Synthesis, Crystal Structure, and Second-Order Nonlinear Optical Properties of New Stilbazolium Salts Zhou Yang,*,† Michael Wo¨rle,‡ Lukas Mutter,† Mojca Jazbinsek,† and Peter Gu¨nter† Nonlinear Optics Laboratory, Institute of Quantum Electronics, ETH Zurich CH-8093, Switzerland, and Laboratory of Inorganic Chemistry, ETH Zurich CH-8093, Switzerland
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 1 83-86
ReceiVed July 12, 2006; ReVised Manuscript ReceiVed October 20, 2006
ABSTRACT: Two derivatives of the highly nonlinear optical crystal DAST (4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate) with m-trifluoromethylbenzene-sulfonate (DSMFS) and p-trifluoromethylbenzenesulfonate (DSPFS) replacing the p-toluenesulfonate of DAST have been synthesized and characterized. X-ray crystallographic analyses revealed that the crystal structure of DSPFS is monoclinic Cc and very similar to that of DAST, whereas DSMFS possesses a centrosymmetric P1 structure with two molecules with different orientations on one site. A Kurtz powder test has shown that DSPFS exhibits a large second-order optical nonlinearity 3 orders of magnitude greater than that of the urea standard. Introduction There has been considerable interest in organic nonlinear optical materials that could be used for applications such as optical signal processing and THz generation in the past decades.1-4 Particularly interesting are the molecular crystalline materials because they have several advantages, for example, high chromophore number density and excellent long-term orientational stability compared to poled polymers.5,6 Especially, 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) was shown to exhibit pronounced bulk second-order nonlinear optical activity with a powder second harmonic generation efficiency 1 × 103 times that of the urea standard at 1907 nm.7-9 Research has shown that DAST has the possibility to be further optimized to get new materials that possess higher second-order optical nonlinearities and/or better characteristics for the growth of high-optical-quality single crystals.10,11 Variation of counterions in organic salts has been evidenced to be a simple and highly successful approach for creating materials with very high second-order nonlinear optical properties.12,13 We have reported the synthesis, crystal growth, and nonlinear optical properties of a series of stilbazolium derivatives that were obtained by modifying the structure of counteranions of the DAST salt.14,15 The results show that minor modifications of the counteranion considerably affect the crystal structure and nonlinear optical activity of stilbazolium derivatives. In this study, we report on the synthesis and investigations of two new DAST derivatives, DSMFS and DSPFS (Scheme 1), where m(p)-trifluoromethylbenzenesulfonate replaces the p-toluenesulfonate of DAST. Experimental Section Materials and Methods. All reagents were purchased as high purity from Aldrich and used without further purification. 1H NMR spectra were recorded on a Brucker 300 MHz spectrometer on DMSO-d6 solutions. UV-vis spectra were recorded by a Perkin-Elmer Lambda 9 spectrometer. Elemental analyses were performed by the Microanalytical Laboratory, ETH. Thermal analysis was conducted on a PerkinElmer TGA-7 and DSC-7 spectrometer at a heating rate of 10 °C/min. Synthesis of DSMFS and DSPFS. 4-N,N-Dimethylamino-4′-N′methyl-stilbazolium m-trifluoromethylbenzenesulfonate (DSMFS) and 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium p-trifluoromethylben* To whom correspondence should be addressed. E-mail: zhouyang@ phys.ethz.ch. † Nonlinear Optics Laboratory. ‡ Laboratory of Inorganic Chemistry.
Scheme 1.
Molecular Structure of DSMFS and DSPFS
zenesulfonate (DSPFS) were synthesized by a condensation reaction of 4-methyl-N-methyl pyridinium m(p)-trifluoromethylbenzenesulfonate, which was prepared from 4-picoline and methyl m(p)-trifluoromethylbenzenesulfonate, and 4-N,N-dimethylamino-benzaldehyde in the presence of piperidine. Methyl m(p)-trifluoromethylbenzenesulfonate was synthesized from m(p)-trifluoromethylbenzenesulfonyl chloride by the method described before.14 After drying and heating at 140 °C for 1 h to get rid of water, purification was effected by recrystallizing at least three times from methanol. DSMFS. Yield: 75%. 1H NMR (300 MHz, DMSO-d6): δ 8.69 (d, 2H, J ) 6.3 Hz, C5H4N), 8.03 (d, 2H, J ) 6.3 Hz, C5H4N), 7.91-7.82 (m, 3H, CH + C6H4SO3-), 7.69 (d, 1H, J ) 8.4 Hz, C6H4SO3-), 7.597.56 (m, 3H, C6H4 + C6H4SO3-), 7.17 (d, 1H, J ) 16.2 Hz, CH), 6.78 (d, 2H, J ) 8.4 Hz, C6H4), 4.15 (s, 3H, NMe), 3.00 (s, 6H, NMe2). C, H, N anal. Calcd for C23H23F3N2O3S: C, 59.47; H, 4.99; N, 6.03. Found: C, 59.53; H, 5.04; N, 6.12. DSPFS. Yield: 62%. 1H NMR (300 MHz, DMSO-d6) δ 8.69 (d, 2H, J ) 6.6 Hz, C5H4N), 8.04 (d, 2H, J ) 6.6 Hz, C5H4N), 7.93 (d, 1H, J ) 16.2 Hz, CH), 7.81 (d, 2H, J ) 8.7 Hz, C6H4SO3-), 7.71 (d, 2H, J ) 8.7 Hz, C6H4SO3-), 7.60 (d, 2H, J ) 9.0 Hz, C6H4), 7.19 (d, 1H, J ) 16.2 Hz, CH), 6.80 (d, 2H, J ) 9.0 Hz, C6H4), 4.17 (s, 3H, NMe), 3.02 (s, 6H, NMe). C, H, N anal. Calcd for C23H23F3N2O3S: C, 59.47; H, 4.99; N, 6.03. Found: C, 59.60; H, 4.93; N, 5.94. Crystal Growth. The experiments were performed in a constant temperature bath of controlled stability (0.01 °C.14 Solutions were prepared using methanol (or ethanol) as a solvent and saturated at 3540 °C. After filtering using 0.2 µm porosity Millipore filters, they were preheated to 45-50 °C for 24 h in sealed crystallizer tubes. This procedure was performed to ensure that all ingredients were dissolved. The programmed cooling rate was 0.2-0.3 °C/day. Powder SHG Measurements. The second harmonic generation (SHG) powder tests were carried out as described previously.14 The microcrystalline powdered samples were prepared by sieving the material to a particle size of 63-90 µm and squeezing it into a 1.00 mm Hellma UV quartz sample cell to give a constant sample thickness. The SHG signals were measured with respect to 63-90 µm powdered urea samples as reference. An idler wave with a wavelength λ ) 1907 nm from an optical parametrical amplifier pumped by an amplified Ti:sapphire laser was used as fundamental wave to determine the nonresonant SHG efficiency. X-ray Crystallography. Data sets were obtained using a Bruker SMART Platform Diffractometer equipped with a CCD detector (graphite-monochromated Mo-KR radiation, λ ) 0.7107 Å) at 293 K
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84 Crystal Growth & Design, Vol. 7, No. 1, 2007
Yang et al.
Figure 1. Crystals of (a) DSMFS and (b) DSPFS. Table 1. Properties of DSMFS, DSPFS, and DAST sample
melting point (°C)
λmaxa (nm)
solubilityb (g/100 g)
SHG efficiencyc
DSMFS DSPFS DAST
270 ( 1 242 ( 1 256 ( 1
475 474 475
3.5 9.1 4.2
0 900 1000
a In methanol solution at room temperature. b In methanol solution at 45 °C. c Relative to urea ) 1.
Table 2. Crystallographic Data of DSPFS, DSMFS, and DAST Crystals
formula FW cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) R1 wR2
DSPFS
DSMFS
DAST
C23H23 F3N2O3S 464.49 monoclinic Cc 10.511(3) 11.229(2) 18.326(3) 90 91.90(1) 90 2161.6 0.055 0.164
C23H23 F3N2O3S 464.49 triclinic P1 7.977(1) 9.713(1) 16.023(2) 98.03(1) 98.32(1) 113.98(1) 1095.0(3)
C23H26N2O3S 410.52 monoclinic Cc 10.365(3) 11.322(4) 17.892(4) 90 92.24 90 2099.7 0.069 0.185
(DSPFS) and 233 K (DSMFS), respectively. The structure was solved by direct methods.16 The R-values of the full-matrix least-squares refinements17 are given in Table 2. All atoms except hydrogen atoms and atoms of disordered molecules were refined anisotropically. H-atoms were placed at calculated positions on the basis of stereochemical considerations and refined according to the riding model. CCDC 614574 and 614575 contain the supplementary crystallographic data for DSMFS and DSPFS, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Results and Discussion Synthesis and Characterization. DSMFS and DSPFS were successfully prepared by a three-step reaction, and their structures were confirmed by 1H NMR and elemental analysis.
Figure 2. Crystal packing diagram: DSPFS with a monoclinic Cc symmetry viewed along the a-axis (up) and along the c-axis (down).
Figure 3. Electron density map in the cation molecule plane in DSMFS. The observed electron density was interpreted by assuming a superposition of two possible cation orientations (solid and dashed bonds) with frequencies of 46 and 54%, respectively.
Products of DSMFS and DSPFS absorb moisture easily to form a hydrate of yellow color. The hydrated form loses all the water after heating for 1 h at 140 °C with a color change from yellow into red (DSMFS) or deep red (DSPFS). Table 1 summarizes the main properties such as melting point and solubility (at 45 °C) of DSMFS and DSPFS compared with DAST. Although DSMFS and DSPFS have very similar structures with only a different substituent position of the counteranion, an almost
New Stilbazolium Salts
Crystal Growth & Design, Vol. 7, No. 1, 2007 85
Figure 4. Crystal packing diagram of DSMFS viewed along the a-axis; each picture shows only one of the two possible orientations of the cations.
30 °C difference in the melting point was observed. However, they show nearly the same λmax in methanol because they have the same cationic chromophore. From the table, we can also see that DSMFS possesses a solubility slightly less than DAST in methanol at the same temperature, whereas DSPFS possesses a high solubility that is more than two times that of DAST. Crystal Growth. The slow cooling method was adapted for the growth of optical quality DSMFS and DSPFS crystals. Although the conditions of the crystal growth have not yet been fully optimized, first red colored hexagonal DSMFS single crystals with sizes of 3 × 3 × 0.2 mm3 could be successfully grown from methanol solution (Figure 1a). In addition to methanol, DSPFS can also be dissolved well in ethanol. Therefore, DSPFS crystal growth experiments from both of these solvents were performed. In ethanol, a lot of small crystals at the bottom of the vessel were always formed, and after a continuous decrease in the temperature, the number of nuclei still increased, indicating a small metastable zone width of the solution and thus difficult control for the growth of the crystals suitable for optical applications. However, it was found that in methanol, DSPFS nucleated even faster than DAST because of its much higher solubility; crystals with sizes of 5 × 5 × 1 mm3 could be easily obtained after a few days (Figure 1b). These bulk crystals still contained lots of defects as a cost of the fast growth. However, crystals with sizes of less than 1 mm always possess a very good optical quality. X-ray Crystallography. Crystallographic data of DSPFS and DSMFS are listed in Table 2, together with the corresponding data of DAST for comparison. DSPFS and DAST have similar crystal packing. A similar structure has also been reported for another DAST derivative: DASC with a p-chlorobenzenesulfonate counteranion.18 Figure 2 shows the crystal structure of DSPFS. The tilting angle between the cation’s long axis and the polar a-axis of the crystal is θ ≈ 20°, which is the equal to the one found in DAST.7 As a result, the optical and nonlinear optical properties of DSPFS crystals are expected to be similar as that of DAST, which is confirmed by the SHG measurement. For DSMFS, the cations are disordered, as can be seen from the electron density map in the plane of the 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium cations (Figure 3). The disorder could be resolved by assuming a superposition of two orientations (46 and 54% frequency) of the 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium cations on the same site, generated by a rotation of 180o around the long axis of the molecule (Figure 4). Kurtz Powder Test. We measured the relative second harmonic generation (SHG) activity of DSMFS and DSPFS using the Kurtz powder technique.19 A fundamental wave with a wavelength λ ) 1907 nm (from an optical parametrical amplifier pumped by an amplified Ti:sapphire laser) was used to determine the SHG efficiency. The backscattered light at the
second-harmonic wavelength (953.5 nm) was detected with a Si photodiode, which was not sensitive to the fundamental wavelength. Contributions from third-harmonic generation (λ ) 635.6 nm) were eliminated with appropriate filters. For a better comparison, we use material that has been sieved to a similar particle size distribution (63-90 nm). By this method, we obtained an SHG efficiency of DSPFS that was 900 times that of urea, whereas DSMFS has shown no SHG efficiency. The result is in agreement with the crystallographic structure determined above: centrosymmetric for DSMFS and noncentrosymmetric monoclinic with similar chromophore packing as in DAST for DSPFS. Conclusion We have synthesized two new organic nonlinear optical crystals: DSMFS and DSPFS. Single crystals of these two materials that have the same cation chromophore but different anions as compared to DAST have been grown from methanol by a slow cooling technique. The solubilities and growth rates have not yet been optimized, but it has been shown that DSPFS can be grown faster than DAST crystals because DSPFS has a higher solubility at the same temperature. X-ray crystallographic analyses and a Kurtz powder test have been carried out on these two materials. It was found that DSPFS, with p-trifluoromethylbenzenesulfonate replacing the p-toluenesulfonate of DAST, exhibits a similar crystal structure and optical nonlinearity as DAST. However, with m-trifluoromethyl-benzenesulfonate as counteranion (DSMFS), the crystal structure shows disorder of the cationic chromophores and a centrosymmetric structure. These results revealed that the crystal packing of stilbazolium salts is very sensitive to the nature of the counteranions. Also, although they have a very similar counteranion structure with only a different position of the F3C- group, the two compounds exhibit a melting point difference of about 30 °C and solubility difference of more than two times. Therefore, further modification of the counteranions is expected to lead to optimized stilbazolium salt structures with improved nonlinear optical properties and physical and chemical properties that are important for single-crystal growth and further material processing for applications. Acknowledgment. This work was supported by the Swiss National Science Foundation. References (1) Bosshard, Ch.; Bo¨sch, M.; Liakatas, I.; Ja¨ger, M.; Gu¨nter, P. In Nonlinear Optical Effects and Materials; Gu¨nter, P., Ed.; SpringerVerlag: Heidelberg, Germany, 2000. (2) Bosshard, Ch.; Sutter, K.; Preˆtre, Ph.; Hulliger, J.; Flo¨rsheimer, M.; Kaatz, P.; Gu¨nter, P. Organic Nonlinear Optical Materials; Gordon and Breach Publishers: Newark, NJ, 1995.
86 Crystal Growth & Design, Vol. 7, No. 1, 2007 (3) Molecular Nonlinear Optics: Materials, Physics, and DeVices; Zyss, J. Ed.; Academic Press: New York, 1994. (4) Materials for Nonlinear Optics: Chemical PerspectiVes; Marder, S. R., Stucky, G. D., Sohn, J. E., Eds.; ACS Symposium Series 455; American Chemical Society: Washington, DC, 1991. (5) Marder, S. R.; Perry, J. W.; Schaefer, W. P. Chem. Mater. 1994, 6, 1137. (6) Coe, B. J.; Harris, J. A.; Clays, A. K.; Olbrechts, G.; Persoons, A.; Hupp, J. T.; Johnson, R. C.; Coles, S. J.; Hursthouse, M. B.; Nakatani, K. AdV. Funct. Mater. 2002, 12, 110. (7) Marder, S. R.; Perry, J. W.; Schaefer, W. P. Science 1989, 245, 626. (8) Pan, F.; Wong, M. S.; Bosshard, Ch.; Gu¨nter, P. AdV. Mater. 1996, 8, 592. (9) Adachi, H.; Takahashi, Y.; Yabuzaki, J.; Mori, Y.; Sasaki, T. J. Cryst. Growth 1999, 198/199, 568. (10) Okada, S.; Masaki, A.; Matsuda, H.; Nakanishi, H.; Kato, M.; Muramatsu, R.; Otsuka, M. Jpn. J. Appl.Phys. 1990, 29, 1112. (11) Coe, B. J.; Harris, J. A.; Asselberghs, I.; Wostyn, K.; Clays, K.; Persoons, A.; Brunschwig, B. S.; Coles, S. J.; Gelbrich, T.; Light, M. E.; Hursthouse, M. B.; Nakatani, K. AdV. Funct. Mater. 2003, 13, 347.
Yang et al. (12) Meredith, G. R. In Nonlinear Optical Properties of Organic and Polymeric Materials; Williams, D. J., Ed.; ACS Symposium Series 233; American Chemical Society: Washington, DC, 1983; pp 2756. (13) Glavcheva, Z.; Umezawa, H.; Mineno, Y.; Odani, T.; Okada, S.; Ikeda, S.; Taniuchi, T.; Nakanishi, H. Jpn. J. Appl. Phys. 2005, 44, 5231. (14) Yang, Z.; Aravazhi, S.; Schneider, A.; Seiler, P.; Jazbinsek, M.; Gu¨nter, P. AdV. Funct. Mater. 2005, 15, 1072. (15) Ruiz, B.; Yang, Z.; Jazbinsek, M.; Gramlich, V.; Gu¨nter, P. J. Mater. Chem. 2006, 16, 2839. (16) Sheldrick, G. M. SHELXL-97 Program for the Refinement of Crystal Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (17) Sheldrick, G. M. SHELXL-97 Program automatic solution of Crystal Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (18) Umezawa, H.; Okada, S.; Oikawa, H.; Matsuda, H.; Nakanishi, H. Bull. Chem. Soc. Jpn. 2005, 78, 344. (19) Kurtz, S. K.; Perry, T. T. J. Apply. Phys. 1968, 39, 3798.
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