Chem. Mater. 1992,4,683-687 organometallic complexes are shown to be sensitive to environmental rigidity over a wide range of viscosity changes.
Acknowledgment* We gratefully the IBM Corps for support of this research- We also thank Union Carbide and Shell Oil Corp. for providing the ERL-4221 and EPON-828 epoxy resin samples, respec-
683
tively, Dr.J. T. Gotro for helpful discussions and Mr. G. Kohut for technical assistance. T.G.K. thanks the IBM Corp. for receipt of an IBM Graduate Fellowship. Registry No. fa~-ClRe(CO)~(phen), 56846-58-3; fac-ClRe(CO)B(Me-phen),131544-70-2; fa~-ClRe(CO)~(Ph~-phen), 140849-51-0; f~c-IRe(CO)~(Ph~-phen), 131544-71-3;fac-BrRe(CO)&Me2-bpy), 122279-31-6;ERL-4221,25085-98-7;Epon 828, 25068-38-6; MeZNCHZPh, 103-83-3.
Nonlinear Optical Properties of Poly ( p-phenylenebenzobisoxazole) Samson A. Jenekhe* and John A. Osaheni Department of Chemical Engineering and Center for Photoinduced Charge Transfer, University of Rochester, Rochester, New York 14627-0166
Jeffrey S. Meth and Herman Vanherzeelef Du Pont Central Research and Development, P.O.Box 80356, Wilmington, Delaware 19880-0356 Received January 9, 1992. Revised Manuscript Received March 4, 1992 The third-order nonlinear optical susceptibility x(~)(-~o;w,w,w) of the conjugated rigid-rod polymer poly(p-phenylenebenzo[1,2-d:5,4-dlbisoxazole) (PBO) has been investigated by third-harmonic eneration spectroscopy and theoretical modeling in the wavelength range 0.S2.4 pm (1.4-0.5 eV). The ~(~fspectrum of PBO exhibits a strong three-photon resonance with a three-photon resonance enhanced x(~)(-~w;w,w,w) esu at 1.2 pm. The nonresonant value at 2.4 pm was 8.1 X 10-l2esu, which is large value of 7.0 X and comparable to some of the best conjugated polymers. The x ( ~dispersion ) data were found to be well described by a theoretical two-level essential states model. These results for the oxygen-containing heterocyclic rigid-rod polymer (PBO) were compared and contrasted with those previously reported for the structurally analogous sulfur-containingpoly(p-phenylenebenmbzobisthiazole)(PBZT). The results indicate that the magnitude of is essentially identical in PBO and PBZT, suggesting the absence of an effect of the heteroatom on the nonlinear optical response of this class of heterocyclic rigid-rod polymers in contrast to prior oligomer model compound studies which had shown a factor of 3 enhancement by the sulfur heteroatom.
Introduction Organic nonlinear optical materials with large thirdorder electronic susceptibility #) are currently receiving much attention from many research groups and laboratories because of the potential these materials hold for diverse applications in photonic devices.l-l5 The advantages of conjugated polymers as third-order nonlinear optical (NLO) materials have been well cited in many recent publication^.'-^ In an effort to understand the molecular design criteria for conjugated polymers with large x ( ~ )several , classes of conjugated polymers'+ and model compounds7have been investigated. Although some molecular design concepts are emerging, polymers that meet the material figure of merit (e.g., the ratio of the real part of X(~)(-O;W,-O,W) to the absorption coefficient a)required for practical photonic device applications are yet to be found. Thus, the challenge of elucidating all the relevant structure-^'^) relationships and molecular design criteria is still an open one. While several new classes of conjugated polymers have recently been synthesized as *Towhom correspondence should be addressed. 'Current address: Department of Applied Sciences, University of Brussels, Pleinlaan 2, B-1050,Brussels, Belgium.
potential NLO materials,8 investigation of the NLO properties of existing polymers can serve as a guide to the (1)(a) Khanarian, G.,Ed. Nonlinear Optical Properties of Organic Materials II, SPIE Proceedings Vol. 1147;The International Society for Optical Engineering: Bellingham,WA, 1990. (b) Prasad, P. N.; Williams, D. J. An Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (c) Bre'das, 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) Marder, S. R., Sohn,J. E., Stuky, G. D.; Eds. Materials for Nonlinear Optics: Chemical Perspectiues; ACS symposium Series No. 455;American Chemical Society: Washington, DC, 1991. (0 Meissier, J., Kajzar, F., Prasad, P., Ulrich, D. R., Eds. Nonlinear Optical Effects in Organic Polymers; Kluwer Academic Publishers: Dordrecht, Holland, 1989. (9) Heeger, A. J., Orenstein, J., Ulrich, D. R., Eds. Nonlinear optical properties of polymers; Material Research Society Proceeding, 1988; vol. 109. (h) Prasad, P. N., Ulrich, D. R., Eds. Nonlinear Optical Properties of Polymers, Plenum: New York, 1988. (i) Chemla, D. S., Zyss, J., Eds. Nonlinear optical properties of organic molecules and crystals; 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.; Roberta,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) Vanheneele, H.; Meth, J. S.;Jenekhe, S. A.; Roberts, M. F. Appl. Phys. Lett. 1991,58,663.
0897-4756/92/2804-0683%03.00/0 0 1992 American Chemical Societv
Jenekhe et al.
684 Chem. Mater., Vol. 4, No. 3, 1992 Chart I
1. eiePB0
I 1-PBZT
3. PBO
4.
5.
PBZT
PBZl
optimization of known structures. One particularly interesting class of conjugated polymers with known large third-order susceptibility and robust physical properties is the heterocyclic rigid-rod polymers whose members include poly@-phenylenebenzo[l,2d:4,5-d'Jbisthiazole-2,6-diyl)(PBZT), poly@-phenylenebenzo[1,2,-d:5,4-d']bisoxazole-2,6,diyl) (PBO), and poly@-phenylenebenzobisimidazole) (PBZI).la-19 Although (3) Oaaheni, J. A.; Jenekhe, S. A.; Vanherzeele, H.; Meth, J. S. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1991,32,154. (b) Yang, C. J.; Jenekhe, S.A.; Vanherzeele, H.; Meth, J. S. Polym. Prepr. (Am. Chem. SOC.,Div. Polym. Chem.) 1991,32, 165. (c) Agrawal, A. K.; Jenekhe, S. A.; Vanherzeele, H.; Meth, J. S. Polym. Prepr. (Am. Chem. SOC.,Div. Polym. Chem.) 1991,32, 124. (d) Jenekhe, S. A.; Roberta, M. F.; Vanherzeele, H.; Meth, J. s. Polym. Prepr. (Am. Chem. SOC.,Div. Polym. Chem.) 1991,32, 140. (4) (a) Vanhemele, H.; Meth, J. S.; Jenekhe, S.A.; Roberts, M. F. J. Oot. SOC.Am. B. in mess. (b) Jenekhe. S. A.:. Yann. - C. J.: Vanherzeele. H.; Meth, J. S.Chem. Mater. 1991,3,985. (5) (a) Oaaheni, J. A.; Jenekhe, S. A,; Vanherzeele, H.; Meth, J. S. J. Phys. Chem., in press. (b) Meth, J. S.;Vanherzeele, H.; Jenekhe, S. A.; Yang, C. J.; Roberts,M. F.; Agrawal, A. K. SPIE Proc. 1991,1560,13-24. (c) Agrawal, A. K.; Jenekhe, S. A.; Vanherzeele, H.; Meth, J. S.J. Phys. Chem., in press. (d) Meth, J. S.; Vanheneele, H.; Chen, W. C.; Jenekhe, S.A. Synth. Met., in press. ( 6 ) (a) Kaino, T.; Kubodera, K. F.; Tomura, S.; Kurihara, T.; Saito, S.; Tsutaui, T.; Tokito, S. Electron Lett. 1987,23, 1095. (b) Kaino, T.; Saito, 5.;Tsutaui, 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 l f , pp 143-147. (d) Kaino, T.; Kubodera, K.; Kobayeshi, H.; Kurihara, T.; Saito,S.; Tsutsui, T.; Tokito, S.;Murata, H. Appl. .. Phys. Lett. 1988, 53, 2002. (7) (a) Reinhart, B. A.; Unroe, M. R.; Evers, R. C.; Zhao, M.; Samoc, M.: Prasad. P. N.: Sinskv. M. Chem. Mater. 1991.3.864. Ib) Zhao. M.: S k o c , M.iPrasad, P. N.: Reinhardt, B. A,; Unrk,'M. R.f Prazak,' M.f Evers, R. C.; Kane, J. J.; Jariwala, C.; Sinsky, M. Chem. Mater. 1990,2, 670. (c) Rao, D. N.; Swiatkiewicz,J.; Chopra, P.; Ghosal, S. K.; Prasad, P. N. Appl. Phys. Lett. 1986,48, 1187. (8) (a) Agrawal, A. K.; Jenekhe, S. A. Macromolecules 1991,24,6806. (b) Agrawal, A. K.; Jenekhe, S. A. Chem. Mater. 1992, 4, 95-104. ( c ) Osaheni, J. A,; Jenekhe, S. A., manuscript in preparation. (d) Yang, C. J.; Jenekhe, 5.A. Chem. Mater. 1991, 3, 878-887. (e) Yang, C. J.; Jenekhe, S. A., manuscript in preparation. (9) (a) Houlding, V. H.; Nahata, A.; Yardley, J. T.; Elsenbaumer, R. L. Chem. Mater. 1990,2, 169. (b) Buchalter, B.; Meredith, G. R. Appl. Opt. 1982,21, 3221. (10) Heflin, J. R.; Cai, Y. M.; Garito, A. F. J. Opt. SOC.Am. B 1991,
8, 2132. (11) Agrawal, G. P.; Cojan, C.; Flytzanis, C. Phys. Reu. B 1978,17,776. (12) (a) Langhoff, P. W.; Epstein, S.T.; Karpus, M. Rev. Mod. Phys. 1972,44,602. (b) Kuyzyk, M. G.; Dirk, C. W. Phys. Reu. A 1990,41,5098. (13) (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) Torruelaa, W. E.; Neher, D.;
Zanoni, R.; Stegeman, G. I.; Kajzar, F.; Leclerc, M. Chem. Phys. Lett.
1990, 175, 11. (14) (a) Hefli, J. R.; Wong, K. Y.; Zamani-Khamiri, 0.; Garito, A. F. Phys. Rev. B 1988,38 1573. (b) Soos, 2. G.; Mcwilliams, P. C. M.; Hayden, G. W. Chem. Phys. Lett. 1990, 171, 14. (c) Dixit N.; Guo, D.; Mazumdar, S. Mol. Cryst. Li9. Cryst. 1991, 194, 33. (15) Messier, J. In ref If, pp 47-60.
s.
the original focus in the early synthesis and processing of these polymers was as light-weight, high-strength, highmodulus, and environmentally resistant materials for structural applications,l6l8these excellent mechanical and thermal properties also make this class of polymers attractive NLO materials. For example, the optical damage threshold of PBZT films has been found to be as high as 50 GW/cm2 for 30 ps pulses at 1.9 pm.2d34aWe2d93d94a and others7c have previously investigated the third-order nonlinear optical properties of PBZT films. We have also found that polymer molecular composites containing PBZT as the rigid-rod component have enhanced NLO propertie~.~d,4a Although the third-order nonlinear optical properties of the rigid-rod polymer PBO have not been reported, those of its oligomer model compound and related conjugated smell molecules have been reported.7ab The second hyperpolarizability (y) of cis-PBO and t-PBZT model compounds, whose structures are shown in Chart I, has also been theoretically calculated.20 One of the interesting results of the experimental model compound studies is the finding that the oxygen-containing model compound has esu ( x ( ~=) 7.3 X a y value of 7.1 X esu) which is a factor of 3 less than the analogous sulfur-containing model c~mpound.'*~~ Such a dramatic effect of the heteroatom on the nonlinear optical response was explained in terms of the greater degree of *-delocalization of the t-PBZT molecule, the greater polarizability of the sulfur atom, and the contribution from the d-orbital of sulfur to the conjugated *-system of the m o l e c ~ l e . ' ~If~ ~this structure-^(^) relationship found for oligomers was also valid for the polymers, it would suggest that the x ( ~of) PBO would be significantly less than that of PBZT. However, it is not at all clear that such an extrapolation of oligomer structure^(^) relationship to the high molecular weight polymers can be made. In this paper, we report the third-order optical susceptibility x ( ~ ) ( - ~ w ; w , w , wspectrum ) of PBO thin films measured by third harmonic generation (THG) spectroscopy in the wavelength range 0.9-2.4 pm (1.4-0.5 eV). The x ( ~ ) dispersion data were analyzed with a theoretical two-level essential states model. We also compare and contrast the ) and theoretical modeling of results of the x ( ~spectrum PBO with those of PBZT as well as with the reported results on the model compounds of PBZT and PBO and show that both the benzobisoxazole and benzobisthiazole molecules are equally important building blocks in the design of polymers with large third-order NLO properties.
Experimental Section Poly@-phenylenebenzobisoxazole) (PBO) with intrinsic viscosity [ q ] of 23 dL/g at 30 "C in methanesulfonic acid was kindly provided by the Polymer Branch of the Air Force Materials Laboratories (Dayton, OH). Using the known Mark-Houwink" relation for this polymer, this intrinsic viscosity translates to a molecular weight of -25 000. Thin films for third harmonic generation experiment were prepared by spin coating of an isotropic solution of PBO. We used the method of reversible Lewis acid coordination com(16) (a) Wolfe, J. F.; Arnold, F. E. Macromolecules 1981,14,909-915. (b) Wolfe, J. F.; Loo, B. H.; Arnold, F. E. Macromolecules 1981, 14, 915-920. (17) Wolfe, J. F. In Encyclopaedia of Polymer Science and Engineering; Wiley: New York, 1988; Vol. 11, pp. 601-635. (18) Krause, S.J.; Haddock, T. B.; Vezie, D. L.; Lenhert, P. G.; Hwang,
W. F.; Price, G. E.; Helminiak, T. E.; O'Brien, J. F.; Adams, W. W. Polymer 1988,29, 1354-1364. (19) (a) Jenekhe, S.A.; Johnson, P. 0.; Agrawal, A. K. Macromolecules 1989,22,3216. (b) Jenekhe, S. A.; Johnson, P. 0. Macromolecules 1990, 23, 4419-4429. (20) Goldfarb, I. J.; Medrano, J. In ref lf, pp 93-99.
Nonlinear Optics of Poly@-phenylenebenzobisoxazole) plexationlSto dissolve the polymer in aluminum chloride/nitromethane to obtain a polymer concentration of -0.4 w t %. The resulting isotropic solution was spin coated onto optically flat, fused silica substrate (5-cm diameter) at 2500 rpm for about 20 s to obtain a thin film of PBO/A1C13 complex. The PBO/A1Cl3 complex was placed in a beaker of water to be decomplexed for 2 days, changing the water at least two times a day to ensure complete removal of AlCl,. Previous studieslShave shown that overnight decomplexationin water is usually sufficient for complete regeneration of the pristine polymer. The film was dried in vacuum oven overnight at 80 OC. The film thickness was measured with an Alpha step (Tencor Instruments) profdometer which has a resolution of 1nm as well as by an optical technique. The thickness of the PBO sample used in this study was 68 nm. Electronic absorption spectrum of the thin film was recorded on a Perkin-Elmer Model Lamda-9 UV-vis-near IR spectrophotometer in the wavelength range 190-3200 nm. Third Harmonic Generation Experiment. Third harmonic generation (THG) experiments were performed with a picosecond laser system which is continuously tunable in the range 0.6-4.0 pm.*l The detailed procedure for the measurement of the ) this laser system has also been third-order susceptibility x ( ~with described in detail else~here.~sfi Briefly, the computer controlled system is based on a high-power modelocked NdYLF laser which synchronouslypumps a dye laser and seeds a NdYLF 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 beam is attenuated to 100 J/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 the 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 in the sample path. In the latter, the sample (fused silica as the reference material and/or the polymer film under study on 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 a 0.5-pm film of poly(benzimidazobenz0phenanthroline) semiladder polymer (BBB) on a silica substrate is used to generate the third harmonic. To improve the signalto-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 effect of fluctuations in both power and pulse width of the fundamental beam are further eliminated. Finally, the standard deviation of this ratio typically is 0.5%, by averaging the ratio of the THG signals over some 250 laser shots. The present THG experiments were performed at a funda) are mental wavelength of 0.9-2.4 pm. The reported x ( ~values average values corrected for absorption at the third harmonic wavelengthsaand were obtained relative to the x ( ~of) fused silica (2.8 X esu at 1.9 pm).sb Because the sample was thin, the optical density was low, so the correction term is still valid, even in the relatively strong absorbing region. Although recent work indicates that the reference value for fused silica may be wrong by a factor of 2,1° it is still used here to allow comparisons to be made to other THG experiments which used the same reference ) are value. Details of the exact determination of the x ( ~values discussed further e l s e ~ h e r e . ~ ~The ~ ~ error + " for the reported x@) values is typically *20% and is mostly due to error in film thickness and index of refraction measurements. The repeatability of individual results for each material is *5%. (21) Vanherzeele, H.Appl. Opt. 1990,29, 2246.
Chem. Mater., Vol. 4, No. 3, 1992 685 25.0 II
0.0 200
400
-
PBZT
600
800
WAVELENGTH, h ( nm )
Figure 1. Optical absorption spectra of thin films of PBO (dashed line) and PBZT (solid line).
Theoretical Modeling The x ( ~dispersion ) of some conjugated polymers such as polyacetylene has been successfully accounted for by the band theory." However, the disperison of the thirdorder susceptibility of most conjugated polymers, including polyanilines,58 polyazomethines,5bpolyquinolines,scpolythiophenes,"J3 and p~lydiacetylenesl~ is best described using the essential states theory.12-14 Following the approach based on the essential states theory that we have previously used to describe the ~(~)(-30;0,0,0) dispersion data of PBZT,&we have used a two-level theoretical model to analyze the x ( ~spectrum ) of PBO. First, the density is independently calculated from a theory based on atomic contributions to the physical density. For liquids and polymers, this calculation is accurate to &5%. From this number, we can calculate the number density of the , 0then ) calcupolymer repeat units. The ~ ( ~ ) ( - 3 0 ; 0 , 0is lated from the formula x ( 3 ) x x x x ( - 3 ~ ; 0 ,= w )l / g N f w 3 f 3 0 Y x r l x ( - 3 0 ; W , W , W )
In this equation, N is the number density of the polymer repeat units, f is the standard Lorentz local field factor, and y is the second hyperpolarizability calculated from the two-level The factor of 1/5 is for orientational averaging of y. We assume that the nonlinearity is dominated by the tensor component along the polymer chain. In addition, recent theoretical work which we have performed to include the effects of inhomogeneous and vibronic broadening on ~(~)(-3w;w,w,w)has led to the use of a hyperbolic secant line shape in place of a lorentzian line shape in the perturbation expressions.& 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 we need to fit the data are the energy of the transition, the phenomenologicalwidth that occurs from the broadening mechanisms, and the strength of the transition dipole moment. Since we are only using a two-level model and we do not have contributions to the nonresonant polarizability from higher lying excited states, the transition dipole moment is artificially increased by the model. The transition dipole moment must be interpreted as a fitting parameter and not as the true transition dipole moment. The fact that the number density is obtained for one polymer repeat unit also emphasizes this interpretation. A similar approach by Messier also led to uncharacteristically large transition dipole moment~.'~ Results and Discussion Figure 1 shows the optical absorption spectra of PBO and PBZT thin films. The absorption band maxima (A-)
Jenekhe et al.
686 Chem. Mater., Vol. 4, No. 3, 1992 Table I. Summary of Linear Optical and Nonlinear Optical Properties of PBO, PBZT, and Model Compounds x ( ~ esu ), resonanceh" E enhanced off-resonance material nm e f y, eeu (at 3X,,) (at 2.4 pm) PBO 427 2.76 7.0 X lo-" 8.1 X 401 PBZT 468 2.48 8.3 X lo-" 6.0 X 438 256 7.1 x 10-35 7.3 x 10-13 cis-PBOa 347.6 331.8 21 x 10-35 20 x 10-13 t-PBZT" 364.6 347.2 332.6
" Data from ref 7a were obtained by degenerate four-wave mixing at 602 nm. 10.0
m
-
6.0
0.0
.I
0.8
Figure 2.
1.2
1.6 WAVELENGTH, X ( p m )
2.0
~ ( ~ ) ( - 3 W , w , w , wspectra ) of PBO (dashed line)
2.4
and PBZT
(solid line).
and the optical bandgap (E,) for both polymers are s u m marized in Table I. The optical absorption spectra of PBO and PBZT are very similar. However, the main difference is that the spectrum of PBO is blue shifted from that of PBZT, resulting in an optical bandgap that is 0.28 eV higher than that of PBZT (Eg= 2.48 eV). Also, PBO has a higher absorption coefficient at A, (2.4 X lo5cm-') than PBZT (1.7 X los cm-'). Above about 0.5 pm there are no absorption features in the linear optical spectra of both polymers, and the absorption coefficients are identical in this wavelength region. A blue shift of the absorption edge and A, of PBO relative to those of PBZT indicates that there is better electron delocalization in PBZT than in PBO. This trend is also seen in the absorption maximum (Amm) of their model compound^.'^ This indicates that there is a direct correlation between the linear optical properties of the model compounds and those of the polymers. Figure 2 shows the x(~)(-~w;w,w,w) spectra of PBO and PBZT in the wavelength range 0.9-2.4 pm. A summary of the data for both polymers and their model com) show strong pounds is given in Table I. The x ( ~spectra three-photon resonance peaks at -1.2 and -1.3 pm respectively for PBO and PBZT in accord with the electronic absorption spectra of these polymers in Figure 1. Notice that since there are no absorption features in the range 0.6-2.4 pm in the electronic spectra of these polymers, the peaks in the x(3 spectra at -3A,, are clearly due to three-photon resonances. The three-photon resonance) for PBO and PBZT are respectively enhanced x ( ~values 7.0 X lo-" and 8.3 X lo-" esu. Away from resonance at 2.4 pm, the nonresonant x ( ~of) PBO and PBZT is respectively 8.1 X and 6.0 X esu. Within experimental errors, the X(~)(-~W;W,W,W) values for both PBO and
PBZT are practically the same at the respective threephoton resonance peaks and off-resonance at 2 pm or higher. As expected from the optical absorption spectra, ) of the three-photon resonance peak of the x ( ~spectrum PBO is blue shifted relative to that of PBZT. Hence, PBO at 2.4 pm is more detuned from ita three-photon resonance than is PBZT at the same wavelength. The nonresonant x(~)(-~w;w,w,o) value of PBO which is highly detuned from three-photon resonance is quite large and comparable to some of the best third-order NLO polymers.'-9 A comparison of the nonresonant ~ (value 3 for PBO (8.1 X esu) can be made to the r e p ~ r t e d 'x(~)(-o;w,-w,o) ~ value (7.3 X esu) for the cis-PBO model compound to estimate the effect of chain length or molecular weight on the third-order optical susceptibility of PBO. Such a comparison indicates a factor of 11enhancement of the nonlinear optical response by polymerization. This enhancement of the x ( ~in) the high molecular weight PBO relative to the oligomer cis-PBO can probably be explained by the increased electronic delocalization in the polymer beyond the polymer repeat unit in the oligomer. However, we must caution that the full understanding of the effect of chain length on the NLO response of PBO would require knowledge of the x ( ~spectrum ) and theoretical essential states analysis of such data which are not currently available for the oligomer. A number of important observations on the third-order NLO properties of PBO and PBZT can be made from these results. First, the similarity of the magnitude of the third-order susceptibility x ( ~of) PBO and PBZT in the resonant and off-resonant regions of the spectrum shows that there is no effect of the heteroatoms on the nonlinear optical response of this class of conjugated polymers. This means that the dominant contribution to the electronic x ( ~of) the polymers is from the conjugated ?r-system. This is in sharp contrast to the findings in oligomer model compound s t ~ d i e s ~where " ~ ~ the sulfur heteroatom enhanced the NLO response by a factor of 3 (see Table I). Second, the observed trend of y or x(3 increase with inof the oligomer model crease in optical bandgap or ,A, compounds7ais completely absent in the polymers. This suggests that although the scaling law of the form (A&" E -" holds for the oligomers t-PBZT and cis-PBO, it fails in the polymers PBZT and PBO. Thus, a struct u r e - ~ 'property ~) relationship found for these oligomer model compounds cannot be extrapolated to the high molecular weight polymers. What this implies is that there is more to polymerization than mere endowment of mechanical strength lacking in the oligomers. Our prior s t ~ d i e s ~of~ ladder , ~ " and semiladder polymers (BBL and BBB) and their oligomer model compound together with the present results suggest that the electronic states in polymers and their contributions to the electronic second hyperpolarizability can be dramatically different from those of oligomers. Such issues as disorder in polymers which can lead to larger local anharmonicity versus crystalline order in oligomers can also significantly change the nonlinear optical response mechanism when going from oligomer to polymer. Third, the absence of any effect of the heteroatoms on the magnitude of the nonlinear optical properties of PBO and PBZT suggests that the NLO properties of the structurally analogous rigid-rod polymer poly@-phenylenebenzobisimidazole) (PBZI) which have not yet been investigated, would be similar to those of PBZT and PBO. It is important to investigate this prediction in order to complete our understanding of the structure-^'^) relationships in this class of conjugated rigid-rod polymers.
-
-
Nonlinear Optics of Poly@-phenylenebenzobisoxazole)
Chem. Mater., Vol. 4, No. 3,1992 687 density calculated for PBO (1.49 g/cm3) and PBZT (1.55 g/cm3) are in good agreement (within &7% error) with reportedzz experimental values of 1.38 and 1.44 g/cm3 respectively for PBO and PBZT.
o i 0
I
I
I
0.5
1
1.5
Energy (eV)
Figure 3. Two-level model fit of the x@)(-~w;w,w,w) spectrum of PBO. Table 11. Two-Level Model Fitting Parameters for Conjugated Rigid-Rod Polymers PBO and PBZT transition physical no. density, dipole energy, width, density, material x1Oz1cm-3 moment, D eV eV g/cm3 0.57 1.49 25.4 3.26 PBO 4.14 0.48 1.55 25.3 2.94 PBZT 4.20 *5% f1.5 *0.05 10.05 15% error
Figure 3 shows the fit to a two-level model of the ~ ( ~ ) ( - 3 O ; w , w , odispersion ) of PBO. The fitting parameters
for PBO along with those previously reportedk for PBZT are collected in Table 11. As seen from the figure, there is a reasonably good fit between the experimental data and the theoretical two-level model. This suggests that the magnitude of the third-order susceptibility of these polymers can be adequately described by considering two essential states of the system, the ground state and the lowest lying transition dipole allowed excited state. A comparison of the fitting parameters shows that both PBO and PBZT have essentially identical number density and transition dipole moment. The energy of transition and the phenomenological width that occurs from the broadening mechanisms are larger in PBO than PBZT, consistent with the absorption spectra of the polymers. The strength of the transition dipole moment and the number density are especially useful parameters for comparing the macroscopic ( ~ (and ~ 9microscopic ( y ) optical nonlinearities of polymers with the same basic structure. The identical nature of these two parameters in PBO and PBZT suggests that there is no noticeable effect of the heteroatoms on the second hyperpolarizability y and the off-resonance x ( ~of) these polymers. This is in contrast with the factor of 3 increase seen in the e~perimental’~ and calculated” second hyperpolarizability in going from the oxygen-containing, benzobisoxazole, to the sulfur-containing, benzobisthiazole oligomer model compounds. Notice that the physical
Conclusions We have measured the third-order optical susceptibility spectrum of thin films of poly@-phenylenebenzobisoxazole) in the wavelength range 0.9-2.4 pm (1.4-0.5 eV) and compared the results with the structurally analogous sulfur-containing poly@-phenylenebenzobisthiazole)in an effort to understand the nature of the nonlinear optical response of the heterocyclic rigid-rod poly(benzobisazo1e) family and role of the heteroatoms. The results show that within experimental errors, both the resonant and nonresonant X ( ~ ) ( - ~ W ; O , W , W ) values of PBO and PBZT are essentially the same, in contrast to the trend seen in the oligomer model compounds which showed a factor of -3 enhancement in ~ ( ~ ) ( - o ; o , - w , w )or y in going from the oxygen-containing benzobisoxazole to the sulfur-containing benzobisthiazole model compounds. The experimental x ( ~ ) dispersion of PBO, like the previously reported data for PBZT, is well described by a theoretical two-level model and the fitting parameters are identical to those of PBZT, suggesting that there is no noticeable effect of the different heteroatoms, S and 0, on the microscopic and macroscopic third-order nonlinear optical properties of these polymers. The present results suggest that although oligomer model compound studies are important for elucidating polymer structure and some physical properties, structure-^(^) relationships derived from oligomers may not be used to predict structure-^(^) relationships in polymers. The measured nonresonant ( x ( ~=) 8.1 X 10-l2esu at 2.4 pm) and three-photon resonant ( x ( ~=) 7.0 X lo-” esu at 1.2 rm) nonlinear optical properties of spin-coated PBO thin films are quite large and comparable to some of the best nonlinear optical polymers. It is also interesting to note that of the three rigid-rod polymers (PBO, PBZT, PBZI) in Chart I, only PBO is apparently moving toward commercialization, by Dow Chemical C O . ~Thus, ~ PBO is a promising third-order nonlinear optical material for further investigation and exploration for photonic device applications.
Acknowledgment. We thank Michael Roberts for preparing the solutions used for preparing the PBO thin films. Work at the University of Rochester was supported by Amoco Foundation and the National Science Foundation (Grant CHE-881-0024). H.V. thanks J. Kelly for the valuable technical assistance. Registry No. PBO (homopolymer), 60871-72-9. (22)Bhaumik, D.;Welsh, W. J.; Jaffe’, H. H.; Mark,J. E. Macromolecules 1981,14, 951. (23) Flam, F. Science 1991,251,874-876.
