From Controllable Attached Isolation Moieties to Possibly Highly

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ARTICLES From Controllable Attached Isolation Moieties to Possibly Highly Efficient Nonlinear Optical Main-Chain Polyurethanes Containing Indole-Based Chromophores Qianqian Li,† Zhen Li,*,† Fanxin Zeng,† Wei Gong,† Zhong’an Li,† Zhichao Zhu,† Qi Zeng,† Shanshan Yu,† Cheng Ye,‡ and Jingui Qin*,† 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: October 2, 2006; In Final Form: NoVember 10, 2006

For the first time, the indole-based NLO chromophores were embedded into the polymer main chain, and different isolation groups were attached to their donor side with the efforts of adjusting the NLO properties of the resultant main-chain polyurethanes, according to the site isolation principle. Thanks to the main-chain structure and the advantages of the indole-based chromophores, all the polymers show excellent transparency, good processability, thermal stability, and relatively good NLO effects. The tested NLO properties of the polymers demonstrate that there is a suitable isolation group present for the sulfonyl-based chromophore to boost its microscopic β value to a possibly higher macroscopic NLO property efficiently.

Introduction Considerable interest has been attracted in the research field of second-order nonlinear optical (NLO) materials due to their huge potential applications such as optical information processing, optical sensing, data storage, and telecommunications. NLO polymers are also considered as viable alternatives to conventional inorganic crystalline materials due to their many advantages over the inorganic materials, in which lithium niobate (LiNbO3) is the typical delegate.1-3 Thanks to the great efforts of scientists, many strategies and approaches have been reported for the development of polymeric NLO materials to meet the basic requirements of practical applications: large macroscopic optical nonlinearity, high physical and chemical stability, and good optical transparency. However, so far, it is still difficult for a NLO polymer to simultaneously demonstrate these properties.4 Recently, by introducing some isolation groups to the NLO chromophore moieties according to the site isolation principle, Dalton and Jen et al. have prepared a series of NLO dendrimers and polymers containing dendronized chromophores in the side chains, which demonstrates a large macroscopic optical nonlinearity due to the reduction of intermolecular electrostatic interactions of the polar chromophore moieties during the poling process.5-7 Also, the stability of the NLO polymers could be achieved through the linkage of more bulky groups and the use of rigid polymeric backbones.8 However, relatively, much less attention has been paid to the optical transparency of the NLO polymers, although the bad transparency generally results in the primary source of optical loss (an important parameter for * Corresponding author. Phone: 86-27-62254108. Fax: 86-27-68756757. E-mail: [email protected]. † Wuhan University. ‡ The Chinese Academy of Sciences.

the practical used NLO materials) at the operation wavelength of electro-optical devices (typically 0.8, 1.3, and 1.5 µm). Actually, there is a so-called nonlinearity-transparency tradeoff present, that is to say, the improvement of the NLO properties often accompanies the decreased transparency.9 On the basis of the work reported in the literature,10-12 in the past several years, we have successfully prepared some series of NLO chromophores, which exhibit large NLO effects and good transparency at the same time, by using the combined conjugation bridge or special electron acceptors.13 More recently, we found that indole moieties are a good construction block for the development of NLO chromophores, and our job demonstrated that the indole-based NLO chromophores display comparable or even superior NLO properties but have blueshifted absorptions (even up to 30 nm), in comparison with their aniline donor analogues. Also, the indole chromophores are thermally stable.14 Thus, it is promising to develop new NLO polymers with good comprehensive properties as mentioned previously, by introducing the indole chromophores to the polymer system. On the other hand, the main-chain NLO polymers, in which different types of chromophore moieties were embedded into the polymer backbone, have demonstrated large second-order optical nonlinearity and stabilized oriented dipoles at elevated temperatures, better than other types of NLO polymers.15,16 Then, what about the main-chain NLO polymers with indole-based chromophores? Perhaps, this kind of polymer would combine the advantages of the main-chain NLO polymers and those of indole chromophores. However, so far, there are no reports concerned with the main-chain NLO polymers containing indole-based chromophores. Also, to minimize the intermolecular electrostatic interactions of the polar chromophore moieties, and to further

10.1021/jp066489l CCC: $37.00 © 2007 American Chemical Society Published on Web 12/24/2006

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SCHEME 1:

improve the macroscopic optical nonlinearity of the resultant polymers, we would like to introduce some isolation groups to the indole-based chromophores, with applying the site isolation principle. Therefore, based partly on our previous research,17 in this work, we prepared a series of main-chain polyurethanes containing indole-based chromophores, and also different isolation moieties that were changed from small atoms such as hydrogen to much larger groups (Scheme 1) were introduced to the chromophore moieties on the donor side with the effort to study the effect of the different sizes of isolation spacers on the resultant NLO properties. The sulfonyl groups were used as the acceptor moieties in this study since they were easily obtained, and widely studied, combined with some advantages such as much wider transparency in the visible region (with a hypsochromic shift of 20-40 nm as compared to their analogues with nitro groups as the acceptor) and synthetic flexibility.18 Polyurethanes were chosen as the polymer backbone because they can form extensive hydrogen bonding between urethane linkages and increase rigidity to prevent the relaxation of the induced dipoles. Also, they could be conveniently synthesized.19 The tested NLO properties demonstrate that the NLO values of the polymers could be controlled, to some degree, by adjusting the size of the isolation spacer, and there is a suitable isolation group to boost the NLO properties of the polymers to possibly high levels. Herein, we would like to report the syntheses, characterization, and NLO properties of these main-chain NLO polymers. Experimental Procedures Materials. N,N-Dimethylformamide (DMF) was dried over and distilled from CaH2 under an atmosphere of dry nitrogen. 2,4-Toluenediisocyanate (TDI) was freshly distilled under reduced pressure before use. Instrumentation. The 1H- NMR spectroscopy study was conducted with a Varian Mercury300 spectrometer using tetramethylsilane (TMS; δ ) 0 ppm) as an internal standard. The Fourier transform infrared (FT-IR) spectra were recorded on a Perkin-Elmer-2 spectrometer in the region of 3000-400 cm-1 on KBr pellets. UV-vis spectra were obtained using a Shimadzu 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 NETZSCH STA449C thermal analyzer at a heating rate of 20 °C/min in argon at a flow rate of 50 cm3/min for thermo-

gravimetric 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 thermometer for measurement of the melting point was uncorrected. The thickness of the films was measured with an Ambios Technology XP-2 profilometer. General Procedure for the Synthesis of Polyurethanes P1-3. NLO chromophores and TDI with equivalent molar ratios were reacted in an appropriate anhydrous DMF solution at 80 °C for about 40 h in an atmosphere of dry nitrogen. After cooling to ambient temperature, the resultant solution was dropped into methanol to remove monomers. The polymer was filtered and dried in a vacuum desiccator. P1. Chromophore 2 (179 mg, 0.48 mmol), TDI (83 mg, 0.48 mmol). Orange solid (125 mg, 48.0%). Mw ) 30 100, Mw/Mn ) 1.16 (GPC, polystyrene calibration). IR (thin film), υ (cm-1): 1727 (CdO), 1599 (sCdCs), 1296, 1130 (-SO2-). 1H NMR (DMSO-d6) δ (ppm): 2.0 (-CH3), 3.5 (-SO2-CH2-), 4.4 (-N-CH2-), 4.5 (-O-CH2-), 4.6 (-O-CH2-), 7.0 (ArH), 7.3 (ArH), 7.7 (ArH), 8.0 (ArH), 8.4 (ArH), 8.5-8.6 (ArH), 8.9-9.0 (-NH-), 9.5-9.6 (-NH-). UVvis (DMF, 0.02 mg/mL): λmax (nm): 395. P2. Chromophore 3 (130 mg, 0.30 mmol), TDI (51 mg, 0.30 mmol). Orange solid (80 mg, 43.6%). Mw ) 27 000, Mw/Mn ) 1.11 (GPC, polystyrene calibration). IR (thin film), υ (cm-1): 1729, 1660 (CdO), 1598, 1534 (sCdCs), 1293, 1127 (-SO2-). 1H NMR (DMSO-d6) δ (ppm): 2.0 (-CH3), 3.7-3.8 (-SO2-CH2-), 4.4 (-N-CH2-), 4.5 (-O-CH2-), 4.7 (-OCH2-), 7.0-7.1 (ArH), 7.4-7.5 (ArH), 7.7 (ArH), 8.0 (ArH), 8.7 (ArH), 8.9-9.0 (-NH-), 9.4-9.6 (-NH-). UV-vis (DMF, 0.02 mg/mL): λmax (nm):401. P3. Chromophore 4 (123 mg, 0.21 mmol), TDI (36 mg, 0.21 mmol). Orange solid (85 mg, 53.6%). Mw ) 23 400, Mw/Mn ) 1.10 (GPC, polystyrene calibration). IR (thin film), υ (cm-1): 1725, 1658 (CdO), 1598, 1529 (sCdCs), 1297, 1129 (-SO2-). 1H NMR (DMSO-d6) δ (ppm): 0.9 (3H, -CH3), 1.21.4 (4H, -CH2-CH2-), 2.0 (3H, -CH3), 3.7 (2H, -SO2-CH2), 4.1 (2H, -N-CH2-), 4.4 (2H, -N-CH2-), 4.5 (2H, -OCH2-), 4.7 (2H, -O-CH2-), 6.9-7.2 (5H, ArH), 7.5-7.8 (5H, ArH), 8.0 (5H, ArH), 8.5-8.6 (2H, ArH), 8.7-8.8 (2H, ArH), 8.9-9.0 (-NH-), 9.4-9.7 (-NH-).UV-vis (DMF, 0.02 mg/ mL): λmax (nm): 397. Preparation of Polymer Thin Films. The polymers were dissolved in DMF (concentration ∼3 wt %), and the solutions were filtered through syringe filters. Polymer films were spincoated onto indium-tin-oxide (ITO)-coated glass substrates, which were cleaned by N,N-dimethyformide, acetone, distilled water, and THF sequentially in an ultrasonic bath before use.

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TABLE 1: Some Characterization Data of Polymers no.

yield (%)

Mwa

Mw/Mna

λmaxb (nm)

Tgc (°C)

Tdd (°C)

Tee (°C)

lsf (µm)

d33g (pm/V)

d33(∞)h (pm/V)

P1 P2 P3

48.0 43.6 53.6

30 100 27 000 23 400

1.16 1.11 1.10

395 (397) 401 (403) 397 (400)

153 140 125

195 193 205

158 157 149

0.36 0.16 0.15

20.6 26.2 11.8

8.0 9.7 4.5

a Determined by GPC in DMF on the basis of a polystyrene calibration. b Maximum absorption wavelength of polymer solutions in DMF, while the maximum absorption wavelength of the corresponding small chromophore molecules in diluted DMF solutions is given in parentheses. c Glass transition temperature (Tg) of polymers detected by the DSC analyses under nitrogen at a heating rate of 10 °C/min. d 5% weight loss temperature of polymers detected by the TGA analyses under argon at a heating rate of 20 °C/min. e Best poling temperature. f Film thickness. g Second harmonic generation (SHG) coefficient. h Nonresonant d33 values calculated by using the approximate two-level model.

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) experiments 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, 7.5 kV at the needle point; and 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. The synthetic route of chromophores is shown in Scheme S1, while that of polymers is shown in Scheme 1. The substituted reaction between 2-chloroethanol and indole or 5-bromoindole was conducted in DMF with the presence of potassium hydroxide as the base to yield compounds S1 and S2, which were used as the starting materials in the next azo coupling reactions to afford compounds 2 and S4, following a similar procedure reported previously.20 3-N-(n-Butane) carbazole boronic acid (S7) was obtained through three steps as shown in Scheme S1, while phenyl boronic acid (S8) was purchased from Aldrich directly. Under the typical conditions of the Suzuki reaction,21 the syntheses of chromophores 3 and 4 were conveniently proceeded (Scheme S1). We could change the isolation groups easily by changing the boronic acid reagents if needed. Here, in chromophores 2-4, the isolation part was adjusted from the very small hydrogen atoms (also, we could consider that there are no isolation moieties) to relatively large carbazolyl ones. Then, we could know the size effect to some degree from the different isolation groups linked to the NLO chromophores. The main-chain polyurethanes, P1-3, were easily obtained from the corresponding chromophores and TDI under similar conditions as reported in the literature for the preparation of polyurethanes.19 Also, as there is nearly no different reactivity between the two hyroxyl groups in the chromophores, it was expected that the chromophore dipoles should be randomly arranged in the polymer backbone (i.e., the dipoles can be headto-tail, tail-to-tail, or head-to-head) as the cases reported in the literature.15,16,22 This structure, perhaps, might benefit from the alignment of chromophore moieties upon poling, and we discuss this point in NLO Properties. Structural Characterization. The chromophores and polymers were characterized by spectroscopic methods, and all give satisfactory spectral data (see Experimental Procedures, Supporting Information, and Table 1 for detailed analysis data). Figures S1-3 show the IR spectra of chromophores 2-4, in which the absorption bands associated with the sulfonyl groups

Figure 1. 1H NMR spectra of chromophore 4 (A) in chloroform-d and P3 (B) in DMSO-d6. The solvent peaks are marked with asterisks.

are at about 1290 and 1130 cm-1. After these chromophores react with TDI, the absorption bands of the sulfonyl groups remain in the IR spectra of the resultant polymers P1-3 (Figures S4-6), while another strong absorption peak appears at about 1725 cm-1, which is attributed to the vibration of the carbonyl group in a urethane group, indicating the formation of urethane linkages during the polymerization process as shown in Scheme 1.19 In all the 1H NMR spectra of the polymers P1-3, the chemical shifts are consistent with the proposed polymer structure as demonstrated in Scheme 1; however, they show an inclination of signal broadening due to polymerization. For example, Figure 1 shows the spectra of chormophore 4 and the corresponding polymer P3, which were conducted in different solvents for their different solubility. It is obvious that there are some small peaks present in the down fields besides those signals derived from the chromophore moieties. These peaks are assigned to the urethane unit formed in the polymerization process,19 further confirming the successful polymerization between chromophore 4 and TDI. All the polymers are soluble in common polar organic solvents such as DMF and DMSO. The UV-vis absorption spectra of the chromophores and polymers are shown in Figure 2. It is easily seen that there are strong absorption peaks with the maximum absorption wavelengths around 400 nm and a band edge of ∼520 nm, due to the π-π* transition of the NLO chromophore moieties (Table 1). According to the literature and our previous case, if not indole groups but aniline ones were used as the donor in the chromophore moieties, the maximum absorptions should be longer than 440 nm, more than 40 nm red-shifted in comparison with our data reported here.17,18,20 Thus, the indole sulfonyl-based chromophores really show a wide transparent window and will benefit their low optical loss during the possible applications. Also, it is noticed that the maximum absorption wavelength of the chromophore molecules (2-4) is similar and that the difference among them is not more than 6 nm, indicating that the introduction of the different isolation groups to the donor side of the chromophore molecules

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Figure 2. UV-vis spectra of DMF solutions of (A) chromophores 2-4 and (B) P1-3. Concentration: 0.02 mg/mL for polymers.

did not affect their electronic structure properties to a large extent, or in other words, they would exhibit similar NLO properties, regardless of the different isolation moieties linked. Thus, it is reasonable for us to focus on the relationship between the size of the isolation groups and the resultant NLO properties of the main-chain polymers. As reported before, the maximum absorption wavelength of the chromophore moieties after introduction to the side-chain polyurethanes often blue-shifted (up to 15 nm) as compared with those of the free chromophore molecules, due to the presence of the electronic interaction between the chromophore moieties and the polymer chain.17,19 Here, the difference of the maximum absorption wavelength is minor (no more than 3 nm), indicating the similarity of the environments of the chromophore moieties either at free molecular state or in polymers. This, perhaps, demonstrates the difference between the side-chain and the main-chain polyurethanes to some degree.23 The molecular weights of polymers were determined by gel permeation chromatography (GPC) with DMF as eluent and polystyrene standards as calibration standards. All the results are summarized in Table 1, and all the polymers possess a relatively high molecular weight (Mw higher than 23 000), which would perhaps facilitate the comparison of their properties on the same level. The polymers are thermally stable, their TGA thermograms are shown in Figure 3, and the 5% weight loss temperature of polymers is listed in Table 1. The glass transition temperature (Tg) of the polymers was investigated using a differential scanning calorimeter (Table 1), and polymers generally have a moderate Tg above 120 °C due to the strong secondary forces between polymer chains. 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 is to investigate the second harmonic generation (SHG) processes characterized by d33, an SHG coefficient. To check the reproducibility of the results, 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 previous papers.17 From the experimental data, the d33 values of P1-3 are calculated at a 1064 nm fundamental wavelength (Table 1). Although the d33 value of the same NLO polymer can be different when measured by different methods or different testing systems at different times, it is reasonable to compare the d33 values of the polymers carrying the similar

Figure 3. TGA thermograms of P1-3 measured in argon at a heating rate of 20 °C/min.

NLO chromophores calculated by the same method using the experimental data obtained from the same apparatus. In our previous cases, the polymers, including polphosphazenes and polysiloxanes, which containes indole sulfonyl-based chromophores, all exhibited smaller d33 values than P1 reported here.17,20 The different point is minor: the chromophore moieties were embedded into the main chain of P1, while our previous polymers contained the chromophore groups as side chains. As mentioned in the Introduction, the polymeric main-chain backbone results in large second-order optical nonlinearity and stabilized oriented dipoles at elevated temperatures.15,16 The obtained results thus confirmed the advantage, thanks to the structure of the random arrangement of the chromophore moieties in the polymer backbone, and similar phenomena were also observed in the literature.22 Also, it was reported that the main-chain second-order nonlinear optical materials are more difficult to be poled than the side-chain ones, which makes it difficult to achieve a high poling efficiency. Thus, these materials are needed to prolong the poling time and optimize the poling conditions. However, on the other hand, this point is just the reason why the main-chain polymers show good stabilized oriented dipoles after poling.15,16

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Figure 4. (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 as a reference.

To show the NLO results more visually, we compared the d33 values of the polymers using that of P1 as a reference (Figure 4A). When the phenyl groups were linked to the indole-based chormophore, the d33 value of P2 increased to 1.27 times that of P1; however, the tested NLO effect decreased dramatically while the isolation group was changed to carbazolyl moieties in P3. This seems strange since the NLO properties were usually improved after bonding some isolation groups to the chromophore moieties, as reported in the literature.7 Thus, perhaps, the linked carbazolyl groups were a little bigger since the introduction of different isolation groups would surely lead to the different molar weights of the obtained chromophores, resulting in the diluted concentration of the active chromophore moieties in the polymer. According to the one-dimensional rigid orientation gas model24

1 d33 ) Nβf2ω(fω)2〈cos3 θ〉 2

(1)

where N is the number density of the chromophore, β is its first hyperpolarizability, f is the local field factor, 2ω is the double frequency of the laser, ω is its fundamental frequency, and 〈cos3 θ〉 is the average orientation factor of the poled film, we compared their NLO results again, using the tested d33 values dividing the concentration of the active chromophore moieties (the green part in the structure of the polymers as shown in Scheme 1) in the polymers, with that of P1 still as a reference. As shown in Figure 4B, the curve was similar to curve A, and the relative d33 value of P3 was still lower than that of P1. Although, as discussed previously, the different isolation groups linked to the indole-based chromophores would not influence the β values of the chromophores to a large degree, as confirmed by their nearly same maximum absorptions observed in the UV-vis spectra, there might be some resonant enhancement due to the absorption of the chromophore moieties at 532 nm. Then, the NLO properties of P1-3 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 as a reference (Figure 4C), and this time, the trend existing in the d33(∞) values was similar to the previous two.

Therefore, from the three curves demonstrated in Figure 4, it is obvious that the d33 value reaches the peak value while the isolation group is a phenyl moiety. In other words, the phenyl moieties are the best isolation groups for this polymer system. Thus, the d33 values of the polymers are not always increasing as the isolation groups are enlarged according to the experimental results, and while further increasing the size of the isolation group without limitation (i.e., P3 here), the tested d33 values will drop to even lower values than that of the origin polymer (P1 here), in which there is no isolation moiety (or hydrogen atoms as the isolation part). Thus, there should be a suitable isolation group present for a special chromophore to boost its fixed µβ value to possibly higher macroscopic NLO activities of the polymers. In fact, these phenomena are reasonable, if we consider them carefully. The linkage of the isolation group to the chromophore moieties in the polymers would mainly cause three impacts: (1) minimize the strong intermolecular dipole-dipole interactions to some degree, leading to the improvement of the d33 values of the polymers as many scientists expected; (2) dilute the active concentration of the chromophore moieties, generally reducing the d33 values; and (3) increase the bulk of the resultant chromophore moieties, making the noncentrosymmetric alignment of the chromophore upon poling in the electronic field more difficult, which would generate complicated effects on the resultant d33 values (at the beginning, the size enlargement would benefit the alignment due to the minimized electrostatic interaction but would restrain the alignment when the size is too bulky). Thus, the macroscale NLO properties of polymers containing chromophores with isolation groups are expected to be heavily related to the subtle difference in architectural design, and there is a balance present according to the previously mentioned three impacts. Or we could say: there is a suitable isolation group to realize this balance, leading to the possible highest macroscopic NLO activities of polymers containing the preferred NLO chromophores. Therefore, on the basis of this point, it is promising that the reported NLO polymers with dendronized chromophores as side chains might achieve even better NLO properties if more suitable isolation groups were introduced. In addition, it should be pointed out that the suitable isolation group might be different for the same NLO chromophore if the linked position in the chromophore or the chemical environments of the chromophore were changed. Thus, to study the structureproperty relationship of NLO polymers in detail and to realize the widely practical application of NLO materials, more work is still needed. Conclusion A series of main-chain polyurethanes (P1-3) containing indole sulfonyl-based chromophores were obtained conveniently. To the best of our knowledge, this is the first time that indolebased chromophores were used to construct the main-chain NLO polymers. Our preliminary results demonstrate the following points: (1) all the polymers exhibit excellent transparency, good processability, thermal stability, and relatively good NLO effects, thanks to the main-chain structure and the special electronic properties of indole moieties. Thus, they could be promising candidates for practical application in photonic fields. (2) The site isolation principle is applied to the main-chain polymers but not the side-chain ones reported in the literature. The NLO properties of the corresponding polymers do not always improve when they are accompanied by the enlargement of the isolation groups linked to the chromophore moieties, due to the complicated impacts that are present by introducing the

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