J. Phys. Chem. B 2001, 105, 4111-4117
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Molecular Orientation in Vacuum-Deposited Peralkyloligosilane Thin Films Yoshiro Ichino,* Nobutsugu Minami, and Tetsuo Yatabe National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba 305-8565, Japan
Kuninori Obata† Photodynamic Research Center, The Institute of Physical and Chemistry Research (RIKEN), Sendai 980-0868, Japan
Mitsuo Kira Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-8578, Japan ReceiVed: July 12, 2000; In Final Form: March 1, 2001
Linear oligosilanes (R(SiR2)10R), permethyldecasilane (R ) CH3), and perpropyldecasilane (R ) C3H7), were vacuum-deposited onto quartz substrates under various conditions. Molecular orientation in the deposited thin films was investigated by X-ray diffractometry and absorption spectroscopy with a varying incident angle to the substrate. Two types of highly ordered multilayered structures, one with molecular orientation normal to the substrate and the other with oblique orientation, were observed in the permethyldecasilane thin films deposited at room temperature. When deposited at -150 °C, on the other hand, the molecular orientation was largely normal to the substrates but the multilayered structure was hardly formed. For perpropyldecasilane thin films, an interdigitated structure with molecular orientation normal to the substrate was found when deposited at and above room temperatures.
1. Introduction Optical and electronic properties of polysilanes, Si-based polymers, have been extensively studied,1 and they were ascribed to the electronic states delocalized along the σ-conjugated silicon backbones. Despite a great number of studies, however, the relationship between the structure and the fundamental photophysics in solid-state polysilanes remains to be established. For such studies, Si-based oligomers (oligosilanes) should provide better opportunities, because, unlike polysilanes, they have definite silicon chain lengths and, hence, well-defined σ-conjugation lengths, which must be important for the fabrication of thin films with well-defined structure and optical properties. Since the first report of the synthesis of permethylated linear oligosilanes,2 various kinds of related compounds were synthesized1,3-6 and examined by optical5-10 and NMR spectroscopy.11 Mesomorphic properties have recently been studied for permethylated oligosilanes with various end groups.3,4,12 Structures and physical properties of these oligomers in thin films also attracted attention in the past decade.13-16 Structures of vacuum-deposited thin films were investigated for permethyldodecasilane, Me(SiMe2)12Me, for which highly ordered multilayered structures were found.13,14 Such well-ordered multilayered structures were also observed in vacuum-deposited thin films of π-conjugated oligomers with varying molecular structures.15 For σ-conjugated, Si-based molecules, on the other * To whom correspondence should be addressed. Present address: National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 3, 1-1-4 Umezono, Tsukuba 305-8563, Japan. E-mail:
[email protected]. Fax: +81-298-61-4860. Phone: +81-298-61-4298. † Present address: Kansai Research Institute Inc., Kyoto Research Park 17, Chudoji, Minami-machi, Shimogyo-ku, Kyoto 600-8813, Japan.
hand, almost no systematic studies have been done about the effect of the chain length or the kind of substituents on the thin film structures. The purpose of this paper is to elucidate the structures of vacuum-deposited thin films of permethyl- and perpropyl-oligosilanes with 10 Si units. X-ray diffractometry (XRD) was used in combination with absorption spectroscopy with a varying angle of the incident light. It has been found that the side chains (i.e., methyl vs propyl) of the oligosilanes as well as their end groups and the deposition temperatures have a strong impact on the structure of their thin films. 2. Experimental Section The chemical structures of the two peralkyldecasilanes (R(SiR2)10R), permethyldecasilane (R ) CH3, hereafter abbreviated to MS10) and perpropyldecasilane (R ) n-C3H7, PrS10), are given in Figure 1. The syntheses of these compounds were described elsewhere.5,12 The products were carefully purified by high performance liquid chromatography. MS10 and PrS10 were vapor-deposited onto quartz substrates with a deposition rate of ca. 1 Å/s at the background pressure of 1 × 10-4 Pa. The temperature of the substrates during deposition was kept at +24 or -150 °C for both oligosilanes and 80 °C only for PrS10. Molecular orders in the films were investigated with an X-ray diffractometer (Rigaku RU-300) in the θ-2θ reflection mode, where the Cu KR line (λ ) 1.542 Å) was used as an X-ray source. Absorption spectra were collected with a UV-visible spectrometer (Shimadzu UV3100PC). The experimental configuration for the absorption spectroscopy is schematically illustrated in Figure 2. Sample films were attached to a rotating holder so as to change the angle of incidence from normal (0°)
10.1021/jp002503x CCC: $20.00 © 2001 American Chemical Society Published on Web 04/20/2001
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Figure 1. Chemical structures of peralkyldecasilanes.
Figure 3. X-ray diffraction profiles of MS10 thin films vacuumdeposited at +24 (a) and -150 °C (b). The humps at around 20° are background signals from quartz substrates. Signal intensities are reduced at 2θ < 6° and are magnified at 2θ > 46°, as indicated in the top of the figure. The diffraction profile at the angle of 33° < 2θ < 46° is not shown.
Figure 2. Schematic illustration of the experimental setup for the p-polarized absorption spectroscopy.
to 45°. A blank substrate was also attached to the same holder as a reference to compensate the incident-angle-dependent baseline. Both the sample and the reference were covered with thin masks with the aperture height of 2 mm. A GlanThompson polarizer was placed in front of the sample films to provide the p-polarized incident light. All measurements were carried out at room temperatures. 3. Results and Discussion 3.1. Permethyldecasilane. XRD profiles for vacuumdeposited MS10 thin films on quartz substrates with various film thicknesses are shown in Figure 3. A series of sharp and intense peaks were clearly observed in Figure 3a for all of the films deposited at room temperatures. The most intense peak for each XRD profile at 2θ ) 4.2° in Figure 3a is reasonably assigned to the 001 diffraction with a d spacing of 20.9 Å and the others in the left-side panel of Figure 3a to its higher-order (00l, l ) 2-5) diffractions. It indicates a highly ordered multilayered structure with an interlayer spacing of 20.9 Å. Another two peaks were also observed in wider angles as shown in the right-side panel of Figure 3a, corresponding to 00l peaks with l ) 11 (d ) 1.90 Å) and 12 (d ) 1.74 Å). These d spacing values are close to the interval between neighboring Si atoms projected on the molecular long axis in the all-trans conformation (2.0 Å), resulting in enhancement of the intensities of such
higher-order diffractions. Absence of 00l peaks with l ) 6-10 is accounted for by the decrease of the structure factors with increasing l. For the film with the thickness of 480 nm, an additional series of diffraction peaks appeared, which is assigned to the 001′ diffraction with a d spacing of 18.5 Å and its higher-order diffractions (00l′, l ) 2-7). The spacing of 18.5 Å in this “secondary” structure is even shorter than that found in the “primary” structure as described above (20.9 Å). Such a secondary structure was also observed for other molecular films such as p-hexaphenyl film.16 We employed atomic force microscopy (AFM) to examine the surface morphology of the deposited films. Fairly flat surfaces containing large domains with the size of up to 10 µm were observed for the films thinner than 200 nm, which is consistent with the highly ordered multilayered structure observed by XRD. In contrast, the surface of the thicker film was found to be covered with many spirally grown, stepwisestructured grains with the size of several micrometers. Its step height was estimated to be 20 Å from a section view of the AFM image, which is close to the interlayer spacing, 20.9 or 18.5 Å, deduced from XRD. Let us discuss molecular orientation in the primary (d ) 20.9 Å) and the secondary (d ) 18.5 Å) multilayer structures in terms of the relationship between the interlayer spacing and the molecular length. The molecular length of MS10 in all-trans backbone conformation, which is represented by the distance between carbon atoms in both methyl end groups, is estimated as 20.8 Å using MM3 geometry optimization. In addition, a loosely helical backbone structure, which was proposed as the
Vacuum-Deposited Peralkyloligosilane Thin Films
Figure 4. Absorption spectra of MS10 thin films vacuum-deposited at +24 (a-c) and -150 °C (d-f) with the angle of the p-polarized incident light to the substrates of 0° (-‚-), 15° (‚‚‚), 30° (- - -), and 45° (s).
conformer having lowest transition energy in solution,5 has the molecular length whose deviation from that of the all-trans conformer is negligibly small. Therefore, one cannot specify the conformer of MS10 in the films, all-trans or helical, solely from the XRD results. The interlayer spacing of 20.9 Å for the primary multilayered structure is very close to the model molecular length, 20.8 Å, indicating that the molecular orientation in this structure is considered to be normal or slightly oblique to the substrate. On the other hand, the spacing of 18.5 Å for the secondary multilayered structure is substantially smaller than the molecular length. This suggests the oblique molecular orientation with a certain tilt angle. The tilt angle is not precisely determined only from the interlayer spacing because the relative position of molecules in neighboring layers still remains uncertain. As shown in Figure 3b, an MS10 film deposited at -150 °C showed much weaker X-ray diffraction peaks, but the peak positions were the same as those of the thickest (480 nm) film deposited at room temperature. As was discussed, these correspond to the interlayer spacing of 20.9 or 18.5 Å. This indicates that, when deposited on cold substrates, MS10 molecules could form a multilayered structure but with much less order than that deposited at room temperature. The poorly ordered structure was also confirmed by AFM, which showed the film surface covered with small grains with the size of no larger than 1 µm. Figure 4 shows the absorption spectra of the MS10 thin films with various incident angles of the p-polarized light from 0° (normal incidence) to 45°. We also measured three types of reflection spectra, reflection at the film surface or the back surface of the sample and that at the surface of the blank substrate, at each incident angle to examine the contribution of reflection to each absorption spectrum. Although the reflection spectrum was incident-angle-dependent, the absorption spectrum corrected for the reflection only exhibited the enhanced absorbance by no larger than 0.02 at the absorption peak. Accordingly, we consider that the contribution of the reflection was negligible in our case, and as a result, the uncorrected spectra are shown in Figure 4. In addition, an interference effect in the
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4113 film was also considered to be negligible, which was confirmed by the absence of the interference pattern in the transmission spectrum at the spectrally transparent region. The left-side panels (Figure 4a-c) for the films deposited at room temperature exhibit a rather complicated dependence of the spectra on the incident angle. In Figure 4a, the peak absorbance simply increases as the incident angle increases while preserving its whole spectral profile. For thicker films, in contrast, the spectral profiles depended strongly on the incident angle as shown in Figure 4 parts b and c, whereas each peak absorbance did not increase so remarkably. These spectroscopic results are not incompatible with the XRD results suggesting the highly ordered multilayered structure with molecular orientation normal to the substrate, when the difference between these experimental methods is taken into consideration. The XRD measurement in a reflection mode is exclusively sensitive to a well-ordered multilayered structure. On the contrary, all molecules in a film can contribute to an absorption spectrum, irrespective of degree of order or molecular orientation. In the case of a molecule like peralkyloligosilane, with its transition dipole moment parallel to the molecular long axis, overall molecular orientation will be obtained from the varying incident-angle absorption spectrum. Absorbance of such molecule oriented with a tilt angle φ from normal to the substrate is described as a function of the incident angle θ as follows:
R(φ,θ) ∝ (1 - 3 cos2φ) cos2 θ + 2 cos2 φ
(1)
For a molecule with normal orientation (φ ) 0°), sin2 θ dependence is derived from eq 1. A first derivative of eq 1
∂R(φ, θ) ∝ (3 cos2 φ - 1) sin 2θ ∂θ
(2)
indicates that an increasing absorption ratio upon increasing incident angle becomes smaller as the tilt angle of the molecule φ (< 54.7°) increases. An unexpectedly small increasing ratio of the absorbance shown in Figure 4c suggests that a large amount of molecules in the film had nonnormal orientation, though they were not as highly ordered in a multilayered structure as the normally oriented molecules. The complicated spectral behavior indicated in Figure 4c is presumably due to the coexistence of several absorption components with their own incident-angle dependencies. However, we consider that the full description of the assignment of the absorption peaks to corresponding molecular orientations is beyond the scope of this paper and will be discussed elsewhere in detail.17 In contrast, for the films deposited at -150 °C, the incidentangle-dependence of the absorption spectra was found to be quite simple, as is shown in the right-side panels in Figure 4. The peak absorbance became considerably stronger as the incident angle increased, whereas the spectral profile remained unchanged. It is noteworthy that this angle dependence is similar to that found for the MS12 films when deposited at room temperature.13 In Figure 5, the peak absorbance is plotted as a function of the incident angle for each film. As described above, when deposited at room temperature, the ratio of the absorbance at 45° incidence to that at normal incidence was about 2 for the thickest film. On the other hand, when deposited at -150 °C, the greatest ratio of 8.3 was obtained, indicating that most of the molecules in these films were in normal orientation to the
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Figure 5. Absorbance at the peak wavelength of the UV absorption band around 280 nm for each MS10 thin film as a function of the incident angle.
substrate. Furthermore, for the films deposited at -150 °C, the peak absorbance at normal incidence was almost constant irrespective of their thickness. Because the absorbance observed at the normal incidence is caused by molecules that are not oriented normal to the substrate, these results indicate that the total amount of such molecules did not increase with the film thickness. This further implies that such oblique orientation took place only at the vicinity of the substrate and that the molecules tended to be oriented normal to the substrate as the distance from the substrate surface increased. Summarizing the results of XRD and absorption spectroscopy, we can conclude the following: molecular orientation in vacuum-deposited MS10 thin films strongly depended on the substrate temperature. MS10 molecules deposited onto lowtemperature substrates were less ordered in terms of the multilayered structure and composed of small grains, but most of them were oriented normal to the substrate. In contrast, those deposited onto room-temperature substrates had two types of the highly ordered multilayered structures, one with molecular orientation normal to the substrate and the other with oblique orientation, but the normal molecular orientation was not dominant. It should be noted that the molecular orientation of MS10 deposited at room-temperature was greatly different from that of MS12,13 in which normally oriented molecules were dominant. This shows that the backbone length of oligosilanes is an important factor for their molecular orientation in vacuumdeposited thin films. 3.2. Perpropyldecasilane. Figure 6 shows XRD profiles of PrS10 thin films vacuum-deposited onto quartz substrates at various substrate temperatures. No diffraction peaks were observed at diffraction angles (2θ) larger than 12°. This is in contrast with MS10, for which the higher order diffraction peaks originating from the multilayer structure were observed. The films deposited at 80 and 24 °C showed quite similar XRD profiles. The most intense peaks appearing at 3.8° correspond to the d spacing of 23.1 Å, and those at 7.7° and 11.5°
Ichino et al.
Figure 6. X-ray diffraction profiles of vacuum-deposited PrS10 thin films at +80 (a), +24 (b) and -150 °C (c). Background signals from quartz substrates have been subtracted. Signal intensities less than 2θ ) 5.4° are reduced at 1/10.
correspond to the second- and the third-order diffractions, respectively. These peaks (00l, l ) 1-3) indicate that a multilayer structure with an interlayer spacing of 23.1 Å exists in PrS10 films, though less ordered than that in MS10 films. If we assume the all-trans conformation for both the silicon backbone and the n-propyl end groups, the molecular length along the long-axis is estimated to be 25.8 Å, which is longer than the interlayer spacing of 23.1 Å mentioned above. We note that this spacing of 23.1 Å was also obtained for 1,10dipropylpermethyldecasilane in its mesophase with hexagonally packed molecular orientation.3 To explain the discrepancy, 2.7 Å, between the molecular length and the interlayer spacing, we propose the interdigitated structure as depicted in Figure 7. The similar structure was previously proposed for permethyldecasilanes with ethyl, propyl, and n-butyl end groups in each mesophase, in which Smectic B molecular packing was found together with a interlayer spacing substantially shorter than each molecular length.3 In this structure, the neighboring n-propyl end groups are aligned parallel with each other, which seems to be quite reasonable because the discrepancy of 2.7 Å is close to the size of propyl end group. The interdigitated structure seems to take place not for methyl but for longer alkyl end groups. It should be mentioned that the above discussion does not exclude the possibility of a loosely helical conformer in the interdigitated structure because its molecular length is quite similar with that of the all-trans conformer. In addition, a helical conformer with a much smaller pitch than that of 7/3 helix might explain the obtained interlayer spacing without introducing the interdigitated structure. Such a conformer, however, should have much higher optical transition energy because of poor σ conjugation, which was inconsistent with our UV absorption results described later. PrS10 films generally gave much weaker diffraction peaks than MS10 films, suggesting less ordered structure. The order
Vacuum-Deposited Peralkyloligosilane Thin Films
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Figure 7. Schematic illustration of the interdigitated structure in PrS10 thin films. Molecular structure of PrS10 was stabilized by MM3 calculation. The distance between two broken lines roughly corresponds to the interlayer spacing.
became even weaker when deposited onto the substrate at -150 °C, as judged from the absence of 00l diffraction peaks (Figure 6c). We should mention the additional peaks other than at the 00l series at 9.3° (d ) 9.5 Å) in Figure 6 parts a and b and at 9.0° (d ) 9.8 Å) in Figure 6c. These d spacings are close to the diameter of PrS10 molecule, 10 Å, estimated from the molecular structure optimized by MM3 calculation. Such additional peaks corresponding to the molecular diameter were also observed for vacuum-deposited 1,10-diphenylpermethyldecasilane thin films.18 We tentatively assign these peaks to the spacing between side-by-side neighboring PrS10 molecules within their bundles lying parallel to the substrate. Further consideration is required to discuss the substrate-temperature dependence of the d spacing. In particular, conformation of propyl side groups in this structure, which significantly affects the side-by-side molecular packing, is unknown. Actually, a twisted conformation of propyl side groups, which is different from the MM3-optimized geometry, was found in perpropylhexasilane with chiral end groups by the X-ray crystallographic analysis of its single crystal.5 Varying-angle absorption spectra of PrS10 thin films are shown in Figure 8. Because the absorption spectrum hardly depended on the incident angle when deposited onto the substrate at -150 °C, only the absorption spectra at the normal incidence is indicated in Figure 8c. Such independence of the incident angle suggests that the PrS10 molecules deposited on the cold substrate were aligned neither normal nor oblique with a small tilt angle to the substrate. The absorption spectra significantly depended on the incident angle when deposited at 80 and 24 °C, as shown in Figure 8 parts a and b, respectively. Despite the similarity of the XRD profiles for both cases, the shapes of the absorption spectra were remarkably different. To discuss it in more detail, we resolved each absorption spectrum into several bands, as indicated in Figure 9. Three Gaussian functions, bands B-D in Figure 9 parts a and b, were commonly used to reproduce each absorption spectrum. Band A, on the other hand, was only seen in Figure 9a. Each set of the varying-angle absorption spectra shown in
Figure 8. Polarized absorption spectra of vacuum-deposited PrS10 thin films at 80 (a) and 24 °C (b) with various incident angles (0°40°) and at -150 °C (c) with normal incidence. All spectra were collected at room temperatures.
Figure 8 parts a and b was found to be well-reproduced by the sum of each component with the fixed peak wavelength and bandwidth. These parameters of each component are summarized in Table 1. The absorption spectrum of the PrS10 film deposited at -150 °C was also resolved into two Gaussian functions, D′ and E, as shown in Figure 9c. The function D′ is quite similar to D, whereas E is apparently different from other components in Figs. 9 parts a and b. In Figure 10, we indicate the dependence of the intensity of each resolved component on the incident angle. Each component grows as the incident-angle increases but its growth rate differs remarkably with one another. Table 1 also summarizes their rates as a ratio of the intensity for 40° incidence to that for the normal incidence and tilt angles calculated by eq 1. Among these components, band B has the largest ratio of 57 when deposited at 80 °C and of 12 at 24 °C, which are due to the extremely low intensities of band B at the normal incidence. This indicates that band B is probably attributed to the absorption of PrS10 molecules oriented normal to the substrate. Taking the XRD results together with a steric effect into consideration, we can conclude that the normally oriented PrS10 molecules are mainly in interdigitated, multilayered alignment. The origins of the other components, bands A, C, and D in Figure 9 parts a and b, whose incident angle-dependence was much weaker than that of band B, are still unidentified. Each
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Figure 10. Intensity of each spectrally resolved peak of the absorption spectra for PrS10 thin films deposited at 24 (a) and 80 °C (b) as a function of the incident angle. Each broken line is a guide of data points for convenience.
Figure 9. Peak-resolved analysis for the absorption spectra of PrS10 thin films with the incident angle of 40°. All resolved components are Gaussian functions, whose peak energies and bandwidths are listed in Table 1.
TABLE 1: Parameters for Each Resolved Component Used for the Analysis of the Absorption Spectra of PrS10 Thin Filmsa
peak energy A (]) B (O) C (0) D (4) a
4.35 eV (285 nm) 4.20 eV (295 nm) 4.15 eV (299 nm) 3.91 eV (317 nm)
ratio tilt-angle (40°/0°) (deg) bandwidth (fwhm) 80 °C 24 °C 80 °C 24 °C 0.10 eV 0.09 eV 0.28 eV 0.18 eV
2.5 57 7.8 3.3
12 4.5 1.7
33 7 19 29
15 25 41
See Figures 9 and 10. Symbols used in Figure 10 are also indicated.
component has a certain intensity at the normal incidence, suggesting that they are not attributed to the absorption of the normally oriented molecules. The peak wavelength of sharp band A is shorter than band B by 10 nm, and those of broad bands C and D are longer. In addition, band A had the weakest incident angle-dependence as shown in Table 1 and only appeared when deposited well above the room temperature. This suggests that molecules whose absorption is attributed to band A are closer packed on high-temperature substrates, and as a result, they are tilted from the normal direction. In accordance with the molecular exciton model,19 the amount of the blueshift of side-by-side packed molecular aggregates with respect to the isolated molecule will be determined both by the intermolecular distance and tilt angle of the molecules against the packing direction. Recently, we have found that the
fluorescence spectra and their transient behaviors of oligosilane aggregates in frozen solvents were well described by this model.10 It can also explain the longest peak wavelength of band A when the effect of the closer packing is assumed to overwhelm that of the tilted alignment. Further discussion of each absorption band observed in vacuum-deposited PrS10 thin films with respect to its molecular orientation (or conformation) is beyond the scope of this paper and will be given elsewhere. Finally, we should mention the incident angle-independent absorption spectrum of the film deposited at -150 °C, which is dominated by the broad, structureless band E (Figure 8c). We may explain it if we assume the random molecular orientation, which seems consistent with the XRD profile suggesting the absence of the ordered multilayer structure. 4. Conclusion We investigated the molecular orientations in vacuumdeposited thin films of linear oligosilanes MS10 and PrS10 by X-ray diffractometry and absorption spectroscopy. Two types of highly ordered multilayered structures, one with molecular orientation normal to the substrate and the other with oblique orientation, was found in the MS10 films deposited at room temperatures. However, a large amount of the molecules were found to have nonnormal orientation. This structure is quite different from that obtained for MS12 thin films deposited at room temperature, in which the highly ordered multilayered structure was found and the normal molecular orientation was dominant.13 When deposited onto cold substrates, on the other hand, the molecules were less ordered in terms of the multilayered structure but were oriented precisely normal to the substrate. Such dependence of the molecular orientation on the molecular length has never been found for other conjugated oligomers. This is probably explained by considering the interaction between the quartz substrate and oligosilane molecules at each temperature. For PrS10 thin films deposited at and above room temperatures, the interdigitated structure with normal molecular orientation was confirmed by resolving the absorption spectra into several bands. When deposited on the cold substrate, on the other hand, neither the multilayered structure nor the molecular orientation normal to the substrate was found. The remarkable difference between the molecular orientations of MS10 and PrS10 in thin films should be explained in terms of the different size of the side and end groups. Longer n-alkyl
Vacuum-Deposited Peralkyloligosilane Thin Films end groups may induce the interdigitated structure with its molecular orientation normal to the substrate. Longer side groups, on the other hand, may prevent the molecules from organizing large domains with multilayered structures. Acknowledgment. We thank Drs. H. Okumoto, M. Shimomura, A. Kaito, N. Tanigaki, J. P. Ni, and Y. Yoshida of NIMC for their helpful suggestions. We also thank Dr. H. Takiguchi (Seiko Epson Inc.) for his kind help in AFM observations. References and Notes (1) Miller, R. D.; Michl, J. Chem. ReV. 1989, 89, 1359 and references therein. (2) Kumada, M.; Tamao, K. AdV. Organomet. Chem. 1968, 6, 19. (3) Yatabe, T.; Kanaiwa, T.; Sakurai, H.; Okumoto, H.; Kaito, A.; Tanabe, Y. Chem. Lett. 1998, 345. (4) Yatabe, T.; Minami, N.; Okumoto, H.; Ueno, K. Chem. Lett. 2000, 742.
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4117 (5) Obata, K.; Kabuto, C.; Kira, M. J. Am. Chem. Soc. 1997, 119, 11345. (6) Obata, K.; Kira, M. Organometallics 1999, 18, 2216. (7) Sun, Y. P.; Hamada, Y.; Huang, L.-M.; Maxka, J.; Hsiao, J. S.; West, R.; Michl, J. J. Am. Chem. Soc. 1992, 114, 6301. (8) Kishida, H.; Tachibana, H.; Sakurai, K.; Matsumoto, M.; Abe, S.; Tokura, Y. J. Phys. Soc. Jpn. 1996, 65, 1578. (9) Obata, K.; Kira, M. Chem. Commun. 1998, 12, 1309. (10) Ichino, Y.; Minami, N.; Yatabe, T. J. Lumin. 2000, 87-89, 727. (11) Maxka, J.; Huang, L.-M.; West, R. Organometallics 1991, 10, 656. (12) Yatabe, T.; Kaito, A.; Tanabe, Y. Chem. Lett. 1997, 799. (13) Yatabe, T.; Shimomura, M.; Kaito, A. Chem. Lett. 1996, 551. (14) Sasaki, D.; Tada, H.; Ishida, K.; Horiuchi, T.; Matsushige, K.; Endo, T.; Kako, M.; Nakadaira, Y. Jpn. J. Appl. Phys. 1998, 37, L953. (15) Lee, S. A.; Yoshida, Y.; Fukuyama, M.; Hotta, S. Synth. Met. 1999, 106, 39. (16) Athouel, L.; Froyer, G.; Riou, M. T.; Schott, M. Thin Solid Films 1996, 274, 35. (17) Ichino, Y.; Minami, N.; J. Phys. Chem. B 2001, 105, 4118. (18) Ichino, Y.; Yatabe, T.; Minami, N.; Jpn. J. Appl. Phys. 2000, 39, L1002. (19) Kasha, M. Pur. Appl. Chem. 1965, 11, 371.
