Chapter 46
Polymers and an Unusual Molecular Crystal with Nonlinear Optical Properties
Downloaded by UNIV OF NORTH CAROLINA on October 23, 2014 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch046
F. Wudl, P.-M. Allemand, G. Srdanov, Z. Ni, and D. McBranch Departments of Chemistry and Physics, University of California, Santa Barbara, CA 93106
In the recent past, conjugated polymers were found to have very fast, subpicosecond nonlinear optic response and χ on the order of 10 esu. We have been working on the synthesis of processible conjugated polymers in relation to their electrical conductivity properties. Once processibility was established, we were able to prepare thin films which were suitable for optical measurements. The syntheses of these conjugated polymers and of the monomers will be described. In a different project involving organic ferromagnetism, we found two compounds whose solid state structure was non-centrosymmetric. In one case the molecule is polar with two dipolar moieties (nitronylnitroxide and nitro) pointing in the same crystallographic direction over the whole lattice. In another case the molecule is symmetrical, yet the lattice is polar. The properties of these molecular solids are described including second harmonic generation (SHG). (3)
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This presentation is divided into two parts, one dealing with third harmonic generation (THG) non linear optical (NLO) materials and the other dealing with second harmonic generation (SHG) materials. A New Conjugated Polymer for T H G Applications Conjugated polymers are very fast response N L O materials. The most studied are the poly(diacetylenes) and polyacetylene. Poly(para-phenylenevinylene) (PPV) is a conjugated backbone polymer which can be processed through a water soluble precursor polymer by use of the Wessling-Zimmerman method(l-3). Once the conjugated backbone is obtained, the yellow polymer has excellent mechanical properties but is intractable. In the recent past, soluble conjugated PPV's have been obtained by the introduction of long chain alkoxy groups(4j). In our hands even the dioctyloxy substituted, high molecular weight, P P V was soluble only in hot chlorobenzene. In order to be able to fabricate optically smooth films by spin casting, we required a genuinely, ambient temperature soluble P P V in solvents such as cyclopentanone. We reasoned that if the two alkoxy groups were of disparate size and if one of the alkoxy groups had a branch, the solubility of the polymer would be
0097-6156/91/0455-0683$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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enhanced considerably because the ensuing asymmetry built onto the macromolecule's stiff backbone would prevent it from packing in an ordered fashion. Noting that hydroquinone monomethyl ether is commercially available and that the 2-ethylhexyl moiety has been used extensively in the past as part of plasticizer ingredients in commercial polymer blends, we decided that the title polymer should be obtained relatively easily and should have the desired properties. Results and Discussion
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The target polymer was produced by the "traditional" precursor approach(l) as depicted in Scheme I, below.
Prec. Pol. e PiOMPV +
R = CH2CH(CH CH3)C4H ; a, R-Cl, MeOO/MeOH; b, CH 0«H 0 Cl-/dioxane; c, THT/MeOH; d, NaOH/MeOH; e, A/l,2,4-trichlorobenzene. 2
9
2
3
Scheme I The polymer obtained by this procedure is a red powder, insoluble in methanol and ethanol but soluble in THF, benzene, chlorobenzene, cyclopentanone and other nonpolar organic solvents. The polymer had a molecular weight of -300,000 with a polydispersity of - 4 as determined by GPC relative to polystyrene standard. A l l spectroscopic properties are in accord with the proposed structure. Some of the properties are solvent dependent. For example, the polymer is thixsotropic in benzene. Very smooth films can be cast from THF. Free standing films have the appearance of "red cellophane". Preparation of the Precursor Polymer A solution of 200 mg (0.39 mmol) of the monomer salt (2)(£)in 1.2 mL dry methanol was cooled to 0° C for 10 min and a cold degassed solution of 28 mg (1.7 equivalents) of sodium hydroxide in 0.7 mL methanol was added slowly. After 10 min the reaction mixture became yellow and viscous. The above mixture was maintained at 0° C for another 2-3 h and then the solution was neutralized. A very thick, gum-like material was transferred into a Spectrapore membrane (MW cutoff 12,000-14,000) and dialysed in degassed methanol containing 1 % of water for 3 days. After drying in vacuo, 70 mg ( 47 % ) of "plastic" yellow material was obtained. U V (CHCI3) 365. IR (film) 740, 805, 870, 1045, 1075, 1100, 1125, 1210, 1270, 1420, 1470, 1510, 2930, 2970, 3020. Soluble in C H C 1 , C H C 1 , C H C 1 , C H C I 3 , E t 0 , T H F . Insoluble in MeOH. 6
5
6
3
3
2
2
In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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WUDL ET AL.
Polymers and an Unusual Molecular
Crystal
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Preparation of Poly [3-Methoxy-6-(2-Ethyl-Hexyloxy)phenylene vinylene A solution of 385 mg (1 mmol) of the precursor polymer prepared above in 120 mL 1,2,4-trichlorobenzene was allowed to reflux under N2 for 48 h. After cooling to R.T., 300-400 mL of cold MeOH was added, the mixture was centrifuged and 230 mg (92 %) of the solid was obtained. U V (CHCI3) 500 nm. IR (film) 695, 850, 960, 1035, 1200, 1250, 1350, 1410, 1460, 1500, 2840, 2900, 2940, 3040 cm" . E l . Anal. Calculated for C17H24O2: C, 78.46; H , 9.25. Found: C, 78.34; H 9.26. N M R shows no resonance due to THT. Maximum conductivity for non-stretched, 12 doped films: 60 S/cm. Downloaded by UNIV OF NORTH CAROLINA on October 23, 2014 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch046
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Molecular Crystals With SHG Properties Two molecular crystals were prepared with the intent to prepare organic ferromagnets. These are the nitronyl nitroxide 1 and l,3,5-tris(tricyanovinyl)benzene (2). Recently, the discovery of short range ferromagnetic interactions (SRFM) (Weiss temperature, θ ~ I K from l/χ vs T, magnetization saturation curves corresponding to S = 2, rather than S = 1/2) in crystals of 1(2 ) was reported(7,& ). Ο
Ι
We, in the process of repeating Awaga's discovery, found that this molecule crystallizes in three different polymorphs. One of these is a polar structured) (orthorhombic, F2dd) and shows an S H G efficiency equivalent to quartz. The efficiency would probably be considerably higher if the fundamental (1.060μπι) were of a different wavelength, since the solid absorbs the second harmonic. A much more interesting discovery is that of SHG by crystals of 2. The latter is devoid of a large dipole moment but crystallizes in a polar space group ÇP2\2\2\). CN
2 This material is a white solid which exhibits a powder SHG efficiency of ~100x quartz. This observation is rather unusual because most molecules designed for the improvement of S H G properties require an extended dipolar structure and consequently have an approximately D2h symmetry, yet 2 has a distorted C 3 symmetry. Elsewhere in this symposium, J.-M. Lehn reported that crystals of 1,3,5triamino-2,5,6-trinitro benzene (TATNB), another molecule of similar symmetry to 2, exhibit a large S H G efficiency. However T A T N B can, in principle, distort to a V
In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES
molecule with a large dipole moment (as shown below), whereas 2 cannot. Therefore 2 is indeed unique in its NLO behavior.
0N Downloaded by UNIV OF NORTH CAROLINA on October 23, 2014 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch046
2
NH
2
0N (-) 2
NH
2
Conclusion We have shown that a new stable, processible polymer can be produced by pushing dissymmetric substitution on the PPV skeleton to an extreme. We have also described our discovery of a molecular crystal which does not absorb in the visible (Xmax 300 nm) and has no obvious large molecular dipole moment, yet shows a substantial signal for the second harmonic of Nd-YAG laser light. Acknowledgments We thank the Air Force Office for Scientific Research (AF49620-88-C-0138), the Office for Naval Research (N00014-83-K-0450) and the National Science Foundation (Grant DMR 88-20933) for support of this research. Literature Cited 1. Wessling, R.A.; Zimmerman, R.G. U. S. Patent, 3401152 (1968); 3404132 (1968); 3532643 (1970); 3705677 (1972). 2. Wessling, R.A. J. Polym. Chem.; Polym. Symp., 1985, 72, 55. 3. Lahti, P.M. Modarelli; D.A. Denton III; F.R. Lenz, R.W.; Karasz, F.E. J. Am. Chem. Soc. 1988,110,7259 and references within to the U. of Massachusettswork 4. Askari, S. H.; Rughooputh, S. D.; Wudl, F. Proceedings of the ACS Division of Polymeric Materials : Science and Engineering, 1988, 59, 1068. 5. Han, C. C.; Jen, Κ. Y.; Elsenbaumer, R. L. Synth. Met., 1989, 30, 123. 6. The salt was prepared by standard literature procedures (see references 1-3, above). 7. Awaga, K.; Maruyama. Y. Chem. Phys. Lett. 1989, 158, 556. 8. Awaga, K.; Maruyama, Y. J. Chem. Phys. 1989, 91, 2743. RECEIVED July 18, 1990
In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
