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Investigation on substituent effect in novel azo-naphthol dyes containing polymethacrylates for nonlinear optical studies
ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 68 (2007) 1812–1820 www.elsevier.com/locate/jpcs
Investigation on substituent effect in novel azo-naphthol dyes containing polymethacrylates for nonlinear optical studies P. Delphia Shalini Rosalyna, S. Senthila, P. Kannana,, G. Vinithab, A. Ramalingamb a
Department of Chemistry, Anna University, Chennai 600 025, India Centre for Laser Technology, Department of Physics, Anna University, Chennai 600025, India
b
Received 4 January 2007; received in revised form 4 May 2007; accepted 4 May 2007
Abstract A novel structurally isomeric and free-radically polymerizable methacrylates bearing azo-naphthol group in the side chain spaced away from the backbone by a hexamethylene spacer and substituted in 4-position with electron-withdrawing and donating substituent were synthesized for NLO applications. These polymers were characterized by UV, IR, 1H-NMR and 13C-NMR spectroscopy. The photoisomerization properties of all the polymers were studied. The glass transition temperature and thermal stability of the polymers were investigated by DSC and TGA, respectively. The third-order nonlinear optical properties of the polymer film were measured by the Z-scan technique using Ar-ion laser and exhibits negative optical nonlinearity. The results revealed that these polymers possess potential applications in nonlinear optics. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Thin films
1. Introduction Dye lasers are the most versatile laser sources and have made significant contribution to science and technology. In order to overcome the problems associated with static or flowing liquid systems, solid matrices containing laser dyes have been developed. The nonlinear optical materials can be used with low intensity laser for applications such as phase conjugation, image processing and optical switching [1,2]. Large nonlinear optical susceptibility resulting from the nonlinear response of organic molecules has attracted much attention. Azobenzene derivatives have been widely investigated as promising systems for various applications such as switching elements for microelectronics, highdensity data storage and nonlinear optics [3–6]. There is a growing interest in azobenzene-containing polymers because of their potential utilities in holographic storage and other optical and photonic applications [7–9]. The influence of the polymer properties on the photochemical and thermal isomerization have been demonstrated by Corresponding author. Tel.: +91 44 22203155; fax: +91 44 22200660.
E-mail address: [email protected] (P. Kannan). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.05.009
kinetic studies on different azo polymers and the rate of isomerization of the azo groups attached covalently to the polymers depends on the structural properties of polymer matrix [10,11]. Most frequently used polymeric material is polymethylmethacrylate (PMMA) [12] due to its best optical transparency for pumping wavelength and resistance to laser damage. The low solubility of dye in PMMA is overcome by introducing modified additives. Azobenzene and its derivatives undergo trans-to-cis isomerization when subjected to UV irradiation [13]. When the azobenzene molecule is attached chemically to a polymer chain, photoinduced isomerization can result in conformational change being induced in the polymer chains. This can cause significant changes to physical properties such as the dipole moment, refractive index and solution viscosity. Many of these changes can be reversed by heat or visible irradiation [14]. Recently, much attention has been focused on materials that have nonlinear optical properties for potential optical switching applications in telecommunication technologies. Organic polymeric solids are widely recognized because of their exceptionally large nonlinear second-order optical properties, more so, the larger variety of asymmetric
ARTICLE IN PRESS P.D. Shalini Rosalyn et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1812–1820
crystal structures are available. Applications envisaged for organic crystals and polymers include amplifiers, frequency doublers [15], waveguides [16], photoinduced poling [17], photorefractivity [18], Q-switches (a method of building up power output in lasers) and filters. The present work deals with synthesis of azo-naphthol dye-based poly(alkyloxymethacrylate)s. The investigation includes characterization using IR, UV–vis, 1H, 13C-NMR spectroscopic techniques; thermal studies and nonlinear characterization using Z-scan technique of these polymeric materials have been discussed. 2. Experimental 2.1. Materials a-Naphthol, benzene, acetone, chloroform, dichloromethane, N,N0 -dimethyl formamide, ethanol, methanol and tetrahydrofuran were purified by the usual procedure and dried before use [19]. HBr 47% (SRL), 1,6-hexanediol (Merck), triethylamine (SD fine) were distilled before use. p-Hydroxybenzoic acid (SRL), aniline (Spectrochem), 4-aminobenzonitrile (Aldrich) and p-toludine (Merck) were used as received. Silica gel (60–120 mesh, SRL) was dried in oven at 110 1C for 1 h and cooled in a desiccator before use. 2,20 -azobis(isobutyronitrile) (AIBN) was used after recrystallization in chloroform–methanol (1:1). 6-Bromohexanol was prepared by using the procedure reported elsewhere [20]. 2.2. Measurements Infrared spectra were recorded on a Nicolet IR (Impact 440) spectrophotometer using KBr pellets. High-resolution 1 H-NMR and 13C-NMR spectra were recorded on a Brucker spectrometer (300 and 75.4 MHz, respectively). Deutrated DMSO was used as a solvent for 1H-NMR spectra, unless otherwise mentioned. The photoisomerization studies of the polymers were carried out by UV irradiation between 240 and 600 nm wavelengths. Trans–cis photochemical isomerization of polymers was investigated by irradiating with Spectroline (New York) low-pressure mercury pencil lamp in a spin-coated quartz plate for 1 min and the UV absorption measured immediately on a Shimadzu UV-260 spectrophotometer. The experiment was repeated until there was no change in absorbance. TGA and DSC studies were carried out on a Mettler Toledo STARc system under nitrogen atmosphere. The heating rate of TGA analysis was done at 20 1C min1 with a nitrogen flow of 20 mL min1. Nonlinear characterization is done using Z-scan technique developed by Mansoor et al. [21]. This technique is a simple but very accurate method to determine both nonlinear index of refraction n2 and nonlinear absorption coefficient b. Nonlinear index of refraction is proportional to the real part of the third-order susceptibility and the nonlinear absorption coefficient is proportional to the imaginary. The Z-scan experiments
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were performed using a 488 nm Ar-ion laser beam (Stabilite 2017) which was focused by 3.5 cm focal length lens. The laser beam waist oo at the focus is measured to be 15 mm and the Raleigh length to be 1.328 mm. 2.2.1. Synthesis of precursors The precursors such as 4-(6-hydroxyhexyloxy)benzoic acid and 4-(6-(methacryloyloxy)hexyloxy)benzoic acid (MHBA) were synthesized by the reported procedures [22]. 2.2.2. Synthesis of 4-(40 -X-phenylazo)-1-naphthol (XQH, CH3, CN)(Ia–Ic) A typical procedure adopted for the preparation of 4-(40 X-phenylazo)-1-naphthol is as follows. Substituted anilines were diazotized using sodium nitrite in the presence of hydrochloric acid followed by coupling with a-naphthol substrate to produce 4-(40 -X-phenylazo)-1-naphthol dye in good yield (80–86%). Its purity was examined by thin layer chromatography (TLC) and purified by column chromatography over silica gel using chloroform/methanol mixture. Ia: 4-(40 -phenylazo)-1-naphthol m.pt: 184–186 1C. IR (KBr) cm1: 1414 (–NQN–), 1316 (C–C), 3239 (–NH/–OH), 1618 (CQO), 1154 (Ar–NQ). 1 H-NMR (DMSO-d6) d: 11.25 (s, 1H, OH), 7.08 (d, 1H, ArH), 7.94 (d, 1H, ArH), 7.66–7.70 (m, 2H, ArH). 13CNMR (DMSO-d6) d: 136.80 (ArC–OH), 131.65 (ArC–H), 148.96 (ArC–NQ), 116.14 (ArC–C). Anal. calcd for C16H12N2O (248.38): C, 77.40; H, 4.87; N, 11.28. Found: C, 77.38; H, 4.85; N, 11.26. Ib: 4-(40 -methylphenylazo)-1-naphthol m.pt: 196–197 1C. IR (KBr) cm1: 1412 (–NQN–), 1315 (C–C), 3244 (–NH/–OH), 1618 (CQO), 1151 (Ar–NQ). 1 H-NMR (DMSO-d6) d: 11.22 (s, 1H, OH), 7.04 (d, 1H, ArH), 7.78 (d, 1H, ArH), 7.63–7.78 (m, 2H, ArH), 8.79 (d, 1H, ArH). 13C-NMR (DMSO-d6) d: 159.84 (ArC–OH), 24.30 (ArC–CH3), 143.95 (ArC–NQ), 114.05 (ArC–C). Anal. calcd for C17H14N2O (262.31): C, 77.84; H, 5.38; N, 10.68. Found: C, 77.79; H, 5.23; N, 10.54. Ic: 4-(40 -cyanophenylazo)-1-naphthol m.pt: 205–206 1C. IR (KBr) cm1: 1407 (–NQN–), 1310 (C–C), 3231 (–NH/–OH), 1618 (CQO), 1155 (Ar–NQ). 1 H-NMR (DMSO-d6) d: 11.3 (s, 1H, OH), 6.66 (d, 1H, ArH), 8.08 (d, 1H, ArH), 7.52–7.68 (m, 2H, ArH). 13CNMR (DMSO-d6) d: 159.64 (ArC–OH), 116.22 (ArC–CN), 143.81 (ArC–NQ), 116.22 (ArC–C). Anal. calcd for C17H11N3O (273.29): C, 74.71; H, 4.06; N, 15.38. Found: C, 74.69; H, 3.98; N, 15.12. 2.2.3. Synthesis of (4-X-phenyl){[4-[4-(6(methacryloyloxy) hexyloxy)benzoyloxy naphthyl]4,4diazene} (XQH, CH3, CN) (IIa–IIc) A solution of N,N-dicyclohexylcarbodiimide (3.71 g, 0.018 mol) and dichloromethane (50 mL) was added dropwise to a stirred solution of MHBA (5.61 g, 0.018 mol), 4-(40 -phenylazo)-1-naphthol (4.469, 0.018 mol) and 4-(dimethylamino)pyridine (2.19 g, 0.018 mol) in dichloromethane (50 mL). This reaction was monitored using
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TLC until completion and was stirred overnight at room temperature. The solution was then filtered and the solvent was removed under reduced pressure. The residue was then purified by using column chromatography on silica gel (chloroform and methanol) to yield the desired monomer. The remaining monomers were prepared in a similar manner using appropriate precursors with 70–76% yield (Scheme 1). IIa: {[4-(4-(6-methacryloyloxyhexyloxy)benzoyloxy)naphthyl]phenyldiazene} m.pt: 201–203 1C. IR (KBr) cm1: 1414 (–NQN–), 1729 (–CQO), 1654 (–CQC–), 1242 (Ar–O–C). 1H-NMR (DMSO-d6) d: 1.27 (s, 3H, CH3–C), 1.60 (s, 2H, CH2QC), 3.64–3.98 (t, 2H, Ar–O–CH2), 7.01–8.03 (m, 9H, ArH), 8.86 (d, 1H, ArH). 13C-NMR (DMSO-d6) d: 25.69 (CH3– C), 28.55 (CQCH2), 164.63 (ArC–O–C), 64.91 (–O–CH2), 144.38 (ArC–NQ). Anal. calcd for C33H33N2O5 (537.63):
C, 73.86; H, 6.01; N, 5.22. Found: C, 73.68; H, 6.05; N, 5.16. IIb:(4-methylphenyl){[4-[4-(6- (methacryloyloxy)hexyloxy) benzoyloxy]4,4 naphthyl]diazene} m.pt: 209–211 1C. IR (KBr) cm1: 1412 (–NQN–), 1735 (–CQO), 1615 (–CQC–), 1259 (Ar–O–C), 1212 (Ar– CH3). 1H-NMR (DMSO-d6) d: 1.32 (s, 3H, CH3–C), 2.16 (s, 2H, CH2QC), 3.59–3.98 (t, 2H, Ar–O–CH2), 7.19–7.91 (m, 9H, ArH), 3.86 (s, 3H, Ar–CH3). 13C-NMR (DMSOd6) d: 25.42 (CH3–C), 28.70 (CQCH2), 166.15 (ArC–O– C), 64.49 (–O–CH2), 145.53 (ArC–NQ). Anal. calcd for C34H35N2O5 (551.65): C, 74.16; H, 6.22; N, 5.09. Found: C, 74.09; H, 6.18; N, 5.02. IIc: (4-cyanophenyl){[4-[4-(6-(methacryloyloxy)hexyloxy) benzoyloxy]4,4naphthyl]diazene} m.pt: 214–216 1C. IR (KBr) cm1: 1417 (–NQN–), 1730 (–CQO), 1660 (–CQC–), 1260 (Ar–O–C), 2235 (Ar–CN).
a NH2
OH
N2Cl HCl
+NaNO2
HO NaOH
+
N
0°C
0°C
N X
X
X
OH
O
b HO
O
+
Br − (CH2)6-OH
Cl
COOH KOH/KI/EtOH
O
HO
THF/TEA
0°C OH
O
O O
O O
O N N
O
CH2Cl2
O
O
b
+ a
DCC/DMAP X
AIBN/THF 60°C
X
H
CH3
CN
Polymer
I
II
III
O CH3 C CH2 O C
O N N
n O
X
OX X=H, CH3,CN Scheme 1. Synthesis of monomers and polymers.
ARTICLE IN PRESS P.D. Shalini Rosalyn et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1812–1820 1
H-NMR (DMSO-d6) d: 1.19 (s, 3H, CH3–C), 1.37 (s, 2H, CH2QC), 3.53–3.73 (t, 2H, Ar–O–CH2), 7.02–7.88 (m, 9H, ArH), 8.76 (d, 1H, ArH). 13C-NMR (DMSO-d6) d: 25.66 (CH3–C), 28.53 (CQCH2), 167.51 (ArC–O–C), 64.44 (–O–CH2), 145.53 (ArC–NQ). Anal. calcd for C34H32N3O5 (562.63): C, 72.71; H, 5.56; N, 7.48. Found: C, 72.66; H, 5.51; N, 7.39.
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I (6 wt%) was dissolved in THF and centrifuged for about 30 min, the supernatant solution was decanted, filtered through a 0.2 mm polyethersulfone membrane filter and then spin coated on a pre-cleaned (washed with dilute chromic acid) glass slides and annealed at 50 1C in vacuum for 2 h. The thickness of the film obtained was found to be 0.53 mm. The remaining polymers II and III films were also prepared in a similar manner.
2.3. Polymerization 3. Results and discussion All the polymers (PI–PIII) were synthesized by a free radical solution polymerization method from the corresponding monomers using AIBN as an initiator in THF at 60 1C. The typical procedure for the synthesis of polymer I is as follows: (4-X-phenyl){[4-[4-(6-(methacryloyloxy)hexyloxy)benzoyloxy]4,4naphthyl]diazene} (XQH) (0.01 mol) and AIBN (0.001 mol) were dissolved in dry THF and a gentle steam of nitrogen was passed into the solution. The solution was kept in an oil bath at 60 1C for 24 h. Then the solution was cooled and poured into excess of methanol to precipitate the product. The crude polymer thus obtained was reprecipitated twice using chloroform and methanol. The purified polymer was dried at 40 1C under vacuum for 48 h (yield 75%). The remaining polymers (PII and PIII) were prepared by adopting the similar procedure using the respective monomers with a good yield of 70–73%. PI: poly{[4-(4-(6-methacryloyloxyhexyloxy)benzoyloxy) naphthyl]phenyldiazene} IR (KBr) cm1: 1509 (–NQN–), 1734 (ester CQO), 1152 (Ar–NQ), 1254 (Ar–O–C). 1H-NMR (DMSO-d6) d: 1.29 (s, 3H, CH3–C), 1.2–1.3 (s, 2H, C–CH2), 4.08 (t, 2H, Ar–O–CH2), 7.03–8.21 (m, ArH). 13C-NMR (DMSO-d6) d: 25.71 (CH3–C), 20.0 (CH2–C), 163.64 (ArC–O–C), 64.91 (–O–CH2), 153.07 (ArC–NQ). PII: poly[(4-methylphenyl){[4-[4-(6- (methacryloyloxy)hexyloxy)benzoyloxy]4,4naphthyl]diazene}] IR (KBr) cm1: 1509 (–NQN–), 1733 (ester CQO), 1153 (Ar–NQ), 1253 (Ar–O–C), 1210 (Ar–CH3). 1HNMR (DMSO-d6) d: 1.25 (s, 3H, CH3–C), 1.1–1.4 (s, 2H, C–CH2), 3.95 (t, 2H, Ar–O–CH2), 6.83–8.13 (m, ArH). 13 C-NMR (DMSO-d6) d: 25.84 (CH3–C), 20.60 (CH2–C), 164.21 (ArC–O–C), 64.21 (–O–CH2), 152.80 (ArC–NQ). PIII: poly[(4-cyanophenyl){[4-[4-(6-(methacryloyloxy)hexyloxy)benzoyloxy]4,4naphthyl]diazene}] IR (KBr) cm1: 1508 (–NQN–), 1732 (ester CQO), 1155 (Ar–NQ), 1259 (Ar–O–C), 2239 (Ar–CN). 1H-NMR (DMSO-d6) d: 1.29 (s, 3H, CH3–C), 1.2–1.3 (s, 2H, C– CH2), 3.66 (t, 2H, Ar–O–CH2), 6.78–8.02 (m, ArH). 13CNMR (DMSO-d6) d: 25.62 (CH3–C), 19.80 (CH2–C), 164.16 (ArC–O–C), 64.61 (–O–CH2), 151.40 (ArC–NQ). 2.4. Film preparation The typical procedure adopted for the preparation of film for optical property studies is as follows: The polymer
3.1. Synthesis and characterization of polymers The synthetic approach used to prepare the monomers and polymers containing azo-naphthol moieties are shown in Scheme 1. The starting material is 4-substituted aniline, in which the amino group was diazotized with sodium nitrite in the presence of three equivalent of hydrochloric acid. The diazonium salt was converted into p-phenylazoa-naphthol by coupling with a-naphthol. The flexible spacer was introduced by alkylation of the compound p-hydoxybenzoic acid with 6-bromohexanol and gave the corresponding 4-(6-hydroxyhexyloxy)benzoic acid. Esterification of the above hydroxylated compound with methacrylic acid yield the MHBA. This was again coupled with p-phenylazo-a- naphthol to yield the polymerizable monomer, (4-x-phenyl){[4-[4-(6-(methacryloyloxy)hexyloxy)benzoyloxy]4,4naphthyl]diazene}. This was polymerized by free radical solution polymerization with AIBN as an initiator in THF. The completion of polymerization was determined by the disappearance of the vinyl proton signals of the methacryloyloxy group at 1.92–1.96 ppm in the 1H-NMR spectrum, 28.8 ppm in the 13 C-NMR spectrum and absorption around 1635 cm1 in the IR spectrum. All the spectral values are in accordance with the structure. Fig. 1 shows the representative 1 H-NMR spectrum of the polymer II. 3.2. Thermal properties 3.2.1. Thermogravimetric analysis The TGA traces of all the polymers are shown in Fig. 2 and their data are given in Table 1. The thermal stability of the polymers was evaluated by 10% and 50% weight loss at minimum temperature. All the polymers were stable up to 230 1C and start to decompose around 263 1C under a ‘two-stage’ decomposition process. The first stage decomposition may be ascribed to the evolution of nitrogen gas by the cleavage of side chain azo group [23]. The final decomposition around 410– 480 11C, may be due to the pyrolytic cleavage of ester linkage of the aromatic backbone. The decomposition of the polymers was almost complete at 700 1C and no further weight loss was observed. Electron-donating and withdrawing nature of the substituents plays a vital role in thermal stability. The electron donating substituent increases the polarity nature of the molecule and leads
ARTICLE IN PRESS P.D. Shalini Rosalyn et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1812–1820
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26.183
7.598
Relative Intensity
1.000
15.325
to more stable and ordered effect. In the case of withdrawing substituent, it is less polar so that the conjugation is less effective and hence the packing efficiency of the molecule is weak. Accordingly, the char yield is reduced compared to the other polymers.
10
8
6
4
2
3.2.2. Differential scanning calorimetry DSC analysis was performed under nitrogen atmosphere at a heating rate of 20 1C min1; the results are summarized in Table 1 and the thermograms are shown in Fig. 3. All the polymers showed two endothermic peaks, one at lower temperature due to glass transition temperature (Tg) and another at high temperature for melting transition temperature (Tm). Polymer containing a strong electron withdrawing substituent (CN) showed higher Tg’s than the electrondonating substituted (CH3) polymers. This may be ascribed to the change in the dipole-moment introduced by substituents (XQH, CH3, CN), i.e. when a substituent was introduced at the terminal position of the molecule that separate the ring structures from one another, which depends on the nature of the substituent (X), the ring-H distance was increased if it was substituted by CH3 or CN. When the size of the substituent increased, the long molecular axis was separated by the amount by which the ring-x bond length with the van der Waals radius of X exceeded the ring-H bond length with the van der Waals radius of H, and the thermal stability and attraction of the molecules were altered according to the nature of substituent [24]. 3.3. Photochemical properties
0
PPM
The photoisomerization study was performed for all the polymer films on the spin-coated quartz plate. The
Fig. 1. 1H-NMR spectrum of polymer II.
I II III
Weight loss (%)
100
III
Exo
II
I
0 100
300
500
700
50
100
Temperature (°C)
150
200
250
Temperature (°C)
Fig. 2. TGA thermograms of polymers I–III.
Fig. 3. DSC thermograms of polymers I–III.
Table 1 TGA and DSC thermal data of the polymers I–III Polymer
TGA X
I II III
H CH3 CN
Total weight loss at 700 1C (%)
Char yield at 700 1C (%)
Weight loss corresponds to (1C) 10%
50%
355 325 230
440 432 430
70 79 78
30 22 22
DSC Tg (1C)
Tm (1C)
46 40 41
85 75 77
ARTICLE IN PRESS P.D. Shalini Rosalyn et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1812–1820
different interval of time. Based on the above observations, the rate of switching time for terminally substituted polymers are in the following order:
UV–visible absorption spectra of the polymers were shown in Fig. 4 and the photoisomerization of polymer II is shown in Fig. 5(a). The trans azobenzene shows an absorption around 375 nm, due to the pp* transition, and its transition moment lies along the molecular long axis of the azobenzene moiety. An increasing trend of cis isomer was also observed at lmax 438 nm due to np* transition of azobenzene moiety. After completion of irradiation with visible light (4350 nm), the absorbance was restored for azobenzene moiety owing to trans–cis photochemical back isomerization. The kinetics (rate) of photoisomerization process is shown in Fig. 5(b). It was carried out to determine the rate of switching time and the effect of terminal pendant substituents for the side chain azobenzene moieties. All the polymers were irradiated at 375 nm of ‘pencil lamp’ (Spectroline, New York) with
CH3 4CN4H This may be attributed that the terminal substituent depends on the size and the effect of withdrawing/donating nature of the resonance. The time taken for total cis transformation depends on the nature and size of the terminal substituent in the side chain azo moiety. Among all the polymers, the bulky terminal substituent (CH3) containing polymer took longer time for the completion of photoisomerization process than the smaller terminal substituents (CN). This reveals that a steric factor hindered two terminal carbons coming closer. If the substituents were small, the steric hindrance was accordingly less, hence the time taken was minimal for cis transformation [25].
2.5
3.4. Nonlinear optical properties The third-order nonlinear refraction index n2, and the nonlinear absorption coefficient b of polymer film were evaluated by the measurements of Z-scan. The experimental set up for Z-scan is shown in Fig. 6. Fig. 7 shows the Z-scan signatures of the polymers. The peak followed by a valley-normalized transmittance obtained from the closed aperture Z-scan data, indicates that the sign of the refraction nonlinearity is negative i.e. self-defocusing. Selfdefocusing effect is attributed to local variation of refractive index with temperature. Fig. 7(b) shows the typical Z-scan data for the open aperture(S ¼ 1) setup for the polymers. The enhanced transmission near the focus is indicative of the saturation of absorption at high intensity. Absorption saturation in the sample enhances the peak and decreases the valley in the closed aperture Z-scan thus distorting the symmetry of
1.5
1.0
I II III
0.5
0.0 350
400
450
500
550
600
Wavelength (nm) Fig. 4. UV–visible absorption spectra of polymers I–III.
1.1
0.7
1.0
0 sec 15 sec 45 sec 75 sec 105 sec 135 sec 165 sec 195 sec 255 sec 315 sec 375 sec 675 sec 975 sec 1275 sec
0.6 0.5 0.4 0.3 0.2
0.9 0.8 Absorbance
Absorbance
2.0
Absorbance
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0.7 0.6 0.5
I III II
0.4 0.3
0.1
0.2 0.1
0.0 250
300
350
400
450
Wavelength (nm)
500
550
600
0
200
400
600
800
1000
1200
Time (Sec)
Fig. 5. (a) UV-irradiation pattern of the polymer II. (b) The rate of photoisomerization of polymers in dependence of time.
1400
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the Z-scan curve about z ¼ 0. The defocusing effect was shown in Fig. 7(a) is attributed to a thermal nonlinearity resulting from absorption of radiation at 488 nm. Localized
absorption of a tightly focused beam propagating through an absorbing polymer medium produces a spatial distribution of temperature in the polymer film and consequently, a spatial variation of the refractive index, that acts as a thermal lens resulting in phase distortion of the propagating beam. |Dj0|, the on-axis phase shift at the focus is related to the difference in the peak and valley transmission, DTpv as: DT pv ¼ 0:406ð1 SÞ0:25 jDj0 j,
(1)
1exp(2r20/o20)
where S ¼ is the aperture linear transmittance with r0 denoting the aperture radius and o0 denoting the beam radius at the aperture in the linear regime. Then nonlinear refractive index is given by n2 ¼ Dj0 l2pI 0 Leff , Fig. 6. Schematic diagram of experimental arrangement for the Z-scan measurement.
where l is the laser wavelength, I0 is the intensity of the laser beam at focus z ¼ 0, Leff ¼ [1exp(aL)/a] is the 1.12
1.8
1.11 I II III
1.10 Normalized Transmittance
Normalized Transmittance
1.6 1.4 1.2 1.0 0.8 0.6 I II III
0.4 0.2
1.09 1.08 1.07 1.06 1.05 1.04 1.03 1.02 1.01 1.00
0.0 -15
(2)
-10
-5
0
5
10
0.99 -15
15
-10
-5
0
z (mm)
5
10
15
z (mm)
1.8
Normalized Transmittance
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
I II III
0.0 -15
-10
-5
0
5
10
15
z (mm) Fig. 7. Z-scan signatures of the polymer films I–III at I0 ¼ 4.38 KW/cm2 (a) closed aperture scan (S ¼ 0.035), (b) open aperture scan (S ¼ 1) and (c) the division of (a) by (b).
ARTICLE IN PRESS P.D. Shalini Rosalyn et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1812–1820 Table 2 Nonlinear optical parameters of the polymer films I–III Polymer X I II III
DTpv n2 105 cm2/W Dn
H 0.725 CH3 1.119 CN 1.116
6.08 9.37 9.35
b cm/W w(3) 10–3 esu
0.2645 1.37 0.4077 0.65 0.4067 0.564
3.061 4.7 4.698
effective thickness of the sample, a is the linear absorption coefficient and L is the thickness of the sample. Generally the measurements of the normalized transmittance versus sample position, for the cases of closed and open aperture, allow determination of the nonlinear refractive index, n2 and the saturation absorption coefficient, b. Here, since the closed aperture transmittance is affected by the nonlinear refraction and absorption, the determination of n2 is less straightforward from the closed aperture scans. It is necessary to separate the effect of nonlinear refraction from that of the nonlinear absorption. A method to obtain purely effective n2 is to divide the closed aperture transmittance by the corresponding open aperture scans. The ratio of open and closed aperture (Figs. 7(a) and (b)] scans are shown in Fig. 7(c). The data obtained in this way reflects purely the effects of nonlinear refraction. The nonlinear absorption coefficient b can be estimated from the open aperture Z-scan data: p b ¼ 2 2DT=I 0 Leff . (3) The real and imaginary parts of the third-order nonlinear optical susceptibility w(3) are defined as Rewð3Þ ðesuÞ ¼ 104 0 c2 n20 n2 =pðcm2 =WÞ,
(4)
Imwð3Þ ðesuÞ ¼ 102 0 c2 n20 Zb=4p2 ðcm=WÞ,
(5)
where e0 is the vacuum permittivity and c is the light velocity in vacuum. The values obtained by Z-scan are shown in Table 2. 4. Conclusion A novel free-radical polymerizable methacryloyloxy group containing azo-naphthyl esters were prepared and their photoisomerization properties were studied. All the spectral data are in accordance with the structure. The thermal properties of the polymers were determined by TGA and DSC. TGA data revealed that the polymers were stable up to 355 1C. All the polymers showed two stage decomposition, first one at lower temperature attributed to the cleavage of azo linkage and the second one at higher temperature ascribed to the pyrolytic cleavage of ester linkage. The DSC results showed that the electron withdrawing substituted polymer (III) showed higher Tg’s than the electron-donating substituted polymer (II). The UV irradiation studies indicated that the photoisomerization time depends on the dipole moment, size and
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donor–acceptor characteristics of the terminal substituents. The time taken for the completion of the photoisomerization for the methyl substituted polymer is much longer than the other polymers. The nonlinear refraction index coefficient, n2 and the nonlinear absorption coefficient, b and susceptibility, w(3) polymer for spin-coated polymer films were measured using the Z-scan technique with 488 nm Ar-ion laser, indicated that all the polymers exhibit negative nonlinear optical properties and the nonlinear absorption can be attributed to a saturation absorption process, while the nonlinear refraction leads to selfdefocusing in those polymers. It is worth noting that the value of nonlinear susceptibility for the cyano-substituted polymer (III) is larger than those of some representative third-order nonlinear optical materials such as organic polymers and organic metal [26,27]. All these experimental results show that these polymers are promising materials for application in nonlinear optical devices. Acknowledgments The authors thank Sophisticated Analytical Instrument Facility, Indian Institute of Technology, Chennai, India for spectral analysis. The authors G.V. and A.R. wish to thank the DAE-BRNS for their financial support. References [1] M.A. Kramer, W.R. Tompkin, R.W. Boyd, Phys. Rev. A 34 (1986) 2026. [2] F. Sagues, R. Albalat, R. Reigada, J. Crusats, J.I. Mullol, J. Claret, J. Am. Chem. Soc. 127 (2005) 5296. [3] S.H. Lee, S. Balasubramanian, D.Y. Kim, N.K. Viswanathan, S. Bian, J. Kumar, S.K. Tripathy, Macromolecules 33 (2000) 6534. [4] B.D. Jung, J. Stumpe, J.D. Hong, Thin Solid Films 441 (2003) 261. [5] H. Wang, Y. He, X. Tuo, X. Wang, Macromolecules 37 (2004) 135. [6] Y. Wu, J. Mamiya, T. Shiono, T. Ikeda, Q. Zhang, Macromolecules 32 (1999) 8829. [7] A. Natansohn, P. Rochon, Chem. Rev. 102 (2002) 1139. [8] L. Nikolova, L. Nedelchev, T. Todorov, T. Petrova, N. Tomova, V. Dragostinova, P.S. Ramanujam, S. Hvilsted, Appl. Phys. Lett. 77 (2000) 657. [9] Y. Zhao, S. Bal, Adv. Mater. 14 (2002) 512. [10] H. Rau, J.F. Rabek (Eds.), Photochemistry and Photophysics, vol. 11, CRC Press, Boca Raton, FL, 1990 Vol II, Ch. 4. [11] G. Somasundaram, A. Ramalingam, J. Lumin. 90 (2000) 1. [12] J.F. Rabek (Ed.), Mechanisms of Photophysical Processes and Photochemical Reactions in Polymers, Wiley, Chichester, 1987 p. 377. [13] P. Ravi, S.L. Sin, L.H. Gan, Y.Y. Gan, K.C. Tam, X.L. Xia, X. Hu, Polymer 46 (2005) 137. [14] M. Moniruzzaman, C.J. Sabey, G.F. Fernando, Macromolecules 37 (2004) 2572. [15] P.M. Balanchard, G.R. Mitchell, Appl. Phys. Lett. 63 (1993) 2038. [16] J. Zyss (Ed.), Molecular Nonlinear Optics, Materials, Physics and Devices, Academic Press, London, 1994. [17] P.N. Butcher, D. Cotter, The Elements of Nonlinear Optics, Cambridge University Press, Cambridge, UK, 1990. [18] I.C. Khoo, M.Y. Shih, A. Shisido, P.H. Chen, M.V. Wood, Opt. Mater. 18 (1) (2001) 85. [19] D.D. Perrin, W.L.F. Armarego, Progress in Polymer Science, third ed., Pergamon Press, New York, 1988, (2), 227.
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[20] S. Senthil, P. Kannan, J. Polym. Sci. A: Polym. Chem. 39 (2001) 2396. [21] M. Sheik-Bahae, A.A. Said, T.-H. Wei, D.J. Hagan, E.W. Van Stryland, IEEE J. Quant. Electron. QE 26 (4) (1990) 760. [22] R.P. Herr, F. Herzog, A. Schuster, Pat. Appl. WO 96 (1996) 10049. [23] K. Rameshbabu, P. Kannan, Liquid Cryst. 31 (6) (2004) 847. [24] S. Kumaresan, P. Kannan, J. Polym. Sci. A: Polym. Chem. 41 (2003) 3194.
[25] O. Exner, S. Bohm, M. Decouzon, J.-F. Gal, P.-C. Maria, J. Chem. Soc. Perkin Trans. 2 (2002) 168. [26] M.G. Kuzyk, C.W. Dirk (Eds.), Characterization Techniques and Tabulation for Organic Nonlinear Materials, Marcel Dekker Inc., New York, 1998. [27] H.S. Nalwa, S. Miyata (Eds.), Nonlinear Optics of Organic Molecules and Polymers, CRC Press, USA, 1997.
Journal of Physics and Chemistry of Solids 68 (2007) 1812–1820 www.elsevier.com/locate/jpcs
Investigation on substituent effect in novel azo-naphthol dyes containing polymethacrylates for nonlinear optical studies P. Delphia Shalini Rosalyna, S. Senthila, P. Kannana,, G. Vinithab, A. Ramalingamb a
Department of Chemistry, Anna University, Chennai 600 025, India Centre for Laser Technology, Department of Physics, Anna University, Chennai 600025, India
b
Received 4 January 2007; received in revised form 4 May 2007; accepted 4 May 2007
Abstract A novel structurally isomeric and free-radically polymerizable methacrylates bearing azo-naphthol group in the side chain spaced away from the backbone by a hexamethylene spacer and substituted in 4-position with electron-withdrawing and donating substituent were synthesized for NLO applications. These polymers were characterized by UV, IR, 1H-NMR and 13C-NMR spectroscopy. The photoisomerization properties of all the polymers were studied. The glass transition temperature and thermal stability of the polymers were investigated by DSC and TGA, respectively. The third-order nonlinear optical properties of the polymer film were measured by the Z-scan technique using Ar-ion laser and exhibits negative optical nonlinearity. The results revealed that these polymers possess potential applications in nonlinear optics. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Thin films
1. Introduction Dye lasers are the most versatile laser sources and have made significant contribution to science and technology. In order to overcome the problems associated with static or flowing liquid systems, solid matrices containing laser dyes have been developed. The nonlinear optical materials can be used with low intensity laser for applications such as phase conjugation, image processing and optical switching [1,2]. Large nonlinear optical susceptibility resulting from the nonlinear response of organic molecules has attracted much attention. Azobenzene derivatives have been widely investigated as promising systems for various applications such as switching elements for microelectronics, highdensity data storage and nonlinear optics [3–6]. There is a growing interest in azobenzene-containing polymers because of their potential utilities in holographic storage and other optical and photonic applications [7–9]. The influence of the polymer properties on the photochemical and thermal isomerization have been demonstrated by Corresponding author. Tel.: +91 44 22203155; fax: +91 44 22200660.
E-mail address: [email protected] (P. Kannan). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.05.009
kinetic studies on different azo polymers and the rate of isomerization of the azo groups attached covalently to the polymers depends on the structural properties of polymer matrix [10,11]. Most frequently used polymeric material is polymethylmethacrylate (PMMA) [12] due to its best optical transparency for pumping wavelength and resistance to laser damage. The low solubility of dye in PMMA is overcome by introducing modified additives. Azobenzene and its derivatives undergo trans-to-cis isomerization when subjected to UV irradiation [13]. When the azobenzene molecule is attached chemically to a polymer chain, photoinduced isomerization can result in conformational change being induced in the polymer chains. This can cause significant changes to physical properties such as the dipole moment, refractive index and solution viscosity. Many of these changes can be reversed by heat or visible irradiation [14]. Recently, much attention has been focused on materials that have nonlinear optical properties for potential optical switching applications in telecommunication technologies. Organic polymeric solids are widely recognized because of their exceptionally large nonlinear second-order optical properties, more so, the larger variety of asymmetric
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crystal structures are available. Applications envisaged for organic crystals and polymers include amplifiers, frequency doublers [15], waveguides [16], photoinduced poling [17], photorefractivity [18], Q-switches (a method of building up power output in lasers) and filters. The present work deals with synthesis of azo-naphthol dye-based poly(alkyloxymethacrylate)s. The investigation includes characterization using IR, UV–vis, 1H, 13C-NMR spectroscopic techniques; thermal studies and nonlinear characterization using Z-scan technique of these polymeric materials have been discussed. 2. Experimental 2.1. Materials a-Naphthol, benzene, acetone, chloroform, dichloromethane, N,N0 -dimethyl formamide, ethanol, methanol and tetrahydrofuran were purified by the usual procedure and dried before use [19]. HBr 47% (SRL), 1,6-hexanediol (Merck), triethylamine (SD fine) were distilled before use. p-Hydroxybenzoic acid (SRL), aniline (Spectrochem), 4-aminobenzonitrile (Aldrich) and p-toludine (Merck) were used as received. Silica gel (60–120 mesh, SRL) was dried in oven at 110 1C for 1 h and cooled in a desiccator before use. 2,20 -azobis(isobutyronitrile) (AIBN) was used after recrystallization in chloroform–methanol (1:1). 6-Bromohexanol was prepared by using the procedure reported elsewhere [20]. 2.2. Measurements Infrared spectra were recorded on a Nicolet IR (Impact 440) spectrophotometer using KBr pellets. High-resolution 1 H-NMR and 13C-NMR spectra were recorded on a Brucker spectrometer (300 and 75.4 MHz, respectively). Deutrated DMSO was used as a solvent for 1H-NMR spectra, unless otherwise mentioned. The photoisomerization studies of the polymers were carried out by UV irradiation between 240 and 600 nm wavelengths. Trans–cis photochemical isomerization of polymers was investigated by irradiating with Spectroline (New York) low-pressure mercury pencil lamp in a spin-coated quartz plate for 1 min and the UV absorption measured immediately on a Shimadzu UV-260 spectrophotometer. The experiment was repeated until there was no change in absorbance. TGA and DSC studies were carried out on a Mettler Toledo STARc system under nitrogen atmosphere. The heating rate of TGA analysis was done at 20 1C min1 with a nitrogen flow of 20 mL min1. Nonlinear characterization is done using Z-scan technique developed by Mansoor et al. [21]. This technique is a simple but very accurate method to determine both nonlinear index of refraction n2 and nonlinear absorption coefficient b. Nonlinear index of refraction is proportional to the real part of the third-order susceptibility and the nonlinear absorption coefficient is proportional to the imaginary. The Z-scan experiments
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were performed using a 488 nm Ar-ion laser beam (Stabilite 2017) which was focused by 3.5 cm focal length lens. The laser beam waist oo at the focus is measured to be 15 mm and the Raleigh length to be 1.328 mm. 2.2.1. Synthesis of precursors The precursors such as 4-(6-hydroxyhexyloxy)benzoic acid and 4-(6-(methacryloyloxy)hexyloxy)benzoic acid (MHBA) were synthesized by the reported procedures [22]. 2.2.2. Synthesis of 4-(40 -X-phenylazo)-1-naphthol (XQH, CH3, CN)(Ia–Ic) A typical procedure adopted for the preparation of 4-(40 X-phenylazo)-1-naphthol is as follows. Substituted anilines were diazotized using sodium nitrite in the presence of hydrochloric acid followed by coupling with a-naphthol substrate to produce 4-(40 -X-phenylazo)-1-naphthol dye in good yield (80–86%). Its purity was examined by thin layer chromatography (TLC) and purified by column chromatography over silica gel using chloroform/methanol mixture. Ia: 4-(40 -phenylazo)-1-naphthol m.pt: 184–186 1C. IR (KBr) cm1: 1414 (–NQN–), 1316 (C–C), 3239 (–NH/–OH), 1618 (CQO), 1154 (Ar–NQ). 1 H-NMR (DMSO-d6) d: 11.25 (s, 1H, OH), 7.08 (d, 1H, ArH), 7.94 (d, 1H, ArH), 7.66–7.70 (m, 2H, ArH). 13CNMR (DMSO-d6) d: 136.80 (ArC–OH), 131.65 (ArC–H), 148.96 (ArC–NQ), 116.14 (ArC–C). Anal. calcd for C16H12N2O (248.38): C, 77.40; H, 4.87; N, 11.28. Found: C, 77.38; H, 4.85; N, 11.26. Ib: 4-(40 -methylphenylazo)-1-naphthol m.pt: 196–197 1C. IR (KBr) cm1: 1412 (–NQN–), 1315 (C–C), 3244 (–NH/–OH), 1618 (CQO), 1151 (Ar–NQ). 1 H-NMR (DMSO-d6) d: 11.22 (s, 1H, OH), 7.04 (d, 1H, ArH), 7.78 (d, 1H, ArH), 7.63–7.78 (m, 2H, ArH), 8.79 (d, 1H, ArH). 13C-NMR (DMSO-d6) d: 159.84 (ArC–OH), 24.30 (ArC–CH3), 143.95 (ArC–NQ), 114.05 (ArC–C). Anal. calcd for C17H14N2O (262.31): C, 77.84; H, 5.38; N, 10.68. Found: C, 77.79; H, 5.23; N, 10.54. Ic: 4-(40 -cyanophenylazo)-1-naphthol m.pt: 205–206 1C. IR (KBr) cm1: 1407 (–NQN–), 1310 (C–C), 3231 (–NH/–OH), 1618 (CQO), 1155 (Ar–NQ). 1 H-NMR (DMSO-d6) d: 11.3 (s, 1H, OH), 6.66 (d, 1H, ArH), 8.08 (d, 1H, ArH), 7.52–7.68 (m, 2H, ArH). 13CNMR (DMSO-d6) d: 159.64 (ArC–OH), 116.22 (ArC–CN), 143.81 (ArC–NQ), 116.22 (ArC–C). Anal. calcd for C17H11N3O (273.29): C, 74.71; H, 4.06; N, 15.38. Found: C, 74.69; H, 3.98; N, 15.12. 2.2.3. Synthesis of (4-X-phenyl){[4-[4-(6(methacryloyloxy) hexyloxy)benzoyloxy naphthyl]4,4diazene} (XQH, CH3, CN) (IIa–IIc) A solution of N,N-dicyclohexylcarbodiimide (3.71 g, 0.018 mol) and dichloromethane (50 mL) was added dropwise to a stirred solution of MHBA (5.61 g, 0.018 mol), 4-(40 -phenylazo)-1-naphthol (4.469, 0.018 mol) and 4-(dimethylamino)pyridine (2.19 g, 0.018 mol) in dichloromethane (50 mL). This reaction was monitored using
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TLC until completion and was stirred overnight at room temperature. The solution was then filtered and the solvent was removed under reduced pressure. The residue was then purified by using column chromatography on silica gel (chloroform and methanol) to yield the desired monomer. The remaining monomers were prepared in a similar manner using appropriate precursors with 70–76% yield (Scheme 1). IIa: {[4-(4-(6-methacryloyloxyhexyloxy)benzoyloxy)naphthyl]phenyldiazene} m.pt: 201–203 1C. IR (KBr) cm1: 1414 (–NQN–), 1729 (–CQO), 1654 (–CQC–), 1242 (Ar–O–C). 1H-NMR (DMSO-d6) d: 1.27 (s, 3H, CH3–C), 1.60 (s, 2H, CH2QC), 3.64–3.98 (t, 2H, Ar–O–CH2), 7.01–8.03 (m, 9H, ArH), 8.86 (d, 1H, ArH). 13C-NMR (DMSO-d6) d: 25.69 (CH3– C), 28.55 (CQCH2), 164.63 (ArC–O–C), 64.91 (–O–CH2), 144.38 (ArC–NQ). Anal. calcd for C33H33N2O5 (537.63):
C, 73.86; H, 6.01; N, 5.22. Found: C, 73.68; H, 6.05; N, 5.16. IIb:(4-methylphenyl){[4-[4-(6- (methacryloyloxy)hexyloxy) benzoyloxy]4,4 naphthyl]diazene} m.pt: 209–211 1C. IR (KBr) cm1: 1412 (–NQN–), 1735 (–CQO), 1615 (–CQC–), 1259 (Ar–O–C), 1212 (Ar– CH3). 1H-NMR (DMSO-d6) d: 1.32 (s, 3H, CH3–C), 2.16 (s, 2H, CH2QC), 3.59–3.98 (t, 2H, Ar–O–CH2), 7.19–7.91 (m, 9H, ArH), 3.86 (s, 3H, Ar–CH3). 13C-NMR (DMSOd6) d: 25.42 (CH3–C), 28.70 (CQCH2), 166.15 (ArC–O– C), 64.49 (–O–CH2), 145.53 (ArC–NQ). Anal. calcd for C34H35N2O5 (551.65): C, 74.16; H, 6.22; N, 5.09. Found: C, 74.09; H, 6.18; N, 5.02. IIc: (4-cyanophenyl){[4-[4-(6-(methacryloyloxy)hexyloxy) benzoyloxy]4,4naphthyl]diazene} m.pt: 214–216 1C. IR (KBr) cm1: 1417 (–NQN–), 1730 (–CQO), 1660 (–CQC–), 1260 (Ar–O–C), 2235 (Ar–CN).
a NH2
OH
N2Cl HCl
+NaNO2
HO NaOH
+
N
0°C
0°C
N X
X
X
OH
O
b HO
O
+
Br − (CH2)6-OH
Cl
COOH KOH/KI/EtOH
O
HO
THF/TEA
0°C OH
O
O O
O O
O N N
O
CH2Cl2
O
O
b
+ a
DCC/DMAP X
AIBN/THF 60°C
X
H
CH3
CN
Polymer
I
II
III
O CH3 C CH2 O C
O N N
n O
X
OX X=H, CH3,CN Scheme 1. Synthesis of monomers and polymers.
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H-NMR (DMSO-d6) d: 1.19 (s, 3H, CH3–C), 1.37 (s, 2H, CH2QC), 3.53–3.73 (t, 2H, Ar–O–CH2), 7.02–7.88 (m, 9H, ArH), 8.76 (d, 1H, ArH). 13C-NMR (DMSO-d6) d: 25.66 (CH3–C), 28.53 (CQCH2), 167.51 (ArC–O–C), 64.44 (–O–CH2), 145.53 (ArC–NQ). Anal. calcd for C34H32N3O5 (562.63): C, 72.71; H, 5.56; N, 7.48. Found: C, 72.66; H, 5.51; N, 7.39.
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I (6 wt%) was dissolved in THF and centrifuged for about 30 min, the supernatant solution was decanted, filtered through a 0.2 mm polyethersulfone membrane filter and then spin coated on a pre-cleaned (washed with dilute chromic acid) glass slides and annealed at 50 1C in vacuum for 2 h. The thickness of the film obtained was found to be 0.53 mm. The remaining polymers II and III films were also prepared in a similar manner.
2.3. Polymerization 3. Results and discussion All the polymers (PI–PIII) were synthesized by a free radical solution polymerization method from the corresponding monomers using AIBN as an initiator in THF at 60 1C. The typical procedure for the synthesis of polymer I is as follows: (4-X-phenyl){[4-[4-(6-(methacryloyloxy)hexyloxy)benzoyloxy]4,4naphthyl]diazene} (XQH) (0.01 mol) and AIBN (0.001 mol) were dissolved in dry THF and a gentle steam of nitrogen was passed into the solution. The solution was kept in an oil bath at 60 1C for 24 h. Then the solution was cooled and poured into excess of methanol to precipitate the product. The crude polymer thus obtained was reprecipitated twice using chloroform and methanol. The purified polymer was dried at 40 1C under vacuum for 48 h (yield 75%). The remaining polymers (PII and PIII) were prepared by adopting the similar procedure using the respective monomers with a good yield of 70–73%. PI: poly{[4-(4-(6-methacryloyloxyhexyloxy)benzoyloxy) naphthyl]phenyldiazene} IR (KBr) cm1: 1509 (–NQN–), 1734 (ester CQO), 1152 (Ar–NQ), 1254 (Ar–O–C). 1H-NMR (DMSO-d6) d: 1.29 (s, 3H, CH3–C), 1.2–1.3 (s, 2H, C–CH2), 4.08 (t, 2H, Ar–O–CH2), 7.03–8.21 (m, ArH). 13C-NMR (DMSO-d6) d: 25.71 (CH3–C), 20.0 (CH2–C), 163.64 (ArC–O–C), 64.91 (–O–CH2), 153.07 (ArC–NQ). PII: poly[(4-methylphenyl){[4-[4-(6- (methacryloyloxy)hexyloxy)benzoyloxy]4,4naphthyl]diazene}] IR (KBr) cm1: 1509 (–NQN–), 1733 (ester CQO), 1153 (Ar–NQ), 1253 (Ar–O–C), 1210 (Ar–CH3). 1HNMR (DMSO-d6) d: 1.25 (s, 3H, CH3–C), 1.1–1.4 (s, 2H, C–CH2), 3.95 (t, 2H, Ar–O–CH2), 6.83–8.13 (m, ArH). 13 C-NMR (DMSO-d6) d: 25.84 (CH3–C), 20.60 (CH2–C), 164.21 (ArC–O–C), 64.21 (–O–CH2), 152.80 (ArC–NQ). PIII: poly[(4-cyanophenyl){[4-[4-(6-(methacryloyloxy)hexyloxy)benzoyloxy]4,4naphthyl]diazene}] IR (KBr) cm1: 1508 (–NQN–), 1732 (ester CQO), 1155 (Ar–NQ), 1259 (Ar–O–C), 2239 (Ar–CN). 1H-NMR (DMSO-d6) d: 1.29 (s, 3H, CH3–C), 1.2–1.3 (s, 2H, C– CH2), 3.66 (t, 2H, Ar–O–CH2), 6.78–8.02 (m, ArH). 13CNMR (DMSO-d6) d: 25.62 (CH3–C), 19.80 (CH2–C), 164.16 (ArC–O–C), 64.61 (–O–CH2), 151.40 (ArC–NQ). 2.4. Film preparation The typical procedure adopted for the preparation of film for optical property studies is as follows: The polymer
3.1. Synthesis and characterization of polymers The synthetic approach used to prepare the monomers and polymers containing azo-naphthol moieties are shown in Scheme 1. The starting material is 4-substituted aniline, in which the amino group was diazotized with sodium nitrite in the presence of three equivalent of hydrochloric acid. The diazonium salt was converted into p-phenylazoa-naphthol by coupling with a-naphthol. The flexible spacer was introduced by alkylation of the compound p-hydoxybenzoic acid with 6-bromohexanol and gave the corresponding 4-(6-hydroxyhexyloxy)benzoic acid. Esterification of the above hydroxylated compound with methacrylic acid yield the MHBA. This was again coupled with p-phenylazo-a- naphthol to yield the polymerizable monomer, (4-x-phenyl){[4-[4-(6-(methacryloyloxy)hexyloxy)benzoyloxy]4,4naphthyl]diazene}. This was polymerized by free radical solution polymerization with AIBN as an initiator in THF. The completion of polymerization was determined by the disappearance of the vinyl proton signals of the methacryloyloxy group at 1.92–1.96 ppm in the 1H-NMR spectrum, 28.8 ppm in the 13 C-NMR spectrum and absorption around 1635 cm1 in the IR spectrum. All the spectral values are in accordance with the structure. Fig. 1 shows the representative 1 H-NMR spectrum of the polymer II. 3.2. Thermal properties 3.2.1. Thermogravimetric analysis The TGA traces of all the polymers are shown in Fig. 2 and their data are given in Table 1. The thermal stability of the polymers was evaluated by 10% and 50% weight loss at minimum temperature. All the polymers were stable up to 230 1C and start to decompose around 263 1C under a ‘two-stage’ decomposition process. The first stage decomposition may be ascribed to the evolution of nitrogen gas by the cleavage of side chain azo group [23]. The final decomposition around 410– 480 11C, may be due to the pyrolytic cleavage of ester linkage of the aromatic backbone. The decomposition of the polymers was almost complete at 700 1C and no further weight loss was observed. Electron-donating and withdrawing nature of the substituents plays a vital role in thermal stability. The electron donating substituent increases the polarity nature of the molecule and leads
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26.183
7.598
Relative Intensity
1.000
15.325
to more stable and ordered effect. In the case of withdrawing substituent, it is less polar so that the conjugation is less effective and hence the packing efficiency of the molecule is weak. Accordingly, the char yield is reduced compared to the other polymers.
10
8
6
4
2
3.2.2. Differential scanning calorimetry DSC analysis was performed under nitrogen atmosphere at a heating rate of 20 1C min1; the results are summarized in Table 1 and the thermograms are shown in Fig. 3. All the polymers showed two endothermic peaks, one at lower temperature due to glass transition temperature (Tg) and another at high temperature for melting transition temperature (Tm). Polymer containing a strong electron withdrawing substituent (CN) showed higher Tg’s than the electrondonating substituted (CH3) polymers. This may be ascribed to the change in the dipole-moment introduced by substituents (XQH, CH3, CN), i.e. when a substituent was introduced at the terminal position of the molecule that separate the ring structures from one another, which depends on the nature of the substituent (X), the ring-H distance was increased if it was substituted by CH3 or CN. When the size of the substituent increased, the long molecular axis was separated by the amount by which the ring-x bond length with the van der Waals radius of X exceeded the ring-H bond length with the van der Waals radius of H, and the thermal stability and attraction of the molecules were altered according to the nature of substituent [24]. 3.3. Photochemical properties
0
PPM
The photoisomerization study was performed for all the polymer films on the spin-coated quartz plate. The
Fig. 1. 1H-NMR spectrum of polymer II.
I II III
Weight loss (%)
100
III
Exo
II
I
0 100
300
500
700
50
100
Temperature (°C)
150
200
250
Temperature (°C)
Fig. 2. TGA thermograms of polymers I–III.
Fig. 3. DSC thermograms of polymers I–III.
Table 1 TGA and DSC thermal data of the polymers I–III Polymer
TGA X
I II III
H CH3 CN
Total weight loss at 700 1C (%)
Char yield at 700 1C (%)
Weight loss corresponds to (1C) 10%
50%
355 325 230
440 432 430
70 79 78
30 22 22
DSC Tg (1C)
Tm (1C)
46 40 41
85 75 77
ARTICLE IN PRESS P.D. Shalini Rosalyn et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1812–1820
different interval of time. Based on the above observations, the rate of switching time for terminally substituted polymers are in the following order:
UV–visible absorption spectra of the polymers were shown in Fig. 4 and the photoisomerization of polymer II is shown in Fig. 5(a). The trans azobenzene shows an absorption around 375 nm, due to the pp* transition, and its transition moment lies along the molecular long axis of the azobenzene moiety. An increasing trend of cis isomer was also observed at lmax 438 nm due to np* transition of azobenzene moiety. After completion of irradiation with visible light (4350 nm), the absorbance was restored for azobenzene moiety owing to trans–cis photochemical back isomerization. The kinetics (rate) of photoisomerization process is shown in Fig. 5(b). It was carried out to determine the rate of switching time and the effect of terminal pendant substituents for the side chain azobenzene moieties. All the polymers were irradiated at 375 nm of ‘pencil lamp’ (Spectroline, New York) with
CH3 4CN4H This may be attributed that the terminal substituent depends on the size and the effect of withdrawing/donating nature of the resonance. The time taken for total cis transformation depends on the nature and size of the terminal substituent in the side chain azo moiety. Among all the polymers, the bulky terminal substituent (CH3) containing polymer took longer time for the completion of photoisomerization process than the smaller terminal substituents (CN). This reveals that a steric factor hindered two terminal carbons coming closer. If the substituents were small, the steric hindrance was accordingly less, hence the time taken was minimal for cis transformation [25].
2.5
3.4. Nonlinear optical properties The third-order nonlinear refraction index n2, and the nonlinear absorption coefficient b of polymer film were evaluated by the measurements of Z-scan. The experimental set up for Z-scan is shown in Fig. 6. Fig. 7 shows the Z-scan signatures of the polymers. The peak followed by a valley-normalized transmittance obtained from the closed aperture Z-scan data, indicates that the sign of the refraction nonlinearity is negative i.e. self-defocusing. Selfdefocusing effect is attributed to local variation of refractive index with temperature. Fig. 7(b) shows the typical Z-scan data for the open aperture(S ¼ 1) setup for the polymers. The enhanced transmission near the focus is indicative of the saturation of absorption at high intensity. Absorption saturation in the sample enhances the peak and decreases the valley in the closed aperture Z-scan thus distorting the symmetry of
1.5
1.0
I II III
0.5
0.0 350
400
450
500
550
600
Wavelength (nm) Fig. 4. UV–visible absorption spectra of polymers I–III.
1.1
0.7
1.0
0 sec 15 sec 45 sec 75 sec 105 sec 135 sec 165 sec 195 sec 255 sec 315 sec 375 sec 675 sec 975 sec 1275 sec
0.6 0.5 0.4 0.3 0.2
0.9 0.8 Absorbance
Absorbance
2.0
Absorbance
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0.7 0.6 0.5
I III II
0.4 0.3
0.1
0.2 0.1
0.0 250
300
350
400
450
Wavelength (nm)
500
550
600
0
200
400
600
800
1000
1200
Time (Sec)
Fig. 5. (a) UV-irradiation pattern of the polymer II. (b) The rate of photoisomerization of polymers in dependence of time.
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ARTICLE IN PRESS P.D. Shalini Rosalyn et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1812–1820
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the Z-scan curve about z ¼ 0. The defocusing effect was shown in Fig. 7(a) is attributed to a thermal nonlinearity resulting from absorption of radiation at 488 nm. Localized
absorption of a tightly focused beam propagating through an absorbing polymer medium produces a spatial distribution of temperature in the polymer film and consequently, a spatial variation of the refractive index, that acts as a thermal lens resulting in phase distortion of the propagating beam. |Dj0|, the on-axis phase shift at the focus is related to the difference in the peak and valley transmission, DTpv as: DT pv ¼ 0:406ð1 SÞ0:25 jDj0 j,
(1)
1exp(2r20/o20)
where S ¼ is the aperture linear transmittance with r0 denoting the aperture radius and o0 denoting the beam radius at the aperture in the linear regime. Then nonlinear refractive index is given by n2 ¼ Dj0 l2pI 0 Leff , Fig. 6. Schematic diagram of experimental arrangement for the Z-scan measurement.
where l is the laser wavelength, I0 is the intensity of the laser beam at focus z ¼ 0, Leff ¼ [1exp(aL)/a] is the 1.12
1.8
1.11 I II III
1.10 Normalized Transmittance
Normalized Transmittance
1.6 1.4 1.2 1.0 0.8 0.6 I II III
0.4 0.2
1.09 1.08 1.07 1.06 1.05 1.04 1.03 1.02 1.01 1.00
0.0 -15
(2)
-10
-5
0
5
10
0.99 -15
15
-10
-5
0
z (mm)
5
10
15
z (mm)
1.8
Normalized Transmittance
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
I II III
0.0 -15
-10
-5
0
5
10
15
z (mm) Fig. 7. Z-scan signatures of the polymer films I–III at I0 ¼ 4.38 KW/cm2 (a) closed aperture scan (S ¼ 0.035), (b) open aperture scan (S ¼ 1) and (c) the division of (a) by (b).
ARTICLE IN PRESS P.D. Shalini Rosalyn et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1812–1820 Table 2 Nonlinear optical parameters of the polymer films I–III Polymer X I II III
DTpv n2 105 cm2/W Dn
H 0.725 CH3 1.119 CN 1.116
6.08 9.37 9.35
b cm/W w(3) 10–3 esu
0.2645 1.37 0.4077 0.65 0.4067 0.564
3.061 4.7 4.698
effective thickness of the sample, a is the linear absorption coefficient and L is the thickness of the sample. Generally the measurements of the normalized transmittance versus sample position, for the cases of closed and open aperture, allow determination of the nonlinear refractive index, n2 and the saturation absorption coefficient, b. Here, since the closed aperture transmittance is affected by the nonlinear refraction and absorption, the determination of n2 is less straightforward from the closed aperture scans. It is necessary to separate the effect of nonlinear refraction from that of the nonlinear absorption. A method to obtain purely effective n2 is to divide the closed aperture transmittance by the corresponding open aperture scans. The ratio of open and closed aperture (Figs. 7(a) and (b)] scans are shown in Fig. 7(c). The data obtained in this way reflects purely the effects of nonlinear refraction. The nonlinear absorption coefficient b can be estimated from the open aperture Z-scan data: p b ¼ 2 2DT=I 0 Leff . (3) The real and imaginary parts of the third-order nonlinear optical susceptibility w(3) are defined as Rewð3Þ ðesuÞ ¼ 104 0 c2 n20 n2 =pðcm2 =WÞ,
(4)
Imwð3Þ ðesuÞ ¼ 102 0 c2 n20 Zb=4p2 ðcm=WÞ,
(5)
where e0 is the vacuum permittivity and c is the light velocity in vacuum. The values obtained by Z-scan are shown in Table 2. 4. Conclusion A novel free-radical polymerizable methacryloyloxy group containing azo-naphthyl esters were prepared and their photoisomerization properties were studied. All the spectral data are in accordance with the structure. The thermal properties of the polymers were determined by TGA and DSC. TGA data revealed that the polymers were stable up to 355 1C. All the polymers showed two stage decomposition, first one at lower temperature attributed to the cleavage of azo linkage and the second one at higher temperature ascribed to the pyrolytic cleavage of ester linkage. The DSC results showed that the electron withdrawing substituted polymer (III) showed higher Tg’s than the electron-donating substituted polymer (II). The UV irradiation studies indicated that the photoisomerization time depends on the dipole moment, size and
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donor–acceptor characteristics of the terminal substituents. The time taken for the completion of the photoisomerization for the methyl substituted polymer is much longer than the other polymers. The nonlinear refraction index coefficient, n2 and the nonlinear absorption coefficient, b and susceptibility, w(3) polymer for spin-coated polymer films were measured using the Z-scan technique with 488 nm Ar-ion laser, indicated that all the polymers exhibit negative nonlinear optical properties and the nonlinear absorption can be attributed to a saturation absorption process, while the nonlinear refraction leads to selfdefocusing in those polymers. It is worth noting that the value of nonlinear susceptibility for the cyano-substituted polymer (III) is larger than those of some representative third-order nonlinear optical materials such as organic polymers and organic metal [26,27]. All these experimental results show that these polymers are promising materials for application in nonlinear optical devices. Acknowledgments The authors thank Sophisticated Analytical Instrument Facility, Indian Institute of Technology, Chennai, India for spectral analysis. The authors G.V. and A.R. wish to thank the DAE-BRNS for their financial support. References [1] M.A. Kramer, W.R. Tompkin, R.W. Boyd, Phys. Rev. A 34 (1986) 2026. [2] F. Sagues, R. Albalat, R. Reigada, J. Crusats, J.I. Mullol, J. Claret, J. Am. Chem. Soc. 127 (2005) 5296. [3] S.H. Lee, S. Balasubramanian, D.Y. Kim, N.K. Viswanathan, S. Bian, J. Kumar, S.K. Tripathy, Macromolecules 33 (2000) 6534. [4] B.D. Jung, J. Stumpe, J.D. Hong, Thin Solid Films 441 (2003) 261. [5] H. Wang, Y. He, X. Tuo, X. Wang, Macromolecules 37 (2004) 135. [6] Y. Wu, J. Mamiya, T. Shiono, T. Ikeda, Q. Zhang, Macromolecules 32 (1999) 8829. [7] A. Natansohn, P. Rochon, Chem. Rev. 102 (2002) 1139. [8] L. Nikolova, L. Nedelchev, T. Todorov, T. Petrova, N. Tomova, V. Dragostinova, P.S. Ramanujam, S. Hvilsted, Appl. Phys. Lett. 77 (2000) 657. [9] Y. Zhao, S. Bal, Adv. Mater. 14 (2002) 512. [10] H. Rau, J.F. Rabek (Eds.), Photochemistry and Photophysics, vol. 11, CRC Press, Boca Raton, FL, 1990 Vol II, Ch. 4. [11] G. Somasundaram, A. Ramalingam, J. Lumin. 90 (2000) 1. [12] J.F. Rabek (Ed.), Mechanisms of Photophysical Processes and Photochemical Reactions in Polymers, Wiley, Chichester, 1987 p. 377. [13] P. Ravi, S.L. Sin, L.H. Gan, Y.Y. Gan, K.C. Tam, X.L. Xia, X. Hu, Polymer 46 (2005) 137. [14] M. Moniruzzaman, C.J. Sabey, G.F. Fernando, Macromolecules 37 (2004) 2572. [15] P.M. Balanchard, G.R. Mitchell, Appl. Phys. Lett. 63 (1993) 2038. [16] J. Zyss (Ed.), Molecular Nonlinear Optics, Materials, Physics and Devices, Academic Press, London, 1994. [17] P.N. Butcher, D. Cotter, The Elements of Nonlinear Optics, Cambridge University Press, Cambridge, UK, 1990. [18] I.C. Khoo, M.Y. Shih, A. Shisido, P.H. Chen, M.V. Wood, Opt. Mater. 18 (1) (2001) 85. [19] D.D. Perrin, W.L.F. Armarego, Progress in Polymer Science, third ed., Pergamon Press, New York, 1988, (2), 227.
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