Synthesis and characterization of photochromic azobenzene cellulose ethers

Article abstract of DOI:10.1016/j.carbpol.2013.08.093

Photochromic azobenzene cellulose ethers were prepared by homogeneous etherification of cellulose with 2,3-epoxypropoxy-azobenzene (EA) in N,N-dimethylacetamide/lithium chloride solution. The EA with epoxy group could highly efficiently react with cellulose to synthesize 3-azobenzyloxy-2- hydroxypropyl-cellulose (Azo-cellulose) ethers with controllable degree of substitution (DS_azo). The DS_azo was in a range of 0.2-2.0 adjusted by the molar ratio of EA to anhydroglucose unit of cellulose. The Azo-celluloses with DS_azo ≥ 0.53 were soluble in aprotic solvents like dimethylsulfoxide. Their chemical structures and properties were characterized by elemental analysis, FT-IR, NMR, and thermogravimetric analysis. They showed reversible trans-cis-trans transition when Azo-cellulose/DMAc solutions were irradiated by successive irradiation of UV and visible light. The transition between trans- and cis- isomers could be effectively controlled by simply adjusting the irradiation time. The photo-stimulated trans-cis-trans conformational change induced conformation transition between rod-like shape of trans-isomer and skewed shape of cis-isomer.

Full text of DOI:10.1016/j.carbpol.2013.08.093

Accepted Manuscript
Title: Synthesis and characterization of photochromic
azobenzene cellulose ethers
Author: Zhijun Li Dong Zhang Jiabao Weng Biao Chen
Haiqing Liu
PII:
S0144-8617(13)00877-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.carbpol.2013.08.093
CARP 8083
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Revised date:
Accepted date:
28-5-2013
20-8-2013
28-8-2013
Please cite this article as: Li, Z., Zhang, D., Weng, J., Chen, B., & Liu, H., Synthesis and
characterization of photochromic azobenzene cellulose ethers, Carbohydrate Polymers
(2013), http://dx.doi.org/10.1016/j.carbpol.2013.08.093
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Synthesis and characterization of photochromic azobenzene cellulose ethers
Zhijun Li, Dong Zhang, Jiabao Weng, Biao Chen and Haiqing Liu*
3
4
5
Fujian Provincial Key Laboratory of Polymer Materials, College of Material Science
and Engineering, Fujian Normal University, Fujian 350007, China
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Corresponding author: Dr. Haiqing Liu, E-mail: haiqing.liu@gmail.com; Tel/Fax:
+86-591-83404938
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11 ABSTRACT
12
Photochromic azobenzene cellulose ethers were prepared by homogeneous
13 etherification of cellulose with 2,3-epoxypropoxy-azobenzene (EA) in
14 N,N-dimethylacetamide/lithium chloride solution. The EA with epoxy group could
15 highly
efficiently
react
with
cellulose
to
synthesize
16 3-azobenzyloxy-2-hydroxypropyl-cellulose (Azo-cellulose) ethers with controllable
17 degree of substitution (DSazo). The DSazo was in a range of 0.2–2.0 adjusted by the
18 molar ratio of EA to anhydroglucose unit of cellulose. The Azo-celluloses with
19 DSazo≥0.53 were soluble in aprotic solvents like dimethylsulfoxide. Their chemical
20 structures and properties were characterized by elemental analysis, FT-IR, NMR, and
21 thermogravimetric analysis. They showed reversible trans-cis–trans transition when
22 Azo-cellulose/DMAc solutions were irradiated by successive irradiation of UV and
23 visible light. The transition between trans- and cis- isomers could be effectively
24 controlled by simply adjusting the irradiation time. The photo-stimulated
25 trans-cis-trans conformational change induced conformation transition between
26 rod-like shape of trans-isomer and skewed shape of cis-isomer.
27 Keywords: Cellulose; azo polymers; photochemistry; photochromic behavior
28
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28 1. Introduction
29
Photoresponsive polymers are very interesting materials whose structure
30 undergoes reversible change under the stimulation of non-invasive and easy available
31 light with appropriate wavelength, leading to variation of its physical properties such
32 as color, shape, dimension, solubility, and viscosity etc. Such polymers have displayed
33 applications in many fields such as photonics, optical storage, controlled medicine
34 delivery and release, surface relief gratings, photoswitchable biomaterials and
35 actuators. Therefore the synthesis, characterization and application of photoresponsive
36 polymers have attracted much research attention from both the academic and
37 industrial circles (Ercole et al., 2010; Hou et al., 2012; Li et al., 2003; Nishi et al.,
38 2011; Pieroni et al., 2000).
39
Among the many photoresponsive polymers, azo-polymers bearing azobenzene
40 moieties have been extensively studied (Hurduc et al., 2007; Xue et al., 2009). The
41 azobenzene moiety has trans- and cis- photoisomers. The former is a more stable state
42 in visible light. Under excitation of ultraviolet light, the trans- changes to cis-
43 structure. A reversible change occurs, i.e. cis- to trans- isomer, when irradiated with
44 visible light or heat (Oertel et al., 2009; Sahu et al., 1995). This characteristic feature
45 makes azo-polymers useful in many new materials, such as optical memory, optical
46 switch and information converter etc.
47
When azo-polymers are used as biomaterials such as photo-regulated in vivo drug
48 carriers, the issues of biocompatibility and biodegradability are highly important
49 (Ciardelli et al., 2013). Grafting of azobenzene molecules onto biopolymers provides
50 a promising solution for the preparation of biocompatible and biodegradable
51 photochromic materials (Asanuma et al., 2007; Hua et al., 2004). Cellulose is one of
52 the most abundant naturally-occurring biopolymers in the world. Due to the good
53 mechanical property, biocompatibility and biodegradability, cellulose has become an
54 important biomaterial (Chang et al., 2011). Its broad applications in novel fields
55 would inevitably lessen the depletion of fossil fuels, and relieve the environmental
56 pollution caused by synthetic polymers. Cellulose molecular chain is composed of
57 anhydroglucose unit (AGU) linked by β-1,4-glycosidic bond. Each AGU has three
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58 hydroxyl groups, which are reactive groups with azobenzene compounds to fabricate
59 cellulose derivatives with chromophores. So far, a few photochromic azo-cellulose
60 derivatives prepared by the esterification reaction have been reported (Arai et al.,
61 1995; Arai et al., 1996; Yashima et al., 1995). However, cellulose derivatives with
62 photochromic moieties linked by ether bond have been less studied. An azocellulose
63 linked by 4-cyanophenylazophenol was prepared via Mitsunobu reaction, which was a
64 heterogeneous reaction and took a long reaction time (2 days) (Yang et al., 2001).
65
Because of cellulose ethers are more stable than cellulose esters in basic
66 conditions, and the preparation conditions for cellulose ethers are more ambient (like
67 allowance of the presence of H2O and O2) than those for cellulose esters when
68 esterification reagents like acryl chlorides and acid anhydride are used. In this work,
69 cellulose was reacted with 2,3-epoxypropoxy-azobenzene through homogeneous
70 etherification reaction in a solvent of N,N-dimethylacetamide/lithium chloride
1
71 (DMAc/LiCl). The structure was studied using elemental analysis, FT-IR, H-NMR,
72 and 13C-NMR spectroscopy. The photochromic property in dilute DMAc solutions, i.e.
73 trans-cis-trans photoisomerization processes upon light irradiation was studied by
74 UV-Vis spectroscopy.
75
76 2. Experimental
77 2.1. Materials
78
Cellulose powder (CF11, Whatman) was used as received. Its molecular weight
79 was ca. 3.1 x 104 g/mol measured with gel permeation chromatography (GPC)
80 (Glasser et al., 2000). N,N-dimethylacetamide (DMAc), N,N-dimethylformamide
81 (DMF), dimethyl sulfoxide (DMSO), 1,2-epoxy-3-chloropropane (ECP), aniline,
82 phenol, benzyltrimethylammonium chloride (TMBAC), anhydrous lithium chloride
83 (LiCl), sodium nitrite (NaNO2), concentrated hydrochloric acid solution (36.5 wt.%),
84 methylene chloride, chloroform and sodium hydroxide flake were purchased from
85 Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. All chemicals are of
86 analytical grade.
87
Cellulose powder and LiCl were vacuum dried at 120oC for about 12 h before use.
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88 DMAc was refluxed overnight at 80oC in the presence of calcium hydride (CaH2),
89 followed by vacuum distillation. Then it was stored in a desiccator and kept over 3Å
90 molecular sieves. All the other reagents were used as received.
91
92 2.2. Synthesis of 2,3-epoxypropoxy-azobenzene (EA)
93
Following a procedure similar to that reported by Yin and coauthors (Yu et al.,
94 2010), EA was synthesized by diazo-coupling reaction between aniline and phenol in
95 the presence of sodium nitrite and hydrochloric acid and the following reaction with
96 ECP in the presence of the phase transferring agent TMBAC and sodium hydroxide
97 (Scheme 1a, b).
1
98
The yield of HA was ca. 93%. H-NMR (400 MHz, CDCl3, δ ppm): 5.57 (1H,
13
99 -OH), 6.93, 7.49, 7.87 (aromatic, 9H). C-NMR (400 MHz, CDCl3, δ ppm): 158.48,
100 152.88, 147.37 130.67, 129.27, 125.20, 122.77, 116.03. FTIR spectrum (cm-1, KBr
101 pellet): 3400 (υO-H); 3157 (υAr-H), 1600 (υC=C), 1502 (υN=N), and 600~850 (γC-H) for the
102 benzene ring.
103
The yield of EA was ca. 60%. 1H-NMR (400 MHz, CDCl3, δ ppm) (Fig. 1a): 2.76
b’
a
b
104 and 2.90 (d, 2H, c’H), 3.37 (m, 1H, H), 4.00 and 4.28 (d, 2H, H), 7.02 (d, 2H, H),
c
105 7.48 (m, 3H, d,eH), 7.90 (d, 4H, H). 13C-NMR (400 MHz, CDCl3, ppm) (Fig. 1b):
106 δ=44.87 (c’C), 50.20 (b’C), 69.20 (a’C), 161.06 (aC), 152.34 (eC), 147.53 (dC), 130.63
107 (hC), 129.26 (gC), 125.20 (cC), 122.82 (fC), 116.03 (bC). FTIR spectrum (KBr pellet)
108 (cm-1): the assignment of most peaks could be referred to that of HA, an additional
109 typical peak at 2925 cm-1 was assigned to υas of -CH2- on the epoxypropyl group.
110
111 2.3. Synthesis of 3-azobenzyloxy-2-hydroxypropyl-cellulose (Azo-cellulose)
112
Cellulose/DMAc-LiCl solution was prepared according to a standard procedure.
113 Briefly, cellulose (1.5 g) was suspended in 66 g of DMAc which was pre-heated to
114 150oC in N2 atmosphere, and kept at this temperature for 30 min under stirring. Then
115 the mixture was cooled to 110oC, followed by an addition of 7.5 g of LiCl. After the
116 mixture was stirred for 1 h at this temperature, it was cooled to room temperature and
117 stirred for ca. 12 h to obtain a transparent 2 wt% cellulose solution.
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118
Azo-cellulose was synthesized according to Scheme 1c. In a typical procedure to
119 prepare Azo-cellulose1, 0.5 mL of distilled water was dropwise added to 20 g of the
120 above prepared cellulose solution containing 0.4 g of cellulose (2.47 mmol AGU),
121 then 1.23 g of NaOH powder (mole ratio of NaOH/AGU=10) was added and stirred
122 for 1 h at room temperature. Subsequently the reaction mixture was heated to 70oC
123 and stirred for 0.5 h. Then 0.95 mL of EA/DMAc solution with concentration of 2.6
124 mol/L was dropwise added and reacted for 7 h at 70oC (mole ratio of EA/AGU=1.0).
125 The reaction was terminated by pouring the solution into 200 mL of diethyl ether. The
126 precipitates were collected by filtration, and were purified by Soxhlet extraction with
127 acetone. The products were vacuum dried at 70oC for 12 h, and showed slight yellow
128 color. The mass yield of Azo-cellulose1 (mAzo-cellulose/mcellulose x 100%) was 105%. A
129 set of eight samples were prepared by adjusting the EA/AGU mole ratio in a range of
130 1 - 21 (Table 1). Their mass yield was in a range of 105 ~320%. It increased with the
131 degree of substitution (DS) of azobenzene moiety.
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132
133
Scheme 1
134 2.4. Characterization
135
13C-NMR spectra of HA, EA, and Azo-cellulose samples were recorded on an
1
136 AVANCE III spectrometer at 25oC operating at 500 MHz. H-NMR spectra were
137 carried out on a Bruker-ARX 500 spectrometer at 22oC operating at 400 MHz. Thermal
138 gravimetric analysis (TGA) was performed on a TA5200/MDSC291 thermogravimetric
139 analyzer at a heating rate of 10oC min−1 from 30 to 600oC in nitrogen atmosphere.
140 FTIR spectra in KBr form were performed on a Nicolet 5700 fourier transform infrared
141 spectrometer. Nitrogen and carbon content were measured on an elemental analyzer
142 (Vario EL III, Hanau, Germany). The DSazo was calculated according to the following
143 equation:
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N%
162(
)
100
144
DSazo

N%
28 254.3(
)
100
145 where N% is the nitrogen content of Azo-cellulose. The DSazo values were listed in
146 Table 1.
147
The solubility of Azo-cellulose was measured by adding 0.01 g of Azo-cellulose
148 to 10 ml of solvents like DMAc, DMF, DMSO, acetone, and chloroform, then stirred
149 at 50oC for 24 h. Dissolution was determined by visual inspection of the
150 disappearance of Azo-cellulose particles. The molecular weight of Azo-celluloses was
151 measured on Waters 1515 gel permeation chromatography with DMF as solvent and
152 eluent, polystyrene was used as standard.
153
Light absorption of HA, EA and Azo-cellulose solutions in DMAc with
154 concentration of 0.12 g/L was recorded on UV-vis spectrophotometer (Lambda 850,
155 PerkinElmer) in a range of 250~600 nm at 28±2 oC. DMAc was used as a reference.
156 In an attempt to measure the DSazo by the calorimetric method, a calibration curve
157 (y=0.152x-0.06, R2=0.994) was obtained by plotting the absorbance at 355 nm against
158 concentration of model compound EA in DMAc (concentration range: 0.01 – 0.16
159 g/L). The amount of azo moieties in the Azo-cellulose was determined according to
160 the absorbance A355 of the Azo-cellulose/DMAc solution. A similar method was
161 applied for an azocellulose reported by Yang and coworkers (Yang et al., 2001). Such
162 obtained DSazo values were listed in Table 1.
163
Photochromic property of Azo-cellulose in solution was measured as follows. The
164 Azo-cellulose/DMAc solution was irradiated with 365 nm light with light intensity of
165 20 mW/cm2 for certain time period, then its light absorption spectrum was recorded
166 immediately. When the spectrum reached equilibrium, the solution was irradiated
167 under visible light of >450 nm in wavelength with light intensity of 35 mW/cm2 for
168 certain time interval. The light source was an incandescent lamp equipped with a
169 long-pass filter, followed by recording of light absorption spectrum.
170
171
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172 Table 1 Nitrogen, carbon content, and DSazo values of azobenzene cellulose
173 (Azo-cellulose)
DSazo values calculated from
Mole ratio of
Colorimetric
Sample
EA/AGU
N/% C%
210
N%
0
1H NMRa
methodb
c
-d
Cellulose
0
1
0
-
c
-d
Azo-cellulose1
Azo-cellulose2
Azo-cellulose3
Azo-cellulose4
Azo-cellulose5
Azo-cellulose6
Azo-cellulose7
Azo-cellulose8
2.35 48.5
3.33 50.0
5.01 52.0
5.33 55.4
5.88 56.4
6.36 58.6
7.01 60.0
8.34 63.8
0.17
-
3
0.28
0.53
0.60
0.73
0.87
1.11
1.98
0.26
0.58
0.63
0.68
0.83
1.03
2.03
0.20
0.48
0.53
0.61
0.75
0.86
1.88
6
9
12
15
18
21
174 aCalculated from 1H-NMR spectra of Azo-cellulose2-7/DMSO-d6 and
175 Azo-cellulose8/CDCl3.
176 bMeasured by the calorimetric method on basis of the absorbance at 355 nm on the
177 UV-Vis spectra, EA was used as a model compound to get the calibration curve.
178 c,dNot measured due to insolubility in DMSO and DMAc, respectively.
179
180 3. Results and discussion
181 3.1. Synthesis of EA
182
In the reaction mixture of EA synthesis, ECP acted as both a reactant and a
183 solvent, so its content was much higher than HA, namely the mole ratio of ECP/HA
184 was 10. HA-OH was altered to HA-O-Na+ after reaction with NaOH, and the HA-O-
185 readily reacted with the epoxide group of ECP. The epoxy ring was closed with the
186 help of NaOH and therefore EA was yielded. The phase transferring agent TMBAC
187 was used to accelerate the reaction and improve the yield. The reaction procedure is
188 shown in Scheme 1b. The 1H-NMR, 13C-NMR and FTIR spectra of EA are displayed
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189 in Fig. 1. Compared to the 1H-NMR of HA, new resonance groups at 2.76-2.90, 3.37,
190 4.00-4.28 ppm appear for the protons of the epoxypropyl group. The resonance at 5.57
191 ppm for the hydroxyl proton is completely absent, suggesting that the hydroxyl group
192 is fully reacted with ECP to produce EA (Fig. 1a). 13C-NMR spectrum (Fig. 1b) shows
193 new resonances at 44.87, 50.20, 69.20 ppm for the carbons of epoxypropyl group. The
a
194 resonance for C shifts from 158.48 to 161.06 ppm after the hydroxyl group of HA
195 altered to epoxypropyoxy group of EA, while the others appear at almost the
196 corresponding positions to those of HA, confirming that EA is produced.
197
198
Fig. 1
199 3.2. Synthesis of Azo-cellulose
200
According to a procedure established by Kondo and coauthors for effectively
201 producing cellulose ethers in solvent LiCl/DMAc (Kondo et al., 1991), a small
202 amount of water was added to the cellulose/DMAc-LiCl solution, in order to allow
203 more NaOH dissolve in the solution. By doing so, the hydroxyl group on cellulose
204 (Cellu-OH) can be effectively activated to react with the epoxy group of EA to
205 produce Azo-cellulose, as shown in Scheme 1c. Due to the steric hindrance effect of
206 the azo side group, further reaction of EA with –OH group in the side chain is less
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207 likely and is not considered here. The DSazo value can be controlled by adjusting the
208 EA/AGU ratio, and a set of Azo-celluloses with DSazo value in a range of 0.17~1.98
209 (measured by elemental analysis) were synthesized as listed in Table 1.
1
210
Fig. 2 displays H-NMR (CDCl3), 13C-NMR (DMSO-d6, CDCl3), and FTIR
211 spectra of Azo-cellulose8. 1H-NMR (Fig. 2a): the three sharp peaks at δ = 6.90, 7.40,
212 7.81 ppm are assigned to the protons on the aromatic azobenzene rings, the broad
213 peak at δ = 3.2~4.8 ppm is for protons on the AGU ring, methylene and methylidyne
214 groups (Zhang et al., 2012). Through integration area, the DSazo value on the cellulose
215 AGU was calculated by the following equation and was listed in Table 1. The three
216 sets of DSazo values calculated from nitrogen content, proton NMR and colorimetric
217 method are close, suggesting all the three methods are effective in obtaining the DSazo
218 values.
9DSazo
1014DSazo
A6.90 A7.40 A
7.81
A6.90 A7.40 A7.81A
3.0~5.0

219
220 where A is the integrated peak area.
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221
222
223
224
225
226
227
228
229
230
231
Fig. 2
13C-NMR spectra (Fig. 2b) were obtained from two solvents, i.e. CDCl3 and
DMSO-d6. Compared to the 13C-NMR spectrum of EA (Fig. 1b), the resonances in
the range of 110-170 ppm are assigned to the carbons on azobenzene rings. Both
spectra from the DMSO-d6 and CDCl3 solvents display almost identical peak signals
in the 110-170 ppm range. However, the characteristic peaks in the range of 60-105
ppm for the carbons in the AGU of cellulose molecules are apparent in the spectrum
from solvent DMSO-d6 (Fox et al., 2011; Zhang et al., 2012), while only weak and
broad peak signals in the range of 60-80 ppm were present for C2,3,5,7 in the AGU
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232
233
234
235
236
237
238
239
240
241
242
243
244
of cellulose molecules, and C9 in the azobenzene moiety in the spectrum from
solvent CDCl3. This is probably due to the very strong peak at δ=77.4 ppm of CDCl3
overlaps and shields the resonances arising from the carbons in the AGU. Compared
to cellulose, FTIR spectrum (not shown here) of Azo-cellulose8 shows several
stronger and new absorption peaks at 2920 cm-1 (υas of -CH2- of epoxypropyl), 3060
cm-1 (υAr-H) and 1600 cm-1 (υC=C) for benzene ring, and 1500 cm-1 (υN=N). These
results prove that azobenzene moieties are successfully introduced onto cellulose
through etherification reaction. The Mw of the Azo-cellulose (4-8) which are soluble
in DMF was measured by GPC (Waters 1515). It was in a range of 2.7~2.8 x 104
g/mol, showing that there was slight reduction in molecular weight as compared to
3.1 x 104 g/mol of cellulose. The heat and NaOH may cause some thermal
degradation and hydrolysis in the 7 h course of etherification reaction.
245 3.3. Solubility of Azo-cellulose
246
Cellulose itself hardly dissolves in conventional solvents due to strong intra- and
247 inter- molecular hydrogen bonds (Song et al., 2008). After derivatization with
248 azobenzene side chains, its solubility obviously changes with DSazo value. All
249 Azo-celluloses could not dissolve in acetone without dependence on DSazo value.
250 Azo-cellulose1-3 with DSazo of 0.17-0.53 even cannot dissolve in highly polar solvent
251 like DMAc and DMF, whereas Azo-cellulose4-8 with DSazo value of 0.6-1.98 are
252 soluble in those aprotic solvents. Azo-cellulose8 is soluble in a less polar solvent
253 chloroform. Apparently their solubility highly depends on the DSazo value. For
254 Azo-cellulose with low DSazo value, the inter- and intra- hydrogen bonding
255 interactions of Azo-cellulose molecular chains are still very strong, resulting in
256 insolubility of Azo-cellulose1-3 in solvent DMAc etc. Such hydrogen bonding
257 interactions decrease with increasing DSazo value, because of the steric hindrance
258 caused by the large azobenzene side groups, leading to improved solubility, as
259 compared to the insolubility of cellulose in any common solvents. Good solubility can
260 benefit the processing of Azo-cellulose in solution.
261
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262 3.4. Thermal stability of Azo-cellulose
263
Fig. 3 shows the TG thermograms of Azo-celluloses and cellulose. Before
264 sharp weight loss, all samples including EA, cellulose and Azo-celluloses underwent
265 4-6% weight loss, which was dedicated to the evaporation of water absorbed on them.
266 The pyrolysis of EA started at 195oC, where its azo bond (N=N) was decomposed into
267 volatile N2, benzene and aniline (Barton et al., 1983). An additional slight weight loss
268 stage initiating at ca. 324oC was observed for EA, which was due to the pyrolysis of
269 epoxypropyl group. Cellulose was thermally degraded in the range of 320 - 400oC to
270 generate H2O, CO, CO2, CH4 and alkanols etc (Hiltunen et al., 2012). The residual
271 content of Azo-celluloses was 13-28% more than that of cellulose at 600oC, and it
272 increased with the DSazo value of EA group, i.e. it was 20.1, 24.9 and 34.0% for
273 Azo-cellulose2, 4, and 8, respectively. This is due to the flame-retardancy effect of
274 azo compounds (Shen et al., 2009), leading to the production of more char residues.
275
The initial thermal decomposition temperature for Azo-cellulose2, 4 and 8 is 225,
276 249, and 213oC, respectively. Thus the thermal stability of Azo-cellulose is 18-54oC
277 higher than that of EA. Two stages of weight loss are present for Azo-celluloses. The
278 first stage appears at ca. 280oC similar to that of EA. The second stage starting from
279 290 oC is due to the decomposition of cellulose main chains. Obviously the thermal
280 stability of Azo-celluloses is lower than that of cellulose. This is because not only the
281 low thermal decomposition temperature of azo groups, but the destruction of chain
282 regularity of cellulose chains after the introduction of EA side groups. The connection
283 between the thermal stability and crystallinity of cellulose has been reported by some
284 researchers. For instance, the regenerated cellulose fibers and the electrospun
285 cellulose nanofibers were less thermal stable than the raw cellulose (Quan et al., 2010;
286 Xiong et al., 2013). This was attributable to the lower crystallinity of regenerated
287 cellulose fibers as compared to that of the raw cellulose.
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288
289
Fig. 3
290 3.5. Photochromic property of HA, EA and Azo-cellulose in DMAc solutions
291 Similar to the UV-Vis spectra of HA and EA solutions, the typical
292 Azo-cellulose8/DMAc solution shows a strong peak at ca. 350 nm and a weak peak at
293 ca. 440 nm (Fig. 4a), corresponding to the respective π-π* and n-π* electron transition
294 of trans-azobenzene structure, further confirming the presence of azo moieties on
295 Azo-cellulose. It has been reported that, in the visible light, the trans- and cis-
296 photoisomers of azobenzene coexist but the former is much more stable than the latter,
297 thus the former is predominant in solution or in bulk material (Nicolas et al., 2006).
298
The UV-Vis spectra of a typical Azo-cellulose8/DMAc solution under irradiation
299 of 365 nm UV light are shown in Fig 5b. The intensity of the strong band at 350 nm
300 progressively decreases with irradiation time, while that of the weak band at 440 nm
301 increases concomitantly. The reason is that more and more trans- isomers transform to
302 cis- ones with the progress of UV light irradiation. In this trans-cis
303 photoisomerization process, the small increase at 440 nm is because both isomers
304 display n-π* transition at the same wavelength of ca. 440 nm (Hurduc et al., 2007).
305 The peak absorbance values vary very little since after irradiation for 10 min,
306 suggesting that the photostationary state is reached. However, the photostationary
307 state of our Azo-cellulose is different from that of some other azo-compounds. For
308 instance, a liquid crystalline dendrimer with azobenzene terminal groups displayed no
15
Page 15 of 25

309 absorbance peak at 350 nm at photostationary state (Liu et al., 2007). This is possibly
310 related to the backbone structure, i.e. rigid main chain structure of cellulose or
311 aromatic polymer may hamper the rotation of azobenzene moieties, whereas flexible
312 molecule structure impart little resistance to the transformation of azobenzene
313 moieties upon light irradiation (Hurduc et al., 2007; Jiang et al., 2010; Liu et al.,
314 2007). Under visible light irradiation, the cis- band at 440 nm decreases, while the
315 trans- band at 350 nm increases (Fig. 4c), indicating the occurrence of a reversible
316 cis-trans isomerization. It takes ca. 25 min to reach complete reversibility, which is ca.
317 15 min longer than that for attaining the photostable trans-cis transition. Such a
318 slower cis-trans reverse rate than trans-cis transition rate is a characteristic
319 phenomenon for azo-bearing polymer (Hurduc et al., 2007). It should be pointed out
320 the other Azo-cellulose/DMAc solutions show similar fully reversible trans-cis-trans
321 transitions under successive irradiation of UV and visible light.
16
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322
323
Fig. 4
324
Due to the peak absorbance values in the course of cis-trans transition changed
325 more steadily (Fig. 4c) than that of trans-cis transition (Fig. 4b), the kinetics of the
326 cis-trans transition of Azo-cellulose in DMAc was further analyzed as an example by
327 plotting the absorbance at 350 nm and 440 nm against irradiation time (Fig. 5). It is
328 found that the content (At-A0) of trans-isomer increases, while that of cis-isomer
329 decreases almost linearly with time under irradiation of visible light. Thus the
330 photoinduced cis-trans transition of Azo-cellulose follows the first order kinetics, as
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331 found for many other azo-polymers (Hurduc et al., 2007; Liu et al., 2007). The
332 photochromism rate constants, i.e. the slopes of the plots, are 2.6 x 10-4 and -2.56 x
333 10-5 s-1 for the π-π* and n-π* transition, respectively. The latter is one order of
334 magnitude less than the former, suggesting the n-π* transition is much less obvious
335 than the π-π* transition. The rate constant of our Azo-cellulose is of the same order as
336 that of cyanophenylazophenol cellulose (Yang et al., 2001). However, the
337 photoisomerization rate of Azo-cellulose is about 103 times less than that of
338 azo-polymer with a flexible aliphatic chain spacer (Liu et al., 2007), due to the steric
339 hindrance of the semi-rigid main chain of cellulose.
340
341
342
Fig. 5
Azobenzene group is a type of chromophore group, whose molecular structure
343 have two isomers, i.e. trans- and cis-, they can reversely change under irradiation of
344 light with suitable wavelength. Such a change leads to conformational switch between
345 a rod-like shape of trans-isomer and a skewed shape of cis-isomer. Since the
346 transition between the two isomers can be conveniently achieved by light irradiation,
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347 such azo-containing biopolymers with photochemically stimulated conformational
348 change can be potentially used to fabricate motor, actuator, and artificial muscle
349 (Yamada et al., 2008). Due to the good biocompatibility and mechanical property of
350 backbone cellulose, the photochromic azo-cellulose ether might show potential
351 applications in biomaterials.
352
353 4. Conclusions
354
We reported a fast and highly efficient introduction of photochromic azobenzene
355 moieties onto cellulose molecular chains through etherification reaction in
356 DMAc/LiCl solvent. The azo-cellulose ethers showed good solubility in most polar
357 organic solvents. The thermal stability of Azo-celluloses was improved as compared
358 to that of 2,3-epoxypropoxy-azobenzene (EA), but was ca. 30oC less than that of
359 cellulose. The Azo-cellulose/DMAc solutions displayed reversible trans-cis-trans
360 photoisomerization under successive irradiation of 365 nm UV light and visible light.
361 The cis-trans isomerization followed the first order transition. In the UV-Vis spectrum,
362 the peak intensity at 350 nm (for π-π* transition of trans-isomer) and 440 nm (for
363 n-π* transition of cis-isomer) concomitantly depresses or increases under light
364 irradiation.
365
366 Acknowledgements
367
This work is supported by the National Basic Research Program of China
368 (2010CB732203), the Natural Science Foundation of Fujian Province (2010J06017),
369 Research Fund for the Doctoral Program of Higher Education of China (No.
370 20123503110003), the National Natural Science Foundation of China (No. 50973019),
371 Program for New Century Excellent Talents in Fujian Province University (JA10059).
372
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466 aqueous solution. Cellulose, 19(1), 189-198.
467
468
469
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469 Figure Captions
470 Scheme 1 Synthetic route of (a) HA, (b) EA and (c) Azo-cellulose.
471 Fig. 1. (a) 1H-NMR and (b) 13C-NMR spectra of EA in CDCl3.
1
472 Fig. 2. (a) H-NMR and (b) 13C-NMR spectra of Azo-cellulose8 (DSazo=1.98) in
473 CDCl3 or DMSO-d6.
474 Fig. 3. TGA curves of EA, cellulose, and Azo-cellulose2, 4, 8 with DSazo=0.28, 0.60,
475 1.98, respectively.
476 Fig. 4. (a) UV-Vis spectra of HA, EA and Azo-cellulose8 (DSazo=1.98) in DMAc
477 solution; UV-Vis spectra of Azo-cellulose8/DMAc solutions after (b) 365 nm UV light
478 and (c) visible light irradiation.
479 Fig. 5. Plots of absorbance change against irradiation time for the
480 Azo-cellulose8(DSazo=1.98)/DMAc solution under visible light irradiation.
481 Absorbance change at (a) 340 nm and (b) 440 nm. A0 and At were peak absorbance
482 values at 0 min and irradiation time t.
483
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483
Graphic abstract
484
485
486
A cellulose ether derivative with azobenzene moiety displayed reversible
487 photochromic property under light irradiation with suitable wavelength. The
488 trans-cis-trans transition led to conformational change between rod-like shape and
489 skewed shape azo-cellulose photoisomers.
490
491
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491
492
Highlights
493 Cellulose ethers bearing azobenzene groups were prepared by homogeneous
494 etherification.
495 Azobenzene celluloses with DSazo≥0.53 showed good solubility in aprotic
496 solvents.
497 Azobenzene celluloses showed reversible trans-cis transition under light
498
irradiation.
499
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