New Polymeric Materials with Cubic Optical Nonlinearities Derived

Chapter 45

New Polymeric Materials with Cubic Optical Nonlinearities Derived from Ring-Opening Metathesis Polymerization Downloaded by IMPERIAL COLLEGE LONDON on February 18, 2015 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch045

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1

1

2

R. H. Grubbs , C. B. Gorman , E. J. Ginsburg , Joseph W. Perry , and Seth R. Marder 2

l

The Arnold and Mabel Beckman Laboratory of Chemical Synthesis, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 2

Partially substituted derivatives of polyacetylene are synthesized via the ring-opening metathesis polymerization (ROMP) of cyclooctatetraene (COT) and its derivatives. Certain poly-COT derivatives afford soluble, highly conjugated polyacetylenes. These materials exhibit large third-order optical nonlinearities and low scattering losses. Organic materials are currently under intense investigation with respect to their potential for nonlinear optical applications (1-5). While the overall prospects for organic materials and their potential merits for nonlinear optical applications have been discussed, the detailed material property requirements for specific device applications are only beginning to be enumerated (6-8). Recent experimental (9-11) and theoretical (12-14) studies indicate that extended electron delocalization leads to large cubic susceptibilities. Materials research efforts are now faced with the challenge to develop materials that have high nonlinear activity and also satisfy stringent requirements, such as low optical absorption and scattering loss, ease of fabrication, and high mechanical, thermal and environmental stability. Polyacetylene, the simplest fully-conjugated organic polymer, displays large third-order optical nonlinearities and high iodine-doped conductivities. Unfortunately, since polyacetylene is an insoluble, unprocessable material with a morphology which is largely fixed during its synthesis, it is difficult to fully exploit all the properties of this potentially useful material. We have shown that ring-opening metathesis polymerization (ROMP) of cyclooctatetraene (COT) produces poly-cyclooctatetraene, a new form of polyacetylene (Figure 1) (15). In this paper, we discuss the ROMP of substituted COTs to form partially substituted polyacetylenes with large third-order optical nonlinearities and greatly improved materials properties relative to polyacetylene. Synthesis of Polymers Polymerizations of substituted COTs are readily accomplished on gram scales in a nitrogen drybox. In a typical polymerization, the tungsten catalyst (16) (2 mg, 2.5 μπιοί) is dissolved in a solution containing 20 μL of tetrahydrofuran and the monomer (yellow liquid, 100 mg, 0.6 mmol). The yellow solution polymerizes over the course 0097-6156/91/0455-0672S06.00/0 © 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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ET AL.

Polymeric Materials with Cubic Optical Nonlinearities

673

of 1-2 minutes, during which time it may be cast onto a variety of substrates. Typically, it is transferred by pipette onto a glass slide where it spreads out to form a film which is 20-200 μπι thick depending on the viscosity of the reaction mixture at the time of the transfer. Polymerization in dilute solution is avoided since a decrease in the monomer concentration at the catalyst center encourages "back-biting" reactions to produce benzene and/or substituted benzenes (Figure 2). This chain transfer reaction does not terminate the polymerization, but it does reduce the molecular weight. The films which are formed can be iodine doped to a conductive state with typical conductivities of 0.1 - 50 fl^cnr (Table I). Nascent poly-COT has a high cis content. Differential scanning calorimetry reveals an irreversible exotherm at 150 °C Q5) corresponding to cis/trans isomerization in polyacetylene. Three of the four cis bonds in the monomer are expected to retain their geometric configuration during polymerization to give a polymer with at least 75% cis configuration. However, although cis/trans isomerization is slow at room temperature (12), the polymerization is exothermic and may induce some isomerization.

Downloaded by IMPERIAL COLLEGE LONDON on February 18, 2015 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch045

1

Properties of Partially Substituted Polyacetylenes Polymerization of substituted COT derivatives results in partially substituted polymers that, in several cases, are soluble and still highly conjugated (18-19). Substitution of polyacetylene via the polymerization of substituted acetylenes results in materials with low effective conjugation lengths as evidenced by their high-energy visible absorption spectra and comparatively low iodine-doped conductivities (20-23). This low conjugation length is presumably due to twisting around the single bonds in the backbone resulting from steric repulsions between the side groups (Figure 3a) (24). Chien has prepared copolymers of acetylene and methyl-acetylene. However, extension of this method to other copolymerizations requires mixing a gas (acetylene) and a liquid (R-acetylene), and this two-phase system is not expected to be wellbehaved (25). In contrast, polymers of substituted COT derivatives have on the average a substituent on every eighth carbon. Thus, in these systems, the predominant steric interaction is between the substituent and a hydrogen on the β-carbons of the backbone (Figure 3b). /z-Alkyl derivatives of COT polymerize to give red materials that are soluble. Upon standing, the polymer solutions turn blue and gel or precipitate if not diluted. (Table I) (12). The color change is proposed to be due to cis-trans isomerization of the polymer in solution (18). A thin film of isomerized poly-w-octylCOT has a broad absorption centered around 650 nm which is comparable to that observed for a thin film of polyacetylene (26). Moreover, in contrast to poly-COT (polyacetylene), which shows large optical scattering due to its crystallinity, the alkylCOT polymers are amorphous and show low scattering losses. Only amorphous halos are observed in the wide angle X-ray profile of these polymers. In general, poly-n-alkylCOTs are soluble in the cis form, amorphous, and highly conjugated as determined by electronic and Raman spectroscopy (Table I). However, these polymers are only barely soluble in the trans form. Placing a secondary or tertiary group adjacent to the polymer chain reduces the effective conjugation length somewhat but affords solubility in both the cis and trans forms of the polymer. Poly-f-butylCOT is freely soluble but yellow-orange in color, indicating a low effective conjugation length. Freely soluble poly-trimethylsilylCOT and poly-seobutylCOT are red in the cis form and purple in the trans form indicative of high conjugation. These polymers can be contrasted with poly-neopentylCOT where the ί-butyl group is spaced one methylene unit away from the polymer chain. The solubility and effective conjugation length of this polymer resemble that of the w-alkyl substituted polymers. Alkoxy substituted polymers such as poly-f-butoxyCOT are also not completely soluble in the trans form.

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

MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES

-fa hv

R*0 = (CF ) CH CO, R = see text 3

2

3


Figure 1. Polymerization of cyclooctatetraenes.

/

\

L M=C

/ R

n

K™= Polymer

R'

/

Propagation Dil. Soin Backbiting and Cycloextrusion

L M n

L M = (see Experimental), R denotes any monosubstituted COT, R' = polymer tail or f-Bu n

Figure 2. Cycloextrusion in dilute solution polymerization of cyclooctatetraenes.


45.


GRUBBS ET AL.

675

Table I. Data for RCOT Polymers R

Abs. Max. after synthesis ,b

Abs. Max. after isom »

Methyl

522 S

....

Λ-Butyl

462

Λ-Octyl

480

w-Octadecyl Phenyl

538 522

i-Butyl

302

614(6-12 hrs) 632 (6-12 hrs) 630 (2-3 hrs) 620(6-12 hrs) 432 (2-3 wks)

s-Butyl

418

556 (2-3 wks)

TMS Neopentyl

380 412,628

512 (2-3 wks) 634(6-12 hrs)


a

a

c

e

Raman vi (A C-C str) cm" 11261132 1132

Raman V2 (Ag C=C str) cm" 1516

σ (S/cm)

1514

0.25-0.7

0.10-0.13

11141128 ... ...h

1485

15-50

0.11-0.19

... ...

0.60-3.65 0.3-0.6

0.13-0.16 0.19-0.28

15391547 1512

esu polymer 0.08 33 20 36 0.15 60 81 0.27 130 0.32 100 160 Nominal uncertainty ±25%


Mol. tract.

ρ

a

a

36

Y'p 10" esu

36

a

a

210 220 280 300

3

The χ( ) and γ values of the copolymers increase substantially with increasing fraction of COT. This increase reflects both the increasing concentration of conjugated units and the increasing conjugation length with higher fraction of COT. It is expected that the nonlinearity of the units of COD in the polymer is negligible compared to that of the nonlinearity of the units of COT; assuming so, one can calculate the hyperpolarizability per unit of COT in the polymer, y , as listed in Table IV. The fact that Y increases with increasing fraction of COT in the polymer shows that the presence of the increased conjugation lengths (segments of 9 and 13 double bonds) results in enhanced nonlinearity. From the solution results, we have estimated the χ( > of a copolymer film with 32% COT to be ~2 x 1 0 esu. By comparison, a solution measurement on β- carotene (11 double bonds) gave a value of χ* ) = 9 x 1 0 esu. Measurements on neat polyacetylene have given a value of 1.3 x 1 0 esu (enhanced by three-photon resonance) at 1907 nm (9-11). Transparent uniform films of these soluble polymers with low scattering losses can be prepared by spin coating. Thus while the χ< ) of the copolymer is modest, this work suggested that the R O M P methodology could be used to produce materials with substantial nonlinearities and is flexible enough to allow tailoring of materials properties. Accordingly, we studied the nonlinear optical properties of some partially substituted polyacetylenes prepared by ROMP. The linear and nonlinear optical properties of films of poly-n-butylCOT were examined. These films were typically prepared by polymerizing the neat monomer and casting the polymerizing mixture either between glass slides, resulting in films of about 20 μπι thickness, or between the fused silica windows of a 100 μπι pathlength demountable optical cuvette. Films cast between substrates were easily handled in air and were very stable for long periods of time (months). In addition, such assemblies were convenient for examination of the optical properties. THG measurements on poly-/i-butylCOT films, referenced to a bare fused silica plate, were made using 1064 nm pulses. These measurements showed that the Ιχ( )Ι values of films of poly-w-butylCOT, ~ l x l 0 " esu, were comparable to that for unoriented polyacetylene at the same wavelength (39). However, comparison of the linear transmission spectra of these materials in the near infrared shows that the partially substituted polyacetylene has greatly improved optical quality. (See Figure 7.) ρ

p

p

3

1 2

3

1 1

9

3

3

1 0



45.

GRUBBS ET AL.


Figure 7. UV-Visible-Near IR spectra of films of poly(COT) (A) and poly(/z-butylCOT) (B).


679

680

MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES

Absorption spectra of polyCOT films show high optical density (1-3 for 20 μπι thick films) even below the true absorption edge (40) in the near IR. The apparent absorption decreases with increasing wavelength but extends out beyond 2000 nm. This apparent absorption is actually due to scattering as shown by laser light scattering observations. We estimate the loss coefficient of poly-COT films to be > 500 c m ' at 1500 nm. The origin of this scattering is certainly due to internal optical inhomogeneities in the polymer associated with the semi-crystalline, fibrillar morphology. In contrast, films of poly-w-butylCOT show very clean transmission in the near IR. Films 100 μπι thick show a sharp absorption edge at -900 nm and little absorption beyond 1000 nm. For poly-n-butylCOT films, we estimate the loss coefficient to be < 0.2 c m at 1500 nm. The greatly reduced scattering loss indicates that partial substitution of polyacetylene with w-butyl groups has resulted in a more homogeneous morphology, approaching that of an amorphous polymer. We have also examined films of poly-TMSCOT. As discussed above, this polymer is completely soluble and can be converted to a fully trans conformation in solution. Films of the trans form of the polymer are then easily produced from solution by casting or spin-coating. THG measurements at 1064 nm on films of poly-TMSCOT give \χ&)\ = 2 ± 1 x 10 * esu. This value is somewhat lower than that of poly-«butylCOT or polyacetylene, consistent with the reduced effective conjugation length inferred from the energy of the absorption maximum, as discussed earlier. The films of poly-TMSCOT prepared from solution are of good optical quality and show scattering losses at least as low as the poly-n-butylCOT films. 1


-1

-

1

Conclusions Ring-opening metathesis polymerization of substituted cyclooctatetraene derivatives yields partially substituted polyacetylenes, many of which are soluble and highly conjugated. Highly conjugated polymers obtained exhibit high optical nonlinearities and low scattering losses. Given the ability to fabricate these polymers into uniform, high quality films with optical nonlinearities comparable to that of polyacetylene, these polymers may be of interest for nonlinear waveguiding experiments. Acknowledgments The research described in this paper was performed, in part, by the Jet Propulsion Laboratory, California Institute of Technology as part of its Center for Space Microelectronics Technology which is supported by the Strategic Defense Initiative Organization, Innovative Science and Technology Office through an agreement with the National Aeronautics and Space Administration (NASA). RHG acknowledges financial support from the Office of Naval Research. S R M thanks the National Research Council and N A S A for a Resident Research Associateship at JPL. EJG thanks IBM for a research fellowship. C B G thanks the JPL for a research fellowship. The authors thank Dr. L . Khundkar, B. G. Tiemann and K. J. Perry for technical assistance. Literature Cited

1. Nonlinear Optical Properties of Organic and Polymeric Materials; Williams, D. J., Ed.; ACS Symposium Series No. 233; American Chemical Society: Washington, DC, 1983. 2. Molecular and Polymeric Optoelectronic Materials: Fundamentals and Applications: Khanarian, G., Ed.; Proc. SPIE Int. Soc. Opt. Eng., No. 682, 1987.


45. GRUBBS ET AL. 3. 4. 5. 6. 7.


8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.


Nonlinear Optical Properties of Polymers: Heeger, A. J.; Orenstein, J.; Ulrich, D. R.; Eds. Materials Research Society Symposium Proceedings, Vol. 109, Materials Research Society: Pittsburgh, PA, 1988. Nonlinear Optical Properties of Organic Molecules and Crystals: Chemla, D. S.; Zyss, J.; Eds.; Academic: Orlando, FL 1987, Vols. 1 and 2. Nonlinear Optical and Electroactive Polymers: Prasad, P. N.; Ulrich, D. R.; Eds.; Plenum: New York, NY 1988, Stegeman, G. I.; Zanoni, R.; Seaton, C. T. in reference 3, p. 53. DeMartino, R.; Haas, D.; Khanarian, G.; Leslie, T.; Man, H. T.; Riggs, J.; Sansone, M.; Stamatoff, J.; Teng,C.;Yoon, H. in reference 3, p. 65 Thackara, J. I.; Lipscomb, G. F.; Lytel, R. S.; Ticknor, A. J. in reference 3, p. 19. Sauteret,C.;Hermann, J. P.; Frey, R.; Pradere, F.; Ducuing, J.; Baughman, R. H.; Chance, R. R. Phys. Rev. Lett. 1976, 36, 956-9. Carter, G. M.; Chen, Y. J.; Tripathy, S. K. Appl. Phys. Lett. 1983, 43, 891-3. Kajzar, F.; Etemad, S.; Baker, G. L.; Messier, J. Synth. Met. 1987, 17, 5637. Agrawal, G. P.; Cojan,C.;Flytzanis, C. Phys. Rev. Β 1978, 17, 776-89. Beratan, D. N.; Onuchic, J. N.; Perry, J. W. J. Phys. Chem. 1987, 91, 26968. Garito, A. F.; Heflin, J. R.; Wong, K. Y.; Zamani, K. O. in reference 3, p. 91. Klavetter, F. L.; Grubbs, R. H. J. Am. Chem. Soc. 1988, 110, 7807-13. Schaverien, C. J.; Dewan, J.C.;Schrock, R. R. J. Am. Chem. Soc. 1986, 108, 2771-3. Chien, J. C. W.; Karasz, F. E.; Wnek, G. E. Nature (London) 1980, 285, 390-2. Ginsburg, E. J.; Gorman, C. B.; Marder, S. R.; Grubbs, R. H. J. Am. Chem. Soc. 1989, 111, 7621-2. Gorman, C. B.; Ginsburg, E. J.; Marder, S. R.; Grubbs, R. H. Angew. Chem. Adv. Mater. 1989, 101, 1603. Zeigler, J. M. U. S. Pat. Appl. 760 433 AO, 21 November 1986; Chem. Abstr. 1986, 20, No. 157042. Zeigler, J. M. Polym. Prepr. 1984, 25, 223-4. Okano, Y.; Masuda, T.; Higashimura, T. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 1603-10. Masuda, T.; Higashimura, T. In Adv. Polym. Sci.; Okamura, S., Ed.; Springer- Verlag: Berlin, 1986; Vol. 81, pp 121-165. Leclerc, M.; Prud'homme, R. E. J. Polym. Sci., Polym. Phys. Ed. 1985, 23, 2021-30. Chien, J. C. W.; Wnek, G. E.; Karasz, F. E.; Hirsch, J. A. Macromolecules 1981, 14, 479-85. Patil, A. O.; Heeger, A. J.; Wudl, F. Chem. Rev. 1988, 88, 183-200. Ito, T.; Shirakawa, H.; Ikeda, S. J. Polym. Sci., Polym. Chem. Ed. 1975, 13, 1943-50. Edwards, J. H.; Feast, W. J.; Bott, D. C. Polymer 1984, 25, 395-8. Bott, D. C. Polym. Prepr. 1984, 25, 219-20. Bohlmann, M. Chem. Ber. 1952, 85, 386-389. Bohlmann, M. Chem. Ber. 1953, 86, 63-69. Bohlmann, M.; Kieslich Chem. Ber. 1954, 87, 1363-1372. Nayler, P.; Whiting, M. C. J. Chem. Soc. Chem. Comm. 1955, 3037-3046. Sondheimer, F.; Ben-Efrian, D.; Wolovsky, R. J. Am. Chem. Soc 1961, 83, 1675-1681. Karrer, P.; Eugster, C. H. Helv. Chim. Acta 1951, 34, 1805-1814. Winston, Α.; Wichacheewa, P. Macromolecules 1973, 6, 200-5.


682 37. 38. 39. 40.

MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES Marder, S. R.; Perry, J. W.; Klavetter, F. L.; Grubbs, R. H. Chem. Mater. 1989, 1, 171-3. Meredith, G. R.; Buchalter, B.; Hanzlik, C. J. Chem. Phys. 1983, 78, 154351. Kajzar, F.; Messier, J. J. Opt. Soc. Am. B: Opt. Phys. 1987, 4, 1040-6. Weinberger, B. R.; Roxlo, C. B.; Etemad, S.; Baker, G. L.; Orenstein, J. Phys. Rev. Lett. 1984, 53, 86-9.


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New Polymeric Materials with Cubic Optical Nonlinearities Derived

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