J . Phys. Chem. 1992, 96, 2830-2836
2830
o.20 0.00
t
1
~
0
100
300
200
m
500
T(k)
Figure 2. Temperature variation of the transverse polaron bandwidth. not excited at low temperatures. Hopping may occur between equivalent chains or inequivalent chains. Although in one direction equivalent chains are spaced only 4 A apart, about the same as the distance between inequivalent chains, t , for the inequivalent chains is considerably larger.19 In further considerations we will deal only with in(19) See, for example: Mizes, H. A.; Conwell, E. M. Phys. Reu. B 1991, 43, 9053.
equivalent chains. For 4lt,l we take the maximum transverse bandwidth calculated for trans-polyacetylene by Vogl and Campbell,200.5 eV. Combining this factor with the other factors in (11.7), and assuming conjugation lengths of 30a and 100a, we find, for T = 0, WT = 5 X lo4 eV, corresponding to 6 K. For a more favorable, but less likely, hop, between two lengths of 30a, WT = 0.017 eV, corresponding to 200 K. We conclude that, when a suitable average over all pairs of conjugation lengths is taken, WT should certainly be less than 150 K, the lowest temperature at which measurements of transverse photoconductivity were taken.8 It is evident that the average hopping rate, and therefore conductivity or photoconductivity, transverse to the chains will be sample dependent. Samples with shorter average conjugation lengths should have higher transverse conductivity. Although we do not have the information on conjugation lengths required to obtain an accurate bandwidth or hopping rate for any given sample, we are able to determine the temperature dependence of the hopping rate, which does not depend on the 0,'s. Using the expression for the hopping rate obtained earlier," we have been able to calculate the temperature dependence of the hopping rate in the range -250 IT I 300 K.*' We find that it agrees well with the temperature dependence of the transverse photoconductivity measured in this range, where it is increasing rapidly. Registry No. trans-Poly(acetylene),25768-71-2. (20) Vogl, P.; Campbell, D. K. Phys. Rev. B 1990, 41, 12797. (21) Conwell, E. M.; Choi, H. Y.; Jeyadev, S., to be published.
Nonlinear Optical Properties of Polyanilines and Derivatives John A. Osaheni, Samson A. Jenekhe,* Department of Chemical Engineering and Center for Photoinduced Charge Transfer, University of Rochester, Rochester, New York 14627-0166
Herman Vanherzeele, Jeffrey S. Meth, Du Pont Central Research and Development, Wilmington, Delaware 19880-0356
Yan Sun, and Alan G. MacDiarmid Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 191 04-6323 (Received: September 23, 1991; In Final Form: November 27, 1991)
The third-order optical susceptibility x ( ~ ) ( - ~ w ; w , w , wof) polyanilines and derivatives has been systematically investigated by picosecond third harmonic generation spectroscopy on spin-coated thin films of the polymers in the wavelength range 0.9-2.4 pm (1.4-0.5 eV). It is shown that the magnitude of x ( ~ ) ( - ~ w ; w , w , wof) this class of polymers is as large as that of other conjugated polymers and that the optical nonlinearity depends on the oxidation level and the derivatization of the pphenylene rings. The dispersion of the optical nonlinearity is dominated by the three-photon resonance to the dipole allowed transition occuring at 1.8 eV, so that the excitonic transition is the major contributor to the optical nonlinearity of polyanilines. Polyemeraldine base, with an oxidation level of about 50%, has a larger optical nonlinearity than the fully oxidized form pemigraniline or poly(phenylani1ine) which we use as a model compound for the fully reduced form. The effects of derivatization are more complex. Methoxy substitutionof the phenyl ring increases the transition moment, which would increase the microscopic nonlinearity. However, these substituents also reduce the number density of the polymer repeat units, which in turn reduces the macroscopic nonlinearity.
-
Introduction Interest in conjugated polymers with large third-order nonlinear optical (NLO) properties for potential applications in photonic devices has grown t r e m e n d o ~ s l ybecause ~~ the third-order NLO effects in these materials are very rapid (subpicosecond). The ease of fabrication and molecular engineering of the materials is an additional advantage of polymers for future nonlinear optical *To whom correspondence should be addressed.
TABLE I: Intrinsic Viscosity of Polyanilines and Derivatives in MSA at 40 O C polymer 7,dL/g polymer 7 , dL/g PEMB 1.63 PDMAB 0.38 POTB 0.72 P4PAB 0.13 PMAB 0.47
devices. In order to synthesize new materials with larger thirdorder susceptibility or optimize existing ones, it is essential to
0022-365419212096-2830$03.00/0 0 1992 American Chemical Society
Nonlinear Optics of Polyanilines
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 2831
CHART I
CHART I1
understand the microscopic mechanism of the third-order N L O response and establish the molecular design criteria. One step in this direction would be to investigate the effects of molecular structure and dispersion on the optical nonlinearity of polymers. Such experimental data are needed for testing theoretical models and for a rational comparison of different materials. Accordingly, we have embarked on a series of systematic aimed at understanding the nature of the third-order N L O response and the underlying s t r u c t u r ~ (relationships ~) in conjugated polymers. Polyanilines represent a versatile family of polymers with interesting electronic, linear optical, and electrochemical properties which can be varied by addition or removal of electrons or protons from the polymer backbone and through derivatization of the pphenylene rings.6.' Additionally, the ease with which the various forms of polyanilines can be prepared makes this family of (1) (a) Marder S. R., Sohn, J. E., Stucky, G. D., Eds. Materials for Nonlinear Optics: Chemical Perspectives; ACS symposium Series No. 455; American Chemical Society: Washington, DC, 1991. (b) Prasad, P. N.; Williams, D. J. An Introduction to Nonlinear Optical Effects in Molecules and Polymers; John Wiley: New York, 1991. (c) Braas, J. L., Chance, R. R., Eds. Conjugated Polymeric Materials: Opportunities in Electronics, Optoelectronics, and Molecular Electronics, Kluwer Academic Publishers: Dordrecht, Holland, 1990. (d) Khanarian, G., Ed. Nonlinear Optical Properties of Organic Materials III; SPIE Proceedings Vol. 1337; The International Society for Optical Engineering: Bellingham, WA, 1990. (e) Khanarian, G., Ed. Nonlinear Optical Properties of Organic Materials II; SPIE Proceedings Vol. 1147; The International Society for Optical Engineering: Bellingham, WA, 1990. (f) Messier, J., Kajzar, F., Prasad, P., Ulrich, D. R., Eds. Nonlinear Optical Effects in Organic Polymers; Kluwer Academic Publishers: Dordrecht, Holland, 1989. (g) Heeger, A. J., Orenstein, J., Ulrich, D. R., Eds. Nonlinear Optical Properties of Polymers; Material Research Society Proceedings, 1988; Vol. 109. (h) Prasad, P. N., Ulrich, D. R., Eds.Nonlinear Opiical Properties of Polymers, Plenum: New York, 1988. (i) Chemla, D. S.,Zyss, J., Eds. Nonlinear Optical Properties of Organic Molecules and Crysfals,Academic Press: New York, 1987; Vols. 1 and 2. (2) (a) Osaheni, J. A.; Jenekhe, S.A.; Vanherzeele, H.; Meth, J. S. Chem. Mater. 1991,3,218-221. (b) Jenekhe, S. A,; Roberts, M.; Agrawal, A. K.; Meth, J. S.; Vanherzeele, H. Mater. Res. SOC.Proc. 1991, 214, 55-59. (c) Agrawal, A. K.; Jenekhe, S. A.; Vanherzeele, H.; Meth, J. S. Chem. Mater. 1991,3,765-768. (d) Vanherzeele, H.; Meth, J. S.;Jenekhe, S.A.; Roberts, . Lett. 1991, 58, 663. M. F. ~ p p lPhys. (3) Osaheni, J. A.; Jenekhe, S. A.; Vanherzeele, H.; Meth, J. S. ACS. Polym. Prepr. 1991,32, 154. (b) Yang, C. J.; Jenekhe, S. A.; Vanherzeele, H.; Meth, J. S. ACS. Polym. Prepr. 1991, 32, 165. (c) Agrawal, A. K.; Jenekhe, S. A.; Vanherzeele, H.; Meth, J. S. ACS. Polym. Prep. 1991, 32, 124. (d) Jenekhe, S. A.; Roberts, M. F.; Vanherzeele, H., Meth, J. S . ACS. Polym. Prepr. 1991, 32, 140. (4) (a) Vanherzeele, H.; Meth, J. S.; Jenekhe, S.A,; Roberts, M. F. J . Opr. Soc. Am. B, in press. (b) Jenekhe, S. A.; Yang, C. J.; Vanherzeele, H.; Meth, J. S. Chem. Mater. 1991, 3, 985-988. ( 5 ) (a) Meth, J. S.; Vanherzeele, H.; Jenekhe, S. A.; Yang, C. J.; Roberts, M. F.; Agrawal, A. K. SPIE Proc. 1991, 1560, 13-24. press. (b) Agrawal, A. K.; Jenekhe, S. A.; Vanherzeele, H.;Meth, J. S. J . Phys. Chem., this issue. ( 6 ) MacDiarmid, A. G.; Chang, J. C.; Hatpern, M.; Huang, W. S.; Mu, S. L.; Somasin, N. L. D.; Wu, W.; Yaniger, S. I. Mol. Cryst. Liq. Crysr. 1985, 121, 173. (7) (a) Sun,Y.; MacDiarmid, A. G.;Epstein, A. J. J . Chem. Sor., Chem. Commun. 1990, 529. (b) do Santons. M. C.; Bridas, J. L. Phys. Rev. Lett. 1989,62, 2499. (8) Cao, Y. Synth. Met. 1990, 35, 319, and references therein.
3
h
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Figure 1. FTIR absorption spectra of (a) P E M B and (b) POTB.
polymers a good candidate for investigating structure-property relationships. The wide range of associated electrical, electrochemical, and optical properties, coupled with the good environmental stability has motivated much research efforts6-10 in developing polyanilines as electronic materials. However, investigation of the NLO properties of polyanilines has been very limited. Earlier photoinduced absorption spectroscopy9 of polyaniline indicated that the material possesses significant NLO properties and this was confirmed by optical Kerr effect measurements at 1.06 pm in N-methylpyrrolidone solutions.1° However, it is only r e ~ e n t l ythat ~ , ~the ~ various forms of polyanilines and derivatives have been systematically investigated by third harmonic generation which probes the purely electronic contributions to the N L O properties. Degenerate four-wave mixing measurements'' of the third-order N L O properties of polyaniline at 0.62 pm have reported x(~)(-w;w,-w,w) to be as large as 8 X 1O-Io esu while that of a model system comprising derivatized polyaniline entrapped in a silica gel hostI2 was 4.8 X esu a t 1.06 pm. In order to understand the origins of the measured x ( ~and ) to correlate the data with molecular structures, it is important to probe the NLO properties of the materials by third harmonic generation (THG) spectroscopy. Such THG spectra of x ( ~are ) needed to test various theoretical models of the NLO response of organic materials and polymers. In this paper, we report the x ( ~ ) ( - ~ w ; w , w , wspectra ) of spincoated thin films of polyanilines and derivatives in the fundamental wavelength range 0.9-2.4 pm (1.4-0.5 eV). We have investigated the effect of oxidation level, 1 - y , in Chart I, on the nonlinear optical properties of polyanilines, from the fully reduced (1 - y = 0 ) , polyleucoemeraldine base (PLEMB), to the fully oxidized (1 - y = l), polypernigraniline (PPGN). Also, thep-phenylene rings that alternate with the nitrogen in the polyaniline backbone were derivatized with different substituents ( R l , R2)as in la-ld in order to investigate the effect of derivatization on nonlinear optical properties. The effect of incorporating an additional phenylene ring into the polyaniline backbone, such as in poly(4phenylaniline), Chart 11, on x@) was also studied and used as a model compound for the fully reduced form of polyaniline. The x ( ~spectra ) for all the polyanilines and derivatives as well as a (9) (a) Epstein, A. J.; Ginder, J. M.; Roe, M. G.; Gustafson. J. L.; Angelopoulos, M.; MacDiarmid, A. G. In ref lg, pp 313-381. (b) McCall, R. P.; Ginder, J. M.; Leng, J. M.; Ye, H. J.; Manohar, S. K.; Masters, J. G . ; Asturias, G. E.; MacDiarmid, A. G.; Epstein, A. J. Phys. Rev. B 1990, 41, 5202. (10) Ermer, S . ; Aron, K. P.; Hansen, G. A,; Lipscomb, G. F.;Lytel, R.; Thakara, J. I.; Ticknor, A. J.; Yaniger, S. I. Proc. 3rd Int. SAMPE Conf., June 20-22, 1989, 429-438. (1 1) Wong, K. S.; Han, S. G.; Vardeny, Z . V. Synrh. Mer. 1991, 41-43, 3209. (12) Mattes, B. R.; Knobbe, E. T.; Fuqua, P. D.; Nishida, C. F.; Chang, E. W.; Pierce, B. M.; Dunn, B.; Kaner, R. B. Synrh. Mer. 1991, 41-43, 3183.
2832 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 theoretical fit of the IX(~)(-~O;W,O,W)( dispersion data are presented and discussed.
Experimental Section Synthesis, Characterization, and Thin Film Preparation. Polyemeraldine salt (hydrochloride) (PEMS) was prepared by the chemical oxidation of aniline with ammonium persulfate in 1 M HCl in air using the literature m e t h ~ d 'and ~ ~ was , ~ subsequently converted to the emeraldine base form (PEMB) by stimng in 0.1 M N H 4 0 H solution. Using similar approaches, poly(o-toluidine) base (POTB),13Cpoly(2-methoxyaniline) base (PMAB), and poly(2,5-dimethoxyaniline) base (PDMAB) were prepared by chemical oxidation of o-toluidine, 2-methoxyaniline, and 2,5dimethoxyaniline respectively. Poly(4phenylaniline) salt (P4PAS) was synthe~ized'~ by the chemical oxidation of 4-phenylaniline with ammonium persulfate in 2 M HCl/acetonitrile solution (68:32 vol/vol) at 0-5 "C.The greenish-brown precipitate was collected and extracted with acetonitrile in a Soxhlet extractor to remove oligomers. It was subsequently converted to the base form (P4PAB) using a procedure similar to PEMB synthesis. The same polymer (P4PAB) can also be prepared from 4-phenylaniline in acetonitrile using copper tetrafluoroborate C U ( B F ~as) ~the oxidizing agent instead of using ammonium persulfate. The fully reduced form of polyaniline, polyleucoemeraldine base (PLEMB), was obtained by the reduction of PEMB with phenylhydrazine in deoxygenated ether.15 The fully oxidized form, polypernigraniline (PPGN), was supplied by MacDiarmid's laboratory, the synthesis and characterization of which have been reported.7a The intrinsic viscosities of the polymers, measured at 40 O C in methanesulfonic acid (MSA) are shown in Table I. The intrinsic viscosity values of 0.72-1.63 dL/g for PEMB and POTB are typical of high molecular weight polyanilines. For example, the N M P soluble fraction of polyemeraldine base has been reportedI6 to have molecular weights as high as 200 000. Infrared absorption spectra were recorded on a Nicolet Model 2SXC Fourier transform infrared (FTIR) spectrometer. The characteristic absorption bands of polyanilines, namely, the C=N in the quinoidal units which appears at about 1600 cm-I, the benzenoid stretches at -1500 cm-I, the N-H stretches at 3400-3100 cm-I, and the aromatic C-H stretches at -3010 cm-I were observed. However, the out-of-plane aromatic C-H bending absorptions of substituted polyanilines appear at different positions depending on the substitution pattern. For example (see Figure l), in POTB, the bands at 810 and 870 cm-I indicate a 1,2,4trisubstituted benzene, whereas the spectrum of polyemeraldine base shows one band a t 830 cm-l because of 1,4-disubstitution pattern,17 in agreement with previous report in the literature.1s-20 The aliphatic C-H stretches around 2900-2800 cm-' due to -CH3 and -OCH3 were observed in POTB, PMAB, and PDMAB but were absent in PEMB as expected. The methoxy derivatives show characteristic C 4 - C asymmetric and symmetric stretches at 1210 (13) (a) MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Somasiri, N. In Conducring Polymer; Alcacer, L.,Ed.; Reidel: Dordrecht, Holland, 1987; pp 105-120. (b) Cao, Y.; Andreatta, A.; Heeger, A. J.; Smith, P. Polymer 1989, 30, 2305. (c) Wei, Y.; Fock, W. W.; Wnek, G. E.; Ray, A.; MacDiarmid, A. G.; Akhtar, M.; Kiss, Z.; Epstein, A. J. Mol. Crysf.Liq. Cryst. 1988, 160, 15 1. (14) (a) Guay, J.; Leclerc, M.; Dao, L. H. J . Elecrroanal. Chem. 1988, 251, 31. (b) Guay, J.; Dao, L. H. J . Elecrroanal. Chem. 1989, 274, 135.
(15) Ray, A.; Asturias, G. E.; Kershner, D. C.; Richter, A. F.; MacDiarmid, A. G.; Epstein, A. J. Synth. Mer. 1989, 29, E141. (16) Tang, X.;Sun, Y.; Wei, Y. Makromol. Chem. Rapid Commun. 1988, 829-834. (17) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectroscopic Identification of Organic Compounds, 4th ed.;John Wiley: New York, 1981; Chapter 3. (18) Wei, Y.; Hsueh, K. F.; Nagy, S.; Ray, A,; MacDiarmid, A. G.; Dykino, J.; Epstein, A. J.; Wnek, G. E. Mater. Res. SOC.Symp. Proc. 1990, 173, 341. (19) (a) Lu, F. L.; Wudl, F.; Nowak, M.; Heeger, A. J. J. Am. Chem. SOC. 1986, 108, 831 I . (b) Wudl, F.; Angus Jr., R. 0.; Lu, F. L.; Allemand, P. M.; Vachon, D. J.; Nowak, M.; Liu, Z. X.;Heeger, A. J. J . Am. Chem. SOC.1987, 109, 3677. (20) Tang, J.; Jing, X.;Wang, B.; Wang, F. Synrh. Mer. 1988, 24, 231.
Osaheni et al. 0
-1 9
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I
I
'i 0
N. 3 00
3225
ZkO
21t75
2100
1?25
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475
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WRVENUMBEA
Figure 2. FTIR absorption spectrum of P4PAB.
and 1030 cm-I, respecti~ely.'~These C 4 - C stretches in PDMAB are stronger than those in PMAB because of the disubstitution in PDMAB, and their intensities are comparable to those of the benzenoid stretches. The IR absorption spectrum of P4PAB, shown in Figure 2, exhibits features similar to what has been previously reported in the 1 i t e r a t ~ r e . IThe ~ ~ C=N stretch which is due to the quinoid segments of the polymer appear at 1602 cm-I. The N-H stretches appear around 3400-3300 cm-l, the C-N stretching of an aromatic amine appears at 1316 cm-'. The peak at 815 cm-I which is characteristic of a 1,4disubstituted benzene ringz1Zindicates para linkages of the monomer units. The presence of imine nitrogen in the polymer backbone was further supported by the fact that the polymer could be further reduced with phenylhydrazine to obtain the fully reduced form (y = 0 in Chart 11). We estimated the oxidation state of the emeraldine base forms of these polymers from the intensity of the I R bands at 1600 cm-I (quinoid) and 1500 cm-I (benzenoid).20*21From this, we found that, 1 - y , in PEMB, POTB, PMAB, and PDMAB is 0.44, 0.42, 0.47, and 0.5 1, respectively. Thin films for third harmonic generation (THG) experiments were prepared by spin coating of N M P solutions of the polymers at 3000 rpm for 60 s onto optically flat fused silica substrates (5 cm in diameter). The film thicknesses were measured with an Alpha-step 200 (Tencor Instruments) profilometer which has a resolution of 1 nm as well as with an optical technique. Thin films of PEMB, POTB, PMAB, and PDMAB were 27,58,40, and 91 nm thick, respectively. Thin films of PPGN and P4PAB were 38 and 501 nm, respectively. Electronic absorption spectra of thin films and dilute solutions in N M P were recorded on a Perkin Elmer Model Lamda-9 UVvis-near-IR spectrophotometer. Thud Harmonic Generation Experiment. The third harmonic generation (THG) experiments were performed with a picosecond laser system which is continuously tunable in the range 0.6-4.0 ~ m . * The ~ detailed procedure for the measurement of the third-order susceptibility x(~)(-~w;w,o,w) with this laser system has also been described in detail e l ~ e w h e r e . ~Briefly, ~ ~ ~ , the ~~ computerantrolled system is based on a high-power modelocked Nd:YLF laser which synchronously pumps a dye laser and seeds a Nd:YLF regenerative amplifier. A Glan Taylor polarizer near the output of the laser system rejects one of the orthogonally polarized output pulses (signal or idler). The remaining output
-
(21) Furukawa, Y.; Ueda, F.; Hyodo, Y.; Haroda, I.; Nakajima, T.; Kawagoe, T. Macromolecules 1988, 21, 1297. (22) Pavia, D. L.; Lampman, G. M.; Kriz, G.S. Inrroducrion fo Organic Laboratory Techniques; a Conrempurary Approach, 3rd ed.;Saunders College Publication: Philadelphia, 1988; p 695. (23) Vanherzeele, H. Appl. Opr. 1990, 29, 2246.
Nonlinear Optics of Polyanilines beam is attenuated to 100 pJ per pulse and its polarization direction is adjusted to vertical by means of a Soleil-Babinet compensator followed by a second Glan Taylor polarizer. The energy per pulse is monitored by a calibrated (cooled) Ge detector, sampling a small fraction (4%) of the beam. The remaining beam is split (90/10) into two beams. The weaker one is used in a reference path, and the stronger one in the sample path. The focusing and collection optics as well as the THG detection equipment (filters, monochromators, photomultiplier tube, and boxcar integrator) in both paths are identical except for a vacuum cell used only in the sample path. In the latter, the sample (fused silica as the reference material and/or the polymer film under study on a silica substrate) is rotated about a vertical axis to generate a THG Maker fringe pattern. The vacuum cell removes undesirable contributions from air to the THG signal from the sample. In the reference path, the THG signal from 0.5 pm film of poly(benzimidazobenzophenanthro1ine) semiladder polymer (BBB)2bon a silica substrate is used to generate the third harmonic. To improve the signal to noise ratio, all laser shots are rejected for which either the energy of the fundamental or third harmonic in the reference arm fall outside predetermined windows. In this way, the instabilities (both in energy per pulse and pulse width) of the laser source are effectively reduced to 1%. By the same token, any possible degradation of the sample in the reference arm can be monitored in real time. By taking the ratios of the THG signals in both arms, the effects of fluctuations in both power and pulse width of the fundamental beam are further eliminated. Finally, the standard deviation of this ratio typically is OS%, by averaging the ratio of the THG signals over some 250 laser shots. The present THG experiments were performed at a fundamental wavelength of 0.9-2.4 pm. The reported x ( ~values ) are average values corrected for absorption at the third harmonic wavelength248and were obtained relative to the x ( ~of) fused silica (2.8 X esu at 1.9 pm).24bBecause the samples were thin, the optical density was low, and so the correction term is still valid, even in the relatively strongly absorbing region. Although recent work indicates that the reference value for fused silica may be wrong by a factor of 2,25it is still used here to allow comparisons to be made to other THG experiments which used the same reference value. Details of the exact determination of the x ( ~ ) values are discussed further e l s e ~ h e r e . ~ The * ~ ~error J ~ for the reported x ( ~values ) is typically f 2 0 % and is mostly due to the error in film thickness and index of refraction measurements. The repeatability of individual results for each material is f5%.
Theory The ~ ( ~ ) ( - 3 w ; w , w , wdispersion ) data were fit to the two-level model based on the essential states theory.2631 First, the density is calculated from a theory based on atomic contributions to the physical density. For liquids and many polymers, this calculation is accurate to f5%. From this number, we can calculate the number density of polymer repeat units. The ~ ( ~ ) ( - 3 w ; w , w , wis) then calculated from the formula x(3)xxxx(-3~;w,~#J)
= (1 /5)Nf,3f3,yxxxx(-3w;w,w,w)
(24) (a) Houlding, V. H.; Nahata, A.; Yardley, J. T.; Elsenbaumer, R. L. Chem. Murer. 1990, 2, 169. (b) Buchalter, B.; Meredith, G. R. Appl. Opt. 1982, 21, 3221. (25) Heflin, J. R.; Cai, Y. M.; Garito, A. F. J . Opt. SOC.Am. B 1991,8, 2132. (26) Langhoff, P. W.; Epstein, S.T.; Karpus, M. Reu. Mod. Phys. 1972, 44, 602. (27) Kuyzyk, M. G.; Dirk, C. W. Phys. Reu. A 1990, 41, 5098. (28) (a) Torruellas, W. E.; Rochford, K. B.; Zanoni, R.; Aramaki, S.; Stegeman, G. I. Opt. Commun. 1991,82. 94. (b) Torruellas, W. E.; Zanoni, R.; Stegeman, G. I.; Mohlmann, G. R.; Erdhuisen, E. W. P.; Horsthuis, W. H. G. J . Chem. Phys. 1991, 94, 6851. (c) Torruelas, W. E.; Neher, D.; Zanoni, R.; Stegeman, G. I.; Kajzar, F.; Leclerc, M. Chem. Phys. Lett. 1990, 175, 1 1 . (29) Heflin, J. R.; Wong, K.Y.; Zamani-Khamiri, 0.;Garito, A. F. Phys. Reo. B 1988, 38, 1573. (30) Soos, Z. G., Mcwilliams, P. C. M.; Hayden, G. W. Chem. Phys. Lett. 1990, 171, 14. (31) Dixit, S. N.; Guo, D.;Mazumdar, S. Mol. Cryst. Liq. Cryst. 1991, 194, 33.
-
7
a
I
- PEMB
WAVELENGTH, X ( nm )
Figure 3. Optical absorption spectra of thin films of (a) PEMB and (b) POTB.
TABLE I 1 Summary of the Optical Absorption Spectra and Resonant x@)Values of Polyanilines and Derivatives x(’) at 3Amp,.b polymer PEMB PPGN POTB PMAB PDMAB P4PAB
A,,. 640 560 605 620 580 324
nm l e v i [1.94] [2.21] [2.05] [2.0] [2.14] [3.83]
aAbsorption coefficient at A,.,
a,” io4 cm-I
3.2 2.8 2.3 9.8 7.0 2.9
esu 31 14 18 39 22 1 .8c
* A t 1.83 pm. C A t0.96 pm
In this equation, N is the number density of polymer repeat units,
f is the standard Lorentz local field factor, and y is the hyperpolarizability calculated from the two-level model. The factor of 1/5 is for orientational averaging of y. We assume that the nonlinearity is dominated by the tensor component along the polyaniline chain. In addition, recent theoretical work that we have performed to include the effects of inhomogeneous and vibronic broadening on ~ ( ~ ) ( - 3 w ; w , w , whave ) led to the use of a hyperbolic secant line shape in place of the Lorentzian line shape in the perturbation expre~sions.~~ The Lorentz local field factors are calculated by using the perturbation expression for the linear polarizability in the two-level model. Thus, the three parameters which we need to fit the data are the energy of the transition, the phenomenological width that occurs from the broadening mechanisms, and the strength of the transition moment. Since we are only using a two-level model, and we do not have the nonresonant contributions to the polarizability from the higher-lying excited states, the transition moment is artificially increased by the model. The transition moment must be interpreted as a fitting parameter and not as the true transition moment. The fact that the number density is obtained for one polymer repeat unit also emphasizes this interpretation. However, since all the polymers have the same basic structure, the variations in the number density are meaningful. A similar approach by Messier also led to uncharacteristically large transition dipole moments.32
Results and Discussion Linear Optical Properties. Figure 3 shows the optical absorption spectra of thin films of PEMB and POTB. These spectra exhibit two absorption bands, one in the UV region near 320 nm (3.9 eV) and the other in the visible region near 620 nm (2 eV), which are characteristic of partially or completely oxidized polyanilines and derivatives. The higher energy absorption band in polyanilines * which defines the optical has been assigned to the 1 ~transition band gap of the materials. The lowest energy absorption band in the visible region (-2 eV) has been shown to be due to excitonic t r a n ~ i t i o n . ~The ~ . ~major ~ changes in the linear optical properties (32) Messier, J . In ref If, pp 47-60. (33) Yue, J.; Wang, Z.; Cromack, K. R.; Epstein, A. J.; MacDiarmid, A. G. J . A m . Chem. SOC.1991, 118, 2665.
Osaheni et al.
2834 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 2.5 1 a
-
...... 8-level model
-Absorption
l P4PAB
Expt.
PMAB
b - PLEMB
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Figure 4. Optical absorption spectra of (a) P4PAB and (b) PLEMB in
0
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0
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5 1
5
4
Figure 7. The I X ( ~ ) ( - ~ W ; ~ , W , W ) ~spectrum of PMAB plotted with a twolevel model fit (dashed line) and optical absorption spectrum (solid line).
Expt.
PEMB
3
2
Energy (eV)
...... 2-level model
-Absorption
1
5.010”’
...... 2-level mod4
-Absorption
Expl.
PDMAB
4.0 10’’’
1
3.2 10‘”
2.8 10’’ 2.4 10”’
3
2.010’”
I
1.8 10”‘
”
1-
1.210”’
8.0 10’” 2
1
0
3
5
4
4.0 10” a
Energy (evl
Figure 5. The 1~(~)(-3w;w,w,w)lspectrum of PEMB plotted with a twolevel model fit (dashed line) and optical absorption spectrum (solid line).
-Absorptlon
0 0
1
0
5
4
E w v (ev)
2-lwei model
POTB
3
2
Expl. 4.0 1 0”
’
Figure 8. The (~(~)(-3w;w,w,w)lspectrum of PDMAB plotted with a two-level model fit (dashed line) and optical absorption spectrum (solid line).
-Absorption
...
PPGN 4
1
’
2-1eveI model Expt.
1
2.0 10.1‘
3.5 1.6 10’”
3
.
-$a
.
0 ‘ 0
‘ 0 1
3
2
4
5
Energy @VI
Figure 6. The ( ~ ( ~ ) ( - 3 w ; w , w , wspectrum )l of POTB plotted with a twolevel model fit (dashed line) and optical absorption spectrum (solid line).
of polyaniline with oxidation level and derivatization occurs as shifts of the position of the “exciton” band compared to the band gap transition. A summary of the optical absorption spectra of all the polyanilines investigated is given in Table 11, including the lowest energy absorption band maximum (&) and the absorption From Table 11, it is seen that neither coefficient (a)at A,,. further oxidation t o P E N nor derivatization of the p-phenylene (34) (a) Kim, Y. H.; Foster, C.; Chiang, J.; Heeger, A. J. Synrh. Mer. 1989, 29, E285. (b) Phillips, S. D.; Yu,G.; Cao, Y.; Heeger,A. J . Phys. Reo. B 1989, 39, 10702. (c) Conwell, E. M.; Duke, C. B.; Paton, A,; Jeyadev, L. J . Chem. Phys. 1988, 88, 3331, and references therein. (d) Stafstrom, S.; Bre’das, J. L.; Epstein, A. J.; Woo, H. S.; Tanner, D. B.; Huang, W. S.; MacDiarmid, A. G. Phys. Reu. Lett. 1987, 59, 1464.
P
2.5 2 1.5
l.zlo’” 8.0 10.’2
1
4.0 10”
t 8
F-
*
0.5 0
0
Figure 9. The I x ( ’ ~ ( - ~ w ; w , w , w ) ~ spectrum of PFGN plotted with a twolevel model fit (dashed line) and optical absorption spectrum (solid line).
rings of polyemeraldine base (PEMB) increases the lowest energy In fact, all have optical spectra that are absorption band A,., blue-shifted from PEMB. As evidenced by values of the absorption coefficients in Table 11, the oscillator strength of the excitonic transition increases dramatically with 2,Sdimethoxy and 2methoxy substitutions. As shown in Figure 4b, the optical absorption of an N M P solution of the fully reduced form of polyaniline, PLEMB, is characterized by a single absorption band in the UV at 324 nm (3.83 eV). The solution optical absorption
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 2835
Nonlinear Optics of Polyanilines
-Abwrpllon
0
Expt.
TABLE III: Fitting Parameters for the x ( ~Dispersion ) of Polyanilines and Derivatives
P4PAB
number transition density, dipole energy, width, density, eV X102’ cm-3 moment, D eV g/cm3
3.0 10’”
1
polymer 2.0 10”
PEMB POTB PMAB PDMAB PPGN
tolerance, 7% 4.0h
0
1
2
3 Enw W )
4
5
I
-
v
spectrum of poly(4-phenylaniline) (P4PAB) which we use as a model compound for the fully reduced form of polyaniline is also shown in Figure 4. The spectrum of P4PAB is very similar to that of PLEMB except that it is slightly red-shifted and has a shoulder in the visible. Nonlinear Optical Properties. Figures 5-10 show the energy dispersion of X ( ~ ) ( - ~ W ; O , W , W ) for the six polyanilines studied, plotted with optical absorption spectrum. The data are plotted at the third harmonic energy, not the fundamental energy, to effect comparison. The magnitude of the optical nonlinearity compares favorably with the reported x(~)(-~w;o,w,w) of some other conjugated polymers.35 For example, films of poly@-phenylene vinylene) (PPV) and its methoxy derivatives have a x ( ~of) 7.8 X lC1*and 5.4 X 1C1Iesu, respectively, at 1.85 pm.35ab However, esu at 1.85 highly oriented PPV films have a x ( ~of) 1.5 X pm,35cwhile that of poly(thiophene vinylene) is 3.2 X lo-” esu at 1.85 pm.35d Ideally, a rational basis for the comparison of the ~ (of9one class of polymers with another would be the x ( ~spectra. ) Unfortunately, most reported x ( ~data ) are generally at a single wavelength in which case,adequate comparison cannot bejustified. As seen in Figures 5-9, the resonance behavior of the nonlinear optical response, which is dominated by the three-photon resonance, tracks the excitonic transition in the polymers very well. This suggests that this transition is dominating the optical nonlinearity in these materials. Theoretical Modeling. The dotted lines in each of Figures 5-9 are the two-level model fits to the data. There appears to be a very good correlation between the model and the data, again suggesting that the exciton transition dominates the nonlinear optical response. The fitting parameters are collected in Table 111. As mentioned previously, the number density is per polymer repeat unit, and is an independently calculated value. Note the correlation between the number density and derivatization. The larger the substituent, the lower the number density. Also notice that the transition dipole moment increases as the electron-donating strength of the substituent increases. This is as expected. While the absolute numbers are not reliable, the trend is meaningful. The energies and widths simply model those of the absorption spectrum. We also report in Table I11 the calculated physical density. The calculated density of PEMB (1.28 g/cm3) is in excellent agreement with the reported36value of 1.25 g/cm3. Effect of Derivatization. The most significant difference in the magnitude of the x ( ~of) these polymers occurs principally near the peak of the three-photon resonance. A summary of the (35) (a) Kaino, T.; Kubodera, K. F.; Tomura, S.; Kurihara, T.; Saito, S.; Tsutsui, T.; Tokito, S. Electron Left. 1987, 23, 1095. (b) Kaino, T.; Saito, S.;Tsutsui, T.; Tokito, S. Appl. Phys. Lett. 1989, 54, 1619. (c) Bubeck, C.; Kaltbeitzel, A.; Lenz, R. W.; Neher, D.; Stenger-Smith, J. D.; Wegner, G . In ref If, pp 143-147. (d) Kaino, T.; Kubodera, K.; Kobayashi, H.; Kurihara, T.; Caito, S.; Tsutsui, T.; Tokito, S.; Murata, H. Appl. Phys. Lett. 1988, 53, 2002. (36) Focke, W. W.; Wnek, G . E. J . Elertroanal. Chem. 1988, 256, 343.
m x 2.0: 0.0 1.0
v
.-. 0-0
PMAB
A-A
POTB
13.8 14.4 16.0 16.2 13.2 f7
1.72 0.62 1.83 0.60 1.85 0.57 1.61 0.72 1.91 0.71 f17 f16
1.28 1.14 1.25 1.29 1.27 f5
PDMAB
3.0-
7
Figure 10. The Ix(~)(-~w;w,w,w)~ spectrum of P4PAB plotted with the optical absorption spectrum (solid line).
8.52 6.57 6.23 5.09 8.46 f5
1, WAVELENGTH, A (pm)
Figure 11. The ~ x ( ~ ) ( - ~ w ; w , w ,spectra w ) ~ of polyaniline derivatives: PMAB, PDMAB, and POTB.
three-photon resonance-enhanced x ( ~values ) in Table I1 shows that PMAB has the largest x ( ~(3.9 ) X lo-” esu) compared with the basic polyemeraldine base and other derivatives studied. This enhancement of x ( ~by ) a 2-methoxy substitution on the p phenylene rings of polyaniline could not have been predicted from the linear optical properties. Although the energy of the exciton band in the optical absorption spectra is smaller in PEMB (1.94 eV) than in PMAB (2.0 eV), the 2-methoxy substituted material has a larger absorption coefficient. The primary difference in the two-level model fitting parameters for PMAB and PEMB lies in the transition dipole moment and the number density N the transition moment is larger in PMAB whereas the number density is larger in PEMB. Figure 11 shows the I x ( ~ ) ( - ~ w ; w , w , w ) ( spectra of the three derivatives of polyemeraldine base: PMAB, PDMAB, and POTB. The magnitude of the third-order susceptibility is in the decreasing order PMAB > PDMAB > POTB throughout the wavelength range in Figure 11. If comparison to the basic polyemeraldine base (PEMB) is made, the order of decreasing third-order susceptibility in the 0.9-2.4 pm wavelength range is PMAB PEMB > PDMAB > POTB. Thus, at the same level of oxidation, either 2,Sdimethoxy or 2-methyl substitution leads to a significant reduction in the third-ordef nonlinear optical properties of polyemeraldine base. Although one might have thought that the strong electron-donating nature of the 2J-dimethoxy substitution on the p-phenylene rings of PEMB would increase the electron density on the polymer backbone and enhance the optical nonlinearity of PDMAB compared with PMAB or PEMB, the contrary experimental results underline the need for systematic studies of the structure-^(^) relationships in order to establish true molecular design criteria for enhancing x ( ~in) polymers. Although 2,Sdimethoxy substitution increases the absorption coefficient and transition moment relative to polyemeraldine base, similar to 2-methoxy substitution, the number density is significantly reduced in the process resulting in a reduction of the macroscopic optical nonlinearity. Effect of Oxidation State. In our earlier communication,2awe alluded to the oxidation state as an important factor in understanding the structure-^'^) relationships in the polyanilines. We now have a more complete picture of the effect of oxidation state on the third-order NLO properties of polyaniline. Figure 12 shows a comparison of the I x ( ~ ) ( - ~ w ; w , w , w ) ~ spectra of the fully oxidized polyaniline (PPGN) and polyemeraldine base ( P E M B ) which is
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Osaheni et al.
2836 The Journal of Physical Chemistry, Vol, 96, No. 7, 1992 0-0
A-A W-W
PEMB PPGN P4PAB
h
/
0.0
I
\
~,
1.20
1.60
2.00
2.40
WAVELENGTH, X (pm)
Figure 12. The (x(’)(-~w;w,w,w)~ spectra of polyaniline at different oxidation levels: PEMB, PPGN, and P4PAB.
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about half-oxidized (1 - y 0.44). The fully oxidized form has a smaller x ( ~value, ) compared with the intermediate oxidized form, essentially throughout the spectrum. At the three-photon resonance peak 3(X,, 1.83 pm), the magnitude of the x ( ~ ) ( 3w;w,w,w) of PEMB is 3.7 X lo-” esu which is a factor of 2.64 larger than that of PPGN at the same wavelength. It was anticipated2athat the all-sp2 nitrogen in PPGN would facilitate greater electronic delocalization and greater polarizability that could enhance the xQ) of PPGN over that of PEMB. Theoretical c a l c ~ l a t i o n shad ~ ~ also predicted that PPGN with degenerate ground state can develop nonlinear solitons with fractional charge, and since the hyperpolarizability values are related to charge ~eparation,~~ actual or virtual photogeneration of soliton pairs with fractional charges in PPGN was expected to intrinsically provide for such a mechanism. On the basis of this, the nonlinear optical properties of PPGN was expected to be the most interesting. The energy dispersion in the nonlinear optical properties of both polyemeraldine base (PEMB) and polypernigraniline (PPGN) are well accounted for by the two-level model fits of the x ( ~dispersion ) in Figures 5 and 9, respectively. However, the two-level model fitting parameters for PEMB and PPGN (Table 11) are all so comparable that they do not provide a basis to account for the observed difference in the magnitude of their three-photon resonant optical nonlinearities. It appears that the best explanation is the reduced electronic delocalization in PPGN due to its nonplanar backbone structure which arises from twists at the imine nitrogens. This is in accord with the blue shift of the optical absorption spectrum of PPGN relative to that of PEMB. The fully reduced polyleucoemeraldine base, PLEMB (1 - y = 0), is expected to have a small magnitude of the third-order susceptibility x ( ~because ) the sp3-hybridized nitrogen prevents electronic delocalization. Hence, the bond additivity model is expected to work well, suggesting that the x ( ~of) PLEMB will be close to that of any substituted benzene because the secondorder hyperpolarizibility (y) would be-dominated by the benzene ring. The nonresonant x(’) value of PLEMB is estimated to be about that of aniline which has a x ( ~value ) of 1.7 X esu (y e s ~ ) . ~We * have not directly measured the x()) = 5.7 X of PLEMB because of its ready oxidation in air. Instead, we have investigated the nonlinear optical properties of poly(4-phenylaniline) (P4PAB) as a way of placing an upper bound on the x ( ~ ) value of PLEMB. From the optical absorption spectra of P4PAB and PLEMB shown in Figure 4, it is clear that the former has
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(37) (a) Garito, A. I.; Heflin, J. R.; Wong, K. Y.; Zamani-Khamiri, 0. In ref Ig, p 91. (b) Sinclair, M.; Moses, D.; Akagi, K.; Heeger, A. J. In ref lg, p 205. (38) Meredith, G.R.; Buchalter, 8.;Hanzlik, C. J . Chem. Phys. 1983, 78, 1543.
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-
p6-
0.80
a slightly greater degree of electronic delocalization than the latter. The x ( ~of) P4PAB is, like PLEMB, expected to be dominated by the second hyperpolarizability of the benzene rings; however, the magnitude of the ~ 0of)P4PAB is expected to be larger than that of PLEMB because of the former’s greater degree of delocalization. Figure 12 also shows the 1 ~ ( ~ ) ( - 3 w ; w , w , w spectrum )l of P4PAB in the wavelength range 0.9-2.4 pm. Clearly, the third-order susceptibility of P4PAB is more than an order of magnitude less than that of PEMB throughout the spectrum. Off-resonance, the ~ ( ~ ) ( - 3 w ; w , w , wvalue ) of P4PAB is esu and at 3h,,, 0.96 pm,the ~(~)(-3w;w,w,w) is 1.8 X esu (Table 11). So we expect the x ( ~value ) of PLEMB to be in the range 1.7 X 1O-I) to esu. The dramatic reduction in the third-order susceptibility of P4PAB, and hence PLEMB, compared with polyemeraldine base is due primarily to the reduction in electronic delocalization. By comparing the optical nonlinearity of P4PAB with its optical absorption spectrum in Figure 10, we see that the nonlinearity roughly follows the absorption at higher energies, but at lower energies there is a peak which occurs at the same energy as the threephoton resonance in PEMB. We attribute this peak to either the oxidized segments in the backbone or acid doping. There is probably a small amount of acid left over in the polymer from the synthesis, which serves as a dopant. Why such a small amount of doping should increase the nonlinearity so much is still unanswered. It is suggestive of the idea that acid doping mimics the oxidation of the PLEMB and PEMB. Doping-dependent x ( ~ ) measurements would be very interesting, and these experiments may be attempted in the future.
Conclusions The third-order nonlinear optical properties of polyanilines and derivatives have been investigated by picosecond third harmonic generation spectroscopy in the wavelength range 0.9-2.4 Hm (1.4-0.5 eV) in an effort to establish the structure-^(^) relationships for this class of polymers and also to provide the data base for the theoretical understanding of the nature of the third-order N L O response in conjugated polymers. Our results show that the optical nonlinearities of polyanilines are comparable to other conjugated polymers throughout the 0.9-2.4-pm spectrum. The magnitude of ~ ( depends 9 on the oxidation state and on the derivatization of thep-phenylene rings. At about the same oxidation State, We found that the X ( 3 ) p ~ ~X ~( 3 ) p ~ > ~ gX ( 3 ) p ~ > X(3)mT9.Whereas the ~ 0of )the fully reduced form of polyanilines is more than an order of magnitude less than the -50% oxidized, the x())of the intermediate oxidation state, PEMB, is a factor of 2.64 more than the fully oxidized, PPGN. Incorporation of an additional phenylene ring into the backbone of polyaniline as in poly(4-phenylaniline) drastically decreases the x ( ~ ) of this class of polymers as a result of poor electronic delocalization. The resonance behavior of the third-order susceptibility of polyanilines was shown to be dominated by the three-photon resonance to the exciton transition in the optical absorption of the materials. Thus, the exciton transition near 2 eV, rather than the r-r* transition, dominates the third-order nonlinear optical properties of polyanilines. A twelevel theoretical model was found to provide an adequate account of the x())dispersion data for the polyanilines.
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Acknowledgment. Work at the University of Rochester was supported by Amoco Foundation and the National Science Foundation (Grant CHE-88 1-0024). The nonlinear optical characterization was carried out at Du Pont. H.V. acknowledges the technical assistance of J. Kelly. Registry No. la, 25233-30-1; l b , 97917-08-3; IC, 99742-70-8; Id, 88374-66-7; P4PAB, 116267-93-7.
~ ~ ~
