Preparation of Main-Chain Polymers Based on Novel Monomers with

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Preparation of Main-Chain Polymers Based on Novel Monomers with D−π−A Structure for Application in Organic Second-Order Nonlinear Optical Materials with Good Long-Term Stability Canbin Ouyang,*,†,‡ Jialei Liu,†,§ Qi Liu,⊥ Yuan Li,‡ Dongdong Yan,‡ Qiuxia Wang,‡ Meixia Guo,‡ and Aocheng Cao*,‡ ‡

Key Laboratory of Integrated Pest Management in Crops, Ministry of Agriculture, Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing 100193, PR China § Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China ⊥ Key Laboratory of Dryland Agriculture, Ministry of Agriculture, Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, PR China S Supporting Information *

ABSTRACT: Main-chain nonlinear optical polymers based on novel chromophores with special structures presented good solubility in most of the organic solvents. Polymers PE-1 and PE-2 attained the thermal decomposition temperatures of 305 and 223 °C and glass transition temperatures of 113 and 108 °C, and exhibited only negligible decay in the SHG signal baked at 85 °C over hundreds of hours, respectively. The SHG coefficients of poled films from polymers PE-1 and PE-2 were 26.3 and 35.8 pm/ V, respectively. These results indicated that this class of polymers can be used in the preparation of organic electro-optic devices. KEYWORDS: NLO, chromophore, polyester, long-term stability, main chain

S

based on this theory, the problem of long-term stability has not yet been resolved. To address the problem, many groups have prepared functional polymers with a variety of structures using traditional host−guest EO systems. The group of Li Zhen at Wuhan University has developed several EO polymers with H-type structures.17,18 This structure offers significant improvements in the EO coefficients and also provides for an optimum trade-off between EO activity and optical transparency. The group of Lee Ju-Yeon at Inje University has reported several kinds of EO polymers with X-type and Y-type structures.19,20 Such EO polymers offer good solubility in common solvents including N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The thermal stability of these EO polymers attained values as high as 240 °C, with a glass transition temperature (Tg) of 114 °C, which is sufficiently high for commercial applications. However, their NLO activities were very poor (3− 30 pm/V). EO polymers with main chain structures are one of the most promising means of simultaneously providing both large EO coefficients and long-term stability. However, such EO

econd-order nonlinear optical (NLO) materials based on organic polymer and small functional molecules have attracted widely attention. These NLO materials have been identified to have potential applications in high-speed electrooptical (EO) conversion devices, such as frequency converters, optical switches and EO modulators.1−3 These organic polymeric NLO materials have also shown prominent merits in EO effect, response time, bandwidth, and driving voltage, over their inorganic counterparts of lithium niobate.4−7 For decades, researchers have been encouraged to synthesize many NLO polymers with large EO coefficients, high optical and thermal stability, as well as good optical transparency to satisfy the requirements of practical applications.8−11 However, the use of NLO polymers remains problematic for two primary reasons: (1) it is difficult to translate efficiently the large first order hyperpolarizability of organic chromophores into large macroscopic EO coefficients of the polymers; (2) organic EO polymers suffer from poor long-term stability. The cause of these deficiencies is that the organic chromophores with large dipole moment improve the intermolecular interactions among the chromophore molecules, and these can cut down the poling efficiency.12−14 Originally, researchers from the group of Jen and Dalton introduced site isolation chemical groups around the chromphores, which can efficiently reduce the interactions.15,16 Though many EO polymers with larger macroscopic EO coefficients have been reported in host−guest systems © XXXX American Chemical Society

Received: January 16, 2017 Accepted: March 14, 2017 Published: March 14, 2017 A

DOI: 10.1021/acsami.7b00742 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. Synthesis of polymers PE-1 and PE-2.

designed polymer structure. As shown in the IR spectra (see Figure S2), the strong carbonyl peak located at 1725 cm−1 for both polymers can be assigned to ester bond, and strong cyan peak at 2225 cm−1 indicate the presence of organic NLO chromophores. These indicate that the inactions of NLO chromophore remained during polymerization. The weightaverage molecular weights (Mw) and number-average molecular weights (Mn) of the polymers were tested by GPC using tetrahydrofuran as the eluent and polystyrene as the standard. The Mn were about 4600 (Mw/Mn = 3.28) and 4300 (Mw/Mn = 4.09) for polymers PE-1 and PE-2, respectively. Although the molecular weights of PE-1 and PE-2 are not very large, the Mw/ Mn values are relatively large because of the rigid structure of the monomers and the rapidly increasing degree of intermolecular interactions with increasing molecular weight. The polymers’ solubility in chloroform, DMF, DMSO and acetone were really good, but their solubility in methanol and diethyl ether were quite poor. The glass transition temperature and thermal degradation pattern were investigated and estimated using DSC and TGA (see Table 1, Figures S3 and S4). According to the TGA

polymers are very difficult to prepare, and are therefore rarely reported. To facilitate the preparation of EO polymers with main-chain structure, we designed and synthesized novel NLO chromophores with carboxyl groups in both donor and acceptor locations at the head and tail of monomers. We selected 1,4-bis(2-hydroxyethoxy)benzene with hydroxyl groups at both ends as the other monomer. The structure and synthesis process of the novel chromophores prepared here are shown in Figure 1 and Figure S1. The synthesis of chromophore a-COOH can be divided into three steps. First, an etherification reaction introduced one carboxyl group in the donor unit. Then, a Knoevenagel condensation reaction was used to link the donor and acceptor units via a carbon−carbon double bond. Finally, a tert-butyl ester in the acceptor was hydrolyzed to introduce the other carboxyl group in the acceptor unit. The chromophore P-COOH has a similar structure with chromophore a-COOH, only the electronic acceptor was changed. Versus the tricyano-furan acceptor, the electron withdrawing ability of tricyanopyrroline became stronger, and this has been reported in our other reports.21 However, the synthesis process was a little more complicated. After linking the donor and acceptor units via the Knoevenagel condensation, tert-butyl bromoacetate was introduced in the acceptor unit by the alkylation reaction of nitrogen. A hydrolysis reaction was also used to remove the protective tert-butanol group. The structures and synthetic route of polymers PE-1 and PE2 are shown in Figure 1. This kind of EO polymer was constructed using the NLO chromophore monomer with carboxyl groups in both ends, and the monomer of 1,4-bis(2hydroxyethoxy)benzene with hydroxyl groups on both ends. The polymerization process was completed using an esterification reaction via 4-dimethylaminopyridine catalyst and dicyclohexylcarbodiimide dehydrating agent. The obtained polymer was then purified with methanol for 48 h by Soxhlet extraction. Because of the strongly rigid structures of the monomers (particularly the chromophores), these reactions provided low yields (32%−37%). 1H NMR, 13C NMR and IR spectra (see the Supporting Information) were employed to confirm the compounds’ chemical structures. The polymer’s 1H NMR spectrum exhibits signal broadening after the polymerization process, whereas the chemical shifts are in line with the

Table 1. Thermal Properties of Polymers PE-1 and PE-2 degradation temperature (°C)b polymer

Tg (°C)a

5 wt %-loss

10 wt %-loss

20 wt %-loss

residue at 500 °C (%)b

PE-1 PE-2

113 108

305 223

342 298

418 321

73 51

a

DSC curves were tested on the equipment of TA Q10 with a increasing rate of 10 °C/min under nitrogen. bTGA curves were tested on the equipment of TA Q50 under the same condition as DSC measurement.

thermogram (see Figure S3), PE-1 is thermally stable up to 305 °C, where its initial weight loss begins near 300 °C. However, polymer PE-2 is thermally stable up to about 223 °C, and its initial weight loss begins near 220 °C. Such a large difference in the thermal stability was due to the different structures of the NLO chromophores. The acceptor of a-COOH in polymer PE1 has a furan structure, which greatly improved the thermal stability of a-COOH. Compared with the ether bond of the B

DOI: 10.1021/acsami.7b00742 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 2. UV−vis absorption spectra of films of (a) PE-1 and (b) PE-2 before and after poling.

assembly systems and novel organic main chain systems have been considered as a dominant research direction in the future, because of their good balance between NLO effect and longterm stability. Presently, the NLO effects of these novel systems are still somewhat low, at about 3−30 pm/V.28 We studied the NLO properties by the Maker-fringes method in this report. Corona poling method is used to orient the chromophores in polymer films prepared by spin coating. In this poling process, the temperature was kept at 5−10 °C higher than the Tg, and the corona voltage was kept at about 4.5 kV, for about 20−40 min. Then after the temperature subsequently cooled to 0 °C, the electric field was removed. Here, the NLO chromophores were oriented in the polymer films, and the UV−visible spectra were employed to confirm the poling efficiency. As shown in Figure 2, the polymers shows a decrease in maximum absorption, which was caused by the birefringence character of the poling films. Table 2 presents the

furan acceptor, the amido bond in the pyrroline acceptor of PCOOH showed poor thermal stability. Though the initial weight loss of PE-2 was not very high (relative to that of PE-1), it is sufficient for their applications in the preparation of electric field poled EO films. As shown in Table 1 and Figure S4, the Tg values of the polymers PE-1 and PE-2are near 113 and 108 °C, respectively. Relative to the large differences in the thermal decomposition temperatures, the differences in Tg values are very small because the similar structures and intermolecular interactions. However, these Tg values are relatively high versus common polyesters, and this can probably be caused by the rigid structure and strong intermolecular interactions of the chromophores.22 In addition, the decomposition temperatures of PE-1 and PE-2 are higher than the corresponding Tg values. Such a result confirms a relatively high temperature of the poling process. The UV−vis spectra of films prepared using PE-1 and PE-2 are shown in Figure 2. The strong absorption peaks are at 525 and 552 nm for PE-1 and PE-2, respectively. These differences in the strong absorption bands were due to the unique structures of the chromophores employed in the polymers. The differences in the strong UV−vis absorption bands of PE-1 and PE-2 could also indicate that P-COOH in polymer PE-2 had a lower delocalized energy than a-COOH in polymer PE-1. A lower delocalized energy level would improve the electron mobility under external electric fields and would generate strong second order NLO characteristic.23,24 The UV−vis absorption peaks of both polymers are very regular with no shoulder peaks. This indicates that the interaction among chromophore molecules is very weak, and no molecular aggregation occurs at this concentration. This can benefit second harmonic generation.25 The NLO effect is the most important property of NLO polymers, and greatly affects their applications. For example, a large NLO effect can reduce the half-wave voltage of organic EO modulators and improve the sensitivity of biological probes. As more and more researchers paid their attentions to this area, large NLO effect has been reported in guest−host systems and NLO dendrimers. Some of these systems have demonstrated NLO effects exceeding 100 pm/V.26 These values are quite substantial when compared with crystalline lithium niobate, which is considered the best inorganic NLO material, and has attained a maximum NLO of 31 pm/V.27 Although self-

Table 2. Optical and NLO Properties of Polymers PE-1 and PE-2 polymer

λmax (nm)a

d33 (pm/V)

Φb

film thickness (μm)c

PE-1 PE-2

525 552

26.3 35.8

0.19 0.22

2.53 2.49

λmax after poling process. bΦ= (1-Aa)/Ab, where Aa and Ab are the maximum absorption values of the EO film after and before corona poling, respectively. cmeasured by Dektak XT profilometer.

a

optical and NLO data of λmax (wavelength of maximum absorption), SHG coefficients (d33) and order parameter values (Φ), as discussed in the following paragraph. The order parameters are related to the polling efficiency, and the estimated values (Φ) are 0.19 and 0.24 for PE-1 and PE-2, respectively. This result indicates that P-COOH in PE-2 was more readily aligned. The UV- visible spectra of the two polymers in solution (see Figure S5) exhibited large differences both in the values of λmax and in the widths of the absorption bands. This is due to the strong dipole interaction of chromophores in the solid state under a high loading density. In addition, AFM in tapping mode was employed to investigate the morphology of polymer films before and after corona poling (see Figure S6). No obvious phase separation or defeats C

DOI: 10.1021/acsami.7b00742 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. (a) Thermal stability and (b) long-term stability of the polymers’ NLO activity.

such as canyons or craters appeared in the films of either PE1or PE-2. Furthermore, the values of d33, measured by Maker-fringes method at a fundamental wavelength of 1550 nm,29 were 26.3 and 35.8 pm/V for PE-1 and PE-2, respectively (see Table 2). Obviously, polymer PE-2 shows a larger NLO activity than PE1. A larger value of Φ obtained in the poling process was one of the reasons. Otherwise, the larger first order hyperpolarizability could be estimated from the strong absorption in the UV−vis spectra. Versus the other NLO polymers with main chain structures reported before,30 the performance of NLO activity of PE-1 and PE-2 was very outstanding. This was due to the novel introduction of an electron acceptor with a multicyan five-membered ring. The main chain monomers resulted in the polymers with large first-order hyperpolarizability. The relatively rigid structure in these polymers avoided the twine inside the molecular chain. The head-to-tail docking dimers of the chromophores were barely formed. These conditions improved the poling efficiency and NLO coefficients. The thermal stability and long-term stability are very important in the preparation and application of EO devices. Excellent thermal stability could improve the level of device processing; and long-term stability could directly determine the service life of EO devices. These have a substantial bearing on the optical loss, operating voltage, bandwidth and type of devices. In situ SHG, the real-time NLO decay situations were investigated at an increasing rate of about 3 °C/min from 20 to 140 °C. As shown in Figure 3a, even at temperature 5 °C greater than Tg, the polymers still present good stability of their dipole alignment. Meanwhile, there was not any SHG decay below 110 °C. This is mainly attributed to the special structure character of the main-chain polymers. In addition, the longterm stability of the polymers is shown in Figure 3b. The normalized SHG signal of PE-1 and PE-2 were tested at 85 °C as a function of baking time. This is a sufficient testing condition for standard EO device applications. As shown in Figure 3b, negligible decreases of d33 values were observed over several hundreds of hours of monitoring. Generally, the dipole alignment in side chain NLO polymers would be losing below Tg. In compare, main chain NLO polymers usually can show us excellent stabilization of the dipole alignment. As such, the good thermal stability of the SHG of PE-1 and PE-2 at a relative high temperature is mainly caused by the reasonable interaction between the chromophore and polymer backbone. Novel main-chain polyester polymers PE-1 and PE-2 with different pendant NLO chromophores were designed and

synthesized. Polymer PE-1 and PE-2 are soluble in most of the organic solvents e.g. chloroform, DMSO, DMF, and acetone. But their solubility in methanol and diethyl ether is very poor. The different chromophore structures employed in PE-1 and PE-2, provided different thermal stabilities, where PE-1 was thermally stability stable up to 305 °C and PE-2 was stable only up to 223 °C. The Tg values measured using DSC of the two polymers were similar, at about 113 and 108 °C for PE-1 and PE-2, respectively, due to their similar structures and intermolecular interactions. The d33 values of the coronapoled polymer films are 26.3 and 35.8 pm/V, respectively, for PE-1 and PE-2. These films exhibited only negligible decay in the SHG signal baked at 85 °C over hundreds of hours. This result indicates that the long-term stability of these polymers is sufficient for EO device applications. This excellent long-term stability is caused by the reasonable interaction between the chromophore and polymer backbone.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00742. Synthetic route; 1H NMR, 13C NMR, and IR spectra; TGA and DSC thermograms, AFM images, and surface roughness data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Canbin Ouyang: 0000-0001-9295-1273 Author Contributions †

C.O. and J.L. have contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (51503215) and Fund of Key Laboratory of Agri-food Safety and Quality, Ministry of Agriculture (2016-KF-14) and the Special Fund of Ministry of Agriculture, China (2130108, 201103027) for financial support. D

DOI: 10.1021/acsami.7b00742 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.7b00742 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX