New photonic and electronic polymers

Materials Science and Engineering B 132 (2006) 8–11

New photonic and electronic polymers Geoffrey A. Lindsay a,∗ , Paul R. Ashley b , Matthew C. Davis a , Andrew J. Guenthner a , Mohan Sanghadasa c , Michael E. Wright a a

Research Department, NAVAIR, MS 6303, 1900 N. Knox Road, China Lake, CA 93555, United States b Research Development and Engineering Command, Redstone Arsenal, AL 35898, United States c The AEgis Technologies Group, Inc., 631 Discovery Drive, Huntsville, AL 35806, United States

Abstract This paper gives an overview of recent work on new organic ␲-electron-conjugated materials prepared and tested in our laboratories. Second-order nonlinear optical polymers for electro-optic modulation were investigated, both guest–host and side-chain chromophoric materials. Chromophores and polymers were synthesized, thin films were prepared, and the following properties were measured: electro-optic coefficient, optical absorption loss and thermal stability. It was concluded that optical modulators fabricated from these materials could be viable components in optical communication systems. Work on novel polyphenylenevinylenes is summarized. Besides their low electrical resistivity, photoluminescent, and electro-active properties, these materials have interesting photoconductive properties that may prove useful in optical control of photonic logic devices. Published by Elsevier B.V. Keywords: Chromophore; Nonlinear optical; Electro-optic; Photo-conductive; Synthesis; Polymer

1. Introduction This paper gives a brief summary of work on ␲-electronconjugated polymers for various photonic applications. The first section covers the synthesis and testing of second-order nonlinear optical polymers that contain discrete chromophores that are either dissolved in a glassy polymer or attached as a side group to the polymer backbone. Preliminary results on films of these materials tailored for electro-optic modulation are reported, as well as oven-aging data on optical modulators made from these materials. A second brief section covers a poly(bis-aminophenylene-co-vinylene) that has potentially useful photoconductive properties for optical control of photonic logic devices. It also has photoluminescent properties for light-emitting devices, and electro-active properties for corrosion control. 2. Second-order nonlinear optical materials 2.1. Background In a Mach–Zehnder electro-optic modulator (MZM), light enters the waveguide, is split into two arms, an electric field is ∗

Corresponding author. Tel.: +1 760 939 1630; fax: +1 760 939 1617. E-mail addresses: [email protected], [email protected] (G.A. Lindsay). 0921-5107/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.mseb.2006.02.025

applied across one (or both) arm(s) to shift the phase of light in one arm relative to the phase in the other arm by 180◦ (that is the half-wave voltage, V␲ ). When light waves combine from the two arms, they interfere destructively if the applied electric field is on (or constructively if the field is off). The frequency of optical intensity modulation can be over 110 GHz [1]. A typical cross-section of an MZM arm is a stack of three polymers (cladding-core-cladding) sandwiched between two electrodes [2]. Light is guided in the nonlinear core material by total internal reflection off the interface with the lower and upper optical cladding layers. The fabrication and testing of actual modulators is rather immature but is rapidly gaining momentum in many laboratories around the world. For example, a recent report shows excellent progress in etching polymer waveguides and integrating light sources on the polymer optical waveguide devices [3]. One of the three most important properties of the MZM core material is the electro-optic (EO) coefficient, r33 (the other two most important properties are optical loss and thermal stability). The r33 attainable in polymer films depends upon the type of asymmetric polarizable chromophore incorporated, the concentration of chromophores, and their degree of polar orientation (in general, the film must have noncentrosymmetry). In this study, the polar order was achieved by the following steps: casting a polymer film from solution, evaporating the solvent, heating the film to the softening temperature (the

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glass transition temperature, Tg ), applying a large electric field (100 V/␮m), and cooling the film well below Tg before removing the applied field to freeze in the polar order. The most easily implemented EO core materials are those of the guest–host type wherein a chromophore is simply dissolved in an amorphous polymer that has a high Tg . The chromophore must be very soluble in the host polymer to achieve high loadings. If the solubility limit is exceeded, chromophores may precipitate or two phases may form that cause light to scatter out of the waveguide (or other problems). As for polar order (the degree of polar alignment), since the chromophores are not locked in a crystal lattice, they can undergo very slow rotational diffusion. Under normal conditions, the rate of rotation diffusion is limited by the rate of segmental motion of the polymer (with a time constant of years at temperatures well below the Tg ). Larger chromophores will have slower rotations. Attaching the chromophores to the polymer backbone will retard their rotation even more. Crosslinking the polymer will add the ultimate stability, but this is difficult to implement because the polar order must be imparted before the film is fully crosslinked. Linking several chromophores together in a branched motif (the “dendrimer” approach) should also prove to be a viable design [4] if the glass transition temperature can be made high enough, or if the matrix can be crosslinked during the poling process.

Silyl ether protection was used on the hydroxyl group for the subsequent Wittig–Aldol reaction to further extended the ␲electron conjugation. A good yield (65%) of a mixture of the Z and E isomers (the “extended aldehyde”) was obtained. The Z:E isomer ratio was 33:67 by 1 H NMR. The extended aldehyde (mixed isomers) was used for the final step of end capping with the electron acceptor. The final step was a Knoevenagel condensation between the extended aldehydes and the known tricyano dihydrofuran electron acceptor [8] using piperidinium acetate as a catalyst to yield the monohydroxy-functional chromophore (CLD-OH). Reslurrying the solid with acetone gave a homogenous highly crystalline powder with purity of >98% by 1 H NMR (corroborated by HPLC). It was estimated from molecular modeling that the E isomer has 65% of the molecular hyperpolarizability of the Z isomer (using semi empirical modeling with the PM3 Hamiltonian in the MOPAC® program). No attempt was made to isolate the isomers due to the amount of work involved and because the mixed isomers would be more soluble than the pure E isomer (which has a higher β). This synthetic route was used to scale-up CLD-OH (over 50 g per batch) for subsequent attachment to polyimide, and for attachment of bulky solubilizing groups. FTC-type chromophores were also scaled up, but this will be the subject of a future paper.

2.2. Synthesis

2.2.2. Side-chain polyimides A new synthetic approach for attaching chromophores to polyimides was published elsewhere [6]. In this case, the chromophore was attached to a hydroxy-functional preformed polyimide (PI) backbone. The PI backbone was functionalized with a new monomer, 3,5-bis(4-aminophenoxy)1-hydroxymethylbenzene, referred to as BHB in this paper. The first step in the synthesis of BHB is the reaction of methyl 3,5-dihydroxybenzoate (Aldrich) with p-fluoronitrobenzene and base. The product from the first step was purified by trituration with hot ethanol. The products from the two remaining reduction steps required only simple filtrations to yield the analytically pure BHB monomer (overall yield from methyl 3,5-dihydroxybenzoate was 65%). BHB was copolymerized with 6FDA and other comonomers to provide polyimides (PI) having a convenient mole percentage of reactive hydroxymethyl side groups (25–50 mol%). A few percent of phthalic anhydride end groups was used to control the molecular weight of the PI. Two methods were explored for the chromophore-attachment chemistry. In one approach, the BHB-PI was reacted with excess of 1,6-diisocyanatohexane in the presence of triethylamine. Complete reaction of the benzyl alcohol group on the PI was confirmed by proton NMR. The polymer retained excellent solubility suggesting that little if any cross-linking occurred during polymer modification. Reaction of the dangling isocyanate on the BHB-PI with the alcohol functional group of the chromophore in the presence of 4-(N,N-dimethylamino)pyridine (DMAP) resulted in the sidechain electro-active polyimide. There was evidence of a small fraction of unreacted isocyanate groups (a residual infrared absorption at 2265 cm−1 ). These may prove useful for subsequent crosslinking reactions (this has not yet been explored).

This section reviews the synthesis of a highly active chromophore useful for guest–host core materials and for attaching to polyimide backbones. The basic structure of an isophoronebased chromophore called “CLD” (sometimes called “CZC”), and that of a thiophene-based chromophore called “FTC,” were reported in the late 1990’s by the Dalton group [5]. Both chromophores were used in this study, but the CLD chromophore is emphasized in this paper. In the present study, bulky groups were attached to the chromophore for the following reasons: (1) to increase the pot-life of concentrated spin-casting solutions, (2) to impart complete solubility of chromophores at high concentrations (up to 35%) in the host polymers, and (3) to eliminate chromophore sublimation from films during high-temperature poling. These chromophores were also attached to polyimides by side-chain chemistry; details was published elsewhere [6]. 2.2.1. Chromophores The synthesis of the basic CLD chromophore used in this study has been published elsewhere [7], and is briefly reviewed here. The chromophore was built up starting with the amine end (the electron donor): the hydroxyl group of commercially available N-ethyl-N-hydroxyethyl aniline was protected to give a quantitative yield of the acetate. Acetate protection was required for the subsequent Vilsmeier–Haack formylation. The formylation proceeded selectively at the 4-position in good yield. Next, in a Knoevenagel reaction the isophorone group was added to the aldehyde to extend the ␲-conjugation. Although the reaction rate was slow, a moderate yield (48%) of the product precipitated from the aqueous work-up making chromatography unnecessary.

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The second approach to chromophore-attachment involved reacting BHB-PI with an excess amount of various diacid chlorides in the presence of 2,6-di-t-butyl-4-methylpyridine and DMAP. The NMR spectrum of the isolated polymer confirmed complete functionalization of the benzyl sites. After attaching the chromophore, a new NMR resonance appeared that was assigned to the –OCH2 – of the ester link with the chromophore. The anticipated increase in molecular weight was observed by GPC. The attachment chemistry did not modify the polydispersity at either stage of the attachment chemistry. Several different chromophores have been successfully attached by this method and will be reported elsewhere. 2.3. Physical characterization 2.3.1. EO coefficient measurement A modified Teng-Man reflection apparatus was used to estimate the r33 electro-optic coefficient [9]. The sample fixture consisted of a film of polymer on indium–tin oxide coated glass with a 200 nm layer of gold deposited on top of the polymer. It was necessary to use a thin ITO coating (>70 -sq.) to eliminate Fabry–Perot effects. The polymer in this fixture was poled by applying 100 V/␮m across the polymer film thickness by means of these contacting electrodes in a dark vacuum chamber. The r33 for a poled film of 35% CLD in a polycarbonate host polymer was 37 pm/V at 1310 nm and 25 pm/V at 1550 nm. The percentage of CLD being rather high and the poling field being rather low resulted in a rather low r33 . For similar films poled at a higher voltage in a Mach–Zehnder modulator, the back-calculated r33 was about 40 pm/V at 1550 nm (the V␲ was about 5 V). The reflection r33 method is presently being modified to ramp the temperature up while monitoring r33 . 2.3.2. Screening for optical absorption loss A procedure for screening optical absorption loss of these EO materials in solution has been published elsewhere [10]. Optical measurements were carried out with a Cary 5 spectrophotometer on solvent-borne polymers and chromophores in a quartz cuvette. With careful procedures in place, a precision of around ±0.2 dB/cm was achieved. The absorption loss for CLD core materials was about 0.7 and 1.0 dB/cm in the 1300 and 1500 nm windows of interest. Results from this screening procedure were about 1.5 dB/cm lower than what was measured in poled waveguides for the guest–host core material. Loss from the side-chain CLD-PI core materials was about 1 dB/cm higher, but these materials have not yet been fully developed and characterized. 2.3.3. Thermal stability Guest–host films can suffer from sublimation of the chromophore at temperatures at or above the Tg (the poling temperature). The chromophore can also undergo chemical degradation due to oxidation and reaction with impurities in the polymer. These effects were measured by monitoring changes in the absorption spectra in thin films as a function of time at elevated temperature for both guest–host and side-chain polyimides (full details to be published elsewhere by Guenthner et al.). Films

Table 1 Oven aging of modulators: V␲ vs. time and temperature Core type

Half-wave voltages 1h

1 month

5 years

CLD-X/APC (a polycarbonate obtained from Aldrich) 80 ◦ C 5 11 17 95 ◦ C 5.5 14 21 110 ◦ C 5.5 34 52

10 years 19 23 56

were held at 150, 170 and 190 ◦ C (30 min at each temperature). The % decrease in absorption (due to sublimation + degradation) was measured at characteristic wavelengths, and for a CLDX/polycarbonate guest–host film, the loss was: 0.1% at 150 ◦ C, 0.3% at 170 ◦ C, and 4% at 190 ◦ C; and for a CLD-PI side-chain film: 0.7% at 150 ◦ C, 2.1% at 170 ◦ C, and 6% at 190 ◦ C. There is evidence that at least part of the higher loss at 190 ◦ C is due to a physical aging mechanism (rather than degradation) that broadens the absorption envelop. New data on a high-activity thiophene-based chromophore (FTC-X) indicate that it has better stability towards oxidative or other chemical degradations at elevated temperatures. FTC is also under investigation in sidechain polyimides. For guest–host core materials, the softening temperature of the film must be at least 70 ◦ C higher (preferably 100 ◦ C higher) than the long-term operating temperature of the modulator to maintain a low operating voltage, because, as the Tg is approached, free volume increases and the chromophores undergo accelerated rotation. A long-term aging study on a first generation DANS-PMMA core material showed that 60% of the original polar order was retained after 8 years at room temperature [11]. Polyimides with side-chain chromophores and crosslinked polymers will do better. We present here aging data at elevated temperatures of a guest–host core material. Mach–Zehnder modulators were fabricated from the CLD-X/APC guest–host core material and oven aged at three elevated temperatures. A model after Dissado-Hill as reported by Singer et al. [12] was used to fit the long-term relaxation data. Data for 1-h and 1-month aging, and extrapolations to 5 and 10 years is summarized in Table 1. Shortly after the temperature was increased, an initial rapid relaxation period (increase in V␲ ) was observed. This initial rapid relaxation upon heating the films could be due to the creation of thermally enlarged nanovoids and the rotation of adjacent chromophores (i.e., the fraction of chromophores near pliable molecular segments surrounding nanovoids that have dimensions appropriate for coupling their motions to the chromophore). One report shows this rapid aging could have been reduced by annealing the modulator for 20 h at the aging temperature while applying the poling field [13]. The side-chain polyimide core materials have just begun to be investigated in modulators. The Tg of these materials was tailored to be about 170 ◦ C, the highest poling temperature found to safely pole the CLD chromophore. Poling temperatures higher than 175 ◦ C tended to give a higher optical loss. The Tg of the chromophore-attached polyimides of this study is about 30 ◦ C higher than the Tg of the guest–host core materials and will thus

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give superior thermal aging performance (this will be reported elsewhere). Using the FTC chromophore will allow even greater poling temperatures (higher Tg ’s and better thermal stability). 3. Polymer with ␲-conjugated backbone: potential applications

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with ␲-conjugated backbones have many interesting and unusual photonic and electronic properties and are finding many new applications. The photoconductivity for a new amino-functional phenylene–vinylene polymer bodes well for application in photonic logic devices. Acknowledgements

3.1. Background Poly(phenylene vinylene)’s, polymers having ␲-electron conjugated backbones, have been known for some time to have interesting electrical and optical properties. The synthesis of an important member of this family, called Poly(2-(N,Ndimethylamino)-1,4-phenylene vinylene), was first prepared in the early 1990’s [14]. Since that time films of this polymer have shown good potential for application in light-emitting devices [15], in protecting metals from corrosion [16], in electrode redox applications (electro-chromic and fluorescent) [17], and most recently in applications requiring photoconductivity [18]. In this limited space paper, only the photoconductivity data will be briefly summarized. 3.2. Photoconductivity of poly(2-(N,N-dimethylamino)-1,4-phenylene vinylene) An optical pump-probe method with subpicosecond time resolution was used to measure the transient photoconductivity of poly(2-(N,N-dimethylamino)-1,4-phenylene vinylene) [18]. The peak photo-conductance was linearly proportional to the laser fluence, and the switching time was nearly independent of the laser fluence and electric bias field. The charge carrier lifetime was less than 2 ps. The lower limit of charge mobility in the case of ultrafast photo-excitation was estimated to be 0.2 cm2 /V s. Films of this material may find application in micro-ring resonators for control of photonic logic devices. 4. Conclusions The outlook for high-bandwidth polymer electro-optic modulators is very promising with a broad vista of new applications on the horizon. New reports on electro-optic coefficients of new organic materials indicate they will be at least an order of magnitude greater than for lithium niobate. For well-known guest–host core materials the thermal stability of polymer modulators is extrapolated to be adequate for 5 years operation at 80 ◦ C, and much better for chromophore-attached-to-polymer core materials. More studies on oven-aged modulators, and more data on photo-stability testing are needed to build the confidence of device engineers. Electro-active photonic polymers

We thank Warren Herman and his group for the photoconductivity measurements; we thank Stephen Fallis, David Irvin, Richard Hollins, Cynthia Webber, Peter Zarras and John Stenger-Smith for synthesizing some of the materials reported herein; we thank Andrew Chafin for molecular orbital calculations; we thank Aaron Sathrum and Raymond Fu for film preparations and measurements; we thank the Office of Naval Research for funding most of the work presented here. References [1] D. Chen, H.R. Fetterman, A. Chen, W.H. Steier, L.R. Dalton, W. Wang, Y. Shi, Appl. Phys. Lett. 70 (1997) 3335. [2] M.-C. Oh, H. Zhang, A. Sze, V. Chuyanov, W.H. Steier, C. Zhang, L.R. Dalton, H. Erlif, B. Tsa, H.R. Fetterman, Appl. Phys. Lett. 76 (2000) 3525. [3] W.M. Diffey, R.H. Trimm, M.G. Temmen, P.R. Ashley, J. Lightwave Technol. 23 (2005) 1787. [4] J. Luo, S. Liu, M. Haller, L. Liu, H. Ma, A.K.-Y. Jen, Adv. Mater. 14 (2002) 1763. [5] L.R. Dalton, W.H. Steier, B.H. Robinson, C. Zhang, A. Ren, S. Garner, A. Chen, T. Londergan, L. Irwin, B. Carlson, L. Fifield, G. Phelan, C. Kincaid, J. Amend, A. Jen, J. Mater. Chem. 9 (1999) 1905. [6] M.E. Wright, S. Fallis, A.J. Guenthner, L.C. Baldwin, Macromolecules 38 (2005) 10014–10021. [7] M.C. Davis, A.P. Chafin, R.A. Hollins, L.C. Baldwin, E.D. Erickson, P. Zarras, E.C. Drury, Synth. Commun. 34 (2004) 3419–3429. [8] G. Melikian, F.P. Rouessac, C. Alexandre, Synth. Commun. 25 (1995) 3045–3051. [9] F. Michelotti, A. Belardini, M.C. Larciprete, M. Bertolotti, A. Rousseau, A. Ratsimihety, G. Schoer, J. Mueller, Appl. Phys. Lett. 83 (2003). [10] A.J. Guenthner, J.M. Pentony, G.A. Lindsay, Proc. SPIE 5517–32 (2004) 175–186. [11] H.T. Man, H.N. Yoon, Appl. Phys. Lett. 72 (1998) 540. [12] R.D. Dureiko, D.E. Schuele, K.D. Singer, J. Opt. Soc. Am. B 15 (1998) 338. [13] Q. Shen, K.Y. Wong, Opt. Commun. 164 (1999) 47. [14] J.D. Stenger-Smith, A.P. Chafin, W.P. Norris, J. Org. Chem. 59 (1994) 6107. [15] J. Stenger-Smith, P. Zarras, L. Merwin, S. Shaheen, B. Kippelen, N. Peyghambarian, Macromolecules 31 (1998) 7566–7569. [16] P. Zarras, J.D. Stenger-Smith, Y. Wei, Electroactive Polymers for Corrosion Control, ACS Symposium Series 843, American Chemical Society, Washington, DC, 2003. [17] T.-Q. Nguyen, B.J. Schwartz, J. Chem. Phys. 116 (2002) 8198. [18] W. Cao, H. Liang, M. Du, Y. Kim, W.N. Herman, C.H. Lee, Polym. Mater.: Sci. Eng. 91 (2004) 805.