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A carbazole-based photorefractive polyphosphazene prepared via post-azo-coupling reaction
REACTIVE & FUNCTIONAL POLYMERS
Reactive & Functional Polymers 66 (2006) 1404–1410
www.elsevier.com/locate/react
A carbazole-based photorefractive polyphosphazene prepared via post-azo-coupling reaction q Li Zhang a, Maomao Huang b, Zhiwei Jiang a, Zheng Yang a, Zhijian Chen b, Qihuang Gong b, Shaokui Cao a,* a
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450052, PR China b Department of Physics, Peking University, Beijing 100871, PR China Received 4 December 2005; received in revised form 21 March 2006; accepted 11 April 2006 Available online 26 May 2006
Abstract A carbazole-based photorefractive polyphosphazene was prepared via post-azo-coupling reaction. The polymer shows a low glass transition temperature of 50 °C, and has an excellent solubility in common organic solvents, which can easily be fabricated into an optically transparent film with long-term stability. The nonlinear optical effects, including two-beamcoupling and four-wave-mixing, were studied at 633 nm under room temperature. A gain coefficient of 79 cm1 and a diffraction efficiency of 18% were observed at zero external electric field without any added plasticizer or sensitizer. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Polyphosphazene; Post-azo-coupling; Carbazole; Photorefractive
1. Introduction Polymeric photorefractive materials have attracted steady interest due to their potential applications and many distinct advantages. Compared to widely investigated host-guest polymer composites [1,2] and fully functionalized polymers [3,4], bi-functional photorefractive polymers, which possess both photoconductivity and electro-optic (EO) effect, have obvious advantages attributed to their high stability and easy synthesis. In the bi-functional q Project supported by the National Natural Science Foundation of China, Project No. 20274042. * Corresponding author. Tel./fax: +86 371 6776 3561. E-mail addresses: [email protected] (L. Zhang), caoshaokui@ zzu.edu.cn (S. Cao).
photorefractive polymer family, carbazole-based polymers have received much attention [5–8] because of the fact that polyvinylcarbazole has previously been used in a high-optical-gain photorefractive polymer composite [9]. But the high glass transition temperature (Tg) of these polymers often made it difficult to pole at room temperature to achieve the alignment of the chromophore. Lowering the Tg by adding plasticizer or introducing a spacer between the main chain and the chromophore normally resulted in the deduction of photorefractive performance due to the relative low content of chromophore. Polyphosphazenes, which contain a very flexible inorganic backbone of alternating phosphorous and nitrogen atoms, are very good candidates for preparing polymeric materials having a low Tg
1381-5148/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2006.04.003
L. Zhang et al. / Reactive & Functional Polymers 66 (2006) 1404–1410
[10]. Moreover, polyphosphazenes possess unusual properties, such as high thermal and oxidative stability, optical transparency from 220 nm to the near-IR region of the backbone, as well as their ease of syntheses [11]. These characteristics are very beneficial to electro-optic applications. In this paper, we report a carbazole-based photorefractive polyphosphazene prepared via post-azocoupling reaction. The result of two-beam-coupling (TBC) and four-wave-mixing (FWM) measurements is also presented as proof of the photorefractive effect. We surprisingly found that the single component polymer exhibited good PR performance at room temperature without an external electric field or pre-poling.
1405
2. Experimental
trometer using CDCl3 as solvent. IR spectra were recorded on a Nicolet-460 FTIR spectrometer on KBr pellets. UV–Vis spectra were recorded with a Shimadzu 3010 UV–Visible spectrometer using THF as solvent, and the mole concentration of all samples was 0.1 mmol/L. Elemental analysis was measured with an MOD-1106 elemental analysis system. Gel permeation chromatography (GPC) analysis was performed on HLC-8220 liquid chromatograph with THF as eluent and UV/RI detection versus polystyrene standards. Differential scanning calorimetry (DSC) analysis was carried out under a nitrogen atmosphere on TA-2920 differential scanning calorimeter at a heating rate of 10 °C/min. Thermogravimetric analysis (TGA) was performed on a Netzsch-209 TGA system at a heating rate of 10 °C/min.
2.1. Materials
2.3. Syntheses
All materials were commercially available and used as received, except that the solvents were purified before use. Tetrahydrofuran (THF) was dried over and distilled from CaH2 under the protection of dry nitrogen.
The synthetic procedure for polyphosphazene is summarized in Scheme 1. N-(hydroxyhexyl)-carbazole (2) and hexachlorocyclotriphosphazene (1) were synthesized by following the literature procedures [5,12]. Poly(dichlorophosphazene) (P1) was firstly prepared via ring-opening polymerization from hexachlorocyclotriphosphazene. Then, polyphosphazene P2 with carbazole side groups was subsequently prepared by the substitution of chloro groups in polymer P1 with a carbazole
2.2. Characterization 1
H (400 MHz) and 31P NMR (162 MHz) spectra were obtained with a Bruker DRX-400 NMR spec-
Cl 1,1,2,2-Tetrachloroethane
PCl5 + NH4 Cl
135oC
Cl Cl
P
Cl
N
N
250 oC
P
P
Cl
N
P1
OR NaH
+ P1
Dry THF
+N
2
P
N
n
n
Cl
1
3
N
Cl
Cl OH N
P
NO2
H 2O/Nitrobenzene, NaDBS
OR 2
P2 N OR
OR' P
N
x
OR
P OR
P3
R= N
y
N
n
3
R' =
N N NO2
Scheme 1. Synthetic route for photorefractive polyphosphazene.
3
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L. Zhang et al. / Reactive & Functional Polymers 66 (2006) 1404–1410
compound, which was carried out in a dry argon atmosphere. Finally, the target polymer P3 was obtained via an azo coupling reaction, in which the carbazole ring was partially functionalized with a nitrophenyl azo group. 2.3.1. Preparation of poly(dichlorophosphazene)(P1) Dry hexachlorocyclotriphosphazene (5 g) was placed in a clean, dry Pyrex tube, which was sealed under vacuum. The sealed tube was placed in a salt bath and heated up to 245 °C for polymerization. The tube was removed after 50 h, cooled to room temperature, and then broken up under an argon atmosphere. Polymer P1 (1.8 g, 37% yield) was obtained by dissolving the solids in 20 mL of benzene and then precipitated from heptane. The pure polymer P1 was stored in dry THF under an inert atmosphere prior to use. 2.3.2. Preparation of polymer P2 N-(hydroxyhexyl)-carbazole (4.46 g, 16.7 mmol) was allowed to react with sodium hydride (0.87 g, 22 mmol) in THF(60 mL) at reflux for 24 h. Then, 20 mL of THF solution of polymer P1 (0.65 g, 11.2 mmol) was added. The mixture was stirred at reflux for 48 h, and was poured into water (500 mL). The precipitate was filtrated, washed with water, and air-dried. The product was purified by succeeding re-precipitations of the THF solution from water, methanol and hexane, respectively. A white powdery product P2 was obtained after drying in vacuum at 40 °C. Yield: 2.9 g, 90%. 1H NMR(CDCl3): 7.9–7.0 (m, Ar–H), 3.6 (s, –CH2– O, N–CH2–), 1.3–0.9 (m, –N–C–(CH2)4–C–O–); 31 P NMR(CDCl3): 8.15 (s). IR (KBr, cm1): 3050, 1596, 1484, 1452 (carbazole); 2933 (alkyl-H); 1326, 1231(P@N); 1052(P–O–C); 750, 723(P–N). Anal. Calcd: C, 74.85; H, 6.98; N, 7.27. Found: C, 74.78; H, 6.91; N, 7.21. 2.3.3. Preparation of polymer P3 4-Nitroaniline (0.276 g, 2 mmol) was dissolved in a solution of concentrated HCl (20 mL). The mixture was cooled with an ice bath to lower than 4 °C, then an aqueous solution containing sodium nitrite (0.16 g, 2.4 mmol) was slowly added. The mixture was stirred for 30 min. Sodium dodecyl benzenesulfonate (NaDBS) (0.2 g) and a solution of polymer P2 (0.58 g, 2 mmol carbazole groups) in nitrobenzene (20 mL) were successively added. The resultant mixture was vigorously stirred at
room temperature for 30 h, and then another part of diazonium salt, prepared from 2 mmol of 4-nitroaniline and 0.16 g of sodium nitrite, was added. The mixture was vigorously stirred at room temperature for another 30 h and then washed with water. The mixture was poured into cold methanol and the precipitate was filtered out, dried and then dissolved in a minimum amount of THF. The product was purified by reprecipitation from methanol for three times, then dried in vacuum at 40 °C. A red solid product P3 was obtained in 45% yield (0.4 g). 1H NMR(CDCl3)d: 8.6–7.0 (m, Ar–H), 3.6 (s, –CH2–O, N–CH2–), 1.3–0.9 (m, – N–C–(CH2)4–C–O–); 31P NMR(CDCl3)d: 8.50 (s). IR (KBr, cm1): 1596 (N@N); 1520, 1340 (NO2); 860(phenyl-H). 2.4. Photorefractive measurement For the photorefractive measurement, P3 dissolved in THF (10 wt %) was filtered through a 0.2 lm pore Teflon membrane filter, and then dipped onto two indium tin oxide (ITO) glass plates at room temperature. Residual solvent was removed by heating the film at around Tg in vacuum for 12 h. The dried sample was warmed on a hot plate and then the melted sample was sandwiched between the two ITO glass plates by applying a gentle pressure. The sandwich was cooled quickly to room temperature, while maintained under pressure, and the thickness of the film was controlled to be 80 lm through a Teflon spacer. The photorefractive property of an 80 lm-thick film of P3 was determined by TBC and FWM experiments. The experimental setup is depicted in Fig. 1. The normal of the sample surface was tilted 35° relative to the bisector of the incident beams, and the external interbeam angle was 20°. In the TBC experiment, two coherent p-polarized He–Ne laser beams at a wavelength of 633 nm overlapped on the sample. The initial power of beam 1 and beam 2 was measured to be 11 and 10 mW, respectively. The TBC coefficient C could be estimated from the following expression: 1 C ¼ ðlnðc0 bÞ lnðb þ 1 c0 ÞÞ; d Here, d is the optical path length, b is the initial beam ratio (in the absence of coupling), and c0 = I/I0 is the beam coupling ratio, where I0 is the signal intensity without the pump beam, and I is the signal intensity with the pump beam.
L. Zhang et al. / Reactive & Functional Polymers 66 (2006) 1404–1410
Fig. 1. Experimental setup for two-beam coupling and four-wave mixing. Only beam 1 and beam 2 are present in the two-beam coupling experiment, which are p-polarized He–Ne laser beams. Beam 3 is the s-polarized probe beam and beam 4 is the diffractive beam. K is the grating wave vector. V is an applied external electric field.
For the FWM experiment, two p-polarized beams with the same intensity of 10 mW were used as writing beams and an s-polarized probe beam with the intensity of 0.5 mW counterpropagated to one of the writing beams. Diffraction efficiency was determined by the ratio of the intensity of the diffracted signal to that of the incident reading beam. 3. Results and discussion 3.1. Synthesis The synthetic route to polyphosphazene is shown in Scheme 1. Polymer P2 was obtained by nucleophilic substitution reaction from polymer P1. In order to achieve the complete displacement of the chlorine atoms in P1, it is necessary to allow P1 to react with a large excess of a sodium alkoxide nucleophile of the carbazole group in a boiling solvent for a longer time [10,13]. Moreover, the introduction of the flexible spacer reduces the steric hindrance of the nucleophile during the substitution, which would also contribute to the complete displacement of the chlorine atoms in P1. 31P NMR spectroscopy was used to monitor the progress of the reaction until no further change in the spectrum occurred. The obtained polymer P2 was
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soluble in organic media and did not cross-link even after precipitations from water or exposure to atmospheric moisture, which illustrated that no residual chlorine atoms in P2 was remaining. As for the synthesis of polymer P3, a post-functionalization method was selected based on our previous experience [14,15]. The direct reaction of the sodium alkoxide of a donor–acceptor-substituted side group with poly(dichlorophosphazene) to prepare an NLO material was reported to be not so successful [16], in which an insoluble, incompletely substituted polymeric precipitate was obtained owing to the intrinsically high polarity of the donor–acceptor structure and the low solubility of sodium alkoxide of the corresponding side group. The post-azo-coupling reaction between polymer P2 and diazonium salt enabled the introduction of an NLO chromophore in situ, through which the cross-linking that resulted from the incomplete substitution of P1 with sodium alkoxide of a push–pull chromophore was avoided. The azo coupling reaction on the carbazole ring is not as easy as that on the benzene ring. For the less reactive carbazole compounds, it was reported that the azo coupling could be made more efficient in a two-phase system in the presence of the phase transfer catalyst NaDBS [17]. Nitrobenzene/water was selected as the reaction medium in the present work for its high dielectric constant and excellent solubility to the precursor polymer P2. Both the bulkiness of the introduced functional group and steric effect of the azo coupling product would make the electrophilic substitution to take place exclusively at the 3-position of carbazole. The resultant P3 was red in color and easily soluble in common organic solvents like chloroform, THF and DMF. A polymer film with high optical quality could be easily fabricated through solution casting. 3.2. Structural characterization of polymers Polymers P2 and P3 were characterized by 1H and 31P NMR spectra, infrared spectra as well as elemental analysis, and the results have been presented in the experimental part. In the 1H NMR spectrum of P3, several new signals appeared in the downfield at around 8.6 ppm that were not found for P2. These new signals were derived from the p-nitrophenyl moiety, which illustrated the successful formation of the azo-carbazole chromophore. In the 31P NMR spectra, a sharp singlet centered at 8 ppm was observed for both P2
L. Zhang et al. / Reactive & Functional Polymers 66 (2006) 1404–1410
and P3, which is typical of alkyloxy-substituted polyphosphazenes. The similarity in the 31P NMR spectra of P2 and P3 presumably revealed a consequence of the similar chemical environment of the phosphorus atoms in polymer P2 and the mixedsubstituted polymer P3. In addition, the singlet resonance also gave an evidence for the complete substitution of the chlorines in polymer P1, which was further supported by the results of elemental analysis of polymer P2. The FTIR spectrum of polymer P3 is shown in Fig. 2 in comparison with that of polymer P2. Polymer P3 showed an intense N@N stretching vibration at 1596 cm1, and the strong absorbencies at 1520 and 1340 cm1 are assignable to the NO2 group, the characteristic absorbance from the phenyl ring was found at 860 cm1, which proved the successful introduction of the nitrophenyl group in polymer P3. In order to determine the chromophore content, a model compound with similar chromophore structure, (4-Nitrophenyl)-[3-[N-(2-Hydroxyethyl)carbazolyl]]-diazene, was synthesized (Scheme 2), following the procedures described in the literature [5]. The chromophore content in the model compound is considered to be 100% (mol%). Fig. 3 shows the UV–Vis spectra of polymers P2 and P3 in comparison with the absorption spectrum of the model compound. After the azo coupling reaction, a broad absorption ranging from 360 to 570 nm with a kmax at 433 nm was observed for polymer P3, which was not found for polymer P2. This
OH N N N NO2 Scheme 2. Structure of model compound.
1.5 Absorbance
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model molecule P3 P2
1.0
0.5
0.0 300
400 500 Wavelength(nm)
600
Fig. 3. UV–Vis spectra of polyphosphazenes P2, P3 and the model compound in THF solution (0.1 mmol/L).
broad absorption peak was derived from the increase in the conjugation length. The chromophore content in P3 was calculated to be 29% through a comparison of the intensities of the maximum absorption peak at 433 nm with that of the model compound, i.e., the value of x in the structure of P3 given in Scheme 1 is 0.29. The molecular weight of P3 was estimated by gel permeation chromatography to be Mn = 22,600, with an Mw/Mn of 2.8. Such typical high molecular weight was in agreement with the expectation for phosphazene polymers [13]. No evidence of degradation was observed during the synthesis and isolation procedures. 3.3. Thermal properties and morphology
Fig. 2. Infrared spectra of polyphosphazenes P2 and P3.
The thermogravimetrical analysis showed that polymer P3 started to decompose at about 300 °C. Its glass transition temperature was found to be 50 °C, as measured by DSC. No melting transition was detected in the DSC curves, which implies that this polymer P3 is amorphous. Compared to P2 (Tg = 38 °C), the conformational stiffness of the
L. Zhang et al. / Reactive & Functional Polymers 66 (2006) 1404–1410
azobenzene group could contribute to P3 an increased Tg. However, the Tg of P3 is still significantly lower than that of the previously reported polymethacrylate containing the same NLO side groups [6,15], which is primarily a consequence of the highly flexible character of the phosphazene backbone coupled with the flexible spacer group. The film made from P3 has excellent optical transparency and retains unabated photorefractive performance for over 1 year under dark conditions. 3.4. Photorefractive properties It is well known that the nonlocal nature is the only feature to distinguish PR gratings from index gratings caused by other mechanisms and thus the asymmetric energy transfer between two writing beams, the so-called two-beam coupling is the most direct evidence for the occurrence of the PR effect. A nonzero and stable TBC signal was observed in the unpoled sample at zero bias field, as shown in Fig. 4, and the coupling gain coefficient C was measured to be 79 cm1. To our knowledge, this is the first report of asymmetric two-beam coupling in centrosymmetric phosphazene-based single component polymer at room temperature. According to the current PR theory, an external electric field to remove inversion symmetry is necessary for the PR effect in organic materials. However, the unique energy exchanges observed in polymer P3 do not meet the above requirement for photorefraction, which may result from the coupling between the space-charge field and light-induced orientational grating. When the sample is illumi-
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nated by the interference pattern, a periodic spacecharge field can arise. The photoisomerization enhances the rotational mobility of the azo groups and makes them rotate easily at a weak spacecharge field. Such a light-induced orientational effect has been observed experimentally and explained theoretically in PR polymeric materials [18–20]. The four-wave-mixing experiment was also performed without an external electric field, and the diffraction efficiency as a function of time is shown in Fig. 5. The grating growth was kept stable after 360 s, and the stable-state diffraction efficiency was measured to be 18%. It seems that the formation of the grating needs a longer time. Possibly, this is due to a reduced photogeneration efficiency or slow photogeneration that resulted from the lack of an extra photo-sensitizer. But after the grating formation, the erasing of the grating was fast when one beam was broken. The effect of external electric field on the photorefractive 2BC gain was studied, and the results are presented in Fig. 6. It is an interesting phenomenon that the gain coefficient reduced quickly at the beginning of an applied electric field of 10 V/lm, and then increased gradually with the further increase of the external electric field. When a 40 V/ lm external electric field was applied, an optical gain of 75 cm1 was observed. Compared to the common photorefractive materials, this phenomenon is somewhat abnormal. For most photorefractive polymers, all the processes responsible for
0.20
0.16
beam1
1.2
Diffraction efficiency
Normalized light intensity(I/I0)
0.18
1.0
0.8
beam off
0.14 0.12 0.10 0.08 0.06 0.04
beam on
0.02
beam2
0.00
0.6 0
500
1000
1500
Time(s) Fig. 4. TBC signal measured in P3 at zero electric field. Beam 1 intensity (upper trace) increases when beam 2 (lower trace) is turned on.
0
2
4
6
8
10
12
14
Time(min) Fig. 5. Diffraction efficiency of the grating as a function of time for P3 without an external electric field. Beam on and off refer to beam 2 turns on and off, respectively.
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L. Zhang et al. / Reactive & Functional Polymers 66 (2006) 1404–1410
Acknowledgement
90 80
This work was financially supported by the National Natural Science Foundation of China, Project 20274042.
Gain coefficient
70 60 50
References
40 30 20 10 0
10
20
30
40
Electric field (V/ m) Fig. 6. TBC gain coefficient as a function of applied field.
space-charge field build-up and chromophores orienting are strongly electric field-dependent, and it is frequently observed that the two-beam coupling gain increases with the electric field. Therefore, the unusual electric field-dependent of TBC observed in the present work may need further study for seeking an explanation. 4. Conclusion Post-azo-coupling reaction was successfully applied to the synthesis of a photorefractive polyphosphazene that contains a hole-transporting carbazole group and an azobenzene chromophore. The polymer shows a low glass transition temperature of 50 °C, and has excellent solubility in common organic solvents, which can easily be fabricated into an optically transparent film with long-term stability. The nonlinear optical effects, including twobeam-coupling (TBC) and four-wave-mixing (FWM), were studied at 633 nm under room temperature. The gain coefficient of 79 cm1 and diffraction efficiency of 18% were observed at zero bias field with the single component polymer. A possible explanation for this unique property is the coupling between the space-charge field and light-induced orientational grating.
[1] W.E. Moerner, S.M. Silence, Chem. Rev. 94 (1994) 127–155. [2] W.E. Moerner, A.G. Jepsen, C.L. Thompson, Ann. Rev. Mater. Sci. 27 (1997) 586–623. [3] W. You, S.K. Cao, Z.J. Hou, L.P. Yu, Macromolecules 36 (2003) 7014–7019. [4] Q. Wang, L.M. Wang, L.P. Yu, Macromol. Rapid Commun. 21 (2000) 723–745. [5] M.S. Ho, C. Barrett, J. Paterson, M. Esteghamatian, A. Natansohn, P. Rochon, Macromolecules 29 (1996) 4613– 4618. [6] C. Barrett, B. Choudhury, A. Natansohn, P. Rochon, Macromolecules 31 (1998) 4845–4851. [7] J. Hwang, J. Sohn, S.Y. Park, Macromolecules 36 (2003) 7970–7976. [8] Y.W. Chen, Y.K. He, F. Wang, H.Y. Chen, Q.H. Gong, Polymer 42 (2001) 1101–1107. [9] K. Meerholz, B.L. Volodin, B. Kippelen, N. Peyghambarian, Nature 371 (1994) 497–500. [10] H.R. Allcock, M.N. Mang, A.A. Dembek, K.J. Wynne, Macromolecules 22 (1989) 4179–4190. [11] H.R. Allcock, R. Ravikiran, M.A. Olshavsky, Macromolecules 31 (1998) 5206–5214. [12] R. De Jaeger, M. Gleria, Prog. Polym. Sci. 23 (1998) 179– 183. [13] M.A. Olshavsky, H.R. Allcock, Macromolecules 25 (1995) 6188–6197. [14] J. Shi, Z.W. Jiang, S.K. Cao, React Funct. Polym. 59 (2004) 87–91. [15] J. Shi, Z.W. Jiang, L. Zhang, S.K. Cao, Chin. J. Polym. Sci. 22 (2004) 551–555. [16] H.R. Allcock, A.A. Dembek, C. Kim, R.L. Devine, Y.Q. Shi, W.H. Steier, C.W. Spangler, Macromolecules 24 (1991) 1000–1010. [17] M. Ellwood, J. Griffiths, J. Chem. Soc., Chem. Commun. (1980) 181–183. [18] P. Cheben, F. del Monte, D.J. Worsfold, D.J. Carlsson, C.P. Grover, J.D. Mackenzie, Nature 408 (2000) 47–64. [19] C.J. Chang, H.C. Wang, G.Y. Liao, W.T. Whang, J.M. Liu, K.Y. Hsu, Polymer 38 (1997) 5063–5071. [20] J. Shi, M.M. Huang, Z.J. Chen, Q.H. Gong, S.K. Cao, J. Mater. Sci. 39 (2004) 3783–3785.
Reactive & Functional Polymers 66 (2006) 1404–1410
www.elsevier.com/locate/react
A carbazole-based photorefractive polyphosphazene prepared via post-azo-coupling reaction q Li Zhang a, Maomao Huang b, Zhiwei Jiang a, Zheng Yang a, Zhijian Chen b, Qihuang Gong b, Shaokui Cao a,* a
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450052, PR China b Department of Physics, Peking University, Beijing 100871, PR China Received 4 December 2005; received in revised form 21 March 2006; accepted 11 April 2006 Available online 26 May 2006
Abstract A carbazole-based photorefractive polyphosphazene was prepared via post-azo-coupling reaction. The polymer shows a low glass transition temperature of 50 °C, and has an excellent solubility in common organic solvents, which can easily be fabricated into an optically transparent film with long-term stability. The nonlinear optical effects, including two-beamcoupling and four-wave-mixing, were studied at 633 nm under room temperature. A gain coefficient of 79 cm1 and a diffraction efficiency of 18% were observed at zero external electric field without any added plasticizer or sensitizer. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Polyphosphazene; Post-azo-coupling; Carbazole; Photorefractive
1. Introduction Polymeric photorefractive materials have attracted steady interest due to their potential applications and many distinct advantages. Compared to widely investigated host-guest polymer composites [1,2] and fully functionalized polymers [3,4], bi-functional photorefractive polymers, which possess both photoconductivity and electro-optic (EO) effect, have obvious advantages attributed to their high stability and easy synthesis. In the bi-functional q Project supported by the National Natural Science Foundation of China, Project No. 20274042. * Corresponding author. Tel./fax: +86 371 6776 3561. E-mail addresses: [email protected] (L. Zhang), caoshaokui@ zzu.edu.cn (S. Cao).
photorefractive polymer family, carbazole-based polymers have received much attention [5–8] because of the fact that polyvinylcarbazole has previously been used in a high-optical-gain photorefractive polymer composite [9]. But the high glass transition temperature (Tg) of these polymers often made it difficult to pole at room temperature to achieve the alignment of the chromophore. Lowering the Tg by adding plasticizer or introducing a spacer between the main chain and the chromophore normally resulted in the deduction of photorefractive performance due to the relative low content of chromophore. Polyphosphazenes, which contain a very flexible inorganic backbone of alternating phosphorous and nitrogen atoms, are very good candidates for preparing polymeric materials having a low Tg
1381-5148/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2006.04.003
L. Zhang et al. / Reactive & Functional Polymers 66 (2006) 1404–1410
[10]. Moreover, polyphosphazenes possess unusual properties, such as high thermal and oxidative stability, optical transparency from 220 nm to the near-IR region of the backbone, as well as their ease of syntheses [11]. These characteristics are very beneficial to electro-optic applications. In this paper, we report a carbazole-based photorefractive polyphosphazene prepared via post-azocoupling reaction. The result of two-beam-coupling (TBC) and four-wave-mixing (FWM) measurements is also presented as proof of the photorefractive effect. We surprisingly found that the single component polymer exhibited good PR performance at room temperature without an external electric field or pre-poling.
1405
2. Experimental
trometer using CDCl3 as solvent. IR spectra were recorded on a Nicolet-460 FTIR spectrometer on KBr pellets. UV–Vis spectra were recorded with a Shimadzu 3010 UV–Visible spectrometer using THF as solvent, and the mole concentration of all samples was 0.1 mmol/L. Elemental analysis was measured with an MOD-1106 elemental analysis system. Gel permeation chromatography (GPC) analysis was performed on HLC-8220 liquid chromatograph with THF as eluent and UV/RI detection versus polystyrene standards. Differential scanning calorimetry (DSC) analysis was carried out under a nitrogen atmosphere on TA-2920 differential scanning calorimeter at a heating rate of 10 °C/min. Thermogravimetric analysis (TGA) was performed on a Netzsch-209 TGA system at a heating rate of 10 °C/min.
2.1. Materials
2.3. Syntheses
All materials were commercially available and used as received, except that the solvents were purified before use. Tetrahydrofuran (THF) was dried over and distilled from CaH2 under the protection of dry nitrogen.
The synthetic procedure for polyphosphazene is summarized in Scheme 1. N-(hydroxyhexyl)-carbazole (2) and hexachlorocyclotriphosphazene (1) were synthesized by following the literature procedures [5,12]. Poly(dichlorophosphazene) (P1) was firstly prepared via ring-opening polymerization from hexachlorocyclotriphosphazene. Then, polyphosphazene P2 with carbazole side groups was subsequently prepared by the substitution of chloro groups in polymer P1 with a carbazole
2.2. Characterization 1
H (400 MHz) and 31P NMR (162 MHz) spectra were obtained with a Bruker DRX-400 NMR spec-
Cl 1,1,2,2-Tetrachloroethane
PCl5 + NH4 Cl
135oC
Cl Cl
P
Cl
N
N
250 oC
P
P
Cl
N
P1
OR NaH
+ P1
Dry THF
+N
2
P
N
n
n
Cl
1
3
N
Cl
Cl OH N
P
NO2
H 2O/Nitrobenzene, NaDBS
OR 2
P2 N OR
OR' P
N
x
OR
P OR
P3
R= N
y
N
n
3
R' =
N N NO2
Scheme 1. Synthetic route for photorefractive polyphosphazene.
3
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L. Zhang et al. / Reactive & Functional Polymers 66 (2006) 1404–1410
compound, which was carried out in a dry argon atmosphere. Finally, the target polymer P3 was obtained via an azo coupling reaction, in which the carbazole ring was partially functionalized with a nitrophenyl azo group. 2.3.1. Preparation of poly(dichlorophosphazene)(P1) Dry hexachlorocyclotriphosphazene (5 g) was placed in a clean, dry Pyrex tube, which was sealed under vacuum. The sealed tube was placed in a salt bath and heated up to 245 °C for polymerization. The tube was removed after 50 h, cooled to room temperature, and then broken up under an argon atmosphere. Polymer P1 (1.8 g, 37% yield) was obtained by dissolving the solids in 20 mL of benzene and then precipitated from heptane. The pure polymer P1 was stored in dry THF under an inert atmosphere prior to use. 2.3.2. Preparation of polymer P2 N-(hydroxyhexyl)-carbazole (4.46 g, 16.7 mmol) was allowed to react with sodium hydride (0.87 g, 22 mmol) in THF(60 mL) at reflux for 24 h. Then, 20 mL of THF solution of polymer P1 (0.65 g, 11.2 mmol) was added. The mixture was stirred at reflux for 48 h, and was poured into water (500 mL). The precipitate was filtrated, washed with water, and air-dried. The product was purified by succeeding re-precipitations of the THF solution from water, methanol and hexane, respectively. A white powdery product P2 was obtained after drying in vacuum at 40 °C. Yield: 2.9 g, 90%. 1H NMR(CDCl3): 7.9–7.0 (m, Ar–H), 3.6 (s, –CH2– O, N–CH2–), 1.3–0.9 (m, –N–C–(CH2)4–C–O–); 31 P NMR(CDCl3): 8.15 (s). IR (KBr, cm1): 3050, 1596, 1484, 1452 (carbazole); 2933 (alkyl-H); 1326, 1231(P@N); 1052(P–O–C); 750, 723(P–N). Anal. Calcd: C, 74.85; H, 6.98; N, 7.27. Found: C, 74.78; H, 6.91; N, 7.21. 2.3.3. Preparation of polymer P3 4-Nitroaniline (0.276 g, 2 mmol) was dissolved in a solution of concentrated HCl (20 mL). The mixture was cooled with an ice bath to lower than 4 °C, then an aqueous solution containing sodium nitrite (0.16 g, 2.4 mmol) was slowly added. The mixture was stirred for 30 min. Sodium dodecyl benzenesulfonate (NaDBS) (0.2 g) and a solution of polymer P2 (0.58 g, 2 mmol carbazole groups) in nitrobenzene (20 mL) were successively added. The resultant mixture was vigorously stirred at
room temperature for 30 h, and then another part of diazonium salt, prepared from 2 mmol of 4-nitroaniline and 0.16 g of sodium nitrite, was added. The mixture was vigorously stirred at room temperature for another 30 h and then washed with water. The mixture was poured into cold methanol and the precipitate was filtered out, dried and then dissolved in a minimum amount of THF. The product was purified by reprecipitation from methanol for three times, then dried in vacuum at 40 °C. A red solid product P3 was obtained in 45% yield (0.4 g). 1H NMR(CDCl3)d: 8.6–7.0 (m, Ar–H), 3.6 (s, –CH2–O, N–CH2–), 1.3–0.9 (m, – N–C–(CH2)4–C–O–); 31P NMR(CDCl3)d: 8.50 (s). IR (KBr, cm1): 1596 (N@N); 1520, 1340 (NO2); 860(phenyl-H). 2.4. Photorefractive measurement For the photorefractive measurement, P3 dissolved in THF (10 wt %) was filtered through a 0.2 lm pore Teflon membrane filter, and then dipped onto two indium tin oxide (ITO) glass plates at room temperature. Residual solvent was removed by heating the film at around Tg in vacuum for 12 h. The dried sample was warmed on a hot plate and then the melted sample was sandwiched between the two ITO glass plates by applying a gentle pressure. The sandwich was cooled quickly to room temperature, while maintained under pressure, and the thickness of the film was controlled to be 80 lm through a Teflon spacer. The photorefractive property of an 80 lm-thick film of P3 was determined by TBC and FWM experiments. The experimental setup is depicted in Fig. 1. The normal of the sample surface was tilted 35° relative to the bisector of the incident beams, and the external interbeam angle was 20°. In the TBC experiment, two coherent p-polarized He–Ne laser beams at a wavelength of 633 nm overlapped on the sample. The initial power of beam 1 and beam 2 was measured to be 11 and 10 mW, respectively. The TBC coefficient C could be estimated from the following expression: 1 C ¼ ðlnðc0 bÞ lnðb þ 1 c0 ÞÞ; d Here, d is the optical path length, b is the initial beam ratio (in the absence of coupling), and c0 = I/I0 is the beam coupling ratio, where I0 is the signal intensity without the pump beam, and I is the signal intensity with the pump beam.
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Fig. 1. Experimental setup for two-beam coupling and four-wave mixing. Only beam 1 and beam 2 are present in the two-beam coupling experiment, which are p-polarized He–Ne laser beams. Beam 3 is the s-polarized probe beam and beam 4 is the diffractive beam. K is the grating wave vector. V is an applied external electric field.
For the FWM experiment, two p-polarized beams with the same intensity of 10 mW were used as writing beams and an s-polarized probe beam with the intensity of 0.5 mW counterpropagated to one of the writing beams. Diffraction efficiency was determined by the ratio of the intensity of the diffracted signal to that of the incident reading beam. 3. Results and discussion 3.1. Synthesis The synthetic route to polyphosphazene is shown in Scheme 1. Polymer P2 was obtained by nucleophilic substitution reaction from polymer P1. In order to achieve the complete displacement of the chlorine atoms in P1, it is necessary to allow P1 to react with a large excess of a sodium alkoxide nucleophile of the carbazole group in a boiling solvent for a longer time [10,13]. Moreover, the introduction of the flexible spacer reduces the steric hindrance of the nucleophile during the substitution, which would also contribute to the complete displacement of the chlorine atoms in P1. 31P NMR spectroscopy was used to monitor the progress of the reaction until no further change in the spectrum occurred. The obtained polymer P2 was
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soluble in organic media and did not cross-link even after precipitations from water or exposure to atmospheric moisture, which illustrated that no residual chlorine atoms in P2 was remaining. As for the synthesis of polymer P3, a post-functionalization method was selected based on our previous experience [14,15]. The direct reaction of the sodium alkoxide of a donor–acceptor-substituted side group with poly(dichlorophosphazene) to prepare an NLO material was reported to be not so successful [16], in which an insoluble, incompletely substituted polymeric precipitate was obtained owing to the intrinsically high polarity of the donor–acceptor structure and the low solubility of sodium alkoxide of the corresponding side group. The post-azo-coupling reaction between polymer P2 and diazonium salt enabled the introduction of an NLO chromophore in situ, through which the cross-linking that resulted from the incomplete substitution of P1 with sodium alkoxide of a push–pull chromophore was avoided. The azo coupling reaction on the carbazole ring is not as easy as that on the benzene ring. For the less reactive carbazole compounds, it was reported that the azo coupling could be made more efficient in a two-phase system in the presence of the phase transfer catalyst NaDBS [17]. Nitrobenzene/water was selected as the reaction medium in the present work for its high dielectric constant and excellent solubility to the precursor polymer P2. Both the bulkiness of the introduced functional group and steric effect of the azo coupling product would make the electrophilic substitution to take place exclusively at the 3-position of carbazole. The resultant P3 was red in color and easily soluble in common organic solvents like chloroform, THF and DMF. A polymer film with high optical quality could be easily fabricated through solution casting. 3.2. Structural characterization of polymers Polymers P2 and P3 were characterized by 1H and 31P NMR spectra, infrared spectra as well as elemental analysis, and the results have been presented in the experimental part. In the 1H NMR spectrum of P3, several new signals appeared in the downfield at around 8.6 ppm that were not found for P2. These new signals were derived from the p-nitrophenyl moiety, which illustrated the successful formation of the azo-carbazole chromophore. In the 31P NMR spectra, a sharp singlet centered at 8 ppm was observed for both P2
L. Zhang et al. / Reactive & Functional Polymers 66 (2006) 1404–1410
and P3, which is typical of alkyloxy-substituted polyphosphazenes. The similarity in the 31P NMR spectra of P2 and P3 presumably revealed a consequence of the similar chemical environment of the phosphorus atoms in polymer P2 and the mixedsubstituted polymer P3. In addition, the singlet resonance also gave an evidence for the complete substitution of the chlorines in polymer P1, which was further supported by the results of elemental analysis of polymer P2. The FTIR spectrum of polymer P3 is shown in Fig. 2 in comparison with that of polymer P2. Polymer P3 showed an intense N@N stretching vibration at 1596 cm1, and the strong absorbencies at 1520 and 1340 cm1 are assignable to the NO2 group, the characteristic absorbance from the phenyl ring was found at 860 cm1, which proved the successful introduction of the nitrophenyl group in polymer P3. In order to determine the chromophore content, a model compound with similar chromophore structure, (4-Nitrophenyl)-[3-[N-(2-Hydroxyethyl)carbazolyl]]-diazene, was synthesized (Scheme 2), following the procedures described in the literature [5]. The chromophore content in the model compound is considered to be 100% (mol%). Fig. 3 shows the UV–Vis spectra of polymers P2 and P3 in comparison with the absorption spectrum of the model compound. After the azo coupling reaction, a broad absorption ranging from 360 to 570 nm with a kmax at 433 nm was observed for polymer P3, which was not found for polymer P2. This
OH N N N NO2 Scheme 2. Structure of model compound.
1.5 Absorbance
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model molecule P3 P2
1.0
0.5
0.0 300
400 500 Wavelength(nm)
600
Fig. 3. UV–Vis spectra of polyphosphazenes P2, P3 and the model compound in THF solution (0.1 mmol/L).
broad absorption peak was derived from the increase in the conjugation length. The chromophore content in P3 was calculated to be 29% through a comparison of the intensities of the maximum absorption peak at 433 nm with that of the model compound, i.e., the value of x in the structure of P3 given in Scheme 1 is 0.29. The molecular weight of P3 was estimated by gel permeation chromatography to be Mn = 22,600, with an Mw/Mn of 2.8. Such typical high molecular weight was in agreement with the expectation for phosphazene polymers [13]. No evidence of degradation was observed during the synthesis and isolation procedures. 3.3. Thermal properties and morphology
Fig. 2. Infrared spectra of polyphosphazenes P2 and P3.
The thermogravimetrical analysis showed that polymer P3 started to decompose at about 300 °C. Its glass transition temperature was found to be 50 °C, as measured by DSC. No melting transition was detected in the DSC curves, which implies that this polymer P3 is amorphous. Compared to P2 (Tg = 38 °C), the conformational stiffness of the
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azobenzene group could contribute to P3 an increased Tg. However, the Tg of P3 is still significantly lower than that of the previously reported polymethacrylate containing the same NLO side groups [6,15], which is primarily a consequence of the highly flexible character of the phosphazene backbone coupled with the flexible spacer group. The film made from P3 has excellent optical transparency and retains unabated photorefractive performance for over 1 year under dark conditions. 3.4. Photorefractive properties It is well known that the nonlocal nature is the only feature to distinguish PR gratings from index gratings caused by other mechanisms and thus the asymmetric energy transfer between two writing beams, the so-called two-beam coupling is the most direct evidence for the occurrence of the PR effect. A nonzero and stable TBC signal was observed in the unpoled sample at zero bias field, as shown in Fig. 4, and the coupling gain coefficient C was measured to be 79 cm1. To our knowledge, this is the first report of asymmetric two-beam coupling in centrosymmetric phosphazene-based single component polymer at room temperature. According to the current PR theory, an external electric field to remove inversion symmetry is necessary for the PR effect in organic materials. However, the unique energy exchanges observed in polymer P3 do not meet the above requirement for photorefraction, which may result from the coupling between the space-charge field and light-induced orientational grating. When the sample is illumi-
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nated by the interference pattern, a periodic spacecharge field can arise. The photoisomerization enhances the rotational mobility of the azo groups and makes them rotate easily at a weak spacecharge field. Such a light-induced orientational effect has been observed experimentally and explained theoretically in PR polymeric materials [18–20]. The four-wave-mixing experiment was also performed without an external electric field, and the diffraction efficiency as a function of time is shown in Fig. 5. The grating growth was kept stable after 360 s, and the stable-state diffraction efficiency was measured to be 18%. It seems that the formation of the grating needs a longer time. Possibly, this is due to a reduced photogeneration efficiency or slow photogeneration that resulted from the lack of an extra photo-sensitizer. But after the grating formation, the erasing of the grating was fast when one beam was broken. The effect of external electric field on the photorefractive 2BC gain was studied, and the results are presented in Fig. 6. It is an interesting phenomenon that the gain coefficient reduced quickly at the beginning of an applied electric field of 10 V/lm, and then increased gradually with the further increase of the external electric field. When a 40 V/ lm external electric field was applied, an optical gain of 75 cm1 was observed. Compared to the common photorefractive materials, this phenomenon is somewhat abnormal. For most photorefractive polymers, all the processes responsible for
0.20
0.16
beam1
1.2
Diffraction efficiency
Normalized light intensity(I/I0)
0.18
1.0
0.8
beam off
0.14 0.12 0.10 0.08 0.06 0.04
beam on
0.02
beam2
0.00
0.6 0
500
1000
1500
Time(s) Fig. 4. TBC signal measured in P3 at zero electric field. Beam 1 intensity (upper trace) increases when beam 2 (lower trace) is turned on.
0
2
4
6
8
10
12
14
Time(min) Fig. 5. Diffraction efficiency of the grating as a function of time for P3 without an external electric field. Beam on and off refer to beam 2 turns on and off, respectively.
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Acknowledgement
90 80
This work was financially supported by the National Natural Science Foundation of China, Project 20274042.
Gain coefficient
70 60 50
References
40 30 20 10 0
10
20
30
40
Electric field (V/ m) Fig. 6. TBC gain coefficient as a function of applied field.
space-charge field build-up and chromophores orienting are strongly electric field-dependent, and it is frequently observed that the two-beam coupling gain increases with the electric field. Therefore, the unusual electric field-dependent of TBC observed in the present work may need further study for seeking an explanation. 4. Conclusion Post-azo-coupling reaction was successfully applied to the synthesis of a photorefractive polyphosphazene that contains a hole-transporting carbazole group and an azobenzene chromophore. The polymer shows a low glass transition temperature of 50 °C, and has excellent solubility in common organic solvents, which can easily be fabricated into an optically transparent film with long-term stability. The nonlinear optical effects, including twobeam-coupling (TBC) and four-wave-mixing (FWM), were studied at 633 nm under room temperature. The gain coefficient of 79 cm1 and diffraction efficiency of 18% were observed at zero bias field with the single component polymer. A possible explanation for this unique property is the coupling between the space-charge field and light-induced orientational grating.
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