4732
J. Phys. Chem. B 2003, 107, 4732-4737
Photorefractive Properties of Poly(siloxane)-triarylamine-Based Composites for High-Speed Applications Daniel Wright,*,† Ulrich Gubler,‡ and W. E. Moerner Department of Chemistry, Stanford UniVersity, Stanford, California 94305-5080
Michael S. DeClue§ and Jay S. Siegel Department of Chemistry, UniVersity of CaliforniasSan Diego, La Jolla, California 92093-0340 ReceiVed: NoVember 13, 2002; In Final Form: March 13, 2003
The photorefractive properties of the hole-transporting polymer poly(methyl-bis-(3-methoxyphenyl)-(4propylphenyl)amine)siloxane (MM-PSX-TAA) doped with the photorefractive chromophore 4-di(2-methoxyethyl) aminobenzylidene malononitrile (AODCST) are presented and compared with results obtained on similar composites based on poly(n-vinylcarbazole) (PVK). The low intrinsic Tg of the new polymer allows the preparation of samples without additional plasticizers. These composites exhibit good chromophore orientational mobility and exhibit photorefractive response times in the millisecond range, among the fastest reported to date.
1. Introduction Photorefractive materials have been studied because of their potential applications in optical data storage and signal processing.1-6 Organic photorefractive materials offer several potential advantages over their inorganic and semiconducting counterparts because of their low cost, ease of processability, and chemical tunability. Current synthetic methods offer the possibility to create a virtually unlimited number of organic materials with varying properties, making possible the realization of samples that meet all design requirements for a particular application. Most of the research carried out in the field of organic photorefractive materials has employed polymer composites in which molecules incorporating all of the necessary processes for photorefraction (charge generation, charge transport, charge trapping, and optical nonlinearity) are doped into a host polymer (which itself may provide one or more of the aforementioned effects). The most commonly used host polymer has been PVK, which acts as a host matrix and hole transporter. PVK has many advantages including its commercial availability, the depth of study on the polymer, and its ability to dissolve polar chromophores. Composites made with this polymer have produced observations of gain coefficients approaching 400 cm-1,7 refractive index modulations (∆n) approaching 10-2,8 and diffraction efficiencies (η) of nearly 100%.1,4-6 Much of the progress in the field of organic photorefractive materials has been accomplished with materials using this polymer, but PVK also has several disadvantages, as discussed in more detail below. In light of the discovery of the “orientational enhancement” effect,9 there is much to be gained in terms of index contrast if * Corresponding author. E-mail:
[email protected]. Tel: 33 01 47 40 55 53. Fax: 33 01 47 40 55 67. † Present address: Laboratoire de Photonique Quantique et Mole ´ culaire, Ecole Normale Supe´rieure de Cachan, 61 avenue du Pre´sident Wilson, 94235 Cachan, France. ‡ Present address: CSEM Alpnach, Untere Gru ¨ ndlistrasse 1, CH-6055 Alpnach, Switzerland. § Present address: Laboratorium fu ¨ r Organische Chemie, Swiss Federal Institute of Technology, ETH Ho¨nggerberg, CH-8093, Zu¨rich, Switzerland.
the Tg of the composite is close to room temperature. Unfortunately, the intrinsic Tg of PVK is greater than 200 °C. Adding the high concentration of chromophore needed for an efficient composite depresses the Tg somewhat, but to obtain a Tg near room temperature, an additional plasticizer is usually added. This additional component occupies volume in the sample without contributing to the photorefractive process directly. It would be preferable to have this volume occupied by additional nonlinear or charge-transporting moieties. Additionally, plasticizers can affect the photorefractive dynamics because of changes in the chromophore orientational mobility, the charge trapping, or charge transport.6 One of the more successful ways around these problems has been to use a siloxane polymer with pendant carbazole groups, usually referred to as PSX.10 The main merit of this polymer is that it has similar hole-transport characteristics to those of PVK11,12 with a lower intrinsic Tg, allowing the preparation of samples with a room-temperature Tg without additional plasticizer.13-15 In addition to enabling highly efficient photorefractive polymer composites, bifunctional systems in which both the nonlinear optical chromophore and the chargetransporting group are covalently bound to the siloxane backbone have been synthesized.16 In addition to the plasticizer-related problems, PVK has a relatively low hole mobility, on the order 10-7 cm2/V‚s.17,18 Organic hole-transporting materials with mobilities many orders of magnitude larger have been reported in the literature;19 these include tris-tolylamine (TTA), which when doped into polycarbonate yields mobilities up to 10-4 cm2/V‚s.20 Thus, the possibility of greatly improving the response time of the photorefractive effect may be possible with the substitution of a better charge-transport agent. The combination of a better charge-transport agent with a polymer that does not require additional plasticizer could lead to an improved composite. In an attempt to improve on the results obtained with PVK and PSX, we have synthesized a new class of charge-transport polymers based on a siloxane backbone with pendant triaryl-
10.1021/jp027456i CCC: $25.00 © 2003 American Chemical Society Published on Web 04/24/2003
Poly(siloxane)-triarylamine-Based Composites
J. Phys. Chem. B, Vol. 107, No. 20, 2003 4733
SCHEME 1: Synthesis of MM-PSX-TAA
Figure 1. Intensity dependence of the photo (open symbols) and dark (filled symbols) conductivity for composites of PVK\AODCST\BBP\C60 (circles), MM-PSX-TAA\AODCST\C60 (30 wt % AODCST, triangles), and MM-PSX-TAA\AODCST\C60 (15 wt % AODCST, squares). E ) 20 V/µm, and λ ) 647 nm.
the velocity with which the charges move through the material, and by trapping effects. Charge generation and transport are reflected in the value of the photoconductivity σp, which in the most basic (bulk, and assuming small absorption, RL , 1) picture can be written as
amine groups. This paper describes measurements made on these polymers doped with the nonlinear optical chromophore AODCST. 2. Results and Discussion MM-PSX-TAA (3) was synthesized on a multigram scale by the method depicted in Scheme 1. A modified Ullmann reaction21 was used to couple 4-bromoaniline to 3-iodoanisole to provide the triarylamine core (1). Compound 1 was then converted to the Grignard followed by a palladium-catalyzed coupling with allyl bromide to provide compound 2. Compound 2 was then attached to commercially available poly(methylhydro)siloxane using a platinum(II) catalyst to provide MM-PSXTAA (3). In addition to MM-PSX-TAA, this synthetic method was applied to provide a small library of PSX-TAA materials.22 The additional methoxy groups were added to improve the solubility of the matrix with polar chromophores. Composites with this class of polymer could be made without the addition of any plasticizer. In general, when doped with a plasticizer and other dopants, the Tg of the resulting composite can often be difficult to measure.23 By contrast, a large reversible change in heat flow is easily observed in the MM-PSX-TAA polymer and in composites made with it. The pure MM-PSX-TAA polymer was found to have a Tg of 25 °C, which is actually too low to be used with the concentrations of chromophore normally employed in photorefractive polymer composites. When 30 wt % of the chromophore AODCST is doped into this polymer, the resulting composite has a Tg near 0 °C. This low Tg leads to fast phase separation of the chromophores in the polymer host and dielectric breakdown at small electric-field strengths (E < 40 V/µm). To circumvent this problem, only 15 wt % of AODCST was used in most of the experiments described below, which results in a composite with a Tg of about 13 °C that has a greater electric-field stability and shelf lifetime. The overall speed of the space-charge field formation is governed by the number of generated charges per unit time,
σp ) neµ )
eµ (ΦRIτ hυ )
(1)
where n is the density of mobile charge carriers, e is the elementary charge, µ is the mobility, Φ is the mobile charge generation quantum efficiency, R is the absorption coefficient, I is the optical intensity, τ is the lifetime of the carrier (the length time that a charge moves before being trapped or recombining), h is Planck’s constant, and ν is the frequency of the light. In the standard model of photorefractivity,2,3 the rate of space-charge field formation is governed in part of the normalized response time tn ) τd, which is in turn proportional to the dielectric relaxation time
τd )
s neµ
(2)
where s is the static dielectric constant of the material. We see from eqs 1 and 2 that this characteristic time is nothing more than the photoconductivity scaled by the static dielectric constant. Thus, we can regard the value of σp as a measure of the speed of the Esc formation. The photoconductive properties of the composites made with MM-PSX-TAA and PVK were examined with an applied field of 20 V/µm, with the results shown in Figure 1. Composites made with MM-PSX-TAA have a greater photoconductivity at all of the intensities used in these experiments even though the intensity dependence strongly deviates from the expected linear dependence. One possible explanation for the sublinear intensity dependence of the photoconductivity is that excess shallow traps are present in the material.24 The photoconductivity is about 1 order of magnitude larger than the dark conductivity in the range of intensities used in photorefractive experiments. A difference of approximately a factor of 10 between the magnitude of the dark and the photoconductivity is a prerequisite to establishing the maximum space-charge field possible in response to inhomogeneous illumination. Transient ellipsometry experiments25 were used to quantify the orientational characteristics of the chromophore in the new
4734 J. Phys. Chem. B, Vol. 107, No. 20, 2003
Figure 2. Normalized transient ellipsometry curves taken at 905 nm for composites of PVK\AODCST\BBP\C60 (circles), MM-PSXTAA\AODCST\C60 (30 wt % AODCST, triangles), and MM-PSXTAA\AODCST\C60 (15 wt % AODCST, squares). E ) 25 V/µm.
Figure 3. Electric-field dependence of the quasi-steady-state (t ) 100 s) refractive index change as measured by transient ellipsometry taken at 905 nm for composites of PVK\AODCST\BBP\C60 (circles), MMPSX-TAA\AODCST\C60 (30 wt % AODCST, triangles), and MM-PSXTAA\AODCST\C60 (15 wt % AODCST, squares).
composite. The results of these measurements are shown in Figures 2 and 3. In Figure 2, we see the time dependence of the normalized (to the quasi-steady-state value) ellipsometry signal for the same samples as in Figure 1. We observe that the orientation is much less dispersive for the MM-PSX-TAA as compared to that of composites made with PVK. The chromophores in the MM-PSX-TAA polymer orient in the 100-µs to 100-ms time range. Chromophores orient faster in the composite made with 30 wt % AODCST owing to its lower Tg. These results contrast with those obtained with PVK-based composite samples in which chromophore orientation takes place over all of the time scales measurable by our setup. In Figure 3, we compare the quasi-steady-state values (t ) 100 s) of composites of MM-PSX-TAA doped with both 15 and 30 wt % AODCST as well as those for the PVK\AODCST\ BBP\C60 composite. We see that there is a reduction of the quasi-steady-state magnitude of the nonlinearity by about a factor of 3 in the MM-PSX-TAA polymer composites doped with 15 wt % AODCST compared to the composite made with PVK. These composites show a field dependence of the ∆n that is close to the quadratic dependence expected for low-Tg nonlinear optical polymers. The first possible explanation for the lower nonlinearity is that it is simply due to the smaller chromophore concentration.
Wright et al.
Figure 4. Electric-field dependence of ∆n measured in degenerate fourwave mixing experiments on composites of PVK\AODCST\BBP\C60 (filled circles) and MM-PSX-TAA\AODCST\C60 (15 wt % AODCST, open circles). I ) 100 mW/cm2, and λ ) 647 nm.
We examined this possibility by taking data on MM-PSX-TAA composites doped with 30 wt % AODCST, which can also be seen in Figure 3. As expected, the nonlinearity doubled compared to that of the 15 wt % sample. However, the difference in chromophore weight percent seems to account only for part of the discrepancy because there still remains a factor of about 2 compared to the PVK composite. This is particularly perplexing because the MM-PSX-TAA composites doped with 30 wt % have a much lower Tg than the samples made with PVK and thus should allow a greater orientation of the chromophores. We have verified that PVK and BBP do not significantly contribute to the birefringence in these composites by performing transient ellipsometry experiments on samples without nonlinear chromophores composed of only PVK\BBP.26 One possible explanation for the reduction in birefringence is a reduction in the volume fraction of the chromophore that could arise if the density of the new polymer were lower than that of PVK. The reduction can also arise from the tendency of strongly dipolar chromophores to order antiparallel with respect to one another, thus forming chromophore aggregates that provide no nonlinear optical effects.8 This effect will be more pronounced in composites with lower Tg since the chromophores have a greater ability to move in the host matrix. Another possible consequence of the lower Tg is that such materials tend to have higher dielectric constants because of their increased ability to change their morphology to compensate a given electric field.27 The greater electric-field screening in the material can reduce the effect of the space-charge field on the chromophores and thereby lower the change in refractive index. The difference in nonlinearity observed in ellipsometric experiments is also apparent when comparing steady-state refractive index contrasts measured by degenerate four-wave mixing. We see in Figure 4 that there is approximately a factor of 3 difference in the value of ∆n within the field range of these measurements. Another difference in the two polymers is the environment that they provide for the charge generator C60. In both composites, 0.5 wt % of the sensitizer was doped into the composites, but the PSX composite had an absorption coefficient of ∼30 cm-1 as compared to the ∼10 cm-1 value observed for the PVK composite. The disparity in the absorption coefficient between the two polymers may be due to a charge complex between the TAA units and C6028 or the difference in the local environment (most importantly arising from a variance of the dielectric constant). The high absorption coefficient for MM-
Poly(siloxane)-triarylamine-Based Composites
Figure 5. Electric-field dependence of the gain coefficient of composites of PVK\AODCST\BBP\C60 (filled circles) and MM-PSXTAA\AODCST\C60 (15 wt % AODCST, open circles). I ) 100 mW/ cm2, and λ ) 647 nm. The solid (dotted) line represents the absorption coefficient of the PVK\AODCST\BBP\C60 (MM-PSX-TAA\AODCST\C60) sample.
J. Phys. Chem. B, Vol. 107, No. 20, 2003 4735 cm2, making this system among the fastest organic photorefractive materials reported to date. When we compare the speeds of the MM-PSX-TAA and PVK composites, the PSX composite has about a factor of 3 greater speed in the intensity range studied. In this comparison, it is important to recall from eq 1 that the photoconductivity (σp) and therefore the speed are proportional to the rate of generation of mobile charges in the material and therefore to the absorption. Consequently, to compare the performance of the different composites, we normalize both the photoconductivity and speed by dividing by the absorption coefficient of the sample. This ensures that the possible differences that are observed are not due to a different degree of charge generation caused simply by a different level of absorption of the sensitizer. Since the absorption is about 3 times larger for the MM-PSX-TAA samples, the normalization accounts for the factor of 3 difference in speed described above. The lack of improvement in normalized speed can be due to several factors. For one, we assumed that the mobility in the MM-PSX-TAA system should be large on the basis of the triphenylamine unit. Direct measurements of the mobility would be needed to prove this assumption. Such measurements are normally carried out with a time-of-flight apparatus19 and could be the goal of future research. If the mobility is indeed found to be larger, then this would point to a difference in the quantum efficiency of charge generation or charge-trapping dynamics. As mentioned above, the local environment of C60 seems to be quite different in the MM-PSX-TAA polymer as compared to that in PVK; this difference might also result in a difference in quantum efficiency. One would need a separate measurement of the quantum efficiency as well (for example, by xerographic discharge measurements19) to test this hypothesis. 3. Conclusions
Figure 6. Intensity dependence of the two-beam coupling speed at an applied electric field of 50 V/µm (filled circles) and of the photoconductivity at an applied electric field of 20 V/µm (open squares) in a MM-PSX-TAA\AODCST\C60 (15 wt % AODCST) composite. λ ) 647 nm.
PSX-TAA samples, in conjunction with the lower nonlinearity mentioned above, pushes the field required for net gain to around 55 V/µm in these samples. The field strength needed for net gain in MM-PSX-TAA samples is rather high compared with the field strength of 20 V/µm needed for the PVK composite. The PR performance of these new materials should be improved if samples with a higher weight percent of chromophore could be prepared. Two-beam coupling measurements were performed over a range of intensities at a constant applied electric field of 50 V/µm to characterize the gain dynamics. Empirical time constants were obtained from the geometric average of double exponential fits, thus τave ) τ1a × τ2b, where a and b are the factional weights of each time constant. In Figure 6, the intensity dependence of the geometrically averaged photorefractive speed (1/τave) and the photoconductivity are compared. Both plots can be fit well with a power law. The fits give the same exponent within experimental error, suggesting that the speed is not hindered by the orientation of the chromophores. This is also reasonable from the ellipsometry data presented above, which demonstrated that a significant portion (∼40%) of the orientation takes place on time scales of less than 10 ms. The average time constants falls below 10 ms for intensities above ∼600 mW/
In conclusion, this new class of polymers has several attractive features when compared to the standard PVK, including no need for additional plasticizers and possibly higher intrinsic hole mobility. When doped with the chromophore AODCST and the sensitizer C60, these composites exhibit grating growth times in the millisecond range at moderate fields and intensities, although the refractive index modulation that is obtained is lower than with PVK composites with similar amounts of chromophore. These composites could be further improved by using the synthetic methods described in this work to attach other moieties to the siloxane backbone. Forming multifunctional polymers16 based on polysiloxane is another possible future direction. 4. Experimental Section NMR spectroscopy (1H and 13C) was recorded on a Varian (Mercury 400 MHz) spectrometer. 1H NMR spectra were referenced to tetramethylsilane (TMS) at 0.00 ppm. 13C NMR spectra were referenced to 77.0 ppm in chloroform-d (CDCl3). High-resolution mass spectra (HRMS) were obtained from the University of California at Riverside’s mass spectrometry facility in the fast atom bombardment (FAB) mode. Infrared (IR) spectra were recorded on a Perkin-Elmer 1420 IR spectrophotometer. Modulated differential scanning calorimetry (MDSC) measurements were recorded using a TA Instruments 2920 MDSC with a sample size ranging between 5 and 20 mg. The glasstransition temperature (Tg) was determined from the inflection point in the plot of heat flow versus temperature. The reversible heat curves were generated at a temperature-rate increase of 4 °C/min.
4736 J. Phys. Chem. B, Vol. 107, No. 20, 2003 Gel permeation chromatograph (GPC) was performed using a Varian 9002 GPC analyzer with Pro Star 310 UV/vis and Varian RI-4 detectors with Varian Star 5.3 system-control software. A Phenogel 5µ linear column with 100-10 000K MW range was used with HPLC-grade tetrahydrofuran (THF) at a flow rate of 0.5 mL/min. Calibration was made with polystyrene standards. All standards and samples were prepared at 1 mg/ mL in THF with an injection volume of 20 µL. Data analysis was preformed using Polymer Laboratories PL Caliber Reanalysis version 7.04. Techniques and Materials. All syntheses were carried out under argon in freshly distilled solvents under anhydrous conditions unless otherwise noted. Commercial chemicals were acquired from Sigma-Aldrich with the exception of dichlorodicyclopentadienyl platinum(II) that was obtained from Strem Chemicals, Inc. All commercial chemicals were used as supplied unless otherwise stated. Tetrahydrofuran (THF) was distilled from sodium/benzophenone; toluene was distilled from calcium hydride. Silica gel (230-425 mesh) for flash chromatography was purchased from Fisher Scientific Company. Thin-layer chromatography (TLC) was performed on aluminum-backed silica gel 250-µm F254 plates from Whatman. 4-Bromophenyl-bis(3-methoxyphenyl) amine (1). The procedure was adapted from Goodbrand.21 To a 500-mL twonecked round-bottom flask equipped with a Dean-Stark trap, condenser, gas inlet, stir bar, and glass stopper was added toluene (60 mL), 4-bromoaniline (3.8 g, 22.3 mmol), 3-iodoanisole (5.85 mL, 49.1 mmol), copper(I) chloride (180 mg, 1.8 mmol), 1,10-phenanthroline (325 mg, 1.8 mmol), and potassium hydroxide (21.5 g, 385 mmol) under a flow of argon. This mixture was heated to reflux (125-130 °C) overnight. Then the reaction was cooled to room temperature. The cooled reaction was then diluted with water (75 mL) and extracted with CH2Cl2 (3 × 100 mL). All CH2Cl2 layers were combined, dried over MgSO4, filtered, and evaporated under reduced pressure to yield a crude purple oil. The crude purple residue was dissolved in a mixture of CH2Cl2/hexanes (50 mL, 1:1 v/v) and filtered through a plug of silica gel. The plug was washed with CH2Cl2/hexanes (1000 mL, 1:1 v/v). The solvent was evaporated to yield a brown oil. Further purification via flash chromatography with silica as the absorbent (100% hexanes, gradient to a mixture of CH2Cl2/hexanes, 2:3 v/v) yielded a light-yellow oil. (6.7 g, 78%). 1H NMR (400 MHz, CDCl3): δ 3.71 (s, 6H), 6.56-6.63 (m, 4H), 6.65 (dd, J ) 8.4, 1.2 Hz, 2H), 6.96 (d, J ) 8.8 Hz, 2H), 7.15 (t, J ) 8.0 Hz, 2H), 7.32 (d, J ) 8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ 55.28, 108.53, 110.11, 115.00, 116.73, 125.44, 129.77, 131.95, 146.53, 148.24, 160.23. HRMS-FAB+ m/z: found 383.0522; calcd (C20H18NO2Br) 383.0521. 4-Allylphenyl-bis(3-methoxyphenyl) amine (2). To a twonecked round-bottom flask equipped with a condenser, gas inlet, stir bar, and glass stopper was added 1 (6.7 g, 17.4 mmol), tetrahydrofuran (75 mL), and magnesium (635 mg, 26.1 mmol, 50 mesh powder) under a flow of argon. This mixture was heated to reflux (65-70 °C). After the reaction had begun refluxing, it was cooled slightly to allow for the addition of allyl bromide (2-3 drops). The reaction was again heated to reflux following the addition of the Grignard initiator allyl bromide. Aliquots of the reaction mixture can be quenched and analyzed by TLC (SiO2, CH2Cl2/hexanes, 2:3 v/v) to determine complete Grignard formation. In general, the Grignard was completely formed after refluxing for 4 h. Then the reaction was cooled to room temperature, and tetrakis(triphenylphos-
Wright et al. phine)palladium(0) (1.0 g, 0.9 mmol) was added followed by allyl bromide (3.0 mL, 34.8 mmol). This mixture was stirred at room temperature overnight. Next, the reaction was quenched by the slow addition of water (50 mL). The mixture was then extracted using CH2Cl2 (3 × 100 mL). All CH2Cl2 layers were combined, dried over MgSO4, filtered, and evaporated under reduced pressure to yield a crude tan oil. Purification via flash chromatography with silica as the absorbent (100% hexanes, gradient to a mixture of CH2Cl2/hexanes, 2:3 v/v) yielded a clear oil. (4.2 g, 70%). 1H NMR (400 MHz, CDCl3): δ 3.35 (d, J ) 6.8 Hz, 2H), 3.72 (s, 6H), 5.05-5.13 (m, 2H), 5.92-6.03 (m, 1H), 6.54 (dd, J ) 8.4, 2.0 Hz, 2H), 6.60-6.68 (m, 4H), 6.997.17 (m, 6H). 13C NMR (100 MHz, CDCl3): δ 39.73, 55.26, 107.69. 109.58, 115.60, 116.26, 124.94, 129.19, 129.54, 134.80, 137.35, 145.34, 148.84, 160.11. HRMS-FAB+ m/z: found 345.1742; calcd (C23H23NO2) 345.1729. Poly(methyl-bis-(3-methoxyphenyl)-(4-propylphenyl)amine)siloxane (3). The procedure was adapted from Strohriegl.10,29 To a 250-mL two-necked round-bottom flask equipped with a condenser, gas inlet, stir bar, and glass stopper under argon was added toluene (50.0 mL) and poly(methylhydro)siloxane (530 µL, 8.9 mmol). Olefin 2 (4.0 g, 11.6 mmol) was then added to the mixture followed by dichlorodicyclopentadienyl platinum(II) (1.0 mg, 0.0025 mmol). The mixture was stirred at elevated temperature (60-65 °C) using an oil bath. The reaction progress was monitored using IR spectroscopy. After an initial reaction time of 20 h, an aliquot of the neat reaction solution was evaporated on NaCl plates, and the IR spectrum was recorded to follow the disappearance of the Si-H stretch at 2150 cm-1. Additional dichlorocyclopentadienyl platinum(II) (ca. 1 mg) was added at regular intervals until the IR spectrum of an aliquot showed no residual Si-H stretch. If the reaction was incomplete, then additional dichlorodicyclopentadienyl platinum(II) (ca. 1.0 mg) was added. This cycle was continued at regular time intervals (ca. 1 h) until the silicon-hydrogen signal was no longer present in the IR spectrum. A total of three additions of dichlorodicyclopentadienyl platinum(II) (3.0 mg, 0.008 mmol) in addition to the initial amount was needed to reach completion. Upon completion, the reaction was cooled to room temperature. Then the reaction solution was added dropwise to an excess of hexanes (75 mL). The precipitate was collected by centrifugation and decanted. Then the precipitated polymer residue was dissolved in the minimum amount of THF (10 mL) needed to dissolve the crude polymer completely. This THF solution was precipitated again into an excess of hexanes (20 mL). The precipitation process was continued until the polymer residue was free of monomers as determined by 1H NMR. A total of three precipitations were needed for a clean product. The residual solvents were removed from the polymer residue under reduced pressure to yield a white solid (2.3 g, 64%). Tg ) 25.0 °C. 1H NMR (400 MHz, CDCl3): δ -0.10-0.28 (br s, 3H), 0.48-0.72 (br s, 2H), 1.52-1.75 (br s, 2H), 2.40-2.63 (br s, 2H), 3.41-3.75 (br s, 6H), 6.39-6.52 (br s, 2H), 6.59-6.68 (br s, 4H), 6.85-7.13 (br m, 6H). 13C NMR (100 MHz, CDCl3): δ 0.01, 17.75, 25.22, 39.05, 55.08, 107.51, 109.46, 116.14, 124.82, 128.99, 129.47, 137.22, 144.92, h n ) 25 000, M h n (theoretical) ) 148.79, 160.02. M h p ) 18 000, M 16 000, M h w ) 93 000, M h n/M h w ) 3.81. IR (neat): νSi-O ) 1050 cm-1, νC-H ) 2930 cm-1. Secondary standard PVK was purified four to seven times by dripping a solution of PVK/toluene into boiling ethanol. The synthesis of AODCST is described in the literature.30 BBP (the plasticizer butyl benzyl phthalate) and C60 were bought commercially and used as received.
Poly(siloxane)-triarylamine-Based Composites Photoconductivity experiments were carried out by a standard DC technique by measuring the current traversing the sample at a certain applied voltage with and without uniform illumination at a given intensity from a Kr+ laser operating at 647 nm. Steady-state currents were transformed into conductivities using Ohm’s law. Ellipsometric measurements were carried out by placing samples between crossed polarizers at an angle of 51° with respect to the laser beam from a temperature-stabilized laser diode operating at 905 nm. A Babinet-Soleil compensator was used to adjust the phase delay between the two polarizations such that the transmitted beam was totally extinguished when zero field was applied. Light transmission through the second polarizer was measured with a silicon photodiode, and the voltage was applied with a response time of around 100 ms by a Trek 10/10 voltage amplifier. Part of the initial beam was diverted by a beam splitter onto a second silicon photodiode to normalize the resulting signal with respect to intensity fluctuations. The change in refractive index ∆n was calculated using
∆n(t) )
λ cos φ ‚arcsinx|Sn| 2πd sin2 φ
where λ is the laser wavelength, φ is the internal angle of incidence (which was 29° for these measurements), d is the sample thickness, and Sn is the normalized signal that is the voltage of the detector during the experiment divided by the voltage for maximum transmission. Photorefractive measurements were carried out with standard two- and four-wave mixing setups.1-6 For both experiments, the same tilted geometry configuration was employed in which two equal intensity p-polarized 647-nm beams from a Kr+ laser intersected in the material at external angles of 30 and 60° with respect to the sample normal. In two-wave mixing measurements, the output powers of the two intersecting beams were monitored after the sample with silicon photodiodes. The gain coefficient was calculated using Ig ) I0/1 + β exp(-ΓL), where Ig is the power of the beam that gains energy, I0 is the total writing beam power, β is the beam ratio, and L is the path length of the beam through the sample. In four-wave measurements, a weak (100× less intense with respect to the writing beams) probe-beam counter propagated one of the writing beams. The diffraction of this beam was monitored with another silicon detector. The probe beam was chopped at a frequency of 10 kHz with an acousto-optic modulator that allowed lock-in detection. The change in refractive index was calculated from the internal diffraction efficiency (diffracted probe beam power divided by transmitted probe power with no diffraction) using
e ‚e ) (π∆nL λ
η ) sin2
1
2
where λ is the laser wavelength and e1 and e2 are unit vectors along the electric field of the incident and diffracted beams, respectively. Acknowledgment. We thank Shaumo Sadhukhan for assisting in sample preparation and R. J. Twieg for providing the
J. Phys. Chem. B, Vol. 107, No. 20, 2003 4737 AODCST chromophore. This work was supported in part by the U.S. Air Force Office of Scientific Research Grant No. F49620-00-1-0038. References and Notes (1) Moerner, W. E.; Grunnet-Jepsen, A.; Thompson, C. L. Annu. ReV. Mater. Sci. 1997, 27, 585-623. (2) Solymar, L.; Webb, D. J.; Grunnet-Jepsen, A. The Physics and Applications of PhotorefractiVe Materials; Clarendon Press: Oxford, U.K., 1996. (3) Gunter, P.; Huignard, J. P. PhotorefractiVe Effects and Materials; Springer-Verlag: New York, 1988; Vol. 61, pp 7-73. (4) Moerner, W. E.; Silence, S. M. Chem. ReV. 1994, 94, 127-155. (5) Kippelen, B.; Meerholz, K.; Peyghambarian, N. An Introduction to Photorefractive Polymers. In Nonlinear Optics of Organic Molecules and Polymers; Nalwa, H. S., Miyata, S., Eds.; CRC Press: Boca Raton, FL, 1997; pp 465-514. (6) Zilker, S. J. ChemPhysChem 2000, 1, 72-87. (7) Wright, D.; Gubler, U.; Roh, Y.; Moerner, W. E.; He, M.; Twieg, R. J. Appl. Phys. Lett. 2001, 79, 4274. (8) Wurthner, F.; Yao, S.; Schilling, J.; Wortmann, R.; Redi-Abshiro, M.; Mecher, E.; Gallego-Gomez, F.; Meerholz, K. J. Am. Chem. Soc. 2001, 123, 2810-2824. (9) Moerner, W. E.; Silence, S. M.; Hache, F.; Bjorklund, G. C. J. Opt. Soc. Am. B 1994, 11, 320-330. (10) Strohriegl, P. Makromol. Chem. 1986, 7, 771-775. (11) Domes, H.; Fischer, R.; Haarer, D. Makromol. Chem. 1989, 190, 165-174. (12) Haarer, D.; Meyer, H.; Strohriegl, P. Makromol. Chem. 1991, 192, 617-629. (13) Zobel, O.; Eckl, M.; Strohriegl, P.; Haarer, D. AdV. Mater. 1995, 7, 911-914. (14) Poga, C.; Lundquist, P. M.; Lee, V.; Shelby, R. M.; Twieg, R. J.; Burland, D. M. Appl. Phys. Lett. 1996, 69, 1047-1049. (15) Schloter, S.; Hofmann, U.; Strohriegl, P.; Schmidt, H. W.; Haarer, D. J. Opt. Soc. Am. B 1998, 15, 2473-2475. (16) Bratcher, M. S.; DeClue, M. S.; GrunnetJepsen, A.; Wright, D.; Smith, B. R.; Moerner, W. E.; Siegel, J. S. J. Am. Chem. Soc. 1998, 120, 9680-9681. (17) Bos, F. C.; Burland, D. M. Phys. ReV. Lett. 1987, 58, 152-155. (18) Bos, F. C.; Guion, T.; Burland, D. M. Phys. ReV. B 1989, 39, 12633-12641. (19) Borsenberger, P. M.; Weiss, D. S. Organic Photoreceptors for Xerography; Marcel Dekker: New York, 1998. (20) Borsenberger, P. M. J. Appl. Phys. 1990, 68, 5188-5199. (21) Goodbrand, H. B.; Hu, N.-X. J. Org. Chem. 1999, 64, 670-674. (22) DeClue, M. S.; Wright, D.; Moerner, W. E.; Siegel, J. S. To be submitted for publication, 2003. (23) Bolink, H. J.; Krasnikov, V. V.; Malliaras, G. G.; Hadziioannou, G. J. Phys. Chem. 1996, 100, 16356-16360. (24) Mahgerefteh, D.; Feinberg, J. Phys. ReV. Lett. 1990, 64, 21952198. (25) Sandalphon; Kippelen, B.; Meerholz, K.; Peyghambarian, N. Appl. Opt. 1996, 35, 2346-2354. (26) Wright, D. Ph.D. Thesis, Stanford University, Stanford, CA, 2001. (27) Aklonis, J. J.; MacKnight, W. J. Introduction to Polymer Viscoelasticity, 2nd ed.; Wiley & Sons: New York, 1983. (28) Hendrickx, E.; Kippelen, B.; Thayumanavan, S.; Marder, S. R.; Persoons, A.; Peyghambarian, N. J. Chem. Phys. 2000, 112, 9557-9561. (29) Lux, M.; Strohriegl, P.; Ho¨cker, H. Makromol. Chem. 1987, 188, 811-820. (30) Diaz-Garcia, M. A.; Wright, D.; Casperson, J. D.; Smith, B.; Glazer, E.; Moerner, W. E.; Sukhomlinova, L. I.; Twieg, R. J. Chem. Mater. 1999, 11, 1784-1791.
