J. Phys. Chem. B 2009, 113, 14943–14949
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Two Types of Nonlinear Optical Polyurethanes Containing the Same Isolation Groups: Syntheses, Optical Properties, and Influence of Binding Mode Zhong’an Li,† Wenbo Wu,† Cheng Ye,‡ Jingui Qin,† and Zhen Li*,† Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials, Department of Chemistry, Wuhan UniVersity, Wuhan 430072, China, and Organic Solids Laboratories, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China ReceiVed: July 27, 2009; ReVised Manuscript ReceiVed: September 24, 2009
In this paper, two new series of main-chain and side-chain second-order nonlinear optical (NLO) polyurethanes have been successfully prepared, in which isolation groups with different sizes were introduced to adjust the NLO property of the resultant polymers, according to the concept of “suitable isolation groups”. The second harmonic generation (SHG) experiments demonstrated that all the polymers exhibited large second harmonic generation (SHG) coefficients of d33 values (up to 105.6 pm/V) with excellent thermal stability and filmforming ability, indicating that the introduction of isolation groups could alleviate the “nonlinearity-stability trade off” efficiently. In addition, for both main-chain and side-chain polymers, BOP acted as the suitable isolation group. It was also found that the positive influence of the linked suitable isolation spacer was more obvious in the main-chain polymers, in comparison with those in the side-chain polymers. Introduction In the past decades, second-order nonlinear optical (NLO) polymeric materials have interested many researchers due to their potential applications in electro-optic devices such as telecommunications, optical data storage, and optical information processing.1-3 For the practical applications, the NLO materials should meet mainly three requirements: large macroscopic optical nonlinearity, high temporal stabilities, and good optical transparency. Thanks to the great efforts of scientists, various types of NLO polymers were prepared to realize these requirements, among which main-chain polymeric systems have received more attention because of their improved orientational stability and high chromophore density compared to guest-host or side-chain systems.4-6 Most types of the reported main-chain polymers were head-to-tail so that all the NLO chromophores were incorporated into the polymeric backbone without any flexible spacers, making the main-chain like a rigid rod.4 Therefore, the sub-Tg relaxation of the poled order was expected to be significantly inhibited, and the temporal stability of the SHG coefficient could be enhanced. However, due to this special rigid rodlike structure, the chromophore dipole moments in the main-chain polymeric system usually pointed in the same direction along the backbone, and large segmental motions of the polymer backbone should be required for poling. Thus, during the poling process, the effective oriented alignment of the dipole moments was very difficult to be realized, resulting in the relatively low poling efficiency.4-7 Different approaches were designed to optimize the structure, and many new types of main-chain polymers have been developed, such as accordion,8 randomly oriented,9 shoulder-shoulder,5a lambdashaped,7b,c and so on. However, most of their synthetic processes were dainty for some special chromophores, and the improved effect was limited in some degree. * Corresponding author. Phone: 86-27-62254108. Fax: 86-27-68756757. E-mail:
[email protected]. † Wuhan University. ‡ The Chinese Academy of Sciences.
Recently, Dalton and Jen et al. reported that the macroscopic nonlinearity of the polymeric materials (including dendrimers) could be even enhanced by controlling the shape of the chromophore according to the site isolation principle.10-13 The introduction of isolation spacers could help to decrease the strong intermolecular dipole interactions between the chromophore groups, leading to the enhancement of poling efficiency. On the basis of their excellent work, since 2006, we first prepared some new side-chain NLO polymers in which different sizes of isolation groups were linked to the chromophore moieties, and the results demonstrated that the macroscopic nonlinearity of NLO polymers could be boosted much higher by bonding “suitable isolation groups” to the NLO chormophore moieties.14,15 Considering the low poling efficiency of main-chain polymers, we wondered whether this problem could be solved by applying the concept of “suitable isolation groups”. From this standpoint, we synthesized a series of mainchain NLO polymers with different sizes of isolation groups introduced.16 The obtained results confirmed our original idea: the poling efficiency could be improved by the introduction of suitable isolation groups. In addition, the most encouraging results were that along with the enhancement of d33 value the orientation stabilities of polymers were also improved.16a To develop more main-chain polymers with encouraging performance, and clearly study the relationship between the structure and properties, in this paper we reported a series of new main-chain NLO polymers P1-P4 (Scheme 1) with different isolation groups introduced. And with the aim to investigate the difference of the effect of the isolation groups, we also synthesized another series of side-chain polymers P5-P8 (Scheme 2) for comparison, containing the same isolation groups corresponding to those of P1-P4. All the obtained polymers were well characterized, and they were easily soluble in common solvents and exhibited good thermal stability and film-forming ability. The NLO results demonstrated that 4-(benzyloxy)phenyl moieties (BOP), in both of the two series of polymers, acted as suitable isolation groups, further confirming the concept of “suitable isolation groups”. Moreover, we
10.1021/jp907135f CCC: $40.75 2009 American Chemical Society Published on Web 10/15/2009
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Li et al.
SCHEME 1
SCHEME 2
found that the same isolation group brought a different effect in different type of polymers; for example, the enhanced degree of d33 values of main-chain polymers was higher than that of side-chain polymers, and the best poling temperature was different. P2 exhibited a very high d33 value (up to 105.6 pm/ V), and to the best of our knowledge, for main-chain NLO polymers this value is the highest one reported so far. Herein, we would like to report the synthesis, characterization, and NLO properties of these polymers. Experimental Section Materials and Instrumentation. N,N-Dimethylformamide (DMF) was dried over and distilled from CaH2 under an atmosphere of dry nitrogen. 2,4-Toluenediisocyanate (TDI) was purified by distillation under reduced pressure before use. 1 H NMR spectra were measured on a Varian Mercury 300 spectrometer using tetramethylsilane (TMS; δ ) 0 ppm) as the internal standard. The Fourier transform infrared (FTIR) spectra were recorded on a PerkinElmer-2 spectrometer in the region of 3000-400 cm-1 on NaCl pellets. UV-visible spectra were obtained using a Schimadzu UV-2550 spectrometer. Gel permeation chromatography (GPC) was used to determine the molecular weights of polymers. GPC analysis was performed on a Waters HPLC system equipped with a 2690D separation module and a 2410 refractive index detector. Polystyrene standards were used as calibration standards for GPC. DMF was used as an eluent, and the flow rate was 1.0 mL/min. Thermal analysis was performed on a NETZSCH STA449C thermal analyzer at a heating rate of 10 °C/min in nitrogen at a flow rate of 50 cm3/min for thermogravimetric analysis (TGA). The thermal transitions of the polymers were investigated using a METTLER differential scanning calorimeter DSC822e under
nitrogen at a scanning rate of 10 °C/min. The thickness of the films was measured with an Ambios Technology XP-2 profilometer. General Procedure for the Synthesis of Polyurethanes P1-P8. Chromophores (1-8) and 2,4-toluenediisocyanate (TDI) with equivalent molar ratios were reacted in appropriate anhydrous DMF solution at 80 °C for 30-36 h in an atmosphere of dry nitrogen. After the solution was cooled to ambient temperature, it was dropped into methanol to remove monomers. The polymer was filtered and dried in a vacuum desiccator. P1. 1 (122.0 mg, 0.247 mmol), TDI (44.5 mg, 0.246 mmol). P1 was obtained as deeply red powder (130.0 mg, 78.1%). Mw ) 32 000, Mw/Mn ) 1.12 (GPC, polystyrene calibration). IR (thin film), υ (cm-1): 1728 (CdO), 1518, 1332 (-NO2). 1H NMR (CDCl3) δ (ppm): 1.0-1.2 (-CH3), 1.2-1.4 (-CH3), 2.0-2.2 (-CH3), 3.4-3.8 (-NCH2-), 3.9-4.2 (-OCH2-), 4.2-4.4 (-OCH2-), 4.4-4.6 (-OCH2-), 6.6-7.0 (ArH), 7.0-7.6 (ArH), 7.7-8.0 (ArH), 8.8-9.2 (-NH-), 9.6-9.9 (-NH-). P2. 2 (129.2 mg, 0.232 mmol), TDI (42.0 mg, 0.232 mmol). P2 was obtained as deeply red powder (151.0 mg, 88.2%). Mw ) 36 300, Mw/Mn ) 1.26 (GPC, polystyrene calibration). IR (thin film), υ (cm-1): 1728 (CdO), 1521, 1336 (-NO2). 1H NMR (CDCl3) δ (ppm): 1.0-1.3 (-CH3), 1.9-2.2 (-CH3), 3.5-3.9 (-NCH2-), 4.1-4.4 (-OCH2-), 4.4-4.6 (-OCH2-), 5.0-5.2 (-OCH2-), 6.6-6.8 (ArH), 6.9-7.6 (ArH), 7.6-8.0 (ArH), 8.8-9.2 (-NH-), 9.6-9.9 (-NH-). P3. 3 (110.0 mg, 0.165 mmol), TDI (30.0 mg, 0.165 mmol). P3 was obtained as deeply red powder (90.0 mg, 64.3%). Mw ) 24 200, Mw/Mn ) 1.10 (GPC, polystyrene calibration). IR (thin film), υ (cm-1): 1727 (CdO), 1518, 1336 (-NO2). 1H NMR (CDCl3) δ (ppm): 1.0-1.3 (-CH3), 1.8-2.2 (-CH2CH2-
Nonlinear Optical Polyurethanes
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TABLE 1: Polymerization Results and Characterization Data Tgb
yield a
a
Tdc
Td
lse
d33f
d33(∞)h g
no.
(%)
Mw
Mw/Mn
(°C)
(°C)
(°C)
(µm)
(pm/V)
N
P1 P2 P3 P4 P5 P6 P7 P8
78.1 88.2 64.3 91.6 87.8 84.8 88.5 89.7
32000 36300 24200 11200 31800 31100 26700 27500
1.12 1.26 1.10 1.02 1.13 1.15 1.15 1.16
140 142 115 125 144 150 123 135
263 265 265 262 277 288 282 291
157 150 130 135 150 155 130 145
0.32 0.23 0.33 0.33 0.41 0.32 0.50 0.52
65.3 105.6 73.9 66.5 60.4 83.8 58.1 63.8
0.446 0.404 0.355 0.346 0.477 0.434 0.375 0.364
(pm/V)
Φi
7.4 12.6 7.7 5.6 16.3 22.6 15.2 16.0
0.16 0.24 0.16 0.14 0.15 0.22 0.20 0.18
a Determined by GPC in DMF on the basis of a polystyrene calibration. b Glass transition temperature (Tg) of polymers detected by the DSC analyses under nitrogen at a heating rate of 10 °C/min. c The 5% weight loss temperature of polymers detected by the TGA analyses under nitrogen at a heating rate of 10 °C/min. d The best poling temperature. e Film thickness. f Second harmonic generation (SHG) coefficient. g The loading density of the effective chromophore moieties. h The nonresonant d33 values calculated by using the approximate two-level model. i Order parameter Φ ) 1 - A1/A0, A1, and A0 are the absorbances of the polymer film after and before corona poling, respectively.
and -CH3), 3.5-3.9 (-NCH2-), 4.0-4.4 (-OCH2-), 4.4-4.6 (-OCH2-), 6.5-7.6 (ArH), 7.6-8.0 (ArH), 8.0-8.4 (ArH), 8.8-9.2 (-NH-), 9.6-9.9 (-NH-). P4. 4 (108.0 mg, 0.157 mmol), TDI (28.4 mg, 0.157 mmol). P4 was obtained as deeply red powder (125.0 mg, 91.6%). Mw ) 11 200, Mw/Mn ) 1.02 (GPC, polystyrene calibration). IR (thin film), υ (cm-1): 1728 (CdO), 1518, 1336 (-NO2). 1H NMR (CDCl3) δ (ppm): 1.0-1.3 (-CH3), 1.6-2.3 (-CH2CH2and -CH3), 3.5-3.8 (-NCH2-), 3.8-4.1 (-OCH2-), 4.2-4.4 (-OCH2-), 4.4-4.6 (-NCH2-), 6.6-7.0 (ArH), 7.1-8.0 (ArH), 8.0-8.2 (ArH), 8.8-9.2 (-NH-), 9.6-9.9 (-NH-). P5. 5 (82.4 mg, 0.182 mmol), TDI (32.9 mg, 0.182 mmol). P5 was obtained as red powder (101.0 mg, 87.8%). Mw ) 31 800, Mw/Mn ) 1.13 (GPC, polystyrene calibration). IR (thin film), υ (cm-1): 1728 (CdO), 1514, 1336 (-NO2). 1H NMR (CDCl3) δ (ppm): 1.1-1.5 (-CH3), 1.8-2.2 (-CH3), 3.5-4.1 (-NCH2- and -OCH2-), 4.2-4.5 (-OCH2-), 6.7-7.2 (ArH), 7.3-7.6 (ArH), 7.6-8.0 (ArH), 8.0-8.4 (ArH), 8.8-9.2 (-NH-), 9.5-9.8 (-NH-). P6. 6 (122.0 mg, 0.238 mmol), TDI (43.0 mg, 0.238 mmol). P6 was obtained as red powder (140.0 mg, 84.8%). Mw ) 31 100, Mw/Mn ) 1.15 (GPC, polystyrene calibration). IR (thin film), υ (cm-1): 1728 (CdO), 1516, 1332 (-NO2). 1H NMR (CDCl3) δ (ppm): 1.9-2.2 (-CH3), 3.6-3.9 (-NCH2-), 4.2-4.4 (-OCH2-), 5.0-5.2 (-OCH2-), 6.8-7.2 (ArH), 7.2-7.6 (ArH), 7.6-7.9 (ArH), 8.2-8.3 (ArH), 8.9-9.2 (-NH-), 9.6-9.8 (-NH-). P7. 7 (89.2 mg, 0.144 mmol), TDI (26.0 mg, 0.144 mmol). P7 was obtained as red powder (102.0 mg, 88.5%). Mw ) 26 700, Mw/Mn ) 1.17 (GPC, polystyrene calibration). IR (thin film), υ (cm-1): 1729 (CdO), 1512, 1336 (-NO2). 1H NMR (CDCl3) δ (ppm): 1.9-2.2 (-CH2CH2- and -CH3), 3.5-3.9 (-NCH2-), 3.9-4.2 (-OCH2-), 4.2-4.4 (-OCH2-), 6.8-7.1 (ArH), 7.3-7.6 (ArH), 7.6-7.9 (ArH), 8.1-8.4 (ArH), 8.9-9.2 (-NH-), 9.6-9.8 (-NH-). P8. 8 (126.0 mg, 0.196 mmol), TDI (35.7 mg, 0.197 mmol). P8 was obtained as red powder (145.0 mg, 89.7%). Mw ) 27 500, Mw/Mn ) 1.16 (GPC, polystyrene calibration). IR (thin film), υ (cm-1): 1724 (CdO), 1514, 1336 (-NO2). 1H NMR (CDCl3) δ (ppm): 1.6-1.8 (-CH2-), 1.8-2.2 (-CH2CH2- and -CH3), 3.5-4.0 (-NCH2- and -OCH2-), 4.2-4.4 (-OCH2-), 4.4-4.6 (-NCH2-), 6.7-7.1 (ArH), 7.1-7.3 (ArH), 7.3-7.5 (ArH), 7.5-7.9 (ArH), 8.1-8.4 (ArH), 8.9-9.2 (-NH-), 9.6-9.8 (-NH-). Preparation of Polymer Thin Films. The polymers were dissolved in THF (concentration ∼3 wt %), and the solutions were filtered through syringe filters. Polymer films were spin coated onto indium-tin-oxide (ITO)-coated glass substrates,
which were cleaned by N,N-dimethylformide, acetone, distilled water, and THF sequentially in an ultrasonic bath before use. Residual solvent was removed by heating the films in a vacuum oven at 40 °C. NLO Measurement of Poled Films. The second-order optical nonlinearity of the polymers was determined by in situ second harmonic generation (SHG) experiment using a closed temperature-controlled oven with optical windows and three needle electrodes. The films were kept at 45° to the incident beam and poled inside the oven, and the SHG intensity was monitored simultaneously. Poling conditions were as follows: temperature, different for each polymer (Table 1); voltage, 7.7 kV at the needle point; gap distance, 0.8 cm. The SHG measurements were carried out with a Nd:YAG laser operating at a 10 Hz repetition rate and an 8 ns pulse width at 1064 nm. A Y-cut quartz crystal served as the reference. Results and Discussion Synthesis. As demonstrated in Scheme S1 (Supporting Information), the boronic acids S4-S6 could be easily obtained, following procedures similar to the literature methods.17 Chromophores S9 and S12 were conveniently yielded by using different structural nitro-based acceptors, under the normal azo coupling reaction conditions. Then these two compounds underwent the following Suzuki coupling reactions with different boronic acids to afford chromophores 1-8 with different isolation groups in high yield, and these chromophores possessed the similar push-pull structure. The target polyurethanes, P1-P8, were synthesized from the corresponding chromophores and TDI under conditions similar to those reported in the literature for the preparation of polyurethanes.18 The structure of main-chain polymers P1-P4 was just like those of randomly oriented main-chain polymers.9 The difference was that the chromophores in P1-P4 were part of the polymer backbone, while in randomly oriented main-chain polymers the whole chromophores were introduced into the polymer backbone. Thus, these new types of polymers were expected to possess both of the merits of main-chain and side-chain NLO polymers. Also, we prepared a series of side-chain polymers P5-P8 for comparison, which exhibited push-pull similar structure to P1-P4. Structural Characterization. The new prepared chromophores and polymers were characterized by spectroscopic methods and all gave satisfactory spectral data (see Experimental Section and Supporting Information for detailed analysis data). The structures of chromophores 1-8 were characterized by 1H NMR, 13C NMR, EI-MS, and elemental analysis. Figure S1 (Supporting Information) showed the IR spectra of chro-
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Li et al. TABLE 2: Maximum Absorption Wavelength (nm) of Chromophores and Polymers in Different Solventsa P1 THF
Figure 1. 1H NMR spectra of polymers P2 (A) and P6 (B) in DMSOd6. The solvent peaks were marked with asterisks (*).
492 (499) 1,4-dioxane 483 (491) CHCl3 497 (506) CH2Cl2 498 (507) DMF 507 (515) DMSO 516 (522)
P2
P3
P4
P5
P6
P7
P8
492 (499) 484 (490) 498 (505) 499 (505) 505 (514) 513 (523)
491 (499) 486 (491) 500 (507) 500 (507) 507 (515) 514 (521)
491 (499) 484 (491) 501 (505) 502 (505) 508 (514) 516 (521)
472 (490) 460 (472) 461 (468) 467 (475) 488 (510) 495 (516)
472 (491) 463 (473) 462 (467) 467 (475) 488 (508) 497 (518)
471 (491) 462 (472) 462 (467) 468 (475) 491 (509) 496 (518)
471 (490) 462 (474) 462 (469) 467 (474) 491 (508) 497 (516)
a The maximum absorption wavelength of polymer solutions have concentrations fixed at 0.01 mg/mL, while the maximum absorption wavelengths of the corresponding small chromophore molecules 1-8 in diluted solutions (1.25 × 10-5 mol/mL) are given in parentheses.
Figure 2. UV-vis absorption spectra of CHCl3 solutions (A) P1-P4 and (B) P5-P8. (0.01 mg/mL).
mophores 1-8, in which the absorption bands associated with the nitro groups were at about 1335 and 1515 cm-1. After these chromophores (1-8) reacting with TDI, it was easily seen that the absorption bands of the nitro groups remained in the IR spectra of the resultant polymers P1-P8 (Figure S2, Supporting Information), while another strong absorption peak appeared at about 1725 cm-1, which should be attributed to the vibration of the carbonyl group in a urethane group, indicating the successful polymerization. In the 1H NMR spectra of all the polymers, the chemical shifts were consistent with the proposed polymer structure as demonstrated in Schemes 1 and 2, however, showing an inclination of signal broadening due to polymerization. No unexpected resonance peaks were observed, and all the peaks could be readily assigned to the resonances of appropriate protons as demonstrated in Schemes 1 and 2. For example, Figure 1 showed the 1H NMR spectra of polymers P2 and P6, which were conducted in DMSO-d6. The signal of the methylene groups linked with the benzene ring in chromophore 2 or 6 at about 5.1 ppm, which was a sharp peak, became very broad after the polymerization procedure. Moreover, it was obvious that there were some small peaks present in the down fields besides those signals derived from the chromophore moieties, which should be assigned to the urethane unit formed in the polymerization process, confirming the successful polymerization between chromophores and TDI. All the polymers were soluble in common polar organic solvents such as THF, DMF, and DMSO. Their solutions could be easily spin-coated into thin solid films, therefore it was convenient to test their NLO properties based on the thin films. The UV-vis absorption spectra of the chromophores and polymers in different solvents were demonstrated in Figures 2 and S3-13 (Supporting Information), and the results were summarized in Table 2. In our previous work, when the isolation groups were introduced via Suzuki coupling reaction, there was a conjugation bridge between isolation and chromophores, which
should affect the electronic structure properties of chromophores to some degree. Since the conjugated degree was different, the maximum absorption wavelengths of polymers were also different. Here, we optimized the structure of isolation groups, and the conjugation bridges were the same. Therefore, P1-P4 or P5-P8 exhibited the same λmax, making it feasible to compare their NLO properties and study the structure-property relationship at nearly the same level. It was easily seen that after being bonded to the polymer chain the maximum absorption wavelength of the chromophore moieties were blue-shifted, in comparison with those of the free chromophore molecules in the same solvents, indicating the presence of the electronic interaction between the chromophore moieties and the polymer chain.18 However, compared to those of side-chain polymers P5-P8 (up to 19 nm in THF), the blue-shifted degree of the main-chain polymers P1-P4 sharply reduced (not larger than 7 nm in THF), which should be attributed to the special structure of main-chain polymer: the whole chromophores were introduced into the polymer backbone. The molecular weights of polymers were determined by gel permeation chromatography (GPC) with DMF as an eluent and polystyrene standards as calibration standards. All the results were summarized in Table 1, and most of the polymers possessed similar molecular weights, which would perhaps facilitate the comparison of their properties on the same level. The polymers were thermolytically resistant, with their TGA thermograms shown in Figure 3, and the 5% weight loss temperatures of polymers were listed in Table 1. The results showed that all the polymers exhibited good thermal stability up to around 270 °C. The glass transition temperature (Tg) of the polymers was investigated using a differential scanning calorimeter (Table 1), and generally their Tg values were relatively high, indicating their temporal ability of the SHG coefficient might also be very good. NLO Properties. To evaluate the NLO activity of the polymers, their poled thin films were prepared. The most convenient technique to study the second-order NLO activity was to investigate the second harmonic generation (SHG) processes characterized by d33, an SHG coefficient. To check the reproducibility, we repeated the measurements several times for each sample. The method for the calculation of the SHG coefficients (d33) for the poled films has been reported in our
Nonlinear Optical Polyurethanes
Figure 3. TGA thermograms of P1-P8, measured in nitrogen at a heating rate of 10 °C/min.
Figure 4. Analysis of the obtained NLO data of polymers (a) P1-P4 and (b) P5-P8. A: comparison of the d33 values of the polymers. B: comparison of the calculated d33 values, which were obtained by using the tested d33 values dividing the concentration of the active chromophore moieties of the polymers. C: comparison of the calculated d33(∞) values according to the approximate two-level model, using P1 for P1-P4 and P5 for P5-P8 as references.
previous papers.19 From the experimental data, the d33 values of P1-P8 are calculated at 1064 nm fundamental wavelength (Table 1). As shown in Table 1, P1-P4 demonstrated relatively high d33 values, and the d33 value of P2 was up to 105.6 pm/V. To the best of our knowledge, this was the highest value among the reported main-chain polymers, indicating the unique advantage of the additional isolation groups. As expected, all the polymers exhibited different d33 values as the isolation groups enlarged. It was not strange because different sizes of isolation groups introduced should affect the resultant properties of polymers in a large degree. Therefore, to study their NLO results visually, we compared the d33 values of the polymers using that of P1 for P1-P4 and P5 for P5-P8 as a reference (the two labeled A curves in Figure 4). Considering the effects of different molar concentrations of the active chromophore moieties in the polymers, we used the tested d33 values dividing the molar concentrations of the active chromophores and compared the results again with that of P1 and P5 as the reference (the two labeled B curves in Figure 4). From A and B curves in Figure 4, it was easily seen that the d33 values were not always increasing as the isolation groups enlarged from BOE to BOC moieties, and for both of these series of polymers, the d33 values reached the peak value while the isolation groups were BOP moieties. Since 2006, we have prepared several series of NLO polymers, and based on the obtained experimental data
J. Phys. Chem. B, Vol. 113, No. 45, 2009 14947 so far, we proposed that there is a suitable isolation group present for a special chromophore at a fixed linkage position, to boost its microscopic β-value to possibly higher macroscopic NLO property in polymers efficiently, which was concluded as the concept of “suitable isolation groups”.14-16 Here, for the same linkage position, there were the same suitable isolation groups (BOP) present even though the type of the two polymers (P2 and P6) was different: one was main-chain, while another was side-chain. Thus, the observed phenomena further proved the previous idea. As there might be some resonant enhancement due to the absorption of the chromophore moieties at 532 nm, the NLO properties of P1-P8 should be smaller as shown in Table 1 (d33(∞)), which were calculated by using the approximate two-level model. Also, we drew the curve, still using that of P1 and P5 as reference (the two labeled C curves in Figure 4), and the trend was similar to the previous two ones. On the other hand, it was noticed that when the isolation groups changed from BOE to BOP moieties the enhanced degree of d33 values was different in two series of polymers. For main-chain polymers, the d33 value of P2 was 1.62 times that of P1, while the d33 value of P5 was 1.39 times that of P6. As we know, the polymer backbone of main-chain polymers was a rigidly rodlike structure, leading to the difficulty of the alignment of the chromophore dipole along an electric field. However, the isolation groups in chromophore moieties could destroy this special structure (rigidly rodlike) to some degree, and they could also depress the serious polymeric chain entanglement efficiently, making the orientation of the chromophore moieties with strong dipole much easier.5c Moreover, according to the site isolation principle, the linked isolation groups could decrease the strong intermolecular dipole interactions between the chromophores, to improve the poling efficiency of polymers.10 Thus, when the size of isolation groups enlarged, there were two positive effects acting on the macroscopic NLO property in main-chain polymers, while for the sidechain polymers, there was only the latter one active. Thus, the enhancement degree of d33 values for main-chain polymers would be better than that of side-chain polymers, while the suitable isolation groups were introduced. To further explore the alignment of the chromophore moieties in the polymers, we measured their order parameter (Φ). After the corona poling, the dipole moments of the chromophore moieties in the polymers were aligned, and the absorption curves decreased due to birefringence.20 From the absorption change, the Φ values for the polymers could be calculated according to the following equation
Φ ) 1 - A1 /A0 where A1 and A0 are the absorbances of the polymer film after and before corona poling, respectively. The calculated Φ values were summarized in Table 1. Figure 5 showed the UV-vis spectra of the films of P2 and P6 before and after corona poling as an example, while the spectra of other polymers were demonstrated as Figures S14-S19 in the Supporting Information. The tested Φ values were well in accordance with their d33 values, and BOP groups were also confirmed as the suitable isolation group in these two series of polymers. P1-P4 exhibited very high Φ values, up to 0.24 for P2, indicating that the introduction of isolation groups improved the poling efficiency of these main-chain polymers. This should be an efficient method to solve the problem encountered in the main-chain polymers: bad oriented alignment of the dipole moments.
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Figure 5. Absorption spectra of P2 and P6 before and after poling.
Li et al. polymers, BOP acted as the suitable isolation group. The d33 value of P2 with BOP as isolation groups was even up to 105.6 pm/V, while its onset temperature for decay was as high as 110 °C. The enhanced nonlinearity and excellent temporal ability indicated that the structure of P1-P4 could give a clue to solve the “nonlinearity-stability trade off” in some degree. In addition, accompanied by the isolation groups changed from BOE to BOP, the enhanced degree of d33 values of the mainchain polymers (P1-P4) was higher than those of the sidechain polymers (P5-P8), indicating that the linked isolation groups could destroy the rigidity of the polymeric backbone and depress the polymer chain entanglement of main-chain polymers. In total, these eight polymers exhibited large NLO values, good film-forming ability, and high long-term temporal abilities, thus they were promising candidates in NLO and optoelectronic applications. Acknowledgment. We are grateful to the National Science Foundation of China (no. 20674059), the program of NCET, and the National Fundamental Key Research Program for financial support. Supporting Information Available: Detailed synthetic procedures and characterization data for the monomers, Figures of FT-IR spectra, UV-vis spectra, and absorption spectra of polymers before and after poling. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 6. Decay curves of the SHG coefficient of polymers P1-P8 as a function of temperature.
The dynamic thermal stabilities of the NLO activities of the polymers were investigated by depoling experiments, in which the real time decays of their SHG signals were monitored as the poled films were heated from 35 to 150 °C in air at a rate of 4 °C/min. Figure 6 showed the decay of SHG coefficient of all the polymers as a function of temperature, and the onset temperatures for decays were found to be around 90-125 °C, indicating that the long-term temporal stability of the polymers was relatively good, making them promising candidates for practical applications. It was a little strange that the temporal abilities of side-chain polymers were a little better than those of main-chain polymers, while in most cases the latter one exhibited excellent temporal abilities, due to its rigid-rod structure, according to the literature.4-6 As discussed in NLO properties, the additional isolation groups introduced into chromophore moieties destroyed the rigidity of the polymer backbone of main-chain polymers, to increase the rotational freedom of chromophores, which led to the relaxation of the oriented dipoles much more easily. These results were in accordance with the relatively higher glass transition temperatures of P5-P8, in comparison with those of P1-P4. Conclusion In this work, a new series of main-chain nonlinear optical polymers (P1-P4) were successfully prepared, in which isolation groups with different sizes were introduced into the chromophore moieties to improve the poling efficiency, according to the concept of “suitable isolation groups”. To investigate the effect of the isolation groups on the different types of polymers, we also synthesized another series of side-chain polymers (P5-P8), containing the same isolation groups. The obtained results further confirmed the concept of “suitable isolation groups”, and for both main-chain and side-chain
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