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J. Phys. Chem. 1991, 95, 8831-8836 treated zeolites, on NMR intensities. The 27AIMAS NMR spectra of the zeolites under study contain one resonance at -60 ppm (Figure IO) due to tetrahedrally coordinated framework aluminum. There is no apparent signal due to extraframework octahedral aluminum species either in the synthesized or in the calcined materials, suggesting a high thermal stability for the CUB-Y material. The catalytic activity of the ZSM-20, CUB-Y, and CSY materials is compared by using n-hexane conversion as a model reaction. Figure 11 shows the apparent first-order plots, and rate constants are given in Table VII. At low contact times or very low conversion, the existence of an induction period in the n-hexane reaction over Y-type faujasite materials is evident.I6 The order of the activity, ZSM-20 > CUB-Y > CSY, is observed for materials having similar composition. This order of activity could arise from differences in crystal quality or from differences in the amounts of residual sodium ions. In all cases crystal quality as revealed by XRD is good, but microporosity is reduced for CSY as compared to the synthetic materials (Figure 3) and this could influence activity. Ion-exchange procedures are extensive but, although there is evidence for differences in sodium content, it is difficult to exclude the effect of traces of sodium ions on catalytic activity. However, the high activity in ZSM-20 and CUB-Y compared with that in CSY could be due to the different ordering of the T atoms, observed by 29SiNMR, resulting in differences
8831
in active site distribution. n-Hexane cracking over CUB-Y material also demonstrates interesting effects on product selectivity and hydrogen-transfer functions, which will be discussed in more detail in a separate communication. Conclusion
Faujasitic zeolites synthesized at higher framework Si/AI ratio may have either cubic or hexagonal (or mixed) symmetry. They can be synthesized with excellent crystallinity and few defects and show higher surface areas and a more uniform composition than materials of similar framework composition made by secondary synthetic methods or by dealumination of zeolite Y. The materials made by primary synthesis appear to have an aluminum distribution different from that of the dealuminated materials in that they have a greater population of Si(lA1) units, and reduced population of Si(OA1) units, as revealed by 29SiMAS NMR. Structures are tentatively proposed to account for the 29SiNMR spectra in the siliceous cubic faujasites. Differences in the aluminum ordering between siliceous faujasites produced by primary synthesis and those produced by secondary synthesis are reflected in the catalytic properties of these zeolites in hydrocarbon transformations.
(16) Dwyer, J.; Karim, K. Work to be submitted for publication. Karim, K. Ph.D. Thesis, UMIST, 1990. (17) heck, D. W. Zeolife Molecuhr Sieves;Wiley: New York, 1974. (18) Werner, P.E. Documentation for TREOR, Smithsonian Institution
Acknowledgment. We thank Crosfield Catalysts (Wamngton), IC1 Chemicals (Wilton), and Shell Research (Amsterdam) for the financial support of K. Karim, N. P. Thompson, and W. J. Smith, respectively. We also thank SERC for provision of a grant (GR/F 22777). Re&@ No. 15-Crown-5,33100-27-5; nitrogen, 7727-37-9; n-hexane,
1984.
110-54-3.
Molecular Orientation In Mlxed ?r-Conjugated Polymer Monolayers Studied by Second Harmonic Generation T. Kurata,* A. Tsumura, H. Fuchigami, and H. Koezuka Materials and Electronic Devices Laboratory, Mitsubishi Electric Corporation, 1 - 1 Tsukaguchi-Honmachi 8-Chome, Amagasaki, Hyogo 661. Japan (Received: January 4, 1991; In Final Form: May 3, 1991)
Stable mixed Langmuir-Blodgett (LB) films, composed of a soluble *-conjugated polymer, poly(3-hexylthiophene) (PHT), and an amphiphilic diacetylene derivative, pentacosa-IO,12-diynoicacid (DA), were prepared. Their structures have been investigated primarily by second harmonic generation (SHG). The mixed monolayers partly contain a double-layered structure consisting of PHT and DA. It has been concluded that the SHG originated from slightly twisted PHT in the layer, which is supported by the calculations of the molecular tilt angle of PHT repeating units. The molecular hyperpolarizability was evaluated at the same time. The UV polymerization of DA molecules inside the LB films and the successive heat treatment have enhanced the SHG intensities. The mechanism of the SHG enhancement will be also discussed.
1. Introduction Organic molecules have attracted much attention for the fabrication of future devices in optical and molecular electronic fields. Among them, *-conjugated polymers are one of the most promising candidates. Their electronic properties can be modified at will by chemical and electrochemical doping methods. The ?r electrons are loosely bound to the backbones, so they can quickly respond to the electric field of irradiated light. It is important to make the thin films of these materials in terms of device fabrications. The Langmuir-Blodgett (LB) technique has been widely studied because of the easy preparation of molecular oriented films. The molecular orientation of LB films has been investigated by various methods. The Fourier transform infrared (IT-IR) technique has been utilized to characterize the monolayers.'*2 The transmission spectra pick up the vibrations (1) Kimura,
F.; Umemura, J.; Takenaka, T. Lungmuir
1986, 2, 96.
0022-365419 1/2095-883 1$02.50/0
having the transition moments parallel to the film, while the vibration with the transition moments normal to the surface can be selectively observed in the reflection-absorption (RA) spectra. Comparison between the two spectra gives the information on molecular ~ r i e n t a t i o n . ~However, ,~ it cannot be often applied to mixed LB films consisting of several molecules when the vibrations due to each component overlap. Second harmonic generation (SHG) is a well-known phenomenon among the second nonlinear optical effects. SHG requires noncentrosymmetry of the samples and is very sensitive to the properties at the interface between the different kinds of materials. This technique has been increasingly utilized to detect the slight change, for example, at the surface of the electrode during (2) Dote, J.; Mowery, R. L. J . Phys. Chem. 1988, 92, 1571. ( 3 ) Chollet, P.-A.; Messier, J.; Rosilio, C. J . Chem. fhys. 1976, 64, 1042. (4) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J . fhys. Chem. 1990, 94, 62.
0 1991 American Chemical Society
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8832 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 electroly~is.~It is, therefore, considered that the SHG method is a powerful tool in the investigation of the orientation of the chromophores in the LB monolayers. Study on the orientation of the chromophores in the LB films was done by Heintz et al. by means of the SHG method? Later, Girling's group evaluated the molecular hyperpolarizability fl of cyanine dyes and its tilt angle using the fmed incident angle (45O) of the fundamental wave.' If molecules in a LB film with only one large hyperpolarizability component are distributed along one direction and are transferred on one side of a substrate, SHG measurement at a fixed angle is sufficient to evaluate the molecular orientation.* However, when molecules in a LB film have a more complicated distribution, it is necessary to analyze the SHG intensity dependence on incident angle. Moreover, when the LB films are deposited on both sides of a substrate, it is inadequate to measure SHG intensities at a fixed angle of incidence. Therefore, it is rather useful for the molecular orientation analysis of LB films to use a method similar to SHG Maker fringe? The SHG Maker fringe is obtained by measuring the incident angle dependence of the SHG intensities for all the combinations of the polarization. There are a few reports about the measurements of the SHG fringe patterns of LB films for the molecular orientation,'OJ' but these LB films consisted of solely dye molecule or both dye molecule and supporting fatty acid. There have been no reports on the molecular orientation of polymer LB films by this technique. In this paper, we have investigated the molecular orientation of the mixed LB films consisting of the soluble poly(3-hexylthiophene) (PHT) and amphiphilic pentacosa-lO,12-diynoic acid (DA) especially by means of SHG measurements. 2. Experimental Section Mixed monolayers consisting of poly(3-hexylthiophene) (PHT) and amphiphilic pentacusa-lO,l2diynoicacid (DA) were prepared. PHT was synthesized according to the method of Yoshin0 et a1.I2 Synthesized PHT was purified and undoped by Soxhelt extraction successively with methanol and acetone. The molecular weight of obtained PHT was determined by gel permiation chromatograrphy using a polystyrene standard. The evaluated number of repeating units in the polymer was -300. DA was purchased from Wako Chemical Ltd. and was used without further purification. The chloroform solution containing both PHT and DA was spread on a subphase with cadmium chloride (3 X IO-' M) and sodium bicarbonate (1 X l V M). Four kinds of mixing ratio of the PHT repeating unit to DA molecule, that is, 1,2, and 4, were chosen. The temperature of the subphase was kept at 20 OC during the deposition. The surface pressure of the monolayer on the subphase was maintained at 15 mN/m, and it was transferred to both sides of a glass substrate by a usual vertical dipping method at the dipping speed of 50 mm/min. Transfer ratios were almost unity for all cases, and all the prepared multilayers were Y-type. After the deposition, as-grown samples were irradiated for 6 min by a high-pressure mercury lamp through a filter (254 nm, 1 mW/cm2) in vacuo and were successively heat-treated at 90 O C for 15 min in air. The UV irradiation polymerized DA molecules inside the LB films to give the so-called blue form poly-DA. The heat treatment transformed the blue form to the red one. ( 5 ) Biwer, B. M.; Pellin, M. J.; Schauer, 1986, 176, 371.
M.W.;Gruen, D.M.Surf. Sci.
Hantz, P.F.; Tom, H.W.K.; Shen, Y. R. Phys. Rev. 1983, A28.1883. N.A.; Kolinsky, P. V.;Eeals, J. D.;Cross,G. H.; Peterson, 1. Thin Solid Films 1985, 132, 101. (8) Kajikawa, K.; Kigata, K.; Takezoe, H.;Fukuda, A. Mol. Crysr. Liq. (6)
( 7 ) Girlin , I. R.; &de,
Crysr. 1990, 182A, 91. ( 9 ) Khanarian, G. Thin Solid Films 1987, 152, 265. (ID) Lupo, D.;Prass. W.;Schenumann, U.; Laschewsky, A.; Ringsdorf, H.;Leodoux, 1. J. Opt. Soc. Am. 1988, B5,300. (1 I ) Leodoux, 1.; Josse, D.;Fremaux, P.;Piel, J.-P.; Post, G.; Zyss, J.; McLean, T.: Hann, R. A.; Gordon, P. F.; Allen, S . Thin Solid Films 1988, 160. 217. ( 12) Sugimoto,
1986, I . 635.
R.; Takeda, S.;Gu, H.B.;Yoshino, K. Chem. Express
Kurata et al.
finer A . 5 3 "
-
r
Iu hi Figure 1. Experimental configuration for SHG measurement.
I C - C I CC&H&W
C&&
DA
I
2
3
4
Rotio of PHT(repeating unit) to DA
Figure 2. Relationship between the area per DA molecule and the ratio of PHT repeating unit to DA. The inset represents the chemical structures of PHT and DA.
Mixed LB films containing PHT and stearic acid at the mixing ratio of unity were also prepared for comparison. Absorption spectra and X-ray diffraction were used for the characterization of the deposited films. The measurement of absorption spectra was carried out by using a photospectrometer (Shmadzu Co. Ltd. Model UV-260).X-ray diffraction patterns were measured to evaluate bilayer distances of LB films by using an X-ray diffraction meter (Rigaku Co. Ltd. Model RAD-C). SHG of the LB films was measured with a Q-switched Nd: YAG laser (Quantel YG-571,1.06 pm, 10-Hz repetition, pulse width 10 ns). The measurement system is shown in Figure 1. The pulse energy and the polarization of the fundamental wave were controlled to be 5 mJ and to be horizontally polarized by use of an attenuator and a Glan laser prism, respectively. The polarization was controlled by rotating a half-wavelength plate. The laser beam was focused on a sample placed onto a rotating stage by a 50 cm focal length lens. Sewnd harmonic radiation generated from the samples was detected by a photomultiplier (Hamamatsu Photonics Model R928) after passing it through an IR cut filter (Schott RG850). a short wavelength cut filter (Toshiba V-Y-42), and a band-pass filter (Toshiba KL-53). The signals from the photomultiplier were processed with a Boxcar averager (Stanford Research Systems Model SRS-250).All the measuring systems were controlled with the aid of a personal computer (Yokogawa- Hew lett-Packard Model 236). 3. Results and Disclrrwrion
Surface pressure (?r)-molecular area ( A ) per diacetylene molecule isotherms at each mixing ratio were measured to obtain information on the monolayer structure. Figure 2 shows the relationship between the molecular area ( A ) and the mixing ratio, where A was obtained by extrapolation of the F A curve to ab-
Molecular Orientation in Mixed Polymer Monolayers
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8833
la1
- I : : :
I : : : : :
: . ; : :
I : : : :
(bl
L
'
P 0
1
2
3
4
5
6
7
Number of Layers Figure 4. Dependence of the absorbance on the number of layers at 550 nm. Samples are as-grown PHT-DA LB films, repeating unit ratios (PHT:DA) (0) 1:1, (0) 2:l,and (A)4:l. Downloaded by UNIV OF SUSSEX on September 8, 2015 | http://pubs.acs.org Publication Date: October 1, 1991 | doi: 10.1021/j100175a076
I5
2D
25
photar Emrgy
Jo
35
(ev)
Figure 3. Absorption spectra of a PHT-DA (1:l) LB film: (a) spectra 1, 2, and 3 correspond to the as-grown, UV-irradiated, and successively heat treated samples, respectively; (b) spectra 4 and 5 correspond to the difference spectra between spectra 1 and 2 and between spectra 1 and 3, respectively.
scissa. A was calculated on the basis of DA molecules. The molecular structures of PHT and DA are shown in the inset of Figure 2. A linearly increased with the addition of PHT until the ratio of the PHT repeating unit to DA reached unity. However, it did not increase in such a way when PHT was further added. These results suggest that the addition of PHT beyond the mixing ratio of unity caused structural change of the monolayer. The absorption spectra of the LB film of the mixing ratio of unity are shown in Figure 3a as a typical case. Spectra 1, 2, and 3 correspond to the as-grown LB film and the UV-irradiated and the successively heat treated films, respectively. The as-grown sample has no absorption due to DA molecules in the measured region, but only the absorption originated from PHT was observed around 2.5 eV (500 nm) as shown in spectrum 1. UV irradiation and heat treatment changed the whole spectra. Difference spectra between the as-grown and the UV-irradiated or the as-grown and the heat-treated LB films are shown as spectra 4 and 5 in Figure 3b. These spectra almost coincide with those of the blue form (UV irradiation) and the red form (heat treatment) of LB films containing only DA, re~pective1y.I~Thus, diacetylene monomers in the mixed LB films can be polymerized in the same way as the LB films containing only diacetylene monomers. These results support no change of PHT during the UV irradiation and the heat treatment. For the as-grown multilayers the absorption peak intensities are linearly proportional to the number of layers as shown in Figure 4. This result guarantees that each layer of the LB film is precisely deposited. Figure 5 shows the typical X-ray diffraction pattern of 29-layer mixed multilayers with the mixing ratio of unity. Although the U V irradiation and the successive heat treatment made the diffraction pattern broaden slightly, the samples so treated still gave clear patterns consisting of seven peaks. The results indicate that the well-ordered layer structure was formed for all samples. Furthermore, neither the UV irradiation nor the heat treatment destroyed the layer structure. Obtained bilayer distances for all of the samples are summarized in Table 1. The bilayer distances of PHT-DA LB films are wider than that of the LB film containing only DA molecules. The UV irradiation slightly lengthened the bilayer distance for all of the (13) Tieke, B.; Wegner, G. J . Polym. Sci., Polym. Chem. Ed. 1979, 17, 1631.
3
2
I ' 4
5
kJ%
6
5
7
IO
2 8 (deg.)
Figure 5. X-ray diffraction pattern of an as-grown PHT-DA ( 1 : l ) LB film (29 layers). TABLE I: Bilayer Distances of PHT-DA LB Films after Each Treatment Measured bv X-rav Diffraction
PHT-DA molar mixing ratio
uv
as-grown
irradiated
56.2 57.3 58.7 58.5
57.0 51.8 59.1 58.4
0:1 1:l
2:1 4:1
heat treated 59.I 59.5 60.2 63.1
mixed LB films. The polymerization of DA by UV irradiation therefore results in compression in the lateral dire~ti0n.I~The heat treatment further made the bilayer distances longer. Figure 6 shows typical SHG fringe patterns of the as-grown monolayer with the mixing ratio of unity deposited on both sides of a glass substrate. Other systems exhibited similar patterns. The appearance of the fringe pattern is attributed to the interference between the second harmonic radiation from the monolayers on both sides of the glass substrate. For all of the SHG measurements, only the p-polarized harmonic radiation was observed for p (P-P) and s (S-P) polarization of the fundamental wave in the high incident angle region. In addition, the SHG signals were independent of the rotation in the vertical plane. These observations indicate that the nonlinear polarization com~
~~~
(14) Tieke, B.; Lieser, G.; Weiss, K. Thin Solid Films 1983, 99, 95.
8834 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
Kurata et al.
(3
I .
s Rolotbn
ow
ldcg)
8
2.
Figure 6. Typical SHG fringe patterns of as-grown PHT-DA (1:l) LB monolayers deposited on both sides of a glass substrate.
'
2 4 6 8 IO I2 I4 PHT repeating units per area ( I O ~ / C ~ ) Figure 8. Square root of SHG intensity vs PHT repeating unit per area plots of PHT-DA LB films for P P polarization. (0),(O), and (A) correspond to the as-grown, UV-irradiated. and heat-treated samples,
Downloaded by UNIV OF SUSSEX on September 8, 2015 | http://pubs.acs.org Publication Date: October 1, 1991 | doi: 10.1021/j100175a076
OOY
respectively.
A
?' o A ; 2 3 4 s 6 7 e '
Fcl*ofiqurs
Fipn7. Dependence of the SHG intensities on the number of layers. Samples are as-grown PHT-DA (I: I ) LB films. (0) and (0)correspond to P-P and S-P polarizations, respectively.
ponents inside the film are randomly distributed and are in-plane isotropic. The LB films, therefore, have only two independent tensor components of nonlinear optical coefficient, that is, d33and 4, (=& = du =,dis) when Kleinman symmetryi5J6is assumed. The SHG intensities contributed from the monolayer on both sides of the glass were roughly 4 t i m e larger than those of the monolayer on the one side for the P-P polarization. This result is caused by the addition of the coherent SH fields generated from LB films deposited on both sides of the substrate. It is pointed out that the SHG intensities from the SHG active layers on both sides of the substrate are not equal for the S-polarization excitation? However, in our system the SHG intensities from the monolayers on both sides of the glass substrate were also 4 times larger than those from the monolayer on the one side for S-P polarization. Figure 7 shows the SHG intensity dependence on the number of layers in the case of as-grown LB films with a mixing ratio of unity. SHG peak intensities a t the incident angle of 60° for P-P and S-P polarization have been plotted there, because the fringe pattern takes a maximum around that angle. The SHG intensities for an odd number of multilayers were larger than those for an even number, a s expected from Y-type deposition. Leodoux et al. have reported that the nonlinear optical effect contributed from the first layer is much larger than that of the other layers.'O Such an effect from the first layer was not observed in our PHT-DA mixed LB films. (15) Kleinman, D. A. Phys. Reu. 1962, 126, 1977. (16) Williams, D.J. Nonlinear Optical Properties of Organic Molecules and Cty$ru/SChemla, D. S.,Zyas, J., Eds.;Academic: New York, 1987; Vol. I , p 405.
"0' PHT2 repeating 4 6 6 units per area IO
I2
I4
(l@/cm*)
Figure 9. Square root of SHG intensity vs PHT repeating unit per area plots of PHT-DA LB films for S-P polarization. (0),(O), and (A) correspond to the as-grown, UV-irradiated, and heat-treated samples, respectively.
Because the SHG intensity increases with the square of effective nonlinear optical coefficient, the square roots of the experimental values ((ZSHG)'/*) should be discussed. In our experiments observed signals include the contribution from the SHG a t the interface between the substrates and the LB films (Figure 6). Such contribution was measured only for P-P polarization, so that the nonlinear optical coefficient due to the interface has only the d33 component." Thus, in the case of P-P polarization, both the nonlinear optical coefficient d3, and d3i of the LB films and the interface nonlinear optical coefficient dg3(interface) contribute to the measured SHG. The square root of the glass SHG intensity owing to the interface should be subtracted from the square root of measured SHG values to obtain the SHG values due only to LB films. In the case of S-P polarization, there is no contribution from the interface nonlinear optic4 coefficient, so that the observed SHG is due to the nonlinear d f k i e n t d3,of LB films themselves. The (ISHG)'/* values for the mixed LB films themselves were (17) Shen, Y . R. The Principles ojNonlinear Optics; Wiley & Sons: New York, 1984; p 479.
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8835
Molecular Orientation in Mixed Polymer Monolayers
I
I
\
I
Downloaded by UNIV OF SUSSEX on September 8, 2015 | http://pubs.acs.org Publication Date: October 1, 1991 | doi: 10.1021/j100175a076
Figure 10. Possible configurations of PHT on the subphase.
plotted against the surface density of the PHT repeating unit on the subphase in Figures 8 and 9 for the P-P and S-P polarization, respectively. In each polarization the intensities are proportional to the PHT repeating units surface density until it reaches 4 X 10I4/cm2, corresponding to the mixing ratio of unity. The increasing SHG intensities diminished at higher PHT concentration. This result corresponds to the behavior in the molecular areamixing ratio characteristics (Figure 2) and indicates that the structure of PHT inside the LB film changes at higher PHT density than mixing ratio of unity. The observed S H G signals from the monolayers have been deduced to be due to PHT from the following points: (1) the SHG intensities increased with the amount of PHT inside the LB films; (2) the LB films with only DA did not give such large S H G signals. The same behavior was also observed for all of the cases. However, the samples after the UV irradiation and the heat treatment gave stronger SHG intensities than did the as-grown ones for each polarization. The LB films with only DA did not again show such S H G for both treatments. It is, therefore, considered that the structural changes of PHT or the modification of the PHT electronic state with the polymerization of DA has made PHT more SHG active. Since PHT-DA LB films have well-ordered layer structure as shown in X-ray diffraction data and PHT has long chain length compared to the length of the DA molecule, the polymer chain is considered to extend in the lateral direction of the plane. Thus, the dominant molecular hyperpolarizability /?is considered to exist not along the polymer chain but along the symmetrical axis of the thiophene ring. This consideration has been supported by the calculation of the molecular hyperpolarizability j3 tensor component using the MNDO molecular orbital method (MOPAC). Figure IO depicts the possible configurations for the PHT on the subphase, where a rectangle stands for the side view of the thiophene ring, a line represents a hexyl chain, and an arrow shows a direction of dominant molecular hyperpolarizability p. Configuration a is to be the most SHG active, since each repeating unit is oriented in the same direction. However, it is implausible because of highly steric hindrance between hexyl chains. The twisting of the thiophene ring in poly(alky1thiophene) has also been observed by NMR.'18 The most plausible configuration for PHT is usually configuration b, where the j3's of the thiophene ring cancel out each other. However, this configuration is not considered to be stable on the water subphase without difficulty. More believable configurations for PHT seem to be configuration c or d. In both configurations, hexyl chains have a tendency to go away from the water surface because of their hydrophobic properties. Configuration c is SHG inactive since each j3 again cancels another. To become SHG active for PHT, the thiophene rings have to twist slightly on the water surface like configuration d. The twisting has been caused by a suitable packing of DA and PHT because the mixed monolayers consisting of PHT and stearic acid have not produced any SHG. This result is consistent with the report that the mixed LB films containing PHT and stearic (18) Love, P.; Sugimoto, R.; Yoshino, K. Jpn. J . Appl. Phys. 1988, 27,
LlS62.
u u
Figure 11. Schematic structures of the mixed Langmuir films containing PHT and DA. (a) and (c) are the side views of tile cases in which the ratios of the PHT repeating unit to DA molecule are 1:1 and 2:1, respectively. (b) is the top view of double-layered structure.
acid have no orientation of PHT.I9 Figure 11 illustrates the structures of the mixed monolayers having PHT and DA on a subphase. When the mixing ratio of PHT to DA is below unity, the Langmuir film has a double-layered structure like Figure 1 la. All of the mixed PHT contributes to the production of SHG, which is supported by the results in both Figures 8 and 9. Structure a should lead to the expansion of the layer distance compared to the LB films containing only DA molecules. Indeed, such expansion was detected by X-ray diffraction. Figure l l b shows the top view of double-layered structure. Each DA molecule stands around the sutfur atom of the thiophene ring as shown in Figure 11b. Therefore, the bilayer distance only expands by about 1 A on going from a pure LB film of the DA to a mixed LB film containing PHT and DA ( 1 : l ) . This double-layered structure may prevent the polymerization of DA because of the larger distance between DA molecules. Ogawa et al. have recently reported that the polymerizable distance between DA molecules is relatively large.20 Even in this structure as illustrated in Figure 1 l a DA molecules are polymerizable. The addition of PHT beyond the mixing ratio of unity did not enlarge the SHG intensity so much. It is, therefore, considered that some parts of further added PHT should be SHG inactive. The possible structure of the Langmuir film with the mixing ratio beyond unity is shown in Figure 1 IC, where some of the mixed PHT is squeezed out from the monolayer to aggregate. This model elucidates the smaller increase in the area per DA molecule (Figure 2). The polymerization of DA molecules in the films increased the bilayer distance (Table I), leading to the shrinkage in the lateral direction. Such shrinkage has compressed the PHT chains, resulting in the enlargement of the PHT twisting. The heat treatment has transformed the blue form of poly-DA into the red one. The heat treatment expanded the bilayer distance further by more than 1 A, resulting in the additional compression of PHT. This is one reason for the SHG enhancement after the UV irradiation and successive heat treatment. However, one question about the DA polymerization increases of SHG intensities for both (19) Watanabe, I.; 94, 8715.
Cheung, J. H.; Rubner, M. F. J . Phys. Chem. 1990,
(20) Ogawa, K.; Tamura, H.; Hatada, M.; Ishihara, T. fungmuir 1988, 4, 903.
J . Phys. Chem. 1991, 95,8836-8839
8836
P-P and S-P polarizations remains: the fl value itself should increase. It is thought that the interaction between PHT and the newly formed r-conjugated poly-DA has enlarged the hyperpolarizability fl of PHT. To confirm the above understanding on the structure for the PHT-DA mixed LB films, we have analyzed the SHG characteristics quantitatively. Our system has only two independent nonlinear optical coefficients, d33and d31. The effective nonlinear optical coefficient is the nonlinear optical coefficient d multiplied by the projection factor p(8) and is described for P-P and S-P polarizations, neglecting local field factor, as
dp(8) = d j 3 sin3 8 + 3d31 cos2 8 sin 8 (P-P polarization) dp(8) = d3l sin 8 (S-P polarization)
(1)
esu (7) 70) (8) A large tilt angle indicates that the thiophene ring is almost laying down in the plane. This result is in good agreement with Figure 1Od configuration expected from SHG characteristics. Therefore, U V irradiation and successive heat treatment have made PHT more twisting and more SHG active. This is the first case for the estimation of 8 for the thiophene ring in the polymer chain to the best of our knowledge. The estimated fi value is smaller esu of 4-(N,N-dimethylamino)-4'-nitrosthan the 450 X tilbene (DANS)21or the 16.7 X esu of 2-methyl-4-nitroaniline (MNA)22but of the same order as the 6.0 X esu of m-nitroaniline ( ~ I - N A or ) ~the ~ 4.5 X esu of urea.24
(2)
4. Conclusion
where 8 is the incident angle of the fundamental wave. To evaluate molecular tilt angle and molecular hyperpolarizability, the simple distribution model was The model has the following assumptions: (1) the molecule has one main component of 8, and the molecular axis is consistent with the direction of the main component of 8; (2) the axis is inclined at an average angle to the surface normal with a random azimuthal angle. These assumptions are rational for our systems having only two independent tensor components of nonlinear optical coefficient and being inplane isotropic. According to above equations, d31 is calculated from S-P polarization first. Second, d3, is estimated from P-P polarization. Finally, hyperpolarizability j3 of the PHT repeating unit and tilt angle $ are obtained
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+
d 3 3 ( = k z )
kNflc0s3 G
d3d=Y2xxxx) = Wj3 sin2 $ cos G
+
p = (4.6 f 1.4)
(3) (4)
A = 2d3,/(2d3, d3))= sin2 J.
(5)
+ = arcsin All2
(6)
where N is the thiophene ring density. Since the contribution of d33to P-P polarization SHG was very little for as-grown and UV-irradiated LB films, d33was not able to be estimated. Only for heat-treated samples having the mixing ratio below unit was it able to be estimated. Estimated molecular hyperpolarizability j3 and molecular tilt angle $ are
X
$ = (77
The PHT-DA mixed LB films have been prepared, and their structures have been investigated primarily by SHG. The well-ordered layer structure has been confirmed by X-ray diffraction and linear dependence of the absorption on the number of deposited layers. The molecular area per DA molecules in the mixed LB films linearly increased with the addition of PHT until the mixing ratio of unity, and it did not increase in such a way beyond the mixing ratio of unity. The mixed PHT-DA LB monolayers have shown the SHG originated from the twisted PHT, and the square roots of SHG intensities for monolayers are proportional to the surface density of the PHT repeating unit. The increasing amount of the SHG intensities diminished at higher PHT concentration beyond the mixing ratio of unity. In the mixed monolayer a part of the DA molecules superimpose onto PHT, and the change for the molecular area and SHG intensities beyond the mixing ratio of unity has been caused by squeezing out some of the PHT from the double layer. The UV polymerization of DA molecules and successive heat treatment have enhanced the SHG intensities. The enhancement has been elucidated by the increase of the twisting of the PHT chain. The estimation of the molecular hyperpolarizability and the averaged tilt angle supports the above considerations. Registry No. PHT, 104934-50-1; DA, 66990-32-7; DA (homopolymer), 66990-33-8. (21) Oudar, J. L. J . Chem. Phys. 1977,67, 446. (22) Teng, C. C.; Carito, A. F. Phys. Rev. 1983, B28,6766. (23) Oudar, J. L.; Chemla, D. S.J . Ch" Phys. 1W7,66. 2664. (24) Ledoux, I.; Zyss, J. Chem. Phys. 1982, 73,203.
Analyds of the Adsorpth of Methane h Silkaifte by a SimpNfied Molecular Dynamics SknulaUon Jeffrey R. Hufton Department of Chemical Engineering, 133 Fenske Building, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: February 12, 1991) Thermodynamic and transport properties of methane adsorbed in the zeolite silicalite have been calculated from a NVT molecular dynamics algorithm. Methane was modeled as a sphere, with potential parameters determined by comparison of theoretical and experimental Henry's constants. The average potential energy and selfdiffusion coefficient of the spherical adsorbate agreed with previous MD results of June et al.' obtained with a five-point methane model. Agreement was also obtained with experimental data. Molecular dynamics (MD) simulation results have recently been reported by June et al.' for methane and xenon adsorbed in silicalite at various temperatures and adsorbed-phase occupancies. In addition to providing insightful thermodynamic and configu( I ) June. R.L.; Bell, A. T.; Theodorou, D. N. J . Phys. Chem. 1990, 91, 8232.
rational results, this very thorough and rigorous investigation also permitted calculation of adsorbate self-diffusion coefficients. In this paper results for the methane/silicalite system obtained with a simplified MD algorithm and potential model are presented. There are several important aspects of the June et a1.l study that differ from the present work. June et al. approximated the adsorbate-adsorbate and adsorbate-adsorbent interactions with
0022-365419 112095-8836$02.50/0 0 1991 American Chemical Society
