New Cross-Linkable Polymers with Huisgen Reaction Incorporating

Article pubs.acs.org/cm

New Cross-Linkable Polymers with Huisgen Reaction Incorporating High μβ Chromophores for Second-Order Nonlinear Optical Applications Clément Cabanetos,† Wissam Bentoumi,‡ Virginie Silvestre,† Errol Blart,† Yann Pellegrin,† Véronique Montembault,§ Alberto Barsella,⊥ Kokou Dorkenoo,⊥ Yann Bretonnière,‡ Chantal Andraud,‡,* Loic Mager,⊥,* Laurent Fontaine,§,* and Fabrice Odobel†,* †

Université de Nantes, CNRS, Chimie et Interdisciplinarité: Synthèse, Analyse, Modélisation (CEISAM), UMR CNRS 6230, 2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France ‡ Université de Lyon, Laboratoire de Chimie, CNRS UMR 5182, Université Lyon I, Ecole Normale Supérieure de Lyon, 46 allée d’Italie, 69007 Lyon, France § LCOMChimie des Polymères, UCO2M, UMR CNRS 6011, Université du Maine, Avenue O. Messiaen, 72085 Le Mans Cedex 9, France ⊥ IPCMSDépartement d’Optique ultrarapide et de Nanophotonique, 23 rue du Loess, 67034 Strasbourg Cedex 2, France S Supporting Information *

ABSTRACT: We report herein the synthesis, the functionalization, and the successful radical polymerization of very nonlinear optical (NLO) active push−pull polyene chromophores (CPO). Second, the thermal Huisgen cyclo-addition cross-linking reaction was implemented, and it proved to be fully compatible with a polyene-based push−pull chromophore. Toward this goal, PMMA-co-CPO-3 and two cross-linkable polymers (PCC1-CPO-3 and PCC2-CPO-3) were first prepared and characterized by a modified Teng and Man technique performed in transmission. These first series of polymers were not compatible with the applied poling conditions because an irreversible film degradation was systematically observed at a temperature significantly lower than the cross-linking temperature. Consequently, a second series of polymers was prepared, in which the cross-linking temperature was decreased by functionalizing acetylenic moieties with ester electron withdrawing groups, which decrease the activation energy of the thermal Huisgen cyclo-addition. These new polymers were stable until the cross-linking reaction, and they exhibit bulk electro-optic coefficients (r33) until 41 pm/V at 1.5 μm. Furthermore, it was shown that the Huisgen cross-linking reaction is compatible with such push−pull polyene-based chromophores, and it systematically enhances the stability of the electro-optic activity because chromophore orientation was maintained up to 96 °C against 70 °C for the same uncross-linked polymer. KEYWORDS: second-order nonlinear optic, methacrylate polymer, Huisgen cyclo-addition reaction, cross-linking, push−pull chomophore, electro-optic, radical polymerization

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INTRODUCTION For the past two decades, organic nonlinear optical (NLO) polymers have emerged as promising candidates for the development of photonic integrated devices.1−3 Prototype polymerbased Mach−Zehnder modulators incorporating organic material were demonstrated.1,4−7 In comparison with NLO inorganic crystals (LiNbO3), organic NLO polymers display many advantages including larger operational bandwidths (greater than 100 GHz), faster response time, smaller dielectric capacity, and better processability.8−10 Such materials are generally composed of push−pull organic chromophores (D−π−A), in which a π-conjugated bridge (π) is end-capped by donor (D) and (A) acceptor moieties, hosted in a polymeric matrix.2,11−14 © 2012 American Chemical Society

To observe the bulk second-order NLO activity, chromophores must be oriented by an external electric field to generate an acentric organization.2,15 However, the thermal and/or temporal instability of poled chromophores organization represents one of the main limitations of NLO organic materials for practical applications.16,17 Indeed, in most organic materials, over time, especially when the temperature goes above room temperature, the chromophores tend to relax back to the centrosymmetric head-to-tail organization as a result of their high ground-state Received: December 1, 2011 Revised: February 23, 2012 Published: February 23, 2012 1143

dx.doi.org/10.1021/cm203590t | Chem. Mater. 2012, 24, 1143−1157

Chemistry of Materials

Article

Figure 1. Illustration of one of the previously studied cross-linkable polymer.

Figure 2. General structures of the CPO1−CPO3 chromophores.

dipole moment leading to an irreversible loss of the overall electro-optic (EO) activity.18 Consequently, it is crucial to design strategies to freeze the chromophores orientation after the poling process.17,19 Among the different approaches reported in the literature, the two most investigated are to embed chromophores into a high glass transition temperature (Tg) polymer18,20−24 or to use cross-linkable matrices.25−32 Previously, we reported a cross-linkable approach based on copper-free thermal Huisgen 1,3-dipolar reaction (Figure 1).33−36

In our previous systems, one cross-linkable group (azide) was borne by the chromophore itself and the complementary group (acetylenic group) was placed on the polymer backbone as a pendant group. Thus, after the poling process, the chromophore orientation was restrained by the formation of new covalent bonds inside the material, since depoling experiments clearly revealed that the thermal stability of the electro-optic (EO) activity was significantly enhanced by the cross-linking reaction. Indeed, the initial EO signal is maintained until 145 °C 1144



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for a cross-linked material compared to 75 °C for a poled-only polymer.35,36 Moreover, large second-order nonlinear optical coefficients (d33) up to 50 pm/V at the fundamental wavelength of 1064 nm have been determined by second harmonic generation (SHG) using a standard Maker fringe technique.37 As a proof of concept, we used the Disperse Red 1 (DR1) chromophore for its ease of synthesis and functionalization and the large amount of information available in the literature about this compound.38 However, its quadratic hyperpolarizability is very much lower than powerful chromophores more recently described in the literature.2,8,13,14,16,19 In this study, we explore the possibility to generalize the above cross-linking strategy to new polymers incorporating for the first time π-conjugated chromophores featuring very high hyperpolarizability values (β). values. This was achieved by replacing the DR1 by a new CPO chromophore, which is composed of a ring-locked polyene spacer end-capped with an indolenine as donor group and a 2-dicyanomethylidene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) as electron acceptor unit (Figure 2). The CPO chromophores were very appealing for our purpose because they can be prepared in multigram scale7,39 within few steps. CPO-1 is characterized by a very high value of hyperpolarizability μβ of 31000 × 10−48 esu, as measured by electricfield-induced second harmonic (EFISH) generation at 1.9 μm. The CPO chromophores can also easily be functionalized by replacing the central chlorine atom by a phenoxy group without altering the nonlinear optical properties.5 We describe herein the synthesis, the functionalization, and the successful copolymerization of CPO derivatives by radical polymerization and the electro-optic characterization of the resulting polymers (Figure 3).

performed on a Bruker Ultraflex III, microTOF Q spectrometer in positive linear mode at 20 kV acceleration voltage with 2,5dihydroxybenzoic acid (DHB) or dithranol as matrix. Samples were injected in DCM. Electron-impact mass spectrometry (EI-MS) and chemical ionization mass spectrometry (CI-MS) were recorded on a Thermo Electron Corporation Polaris Q and DSQII. Elemental analyses were carried out by combustion, using a CHN 2400 analyzer for carbon, hydrogen, and nitrogen, and by pyrolysis using a O Vario EL III analyzer for oxygen. Thin-layer chromatography (TLC) was performed on aluminium sheets precoated with Merck 5735 Kieselgel 60F204. Column chromatography was carried out with Merck 5735 Kieselgel 60F (0.040−0.063 mm mesh). Air sensitive reactions were carried out under argon in dry solvents. UV−visible absorption spectra were recorded on a UV-2401PC Shimadzu spectrophotometer. Fourier transform infrared (FTIR) spectra were performed using a IR absorption spectrometer based on an amplified femtosecond Ti:sapphire laser system (coherent Vitesse oscillator, Clark-MXR CPA 1000 amplifier, 1 kHz repetition rate at 800 nm, 100 fs pulse-width, 900 μJ/pulse). Molecular weights and molecular weight distributions were measured using size exclusion chromatography (SEC) on a system equipped with a SpectraSYSTEM AS 1000 autosampler, with a guard column (Polymer Laboratories, PL gel 5 μm guard column) followed by two columns (Polymer Laboratories (PL), 2 PL gel 5 μm MIXED-D columns), with a SpectraSYSTEM RI-150 detector. THF was used as the eluent at a flow rate of 1 mL/min at 35 °C. Polystyrene standards (580−483 × 103 g/mol) were used to calibrate the SEC. Thermal analyses were performed using a TA Instruments Q500 in a nitrogen atmosphere at a heating rate of 10 °C/min. Solutions were filtered through a syringe filters PTFE membrane (pore size 0.2 μm) purchased from VWR. Kinetic studies on cross-linking Huisgen reaction were carried out with polymer film prepared on glass substrate. The light source for the ellipsometry measurement is a continuous wave (CW) laser diode, operating at 1.5 μm. The electrooptic response is detected via a lock-in amplifier synchronized with the modulated voltage applied on the sample under test. The hot-plate holding the sample allows the control of the temperature during the measurement (T45 = 20 °C). The high voltage amplifier, delivering the potential for the EO measurement, is also used for the poling operation. To confirm the effectiveness of the EO response, we checked that the signal amplitude does not depend on the frequency (sweep from 100 Hz to 800 Hz), but vary linearly with the amplitude of the modulated applied electric field. The NLO activity of the chromophores was measured by the conventional electric-field-induced second harmonic (EFISH) generation technique, which provides access to the μβ product (μ being the dipole moment, β the vector part of the hyperpolarizability). A light beam emitted by a nanosecond pulsed Nd:YAG laser, Ramanshifted to ω = 1907 nm is used to generate the second harmonic. A this wavelength, the absorption of the fundamental light beam at ω and of the generated signal at 2ω are avoided . A high-voltage signal is applied to the solution containing the chromophores, synchronous with the laser pulses, in order to orient the chromophores and break the initial centrosymmetry. The generated second harmonic intensity is measured as a function of the length of the optical path, and the resulting curves (Maker fringes) are then analyzed and compared with a reference (quartz crystal) to determine the μβ value for the chromophore. The measurements are repeated at various chromopore concentrations to detect any aggregation problem (which can be recognized by an abnormal drop in the SHG amplitude). Compound CPO-2tripleCH3. To a solution of carboxylic acid, chromophore CPO-2 (0.5 g, 0.75 mmol, 1.0 equiv), and but-2-yn-1-ol (71 mg, 0.84 mmol, 1.1 equiv) in 10 mL of tetrahydrofuran (THF) was added N-methylmorpholine (0.23 g, 2.2 mmol, 3.0 equiv) and then (4,6dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMTMM) (0.4 g, 1.4 mmol, 1.8 equiv). The mixture was stirred overnight at room temperature (RT). Solvents were then evaporated under reduced pressure to give a crude product, which was purified by silica gel column chromatography using ethyl acetate/petroleum ether (3/7) as eluting system, leading to compound CPO-2tripleCH3 (430 mg, 80%). 1 H NMR (500 MHz, CDCl3), δ (ppm): 8.12 (d, 1H, J = 15.31 Hz);

Figure 3. Generic structure of the CPO derivatives reported herein.

We demonstrate for the first time that such push−pull polyenebased chromophores are fully compatible not only with free radical polymerization but also with the copper-free Huisgen cyclo-addition and that the latter cross-linking reaction significantly improves the thermal orientation stability of the poled chromophores embedded in a methacrylate polymer matrix.

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EXPERIMENTAL SECTION

Materials. Compounds 1,40 2,39 3,39 4,41 6,42 TCF,43 CPO-1,39 CPO-2,7 CPO-2N3,7 and DA44 were prepared according to the methods described in the literature. Chemicals were purchased from Acros or Aldrich and used as received. Methods. 1H and 13C NMR spectra were recorded on Bruker Avance 300 and Avance III 500. Chemical shifts for 1H NMR spectra are referenced relative to residual proton in the deuterated solvent (CDCl3 δ = 7.26 ppm and (CD3)2SO δ = 2.50 ppm). Matrix-assisted laser desorption ionization time of flight (MALDI-TOF) analyses were 1145



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Scheme 1. Synthesis of Chromophores CPO-1 and CPO-2

7.94 (d, 1H, J = 13.13 Hz); 7.06 (t, 1H, J = 7.35 Hz); 6.84 (d, 1H, J = 7.84 Hz); 6.34 (d, 1H, J = 15.31 Hz); 5.67 (d, 1H, J = 13.13 Hz); 4.63 (q, 2H, J = 4.00 Hz); 3.79 (m, 2H); 2.80 (m, 2H); 2.40−2.00 (m, 4H); 1.85−1.40 (m, 24H); 1.04 (s, 9H). 13C NMR (75 MHz, CDCl3), δ (ppm): 175.82; 172.59; 172.10; 164.99; 146.69; 143.83; 142.74; 139.34; 135.99; 135.83; 127.80; 127.29; 125.54; 122.14; 121.94; 121.64; 112.81; 111.98; 111.29; 110.20; 107.74; 95.94; 95.13; 59.97; 53.02; 52.41; 51.00; 47.14; 42.51; 41.96; 33.27; 31.96; 27.93; 27.80; 27.15; 26.96; 26.82; 26.68; 26.18; 25.96; 24.03. HRMS-MALDI: m/z calcd for C44H49ClN4O3, 716.3488 (M+); found, 716.3496. FT-IR (KBr, cm−1): 2931 (νst(CH2)); 2215 (νst(CN)); 1712 (ester νst(CO)). UV−Vis (CH2Cl2): λmax (ε (mol−1 L cm−1)) = 829 nm (113000). General Procedure for Chlorine Substitution on Isophorone. Cesium carbonate (1.3 equiv) was added to a solution of compound 5 (1.1 equiv) in acetonitrile (generally V = 10 mL) at 0 °C under argon. After 30 min of stirring at 0 °C, a solution of chromophore CPO-1, CPO-2tripleCH3, or CPO-2N3 (1 equiv) in acetonitrile (generally V = 5 mL) was added. The mixture was allowed to warm up to RT over an hour and stirred overnight. The mixture was poured in water and dichloromethane. The two layers were separated, and the organic one was washed with brine twice and dried over anhydrous MgSO4. After filtration and evaporation of solvent, the crude product was purified by silica gel column chromatography eluted with ethyl acetate/petroleum ether: 5/5. A dark purple solid was obtained. Compound CPO-3. (Yield 88%): 1H NMR (300 MHz, CDCl3), δ (ppm): 7.61 (d, 1H, J = 15.42 Hz); 7.36 (m, 4H); 7.15 (m, 5H); 6.85 (m, 3H); 6.24 (d, 1H, J = 15.42 Hz); 5.55 (d, 1H, J = 13.30 Hz); 4.92 (q, 2H, J = 16.13 Hz); 4.13 (q, 1H, J = 7.15 Hz); 3.61 (m, 2H); 2.63 (m, 4H); 2.2−1.7 (m, 5H); 1.42 (bd, 6H); 1.35−1.25 (m, 8H); 1.01 (s, 9H). 13C NMR (75 MHz, CDCl3), δ (ppm): 176.22; 173.06; 164.36; 161.03; 157.78; 143.76; 142.27; 139.60; 135.65; 135.32; 133.33; 129.94; 129.11; 128.22; 127.94; 126.49; 122.50; 122.30; 122.01; 114.47; 112.54; 111.63; 109.52; 107.93; 96.13; 92.82; 61.91; 47.00; 42.80; 34.40; 32.46; 31.11; 28.03; 27.44; 26.79; 25.42; 25.11. HRMS-MALDI: m/z calcd for C50H52N4O3, 756.4034 (M+); found, 756.4018. FT-IR (KBr, cm−1): 2927 (νst(CH2)); 2218 (νst(CN)). UV−Vis (CH2Cl2): λmax (ε (mol−1 L cm−1)) = 804 nm (84000). Compound CPO-3tripleCH3. (Yield 85%): 1H NMR (500 MHz, CDCl3), δ (ppm): 7.64 (d, 1H, J = 15.34 Hz); 7.46 (m, 1H); 7.22 (t, 1H, J = 7.5 Hz); 7.13 (d, 2H, J = 8.5 Hz); 7.05 (m, 1H); 6.99 (t, 1H, J = 7.0 Hz); 6.90 (d, 2H, J = 8.5 Hz); 6.76 (m, 1H); 6.26 (d, 1H, J = 15.34 Hz); 5.57 (d, 1H, J = 13.42 Hz); 4.63 (q, 2H, J = 2.5 Hz); 3.73 (t, 2H, J = 7.5 Hz); 3.67 (t, 1H, J = 6.5 Hz); 3.61 (t, 2H, J = 6 Hz); 2.86−2.71 (m, 2H); 2.64 (t, 2H, J = 8 Hz); 2.36−2.35 (m, 2H); 2.24− 2.02 (m, 2H); 1.73 (m, 6H); 1.55 (s, 2H); 1.43 (s, 8H); 1.25 (m, 8H); 1.09 (s, 9H). 13C NMR (125 MHz, CDCl3), δ (ppm): 176.26; 172.89; 172.52; 164.94; 161.27; 157.86; 153.71; 149.87; 143.12; 142.14; 135.67; 134.04; 129.96; 129.47; 128.11; 122.32; 121.98; 121.91;

121.79; 115.22; 114.50; 113.43; 112.61; 111.74; 109.18; 107.98; 96.01; 94.88; 83.29; 73.06; 62.26; 61.92; 60.37; 53.40; 52.82; 47.20; 43.00; 34.42; 33.97; 33.70; 32.54; 31.12; 28.01; 27.93; 27.50; 26.84; 26.81; 26.57; 26.27; 25.48; 25.21; 24.58; 24.44. HRMS-MALDI: m/z calcd for C53H60N4O5Na, 855.4456 (M + Na); found, 855.4432. FT-IR (KBr, cm−1): 2930 (νst(CH2)); 2215 (νst(CN)); 1722 (νst(CO)). UV− Vis (CH2Cl2): λmax (ε (mol−1 L cm−1)) = 818 nm (101000). Compound CPO-3N3. (Yield 83%): 1H NMR (500 MHz, CDCl3), δ (ppm): 7.64 (d, 1H, J = 15.32 Hz); 7.47 (d, 1H, J = 13.30 Hz); 7.23 (t, 1H, J = 7.40 Hz); 7.12 (d, 2H, J = 8.50 Hz); 6.99 (t, 1H, J = 7.41 Hz); 6.90 (d, 2H, J = 8.50 Hz); 6.77 (d, 1H, J = 7.90 Hz); 6.24 (d, 1H, J = 15.32 Hz); 5.57 (m, 2H); 3.73 (t, 2H, J = 7.0 Hz); 3.60 (t, 2H, J = 5.5 Hz); 3.49 (s, 1H); 3.34 (m, 4H); 2.84 (d, 1H, J = 13.5 Hz); 2.74 (d, 1H, J = 16.5 Hz); 2.64 (t, 2H, J = 7.5 Hz); 2.14 (m, 4H); 1.77 (m, 9H); 1.54 (s, 4H); 1.42 (s, 6H); 1.27 (m, 6H); 1.08 (s, 9H). 13C NMR (125 MHz, CDCl3), δ (ppm): 176.28; 172.85; 172.41; 164.96; 161.29; 157.86; 143.18; 142.12; 139.87; 135.68; 134.11; 129.96; 128.10; 122.32; 121.96; 121.92: 121.80; 114.52; 113.44; 112.64; 111.77; 109.15; 108.05; 96.00; 94.93; 92.31; 61.01; 53.40; 52.64; 50.88; 49.53; 47.21; 43.01; 42.86; 41.36; 37.29; 36.25; 34.40; 32.56; 31.11; 28.80; 27.95; 27.53; 26.82; 26.43; 25.48; 25.18; 22.61; 22.32; 20.43;19.43;14.36. HRMS-MALDI: m/z calcd for C52H62N8O4, 862.4889 (M+); found, 862.4886. FT-IR (KBr, cm−1): 2928 (νst(CH2)); 2215 (νst(CN)); 2093(νst(N3)); 1652, 1506 (amide νst(CO)). UV−Vis (CH2Cl2): λmax (ε (mol−1 L cm−1)) = 819 nm (101000). General Procedure for the Esterification of Methacryloyl Chloride. To a solution of chromophore CPO-3, CPO-3tripleCH3, or CPO-3N3, generally 500 mg (1.0 equiv) in dry dichloromethane (20 mL), and freshly distilled triethylamine (2 mL) cooled at 0 °C by an ice bath, distilled methacryloyl chloride (1.1 equiv) was added dropwise. The ice bath was removed, and the solution was stirred overnight. Water was added, and the aqueous layer was extracted twice with dichloromethane. The combined organic layers were dried over MgSO4 and filtered, and the solvents were removed under reduced pressure. The crude product was then purified by column chromatography on silica gel, eluted with ethyl acetate/petroleum ether: 3/7. A dark green/purple solid was obtained and stored under argon in a refrigerator. Compound CPO-3MA. (Yield 82%): 1H NMR (500 MHz, CDCl3), δ (ppm): 7.61 (d, 1H, J = 15.51 Hz); 7.34 (m, 4H); 7.19 (m, 4H); 7.11 (d, 2H, J = 8.50 Hz); 7.00 (t, 1H, J = 7.47 Hz); 6.88 (d, 2H, J = 8.56 Hz); 6.81 (d, 1H, J = 7.87 Hz); 6.25 (d, 1H, J = 15.51 Hz); 6.10 (s, 1H) ; 5.56 (s, 1H); 5.54 (d, 1H, J = 15 Hz); 4.95 (d, 1H, J = 16.5 Hz); 4.86 (d, 1H, J = 16.5 Hz); 4.08 (t, 2H, J = 6.50 Hz); 2.64 (m, 5H); 2.13 (dd, 1H, J = 15.5 Hz, J = 12.5 Hz) ; 1.94 (m, 6H); 1.42 (d, 6H, J = 6.50 Hz); 1.3 (d, 6H, J = 11 Hz); 1.3 (s, 9H). 13C NMR (125 MHz, CDCl3), δ (ppm): 176.17; 173.09; 167.35; 164.31; 160.88; 157.89; 143.79; 142.20; 139.60; 136.36; 135.36; 134.97; 133.20; 129.97; 129.12; 128.21; 127.94; 126.50; 125.39; 122.48; 122.28; 1146



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Scheme 2. Synthesis of Cross-Linkable Chromophores CPO-2tripleCH3 and CPO-2N3

Scheme 3. Preparation of the Methacrylate (MA) Monomers CPO-3MA, CPO-3tripleCH3-MA, and CPO-3N3-MA

1.08 (s, 9H). 13C NMR (125 MHz, CDCl3), δ (ppm): 176.25; 173.16; 172.86; 172.49; 167.34; 164.99; 161.20; 157.95; 143.09; 142.05; 139.86; 136.36; 134.95; 134.09; 129.99; 128.09; 125.38; 122.34; 121.96; 121.92; 121.73; 114.58; 113.43; 112.62; 111.74; 109.18; 107.98; 95.99; 94.89; 92.37; 83.27; 73.06; 63.44; 60.35; 52.82; 52.65; 47.20; 43.00; 42.83; 33.96; 33.69; 32.53; 31.14; 30.41; 27.99; 27.89; 27.50; 26.83; 26.81; 26.57; 26.27; 25.47; 25.21; 24.57; 24.43; 18.30; 14.24. HRMS-MALDI: m/z calcd for C57H64N4O6, 900.4820 (M+); found, 900.4796. FT-IR (KBr, cm−1): 2930 (νst(CH2)); 2218 (νst(CN)); 1712 (νst(CO)). UV−Vis (CH2Cl2): λmax (ε (mol−1 L cm−1)) = 818 nm (101000). Compound CPO-3N3-MA. (Yield 75%): 1H NMR (500 MHz, CDCl3), δ (ppm): 7.64 (d, 1H, J = 15.32 Hz); 7.47 (d, 1H, J = 13.30 Hz); 7.22 (t, 1H, J = 6.5 Hz); 7.12 (m, 3H); 6.99 (t, 1H, J = 7.41 Hz); 6.91 (d, 2H, J = 8.50 Hz); 6.78 (d, 1H, J = 7.90 Hz); 6.24 (d, 1H, J = 15.32 Hz); 5.65−5.55 (m, 2H); 4.07 (t, 2H, J = 6.56 Hz); 3.74 (m, 2H); 3.58 (m, 1H); 3.34 (m, 4H); 2.86−2.71 (m, 2H); 2.64 (m, 2H);

122.01; 114.56; 113.24; 112.44; 111.55; 109.63; 107.91; 96.13; 96.05; 93.07; 63.46; 53.12; 46.99; 42.84; 32.47; 31.14; 30.41; 28.09; 28.02; 27.44; 26.80; 26.77; 25.46; 25.14; 18.31. HRMS-MALDI: m/z calcd for C 54 H 56 N 4 O 4 , 824.4296 (M+); found, 824.4298. FT-IR (KBr, cm−1): 2927 (νst(CH2)); 2218 (νst(CN)); 1712 (νst(CO)). UV−Vis (CH2Cl2): λmax (ε (mol−1 L cm−1)) = 804 nm (85000). Compound CPO-3tripleCH3-MA. (Yield 77%): 1H NMR (500 MHz, CDCl3), δ (ppm): 7.64 (d, 1H, J = 15.18 Hz); 7.46 (d, 1H, J = 13.26 Hz); 7.22 (t, 1H, J = 6.65 Hz); 7.13 (m, 3H); 6.99 (t, 1H, J = 7.03 Hz); 6.91 (d, 2H, J = 8.5 Hz); 6.76 (d, 1H, J = 7.90 Hz); 6.25 (d, 1H, J = 15.18 Hz); 6.1 (s, 1H); 5.57 (d, 1H, J = 11 Hz); 5.56 (t, 1H, J = 1.5 Hz); 4.63 (q, 2H, J = 2.5 Hz); 4.12 (q, 1H, J = 7.03 Hz); 4.08 (t, 2H, J = 6.5 Hz); 3.73 (t, 2H, J = 7.25 Hz); 2.83 (d, 1H, J = 14 Hz); 2.74 (d, 1H, J = 15 Hz); 2.64 (t, 2H, J = 7.25 Hz); 2.37−2.29 (m, 2H); 2.22−2.08 (m, 2H); 1.94 (bs, 2H); 1.91 (dd, 2H, J = 7.28 Hz, J = 7 Hz); 1.85 (t, 2H, J = 2.5 Hz);1.8−1.6 (m, 8H); 1.43 (m, 6H); 1.26 (m, 6H); 1147



Article

Scheme 4. Synthesis of EO polymers PMMA-co-CPO-1, PCC-CPO-1, and PCC-CPO-2

Table 1. Specific Properties of the Polymers entry

polymer

x′; y′; z′

Mn (g/mol)

PDI

% wta

Tg (°C)b

Tcross (°C)c

Td (°C)d

1 2 3

PMMA-co-CPO-3 PCC1-CPO-3 PCC2-CPO-3

0.97; 0.03; 0 0.32; 0.64; 0.04 0.67; 0.30; 0.03

7500 8900 6700

1.5 1.7 2.4

17 19 17

100 100 111

177 162

263 270 215

a

wt % of chromophore. bTg = glass transition temperature. cTcross = cross-linking temperature. dTd = decomposition temperature (Td measured at 5 wt % decomposition). 2.18 (m, 4H); 1.91 (m, 5H); 1.73 (m, 8H); 1.43 (m, 8H); 1.27 (m, 6H); 1.01 (s, 9H). 13C NMR (125 MHz, CDCl3), δ (ppm): 176.30; 172.75; 172.46; 167.36; 165.21; 161.35; 157.96; 149.88; 143.12; 142.01; 136.34; 134.94; 139.99; 128.17; 128.11; 125.40; 122.41; 121.96; 121.91; 114.60; 113.52; 112.74; 111.85; 109.01;

108.13; 95.97; 95.35; 92.01; 63.43; 54.38; 53.40; 52.37; 49.50; 47.28; 43.00; 37.23; 36.82; 36.21; 32.54; 31.13; 30.83; 30.39; 28.81; 28.02; 27.91; 27.52; 26.84; 26.81; 26.75; 26.44; 25.47; 25.21; 25.17; 18.30. HRMS-MALDI: m/z calcd for C56H66N8O5, 930.5151 (M+); found, 930.5118. FT-IR (KBr, cm−1): 2929 1148



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Figure 4. Differential scanning calorimetry trace of the polymer PCC1-CPO-3 recorded under nitrogen flow with a scanning rate of 10 °C/min. (νst(CH2)); 2217 (νst(CN)); 2099 (νst(N3)); 1714 (νst(CO)). 1652, 1506 (amide νst(CO)). UV−Vis (CH2Cl2): λmax (ε (mol−1 L cm−1)) = 819 nm (102000). Compound 7. To a solution of pent-2-ynoic acid (10 g, 0.1 mol, 1.0 equiv) and ethylene glycol (V = 100 mL) was added p-toluenesulfonic acid monohydrate (3.54 g, 20 mmol, 0.2 equiv). The mixture was stirred at 60 °C for 24 h and then poured into 100 mL of saturated aqueous sodium bicarbonate solution. The organic layer was extracted four times with 30 mL of ethyl acetate. The combined organic layers were dried over magnesium sulfate and solvents were removed under reduced pressure. A colorless oil was obtained (11.3 g, 78%). 1H NMR (300 MHz, CDCl3), δ (ppm): 4.21 (m, 2H); 3.78 (m, 2H); 2.28 (q, 2H, J = 7.53 Hz); 1.15 (t, 3H, J = 7.53 Hz). 13C NMR (75 MHz, CDCl3), δ (ppm): 153.82; 91.40; 71.99; 66.96; 63.49; 60.32; 12.30. CI-MS: m/z calcd for C7H10O3, 142.06 (M+); found, 143.02 (M + H). Compound Triple-ester-MA. To a solution of compound 7 (4.6 g, 32 mmol, 1.0 equiv) in THF (50 mL) and triethylamine (7 mL)

Table 2. Evolution of Wavelengths and Molar Extinction Coefficients of Chromophores CPO-1 and CPO-2 Further to Chemical Modifications entry

chromophore

1 2 3 4 5 6 7 8 9 10

CPO-1 CPO-3 CPO-3MA CPO-2 CPO-2tripleCH3 CPO-2N3 CPO-3tripleCH3 CPO-3N3 CPO-3tripleCH3-MA CPO-3N3-MA

λ

max

(nm)

810 804 804 829 829 829 818 818 818 818

ε

max

(L mol−1 cm−1) 92000 84000 85000 112000 113000 112000 102000 101000 101000 102000

Figure 5. UV−Vis absorption spectrum of CPO-1 recorded in dichloromethane. 1149



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Figure 6. Illustrations of the different patterns of the measurement cells.

Figure 7. Optical microscopy (×50) pictures of the upper surface of PCC1-CPO-3 (right) and of PCC2-CPO-3 (left).

Table 3. Influence of the Cladding Layer on the Alteration Temperature under DC Currenta

Table 4. EO Coefficients Determined by mTMT pattern of cell

entry

polymer

Pockels cell

Tal (°C)b

entry

polymer

1 2 3 4 5 6 7 8

PMMA-co-CPO-3

UC DC WC LC UC DC LC DC

n.o.c n.o. 85 85 105 125 115 120

1 2 3 4 5 6

PMMA-co-CPO-3

PCC1-CPO-3

PCC1-CPO-3 PCC2-CPO-3

UC DC UC DC LC DC

Tpol (°C)a r33 (pm/V)b 110 110 100 120 110 115

20.3 18.2 30.9 16.1 16.8 10.2

stability (°C)c 73 72 73 76 75 74

a

a

Tpol = highest temperature applied during the poling process. r33 coefficients measured at 1.5 μm. cstability = temperature at which 5% of the initial SHG signal is lost.

cooled at 0 °C with an ice bath, distilled methacryloyl chloride (4.8 mL, 48 mmol, 1.5 equiv) was added dropwise. The ice bath was removed, and the solution was stirred for 15 h. Water was added, and the aqueous layer was extracted twice with dichloromethane. The organic layer was dried on MgSO4 and filtered, and the solvents were removed under reduced pressure. The oily residue was then purified by column chromatography on silica gel, eluted with Et2O/petroleum ether: 5/95. A colorless oil was obtained and stored under argon in a refrigerator (6 g, 59%). 1H NMR (300 MHz, CDCl3), δ (ppm): 6.12

(m, 1H); 5.90 (m, 1H); 4.38 (m, 4H); 2.33 (q, 2H, J = 7.53 Hz); 1.93 (s, 3H); 1.19 (t, 3H, J = 7.53 Hz). 13C NMR (75 MHz, CDCl3), δ (ppm): 167.06; 153.52; 135.78; 126.26; 91.60; 72.04; 63.18; 62.12; 60.39; 18.25; 12.45. CI-MS: m/z calcd for C11H14O4, 210.08 (M+); found, 228.18 M + NH4). General Procedure of Polymerization. Chromophore CPO3MA, CPO-3tripleCH3-MA, or CPO-3N3-MA (20% wt, generally 200 mg, 1 equiv), compound N3MA, CH3-triple-MA, Triple-ester-MA, and/or MMA and AIBN (10% mol) were dissolved in 10 mL of THF in a sealed tube. The mixture was degassed by three freeze−pump− thaw cycles and heated at 70 °C for 18 h in the dark. After cooling

PCC2-CPO-3

b

Samples heated to 180 °C. bTal = temperature of alteration. cn.o. = not observed.

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Polymer PCC4-CPO-3. (Yield 62%): Triple-ester-MA (7.9 equiv); MMA (18.8 equiv). 1H NMR (300 MHz, CDCl3), δ (ppm): 7.67−7.40 (m, 2H); 7.20−6.70 (m, 6H); 6.20 (m, 1H); 5.59 (m, 1H); 4.36 (bs, 20H); 4.20 (bs, 20H); 3.59 (bs, 123H); 2.39 (m, 22H); 2.00−1.70 (m, 87H); 1.40 (m, 19H); 1.24 (m, 49H); 1.10−0.70 (m, 154H). FT-IR (KBr, cm−1): 2990 (νst(CH2)); 2238 (νst(CN)); 1729 (ester νst(CO)). UV−Vis (CH2Cl2): λmax (%w) = 819 nm (17). SEC (polystyrene): Mn (g/mol), 7000; PDI = 1.3. TGA-DSC (10 °C/min): Tg = 99 °C; Tc = 168 °C; Td = 212 °C. Polymer PCC5-CPO-3. (Yield 74%): MMA (15.4 equiv); N3MA (10.6 equiv). 1H NMR (300 MHz, CDCl3), δ (ppm): 7.61 (m, 1H); 7.32 (m, 2H); 7.20 (m, 2H); 7.12−6.90 (m, 2H); 6.90−6.75 (m, 2H); 6.24 (d, 1H, J = 13.24 Hz); 5.54 (d, 1H, J = 13.24 Hz); 4.91 (q, 2H, J = 17.01 Hz); 4.03 (bs, 18H); 3.59 (m, 69H); 3.42 (m, 12H); 2.59 (m, 4H); 2.10−1.70 (m, 77H); 1.50−1.10 (m, 27H); 1.01 (bs, 34H); 0.84 (bs, 47H). FT-IR (KBr, cm−1): 2950 (νst(CH2)); 2222 (νst(CN)); 2099 (νst(N3)); 1729 (ester νst(CO)). UV−Vis (CH2Cl2): λmax (% w) = 805 nm (19). SEC (polystyrene): Mn (g/mol), 8700; PDI = 1.9. TGA-DSC (10 °C/min): Tg = 100 °C; Td = 242 °C. Polymer PCC6-CPO-3. (65Yield %): MMA (15.8 equiv); N3MA (11.7 equiv). 1H NMR (300 MHz, CDCl3), δ (ppm): 7.80−7.40 (m, 1H); 7.13 (m, 3H); 6.20 (m, 1H); 5.58 (m, 1H); 4.04 (s, 20H); 3.59 (bs, 49H); 3.42 (bs, 18H); 2.90−2.50 (m, 3H); 2.00−1.60 (m, 74H); 1.40 (m, 12H); 1.25 (m, 11H); 1.10−0.70 (m, 82H). FT-IR (KBr, cm−1): 2950 (νst(CH2)); 2222 (νst(CN)); 2099 (νst(N3)); 1729 (ester νst(CO)). UV−Vis (CH2Cl2): λmax (% w) = 820 nm (18). SEC (polystyrene): Mn (g/mol), 9100; PDI = 1.76. TGA-DSC (10 °C/min): Tg = 101 °C; Td = 234 °C.

Figure 8. Illustration of new materials developed to decrease the activation energy of the Huisgen cross-linking reaction. back to RT, the mixture was precipitated by dropping in methanol (120 mL). The solid was washed twice, then solubilized in dichloromethane and precipitated in methanol. After filtration on Millipore, the solid was dried for 12 h at RT under reduced pressure. A green powder was obtained. Polymer PMMA-co-CPO-3. (Yield 68%). 1H NMR (300 MHz, CDCl3), δ (ppm): 7.60 (m, 1H); 7.32 (m, 2H); 7.20 (m, 2H); 7.10 (bd, 2H, J = 7.5 Hz); 7.00 (m, 2H); 6.90−6.78 (m, 3H); 6.24 (d, 1H, J = 14.61 Hz); 5.54 (d, 1H, J = 13.34 Hz); 4.94 (q, 2H, J = 17.23 Hz); 3.90 (bm, 3H); 3.59 (bm, 102H); 2.59 (bm, 5H); 1.81 (bm, 66H); 1.41−1.22 (bm, 30H); 1.01 (bs, 40H); 0.83 (bm, 55H). FT-IR (KBr, cm−1): 2924 (νst(CH2)); 2222 (νst(CN)); 1729 (νst(CO)). UV−Vis (CH2Cl2): λmax (%w) = 804 nm (17). SEC (polystyrene): Mn (g/mol), 7500; PDI = 1.5. TGA-DSC (10 °C/min): Tg = 100 °C; Td = 263 °C. Polymer PCC1-CPO-3. (Yield 65%): N3MA (8 equiv); CH3-tripleMA (16 equiv). 1H NMR (300 MHz, CDCl3), δ (ppm): 7.60 (m, 1H); 7.33 (m, 2H); 7.20 (m, 2H); 7.10 (m, 2H); 7.02 (m, 1H); 6.90−6.80 (m, 3H); 6.24 (d, 1H, J = 16.22 Hz); 5.55 (d, 1H, J = 12.12 Hz); 4.92 (q, 2H, J = 16.99 Hz); 4.56 (m, 27H); 4.06 (bm, 13H); 3.65 (bs, 6H); 3.44 (m, 8H) ; 2.60 (bm, 3H); 1.86 (bm, 79H); 1.45− 1.20 (m, 33H); 1.01 (bs, 74H). FT-IR (KBr, cm−1): 2923 (νst(CH2)); 2222 (νst(CN)); 2099 (νst(N3)); 1733 (ester νst(CO)). UV−Vis (CH2Cl2): λmax (% w) = 805 nm (19). SEC (polystyrene): Mn (g/mol), 8900; PDI = 1.7. TGA-DSC (10 °C/min): Tg = 100 °C; Tc= 177 °C; Td =270 °C . Polymer PCC2-CPO-3. (Yield 68%): MMA (22.3 equiv); N3MA (10 equiv). 1H NMR (300 MHz, DMSO), δ (ppm): 7.83 (m, 1H); 7.50−6.50 (m, 15H); 6.25 (m, 2H); 6.04 (m, 2H); 4.59 (bs ; 3H); 3.96 (bs, 31H); 3.45 (bm, 121H); 3.32 (bs, 27H); 2.00− 0.50 (bm, 370H). FT-IR (KBr, cm−1): 2950 (νst(CH2)); 2221 (νst(CN)); 2100 (νst(N3)); 1729 (ester νst(CO)). UV−Vis (CH2Cl2): λmax (% w) = 818 nm (17). SEC (polystyrene): Mn (g/mol), 6700; PDI = 2.4. TGA-DSC (10 °C/min): Tg = 111 °C; Tc= 162 °C; Td = 215 °C.

■

RESULTS AND DISCUSSION Synthesis and Characterizations. Chromophores CPO-1 and CPO-2 were synthesized as described,5 following the general route laid out in Scheme 1. The chromophores CPO-1 and CPO-2 were prepared in a twostep procedure starting with the initial condensation of TCF to 1,40 leading to the common intermediate 2 in 91% yield. The latter was engaged in a further Knoevenagel condensation with the indolinium salt 441 or 5 to afford the chromophores CPO-1 or CPO-2 in excellent yields. Subsequently, the carboxylic acid group, activated by dimethoxytriazine-N-methylmorpholine (DMTMM),46,47 was esterified or amidified by but-2-yn-1-ol or 3-azidopropan-1-amine, respectively, to install appropriate reactive groups involved in the Huisgen cyclo-addition (Scheme 2). Afterwards, the chlorine substitution on the polyene spacer was performed for both chromophores with 4-(3hydroxypropyl)phenol 642 in the presence of cesium carbonate (Cs2CO3), as described previously for other CPO derivatives (Scheme 3).5 Finally, methacrylate monomers CPO-3MA, CPO-3tripleCH3MA, and CPO-3N3-MA, containing the chromophore, were synthesized in gram-scale by nucleophilic substitution reaction with methacryloyl chloride (Scheme 3).

Scheme 5. Preparation of Triple-ester-MA Monomer

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and consequently enhance poling efficiency,55−59 we have decided to use a transparent thermosetting epoxy resin, developed by RBnano,60 deposited according to four different patterns (Figure 6). EO polymers display good solubility in o-dichlorobenzene (ODB), especially those incorporating CPO-1 derivative chromophores (PMMA-co-CPO-3 and PCC1-CPO-3), since nonsaturated concentrations up to 250 g/L have been reached as compared to 180 g/L for PCC2-CPO-3. Nevertheless, only PCC1-CPO-3 presents a sufficient adhesion on ITO glass substrate to allow the fabrication of the measurement cells with whole patterns (WC, UC, LC, and DC; Figure 6). Thanks to the deposition of an epoxy under-layer, UC and DC measurement cells were, however, successfully prepared with polymers PMMA-co-CPO-3 and PCC2-CPO-3. In all cases, homogeneous films with high optical quality (no light scattering) and thicknesses from 1.8 to 3.2 μm were produced. Chromophores were then poled for 30 min via contact poling at 80 °C by applying an electric field (20−40 V/μm). Then, the temperature was raised for PCC1-CPO-3 and PCC2-CPO-3 samples to bring about the cross-linking reaction. Unfortunately, a surface degradation was systematically observed preventing the EO characterization (Figure 7). A deeper study under DC current revealed that the latter not only depends on the nature of the EO polymer but also on the arrangement of the RBnano cladding layer (Table 3). We note that the degradation of the films was only observed with the cross-linkable polymers, since no alteration of the films with PMMA-co-CPO-3 was recorded (Table 3, entries 1 and 2). Furthermore, the use of epoxy resin on both sides raises the temperature at which this degradation (Tal) is monitored (entries 4 vs 5 and 7 vs 8). Consequently, PCC1-CPO-3 and PCC2-CPO-3 were poled 5 °C below their respective Tal and the r33 EO coefficients were determined (Table 4). The EO coefficients (r33) of the first series of polymers PMMAco-CPO-3, PCC1-CPO-2, and PCC2-CPO-3 range from 10 to 31 pm/V (Table 4). The highest values, reached with the CPO-1 chromophore, can be linked to the improved solubility of the corresponding polymers, probably due to reduced interchromophore electrostatic interactions. Moreover, poling efficiency seems to be negatively affected by total thicknesses (including EO film and cladding layers) because r33 coefficients measured for DC Pockels cells are systematically inferior to those determined for cells built with a single cladding layer (LC and UC). Finally, the dynamic thermal stabilities of the poled films clearly revealed that the cross-linking reaction had not occurred under these conditions because the chromophore orientation start to relax at 73 °C for both cross-linkable (PCC1-CPO-2 and PCC2-CPO-3) and the poled only PMMA-co-CPO-3 polymers. Furthermore, IR spectra of the films confirm the presence of the intense absorbance of the azido groups. The maximum heating temperature being limited by the degradation of cells under DC current, several trials were, however, performed by increasing the curing time at Tpol, but the crosslinking reaction did not occur. Consequently, it was decided to decrease the temperature of the Huisgen cyclo-addition by changing the substituents on the alkyne groups.61−63 New Strategy to Decrease the Cross-Linking Temperature: Synthesis and Characterizations. We previously reported that the cross-linking temperature can be tuned by changing the substituent on the alkyne.34,36 Indeed, it is wellaccepted and experimentally verified that Huisgen cyclo-addition reaction is generally faster when acetylenic moieties are functionalized with electron-withdrawing groups (EWG) rather than

First, the polymer PMMA-co-CPO-3 was prepared as a reference material by radical polymerization, initiated by azobisisobutyronitrile (AIBN). Then, the copolymers incorporating the cross-linking groups (alkyne and azido) PCC1-CPO3 and PCC2-CPO-3 were also successfully prepared in good yields according to the same reaction conditions (Scheme 4). It is worth noting that methacrylate copolymers containing such polyene-based push−pull NLO chromophores have never been reported thus far. The number-average molecular weight (Mn) and the polydispersity index (PDI) were measured by size exclusion chromatography (SEC) versus polystyrene standards. The percentage of chromophore incorporated in the polymer, theoretically equal to 20% in weight, was determined by two different methods, namely UV−visible spectrophotometry and 1H NMR spectroscopy. The decomposition temperature (Td) of the polymers was recorded by thermogravimetric analysis (TGA), while the Tg and the cross-linking temperature (Tcross) were determined by differential scanning calorimetry (DSC) (Table 1). The successful preparations of the polymers PMMA-co-CPO3, PCC2-CPO-3, and PCC1-CPO-3 indicate that the CPO chromophores are stable in presence of free-radicals. The chromophore incorporation in the polymers (from 17 to 19 wt %) is very close to the percentage in the feed (20 wt %). Second, the cross-linking temperatures (Tcross ∼ 160 °C) of PCC2-CPO-3 and PCC1-CPO-3 are distinct from the respective glass transition (Tg ∼ 100 °C) and material decomposition (Td > 210 °C) temperatures. These characteristics enable an efficient poling of the chromophores and a subsequent quantitative crosslink before the degradation of the material. As an example, the DSC trace of PCC1-CPO-3 is shown in Figure 4. The typical DSC trace features a first endothermic breakdown around 100 °C attributed to the glass transition temperature (Tg) of the polymer (PCC1-CPO-3). Then, the first intense exothermic peak at around 177 °C is assigned to the Huisgen cross-linking reaction.48 Finally, according to previously reported studies, exothermic peaks around 240 °C are attributed to the decomposition of triazole probably into nitrene.49,50 Furthermore, DSC and TGA analyses revealed that the incorporation of CPO-3tripleCH3 derivatives, possessing a flexible aliphatic linker between the chromophore backbone and the cross-linking group, seems to lower both Tcross and degradation temperatures (Table 1, entry 2 vs 3). Electro-Optic Measurements. First, the UV−Vis spectra of the chromophores recorded in dichloromethane revealed that, as expected, among all of the chemical modifications realized on chromophores CPO-1 and CPO-2, the chlorine nucleophilic substitution with dihydroxy-compound 6 causes only a slight modification of the charge-transfer band of the chromophores (Table 2). A typical spectrum of this family of chromophores is shown in Figure 5. Second-order nonlinear activity of the above synthesized polymers was then characterized using a modified Teng and Man technique (mTMT) in which the measurements were performed in transmission.51−54 This single wavelength transmission ellipsometric configuration requires that the sample of the EO polymer film is sandwiched between two transparent electrodes (Figure 6). To this end, solutions of PMMA-coCPO-3, PCC1-CPO-3, and PCC2-CPO-3 in o-dichlorobenzene (ODB) were spin-coated over indium tin oxide (ITO) glass sheets (3 × 3 cm) before a semitransparent layer of gold was sputtered as a top electrode. On the other hand, taking into consideration that cladding layers may limit charge injections 1152



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Scheme 6. Synthesis of DA (PTSA = para toluene sulfonic acid)

Scheme 7. Copolymerization of PCC4-CPO-3, PCC5-CPO-3, and PCC6-CPO-3

with electron releasing groups.61,64 Thus, two approaches based on the use of ester groups as EWG were investigated (Figure 8). The first strategy (approach A in Figure 8) consists of preparing a copolymer containing a new propargylic monomer

substituted by an ester group instead of a silyl group and a CPO chromophore carrying an azido group.34,35 The second strategy (approach B in Figure 8) is based on a mixture of the EO polymer with a doping agent (DA), carrying several electron-deficient 1153



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alkyne groups.30,36,65 This second strategy also presents the real advantage of preventing a premature cross-linking of the polymer; a potential problem that can be met because the alkyne reactivity is enhanced compared to the previous polymers. Monomer Triple-ester-MA (approach A) was prepared according to a work published by Nakagawa and co-workers dealing with the synthesis of methacrylates having an acetylene moiety activated by an ester (Scheme 5).64 In parallel, DA1 was prepared according to a protocol published by Duran and co-workers (Scheme 6).44 Thereafter, the preparation of three new polymers was performed by radical copolymerization initiated by AIBN in THF (Scheme 7). They were obtained in yields ≥62% and characterized by previously mentioned methods (Table 5). Number-average molecular weight and polydispersity indexes are in the same range as those measured for PMMA-co-CPO-3,

to note that the DA does not significantly modify the Tg of the polymer, inferring that the former displays relatively insignificant plastifying effect. The functionalization of alkyne moieties by electron withdrawing groups also decreases the temperature of the very intense exothermic peak (around 210 °C, Figure 9) attributed to the decomposition of the triazole ring.49,50 Electro-Optic Characterizations. Polymers and mixtures were spin-coated over ITO glass sheets from filtered solutions of 250 g/L in ODB, through 0.2 μm PTFE membranes. EO materials display high solubility and sufficient adhesion on glass substrate allowing to produce films with high optical quality (no light scattering) and with thickness up to 3.5 μm. Based on the previous EO measurements with PMMA-co-CPO-3, PCC1CPO-3, and PCC2-CPO-3 (Table 4), it was decided to prepared only UC type measurement cells. Consequently, RBnano transparent thermosetting epoxy resin was spun cast over EO film before sputtering the transparent top gold electrode. On the other hand, a sample of each EO polymer was heated under DC current (20 V/μm) to 150 °C to determine the highest temperature that can be applied without degrading the measurement cells (Table 7). Once again, the degradation of Pockels cells was only monitored with cross-linkable EO materials (Table 3). The polymer films were therefore poled at 70 °C for 30 min and cross-linked 5 to 10 °C under the determined Tal until the complete disappearance of the azido stretch absorption band (≈2100 cm−1), as monitored by IR spectroscopy. Gratifyingly, with the second series of polymers, the cross-linking reaction could be quantitatively performed after the poling of the chromophores and without degrading the film. Besides, the chromophores have not reacted with the azido groups upon heating, as the green color and the absorption spectrum of the film remained alike upon cross-linking. Furthermore, the bulk electro-optic activity of the material is maintained upon cross-linking attesting that the chromophores were not degraded. The highest r33 EO coefficient, with a value up to 41 pm/V, has been reached with polymer PCC5-CPO-3 (Table 8, entry 2). This value is much above than that recorded with previously described DR1-based cross-linkable polymers (r33 ∼ 10 pm/V).32 Moreover, the mixtures of the polymers with the doping agent (MCPO-5 and MCPO-6) afford lower nonlinear susceptibilities, but this is consistent and proportional to the lower molar fractions of chromophores in the mixture (due to the incorporation of DA) (Table 8, entries 4 and 5). The electro-optic coefficients of these polymers are significantly much lower than those previously reported with highly performing chromophores such as FTC derivatives, which attain values up to 100 pm/V.2 This is most certainly the consequence of an unoptimized poling process and particularly a more effective cladding layer, such as a silica-based polymer,66 would most probably limit charge injection in the polymer and boost the overall bulk EO coefficient that can be reached. Finally, the depoling experiments show that Huisgen crosslinking reaction (PCC4-CPO-3, MCPO-5, and MCPO-6) enhances the thermal stability in comparison with PCC5-CPO-3 and PCC6-CPO-3 (Table 8 and Figure 10). Indeed, the chromophores start to relax around 70 °C for poled only polymers, whereas PCC4-CPO-3 keeps its bulk second nonlinear activity until 85 °C and the mixtures MCPO-5 and MCPO-6 remain stable up to 96 °C. The lower thermal stability of the chromophore orientation in these polymers with respect to those measured previously with our DR1-based cross-linkable methacrylates33,34 can be certainly attributed to the high flexibility of the

Table 5. Specific Properties of Polymers PCCn-CPO-3 (n = 4−6) entry

polymer

x′; y′; z′

Mn (g/mol)

PDI

% wta

1 2 3

PCC4-CPO-3 PCC5-CPO-3 PCC6-CPO-3

0.28; 0.68; 0.04 0.57; 0.39; 0.04 0.55; 0.41; 0.04

7000 8700 9100

1.3 1.9 1.76

17 19 18

a

wt % in chromophore.

PCC1-CPO-3, and PCC2-CPO-3. Moreover, chromophore weight percentages are once again close to the theoretical incorporation rate fixed at 20 wt %. Subsequently, polymers PCC5CPO-3 and PCC6-CPO-3 were mixed in dichloromethane with DA in equimolar amount (versus the number of cross-linking groups) then evaporated under reduced pressure to determine decomposition (Td) and cross-linking temperatures (Tcross) (Table 6 and Figure 9). These results first confirm that it is possible to tune the cross-linking temperature by changing the nature of the subTable 6. Specific Properties of Polymer PCC-CPO4 and Mixtures (Polymer + DA) MCPO5 and MCPO6 entry

materials

1 2 3 4 5

PCC4-CPO-3 PCC5-CPO-3 PCC6-CPO-3 MCPO-5 (PCC5-CPO-3 + DA) MCPO6 (PCC6-CPO-3 + DA)

Tg (°C)a Tcross (°C)b Td (°C)c 99 98 101 97 102

168

108 108

212 242 234 217 208

a Tg = glass transition temperature. bTcross = cross-linking temperature measured at the maximum of the exothermic peak on the DSC trace. cTd = decomposition temperature determined at 5% weight loss.

stituents borne by complementary reactive groups. Indeed, Tcross dropped from 177 °C for PCC1-CPO-3 to 108 °C for mixtures MCPO-5 (or MCPO-6, Table 6, entries 4 and 5). Furthermore, even if the maximum of the exothermic peak attributed to the cross-linking reaction48 is observed at 168 °C for PCC4-CPO-3 (Table 6, entry 1), the latter is relatively broad because the full width at half-maximum (fwhm) is reported to be around 120 °C. This is probably due to an antagonist effect resulting from the functionalization of alkyne by an ester (EWG) and the ethyl group, which decreases the reactivity owing to steric hindrance and electron inductive effect. On the other hand, it is interesting 1154



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Figure 9. Differential scanning calorimetry trace of the mixture MCPO-5 (PCC5-CPO-3 + DA) recorded under nitrogen flow with a scanning rate of 10 °C/min.

Table 7. Determination of the Alteration Temperature for Each EO Material (Polymer and Mixture)

latter. Indeed, we previously showed that the rigidity of the linker has a large impact on the overall stability, most probably by controlling the degree of movement of the locked chromophore.34 Moreover, this flexibility can also explain the fact that MCPO-5 and MCPO-6 are characterized by the same thermal stability (exclusively induced by DA). This finding reveals again that the functionalization of the chromophore with a cross-linking group is not fully necessary to take advantage of the cross-linking process.

EO material PCC4-CPO-3 PCC2-CPO-3 PCC6-CPO-3 MCPO-5 MCPO-6 Tal (°C)

110

>150

>150

115

102

Table 8. EO Coefficients Determined by mTMT entry

material

% mol CPO

Vpol (V/μm)a

r33 (pm/V)b

stability (°C)c

1 2 3 4 5

PCC4-CPO-3 PCC5-CPO-3 PCC6-CPO-3 MCPO-5 MCPO-6

3.2 3.5 3.2 2.3 2.1

50 50 45 80 44

30.5 41 32 24.7 21.4

85 69 73 96 95

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CONCLUSIONS In this work, we have successfully transposed our previously reported cross-linking strategy (PCC01, Figure 1) to new and very electro-optically active polymers. Toward this goal, we first show that long polyene push−pull chromophores are compatible with free radical polymerization. Second, we demonstrate that the thermal Huisgen cyclo-addition was fully selective with alkyne since the azido cross-linking groups do not react with the double bonds of the CPO chromophores. Indeed, it was previously anticipated that the latter Huisgen cross-linking reaction was incompatible with such chromophores.67 In this study, we have successfully prepared and characterized a series of new copolymers incorporating CPO chromophore, ethynyl, and azido groups. Furthermore, we show that the cross-linking temperature could be dramatically decreased by functionalizing the acetylenic moieties with an ester group. These new polymers exhibit bulk electro-optic coefficients (r33) up to 41 pm/V, which is a considerable improvement over those incorporating DR1 chromophores. Furthermore, it was shown that the Huisgen cross-linking reaction systematically enhances the stability of the electro-optic activity since chromophore orientation was maintained up to 96 °C against 70 °C for a poled only polymer. We are very confident that the EO stability could be enhanced much further by changing the long and floppy spacer between the chromophore

Vpol = poling-induced tension. br33 coefficients measured at 1.5 μm. Stability = temperature at which 5% of the initial SHG signal is lost.

a c

Figure 10. Thermal stability of poled film of the polymers upon heating at a rate of 2 °C/min.

aliphatic linker connecting the azido group to the chromophore backbone, which allows a significant mobility of the 1155



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and the cross-linking groups with a more rigid one. Work is under way to develop such new materials.

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ASSOCIATED CONTENT

* Supporting Information S

1 H NMR and 13C spectra of all the new CPO chromophores, along with their mass spectra. 1H NMR and IR spectra, size exclusion chromatography, TGA, and DSC analyses of the polymers. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +33 (0)4 72 72 83 98. Fax: +33 (0)4 72 72 88 60. E-mail: [email protected]. *Tel: +33 (0) 3 88 10 70 90. Fax: +33 (0) 3 88 10 72 45. E-mail: [email protected]. *Tel: +33 (0)2 43 83 33 25. Fax: +33 (0)2 43 83 37 54. E-mail: [email protected]. *Tel: +33 (0)2 51 12 54 29. Fax: +33 (0)2 51 12 54 02. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Agence Nationale de la Recherche (ANR-Télécom with project ModPol) and Région Pays de la Loire (MILES-MATTADOR program) are gratefully acknowledged for the financial support of this research. RBnano is thanked for a generous gift of the transparent thermosetting epoxy resin used for the fabrication of measurement cells.

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New Cross-Linkable Polymers with Huisgen Reaction Incorporating

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