and PL (b) spectra in a film state. Only PF2 (8%) showed different absorption spectrum as shown in Figure 4(a). The observation of a tail over 420 nm is probably due to the interchain interaction between phenoxazine rich sequences. The existence of phenoxazine significantly influenced PL spectra in the film state (Figure 4(b)). PF-b-PTAA shows similar PL spectrum to that of solution indicating that interchain interaction is negligible because of the bulky substituent. However, only 0.2% of phenoxazine moiety drastically changed the PL spectrum, i.e., the spectrum showed emission peaks at 426, and 450 nm with a tail. In the case of PF2 (8%), further red shift was observed

showing emission maximum only at 463 nm, indicating the effective energy transfer. Although the content of phenoxazine unit in PF-Ph-PTAA was relatively high (17%) as a result of the low molecular weight nature, it showed similar PL spectrum to that of PF2 (0.2%). This is because phenoxazine unit is isolated in single polymer chain nevertheless the high content, and/or microphase separated structure is established in PF-Ph-PTAA film.

The electrochemical properties of the copolymers were investigated by cyclic voltammetry (CV) with a reference electrode of Ag/AgCl. The oxidation process of PF-PhPTAA started around 0.84 V, and the reversible redox peak in the reduction process appeared at around −0.71 V. The energy levels of the highest occupied molecular orbital (HOMO) of the PF-Ph-PTPA was estimated from its oxidation potential according to an empirical formula, E_HOMO = −e(E_ox + 4.4) V. HOMO level of the block copolymer was in -5.24 eV. The levels of the lowest unoccupied molecular orbital (LUMO) for the polymer were calculated with the reduction potential Ered from the empirical formula E_LUMO = -e(E_red + 4.4) V. LUMO level of the block copolymer was in -3.69 eV. This result was almost consistent with that of PF-b-PTAA [10].

3.3. EL Properties

The performance of the EL devices based on PF-PhPTAA with the device structure of ITO/PEDOT:PSS/ polymer/LiF/Al was evaluated. The device with PF-PhPTAA blended with 20 wt% of PF-b-PTAA was also evaluated in order to elucidate the chemical environment where phenoxazine moiety locates. Figure 5 shows luminance vs. voltage (a), and current efficiency vs. current density (b) characteristics, respectively. Although the maximum luminance for the device based on PF-PhPTPA with 20 wt% of PF-b-PTPA (1710 cd/cm²) is slightly higher than neat PF-Ph-PTPA, both devices afforded the similar current efficiency profiles. As discussed in PL section, phenoxazine unit is isolated in single polymer chain nevertheless the high content. Therefore, the environment of emitting centers in both devices is similar each other. Compared with the device based on PF-b-PTAA [10], EL performance is slightly worse. It is speculated that this is due to the low molecular weight nature of PF-Ph-PTAA, which results in the unclear microphase separation.

Figure 6 shows EL spectra for fabricated devices. Compared with the device based on PF-b-PTAA, all the devices based on phenoxazine containing polymers show ca. 50 nm red-shift. It is indicated that emission sites predominantly located in the vicinity of phenoxazine moiety. Because of shallow HOMO level of phenoxazine unit [14], it works as a hole trap resulting in the efficient recombination of hole and electron. Contrary to PL spectra, PF-Ph-PTAA showed similar EL spectrum to PF2 (8%), where a tail was observed in the long wavelength region. It is possible that the interaction of excitons exists because of the local high concentration of phenoxazine moiety.

4. Conclusion

A block copolymer consisting of electron transporting PF and hole transporting PTAA functionalized with green emitting phenoxazine moiety at the junction point of two blocks was successfully prepared via the combination of Suzuki, and C-N coupling polymerization based on the novel molecular design of full functional polymers for the increase the efficiency in PLED. Prepared polymers exhibited similar optical properties in chloroform solutions. On the other hand, in a film state, the existence of phenoxazine unit drastically changed PL spectra. Although the content of phenoxazine unit in PF-Ph-PTAA was relatively high (13%), it showed similar PL spectrum

to that of PF2 (0.2%) indicating that phenoxazine unit is isolated in single polymer chain nevertheless the high content. EL device based on PF-Ph-PTAA showed green-emission, suggesting that emission sites predominantly located in the vicinity of phenoxazine moiety because of its shallow HOMO level. Our molecular design presented here makes it possible to control the emission site, and to afford more efficient materials for EL applications.

REFERENCES

NOTES

^*Corresponding author.