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Chem. Mater. 1999, 11, 1966-1968
Synthesis and Characterization of Sterically Stabilized Second-Order Nonlinear Optical Chromophores Cheng Zhang, Albert S. Ren, Fang Wang, Jingsong Zhu, and Larry R. Dalton* Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, California 90089-1661
J. N. Woodford and C. H. Wang Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304 Received April 21, 1999 Revised Manuscript Received June 7, 1999 Organic second-order nonlinear optical (NLO) materials have received increasing attention for applications involving signal processing and telecommunications.1 Like semiconductor electronic materials, second-order NLO (electrooptic) materials can be used to fabricate a wide range of devices. Functions performed, by these devices, include the following: electrical to optical signal transduction; radiowave to millimeter wave electromagnetic radiation (signal) detection; radiowave to millimeter wave signal generation (broadcasting); optical and millimeter wave beam steering; and signal processing such as analog to digital conversion, ultrafast switching of signals at nodes of optical networks, and highly precise phase control of optical and millimeter wave signals. One of the challenges of the this area of research is to design and synthesize second-order NLO chromophores (the active components of second-order nonlinear optical materials) that simultaneously possess large first molecular hyperpolarizabilities (β), good chemical and thermal stability, and optical transparency at optical communication wavelengths (1.3 and 1.55 µm). Chromophores with β values many times those of the well-known Disperse Red 19 dye are required to obtain electrooptic coefficients comparable to or higher than those of the leading commercial material crystalline lithium niobate. Chromophore intermolecular electrostatic interactions prevent the simple scaling of molecular optical nonlinearity into macroscopic optical nonlinearity. Such interactions strongly attenuate the efficient induction of acentric chromophore order (hence, electrooptic activity) by electric field poling or selfassembly methods.2 Previous studies3 indicate that the β value for a chromophore can be increased by using a diene moiety in place of thiophene in the conventional phenylethenylenethiophene π-conjugated bridge. Moreover, this enhancement in β can be accomplished without an (1) (a) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; John Wiley & Sons: New York, 1991. (b) Dalton, L. R.; Harper; A. W.; Ghosn; R.; Steier, W. H.; Ziari, M.; Fetterman, H.; Shi, Y.; Mustacich, R. V.; Jen, A. K.; Shea, K. J. Chem. Mater. 1995, 7, 1060-1080. (c) Dalton, L. R.; Harper, A. W.; Ren, A.; Wang, F.; Todorova, G.; Chen, J.; Zhang, C.; Lee, M. Ind. Eng. Chem. Res. 1999, 38, 8. (2) Dalton, L. R.; Harper, A. W.; Robinson, B. H. Proc. Natl. Acad. USA 1997, 94, 4842-4847.
Figure 1. Examples of backward and forward synthesis of phenylpolyene-bridged chromophore.
increase in the wavelength of the charge-transfer absorption λmax. However, the resulting phenylpolyene bridge has poor thermal stability unless the polyene structure is sterically protected. The synthesis of various sterically protected (ring-locked) phenylpolyene chromophores involves cyclic enones such as isophorone, verbenone, and double-ring locked dienone as starting materials and intermediates.4 The Knovenegal coupling reaction between enones and electron acceptors is the critical step in both backward and forward methods reported (see Figure 1 for examples). The low reactivity of enone severely limits the choice of acceptor5 and therefore has become the bottleneck in the development of phenylpolyene-bridged high-β chromophores. In this communication, we report a synthesis of the ring-locked aminophenyldienal donor bridge, which has a very high Knovenegal reactivity, thereby allowing nearly all acceptors bearing acidic methyl or methylene groups to (3) Marder, S. R.; Cheng, L.-T.; Tiemann, B. G. A. C.; Friedli, B.D. M.; Perry, J. W.; Skindhoj, J. Science 1994, 263, 511-514. (b) Jen, A. K.-J.; Wong, K. Y.; Rao, V. P.; Drost, K.; Cai, Y. M. J. Electron. Mater. 1994, 23, 653-657. For a comprehensive discussion on propertystructure correlation, see: Zhang, C. Ph.D. Dissertation, University of Southern California; 1999; Chapter 2. (4) (a) Cabrera, I.; Althoff, O.; Man, H.-T.; Yoon, H. N. Adv. Mater. 1994, 6, 43-45. (b) Ermer, S.; Lovejoy, S. M.; Leung, D. S.; Warren, H.; Moylan, C. R.; Twieg, J. T. Chem. Mater. 1997, 9, 1437-1442. (c) Shu, C.-F.; Tsai, W. J.; Chen, J.-Y.; Jen, A. K.-Y.; Zhang, Y.; Chen, T.-A. J. Chem. Soc., Chem. Commun. 1996, 2270-2280. (d) Chen, J.; Zhu, J.; Harper, A. W.; Wang, F.; He, M.; Mao, S. H.; Dalton, L. R.; Chen, A.; Steier, W. H. Polymer Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1997, 215-216 (ACS Meeting Las Vegas, 1997). (e) Shu, C.F.; Shu, Y.-C.; Gong, Z.-H.; Peng, S.-M.; Lee, G.-H.; Jen, A. K. J. Chem. Mater. 1998, 10, 3284-3286. (5) With the forward method only malononitrile has been successfully utilized. With the backward method only malononitrile, isoxazolone, and thiobarbituric acid have been reportedly utilized.
10.1021/cm9902321 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/10/1999
Communications
Chem. Mater., Vol. 11, No. 8, 1999 1967 Scheme 1. Synthesis of New Phenyltriene-Bridged Chromophoresa
a Reaction conditions: (a) DMF, NBS; (b) tert-dimethylsilyl chloride, imidazole, DMF; (c) n-BuLi, THF; (d) MeMgI, THF; (e) 1 N HCl, acetone; (f) Ac2O; (g) POCl3, DMF; (h) one of the acceptors, EtOH, base.
be coupled. This methodology broadens the scope of polyene-bridged chromophores without significantly sacrificing thermal stability or optical transparency. This synthetic approach leads to the development of device-quality NLO chromophores possessing µβ values (where µ is the chromophore dipole moment) of 15 000 × 10 -48 esu or greater at 1.9 µm as determined by the electric field induced second harmonic (EFISH) generation technique. 6 The synthesis of the donor bridge is shown in Scheme 1. The starting material is N-phenyldiethanolamine (1), which is dihydroxy functionalized to allow for the covalent incorporation of resulting chromophores into various polymer matrices. Compound 1 was brominated with N-bromosuccinimide in quantitative yield. The two hydroxy groups in compound 2 were protected with tertbutyldimethylsilyl (TBDMS) which is chosen because it can tolerate lithiation and Grignard reaction conditions and can be cleaved under a condition mild enough for the vulnerable aminophenylpolyene 6 to survive. Reaction of the enone 4 with methylmagnesium iodide followed with acidic workup gave a mixture of isomers 6, which is consistent with the literature result for a similar reaction.7 Due to electron donation from the amino group, the polyene 5 is very rich in electron density and can undergo polymerization in heated acidic solution. Care must be taken in the dehydration of the tertiary alcohol produced in the Grignard reaction. Direct formylation of polyene 6 by the Vilsmeier-Haack reaction was tried first and very low conversion (∼25%) was obtained. The low conversion was caused by the TBDMS protecting group, which is electron-donating and makes the aniline 6 very basic. It was found that bases such as triethylamine could destroy the formylating intermediate produced from DMF and phosphorus oxychloride. Therefore, compound 6 was deprotected (6) Zhang, C.; Ren, A. S.; Wang, F.; Dalton, L. R.; Lee, S.-S.; Garner, S. M.; Steier, W. H. Polymer Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1999, 40 (1), 49-50. (7) Pines, H.; Kozlowski, R. H. J. Am. Chem. Soc. 1956, 78, 37763783.
Table 1. Physical Properties of Representative Chromophoresa
a T s were obtained from differential thermal analysis, N 10 d 2 °C/min. β values are estimated by assuming the HRS intensity is dominated by the tensor component associated with the charge transfer axis. In this case, βHRS = (6/35)1/2βEFISH.
and reprotected with the electron-withdrawing acetyl group to reduce the basicity of aniline. Although polyene 7 was a mixture of isomers, its Vilsmeier-Haack formylation gave only one isomer 8 due to fast isomerization of 7 under the acidic reaction condition.7 Aldehyde 8 has a very high Knovenegal reactivity and is coupled with various acceptors to give different chromophores. Three representative chromophores made by coupling with weak, medium strong, and strong acceptors are shown in Table 1. We also synthesized chromophore D to compare the thermal stability of ringlocked triene chromophores with a simple triene chromophore. With ring-locking to sterically protect the polyene structure, the thermal stability is improved by
1968 Chem. Mater., Vol. 11, No. 8, 1999
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almost 60 °C to 271 °C (comparing C with D), which is sufficient to survive temperatures encountered in electric field poling and lattice hardening processes. The low decomposition temperature, Td, of B is due to the isoxazolone acceptor which decomposes around 200 °C, independent of the structure of donor bridge.4d Shu et al. have shown that the twisting angle between the phenyl ring and the (fused) cyclohexene ring is only 4° in their phenyltriene-bridged dicyano chromophore.4e The twisting angle in chromophore C is expected to be even smaller since tricyanofuran (TCF) is a much stronger acceptor and a strong acceptor reduces the twisting angle.8 The µβ value of chromophore C was measured in tetrahydrofuran (THF) at 1.9 µm using EFISH generation and d11 of quartz ) 0.4 pm/V as reference.9 Chromophore C has a µβ of 6252 × 10-48 esu, which is very impressive considering the low absorption wavelength of the chromophore. The hyper-Rayleigh scattering intensity of each of the chromophores listed in Table 1 has been measured using 1907-nm laser light. The values of the molecular hyperpolarizabilities β in CHCl3 are given in Table 1. The technique and the method of analysis for HRS have been described elsewhere.10,11 The discussion of HRS of chromophores A-D and other chromophores synthesized in this series will be presented elsewhere.12 We have chosen C for an electrooptical study in the PMMA composite. We obtain r33 of 48pm/V at 1.06 µm for a 15wt % loaded PMMA film.13 The chemical stability of C is excellent. It can survive basic conditions (it was
prepared under a basic reaction condition) and strong acidic conditions (0.1 N HCl/acetone) for 24 h without any detectable decomposition. In summary, a general synthetic method for preparing ring-locked polyene donor-bridged chromophores has been developed. We expect this methodology to be applicable to the syntheses of other polyene systems such as single-ring tetraene14 and two-ring and threering-locked systems, thereby making this type of bridge very attractive for synthesizing chromophores with exceptionally large nonlinearly and good thermal and chemical stability.
(8) Cheng, L.-C.; Tam, W.; Marder, S. R.; Stiegman, A. E.; Rikken, G.; Spangler, C. W. J. Phys. Chem. 1991, 95, 10643-10652. (9) Serbutoviez, C.; Bosshard, C.; Kno¨pfle, G.; Wyss, P.; Preˆtre, P.; Gu¨nter, P.; Schenk, K.; Solari, E.; Chapuis, G. Chem. Mater. 1995, 7, 1198-1206. (10) Pauley, M. A.; Wang, C. H. Chem. Phys. Lett. 1997, 280, 544. (11) Pauley, M. A.; Wang, C. H. Rev. Sci. Instrum. 1999, 70, 1277. (12) Woodford, J. N.; Wang, C. H.; Zhang C.; Dalton, L. R. Manuscript in preparation.
(13) Measured by the ATR method. See: Mao, S. S. H.; Ra, Y.; Zhang, C.; Dalton, L. R.; Chen, A.; Garner, S.; Steier, W. H. Chem. Mater. 1998, 10, 146-155. (14) Zhang, C. Ph.D. Dissertation, University of Southern California, 1999; Chapter 4. The methodology has been applied to the synthesis of a phenyltetrene-bridged TCF chromophore, which has a µβ of 19161 × 10-48 esu at 1.9 µm in THF and gave 85 pm/V in PMMA composite. A Mach-Zelinder interferometer fabricated from this material requires 0.8 V to achieve a phase shift of π.
Acknowledgment. We gratefully acknowledge support for this work by the National Science Foundation (DMR-9528021 and DMR-9818179) and by the U.S. Air Force Office of Scientific Research (F49620-95-1-0450, F49620-96-1-0035, and F49620-97-1-0307). We thank Mr. Ilias Liakatas and Professor Peter Gu¨nter for the EFISH measurements and Dr. Sang-Shin Lee and Professor William H. Steier for electrooptic coefficient, r33, measurements. C.H.W. acknowledges the support of a grant for the Nebraska Research Initiative. Supporting Information Available: Experimental information for the compounds discussed in this communication. This material is available free of charge via the Internet at http://pubs.acs.org. CM9902321
