Electronic Energy Transfer in Oriented Bilayer Films of Polysilanes

layer by spin-casting a solution of poly(methyloctadecylsilylene) (PMOdS). The bilayer films were characterized with polarized UV absorption and polar...
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J. Phys. Chem. B 1999, 103, 8467-8473

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Electronic Energy Transfer in Oriented Bilayer Films of Polysilanes A. Kaito,* N. Tanigaki, D. Hajiheidari,† T. Yatabe, and Y. Tanabe National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ReceiVed: May 19, 1999; In Final Form: July 29, 1999

The intermolecular energy transfer was studied in a bilayer film of polysilanes. The highly oriented films of poly(diethylsilylene) (PDES) were prepared by the mechanical deposition technique, which had been originally developed by Wittmann and co-workers. The poorly oriented layer was formed on the highly oriented PDES layer by spin-casting a solution of poly(methyloctadecylsilylene) (PMOdS). The bilayer films were characterized with polarized UV absorption and polarized fluorescence spectroscopy. It was shown that the light absorption in the PMOdS layer contributed to the fluorescence intensity of the PDES layer and that the electronic energy was transferred from the PMOdS layer to the PDES layer. Although the poorly oriented PMOdS layer absorbs both parallel-polarized light and perpendicular-polarized light, the highly oriented PDES layer emits only light polarized parallel to the orientation direction of the silicon main chains. Thus, the bilayer film has a function of rotating the polarization direction and of the isotropic-to-polarized light conversion. The fluorescence intensity of PDES was shown to increase with an increase in the thickness of the PMOdS layer, but the increase in the fluorescence intensity saturates above 500 nm. The saturation is caused by the limitation of the migration distance of excitons. The polarization of the excitation light influenced the emission intensity when the thickness of the PMOdS layer was thinner than 100 nm, suggesting that the excitons move between the sites having similar orientations of transition dipole moments. The possible mechanism of exciton migration was discussed in terms of the segment models, in which the molecular chains are separated into the ordered segments with various lengths.

Introduction Polysilanes are chainlike polymers composed of the silicon backbones and having organic substituents. Considerable attention has been given to the fundamental properties of polysilanes, such as electron conductivity, photoconductivity, thermochromism, photoresists, luminescence, and nonlinear optical properties, owing to their potential applications.1-12 The electronic properties of polysilanes originate from the conjugation of σ-electrons in the silicon-silicon linkages, which gives rise to light absorption in the UV region. The σ-conjugated system of polysilanes also exhibits an efficient emission in the UV and near UV regions, which lets us use them as the lightemitting device (LED).7-12 Polysilanes are not only used as a hole-transporting layer in the multilayered LED device7,8 but also applied to a single-layered LED device owing to their lightemissive and hole-transporting functions.9-12 The electronic structures of polysilanes depend on both substitutents and the conformation of molecular chains. The electronic spectroscopy of polysilanes was discussed on the basis of the segment model.13 The electronic excitation was shown to localize in the chain segments of various lengths, leading to the broad energy distribution in the excited states of polysilanes in dilute solution. If the shorter segments with higher energy are excited, the energy transfer occurs from the shorter segments to the longer segments. The fluorescence is emitted primarily from the longest segment owing to the rapid energy transfer.13 The energy transfer was shown to occur between phases in the polysilane films.14 The photoexcitation energies are transferred * To whom correspondence should be addressed. † On leave from Isfahan University of Technology, Jahad Daneshgahi, Isfahan, Iran.

from the disordered phase of poly(di-n-hexylsilylene) (PDHS) to the ordered phase made of all-trans segments.14 The possibility of intermolecular energy transfer has been studied using bicomponent materials consisting of polysilanes having different substituents.15-17 The energy transfer distance between polysilane layers was examined by employing heteropolysilane Langmuir-Blodgett (LB) films.15 It was estimated that energy was transferred from a poly(alkylsilane) layer to a poly(arylsilane) layer through an insulator layer thinner than 2.2 nm. The fluorescence spectroscopy of the blends of polysilane was discussed in relation to the miscibility of the blends.16 In a miscible blend, such as the PDHS/poly(methyln-propylsilylene) blend, energy is transferred from polysilane with a higher electronic energy to that with lower energy. The intermolecular energy transfer between PDHS and poly(diphenylsilylene) was examined in the mixed powder form by the time-resolved photoluminescence measurements.17 Montali et al. have recently reported a photoluminescent polarizer, which is based on the polarizing energy transfer.18 The materials were constituted of randomly oriented sensitizer molecules and a uniaxially oriented photoluminescent polymer. The former absorbs isotropic light, and then the energy transfer occurs to the latter, from which a highly polarized light is emitted. In this work, we studied the energy transfer in the bilayer films made of a highly oriented poly(diethylsilylene) (PDES) layer and a poorly oriented poly(methyloctadecylsilylene) (PMOdS) layer, aiming at clarifying the polarization characteristics of the intermolecular energy transfer between the two polysilane layers. As PDES is not soluble in organic solvents, it is possible to prepare the PMOdS film on the PDES film by solution casting. The oriented films of PDES could be prepared

10.1021/jp991620t CCC: $18.00 © 1999 American Chemical Society Published on Web 09/16/1999

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by the mechanical deposition technique originally proposed by Wittmann and co-workers.19-21 In this method, ultrathin oriented films of intractable polymers could be deposited onto the substrate by sliding the polymer disk against the substrate. We have succeeded in preparing the oriented films of insoluble polysilanes by the mechanical deposition technique.22-24 Experimental Section PMOdS was synthesized by the Wurtz coupling reaction. (Dichloromethyl)octadecylsilane was added to molten sodium in refluxing toluene. The reaction was carried out for 20 h at 111 °C. Pouring the solution into methanol solidified the white powder, after the reaction mixture was filtrated and washed with water. The molecular weight distribution was measured using gel permeation chromatography (GPC) equipment calibrated with polystyrene standards. A bimodal molecular weight distribution was exhibited on the GPC curve of the polymer product, which was separated into two fractions with different molecular weights by repeated fractional precipitation from hexane with 2-propanol. The higher molecular weight fraction with Mw ) 400 000 was used for the experiment. PDES was obtained by the Wurtz coupling reaction of dichlorodiethylsilane. A higher molecular weight fraction of PDES, which was insoluble in organic solvents, was used for the experiment. The highly oriented films of PDES were prepared by the mechanical deposition technique, as described in ref 24. The powder of PDES was compressed into a disk at about 235 MPa under vacuum. The PDES disk was slid onto a quartz substrate at a velocity of 1 m/min and a pressure of 1 MPa. The temperature of the substrate was controlled at 210 °C during the deposition. The mechanical deposition of PDES was carried out under a nitrogen atmosphere. To evaluate the thickness of the film, the FTIR spectrum of the PDES film was measured in the CH2 stretching region. The integrated absorbance was referred to the thickness-absorbance relationship obtained for PDES powder in the KBr disk. The linear relationship was obtained between the absorption intensity of FTIR spectra and the weight of PDES per unit area in the range 0-10-4 g/cm2. The thickness (weight of PDES per unit area) of the mechanicaldeposition film of PDES was roughly estimated to be (2.53.0) × 10-6 g/cm2 (25-30 nm). PMOdS films were formed on the mechanical-deposition film of PDES by spin-casting the hexane solution of PMOdS at 1500 rpm. The film thickness of PMOdS is controlled by the concentration. As the surface of thin PMOdS was too soft to be probed, the thickness was evaluated from the UV absorption intensity integrated in the spectral range 250-360 nm, relative to the absorption intensity of the reference film with a known thickness. A 630 nm thick reference film was spin-cast on a quartz plate with a size of 10 × 40 mm, and the weight of the film was carefully measured by a sensitive balance. The thickness of the reference film was calculated from the measured weight of the film, density (0.89 g/cm3), and area of the film. The polarized UV spectra were measured with a Shimadzu MPS-2000 spectrophotometer and a Glan-Thompson polarizing prism. The polarized fluorescence spectra were measured with a fluorescence spectrophotometer FP777 (Japan Spectroscopic Co. Ltd.), equipped with two Glan-Thompson polarizing prisms. The optical system for the polarized fluorescence measurements is shown in Figure 1, in which the Z- and X-axes correspond to the draw direction (DD) and the transverse direction (TD), respectively. The fluorescence intensity, Iij (i, j ) X, Y, Z) represents the intensity of a component of polarized

Figure 1. Optical arrangement for the polarized fluorescence measurements.

Figure 2. Polarized UV spectra of the mechanical-deposition film of PMOdS. (s) parallel polarization, (- - -) perpendicular polarization.

fluorescence, which is measured with setting excitation polarization to the i-axis and emission polarization to the j-axis. The excitation beam was incident normal to the film plane, and the fluorescence was detected at a 45° angle from the excitation beam. The polarization for the fluorescence beam was set normal to the optical base, whereas the polarization for the excitation beam was rotated. Four components of polarized fluorescence, Izz, Izx, Ixz, and Ixx, can be detected by rotating the polarizer for the excitation beam and the sample direction. The polarization characteristics of the excitation spectrometer were corrected with a measured intensity of the polarized fluorescence of the isotropic sample. Results and Discussion Figures 2 and 3 show the polarized UV spectra and the polarized fluorescence spectra, respectively, of the mechanically deposited PDES film. Although the absorbance is sensitive to the substrate temperature during deposition,24 the UV and fluorescence spectra are reproducible if the films are prepared in the same conditions. The PDES film shows a sharp absorption at 365 nm and an emission at 372 nm. A strong absorption band is observed for the parallel polarization, whereas any significant absorption intensity is not detected for the perpendicular polarization. A strong emission band is observed when the polarization directions for the excitation and emission are parallel to the sliding direction. But the emission intensity decreases by more than one order of magnitude if either one of the polarization directions is perpendicular to the sliding direction. The highly polarized UV and fluorescence spectra show that the molecular chains of PDES are highly oriented in the sliding direction during deposition. Figure 4 shows the polarized UV spectra of the PMOdS/ PDES bilayer film. PMOdS shows a broad absorption band at 300-320 nm, as is observed for the cast film of PMOdS without

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Figure 5. Measurements of fluorescence spectra of the PMOdS/PDES film: (a) with spacer; (b) without spacer.

Figure 3. Polarized fluorescence spectra of the mechanical-deposition film of PDES excited at λex ) 355 nm: (s) Izz (parallel excitation and parallel emission); (- - -) Izx (parallel excitation and perpendicular emission); (- ‚ -) Ixz (perpendicular excitation and parallel emission); (- - -) Ixx (perpendicular excitation and perpendicular emission).

Figure 6. Fluorescence spectra of the PMOdS/PDES film excited at λex ) 310 nm (thickness of PMOdS: 135 nm): (s) without spacer; (- - -) with spacer.

Figure 4. Polarized UV spectra of the PMOdS/PDES film (thickness of PMOdS: 135 nm): (s) parallel polarization; (- ‚ -) perpendicular polarization.

PDES. The polarized UV spectrum of the bilayer film is approximated to be a combination of the spectra of the respective polysilane layers. The absorption peak of PDES slightly shifts to the shorter wavelength region and becomes broader in the bilayer films with respect to the spectrum of the PDES film without PMOdS. The PDES layer is oriented in the bilayer film as highly as in the PDES film without PMOdS, whereas the degree of orientation is low in the PMOdS layer. When the polymer film is cast on the highly oriented polymer substrate, molecular orientation is generally induced in the film, resulting in the parallel alignment of molecular chains to the orientation direction of the substrate polymer.25-28 But this does not hold true for the case of PMOdS having long side chains, because two modes of orientation compete with each other.29 One is the orientation of main chains to the mechanical direction, and the other is the orientation of side chains to the mechanical direction, which forces main chains to orient perpendicular to the mechanical direction. The side-chain orientation dominates over the main-chain orientation in the case of PMOdS films crystallized on the mechanically deposited poly(dimethylsilylene) film.29 The main-chain orientation is prevailing in the PMOdS film cast on the PDES films. The competition of the

two orientation modes lowers the degree of molecular orientation. Figure 5 shows the measurements of the fluorescence spectra of the PMOdS/PDES film. The PMOdS layer directly contacts with the PDES film in Figure 5b, while an air gap of 100 µm was introduced between the films of PDES and PMOdS in Figure 5a, as described below. PMOdS and PDES films were prepared separately on different quartz substrates. Small pieces of spacer film (100 µm thick) were pasted on the edges of one of quartz plates. Then quartz plates were placed with the coated surfaces facing each other so as that the spacer provides the required air gap between the polysilane layers. The fluorescence spectrum originates not only from the excitation of the PDES and PMOdS layers by the incident beam (λex ) 310 nm) but also from the excitation of the PDES layer by the emission light (348 nm) of the PMOdS layer in the sample with spacer (Figure 5a). In addition to them, the electronic energy transfer from PMOdS to PDES is expected to affect the fluorescence spectrum in the case of the sample without spacer (Figure 5b). Figure 6 shows the fluorescence spectra of the PMOdS/PDES films with and without spacer. The PMOdS layers in the two samples are as thick as each other. The fluorescence intensity of PMOdS is much higher than the intensity of PDES in the sample with spacer. This is because the absorbance of the PDES layer is extremely low at the excitation wavelength (310 nm). The fluorescence intensity of PMOdS, however, decreases and that of PDES increases in the PMOdS/PDES film without spacer relative to the sample with spacer. The electronic energy transfer from the PMOdS layer to the PDES layer is evident from the change in the relative fluorescence intensity. The excitons created in the PMOdS layer migrate to the PDES layer and thereby contribute to the emission from the PDES layer.

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Figure 7. Polarized fluorescence spectra of the PMOdS/PDES film excited at λex ) 310 nm (thickness of PMOdS: 135 nm): (a) parallelpolarized excitation; (b) perpendicular-polarized excitation. Key: (s) parallel-polarized emission; (- ‚ -) perpendicular-polarized emission.

Figure 8. Polarized excitation spectra detected by the parallel-polarized emission at λfl ) 372 nm. (a) Parallel-polarized excitation; thickness of PMOdS layer: 0, 53, 67, 725 nm. (b) Perpendicular-polarized excitation, thickness of PMOdS layer: 0, 53, 67, 106, 725 nm.

The polarized fluorescence spectra of the PMOdS/PDES film are shown in Figure 7. The fluorescence spectra are composed of a weak band of PMOdS and an intense band of PDES. The fluorescence band of the PMOdS layer is poorly polarized, reflecting the low degree of orientation, whereas that of the PDES layer is highly polarized parallel to the sliding direction irrespective of the polarization direction of the excitation light. The PMOdS layer is selectively excited at 310 nm, and the excitation of the PDES layer has only a minor effect on the fluorescence spectrum, because the absorbance of the PDES layer is much lower than that of the PMOdS layer at 310 nm. The poorly oriented PMOdS layer absorbs both parallelpolarized light and perpendicular-polarized light. The excitons created by the light absorption migrate to the highly oriented PDES layer, which emits only the light polarized parallel to the sliding direction. Thus, the excitation of the PMOdS layer by the perpendicular-polarized light contributes to the parallelpolarized emission from the PDES layer. Figure 8 shows the polarized excitation spectra of the PMOdS/PDES film under parallel-polarized detection at the fluorescence wavelength of PDES (λfl ) 372 nm). The excitation spectra under parallel excitation (Figure 8a) are composed of an intense peak (363 nm) and a broad shoulder (300-320 nm) originating from the excitation of the PDES and PMOdS layers, respectively, while the spectrum under perpendicular excitation (Figure 8b) shows only a band due to the PMOdS layer. The excitation spectra are intensified with an increase in the thickness of the PMOdS layer, suggesting that the excitation of the

Figure 9. Corrected intensity of polarized fluorescence (λfl ) 372 nm): (- ‚ -) PDES film, λex ) 355 nm, Izz (parallel excitation and parallel emission); (s) PMOdS/PDES film, λex ) 310 nm, Izz (parallel excitation and parallel emission); (- - -) PMOdS/PDES film, λex ) 310 nm, Ixz (perpendicular excitation and parallel emission). Thickness of PMOdS layer: (a) 53 nm; (b) 725 nm.

PMOdS layer contributes to the parallel-polarized emission from the PDES layer. The excitation spectra of the PMOdS/PDES films also support the occurrence of electronic energy transfer from the PMOdS layer to the PDES layer. The efficiency of energy transfer was studied by comparing the fluorescence intensity of the PMOdS/PDES film with that of the PDES film. The light intensity of the excitation beam was corrected by using the fluorescence intensity of the standard sample, Rhodamin B in this experiment. But the observed intensity, I, should be corrected for the difference in the absorbed light intensity, IA, to compare the fluorescence spectra for which the absorbance at the excitation wavelength is different. The corrected intensity of the fluorescence spectra, IC, was obtained by dividing the observed fluorescence spectrum, I, by the absorbed light intensity, IA, at the excitation wavelength.

IC ) I/IA ) I/(1.0 - 10-A)

(1)

where A is the absorbance at the excitation wavelength. The corrected fluorescence spectra are shown in Figure 9. The fluorescence intensity of the PMOdS/PDES film excited at the absorption maximum of PMOdS is lower than the fluorescence intensity of the PDES film excited at 355 nm. The decrease of fluorescence intensity is caused by the emission from the PMOdS layer and the thermal relaxation during the migration

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I(t) ) I0

∫0texp(-Y/a) exp(-(t - Y)/d) dY

∝ I0 (exp(-t/a) - exp(-t/d))

Figure 10. Fluorescence intensity vs thickness of the PMOdS layer (λex ) 310 nm, λfl ) 372 nm): (O) Izz (parallel excitation and parallel emission); (4) Ixz (perpendicular excitation and parallel emission).

Figure 11. Schematic representation of the exciton migration in the PMOdS layer.

of excitons. When the PMOdS layer is thin (Figure 9a), the efficiency of energy transfer depends on the excitation polarization. The energy transfer to the site with the same polarization is more efficient than the energy transfer accompanied by the rotation of polarization. For the sample with a thicker PMOdS layer (Figure 9b), however, the efficiency of energy transfer is not much affected by the excitation polarization. The efficiency of the energy transfer is reduced with an increase in the thickness of the PMOdS layer. The effect of thickness of the PMOdS layer on the fluorescece intensity of PDES is shown in Figure 10. The fluorescence intensity of PDES increases with the increasing thickness of the PMOdS layer. The excitation of the PMOdS layer contributes to the fluorescence intensity of PDES due to the energy transfer from the PMOdS layer to the PDES layer. The increase in the fluorescence intensity, however, saturates above 500 nm. The saturation is caused by the limitation of the exciton migration distance and the saturation of absorbed light intensity. Figure 11 shows a schematic representation of exciton migration in the PMOdS layer. The amount of excitons, which are created at a point Y in the PMOdS layer and migrate to the interface of the PDES layer, t, is proportional to the light intensity at Y and the probability of exciton migration from Y to t. The light energy inside the PMOdS layer decreases with increasing distance owing to the light absorption. If the probability of exciton migration is assumed to decrease in accordance with the exponential function of the migration distance, the fluorescence

(2)

where t is the thickness of the PMOdS layer and a and d stand for the light penetration distance and the exciton migration distance, respectively, in the PMOdS layer. The value of a can be calculated from the absorbance A measured for a PMOdS film of known thickness and is found to be 0.362 µm. The value of d ) 2 µm is obtained from the fitting of experimental fluorescence intensities to eq 2 (Figure 9). The excitons created in the PMOdS layer migrate in the long distance, until they are annihilated by its own emission and thermal relaxation. The fluorescence of PDES increases with the increasing amount of excitons migrating to the interface of the two polysilane layers. The exciton migration distance has been extensively examined by employing the multicomponent LB films. The fluorescence quenching of a monomethin-oxacyanine dye by monomethin thiacyanine was studied in the LB films,30 and the intermolecular energy transfer between dyes was shown to occur on a scale of 10 nm. The energy transfer distance between the two polysilane layers was estimated to be less than 2.2 nm,15 from the measurement of fluorescence spectra of heteropolysilane LB films having an insulator layer. A single-step energy transfer cannot explain the long migration distance in the PMOdS layer, but the excitons migrate in the wide range of the PMOdS layer by multistep hopping. The fluorescence spectroscopy of polysilanes has been discussed in terms of the segment model, in which the molecular chains are separated into ordered segments with various lengths.13 The segment length determines the energy of the ground and excited electronic states. The longer segments exhibit the electronic transition at longer wavelength, and the distribution of segment length contributes to the broadening of the electronic spectra. Elschner et al. reported the site-selective fluorescence of polysilanes, and the experimental results are successfully compared with the results of Monte Carlo computer simulations of the random walk of excitations of hopping sites.31 Suto et al. studied the time-resolved luminescence spectra as well as the site-selective luminescence of PDHS and explained the results by the exciton dynamics based on the segment model.32,33 They showed that the long-range dipole-dipole interaction plays an essential role in the movement of excitons.33 The segments are considered to be the hopping sites of the energy transfer, and the excitons move from a shorter segment to the longer one; from the higher energy site to the lower energy state. The excitons repeat hopping not only between the sites on the same molecular chains separated by the conformational defects but also between the sites located on the different molecular chains. The fluorescence depolarization during the course of energy transfer is examined as a function of thickness of the PMOdS layer. The effect of excitation polarization on the intensity of parallel-polarized emission is studied in terms of the degree of polarization, P, which is expressed as

P ) (ICxz - ICzz)/(ICxz + ICzz)

(3)

where ICzz and ICxz are the corrected fluorescence intensities of the parallel-polarized (Z-axis polarized) emission of PDES under parallel (Z-axis) and perpendicular (X-axis) excitation, respectively, at the excitation wavelength of PMOdS absorption maximum (λex ) 310 nm). The corrected fluorescence intensities

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Figure 12. Degree of polarization vs thickness of the PMOdS layer (λex ) 310 nm, λfl ) 372 nm.).

are obtained by dividing the observed fluorescence intensity by the excitation absorption intensity.

ICxz ) Ixz/(1.0 - 10-Ax) I

C zz

) Izz/(1.0 - 10

-Az

)

(4) (5)

where Izz and Ixz are the observed fluorescence intensities for the parallel and perpendicular excitations, respectively, and Az and Ax are the absorbances of the polarized UV spectra of the PMOdS/PDES layer. The degree of polarization is shown as a function of thickness of the PMOdS later in Figure 12. The degree of polarization is higher than 0.1, for the PMOdS layer thinner than 50 nm, but the value of P approaches zero with the increasing thickness of the PMOdS layer. When the PMOdS layer is thin, the direction of the electric transition moment is retained during the exciton migration in the PMOdS layer. As the PMOdS layer becomes thicker, the transition moment direction is randomized during the course of the exciton migration. Even when the PMOdS layer is thinner than 50 nm, the layer is still thicker than the energy transfer distance evaluated by the LB techniques. The excitons migrate in the PMOdS layer 50 nm thick by at least a several-steps energy transfer with the partial retention of the transition dipole moment direction. The result suggests that the exciton migration occurs selectively between the sites with the similar orientation of the transition dipole moment. This view is in agreement with the dipoledipole interaction mechanism in the exciton dynamics. As the PMOdS layer becomes thicker, the direction of the excitation polarization is randomized during the exciton migration owing to the increase of hopping steps and the mobility of the transition dipole moments. The degree of polarization was studied as a function of excitation energy,34 and it was shown that the degree of polarization increases when the excitation energy is lowered beyond the mobility edge. The results obtained in this work are in accordance with this tendency. Conclusions The bilayer films of polysilanes made of a highly oriented layer and a poorly oriented layer can be prepared by spin-casting a PMOdS solution on the highly oriented PDES films, which is prepared by the mechanical deposition technique. The electronic energy transfer is shown to occur at the interface of the bilayer films from the poorly oriented PMOdS layer to the highly oriented PDES layer. The poorly oriented PMOdS layer

absorbs both parallel-polarized light and perpendicular-polarized light, whereas the highly oriented PDES layer emits only the light polarized parallel to the orientation direction. Thus, the bilayer film has a function of rotating the polarization direction and of the isotropic-to-polarized light conversion. The emission from the PDES layer is intensified with an increase in the thickness of the PMOdS layer in the thickness range 0-500 nm. It is interpreted that the excitons formed in the PMOdS layer migrate to the PDES layer and contribute to the emission from the PDES layer. The long migration distance in the PMOdS layer cannot be explained by a single-step energy transfer, but the excitons migrate in the wide range in the PMOdS layer by the multistep hopping. The polarization characteristics of the energy transfer suggest that the excitons move selectively between the sites with a similar orientation of the transition dipole moment. As the PMOdS layer becomes thicker, however, the direction of the excitation polarization is randomized during the exciton migration owing to the increase of hopping steps and the mobility of the transition dipole moments. References and Notes (1) Miller, R. D.; Michl, J. Chem. ReV. 1989, 89, 1359. (2) Kepler, R. G.; Zeigler, J. M.; Harrah, L. A.; Kurtz, S. R. Phys. ReV. B 1987, 35, 2818. (3) Miller, R. D.; Wallraff, G.; Clecak, N.; Soriyakumaran, R.; Michl, J.; Karatsu, A. J.; Mckinley, A. J.; Klingensmith, K. A.; Downing, J. Polym. Eng. Sci. 1989, 29, 882. (4) Abkowitz, M. A.; Stolka, M. Synth. Met. 1996, 78, 333. (5) Kabeta, K.; Wakamatsu, S.; Sugi, S.; Imai, T. Synth. Met. 1996, 82, 201. (6) Kani, R.; Nakano, Y.; Yoshida, H.; Mikoshiba, S.; Hayase, S. J. Polym. Sci., Polym. Chem. 1997 35, 2355. (7) Suzuki, H.; Meyer, H.; Hoshino, S.; Haarer, D. J. Appl. Phys. 1995, 78, 2684. (8) Suzuki, H.; Hoshino, S. J. Appl. Phys. 1996, 79, 858. (9) Suzuki, H. AdV. Mater. 1996, 8, 657. (10) Ebihara, K.; Koshihara, S.; Miyazawa, T.; Kira, M. Jpn. J. Appl. Phys. 1996, 35, L1278. (11) Xu, Y.; Fujino, T.; Naito, H.; Oka, K.; Dohmaru, T. Chem. Lett. 1998, 299. (12) Yuan, C.-H.; Hoshino, S.; Toyoda, S.; Suzuki, H.; Fujiki, M.; Matsumoto, N. Appl. Phys. Lett. 1997, 71, 3326. (13) Klingensmith, K. A.; Downing, J. W.; Miller, R. D.; Michl, J. J. Am. Chem. Soc. 1986, 108, 7438. (14) Isaka, H.; Matsumoto, N. J. Appl. Phys. 1990, 68, 6380. (15) Kani, R.; Nakano, Y.; Majima, Y.; Hayase, S. Macromolecules 1996, 29, 4187. (16) Radhakrishnan, J.; Tanigaki, N.; Kaito, A. Polymer 1999, 40, 1381. (17) (a) Aihara, S.; Kamata, N.; Ishizaka, W.; Umeda, M.; Nishibori, A.; Terunuma, D.; Yamada, K. Jpn. J. Appl. Phys. 1998, 37, 4412. (b) Aihara, S.; Umeda, M.; Nishibori, A.; Nagumo, K.; Kamata, N.; Terunuma, D. Technol. Rep. IEICE 1999, OME98-132, 9. (18) Montali, A.; Bastiaansen, C.; Smith, P.; Weder, C. Nature 1998, 392, 261. (19) Wittmann, J. C.; Smith, P. Nature 1991, 352, 414. (20) Fenwick, D.; Ihn, K. J.; Motamedi, F.; Wittmann, J. C.; Smith, P. J. Appl. Polym. Sci. 1993, 50, 1151. (21) Dietz, P.; Hansma, P. K.; Ihn, K. J.; Motamedi, F.; Smith, P. J. Mater. Sci. 1993, 28, 1372. (22) Tanigaki, N.; Yase, K.; Kaito, A.; Ueno, K. Polymer 1995, 36, 2477. (23) Tanigaki, N.; Yase, K.; Kaito, A. Thin Solid Films 1996, 273, 263. (24) Tanigaki, N.; Kyotani, H.; Wada, M.; Kaito, A.; Yoshida, Y.; Han, E.-M.; Abe, K.; Yase, K. Thin Solid Films 1998, 331, 229. (25) Frey, H.; Sheiko, S.; Moeller, M.; Wittmann, J. C. AdV. Mater, 1993, 5, 917. (26) Sheiko, S.; Blommers. B.; Frey, H.; Moeller, M. Langmuir 1996, 12, 584. (27) Frey, H.; Moeller, M.; Turetskii, A.; Lotz, B.; Matyjaszewski, K. Macromolecules 1995, 28, 5498. (28) Tanigaki, N.; Yoshida, Y.; Yase, K.; Kaito, A.; Kyotani, H. Mol. Cryst. Liq. Cryst. 1997, 294, 39. (29) Kaito, A.; Yatabe, T.; Ohnishi, S.; Tanigaki, N.; Yase, K. Macromolecules, in press. (30) Fromherz, P.; Reinbold, G. Thin Solid Films 1988, 160, 347.

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