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Synthesis and characterization of new electroactive polymers
Synthetic Metals, 29 (1989) E463-E470
E463
SYNTHESIS AND CHARACTERIZATION OF NEW ELECTROACTIVE POLYMERS
L. P. Yu and L. R. Dalton Department of Chemistry, University of Southern California, Los Angeles, CA 90089-0482, U.S.A.
ABSTRACT Optical quality thin films of polyquinoxaline ladder polymers have been prepared by thermal treatment of open-chain precursor polymers derivatized with a variety of substituents to improve solubility. Polycondensation synthesis is emplo}ed to avoid problems of polymer branching and disorder. Both alkyl derivatization which improves solubility with little effect upon electronic properties and derivatization with vinylamine side groups which influences both physical and electronic properties are discussed. The planar conformation of ladder polymers optimizes ~-orbital overlap; thus, large optical nonlinearities are anticipated. Third order susceptibilities as high as 3 x 10-9 esu for pristine polymers have been measured by degenerate four wave mixing and third harmonic generation.
*Abbreviations used: X(3) = third order optical susceptibility, 0t = linear absorption; DFWM = degerate four wave mixing, FTIR = Fourier transform infrared spectroscopy, NMR = nuclear magnetic resonance spectroscopy, TGA = thermal gravimetric analysis, DMF = dimethylformamide, DMSO = dimethyl sulfoxide, DMAC = dimethylacetamide, TAB = 1,2,4,5tetraaminobenzene, DABD = 3,3'-diaminobenzidine, DCDEBQ = 2,5-dichloro3,6-bis(2-diethylaminovinyl) benzoquinone, DCDPBQ = 2,5-dichloro-3,6-bis(2d i p r o p y l a m i n o v i n y l ) benzoquinone, DCPBQ = 2,5-dichloro-3,6-bis(2p i p e r i d i n o v i n y l ) b e n z o q u i n o n e , DCHBQ = 2,5-dichloro-3,6-bis (hexamethyleneiminylvinyl)-l,4-benzoquinone, n = refractive index, OCDCDEBQ-TAB = open-chain precursor p o l y m e r p r e p a r e d by the polycondensation of DCDEBQ and TAB, L-DCDEBQ-TAB = ladder polymer prepared by thermal treatment of OC-DCDEBQ-TAB, L-DCDPBQ-TAB -- ladder polymer prepared by the polycondensation of DCDPBQ and TAB, SCE = saturated calomel electrode, THG = third harmonic generation. 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
Ea6a
INTRODUCTION The planar conformation of ladder polymers optimizes the overlap of ~orbitals in the polymer backbone thus enhancing electron delocalization [1]. For nitrogen containing ladder polymers, electron delocalization is also dependent upon the state of nitrogen [1,2] with the greatest delocalization observed for the imine form. Electronic properties are expected to depend upon both protonic and electronic doping [1,3]. From the preceding theoretical analysis, polyquinoxaline ladder polymers would appear to be excellent candidates for nonlinear optical and electroactive materials. The synthesis of prototype ladder polymers was effected some years ago [4]. Unfortunately, the generally observed poor solubility and low molecular weights have discouraged consideration of these materials either as structural or electroactive materials. The physical properties of the prototype ladder polymers are quite reasonable when one realizes that the ~-orbitals which generate interesting electronic properties are likely to participate in strong interchain interaction. It is reasonable that solubility can be improved by utilizing the steric interactions associated with substituents to destabilize polymer-polymer interactions. Moreover, solubility will likely improve if conformational constraints are relaxed. For an open-chain polymer, solubility should improve due to increased entropy and less favorable interchain orbital interaction. Thus, a logical route to preparation of ladder polymers in desired morphologies such as thin films would appear to be via a derivatized openchain precursor polymer route. We undertook the investigation of such an approach starting with the preparation of oligomers, precursor polymers, and ladder polymers derivatized with alkyl substituents [5]. We have demonstrated the fabrication of optical quality thin films of precursor polymers and the conversion of these to ladder structures (including prototype or underivatized ladders) by thermal processing. Indeed, asymmetrically derivatized materials have been used to fabricate films by Langmuir-Blodgett methods. For alkyl-derivatization, solubility is improved with little effect upon electronic properties. A somewhat more intriguing example is that of vinylamine derivatization where both physical and electronic properties are influenced by the side groups. These materials that will occupy our attention for the remainer of this discussion.
RESULTS Synthesis and molecular characteriza¢ion The general scheme for the preparation of derivatized dichloroquinone monomers, precursor polymers, and ladder polymers is shown in Fig. 1. All
E465
of the materials shown in Fig. 1 have been prepared. Because of space limitations we restrict this discussion to one representative example; namely, the preparation of the ladder polymer L-DCDEBQ-TAB and all of its precursor materials.
Cl
. R
I
+ HNR" + CH3CHO
~ O.;~,,~CI RT
R'= Mez,Et2, D , ~ , ~ , Pr 2, 8u2, 3 )
I,,~-'J0 H2N~. ,,NH2 DMF Co.,~,l '~'~R ,-"J01+ % H 2 RT
HoN
R = CH=CHNR"
R NH2 0
R
Fig. 1. General scheme for preparation of vinylamine-derivatized polymers.
The DCDEBQ monomer was prepared according to the method of Buckley et. al. [6] by adding diethylamine (5.8 g) to a stirred solution of chloranil (4.92 g) and freshly distilled acetaldehyde (2.9 g) in 1500 mL of toluene at 30 C. After 5 hrs, the amine salt was extracted with 0.5 N sulfuric acid and the toluene layer was concentrated by evaporation. The product was purified by chromatographic separation using deactivated alumina (yield 45%). The melting point of the product was observed to be 129-131 C. Elemental analysis (Galbraith): Calc. for C18H24N2C1202: C, 58.22; H, 6.47; N, 7.55; Cl, 19.14; found: C, 58.30; H, 6.53; N, 7.52; CI, 19.16%. 1H NMR (CDCI3): 1.2 (t, 12H, -CH3), 3.4 (q, 8H, -CH2-), 5.6 (d, 2H, =CHN-), 8.5 ppm (d, 2H, -HC=C). 13C NMR (CDC13): 12 (2C, -CH3), 14 (2C, -C'H3), 42 (2C, -CH2-), 51 (2C, -C'H2-), 94.5 (2C, -H13C=C), 119 (2C,13C=CC1), 139.5 (2C, 13C--O), 153 (2C, =13CHN), 178.6 ppm (2C, =13CC1).
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The TAB monomer was prepared according to literature procedure [7] just before use to avoid air oxidation. Elemental analysis: Calc. for C6H10N4: C, 52.17, H, 7.24; N, 40.58; found: C, 52.11; H, 7.09; N, 40.31%. The open-chain precursor polymer OC-DCDEBQ-TAB was prepared from the reaction of DCDEBQ with TAB in DMF solution carried out in a controlled atmosphere (nitrogen or helium) Vacuum Atmospheres glove box with constant monitoring of oxygen and moisture levels. To 100 mL of DMF was added 5.000 g of DCDEBQ and 1.855 g of TAB; the solutions was then stirred at room temperature overnight. The solution was poured into 500 mL of chloroform and the precipitate was collected by filtration, washed with chloroform and ethanol, and the dark brown prepolymer was dried at 50 C under vacuum (yield 95%). Solubilities in common organic solvents are as follows: 1.14 g/l, DMF; 0.81 g/1 DMSO; 1.05 g/l, DMAC; 0.63 g/l, m-cresol; 0.15 g/l, [(CH3)2N]3PO. An intrinsic viscosity of 1.4 dl/g was measured in DMF; thin films (0.5-5 microns) can be easily prepared by spreading DMF/prepolymer solution on a glass slide and removing the solvent. Elemental analysis: Calc. for (C24H34N6C1202)n: C, 56.58; H, 6.68; N, 16.50; C1, 13.95; found: C, 56.89; H, 6.45; N, 16.69; C1, 13.71%. 1H NMR (d6-DMSO): 1.2 (-CH3), 2.9 (-CH2-), 3.8 (-NH2), 5.87 (=CHN-), 6.8-7.0 (aromatic H), 8.4 ppm (HC=C). The ladder polymer L-DCDEBQ-TAB can be prepared in either of two ways. The first is to reflux the prepolymer in DMF solution overnight. A black powder precipitates out and is collected directly from DMF, purified by using ethanol in a soxhlet extractor and then dried at 100 C under vacuum (yield 90%). The second method involves heating a prepolymer film or powder to 140 C for 6 hrs. and at 260 C for 10 hrs. FTIR shows that the same material is obtained by either approach. Elemental analysis: Calc. for (C24H30N6C12)n: C, 60.88; H, 6.34; N, 17.75; C1, 15.01; found: C, 65.13; H, 4.49; N, 16.60; C1, 13.97; Calc. best fit structure, (C24H22N5.4CI1.7)n: C, 65.13; H, 4.93; N, 16.95; Cl, 13.53%. TGA, together with mass spectroscopic analysis of elimination products, suggests that water, HCI, and some ammonia are given off in the transformation of precursor polymer to fused ring ladder polymer. FTIR data provides strong evidence that the second condensation step has occurred to an appreciable extent as the C=O stretch at approximately 1650 cm -1 has disappeared in the heat treated material as has the NH2 bend (1510 cm-1); a C=N stretch ( 1625 cm -1) appears in the product. As seen in the electron micrographs presented in Fig. 2, thermal transformation can result is disruption of the film surface. In practice, we have not observed significant degradation in optical quality. A red-shift in the optical spectrum (~-x* interband transition) was also observed upon transformation of prepolymer to
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ladder polymer. By coating platinum electrodes with a thin film of ladder polymer we have investigated redox properties by cyclic voltammetry; the oxidation potential is found to be +1.1 V versus the SCE. Both the optical band gap and the oxidation potential are somewhat larger than expected from theory which may imply a less than perfect ladder lattice perhaps due to incomplete condensation or air oxidation.
Fig. 2. Electron micrographs of diethylamine-derivatized prepolymer (smooth surface) and ladder polymer (rough surface); ladder polymer was prepared by heating the prepolymer film.
Measurement of optical nonlinearity Ancillary_ measurements. Refractive indices were measured by measuring back reflection from the polymer-air interface at the operating wavelength (e.g., 532 rim). This reflection could be measured with an accuracy better than 3%. Film thicknesses were measured with a step profiler (Tencor Alpha Step Profiler Model 1000) and were verified by electron microscopy. M e a s u r e m e n t of third o r d e r susceptibilities. Although some measurements of optical nonlinearity were effected by third harmonic generation (THG), the bulk of our analysis has relied upon degenerate four
E468
wave mixing (DFWM) measurements. Three input beams are formed from 532 nm laser pulses having energy approximately 2 mJ and duration approximately 25 psec. In this geometry, delaying each of the three input beam pulses separately gives three different and independent sets of data for the strength and polarization of the fourth or signal beam. We have performed experiments with essentially all combinations of beam polarization and delay [8]. Representative susceptibility values are given in Table 1. TABLE 1 Optical nonlinearity data for representative vinylamine polymers. Material L-DCDEBQ-TAB (air-oxidized) OC-DCDEBQ-TAB L-DCDEBQ-TAB L-DCDPBQ-TAB L-DCPBQ-TAB L-DCPBQ-DABD L-DCHBQ-TAB
n
X(3) xl0-10(esu)
0c (lain-l)
1.75
0.6
1.1
1.70 1.80 1.82 1.83 1.80 1.80
1.0 3.0 2.8 12.5 12.8 11.2
1.7 1.3 1.4 1.4 1.7 1.4
The values given in Table 1 are for pristine polymers. Chemically doping polymers with electron accepting or donating dopants, (e.g., nitrosyl tetrafluoroborate), resulted in an increase in third order susceptibility. However, the resulting materials have not been characterized structurally at this time. DISCUSSION The third order susceptibilities measured for vinylamine ladders are quite impressive even when it is noted that the resonance contribution to these values is not negligible. A value of 2 x 10-9 esu is observed for BBL but the linear absorption at 532 nm is an order of magnitude greater (18.3 ~m -1) than for vinylamine-derivatized ladders. We have also conducted an extensive p r o g r a m p r e p a r i n g p o l y t h i o p h e n e and p o l y t h i o p h e n e / p o l y a n i l i n e copolymers. The partially resonance-enhanced values for these polymers is on the order of 5 x 10-10 esu which is a factor of 2 less than observed for the vinylamine ladders. As seen from Table 1, X(3) values vary with the extent of air oxidation of nitrogens. Oxygen elemental analyses (performed by Galbraith) and EPR g-values provide qualitative insight into the extent of
E469 oxidation and it is likely that up to 50% of the nitrogens can be oxidized; however, quantitative accuracy is not sufficiently good to permit totally unambiguous conclusions to be drawn from the data. The qualitative trend suggests that air oxidation interrupts ~-orbital overlap and limits electron delocalization. Incomplete condensation has the same effect and it may be difficult to separate these two effects at this time. The similarity of the values for the L-DCPBQ-DABD and L-DCPBQ-TAB polymers is suggestive of such interruption as the DABD portion is known to exist as a nonplanar segment. Comparison of data for open-chain prepolymer and fused-ring ladder polymer suggests that increased orbital overlap in the latter results in an enhanced value of third order susceptibility. From Table 1, it appears that larger values of susceptibility are routinely observed for the alicyclic vinylamine substituents when compared to dialkyl vinylamine-derivatized materials. NMR studies indicate substantially greater side group dynamics for the latter materials. It is not unreasonable that side group dynamics modulate the conjugation of vinylamine 7r orbitals with those of the polymer backbone. In this regard, it is interesting to note that not only is the electronic component diminished but the thermal component is enhanced consistent with the preceding speculation. When the various data sets recorded in this study are considered and compared with data for other ladder polymers, it appears that charge transfer involving the vinylamine substituents plays a role in enhancing optical nonlinearity. Intermolecular charge transfer involving electron donating dopants also appears to enhance nonlinearity. Although the values for third order susceptibility and optical transparency are not sufficiently good to seriously consider device application at this time, one speculation is of interest, at least academically. It can be noted that the linear index of refraction for several ladder polymers matches that of polystyrene. We have succeeded in synthesizing copolymers of ladder polymers with polystyrene and are currently imbedding ladder polymer coated polystyrene spheres of various sizes in different composite matrices. At low laser intensities, these matrices should transmit light but frustrated scattering may occur at higher light intensities. Clearly, one of the advantages of polymeric systems that deserves further investigation is the range of interesting and convenient processing options. ACKNOWLEDGEMENTS We are indebted to Dr. Donald Ulrich for motivating this research and to Professor R. W. Hellwarth and his research group for nonlinear optical measurements. We wish to thank M. R. Unroe and her colleagues at
E470 AFWAL/MLBP for analytical studies which complemented those carried out in our laboratory and at Galbraith. The advice and assistance of coworkers Drs. D. Polis, D. Vachon, M. McLean, R. Honeychuck and graduate students R. Montgomery and Linda Sapochak is appreciated. This work was supported by Air Force Office of Scientific Research contracts F49620-87-C0100 and F49620-88C-0071 and by National Science Foundation grant DMR-8206053. REFERENCES 1. S. Kivelson and O. L. Chapman, Phys. Rev. B, 28 (1983) 723; D. S. Boudreaux, R. R. Chance, J. F. Wolf, L. W. Shacklette, J. L. Bredas, B. Th(~mans, J. M. Andr~ and R. Silbey, 1. Chem. Phys., 85 (1986) 4584; R. R. Chance, D. S. Boudreaux, J. F. Wolf, L. W. Shacklette, R. Silbey, B. Themans, J. M. Andr~ and J. L. Br~das, Synth. Met., 15 (1986) 105; J. L. Bredas, R. R. Chance, R. H. Baughman and R. Silbey, 1. Chem. Phys., 76 (1982) 3673; J. P. Lowe, S. S. Kafadi and J. P. LaFemina, |. Phys. Chem., 90 (1986) 6602. 2. M.J. Rice and E. J. Mele, Phys. Rev. Lett., 49 (1982) 1455; W, Forner, M. Seel and J. Lakik, T. Chem. Phys., 84 (1986) 5910. 3. C. P. DeMelo and tL Silbey, Chem. Phys. Lett., 140 (1987) 537. 4. J. K. Stille and E. Mainen, Polym Lett., 4 (1966) 39; J. K. StiUe and E. Mainen, Macromolecules, 1 (1968) 36; J. K. Stille and M. E. Freeburger, |. Polym. Sci. A-l, 6 (1968); J. Szita and C. S. Marvel, |. Polym. Sci. A-l, 7 (1969) 3203; M. Okada and C. S. Marvel, 1. Polym. Sci. A-l, 9 (1968) 1774; O. K. Kim, L Polym. Sci. Polym. Lett. Ed., 23 (1985) 137. 5. L. R. Dalton, Proc. SPIE, 682 (1986) 77; L. R. Dalton, J. Thomson and H. S. Nalwa, Polvmer. 28 (1987) 543; L. R. Dalton, in P. N. Prasad and D. R. Ulrich (eds.), Nonlinear Ovtical and Electroactive PolvmersA988.Plenum Press, New York, pp. 243-271; L. tL Dalton, NATO A~I ,~eri~, to be published. 6. D. Buckley, H. B. Henbest and P. Slade, |. Chem. Soc., (1957) 4891. 7. R. Nietzki and A. Schedler, Ber., 30 (1897) 1666; H. A. Vogel and C. S. Marvel, |. Polym. Sci., 50 (1961) 511. 8. R. W. Hellwarth et. al., to be published.
E463
SYNTHESIS AND CHARACTERIZATION OF NEW ELECTROACTIVE POLYMERS
L. P. Yu and L. R. Dalton Department of Chemistry, University of Southern California, Los Angeles, CA 90089-0482, U.S.A.
ABSTRACT Optical quality thin films of polyquinoxaline ladder polymers have been prepared by thermal treatment of open-chain precursor polymers derivatized with a variety of substituents to improve solubility. Polycondensation synthesis is emplo}ed to avoid problems of polymer branching and disorder. Both alkyl derivatization which improves solubility with little effect upon electronic properties and derivatization with vinylamine side groups which influences both physical and electronic properties are discussed. The planar conformation of ladder polymers optimizes ~-orbital overlap; thus, large optical nonlinearities are anticipated. Third order susceptibilities as high as 3 x 10-9 esu for pristine polymers have been measured by degenerate four wave mixing and third harmonic generation.
*Abbreviations used: X(3) = third order optical susceptibility, 0t = linear absorption; DFWM = degerate four wave mixing, FTIR = Fourier transform infrared spectroscopy, NMR = nuclear magnetic resonance spectroscopy, TGA = thermal gravimetric analysis, DMF = dimethylformamide, DMSO = dimethyl sulfoxide, DMAC = dimethylacetamide, TAB = 1,2,4,5tetraaminobenzene, DABD = 3,3'-diaminobenzidine, DCDEBQ = 2,5-dichloro3,6-bis(2-diethylaminovinyl) benzoquinone, DCDPBQ = 2,5-dichloro-3,6-bis(2d i p r o p y l a m i n o v i n y l ) benzoquinone, DCPBQ = 2,5-dichloro-3,6-bis(2p i p e r i d i n o v i n y l ) b e n z o q u i n o n e , DCHBQ = 2,5-dichloro-3,6-bis (hexamethyleneiminylvinyl)-l,4-benzoquinone, n = refractive index, OCDCDEBQ-TAB = open-chain precursor p o l y m e r p r e p a r e d by the polycondensation of DCDEBQ and TAB, L-DCDEBQ-TAB = ladder polymer prepared by thermal treatment of OC-DCDEBQ-TAB, L-DCDPBQ-TAB -- ladder polymer prepared by the polycondensation of DCDPBQ and TAB, SCE = saturated calomel electrode, THG = third harmonic generation. 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
Ea6a
INTRODUCTION The planar conformation of ladder polymers optimizes the overlap of ~orbitals in the polymer backbone thus enhancing electron delocalization [1]. For nitrogen containing ladder polymers, electron delocalization is also dependent upon the state of nitrogen [1,2] with the greatest delocalization observed for the imine form. Electronic properties are expected to depend upon both protonic and electronic doping [1,3]. From the preceding theoretical analysis, polyquinoxaline ladder polymers would appear to be excellent candidates for nonlinear optical and electroactive materials. The synthesis of prototype ladder polymers was effected some years ago [4]. Unfortunately, the generally observed poor solubility and low molecular weights have discouraged consideration of these materials either as structural or electroactive materials. The physical properties of the prototype ladder polymers are quite reasonable when one realizes that the ~-orbitals which generate interesting electronic properties are likely to participate in strong interchain interaction. It is reasonable that solubility can be improved by utilizing the steric interactions associated with substituents to destabilize polymer-polymer interactions. Moreover, solubility will likely improve if conformational constraints are relaxed. For an open-chain polymer, solubility should improve due to increased entropy and less favorable interchain orbital interaction. Thus, a logical route to preparation of ladder polymers in desired morphologies such as thin films would appear to be via a derivatized openchain precursor polymer route. We undertook the investigation of such an approach starting with the preparation of oligomers, precursor polymers, and ladder polymers derivatized with alkyl substituents [5]. We have demonstrated the fabrication of optical quality thin films of precursor polymers and the conversion of these to ladder structures (including prototype or underivatized ladders) by thermal processing. Indeed, asymmetrically derivatized materials have been used to fabricate films by Langmuir-Blodgett methods. For alkyl-derivatization, solubility is improved with little effect upon electronic properties. A somewhat more intriguing example is that of vinylamine derivatization where both physical and electronic properties are influenced by the side groups. These materials that will occupy our attention for the remainer of this discussion.
RESULTS Synthesis and molecular characteriza¢ion The general scheme for the preparation of derivatized dichloroquinone monomers, precursor polymers, and ladder polymers is shown in Fig. 1. All
E465
of the materials shown in Fig. 1 have been prepared. Because of space limitations we restrict this discussion to one representative example; namely, the preparation of the ladder polymer L-DCDEBQ-TAB and all of its precursor materials.
Cl
. R
I
+ HNR" + CH3CHO
~ O.;~,,~CI RT
R'= Mez,Et2, D , ~ , ~ , Pr 2, 8u2, 3 )
I,,~-'J0 H2N~. ,,NH2 DMF Co.,~,l '~'~R ,-"J01+ % H 2 RT
HoN
R = CH=CHNR"
R NH2 0
R
Fig. 1. General scheme for preparation of vinylamine-derivatized polymers.
The DCDEBQ monomer was prepared according to the method of Buckley et. al. [6] by adding diethylamine (5.8 g) to a stirred solution of chloranil (4.92 g) and freshly distilled acetaldehyde (2.9 g) in 1500 mL of toluene at 30 C. After 5 hrs, the amine salt was extracted with 0.5 N sulfuric acid and the toluene layer was concentrated by evaporation. The product was purified by chromatographic separation using deactivated alumina (yield 45%). The melting point of the product was observed to be 129-131 C. Elemental analysis (Galbraith): Calc. for C18H24N2C1202: C, 58.22; H, 6.47; N, 7.55; Cl, 19.14; found: C, 58.30; H, 6.53; N, 7.52; CI, 19.16%. 1H NMR (CDCI3): 1.2 (t, 12H, -CH3), 3.4 (q, 8H, -CH2-), 5.6 (d, 2H, =CHN-), 8.5 ppm (d, 2H, -HC=C). 13C NMR (CDC13): 12 (2C, -CH3), 14 (2C, -C'H3), 42 (2C, -CH2-), 51 (2C, -C'H2-), 94.5 (2C, -H13C=C), 119 (2C,13C=CC1), 139.5 (2C, 13C--O), 153 (2C, =13CHN), 178.6 ppm (2C, =13CC1).
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The TAB monomer was prepared according to literature procedure [7] just before use to avoid air oxidation. Elemental analysis: Calc. for C6H10N4: C, 52.17, H, 7.24; N, 40.58; found: C, 52.11; H, 7.09; N, 40.31%. The open-chain precursor polymer OC-DCDEBQ-TAB was prepared from the reaction of DCDEBQ with TAB in DMF solution carried out in a controlled atmosphere (nitrogen or helium) Vacuum Atmospheres glove box with constant monitoring of oxygen and moisture levels. To 100 mL of DMF was added 5.000 g of DCDEBQ and 1.855 g of TAB; the solutions was then stirred at room temperature overnight. The solution was poured into 500 mL of chloroform and the precipitate was collected by filtration, washed with chloroform and ethanol, and the dark brown prepolymer was dried at 50 C under vacuum (yield 95%). Solubilities in common organic solvents are as follows: 1.14 g/l, DMF; 0.81 g/1 DMSO; 1.05 g/l, DMAC; 0.63 g/l, m-cresol; 0.15 g/l, [(CH3)2N]3PO. An intrinsic viscosity of 1.4 dl/g was measured in DMF; thin films (0.5-5 microns) can be easily prepared by spreading DMF/prepolymer solution on a glass slide and removing the solvent. Elemental analysis: Calc. for (C24H34N6C1202)n: C, 56.58; H, 6.68; N, 16.50; C1, 13.95; found: C, 56.89; H, 6.45; N, 16.69; C1, 13.71%. 1H NMR (d6-DMSO): 1.2 (-CH3), 2.9 (-CH2-), 3.8 (-NH2), 5.87 (=CHN-), 6.8-7.0 (aromatic H), 8.4 ppm (HC=C). The ladder polymer L-DCDEBQ-TAB can be prepared in either of two ways. The first is to reflux the prepolymer in DMF solution overnight. A black powder precipitates out and is collected directly from DMF, purified by using ethanol in a soxhlet extractor and then dried at 100 C under vacuum (yield 90%). The second method involves heating a prepolymer film or powder to 140 C for 6 hrs. and at 260 C for 10 hrs. FTIR shows that the same material is obtained by either approach. Elemental analysis: Calc. for (C24H30N6C12)n: C, 60.88; H, 6.34; N, 17.75; C1, 15.01; found: C, 65.13; H, 4.49; N, 16.60; C1, 13.97; Calc. best fit structure, (C24H22N5.4CI1.7)n: C, 65.13; H, 4.93; N, 16.95; Cl, 13.53%. TGA, together with mass spectroscopic analysis of elimination products, suggests that water, HCI, and some ammonia are given off in the transformation of precursor polymer to fused ring ladder polymer. FTIR data provides strong evidence that the second condensation step has occurred to an appreciable extent as the C=O stretch at approximately 1650 cm -1 has disappeared in the heat treated material as has the NH2 bend (1510 cm-1); a C=N stretch ( 1625 cm -1) appears in the product. As seen in the electron micrographs presented in Fig. 2, thermal transformation can result is disruption of the film surface. In practice, we have not observed significant degradation in optical quality. A red-shift in the optical spectrum (~-x* interband transition) was also observed upon transformation of prepolymer to
Ea67
ladder polymer. By coating platinum electrodes with a thin film of ladder polymer we have investigated redox properties by cyclic voltammetry; the oxidation potential is found to be +1.1 V versus the SCE. Both the optical band gap and the oxidation potential are somewhat larger than expected from theory which may imply a less than perfect ladder lattice perhaps due to incomplete condensation or air oxidation.
Fig. 2. Electron micrographs of diethylamine-derivatized prepolymer (smooth surface) and ladder polymer (rough surface); ladder polymer was prepared by heating the prepolymer film.
Measurement of optical nonlinearity Ancillary_ measurements. Refractive indices were measured by measuring back reflection from the polymer-air interface at the operating wavelength (e.g., 532 rim). This reflection could be measured with an accuracy better than 3%. Film thicknesses were measured with a step profiler (Tencor Alpha Step Profiler Model 1000) and were verified by electron microscopy. M e a s u r e m e n t of third o r d e r susceptibilities. Although some measurements of optical nonlinearity were effected by third harmonic generation (THG), the bulk of our analysis has relied upon degenerate four
E468
wave mixing (DFWM) measurements. Three input beams are formed from 532 nm laser pulses having energy approximately 2 mJ and duration approximately 25 psec. In this geometry, delaying each of the three input beam pulses separately gives three different and independent sets of data for the strength and polarization of the fourth or signal beam. We have performed experiments with essentially all combinations of beam polarization and delay [8]. Representative susceptibility values are given in Table 1. TABLE 1 Optical nonlinearity data for representative vinylamine polymers. Material L-DCDEBQ-TAB (air-oxidized) OC-DCDEBQ-TAB L-DCDEBQ-TAB L-DCDPBQ-TAB L-DCPBQ-TAB L-DCPBQ-DABD L-DCHBQ-TAB
n
X(3) xl0-10(esu)
0c (lain-l)
1.75
0.6
1.1
1.70 1.80 1.82 1.83 1.80 1.80
1.0 3.0 2.8 12.5 12.8 11.2
1.7 1.3 1.4 1.4 1.7 1.4
The values given in Table 1 are for pristine polymers. Chemically doping polymers with electron accepting or donating dopants, (e.g., nitrosyl tetrafluoroborate), resulted in an increase in third order susceptibility. However, the resulting materials have not been characterized structurally at this time. DISCUSSION The third order susceptibilities measured for vinylamine ladders are quite impressive even when it is noted that the resonance contribution to these values is not negligible. A value of 2 x 10-9 esu is observed for BBL but the linear absorption at 532 nm is an order of magnitude greater (18.3 ~m -1) than for vinylamine-derivatized ladders. We have also conducted an extensive p r o g r a m p r e p a r i n g p o l y t h i o p h e n e and p o l y t h i o p h e n e / p o l y a n i l i n e copolymers. The partially resonance-enhanced values for these polymers is on the order of 5 x 10-10 esu which is a factor of 2 less than observed for the vinylamine ladders. As seen from Table 1, X(3) values vary with the extent of air oxidation of nitrogens. Oxygen elemental analyses (performed by Galbraith) and EPR g-values provide qualitative insight into the extent of
E469 oxidation and it is likely that up to 50% of the nitrogens can be oxidized; however, quantitative accuracy is not sufficiently good to permit totally unambiguous conclusions to be drawn from the data. The qualitative trend suggests that air oxidation interrupts ~-orbital overlap and limits electron delocalization. Incomplete condensation has the same effect and it may be difficult to separate these two effects at this time. The similarity of the values for the L-DCPBQ-DABD and L-DCPBQ-TAB polymers is suggestive of such interruption as the DABD portion is known to exist as a nonplanar segment. Comparison of data for open-chain prepolymer and fused-ring ladder polymer suggests that increased orbital overlap in the latter results in an enhanced value of third order susceptibility. From Table 1, it appears that larger values of susceptibility are routinely observed for the alicyclic vinylamine substituents when compared to dialkyl vinylamine-derivatized materials. NMR studies indicate substantially greater side group dynamics for the latter materials. It is not unreasonable that side group dynamics modulate the conjugation of vinylamine 7r orbitals with those of the polymer backbone. In this regard, it is interesting to note that not only is the electronic component diminished but the thermal component is enhanced consistent with the preceding speculation. When the various data sets recorded in this study are considered and compared with data for other ladder polymers, it appears that charge transfer involving the vinylamine substituents plays a role in enhancing optical nonlinearity. Intermolecular charge transfer involving electron donating dopants also appears to enhance nonlinearity. Although the values for third order susceptibility and optical transparency are not sufficiently good to seriously consider device application at this time, one speculation is of interest, at least academically. It can be noted that the linear index of refraction for several ladder polymers matches that of polystyrene. We have succeeded in synthesizing copolymers of ladder polymers with polystyrene and are currently imbedding ladder polymer coated polystyrene spheres of various sizes in different composite matrices. At low laser intensities, these matrices should transmit light but frustrated scattering may occur at higher light intensities. Clearly, one of the advantages of polymeric systems that deserves further investigation is the range of interesting and convenient processing options. ACKNOWLEDGEMENTS We are indebted to Dr. Donald Ulrich for motivating this research and to Professor R. W. Hellwarth and his research group for nonlinear optical measurements. We wish to thank M. R. Unroe and her colleagues at
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