Composites - ACS Symposium Series (ACS Publications)

Chapter 34

Composites Novel Materials for Second Harmonic Generation Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 1, 2015 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch034

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C. B. Aakeröy , N. Azoz , P. D. Calvert , M . Kadim , A. J . McCaffery , and K. R. Seddon 1

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School of Chemistry and Molecular Sciences, University of Sussex, Falmer, Brighton BN1 9QJ, United Kingdom Arizona Materials Laboratory, University of Arizona, Tucson, AZ 85712

Herein is described a new class of materials for second harmonic generation (SHG), in which microcrystals of an SHG-active material (guest) are deposited within a polymer matrix (host) in an aligned fashion. The guest crystallites range from 3-nitroaniline (mNA) to a new class of hydrogen-bonded dihydrogenphosphate salts, [AH][H PO ] (where A - an amine). These latter materials have a range of physical properties that make them highly suited as SHG-active guest crystals. The guest crystals are aligned within the polymer matrix, using a thermal gradient technique, a method which produces transparent, non-scattering, flexible, SHG-active composites, with excellent temporal stability. 2

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The field of nonlinear optics (NLO) is currently one of the most active in terms of research intensity and funding. However, despite many heroic efforts, both theoretical and experimental, the most common SHG-active materials that are in use today, potassium dihydrogenphosphate (KDP) and lithium niobate(V), LiNb0 , were discovered when the field was in its infancy. Efforts now have to be concentrated on the development of processable, nonlinear optical materials, and this has to be achieved on a very short time scale. 3

Material Requirements for Second-Harmonic Generation In order to bridge the gap between an SHG-active material and one optimized for use in an optoelectronic device, many compounds have been synthesized, characterized, modified and then ultimately rejected during the past decade (1-3). This is not surprising, since the ideal material must fulfil a plethora of stringent requirements (4-7). The most critical condition for an SHG-active material is that it must form noncentrosymmetric structures; however, thermal stability, involatility, transparency, lack of colour, mechanical strength and crystal habit are also crucial properties for materials to be incorporated into practical devices. We present here an overview of our recent work, in which two novel approaches to new materials for SHG have been combined to yield composites exhibiting quite remarkable optical properties.

0097-6156/91/0455-0516$06.00/0 © 1991 American Chemical Society In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 1, 2015 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch034

Hydrogen-Bonded Salts: Novel SHG-Active Compounds Current Materials. Even though the nonlinear coefficients among inorganic materials are generally significantly lower than those found for organic materials, it has been possible to grow good quality single crystals of several inorganic compounds, making them available as bulk media for utilization in conversion processes in laser-operated systems. However, there has been a great increase in interest in organic materials for SHG and, as a result, many novel organic materials have been synthesized and characterized during the last fifteen years (8-13). There has also been much recent interest in organometallic compounds (14.15). but Kanis et al. (Boston ACS Meeting, 1990, Abstract INOR 472) have cast doubts on their practicality. Unfortunately, various factors are hampering the efficiency of materials from these principal classes. Inorganic compounds often exhibit low χ( ) values, restricted birefringence, and limited solubility. Organic molecules are, potentially, more versatile due to larger β-values, and the possibilities of specifically designing molecules for high SHG activity, e.g. combining large polarizability with the presence of substituents capable of charge transfer. However, they frequently suffer from volatility, low thermal stability and mechanical weakness. Organometallic compounds are usually strongly coloured. 2

Novel Materials. Despite the fact that many organic molecules have very high β-values, their x( )-values are often very small. The reason for this is that a high /3-value is, usually, accompanied by a large molecular dipole moment. The large dipole moment encourages the molecules to form pairs, aligned in an antiparallel fashion, which usually favours centrosymmetric crystal forms, thereby ruling out the possibility of SHG-activity. If highly polarizable organic molecules, with large second-order molecular coefficients, could be prevented from forming unfavourable crystalline structures, their full potential could then be utilized. One very successful approach has been to incorporate these molecules into zeolitic frameworks (16). Our approach has been to incorporate anions and cations into the crystal structure which are capable of forming a strong, three-dimensional network of hydrogen bonds, in the hope that this additional lattice force would overwhelm the propensity for dipole alignment and thus increase the probability of forming noncentrosymmetric crystals. To this end, we have designed a new range of salts [AH][H POJ (17), combining a cation derived from an organic amine (e.g. A = benzylamine, 3-hydroxy-6-methylpyridine, or piperidine), with an inorganic anion, dihydrogenphosphate, which is capable of forming strong hydrogen-bonded crystal structures. The only previously known compound of this type was L-argininium dihydrogenphosphate monohydrate, [(H N) CNH(CH ) CH(NH )COO][H P0 ].H 0 2

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(Ifi). In initial studies, two dozen salts of formula [AH][H P0 ] (A = primary, secondary, or tertiary amine) were prepared and screened for SHG activity, using the powder technique (19). The measured SHG intensities of the organic salts of dihydrogenphosphate (17) are, in general, not particularly high (in the range 0.2-5, relative to a-Si0 ). However, this is not surprising, as the amines selected are not specifically designed to produce large nonlinear effects. There is a high incidence (eight out of twenty-four) of SHG-active materials among this class of materials. A success rate of 33% with regard to noncentrosymmetric structures is significantly higher than the expected statistical average {oft quoted as 20%(L2)}; a 50% success rate was found for a recently reported series of stilbazolium salts (20). Clearly, these dihydrogenphosphates must only represent a small fraction of the total number of SHG-active materials in this 2

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In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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class of salts. Even though the materials studied exhibit rather small nonlinear responses, they are all colourless, chemically stable, in vola tile and soluble, and also show a propensity for the growth of good quality crystals. Structure and Hydrogen Bonding. In order to provide information about the presence and extent of hydrogen bonding within these novel salts, X-ray crystallographic studies were undertaken on single crystals of five of these dihydrogenphosphate salts (17). It was found that each structure was dominated by chains or sheets (e.g. Figure 1) of dihydrogenphosphate anions, invariably held together by short hydrogen bonds (17). Not only were strong hydrogen-bonded networks between the anions detected, but the disposition of the cations was dominated by strong hydrogen bonds between the cations and the anion lattice. Lattice Energy Calculations. Even though the crystal structures of the dihydrogenphosphate salts contain a number of seemingly strong hydrogen-bonded interactions, no explicit information about the energetic contribution made by hydrogen bonding to the overall lattice energy of the materials can be obtained from the crystal structures alone. In order to acquire this information, lattice energy calculations were carried out on four dihydrogenphosphate salts (Aakerôy, C ; Leslie, M . ; Seddon, K.R., to be published). The calculations were performed with the CASCADE suite of programmes, written and developed by Leslie at SERC Daresbury Laboratory (21), and designed specifically for the facilities of the CRAY-1 computer. The results of these calculations are summarized in Table I. The calculated lattice energies, i / , of the four salts show that three of them have very comparable values, whereas the lattice energy of 3-hydroxy-6-methylpyridinium dihydrogenphosphate is significantly lower. This salt also has the largest unit cell volume per empirical formula unit, which is a measure of the packing efficiency throughout the structure. The presence of the methyl group increases the bulk of the cation, and makes close-packing of the ions more difficult. Based on a wide range of experimentally determined values for hydrogen bond energies between ions (which are significantly higher than corresponding values for hydrogen bonds between neutral molecules) (22), each O-Η...Ο interaction was assigned an energy content of 35 kJ mol" , and each N-H...O interaction was assigned a value of 30 kJ m o i . By using these values (which underestimate the probable true values by approximately 50%), combined with the appropriate number and type of hydrogen bonds in each salt, an approximate minimum estimate of the total hydrogen bond energy, E ^ , for each salt was obtained, Table I. As shown in Table I, the energetic contributions made by hydrogen bonding to the total lattice energy, U of organic salts of dihydrogenphosphate is considerable. The minimum contributions, ô^g, lie in a range of 20-25%. c a l

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toV

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Hydrogen bond contributions (kJ moP ) to the total lattice energy of four dihydrogenphosphate salts, [AH][H P0 ] a

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Piperidine 3-HOpy 3-H0-6-Mepy 4-H0py

"cal

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500 545 410 515

130 135 135 135

630 680 545 650

21 % 20 % 25 % 21 %

0.2139 0.1996 0.2159 0.2039

Energy terms are defined in the main text. 3-HOpy » 3-hydroxypyridine; 4-HOpy = 4-hydroxypyridme; 3-HO-6-Mepy = 3-hydroxy-6-methylpyridine. V = unit cell volume; Ζ = number of empirical formula units per unit cell; units of nm .

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It should be emphasized that the estimated relative contributions of Ô^Q represent the lowest possible level, as we have consistently (and quite deliberately) adopted values, at every stage of these calculations, that have minimized the magnitude of the hydrogen bond interactions; a more realistic consideration of 6^ would place it significantly above 30%. Although hydrogen bonding itself should not have a preference for symmetric or asymmetric structures, we believe the primary effect of hydrogen bonding interactions, in these salts, on the packing of a structure is to overwhelm the dipole-dipole interactions, which do have a preference (for a centrosymmetric structure). The results presented here would indicate that the hydrogen bonding, in removing a preference for centrosymmetry, will appear to favour noncentrosymmetry. Certainly, the size of its contribution to the overall lattice energy leaves beyond any reasonable doubt the fact that it must have a deterministic effect on the final structure. Indeed, the prevailing factor in the structures of these salts is the hydrogen bonding within the three-dimensional network of the anions, which itself determines the final locations of, and interactions with, the cations. Composites from Melts: A New Class of SHG-Active Materials Rationale. In order to prepare a processable SHG-active material, it is highly desirable to improve on the physical and chemical parameters of current materials. In nature, many composites (e.g. bone, teeth, and shells) exist with very high loadings of guest crystals within a host matrix (often approaching loadings of 95%) (23). This enables nature to combine the desirable properties of both guest and host in a new composite material. Moreover, in the natural materials, a remarkable degree of alignment of the crystals of guest material is often achieved. Our approach was to mimic nature, and to create a new class of materials, 'tailor-designed' for a combination of optical and mechanical properties. The optical (in the cases described here, we limit the optical properties to NLO properties, and specifically SHG properties - this is not an inherent limitation on the technique, which could be employed in many other optical {and electrical} applications) properties are to be provided by the guest crystals and the physical strength and flexibility to be provided by the host polymer. The main difficulty with such an approach lies in trying to align the SHG-active guest crystals within the polymer matrix. Unless alignment is achieved, light scattering from the microcrystals (due to disorientation, reflection and refraction) will render the composite useless. In addition, it is very important to maximize the loading degree (i.e. the guest/host ratio), as the total nonlinear response is proportional to the amount of SHG-active material present. Finally, the importance of matching the refractive indices of the guest and host materials cannot be overemphasized. Early attempts at aligning molecules within a polymer matrix involved film stretching (24.25) or electric field poling (26-28). but neither method initially met with significant success. However, recent studies of SHG-active polymers (29.30) and low-concentration guest-host composites (31.32) have resulted in superior materials with greatly improved temporal stability. Preparation of SHG-Active Composites. We have developed a new technique, including the construction of a device, which has made it possible to grow crystals of 3-nitroaniline (mNA), in a matrix of poly(methyl methacrylate) (PMMA) or poly(vinylcarbazole) (PVK) in an aligned fashion, to produce transparent, SHG-active composites (33). This approach is based upon the Temperature



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Gradient Zone Melting (TGZM) method (34.35). which is well known in many related applications, but does not appear to have been applied to the production of composites, particularly for electrooptic applications. The composites were prepared from solutions of mNA and PMMA with varying loading degrees (between 30 wt % and 90 wt %), using toluene as a solvent. Thin films (30-40 μπι) were cast on a glass slide and the solvent was allowed to evaporate. The film was then covered with a second slide and placed in the sample channel at the heated end of the thermal gradient device. For the mNA/PMMA composites, the heated section was maintained at 150 *C, which is above both the melting point of the mNA crystals (114 *C) and the glass transition point (T ) of the polymer (105 *C), but below the decomposition point of both materials. The cooled block was kept at 20 *C, well below the the melting point of mNA and the Τ of PMMA. The softened, %ut not completely melted, sample was then drawn slowly from the hot end across the thermal junction. By optimizing the relevant variables (e.g. loading degree, temperature differential, and drawing speed), the guest material crystallizes in a line within the polymer as it traverses the thermal gradient. The crystals of mNA adopt a needle-like habit, which is strongly aligned along the drawing axis (the direction of the thermal gradient). The degree of crystal alignment in the resulting composite is a critical function of the variables listed above, which must be individually optimized for each guest-host system. g

Characterization of SHG-Active Composites. The alignment of the guest crystals within the polymer matrix is the dominating factor in terms of eliminating light scattering from a composite. This is clearly illustrated in Figure 2, which show the angular distribution of the second harmonic (SH) intensity as a function of the alignment of the sample. Indeed, even better results were achieved when using PVK (as opposed to PMMA), as its refractive index is a better match for that of mNA. Well-aligned samples of mNA/PVK display an SHG intensity which is approximately 600 times that of a powdered sample of KDP (sample thickness = 250 /mi); further improvement of the SHG-efficiency can be anticipated by using phase-matching techniques, such as birefringence or host-index modification. These new composites have excellent chemical and optical stability. The samples prepared have shown no change in transparency, composition or SHG-efficiency (or directionality) over a period of more than two years. In contrast to electric field poled composites (whose SHG activity usually decays in a period measured in hours or days, rather than years), these materials have superior temporal stability. Composites from Solution: A Superior Class of SHG-Active Composites Rationale. The composites described in the previous section represent an important new discovery. However, although extremely promising, the approach places several constraints on the guest material which will provide the nonlinear response. In addition, a prerequisite of the method is that the guest material melts cleanly at temperatures below the melting point of the polymer host. As, in a real device, the requirement for thermal stability may approach 320 *C (see Lytel and Lipscomb, elsewhere in this volume), this places an almost prohibitive restriction upon potential organic guest materials, both in terms of preparing the composites from a melt and in terms of the stability of the guest under operating and assembly conditions. For these reasons, we have developed a method for preparing composites which does not entail melting the guest material; in principle, a refractory material can be incorporated with this new methodology (vide infra), providing that it is soluble in a solvent in which the polymer also dissolves.



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Figure 2a. Angular dependence of SHG response (relative to KDP) of an mNA/PMMA (50 wt %) composite; this sample contains sphemlitic crystals.



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Figure 2b. Angular dependence of SHG response (relative to KDP) of an mNA/PVK (50 wt %) composite; this sample contains poorly aligned needle crystals.


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Figure 2c. Angular dependence of SHG response (relative to KDP) of an mNA/PVK (50 wt %) composite; this sample contains well-aligned needle crystals.

Preparation of SHG-Active Composites. A thermal gradient apparatus was utilized to grow aligned crystals from solution (rather than from melt, as described above), by drawing the sample from the cold side (below a temperature at which the solvent evaporates at a significant rate) towards the hot end. Using this novel solution-based method, we have been able to incorporate [ C H C H N H ] [ H P 0 ] , BADP (77), an SHG-active material, into polymeric hosts such as poly(acrylamide) (PAA) and poly(ethylene oxide) (PEO) to produce transparent, colourless, low scattering SHG-active composites with excellent temporal stability. 6

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Characterization of SHG-Active Composites. The nonlinear response of a well-aligned BADP/PAA composite is fifty-times higher than that of a powdered sample of the guest material, BADP, itself. This is the first example of a solution-grown SHG-active composite containing thermally aligned microcrystals. In contrast to the mNA/PVK composites, which are yellow, these films are completely colourless. Moreover, they are transparent, and do not seem to be damaged by the incident laser beam, even at high intensities.



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The properties which make benzylammonium dihydrogenphosphate such a good guest material in the above composite are a combination of good solubility, transparency, suitable refractive index (which improves the refractive index matching between guest and host), and a propensity to form needle-like crystals. The last factor appears to be important for obtaining an alignment of the SHG-active crystals within a polymer matrix (disc-like crystals cannot be easily aligned). Angular distribution measurements of the SHG were carried out on BADP/PAA composites, in a manner similar to that described for mNA/PMMA. Analogous behaviour was observed, and Figure 3 illustrates the distribution obtained from a well-aligned sample. In this case, the angular distribution of the SH is very narrow, and most of the SH flux is confined to a narrow cone in the forward direction.

Figure 3. BADP/PAA crystals.

Angular dependence of SHG response (relative to a-Si0 ) of a (70 wt %) composite; this sample contains well aligned needle 2


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Summary The composite materials described here have a wide range of chemical and physical properties which make them prime candidates for the incorporation into optoelectronic devices. The development of two synthetic routes, melt-growth and solution-growth, have expanded the range of potential guest materials from organic molecular solids to ionic compounds. In addition, a range of different polymers can be utilized as host material. The development of novel SHG-active dihydrogenphosphate salts also means that we can achieve a fine tuning between physical properties of the guest and host. Acknowledgments We are indebted to BP Venture Research for funding this work, and to the Iraqi Government for two research scholarships (to N.A. and M.K.). Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Nonlinear Optical Properties of Organic Molecules and Crystals; Chemla, D.S.; Zyss, J . , Eds.; Academic Press: Orlando, 1987; Vols 1 and 2. Nonlinear Optical Properties of Organic and Polymeric Materials; Williams, D.J., Ed.; ACS Symp. Ser. 233; American Chemical Society: Washington D.C., 1983. Rez, I.S. Sov. Phys. Usp. 1968, 10, 759. Zyss, J . J. Mol. Electron. 1985, 1, 24. Williams, D.J. Angew. Chem. Int. Ed. Engl. 1984, 23, 690. Hulme, K.F. Rep. Progr. Phys. 1973, 36, 497. Allen, S.; Murray, A.T. Phys. Scr. 1988,T23,275. Nicoud, J.F. Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt. 1988, 156, 257. Nicoud, J.F. In Nonlinear Optical Properties of Organic Materials; Khanarian, G., Ed.; SPIE: Bellingham, Washington, 1988, Vol. 971; p. 681. Twieg, R.J.; Azema, Α.; Jain, K.; Cheng, Y.Y. Chem. Phys. Lett. 1982, 92, 208. Jain, K.; Crowley, J.I.; Hewig, G.H.; Cheng, Y.Y.; Twieg, R.J. Opt. Laser Technol. 1981 [Dec], 297. Singer, K.D.; Sohn, J . E . ; King., L.A.; Gordon, H.M.; Katz, H.E.; Dirk, C.W. J. Opt. Soc. Am. Β 1989, 6, 1339. Garito, A.F.; Singer, K.D. Laser Focus 1982, 18, 59. Tam, W.; Wang, Y.; Calabrese, J.C.; Clement, R.A. In Nonlinear Optical Properties of Organic Materials; Khanarian, G., Ed.; SPIE: Bellingham, Washington, 1988, Vol. 971; p. 107. Green, M.L.H.; Marder, S.R.; Thompson, M.E.; Bandy, J.Α.; Bloor, D.; Kolinsky, P.V.; Jones, R.J. Nature 1987, 330, 360. Cox, S.D.; Gier, T . E . ; Stucky, G.D.; Bierlein, J . J. Am. Chem. Soc. 1988, 110, 2986. Aakerôy, C.B.; Hitchcock, P.B.; Moyle, B.D.; Seddon, K.R. J. Chem. Soc., Chem. Commun. 1989, 1856. Xu, D.; Jiang, M.; Tan, Z. Huaxue Xuebao 1983, 41, 570; Acta Chim. Sinica.., 1983, 2, 230. Kurtz S.K.; Perry, T.T. J. Appl. Phys. 1968, 39, 3798. Marder, S.R.; Perry, J.W.; Schaefer, W.P. Science 1989, 245, 626. Leslie, M. SERC Daresbury Lab. Rept., DL-SCI-TM31T, 1982.



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22. Meot-Ner (Mautner), M. In Molecular Structure and Energetics; Liebman, J.F.; Greenberg, Α., Eds.; VCH: New York, 1987, Vol.4; pp. 72-103. 23. Biomineralization; Mann, S.; Webb, J.; Williams, R.J.P., Eds.; VCH: Weinheim (Ger.), 1989. 24. Azoz, N.; Calvert, P.D.; Moyle, B.D. In Organic Materials for Non-Linear Optics; Hann, R.A; Bloor, D., Eds.; Royal Soc. Chem.: London, 1989; pp. 308-314. 25. Calvert, P.D.; Moyle, B.D. Mat. Res. Soc. Symp. 1988, 109, 357. 26. Singer, K.D.; Sohn,J.E.;Lalama, S.J. Appl. Phys. Lett. 1986, 49, 248. 27. Pantelis, P.; Davies, G.J. US Patent 4748074, 1988. 28. Pantelis, P.; Davies, G.J. US Patent 4746577, 1988. 29. Eich, M.; Reck, B.; Yoon, D.Y.; Willson, C.G.; Bjorklund, G.C. J. Appl. Phys. 1989, 66, 3241. 30. Singer, K.D.; Kuzyk, M.G.; Holland, W.R.; Sohn, J.E.; Lalama, S.J.; Comizzoli, R.B.; Katz, H.E.; Schilling, M.L. Appl. Phys. Lett. 1988, 53, 1800. 31. Miyazaki, T.; Watanabe, T.; Miyata, S. Jpn. J. Appl. Phys. 1988, 27, L1724. 32. Lytel, R.; Lipscomb, G.F.; Stiller, M.; Thackara, J.I.; Ticknor, A.J. In Nonlinear Optical Properties of Organic Materials; Khanarian, G., Ed.; SPIE: Bellingham, Washington, 1988, Vol. 971; p. 218. 33. Azoz, N.; Calvert, P.D; Kadim, M.; McCaffrey, A.J.; Seddon, K.R. Nature, 1990, 344, 49. 34. Pfann, W.G. Zone Melting; 2 Edit.; Wiley: New York, 1958; pp. 254-268. 35. Herington, E.F.G. Zone Melting of Organic Compounds; Blackwell: Oxford, 1963. nd

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