Communication

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DOI: 10.1002/marc.200600381

Summary: The first example of the synthesis of acrylonitrile copolymers with porphyrin pendants and the subsequent electrospinning of the resultant copolymers into nanofibers is presented in this communication. Vinyl porphyrin monomers have been synthesized and copolymerized with acrylonitrile through solution polymerization. FT-IR, NMR, UV-vis, and fluorescence spectroscopy are used to characterize the copolymers. Preliminary quantum chemical calculations have also been carried out to reveal the activity of the vinyl porphyrin monomers. Nanofibers with a diameter of around 330 nm are prepared by electrospinning the copolymer solutions. Their morphology and porphyrination are clearly observed by fieldemission scanning electron microscopy and fluorescence microscopy. It is speculated that this type of nanofiber may be a latent support of porphyrins for various purposes such as catalysis, molecular imprinting, sensors, and light/energy conversion. The formation of luminescent nanofibers from porphyrinated polymers

Porphyrinated Nanofibers via Copolymerization and Electrospinning Ling-Shu Wan,1 Jian Wu,*2 Zhi-Kang Xu*1 1

Institute of Polymer Science, and Key Laboratory of Macromolecular Synthesis and Functionalization (Ministry of Education), Zhejiang University, Hangzhou 310027, P. R. China Fax: þ 86 571 8795 1773; E-mail: [email protected] 2 Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China Fax: þ 86 571 8795 1773; E-mail: [email protected]

Received: June 2, 2006; Revised: July 9, 2006; Accepted: July 13, 2006; DOI: 10.1002/marc.200600381 Keywords: electrospinning; luminescence; nanofiber; polyacrylonitrile; porphyrin

Introduction Electrospinning is a simple, convenient, and versatile technique for generating fibers with diameters that range from several micrometers to tens of nanometers.[1–4] The collected fibrous membranes (or mats) have many exciting characteristics such as very high surface area to volume ratio, good mechanical strength, excellent flexibility, and large porosity. Therefore, the preparation and utilization of electrospun nanofibers have become a subject of strong research with worldwide participation in both industrial and academic laboratories, and many outstanding improvements have been achieved in the most recent years. These advances in electrospinning have made it possible to facilely fabricate fibers with

Macromol. Rapid Commun. 2006, 27, 1533–1538

a great variety of morphologies, for example, highly aligned and spaced linear fibers, orthogonal assemblied fibers,[5,6] three-dimensional structured fibers,[7] porous fibers,[8] bicomponent fibers,[9] coaxial fibers,[10] core-sheath fibers,[11] and so on. At the same time, more and more materials have been applied in electrospinning.[12–25] Most recently, some inorganic materials such as metal particles and carbon nanotubes have been electrospun into composite nanofibers.[12,13] Among various polymers, acrylonitrile-based homopolymer and copolymers have been widely used for electrospinning because of their superior fiber-forming property.[13–25] Up to now, nanofibrous materials with reinforcing, superhydrophobic, and/or catalytic properties have been reported, nevertheless, ensuring specific functionality to

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polyacrylonitrile nanofibers is still interesting.[13–25] It is well known that porphyrins play important roles in biological processes and porphyrin polymers have found increasing applications in many areas such as molecular recognition or molecular imprinting,[26] sensors,[27] light-emitting and energy/electron transfer materials,[28] interactions with biological systems,[29] and enzyme mimics for catalysis.[30] Combining the merits of electrospinning with the bioinspired applications of porphyrin polymers may generate functionalized nanofibers for more multiple purposes. Great efforts have been devoted to the design and synthesis of porphyrin polymers over the past decades. Up to now, several kinds of porphyrin polymers have been reported. Among them, conjugated porphyrin polymers have been synthesized for energy/electron transfer in which the porphyrin acts as a core, main chain unit, or pendant.[28] Generally, such polymers together with some oligomers require complicated synthesis processes, and the disadvantage of a low molecular weight makes them unsuitable for electrospinning. Materials with coordinative bonding to a porphyrin have also been investigated, however, leaching has been found to be a problem during applications.[31] Thus, porphyrins covalently incorporated in a copolymer may be preferable. In our previous work, acrylonitrilebased copolymers that contain reactive carboxy groups or possessing good biocompatibility have been synthesized and fabricated into nanofibrous membranes with promising characteristics.[25] Herein, we describe the synthesis of copolymers of vinyl porphyrins with acrylonitrile, and the electrospinning of the resultant copolymers into nanofibers.

Experimental Part Acrylonitrile (AN) and dimethyl sulfoxide (DMSO) were commercially obtained from Shanghai Chemical Agent Co. (China) and distilled at reduced pressure before use. Azoisobutyronitrile (AIBN) was recrystallized. 5-(p-Aminophenyl)-

10,15,20-triphenylporphyrin (ATPP) was prepared using a reported procedure.[32] Solvents employed and other chemicals were of analytical grade and used as received without further purification. The synthesis of 5-(p-methacrylamidophenyl)-10,15,20triphenylporphyrin (MATPP) is briefly described as follows. ATPP (1.048 g, 1.66 mmol) was dissolved in dichloromethane (147 mL), then methacrylic chloride (0.864 g, 8.3 mmol) and triethylamine (0.948 g, 9.1 mmol) were added slowly to the solution and stirred at room temperature. After 30 min, the reaction mixture was washed with water and evaporated under reduced pressure to give the crude purple product, which was then purified by column chromatography (silica gel, CH2Cl2/ petroleum ether ¼ 3:1 v/v). The yield of the porphyrin was found to be 0.727 g (63%). 1 H NMR (500 MHz, CDCl3): d ¼ 8.86 (m, 8H, pyrrole b-H), 8.20 (m, 8H, ortho-H phenyls), 7.95 (d, 2H, J ¼ 8.4 Hz, meta-H phenyl bonded to carboxyamide group), 7.84 (s, 1H, CONH), 7.75 (m, 9H, meta/para-H phenyls), 5.96 (s, 1H, CH , Z), 5.58 (d, 1H, J ¼ 1.2 Hz, CH , E), 2.20 (s, 3H, CH2 CCH3),
2.77 (s, 2H, pyrrole NH). ZnMATPP was prepared according to common procedures. The yield of the metal complexes was almost quantitative. Solution copolymerization of AN with porphyrins initiated with AIBN was performed at 60 8C in DMSO. In a typical procedure, AN (0.65 mL, 9.95 mmol), ZnMATPP (38.0 mg, 0.05 mmol), and AIBN (32.8 mg, 0.2 mmol) were dissolved in DMSO (4 mL). The solution was placed in a glass ampoule, degassed three times, sealed, and polymerized for 24 h. The solution was then poured into deionized water and the precipitated polymer was separated by filtration. An extraction study was conducted whereby an amount of polymer was placed in a Soxhlet extractor with chloroform for 48 h, to allow full removal of the residual porphyrin monomer. The polymer was then dried under vacuum, and the yield was determined gravimetrically. The electrospinning procedure reported previously was used to produce nanofibers. Porphyrin polymer was dissolved in dimethylformamide (DMF) at 60 8C with gentle stirring for 6 h to form a homogeneous solution. Electrospinning was performed after air bubbles were removed completely. The electrospinning apparatus consists of a plastic syringe, a blunt-

Scheme 1. Schematic representation of synthesis and molecular structure of the porphyrin copolymers. Macromol. Rapid Commun. 2006, 27, 1533–1538

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end stainless steel needle (12#, inner diameter is 1.2 mm), a ground electrode (aluminium sheet on a flat glass), and a high voltage power supply (GDW-a, Tianjin Dongwen Highvoltage Power Supply Plant, China) with a low current output. A positive voltage (10 kV) was applied to the polymer solution and the distance between the syringe tip and the collector surface was ca. 15 cm. The flow rate of the polymer solution was kept at 0.6 mL  h
1 by a microinfusion pump (WZ-50C2, Zhejiang University Medical Instrument Co., LTD, China). The resultant nanofibers were dried under vacuum at 60 8C to remove residual solvent. The diameter of the fibers was determined from field-emission scanning electron microscopy (FESEM) micrographs. FT-IR spectra were recorded with a Bruker Vector 22 spectrometer and 1H NMR spectra were measured in DMSOd6 on a Bruker (Advance DMX500) NMR spectrometer. Singlet absorption and emission spectra measurements were carried out on a UV-vis spectrophotometer (756PC, Shanghai Spectrum Instruments, Co., Ltd.) and Shimadzu RF-3510PC fluorescence spectrophotometer using matched quartz cells of 1 cm path length, respectively. The intrinsic viscosity [Z] was measured in DMSO at 30  0.05 8C using an Ubbelohde viscometer. An FESEM (Sirion-100, FEI, USA) and a fluorescence microscope (BX51T-3200, Olympus) were applied to observe the morphologies of the fibers. Optimized structures of ZnMATPP and MATPP were obtained using a suite of Gaussian 98 programs using Hartree-Fock (HF) theory calculations with 3-21 basis sets.

Results and Discussion 5-(p-Aminophenyl)-10,15,20-triphenylporphyrin (ATPP) has been synthesized according to the reported procedure.[32] Reaction of ATPP with methacrylic chloride followed by the insertion of zinc produces ZnMATPP, a vinyl zinc porphyrin, which can be effectively copolymerized with acrylonitrile by solution polymerization in DMSO (Scheme 1). This process ran in a simple and inexpensive way. After copolymerizing for 24 h at 60 8C, the yield is about 90%, as shown in Table 1. Extraction was conducted in a Soxhlet extractor to remove residual porphyrin monomer. The nearly colorless extract indicates a high conversion ratio of the porphyrin monomer.

Figure 1. 1H NMR spectra of a) MATPP in CDCl3 and b) PAN, c) PANCPP (the copolymer containing MATPP), and d) PANCZnPP (the copolymer containing ZnMATPP) in DMSO-d6.

FT-IR, 1H NMR, UV-vis, and fluorescence spectroscopy have bee used to characterize the resultant polymers. Typical results shown in Figure 1 and 2 confirm that the copolymer contains porphyrin pendants. The porphyrin

Table 1. Copolymerization behaviors of acrylonitrile with porphyrin. Entrya)

1 2 3

Monomer

AN AN þ MATPP AN þ ZnMATPP

xporphyrinb)

Yield

[Z]

%

mL  g
1

UV

91.8 91.7 87.8

66.9 95.3 126.7

– 0.035 0.038

1

H NMR – 0.044 0.048

a)

Conditions for copolymerization: T ¼ 60 8C, CAIBN ¼ 2 mol-%, Cmonomer ¼ 2.5 mol  L
1, CAN/Cporphyrin ¼ 99.50/0.50 (mol/ mol), t ¼ 24 h, solvent is DMSO. b) The molar content of the porphyrin moieties in the copolymers is calculated from UV spectra and 1H NMR, respectively. Macromol. Rapid Commun. 2006, 27, 1533–1538

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Figure 2. Fluorescence emission spectra of porphyrin monomers and copolymers in DMSO. PANCPP: the copolymer that contains MATPP, PANCZnPP: the copolymer that contains ZnMATPP. ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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pendants give resonance peaks from 7.75 to 8.86 in the 1H NMR spectra (Figure 1(c) and (d)). The peak at d ¼
2.77 is attributed to the protons in pyrrole for the metal-free porphyrin. The intrinsic viscosity has been measured in

DMSO at 30 8C to estimate the molecular weight. The intrinsic viscosity increases with the introduction of porphyrin, especially for the copolymer that contains ZnMATPP. This might be attributed to the interaction of the

Figure 3. FESEM micrographs of A) microspheres electrospun from a 5 wt.-% copolymer solution and C) nanofibers electrospun from a 15 wt.-% solution. Fluorescence microscope images (100) of those electrospun from B) 5 wt.-% and D) 15 wt.-% solutions. Here the copolymer contains MATPP. Micrographs E) to H) correspond to the copolymer that contains ZnMATPP. Polymer solutions were directly electrospun onto a small cover glass for fluorescence microscope observation. The wavelength of excitation is 488 nm. Electrospinning was performed for B) 20, D) 1, F) 5, and H) 1 min for the fluorescence microscope observations. Macromol. Rapid Commun. 2006, 27, 1533–1538

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metalloporphyrin with the solvent (DMSO). The molar content of ZnMATPP in the copolymers calculated from 1H NMR is 0.048, which is very close to 0.050 (the content in the feed). The metal-free porphyrin, MATPP, also shows a high conversion ratio. As far as we know, there are no published results on the copolymerization behaviors of a versatile monomer with both porphyrin and metalloporphyrin. Optimized structures of ZnMATPP and MATPP have been obtained using a suite of Gaussian 98 programs using Hartree-Fock (HF) theory calculations with 3-21 basis sets. It is found that the planarity of the porphyrin ring is almost the same for ZnMATPP and MATPP, thus they ought to show similar copolymerization activity with acrylonitrile from the point of view of stereochemistry. On the other hand, it can be seen from Table 1 that both the contents of ZnMATPP and MATPP in the corresponding copolymers obtained from UV-vis spectra are slightly lower than those calculated from 1H NMR spectra. This is probably a result of site inhomogeneity of the porphyrins in the copolymers. Nanofibers have been successfully prepared from the resultant porphyrin polymers by electrospinning. It has been reported that many factors can influence the diameters and the morphologies of nanofibers during the electrospinning process. Among them, polymer concentration is one of the most important factors for a certain polymer solution. In our cases, solutions with 5 wt.-% porphyrin polymer in DMF result in microspheres with diameters between 0.5 and 2 mm (Figure 3(A) and (E)). Recently, the preparation of bowl-shaped aggregates by self-assembly has been reported by Liu et al.[33] It is speculated here that luminescent nanospheres or even hollow/porous nanospheres could be facilely produced by this method by regulating the conditions of electrospinning, which might have potential applications such as catalysis. By increasing the concentration of solutions from 5 to 15 wt.-%, uniform nanofibers with a diameter of around 330  34 nm can be fabricated, as indicated in Figure 3(C) and (G). By changing the parameters, such as solution concentration and molecular weight of the polymer, fibers with different diameters can be prepared to meet the requirements for various purposes. Porphyrins have been used as red emitting materials that have reasonable fluorescence efficiency and good thermal stability.[28] Fluorescence spectra confirm the luminescence of the studied porphyrin polymers (Figure 2, emitted at about 651 nm for metal-free porphyrin and about 607 nm for zinc porphyrin when excited at 420 nm, the lmax of MATPP). Similarly, the luminescence of these microspheres or nanofibers bonded with porphyrin pendants is demonstrated by fluorescence microscopy, as shown in Figure 3(B), (D), (F), and (H). It can be seen that red light is emitted uniformly. Furthermore, the fluorescence images demonstrate that electrospinning for different times could produce fibrous materials with various packing densities of Macromol. Rapid Commun. 2006, 27, 1533–1538

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nanofibers. Besides being used as emitting materials, these luminescent nanofibers might be applied in many areas. For example, this characteristic could make the auto-oxidative destruction of the supported porphyrins visually detectable if fluorescence quenching takes place during the process of catalysis. It should be noted that the porphyrinated nanofibers with metal complexes could also be prepared. Since the metal center is very important for the functions of porphyrins (for example, aluminium porphyrin is useful as an initiator of polymerization[34]), porphyrin complexes with other metals such as Fe, Mn, Ni, Co, Cu, and Al could be prepared and the corresponding polymers electrospun into nanofibers based on this method. On the other hand, as is known, some other versatile monomers (e.g., styrene) are easy to copolymerize with vinyl porphyrin,[35] which could also extend the applications of the porphyrinated nanofibers.

Conclusion In this communication, the preliminary results of the synthesis of porphyrin polymers that have a good fiber-forming property, and the electrospinning of the resultant copolymer into luminescent nanofibers is described. The copolymerization of acrylonitrile with vinyl porphyrin through solution polymerization has a high yield and porphyrin conversion ratio. Electrospinning the copolymer solutions at low concentration only gives microspheres, but nanofibers can be obtained by increasing the solution concentration. It is speculated that this type of nanofiber may be a latent support of porphyrins for various purposes such as catalysis, molecular imprinting, sensors, and light/energy conversion.

Acknowledgements: Financial support from the National Natural Science Foundation of China (Grant no. 50473038) and the National Natural Science Foundation of China for Distinguished Young Scholars (Zhi-Kang Xu) are gratefully acknowledged.

[1] D. Li, Y. N. Xia, Adv. Mater. 2004, 16, 1151. [2] Z. M. Huang, Y. Z. Zhang, M. Kotaki, S. Ramakrishna, Compos. Sci. Technol. 2003, 63, 2223. [3] A. Frenot, I. S. Chronakis, Curr. Opin. Colloid Interface Sci. 2003, 8, 64. [4] [4a] D. H. Reneker, A. L. Yarin, H. Fong, S. Koombhongse, J. Appl. Phys. 2000, 87, 4531; [4b] A. L. Yarin, S. Koombhongse, D. H. Reneker, J. Appl. Phys. 2001, 89, 3018; [4c] X. Lu, X. Liu, L. Wang, W. Zhang, C. Wang, Nanotechnology 2006, 17, 3048. [5] Y. Dzenis, Science 2004, 304, 1917. [6] A. Theron, E. Zussman, A. L. Yarin, Nanotechnology 2001, 12, 384. [7] D. Li, G. Ouyang, J. T. McCann, Y. N. Xia, Nano Lett. 2005, 5, 913. ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1538

L.-S. Wan, J. Wu, Z.-K. Xu

[8] J. T. McCann, M. Marquez, Y. N. Xia, J. Am. Chem. Soc. 2006, 128, 1436. [9] T. Lin, H. X. Wang, X. G. Wang, Adv. Mater. 2005, 17, 2699. [10] [10a] X. F. Lu, D. L. Zhang, Q. D. Zhao, C. Wang, W. J. Zhang, Y. Wei, Macromol. Rapid Commun. 2006, 27, 76; [10b] X. F. Lu, Q. D. Zhao, X. C. Liu, D. J. Wang, W. J. Zhang, C. Wang, Y. Wei, Macromol. Rapid Commun. 2006, 27, 430. [11] M. Wei, J. Lee, B. W. Kang, J. Mead, Macromol. Rapid Commun. 2005, 26, 1127. [12] [12a] Z. Y. Li, H. M. Huang, C. Wang, Macromol. Rapid Commun. 2006, 27, 152; [12b] X. F. Lu, Y. J. Zhao, C. Wang, Y. Wei, Macromol. Rapid Commun. 2005, 26, 1325; [12c] W. J. Jin, H. K. Lee, E. H. Jeong, W. H. Park, J. H. Youk, Macromol. Rapid Commun. 2005, 26, 1903; [12d] H. S. Kim, H. J. Jin, S. J. Myung, M. S. Kang, I. J. Chin, Macromol. Rapid Commun. 2006, 27, 146. [13] J. J. Ge, H. Q. Hou, Q. Li, M. J. Graham, A. Greiner, D. H. Reneker, F. W. Harris, S. Z. D. Cheng, J. Am. Chem. Soc. 2004, 126, 15754. [14] E. Zussman, A. L. Yarin, A. V. Bazilevsky, R. Avrahami, M. Feldman, Adv. Mater. 2006, 18, 348. [15] C. Drew, X. Liu, D. Ziegler, X. Y. Wang, F. F. Bruno, J. Whitten, L. A. Samuelson, J. Kumar, Nano Lett. 2003, 3, 143. [16] K. Acatay, E. Simsek, C. Ow-Yang, Y. Z. Menceloglu, Angew. Chem. Int. Ed. 2004, 43, 5210. [17] [17a] S. F. Fennessey, R. J. Farris, Polymer 2004, 45, 4217; [17b] X. H. Qin, Y. Q. Wan, J. H. He, J. Zhang, J. Y. Yu, S. Y. Wang, Polymer 2004, 45, 6409; [17c] V. E. Kalayci, P. K. Patra, Y. K. Kim, S. C. Ugbolue, S. B. Warner, Polymer 2005, 46, 7191. [18] S. Y. Gu, Q. L. Wu, J. Ren, G. J. Vancso, Macromol. Rapid Commun. 2005, 26, 716. [19] Y. Dror, W. Salalha, R. L. Khalfin, Y. Cohen, A. L. Yarin, E. Zussman, Langmuir 2003, 19, 7012. [20] H. Q. Hou, D. H. Reneker, Adv. Mater. 2004, 16, 69.

Macromol. Rapid Commun. 2006, 27, 1533–1538

www.mrc-journal.de

[21] C. Kim, S. H. Park, J. K. Cho, D. Y. Lee, T. J. Park, W. J. Lee, K. S. Yang, J. Raman Spectrosc. 2004, 35, 928. [22] H. Q. Hou, J. J. Ge, J. Zeng, Q. Li, D. H. Reneker, A. Greiner, S. Z. D. Cheng, Chem. Mater. 2005, 17, 967. [23] E. J. Ra, K. H. An, K. K. Kim, S. Y. Jeong, Y. H. Lee, Chem. Phys. Lett. 2005, 413, 188. [24] S. Y. Gu, J. Ren, G. J. Vancso, Eur. Polym. J. 2005, 41, 2559. [25] [25a] L.-S. Wan, Z.-K. Xu, H.-L. Jiang, Macromol. Biosci. 2006, 6, 364; [25b] Z.-G. Wang, Z.-K. Xu, L.-S. Wan, J. Wu, C. Innocent, P. Seta, Macromol. Rapid Commun. 2006, 27, 516; [25c] P. Ye, Z.-K. Xu, J. Wu, C. Innocent, P. Seta, Macromolecules 2006, 39, 1041; [25d] X.-J. Huang, Z.-K. Xu, J. Wu, C. Innocent, P. Seta, Macromol. Rapid Commun. 2006, 27, 1341. [26] J. D. Lee, N. T. Greene, G. T. Rushton, K. D. Shimizu, J. I. Hong, Org. Lett. 2005, 7, 963. [27] J. M. Bedlek-Anslow, J. P. Hubner, B. F. Carroll, K. S. Schanze, Langmuir 2000, 16, 9137. [28] [28a] F. S. Lu, S. Q. Xiao, Y. L. Li, H. B. Liu, H. M. Li, J. P. Zhuang, Y. Liu, N. Wang, X. R. He, X. F. Li, L. B. Gan, D. B. Zhu, Macromolecules 2004, 37, 7444; [28b] B. S. Li, X. J. Xu, M. H. Sun, Y. Q. Fu, G. Yu, Y. Q. Liu, Z. S. Bo, Macromolecules 2006, 39, 456; [28c] B. S. Li, J. Li, Y. Q. Fu, Z. S. Bo, J. Am. Chem. Soc. 2004, 126, 3430. [29] M. Kepczynski, A. Karewicz, A. Gornicki, M. Nowakowska, J. Phys. Chem. B 2005, 109, 1289. [30] X. Q. Yu, J. S. Huang, W. Y. Yu, C. M. Che, J. Am. Chem. Soc. 2000, 122, 5337. [31] O. Nestler, K. Severin, Org. Lett. 2001, 3, 3907. [32] J. Wu, W. M. Shi, D. Wu, Chem. Lett. 2004, 33, 460. [33] X. Y. Liu, J. S. Kim, J. Wu, A. Eisenberg, Macromolecules 2005, 38, 6749. [34] M. H. Chisholm, D. Navarro-Llobet, W. J. Simonsick, Macromolecules 2001, 34, 8851. [35] K. Aramata, A. Kajiwara, A. Hashidzume, Y. Morishima, M. Kamachi, Polym. J. 1998, 30, 702.

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