Synthesis and characterization of new aromatic polyimides containing well-defined conjugation units

A series of aromatic polyimides composed of well-defined conjugation units were synthesized from 5,5'-bis(4-aminophenyl)- 2,2'-bifuryl (PFDA) and 2,2'-bis(furyl) benzidine (FurylBZ) with various dianhydrides. The synthesized polyimides emit blue to green light with a quantum yield of 7.3-14.9%, depending on the polymer backbone. In particular, PFDA-based polymers exhibit extremely narrow photoluminescence. The structure, thermal stability, refractive index and dielectric properties of the polymer films were also determined. INTRODUCTION Aromatic polyimides have many useful characteristics such high thermal stability, good mechanical properties and low dielectric constants (1-6). In addition, they are easy to process. These characteristics have made polyimides very attractive to both academia and industry and, in particular, have caused them to be widely used in the microelectronic industry as interdielectric, passive, and alpha-particle protecting layers, as well as flexible carrier substrate films (1-6). Another area that has seen intense investigation is the search for functional organic and polymeric materials for use in the fabrication of light-emitting devices (LEDs) (7- 10). This work began with the discovery by Tang et cd. (7) and Burroughes et al. (8) of the light-emitting characteristics of tris(8-quinolinolato)aluminium and poly(p-phenylenevinylene) (PPV). Subsequent studies have been directed toward finding ways to improve light-emitting performance and reliability, as well as to control the wavelength and band width (9, 10). One of the major problems in the application of such materials to organic LEDs is their degradation under continuous operation, which is caused by Joule heating (11). However, the shortcomings of these materials may be overcome by the incorporation of light-emitting moieties into an aromatic polymer backbone, such as that of polyimides. This approach has been adopted by several research groups in recent attempts to synthesize light-emitting polyetherimides and polyimides (11-15). Mal'tsev et al. (11, 12) have synthesized polyetherimides containing 9,10-bis(phenylthio)anthracene units, which emit blue-green light in the range 420-700 nm. Kakimoto and coworkers (13) have reported a series of polyimides synthesized from 2,5-bis(4-aminostyryl) pyrazine, a light-emitting diamine monomer derived from 2,5-distyrylpyrazine. These polymers emit orange-red light with an emission maximum at around 560 nm (= [lambda]^sub max^). It was found that the value of [lambda]^sub max^ is independent of the number of dianhydride units, but the emission intensity is strongly dependent on the number of these units. We have reported two new [pi]-conjugated diamines, 5,5'-bis(4- aminophenyl)-2,2'-bifuryl (PFDA) and 2,2'-bis(furyl)benzidine (FurylBZ), and their polyimides (PIs), prepared from pyromellitic dianhydride (PMDA) (14, 15). These monomers and their polymers strongly emit blue light. In this study, we further investigate the PDFA and PurylBZ emitters and extend the synthesis of their PIs to other aromatic dianhydrides such as 4,4'-oxydiphthalic anhydride (ODPA) and hexafluoroisopropylidene-2,2-bisphthalic anhydride (6F), yielding the polyimides ODPA-PFDA PI, 6F-PFDA PI, ODPA-FurylBZ PI, and 6F- Fury1BZ PI. The UV-visible absorption and light-emitting characteristics of the resulting PI films were investigated, and the quantum efficiency in the light emission was determined. In addition, the structure, thermal stability, refractive index and dielectric constant of the polymer films were determined. EXPERIMENTAL Dianhydride monomers were purchased from Chriskev Company (USA) and purified using conventional methods just prior to use. N-Methyl- 2-pyrrolidone (NMP) (Aldrich Chemical Co., USA) was purified by distillation over calcium hydride under reduced pressure. All the other chemicals used in this study were obtained from Aldrich, and used without further purification. PFDA and FurylBZ diamines were prepared in accordance with the synthetic methods reported previously (14, 15). An equivalent mole of ODPA monomer was slowly added to an equivalent mole of PFDA dissolved in dry NMP with vigorous stirring in a glove box filled with dry nitrogen gas. Once the addition was completed, the reaction flask was capped tightly and stirring was continued for 1 day to completely homogenize the polymerization mixture, yielding ODPA-PFDA poly (amic acid) (PAA). Each PAA solution was filtered through a 1.0-([mu]m Fluoropore membrane, and then sealed tightly and stored in a refrigerator of -4[degrees]C before use. The other PAAs in solution were prepared in the same manner: 6F-PFDA PAA, ODPA-Fury1BZ PAA, and 6F-FurylBZ PAA. In addition, PMDA-PFDA PAA and PMDA-FurylBZ PAA solutions were prepared. The solid content in each PAA solution was approximately 10 wt%. The intrinsic viscosity was measured in NMP at 25.0[degrees]C using an Ubbelohde capillary viscometer. The PAA solutions were spin-coated onto glass and quartz substrates, then dried at 80[degrees]C for 1 h. The dried films were imidized at 300[degrees]C under a dried nitrogen gas flow, giving PI films of ca. 10 [mu]m thick (see Fig. 1). The imidization protocol used was 150[degrees]C/30 min, 230[degrees]C/30 min, and 300[degrees]C/60 min. The PI films were removed from the glass substrates with the aid of deionized water, then dried for 2 days at 50[degrees]C in vacuum. In addition, PI films of thickness 0.1-2 [mu]m were prepared on quartz substrates for UV-visible spectroscopic measurements. UV-visible spectroscopic measurements were performed using a Hewlett-Packard spectrometer, and photoluminescence (PL) spectra were measured at room temperature using a fluorescence spectrophotometer (SLM 8000(TM)) with a Xenon lamp. The excitation wavelength employed for the PI films was 380 nm. The excitation spectrum was also monitored at the wavelength at which the maximum PL peak appeared. The relative PL quantum yield ([Phi]f) was measured according to the methods reported previously (14, 15). Refractive indices were measured using a prism coupler (16-18) equipped with a cubic zirconia prism (refractive index = 2.1677 at 632.8 nm) and a He-Ne laser source of wavelength 632.8 nm (474.08 THz). Wide angle X-ray diffraction (WAXD) measurements were performed at room temperature in both reflection and transmission geometry, using a rotating anode based diffractometer (Rigaku RINT- 2500) with a CuK[alpha] radiation source. WAXD patterns were deconvoluted using an interactive non-linear-least-squares fitting technique based on pseudo-Voight functions and one linear baseline (18, 19). Thermal stability was measured over 50-800[degrees]C using a Seiko thermogravimeter (TG/DTA6300). Nitrogen gas was purged with a flow rate of 100 cc/min, and a ramping rate of 5.0[degrees]C/min was employed. RESULTS AND DISCUSSION The conformations of the PFDA and FurylBZ monomers were investigated using the semi-empirical AMI method of a molecular simulations package (Molecular Simulations Co., San Diego, USA). For PFDA, the trans- and cis-isomeric forms were found to be possible stable structures (see Fig. 2). The trans-isomer is more stable than the cis-isomer, although their energy difference is only 3.9 kcal/ mol. In addition, all of the phenyl and furyl rings in the PFDA are nearly coplanar, regardless of isomeric structure. In contrast, FurylBZ has three possible isomeric structures, as illustrated in Fig. 3. The (a)-isomeric form is slightly more stable than the (b)- and (c)-isomers, which are almost equally stable, with an energy difference of only 0.5 kcal/mol. It is probable that these conformational structures are retained in the backbones of the polyimides prepared from these monomers. The intrinsic viscosity was measured in NMP at 25.0[degrees]C to be 0.634 dL/g for ODPA-PFDA PAA, 0.593 dL/g for 6F-PFDA PAA, 0.661 dL/g for PMDA-PFDA PAA, 0.564 dL/g for ODPA-FurylBZ PAA, 0.578 dL/g for 6F-FurylBZ PAA, and 0.545 dL/g for PMDA-FurylBZ PAA. These results indicate that all of the PAA precursors were synthesized with reasonably high molecular weights. The PAA solutions were spincast, dried, and thermally converted to the corresponding PIs in thin films. All the resulting PI films showed considerably greater resistance to breakage than the brittle PMDA-PFDA PI film. Figure 4 shows the UV-visible absorption and PL spectra of the ODPA-PFDA PI and 6F-PFDA PI films, which are compared with the spectra of PFDA in dioxane and PMDA-PFDA PI in film form. The PI films exhibit broad featureless absorptions for wavelengths less than 520 nm. In general, the chromophoric aromatic imide rings of conventional aromatic polyimides cause them to absorb at wavelengths less than 470 nm. This absorption overlaps with that of the PFDA Fig. 1. Chemical structure of the synthesized light-emitting polyimides. Fig. 2. Possible isomeric structures of PFDA: (a) trans-isomer (T^sub 1^ = 0- 10[degrees] and T^sub 2^ = 0-20[degrees]): (b). cisisomer (T^sub 1^ = 170-180[degrees] and T^sub 2^ = 0- 20[degrees]). T^sub i^ = tortional angle. unit, causing the PI films to exhibit a broader absorption spectrum than pure PFDA. The optical band gap of PFDA, which corresponds to the [pi]-[pi]* transition of the conjugated system, is estimated to be 2.96 eV by extrapolati\on of the absorption spectrum to long wavelengths. This band gap is larger than that of PPV (2.5-2.6 eV) (20). The polyimides in the films emit the strongest PL spectra when excited at 380 nm. Both ODPA-PFDA PI and 6F-PFDA PI exhibit single emission peaks rather than multiple peaks, in contrast to PMDA-PFDA PI, which has a PL spectrum very similar to that of PFDA. The emission peak is centered at 480 nm (blue-green light) for ODPA- PFDA PI and at 460 nm (blue light) for 6F-PFDA PI. These emission peaks are red-shifted compared to those of PFDA and PMDA-PFDA PI. This shift may be due in part to the electron affinities of the ODPA and 6F units, which influence the [pi]-[pi]* transition of PFDA units on the polymer backbone, and also to the dilution of the light- emitting PFDA unit by ODPA and 6F units. In addition, the full- width at half maximum (FWHM) of the emission peaks of ODPA-PFDA PI and 6F-PFDA PI are estimated to be 26 nm and 23 nm, respectively. These emission peaks are much narrower than those of PFDA and PMDA- PFDA PI (56 nm and 63 nm FWHM, respectively). It is noteworthy that ODPA-PFDA PI and 6F-PFDA PI exhibit narrower PL spectra than any other light-emitting polymer studied to date. Figure 5 shows the UV-visible absorption and PL spectra of the films of ODPA-FurylBZ PI and 6F-FurylBZ PI, which are compared to those of FurylBZ in dioxane and PMDA-FyrylBZ Pl in film form. The PI films show broad absorptions at wavelengths less than 400 nm in comparison to the absorption of FurylBZ monomer. FurylBZ has an optical band gap of 3.42 eV, which corresponds to the [pi]-[pi]* transition of the conjugated system. This value is much larger than the band gaps of its structural isomers PFDA and PPV. The following two factors are possible causes of the large band gap of FurylBZ. First, it is possible that the [pi]-conjugation forms only along the kinked structure composed of the benzidine main unit and the two furyl side groups. This kinked [pi]-conjugated structure would have a relatively large band in the [pi]-[pi]* transition. Second, in order to minimize steric hindrance, the ring components of FurylBZ are not copolanar. In comparison to the absorption of FurylBZ monomer, its PI films show broad absorptions at wavelength less than 400 nm. Here, the band gaps of the PIs would not be estimated because the measured band gaps are not those of the pure FurylBZ, but also contain contributions due to the chromophoric aromatic imide rings, as mentioned above for the case of PFDA-based PIs. The FurylBZ-based polyimides in films also emit the strongest PL spectra when excited at 380 nm. For both ODPA-FurylBZ PI and 6F- FurylBZ PI, the PL spectra exhibit single emission peaks rather than multiple peaks, which is similar to the behavior of FurylBZ but different from that of PMDA-FurylBZ PI. ODPA-FurylBZ PI gives an emission peak centered at 530 nm (green light), and 6F-FurylBZ PI gives a peak centered at 525 nm (green light). The emission peaks of these polyimides are highly red-shifted compared to those of the FurylBZ and PMDA-FurylBZ PI. This shift may be due in part to the electron affinities of the ODPA and 6F units, which influence the [pi]-[pi]* transition of the FurylBZ units on the polymer backbone, and to the dilution of the light-emitting FurylBZ unit by ODPA and 6F units, as mentioned above in relation to the photoluminescent characteristics of the PFDA-based polyimides. In the emission spectra, ODPA-FurylBZ PI has a peak width of 121 nm FWHM, whereas 6F- FurylBZ PI has a width of 74 nm FWHM. These emission peaks are broader than those of FurylBZ and PMDA-FurylBZ PI (73 nm and 64 nm FWHM, respectively), and is also broader than those of the PFDA- based polyimides. Although the PL spectra of the FurylBZ polyimides are broader than those of the PFDA-based polyimides, they are still narrower than those of all other light-emitting polymers reported in the literature. Fig. 3. Possible isomeric structures ofFurylBZ: (a), T^sub 1^ = T^sub 2^ = 30[degrees] and T^sub 3^ = 50[degrees]: (b). T^sub 1^ = 30[degrees]. T^sub 2^ = 150[degrees], and T^sub 3^ = 50[degrees]: (c), T^sub 1^ = 30[degrees]. T^sub 2^ = 150[degrees], and T^sub 3^ = 130[degrees]. T^sub 1^ = tortional angle. Fig. 4. UV-uisible absorption (Abs) and photoluminescence (PL) spectra of PFDA (1.5 x 10^sup -5^ g/mL in 1,4-dioxane) and of polyimides in thin films. PFDA was excited at 340 nm, whereas the polyimides were excited at 380 nm. Fig. 5. UV-visible absorption (Abs) and photoluminescence (PL) spectra of FurylBZ (1.5 x 10^sup -5^ g/mL in 1,4-dioxane) and of polyimides in thin films. FurylBZ was excited at 340 nm, whereas the polyimides were excited at 380 nm. In addition, PL quantum yield [Phi]^sub f^ measurements were conducted for all PIs in films. The results are listed in Table 1. The OPDA-PFDA PI and 6F-PFDA PI films have high quantum yields ([Phi]^sub f^ = 7.3% and 14.9%, respectively) compared to the quantum yield of the PMDA-PFDA PI film ([Phi]^sub f^ = 1.2%). In particular, the [Phi]^sub f^ value of 6F-PFDA PI is larger than that of poly(2-methoxy-5-(2'-ethyl)hexyloxy-p-phenylenevinylene) ([Phi]^sub f^ = 8.5-11.5%) (21), which emits orange light. On the other hand, the OPDA-FurylBZ PI and 6F-FurylBZ PI films have yields of [Phi]^sub f^ = 7.6% and 8.3%, respectively. These values are slightly higher than the value for the PMDA-FurylBZ PI film ([Phi]^sub f^ = 7.4%). In comparison to the quantum yields of PFDA ([Phi]^sub f^ = 92.0%) and FurylBZ in 1,4-dioxane ([Phi]^sub f^ = 52.0%), the PI films show relatively low quantum yields. This might be due to the morphological structures formed in the PI films. X-ray diffraction analyses were performed to elucidate the morphological structures in the films, and the X-ray diffraction patterns obtained are compared in Fig. 6. For the PMDA-PFDA PI film, well-resolved multiple (00l) peaks, as well as some (hkl) peaks, are detected in the transmission pattern, whereas only (hkl) peaks are observed in the reflection pattern. These patterns indicate that parts of the PI film are structurally ordered, and that the polymer chains are highly aligned in the film plane. We can obtain some structural information from these X-ray patterns, although the full crystal structure is not yet determined. The diffraction peak at 4.11[degrees] (2[theta]) (21.5 [Angstrom] d-spacing), which corresponds to the (001) diffraction, is estimated from the Scherrer relation to have a very high coherence length of 168 [Angstrom] (18, 22). The (hkl) diffractions in the reflection pattern have a coherence length of about 42 [Angstrom] (36-48 [Angstrom]). We therefore conclude that this PI film contains structurally ordered phases with a dimension of approximately 168 x 42 x 42 [Angstrom]. The overall crystallinity is estimated to be 26% from the reflection pattern, and 36% from the transmission pattern. Structural ordering is also observed in the ODPA-PFDA PI film. However, a low coherence length of less than 52 [Angstrom] is obtained for all of the diffraction peaks, and the overall crystallinity in the film is estimated to be only 13-15% from the reflection and transmission patterns. Table 1. Quantum Yields of PFDA- and FurylBZ-Based Polyimides in Photoluminescence. In contrast, the 6F-PFDA PI film is almost amorphous. Only a weak diffraction indicating short-range ordering is detected at 5.58[degrees]. The coherence length of this diffraction is only 37 [Angstrom], which is slightly larger than the length of just one repeat unit in the backbone. The mean interchain distance is estimated to be 5.8-5.9 [Angstrom], which is larger than the interchain distances of the other two polyimides (4.2-4.5 [Angstrom]). Similar featureless X-ray patterns are observed for all of the FurylBZ-based PI films (see Fig. 7). Fig. 6. Wide angle X-ray diffraction patterns of PFDA-based polyimides in thin films: Refl, reflection pattern: Trans, transmission pattern. A CuK^sub [alpha]^ radiation source was employed. In conclusion, we find that a higher degree of structural ordering gives a lower quantum yield. This behavior may be linked to charge transfer between the dianhydride unit and diamine unit in the polyimides, which in general are electronically deficient and rich, respectively. Both intra- and inter-molecular charge-transfer (CT) interactions between dianhydride and diamine units are known to be favorable when the polymer undergoes structural ordering (23-26). Thus, the low quantum yield in the highly ordered PMDA-PFDA PI might be due to a large amount of energy transfer from the excited PFDA units in the polymer backbone to the CT sites, which give a weak emission. In contrast, this CT interaction is relatively weak in the ODPA-PFDA PI and 6F-PFDA PI films, as well as the FurylBZ-based PI films, which are less ordered. In addition, the relatively large size of the ODPA, 6F, and FurylBZ units in the polymer backbone dilute the light-emitting units in the film specimen. These morphological features may contribute positively to the emission of the PFDA and FurylBZ units in the backbone, enhancing the quantum yield. Fig. 7. Wide angle X-ray diffraction patterns of FurylBZ-based polyimides in thin films: Refl, reflection pattern: Trans, transmission pattern. A CuK^sub [alpha]^ radiation source was employed. Table 2. Optical, Dielectric, and Thermal Properties of PFDA- and FurylBZ-based PI Films. We also measured a range of optical and dielectric properties of the PI films. These results are presented in Table 2, which also includes the thermal stabilities of the films. The refractive indices of the films vary from 1.608 to 1.840, and the dielectric constants from 2.586 to 3.386, depending on film type and orientation. In all films, the values of the refractive index and dielectric constant in the film plane (n^sub xy^ and [epsilon]^sub xy^) are larger than the respective out-of-plane (n^sub z^ and [epsilon]^sub z^) value\s. This anisotropy in the refractive index ([Delta]) and dielectric constant ([Delta][epsilon]) is attributed to the preferential in-plane orientation of the polymer chains in the films. Measurements could not be carried out for PMDA-PDA PI because of its brittleness. However, this polymer film is expected to have the largest refractive index, out-of-plane birefringence, and dielectric constant of the polyimides studied because of its rigid and extended chain. Tests on the thermal stability of the polyimides showed them to be stable up to 370-396[degrees]C, depending on polymer type. The stability of these polymers is due to the stable phenyl, imide and furyl constituents in the polymer backbone. CONCLUSIONS Light-emitting polyimides were prepared from PFDA and FurylBZ monomers possessing well-defined conjugation lengths. They emit blue to blue-green light with a quantum yield of 1.2-14.9%, depending on the dianhydride and diamine units in the polymer backbone. In particular, 6F-PFDA PI exhibits a very narrow band of blue light with a quantum yield of 14.9%. The polymers are thermally stable up to 370-396[degrees]C. These new polyimides are therefore potential candidate materials for fabricating optoelectronic devices that can emit blue and blue-green light. In particular, 6F-PFDA PI and ODPA- PFDA PI are good candidate materials for fabricating laser diodes emitting blue and blue-green light, respectively, because of their very narrow PL spectrum characteristics. 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REE* Department of Chemistry School of Environmental Science & Engineering Polymer Research Institute Center for Integrated Molecular Systems Division of Molecular & Life Sciences (BK21 Program) Pohang University of Science and Technology San 31, Hyoja-dong, Pohang 790- 784 South Korea To whom correspondence should be addressed. E-mail: ree@postech.edu Copyright Society of Plastics Engineers Jun 2003 Publication date: 2003-06-01