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Zhi Yuan Wang, Naiheng Song, Andrew M. R. Beaudin, Yaowen Bai, Cara A. M. Weir, Liqiu Men,Ying Xiong and Jian Ping GaoDepartment of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6
Introduction Development of new organic nonlinear optical (NLO) materials with high electro-optical (EO) coefficients is an active research field aimed at meeting the requirements of NLO and EO applications. One of the EO applications is high-speed EO modulators and switches for fiber-optic networks, which can embed analog or digital information onto a laser beam.1 Electro-optic polymers are well suited to this application, as they are able to achieve higher EO coefficients, lower dielectric constants (thus higher bandwidths), and are easier to prepare and process than the inorganic NLO crystals such as LiNbO3 used presently in optical odulators.1,2 When an NLO chromophore is incorporated into a polymer matrix and the required noncentrosymmetry is achieved, the material is optically nonlinear and would have an EO coefficient.3 Some charge separated, donor-(π-electron bridge)-acceptor types of chromophores have sufficiently large µβ values and thus exhibit the second-order NLO effect.2-4 The scalar µβ represents the averaged orientation of the chromophore in poled media where µ is the ground state dipole moment and β is the nonlinear second-order molecular hyperpolarizability. Based on the structural similarity to one of the highest µβ chromophores,5 chromophores 1a,b (Scheme 1) were selected for potential EO application. This type of zwitterionic chromophore has previously been shown to have a large theoretical β value (1270 x 10-30 cm5 e.s.u.-1 at 1.06 µm)4,6, while µ varies depending on ocal environmental conditions.7 Despite these attributes these chromophores have long been neglected, presumably due to their unavailability and lack of the functionality for incorporation into a polymer host via a covalent bond. Attempted synthesis of these chromophores using the literature procedures4,8 resulted in low yields of only 15 to 30%, and long reaction times (5 to 14 days). In order to investigate the utility of these chromophores for EO device applications they need to be made more accessible and functionlizable.
Sample Preparation and Measurement Polymer 2 was dissolved in DMF (about 5 wt%) and cast onto ITO glass. After drying in oven at 150 °C overnight, the sample film with a thickness of 3.6 microns was coated with a layer of gold by sputtering. The sample cell was placed in a home-made thermo-electrical poling station under nitrogen and heated from room temperature up to 226 °C at voltages going from 50 V up to 350 V over 50 min. The measurement of the EO response of the poled polymeric films is performed in reflection using a Teng-Man setup9 at 1550 nm.
Results and Discussion In order to maximize the yield the syntheses of 1a and 1b were repeated several times, using the different amounts of starting materials for each reaction. The conditions providing the highest yield for 1a used 1 equivalent of N-octyl-4-methylpyridinium bromide, 2 equivalents of lithium-TCNQ adduct (LiTCNQ) and 2 equivalents of DBU. Once the reactants were dissolved in refluxing MeCN, DBU was added slowly in intervals of 3 drops every 15 minutes over a period of 1.5 hours. The reaction proceeded for a total of 14 hours and the product that had precipitated from solution was collected. Following purification in which the crude product was washed repeatedly with boiling methanol, 1a was obtained in 97% yield and fully characterized by IR (2132 and 2175 cm-1 attributed to the CN group) and UV-vis (λmax 654 nm).
Chromophore 1b had IR and UV-vis spectra similar to those of 1a, showing C≡N peaks at 2128 and 2175 cm-1 in IR and λmax at 654 nm. These chromophores are quite thermally stable but labile to UV light and oxygen.10 Since chromophore 1b contains a hydroxyl group, a simple way to incorporate this chromophore into a polymer is through the ester bond formation. Accordingly, polyimide 2 was obtained by grafting the precursor acid-containing polyimide with 1b with aid of a coupling agent. A small amount (10 mol%) of the benzocyclobutenone (BCBO) group was introduced in the polymer backbone, which can crosslink the polymer at elevated temperature above 200 °C during thermo-electrical poling. In addition, the residual carboxylic acid groups in the polymer were blocked as esters derived from a phenol. Polymer 2 contained 8.8 % by weight of 1b, as assessed by UV-vis calibration method, 20 mol % of BCBO crosslinker and 40 mol % of phenol blocking group. This polymer began to crosslink at 200 °C and showed a glass transition temperature (Tg) of 224 °C after the first scan by DSC. Polymer 2 has a good solubility in DMF and other amide solvents and can be cast into films on substrates such as glass, silicon wafer and ITO electrode.
NLO Properties The experiment for EO measurement using the Teng-Man method induces a phase difference between the ordinary and extraordinary rays in the material. The static phase difference between these rays is adjusted. The application of modest voltage to the EO film under these circumstances produces phase changes which are linearly related to the applied voltage. With this assumption, r33 can be calculated from the equation (1), in which Vm is the modulation voltage, the wavelength of measurement and I m and Io are the intensity modulation and half-intensity, respectively. The experiment described above measures some combination of EO tensor coefficients, i.e. an effective EO coefficient. There is thus an additional assumption implicit in equation (1). In our case, we assume no prior axial order, in which case we can safely assume r33 = 3r13.
The poled polymer 2 was measured for its r33 to be 25 pm/V at 1550 nm. Considering that there is only a small amount of chromophore (8.8 wt%) incorporated in the polymer and that we are using a long avelength(1550 nm), the measured EO coefficient is relatively high for this type of NLO polymer. The r33 value remained unchanged over a period of 3 months when the poled sample of polymer 2 was left in the dark at room temperature. The EO coefficient remained nearly 90% of the initial value after the sample was left at 85 °C for 200 hours and 60% of the initial value after 800 hours. The results have demonstrated that the zwitterionic chromophore (e.g., 1a,b) can be synthesized readily in high yields and incorporated in various polymer hosts. By increasing the chromophore content in the polymer and further optimizing the poling conditions, it is highly conceivable to obtain an NLO polymer with EO coefficient over 50 pm/V. This work was funded by NSERC and Nortel Networks.
References[1] Lee, M., Katz, H. E., Erben, C., Gill, D. M., Gopalan, P., Herber, J. D., McGee, D. J. Science, 298, 1401-1403 (2002).[2] Dalton, L. Polymers for Photonics Applications I, Adv. Polym. Sci., 158, 1-86 (2002).[3] Chen, A., Chuyanov, V., Garner, S., Zhang, H., Steier, W. H., Chen, J., Zhu, J., Wang, F., He, M., Mao, S.S. H., Dalton, L. R. Optics Lett. 23, 6, 478-480 (1998).[4] Ashwell, G. J. Thin Solid Films, 186, 155-165 (1990).[5] Szablewski, M., Thomas, P. R., Thornton, A., Bloor, D., Cross, G. H., Cole, J. M., Howard, J. A. K.,Malagoli, M., Meyers, F., Bredas, J., Wenseleers, W., Goovaerts, E. J. Am. Chem. Soc. 119, 3144-3154(1997).[6] Ashwell, G. J. Mat. Res. Soc. Symp. Proc. 173, 507-512 (1990). .[7] Cross, G.H., Hackman, N-A., Thomas, P. R., Szablewski, M., Pålsson, L-O., Bloor, D. Opt. Mater. 22, 29-37 (2002).[8] Ashwell, G. J. Eur. Patent EP 0391631 A1, 1990.[9] Teng, C. C. Man, H. T. Appl. Phys. Lett., 56, 1734 (1990).[10]Weir, C. A. M., Hadizad, T., Beaudin, A. M. R., Wang, Z. Y. Tetrahedron Lett. 44, 4697-4700 (2003).
论文来源：International Symposium on Polymer Chemistry,June,2004 |
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