Synthesis and Properties of Zwitterionic Nonlinear Optical Chromophores with Large Hyperpolarizability for Poled Polymer Applications Andrew M. R. Beaudin, ² Naiheng Song, ² Yaowen Bai, ² Liqui Men, ² Jian Ping Gao, ² Zhi Yuan Wang,* ,² Marek Szablewski, ‡ Graham Cross, ‡ Wim Wenseleers, § Jochen Campo, § and Etienne Goovaerts § Department of Chemistry, Carleton UniVersity, 1125 Colonel By DriVe, Ottawa, Ontario, Canada K1S 5B6, Department of Physics, UniVersity of Durham, Durham, United Kingdom DH1 3LE, and Department of Physics, UniVersity of Antwerp (campus Drie Eiken), UniVersiteitsplein 1, B-2610 Antwerpen, Belgium ReceiVed August 7, 2005 ReVised Manuscript ReceiVed NoVember 10, 2005 The electric-field-dependent change in refractive index in nonlinear optical (NLO) materials can be utilized for electrical-to-optical signal conversion, such as fast electroop- tic (EO) modulators needed in optical telecommunication. 1 Compared to inorganic NLO crystals (e.g., LiNbO 3 ), organic NLO materials offer advantages such as higher EO coef- ficients, lower dielectric constants, and good processibility. 2 To obtain a large EO response, chromophores with a high molecular nonlinearity (hyperpolarizability ) need to be oriented to form a macroscopically noncentrosymmetric material. For chromophores with a large dipole moment (µ), one way to achieve such a polar ordering is by electric-field poling of the chromophores in a polymer matrix. Conse- quently, the main figure of merit describing the performance of NLO chromophores in such EO polymer applications is the scalar product µ. Among many types of organic NLO chromophores, zwitterionic derivatives of 7,7,8,8-tetracy- anoquinodimethane (TCNQ) such as (Z)-4-[1-cyano-3-(di- ethylamino)-2-propenylidene]-2,5-cyclohexadiene-1-ylidenepro- panedinitrile (DEMI; Figure 1) are known to possess very high molecular hyperpolarizabilities and show the largest µ values reported to date. 3 To be useful for EO applications, the chromophores need to be either doped in a medium (e.g., polymer or sol-gel glass) or covalently linked to a polymer and oriented under an electric field. While the zwitterionic nature of the NLO chromophores is in favor of attaining a large µ value, it can also present limitations for EO applications as a result of poor solubility and strong dipole-dipole interactions. For example, DEMI was reported to have a high µ 0 value of 9500 × 10 -48 esu ( 0 ) static hyperpolarizability). 3 An analogue, picolinium quinodimethane (PQDM) 2a (Figure 1), was calculated to have a static molecular hyperpolariz- ability of 1270 × 10 -30 esu. 4 However, both chromophores are highly crystalline and have no functional groups (e.g., OH or NH 2 ), making it very difficult to either physically or chemically incorporate them into a host polymer. A simple method to assess new chromophores for potential use as NLO materials is to measure the EO response of a chromophore-doped polymer after poling. However, the amount of chromophores that can be doped in a polymer is limited due to the tendency of phase separation. In com- parison, functionalized chromophores are more desirable, because a relatively large amount of chromophores can be incorporated into a host polymer via covalent bonds to form a processable homogeneous polymer. To explore the potential of zwitterionic chromophores, it is necessary to obtain and evaluate the functionalized chromophores. We report herein the synthesis of a series of PQDM chromophores and studies on the molecular hyperpolarizability () and EO coefficients (r 33 at 1550 nm) of the PQDM-doped poly(ether sulfone) (PES). Chromophore Design and Synthesis. The synthesis of PQDM chromophores is based on the condensation reaction of a picolinium salt and TCNQ. Chromophore 2a was previously prepared in a low yield (15 to 30%) over a long reaction time (5-14 days). 4 It was later found that by using 2 molar equiv of LiTCNQ (the adduct from TCNQ and LiI) and an amine base, such as 1,8-diazabicyclo[5.4.0]undec-7- ene (DBU), the reaction yield for 2a could be improved to 97%. 5 To introduce a functional group into PQDM chro- mophores, the picolinium moiety in PQDM was chosen as the site of modification. Thus, using the same strategy as that reported in ref 6, alkyl bromides readily reacted with 4-picoline in boiling anhydrous ethanol to produce the corresponding N-alkylated picolinium salts (1a-i). The subsequent reactions with TCNQ or LiTCNQ in the presence * To whom correspondence should be addressed. E-mail: wangw@ ccs.carleton.ca. ² Carleton University. ‡ University of Durham. § University of Antwerp. (1) (a) Dalton, L.; Harper, A.; Ren, A.; Wang, F.; Todorova, G.; Chen, J.; Zhang, C.; Lee, M. Ind. Eng. Chem. Res. 1999, 38, 8. (b) Shi, Y.; Lin, W.; Olson, D.; Bechtel, J. Appl. Phys. Lett. 2000, 77, 1. (c) Ma, H.; Chen, B.; Sassa, T.; Dalton, L.; Jen, A. J. Am. Chem. Soc. 2001, 123, 986. (2) Mahapatra, A.; Murphy, E. Opt. Fiber Telecommun. IV 2002, 258. (3) Szablewski, M.; Thomas, P.; Thornton, A.; Bloor, D.; Cross, G. H.; Cole, J.; Howard, J.; Malagoli, M.; Meyers, F.; Bredas, J.; Wenseleers, W.; Goovaerts, E. J. Am. Chem. Soc. 1997, 119, 3144. (4) (a) Ashwell, G. J. Thin Solid Films 1990, 186, 155. (b) Ashwell, G. J. Eur. Patent EP 0 391 631 A1, 1990. (5) (a) Weir, C.; Hadizad, T.; Beaudin, A.; Wang, Z. Y. Tetrahedron Lett. 2003, 44, 4697. (b) Weir, C. M. Sc. Thesis, Carleton University, Ottawa, Canada, 2003. (6) (a) Kay, A. J.; Woolhouse, A. D.; Gainsford, G. J.; Haskell, T. G.; Barnes, T. H.; McKinnie, I. T.; Wyss, C. P. J. J. Mater. Chem. 2001, 11, 996. (b) Kay, A. J.; Woolhouse, A. D.; Zhao, Y.; Clays, K. J. Mater. Chem. 2004, 14, 1321. Figure 1. Zwitterionic NLO chromophores. 1079 Chem. Mater. 2006, 18, 1079-1084 10.1021/cm051758q CCC: $33.50 © 2006 American Chemical Society Published on Web 02/09/2006