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Novel Porphyrinated Polyimide Nanofibers by Electrospinning Yuan-Yuan Lv,† Jian Wu,*,† Ling-Shu Wan,‡ and Zhi-Kang Xu*,‡ Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, Institute of Polymer Science, and Key Laboratory of Macromolecular Synthesis and Functionalization (Ministry of Education), Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China ReceiVed: NoVember 2, 2007; ReVised Manuscript ReceiVed: May 16, 2008
A series of copolymers based on diaminotetraphenylporphyrin (DATPP), oxidianiline (ODA), and pyromellitic dianhydride (PMDA) were synthesized. The resulting copoly(amic acid)s (CPAAs) were electrospun into uniform nanofibers. Subsequently, copolyimide (CPI) nanofibers were obtained from the CPAA nanofibers by thermal imidization. The morphologies and luminescence of these nanofibers were characterized by field emission scanning electron microscopy (FESEM) and confocal laser scanning microscopy (CLSM). It was found from the CLSM analyses that fluorescence quenching took place after the nanofibers were imidized. This could be attributed to the photoinduced electron transfer from excited porphyrin units to diimide acceptor groups. Thermogravimetric (TG) analyses indicated the CPI nanofibers possessed excellent thermal stability. This kind of CPI nanofibers with high temperature resistance and unique photoelectric properties could be potential materials for optical switches, catalysis, nonlinear optics, and molecular wires used in normal and especially in high temperature circumstances. Introduction Electrospinning has been suggested as a useful method to prepare nonwoven fabrics of submicron or nanoscale fibers,1,2 which have high porosity and large surface-to-area ratio, small pore size between depositing fibers of the electrospun mats, and vast possibility for surface functionalization.3 These characteristics make the nonwoven fabrics attractive for many applications, such as functional membranes,4,5 photocatalysts,6,7 biosensors,8–10 and nanoelectronics.11 Among various polymers studied with electrospinning, polyimides have been very interesting because they constitute an important class of polymers due to their superior thermal and chemical resistance as well as mechanical properties that can be used in various fields.12,13 Polyimide nanofibers based on pyromellitic dianhydride (PMDA) and 4,4′-oxidianiline (ODA) (PMDA-ODA) with ultralow dielectric constant were prepared by electrospinning the precursor of polyimide, a poly(amic acid) (PAA) solution, with subsequent thermal imidization,14,15 and the obtained nanofibers has many attractive applications.16–19 During the last decades, much attention has been paid to design and synthesize porphyrin-functionalized polymers for potential applications in solar energy conversion,20 electron and energy transfer,21 nonlinear optics,22 molecular wires,23 and photodynamic therapy for solid tumors.24 Incorporating porphyrin rings into the polyimide backbones to obtain some materials used as photoconductive materials,25 charge, and energy transfer studies26,27 has also raised a great deal of interest recently. In our previous work, acrylonitrile-based copolymers with porphyrin pendants have been synthesized and electrospun into nanofibers.6 The work reported here was motivated by combining the merits of polyimides with the distinctive characteristics of porphyrin to prepare nanofibers with high tem* Correspondingauthors.Fax:+8657187951773.E-mail:
[email protected] (Z.-K.X.),
[email protected] (J.W.). † Department of Chemistry, Zhejiang University. ‡ Institute of Polymer Science, and Key Laboratory of Macromolecular Synthesis and Functionalization (Ministry of Education), Zhejiang University.
TABLE 1: Synthesis Parameters and Viscosities of the Copoly(amic acid)s (CPAAs) samples PAA
PMDA (mol %)
ODA (mol %)
100
100
CPAA-1 CPAA-2
100 100
95 90
CPAA-3 CPAA-4
100 100
95 90
a
[η]a (dL/g)
DATPP (mol %)
0.545 trans-DATPP 5 10 cis-DATPP 5 5
0.303 0.277 0.309 0.257
Intrinsic viscosity determined in DMAc at 30 °C.
TABLE 2: UV-Vis Absorption Maximum of the CPAAs in DMAc CPAs
Soret band (nm)
Q-band (nm)
DATPP/ODA (mol %) (in polymer)a
trans-DATPP CPAA-1 CPAA-2
423.9 422.2 422.3
519, 553, 592, 651 517, 551, 590, 651 517, 551, 590, 651
0.036 0.087
cis-DATPP CPAA-3 CPAA-4
422.2 422.4 422.4
519, 553, 592, 651 517, 551, 590, 651 517, 551, 590, 651
0.039 0.092
a Calculated from the UV-visible spectra of CPAAs. The extinction coefficient values are 1.34 × 105 M-1 cm-1 (for CPAA-1 and CPAA-2) and 1.35 × 105 M-1 cm-1 (for CPAA-3 and CPAA-4), respectively.
perature resistance and unique photoelectric properties. Herein, we describe the synthesis and preparation of porphyrinated polyimide nanofibers. In addition, the thermal properties and the fluorescent performances of these nanofibers were also studied. Experimental Details Materials. 5,15-Bis(4-aminophenyl)-10,20-diphenylporphyrin (trans-DATPP) and 5,10-bis(4-aminophenyl)-15,20-diphenylpor-
10.1021/jp7105549 CCC: $40.75 2008 American Chemical Society Published on Web 06/28/2008
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SCHEME 1: Schematic Representation for the Synthesis and Molecular Structures of Porphyrinated Copolymers
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Figure 1. 1H NMR spectra of (a) CPAA-1 (copoly(amic acid) containing trans-DATPP) and (b) CPAA-3 (copoly(amic acid) containing cisDATPP) in DMSO-d6. The asterisk indicates solvent impurity.
Figure 3. Fluorescence spectra of CPAAs in DMAc (excited at 550 nm). Figure 2. UV-vis absorption spectra of CPAAs in DMAc.
phyrin (cis-DATPP) were synthesized according to the method described by Luguya et al.28 Pyromellitic dianhydride (PMDA) and oxidianiline (ODA) were commercially obtained from Shanghai Chemical Agent Co. (China) and purified through sublimation above their gasification temperatures before use. N,N′-Dimethylacetamide (DMAc, analytical reagent grade, purchased from Shanghai Chemical Agent Co. (China)) was dried over 4Å molecular sieves before use. All other reagents were analytical quality and were used as received without further purification. Synthesis of Porphyrinated Poly(amic acid)s. Diaminomonomers, DATPP and ODA, with different molar ratios were
dissolved in dry DMAc. PMDA was then added slowly to these solutions under vigorous stirring. The mixtures were stirred at 0 °C for 2 h, and then at 25 °C for another 16 h in a N2 atmosphere. Copoly(amic acid)s (CPAAs), “precursors” of the expected copolyimides, were obtained by pouring the final reaction mixtures into cold methanol and drying the precipitates under reduced pressure at 60 °C. Electrospinning. A porphyrinated CPAA solution was put into a plastic syringe with a blunt-end stainless steel needle (no. 12, inner diameter is 1.2 mm). A ground electrode (aluminum sheet on a flat glass), connecting to a high voltage power supply (GDW-a, Tianjin Dongwen High Voltage Power Supply Plant, China) with a low current output of about 0.02 mA was used
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TABLE 3: Quantum Yield of the CPAAs in DMAc sample
A550c
F550c
Φa
TPP trans-DATPP CPAA-1 CPAA-2 cis-DATPP CPAA-3 CPAA-4
0.050 0.045 0.024 0.038 0.037 0.047 0.050
4598.191 4864.398 1593.590 3733.723 3582.819 3618.327 4598.191
0.150b 0.176 0.109 0.160 0.158 0.125 0.150
a Relative to the fluorescence of TPP. b See ref 30. c The excited wavelength is 550 nm.
Figure 5. Thermogravimetry curves of CPI nanofibers.
Characterization. The inherent viscosity of the resultant CPAA was measured at a concentration of 0.5 g/dL in DMAc at 30 °C. FT-IR spectra were recorded on a spectrometer (Nicolet, Nexus-470, USA) with the accessories of attenuated total reflectance (ATR). 1H NMR spectra were measured in DMSO-d6 on a Bruker (Advance DMX500) NMR spectrometer. Singlet absorption and emission spectra measurements were carried out on a UV-visible spectrophotometer (756PC, Shanghai Spectrum Instruments, Co., Ltd.) and Shimadzu RF3510PC fluorescence spectrophotometer, using matched quartz cells of 1 cm path length, respectively. Thermogravimetric analyses (TGA) were carried out at a heating rate of 10 deg/ min in N2 with a NETZSCH STA 409 PC/PG thermogravimetric analyzer. Field emission scanning electron microscope (FESEM, Sirion-100, FEI, USA) was applied to observe the morphologies of the nanofibers. Confocal laser scanning microscopy (CLSM) was performed with a Leica TCS SP5 confocal setup mounted on a Leica DMI 6000 CS inverted microscope (Leica Microsystems, Wetzlar, Germany) and was operated under the Leica Application Suite Advanced Fluorescence (LASAF) program. Results and Discussion
Figure 4. FT-IR spectra of CPAAs and CPIs containing (a) transDATPP and (b) cis-DATPP.
as collector. A positive voltage (16 kV) was applied to the polymer solution and the distance between the syringe tip and the collector surface was ca. 12 cm. The feed rate of polymer solution was kept at 1.0 mL/h by a microinfusion pump (WZ50C2, Zhejiang University Medical Instrument Co., LTD, China). The electrospinning process was conducted in air. The resulting nanofibers were dried under vacuum at 60 °C to remove the residual solvent. Imidization. Imidization of the as-spun CPAA nanofibers was performed through heating step by step under N2 atmosphere at 80 (0.5 h), 160 (1 h), and 250 °C (4 h). The diameters of these nanofibers and their distribution were determined from field-emissionscanningelectronmicroscopy(FESEM)micrographs.
Synthesis and Characterization of Porphyrin-Containing Copoly(amic acid)s. Copolyimides containing trans-DATPP or cis-DATPP moieties could be synthesized by a two-step reaction procedure similar to the synthesis of common polyimide. Scheme 1 illustrates the synthesis of the target polymers. The first step of the reaction led to copoly(amic acid)s (CPAAs), the “precursors” of the corresponding copolyimides. Polymerization between PMDA with trans-DATPP or cis-DATPP and ODA (Scheme 1) was performed at various molar ratios (Table 1) in DMAc. The incorporation of porphyrin into the CPAAs was confirmed by 1H NMR analysis (Figure 1). The characteristic peak at δ -2.82 or -2.83 was attributed to the internal N-H of the DATPP (Figure 1). Peaks at δ 10.41-10.82, which refer to -COOH, were divided into several signals in both parts a and b of Figure 1 because of the random configurations and the repeated units of CPAAs. Figure 1a corresponding to CPAA containing trans-DATPP showed a simpler spectrum because it has more symmetrical backbones than that containing cisDATPP (Figure 1b).
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Figure 6. FESEM micrographs of (A) CPAA-1 (Φ ) 156-180 nm), (B) CPI-1 (Φ ) 141-165 nm), (C) CPAA-2 (Φ ) 221-245 nm), (D) CPI-2 (Φ ) 201-225 nm), (E) CPAA-3 (Φ ) 211-235nm), (F) CPI-3 (Φ ) 186-210 nm), (G) CPAA-4 (Φ ) 231-255 nm), and (H) CPI-4 (Φ ) 206-230 nm).
Table 1 shows the intrinsic viscosities of the resulting CPAAs in DMAc at 30 °C. It was found that the intrinsic viscosities of all CPAAs were lower than that of common poly(amic acid) (PAA). It might be possible that the bulky porphyrin groups lead to a lower reactivity of the comonomer, which makes the
porphyrinated polymers show lower molecular weights and hence lower viscosity. UV-visible absorption spectra of the CPAAs are shown in Figure 2 and Table 2. It can be seen that CPAAs containing trans-DATPP (CPAA-1, CPAA-2) and cis-DATPP (CPAA-3,
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Figure 7. LSCM images (×100) of the porphyrinated nanofibers (I) CPI-2 and (J) CPI-4. The wavelength of excitation is 488 nm.
Figure 8. Fluorescence intensity of the CPAA and CPI nanofibers (A) and dense films (B) investigated by LSCM. The wavelength of excitation is 488 nm and the scope of fluorescence signal for investigation ranges from 650 to 700 nm.
CPAA-4) were similar to each other as well as similar to the monomers, trans-DATPP and cis-DATPP, respectively. The molar ratios of DATPP to ODA in the CPAAs are shown in Table 2 and were calculated from the absorption spectra. It was found that the calculated molar ratios of DATPP to ODA were close to those in feed, indicating a high conversion ratio of the porphyrin monomers. Fluorescence spectroscopies were used to investigate the photophysical behaviors of these CPAAs (Figure 3). Differing from the absorption spectra, the emission spectra of these CPAAs were markedly broadened and more similar to that of tetraphenylporphyrin (TPP) than those of the starting monomers when excited at 550 nm. Fluorescence quantum yields were calculated with the method previously described by Demas and Crosby.29 The quantum yield standard used was a freshly prepared solution of free base tetraphenylporphyrin (TPP) in DMAc and the Φref was taken to be 0.15.30 The results were shown in Table 3. The quantum yields for both the CPAAs increased slightly with increasing the contents of DATPPs. This
Lv et al. demonstrated that there was no obvious interchain or intrachain aggregation in solution between the porphyrin rings that could self-quenched the fluorescence. Preparation and Characterization of Porphyrinated Polyimide Nanofibers. Nanofibers were successfully prepared from the resulting CPAAs by electrospinning a 15 wt % solution in DMAc. To prepare polyimide nanofibers with high temperature resistance, imidization of the as-spun CPAA nanofibers was performed by heating step by step under N2 atmosphere at 80 (0.5 h), 160 (1 h), and 250 °C (4 h). CPAA nanofibers were converted into CPI nanofibers through the imidization process, which was confirmed by the complete disappearance of the amide absorption bands at 1710 and 1660 cm-1 and the appearance of the imide absorption bands at 1775, 1725, 1380, and 725 cm-1 in the FT-IR spectra (Figure 4). Figure 5 represents the thermogravimetric curves of the CPI nanofibers. It was clear that the four CPI nanofibers displayed good thermal stabilities up to approximately 500 °C, indicating the electrospun nanofibers preserve the typical nature of aromatic polyimide materials. The CPI nanofibers containing cis-DATPP were slightly more stable than that containing trans-DATPP at the same level of porphyrin content. It was of interest to notice that the TG curves of the CPI nanofibers did not show the thermolysis temperatures of the starting DATPPs (trans-DATPP and cis-DATPP) at around 380 °C. It might thus be concluded that the thermal stability of porphyrin units in the CPIs was improved through imidization. FESEM micrographs of the CPAA and CPI nanofibers (Figure 6) showed that the fiber diameter increased with the contents of DATPP. The average diameters of CPAA nanofibers ranged from about 150 to 260 nm while those of CPI nanofibers were between 140 and 230 nm. In other words, the nanofibers shrank obviously after the thermal condensation process. However, the CPI nanofibers were still uniform and continuous after imidization. Porphyrins have been used as red emitting materials with reasonable fluorescence efficiency and good thermal stability.31–33 The luminescence properties of the porphyrinated polyimide nanofibers were demonstrated by fluorescence microscopy, as shown in Figure 7. It can be seen that the nanofibers emitted red light uniformly. Figure 8 shows the fluorescence intensities of the CPAA and CPI nanofibers recorded by a laser scanning confocal microscope (LSCM). The corresponding dense films were also investigated as controls. There was a decrease of fluorescence intensity after imidization for both nanofibers and dense films. This indicated that fluorescence quenching for the porphyrin took place. This could be attributed to the electron transfer from a porphyrin donor to a diimide acceptor group, a five-atom ring, which was formed after imidization. Because the aromatic polyimides contain an alternating sequence of electron-rich donor and electron-deficient acceptor subunits, there should be charge-transfer complexes formed between their polymer chains.34–36 It is well-known that porphyrin is a electron-rich moiety, and when porphyrin was incorporated into polyimide chains, there would form charge-transfer complexes that cause some extraodionary photophysical porperties.25 Furthermore, there was another interesting phenomenon as observed from Figure 8. The fluorescence intensities of the dense films were significantly weaker than those of the nanofibers. This result suggested a superior luminescence characteristic of the polyimide nanofibers. It should be noted that these luminescent nanofibers might be applied in many areas. For example, they could be useful in the detection of some metal ions such
Novel Porphyrinated Polyimide Nanofibers as mercury because fluorescence quenching could be visually detected when the porphyrin units coordinate with Hg(II). Conclusion Two kinds of copolyimide (CPI) nanofibers containing porphyrin units were prepared by electrospinning from copoly(amic acid) (CPAA) solutions followed by imidization through thermal curing. The luminescence properties of the obtained nanofibers and corresponding dense films were demonstrated by fluorescence microscopy. Confocal laser scanning microscopy showed that there was a decrease of fluorescence intensity after imidization for both nanofibers and dense films, indicating the fluorescence quenching for the porphyrin units. This was attributed to the electron transfer from a porphyrin donor to a diimide acceptor group. Furthermore, the fluorescence intensities of the dense films were significantly weaker than the nanofibers. Thermogravimetric analyses demonstrated the CPI nanofibers possessed excellent heat-resistant property. These characteristics indicated the porphyrinated CPI nanofibers have the merits of both porphyrins and polyimides. Potential applications in the fields including chemical sensors, molecular wires, and photoresponsive materials for this type of luminescent nanofibers with high temperature resistance will be explored. Acknowledgment. Financial support from the National Natural Science Foundation of China for Distinguished Young Scholars (Grant No. 50625309) and the Foundation of Key Laboratory of Advanced Textile Materials and Manufacturing Technology (Zhejiang Sci-Tech University), Ministry of Education (Grant No. 2007005) is gratefully acknowledged. References and Notes (1) Deitzel, J. M.; Kleinmeyer, J.; Harris, D.; Beck Tan, N. C. Polymer 2001, 42, 261. (2) Li, D.; Xia, Y. N. AdV. Mater. 2004, 16, 1151. (3) Shin, Y. M.; Hohman, M. M.; Brenner, M. P.; Rutledge, G. C. Polymer 2001, 42, 9955. (4) Megelski, S.; Stephens, J. S.; Chase, D. B.; Rabolt, J. F. Macromolecules 2002, 35, 8456. (5) Li, D.; Wang, Y.; Xia, Y. N. Nano Lett. 2003, 3, 1167. (6) Wan, L. S.; Wu, J.; Xu, Z. K. Macromol. Rapid Commun. 2006, 27, 1533. (7) Kedem, S.; Schmidt, J.; Paz, Y.; Cohen, Y. Langmuir 2005, 21, 5600. (8) Wang, X. Y.; Kim, Y. G.; Drew, C.; Ku, B. C.; Kumar, J.; Samuelson, L. A. Nano Lett. 2004, 4, 331.
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