Coexistence of Different Conformer Forms in Nanosize Poly(di-n

Mar 3, 2007 - Photoluminescence and photoluminescence excitation spectra of poly(di-n-hexylsilane) (PDHS) embedded in nanoporous silica material SBA-1...
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J. Phys. Chem. C 2007, 111, 4717-4721

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Coexistence of Different Conformer Forms in Nanosize Poly(di-n-hexylsilane) A. Dementjev,† V. Gulbinas,*,† L. Valkunas,† N. Ostapenko,‡ S. Suto,§ and A. Watanabe| Institute of Physics, SaVanoriu AVenue 231, 02300 Vilnius, Lithuania, Institute of Physics, National Academy of Science Ukraine, Nauky AVenue 46, 03028 KyiV, Ukraine, Department of Physics, Graduate School of Science, Tohoku UniVersity, Sendai 980-8578, Japan, and Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Sendai 980-8577, Japan ReceiVed: September 19, 2006; In Final Form: January 19, 2007

Photoluminescence and photoluminescence excitation spectra of poly(di-n-hexylsilane) (PDHS) embedded in nanoporous silica material SBA-15 with a pore diameter of 10 nm were analyzed at different temperatures ranging from 5 to 320 K. The optical spectra of this composite are structured containing three bands, attributed to different structural forms of the polymer coexisting in a restricted pore volume: the polymer chains in the trans and gauche conformations, and their aggregates. Different structural forms are spatially separated and weakly coupled; therefore, several bands are distinguished in the photoluminescence spectra at low temperatures when several forms coexist. For the first time two termochromic transitions of nanosize PDHS were observed at about 265 and 320 K for a single polymer chain and for aggregates, respectively.

Introduction Polysilanes belong to silicon-organic polymers, which consist of σ-conjugated Si backbone and of organic side groups. They show a remarkable photoluminescence (PL) in the UV region.1 Apart from high PL quantum efficiency, the high mobility of holes is also noteworthy for polysilanes;2 thus, these polymers are promising in construction of emitting or transport layers for electroluminescence devices.3,4 Evidently, the structural organization of these polymers in the solid state predetermines the functioning of polymer-based devices; therefore, understanding of the optical and electric features depending on the polymer structural arrangement is an important issue. Current trends in designing nanostructure materials for various applicative purposes can be achieved by means of different technological approaches. Embedding of polysilanes in nanoporous materials such as MCM-41 and SBA-15 is an effective way of producing and controlling of nanostructure composites. Poly(di-n-hexylsilane) (PDHS), which is a thermochromic polymer, is one of the most widely studied silicon-organic polymers.1 Its electronic transitions are variable depending on the changes of temperature-dependent conformations both of the main chain and of the side chains.1,5 The X-ray diffraction analysis of the PDHS films evidence a planar all-trans conformation of the Si backbone, and a slightly tilted orientation of the hexyl side chains with respect to the polymer chain at temperatures below 300 K.5 Transition from the trans conformation to the disordered gauche conformation occurs above 315 K as a result of disordering of the hexyl side chains.5 Similar conformational changes with corresponding modifications of electronic transitions are also expected for the confined polymers. However, transition energies and, thus, characteristics of conformers may differ significantly in this case. As it was recently demonstrated, * To whom correspondence may be addressed. E-mail: [email protected]. † Institute of Physics. ‡ National Academy of Science Ukraine. § Graduate School of Science, Tohoku University. | Institute of Multidisciplinary Research for Advanced Materials, Tohoku University.

only a single macromolecule chain may fit into pores of small diameters (2.8 nm) and only a small part of the chain undergoes significant local conformational changes due to polymer interaction with SiOH groups of the pore surface.6-8 By increasing the pore diameter, more polymer chains may fit into the pore. As a result of interaction of different polymer chains, the spectral changes related to different conformations, packaging, orientation, and interchain coupling are expected. We have studied how the confined geometry of the PDHS polymer located in silica pores of 10 nm in diameter affects the conformational structure of the polymer chains. Photoluminescence (PL) and photoluminescence excitation spectra (PLE) in the temperature range of 5-320 K are presented. It is demonstrated that due to both polymer-polymer and polymersurface interactions the optical spectra of such a composite significantly differ from the spectra of composites with only a single chain in a pore.6,7 We show that the PL properties of the composite with the pore diameter of 10 nm are determined by coexistence of weakly coupled structural forms of the polymer with spectral properties of the isolated gauche and trans conformations and of the aggregated states. We discovered two thermochromic transitions between these forms in the same sample. For the first time we clearly demonstrate the thermochromic transition between trans and gauche conformations of the isolated polymer chains embedded in nanoporous material and possibility of coexistence of the three polymer forms in the same pore. It should be mentioned that the origin of the thermochromic transition was a subject of extensive discussions even for the PDHS in solution.1 Experimental Methods Details of preparation of the nanoporous silica material SBA15 with the pore diameter of 10 nm have already been described.9 As an example, a certain quantity of triblock copolymer Pluronic 123 (poly(ethyelene oxide-block-propylene oxide-block-ethylene oxide) was dissolved in deionized water by adding HCl. The solution was stirred until the polymer was fully dissolved. Then the silica source (TEOS) was added, and

10.1021/jp0661239 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/03/2007

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Dementjev et al.

Figure 1. PL and PLE spectra of PDHS/SBA-15 composite measured at T ) 5 K after rapid cooling. The PL spectrum was measured under excitation at λe ) 313 nm; the PLE detection wavelengths are indicated by arrows. The inset shows the structural formula of PDHS.

solution was stirred at 303 K for 8 h. Next, the temperature was increased to 333 K and the solution stirred again for 24 h. Afterward the obtained mixture was filled in a hydrothermal bomb, where it was kept for 24 h at 393 K. Finally the samples were cooled to ambient temperature and filtered to receive a solid powder. All reagents used for synthesis were chosen to be HPLC grade. The prepared samples were calcined in an oven at 373 K and, subsequently, in the dry air at 813 K for 23 h to remove the surfactant. The removal of the template was controlled by the FTIR spectroscopy (Bruker IFS 66). The structure of the calcined samples was characterized by X-ray diffraction (XRD) and by the nitrogen gas absorption. The size of the pores was calculated using the BJH method applied to nitrogen desorption isotherms. The measurements demonstrated quite narrow distribution of the pore diameter sizes. As defined from the transmission electron microscopy (TEM) investigations, SBA-15 consists of highly regular uniform sized pores with equal interpore distances. To incorporate the PDHS polymer (Mw ) 53 600) into the pores, the prepared silica matrixes were immersed in the 1 wt % solution of the polymer in toluene and slowly stirred in the dark at 293 K for several hours and then kept in the dark until the solvent evaporated. Then the composite was twice washed in the dark for approximately 2 h by stirring in the fresh toluene to remove the polymer from the exterior surface. To define the location of the polymer in the pores, the X-ray diffraction method was used, which allowed us to monitor the characteristic silica framework structure. The PL and PLE spectra were measured in slow and fast cooling regimes by using two different setups. The Perkin-Elmer LS 50 B spectrometer and an optical closed-cycle helium cryostat were used for studies in the slow cooling regime in the rage of 10-320 K. The samples were cooled with the rate less than 0.5 K/min, and furthermore measurements were performed after 20 min equilibration at any given temperature. The DFS-12 spectrometer and the helium cryostat were used for sample studies at 5 K in the fast-cooling regime. The cooling rate was ∼100 K/min. The PL and PLE spectra were obtained by a monochromator using a Xenon lamp as a source for excitation. Results The PL and PLE spectra of the rapidly cooled PDHS/SBA15 composite measured at T ) 5 K under excitation at λe ) 313 nm are presented in Figure 1. The structural formula of PDHS is shown in the insert of Figure 1. The PL spectrum consists of three bands at 337, 355, and 369 nm defined as PL1,

Figure 2. PL and PLE spectra of the PDHS/SBA-15 composite measured at different temperatures after slow cooling. The PL spectra were measured under excitation at λe ) 313 nm; the PLE detection wavelengths are indicated by arrows. The fluorescence spectrum at 10 K is also shown by a dashed line in a.

PL2, and PL3, respectively. The PLE spectra demonstrate distinct differences by choosing different PL bands for detection. Namely, the PLE bands at 318 (PLE1), 345 (PLE2), and 357 nm (PLE3) correspond to the PL1, PL2, and PL3 bands of photoluminescence, respectively. It is noteworthy that all three PL bands appear only by using the rapid sample cooling. If the sample is cooled slowly from room temperature to 10 K, the PL spectrum contains only two bands at 355 (PL2) and 369 nm (PL3) (see Figure 2). However, it is not the case for the PLE spectra, where all three characteristic bands are distinguished by detecting luminescence within the PL3 band. No qualitative changes in both PL and PLE spectra are observed in the temperature range from 10 to 200 K by using a slow-cooling procedure. Spectral changes are demonstrated in Figure 2a. The PL2 and PL3 bands are observable at 200 K with a slight red shift of their maxima in comparison with those measured at 10 K, while the PL1 band, as it was already outlined, is absent. The PLE spectrum detected at the PL2 band (at 360 nm) clearly reveals a strong PLE2 band and a weak PLE1 band. All three excitation bands may be distinguished by using the PL3 band (at 375 nm) for detection. The spectra at 290 K are qualitatively different, the PL1 band emerges, and the PL2 band is absent (Figure 2b). The PLE spectrum detected within the PL1 band (345 nm) contains a single PLE1 band, while detection at 375 nm reveals PLE1 and

Different Conformer Forms in Nanosize PDHS

Figure 3. PL spectra of the PDHS/SBA-15 composite in the 220290 K temperature range under excitation at λe ) 313 nm. The inset shows the integrated PL1 band intensity measured by sample cooling and heating.

PLE3 bands. Thus, it can be concluded that polymer forms responsible for the PL2 and PLE2 bands are absent at 290 K. At 310 K the spectra show the same PL and PLE bands, however, with changes in their relative intensities (Figure 2c). The PL3 band is much weaker, and the PLE spectrum detected within the PL3 band clearly indicates the presence of the PLE3 band only. Changes in the PL spectrum in the range of 200-290 K, where the most dramatic modification of the spectra is expected, are shown in Figure 3. Changes in the integrated PL1 band intensity caused by cooling and heating of the sample are demonstrated in the insert of Figure 3. By increasing the temperature, the PL2 band is gradually replaced by the PL1 band accompanied by the decrease in the intensity of the PL3 band. However, both spectral changes can hardly be related to the same process, since the decrease in the PL3 band intensity persists by increasing the temperature above 270 K when the PL2 band is already absent (see Figure 3). It is noteworthy that the PL3 band disappears completely by increasing the temperature above 310 K. Moreover, heating above 310 K leads to irreversible changes in the PL spectrumsthe PL3 band does not appear after sequential cooling of the sample. On the contrary, temperature-induced spectral modifications below 310 K are completely reversible, the samples may be cooled and heated several times, and the spectra remain almost the same at any given temperature. However, spectral changes obtained by heating and cooling are slightly shifted on the temperature scale. It should also be noted that relative intensities of different bands slightly differ depending on the sample positioning in the fluorimeter and on the cooling-heating history. Discussion We naturally relate three bands of the PL and PLE spectra coupled in pairs to three different polymer forms/conformations present inside the pore of 10 nm in diameter. Similar coexistence of different forms of PPV and alcane polymers in similar pores has been recently reported.10,11 The difference in the PL and PLE spectra obtained at different excitation or detection wavelengths enables us to decompose the PLE and PL spectra into three spectral components corresponding to three polymer forms. This decomposition was performed by subtracting spectra measured at different excitation or detection wavelengths one from another after their normalization giving equal intensity for one of the components. Such subtraction procedure eliminates one of the components from the spectra, but it suffers from some uncertainty due to uncertain band normalization. To minimize possible errors, the decomposition procedure was performed for

J. Phys. Chem. C, Vol. 111, No. 12, 2007 4719

Figure 4. PL and PLE spectra of different conformational forms of PDHS in SBA-15 obtained by decomposition of PL and PLE spectra. Aggregate and gauche conformer spectra correspond to 280 K temperature, while trans conformer spectra correspond to 200 K.

Figure 5. Temperature dependence of the maxima positions of the PL and PLE bands of different conformational forms of PDHS in SBA15.

several pairs of spectra measured at different probe wavelengths and thus having different relative intensities of spectral components. The spectra obtained from such global analysis corresponding to all three forms/conformations are presented in Figure 4, and the temperature dependence of the maxima of the attributed bands is shown in Figure 5. Polymorphism of PDHS was also disclosed in bulk polymer films1,12 and was already reported for PDHS/SBA-15 composites.13 Absorption bands with maxima at 316 and 364 nm and the PL bands at 343 and 375 nm, respectively, were identified in the PDHS films.1,12 The short-wavelength absorption band at 316 nm and the corresponding PL band at 343 nm were assigned to the gauche conformation of the Si backbone. At room temperature the shortest wavelength absorption and fluorescence bands in PDHS/SBA-15 composite appear in very close spectral positions and, thus, should be also assigned to the gauche conformation. These species show the broadest absorption and PL bands and the largest Stokes shift and also experience the strongest spectral shifts with temperature (Figure 4). All these features are in line with attribution to the disordered structure of this phase. The longer wavelength absorption and corresponding PL components at 364 and 375 nm, respectively, observed in films were assigned to the crystalline phase formed by polymer chains in the trans Si backbone conformation.1,12 The longest wavelength spectral components observed in PDHS/SBA-15 composite are in almost the same positions. Since the real crystalline form can hardly exist in small pores, we attribute this spectral component to the aggregate states. Interchain polymer aggregates with similar PL spectrum were observed at 10 K in

4720 J. Phys. Chem. C, Vol. 111, No. 12, 2007 PDHS solution in toluene at high polymer concentrations.13 The possibility of the PDHS aggregation in a solid solution was also discussed.14 The presence of the third pair of bands situated between the gauche band and the aggregate-phase band makes the main difference between the spectra of the PDHS film and the PDHS/ SBA-15 composite. It has been recently suggested to assign the absorption and PL bands observed in the PDHS/SBA-15 composites at about 345 and 355 nm, respectively, to the trans conformation of individual chains.13 These bands appear at shorter wavelengths than the corresponding bands of the crystalline trans phase or aggregates. Evidently the interchain exciton interaction results in shifting of the absorption and PL bands of the aggregates and crystallites to the long-wavelength side. This assignment is in agreement with observation of these bands in the PDHS/MCM-41 composite with the pore diameter of 2.8 nm, where only one polymer chain may be inserted and the aggregate state cannot be composed.6 Thus, the analysis of the absorption and fluorescence properties of the PDHS/SBA-15 composites allows us to relate three different spectral forms to three different polymer conformations coexisting inside a pore. We discuss the possible arrangement of polymer chains of different forms inside the pore. Pores of CBA-15 are linear and have hexagonal symmetry thus, determining orientation and geometry of polymer chains situated inside. According to the size of the polymer chains, about six chains may fit into the pore of 10 nm in diameter. Polymer chains near the pore boundaries and in the pore center evidently should have different properties. It has been shown that the polymer interaction with the quartz surface leads to formation of the gauche conformation.12 Moreover, only the gauche conformation was recorded spectroscopically at room temperature in films with thickness below about 10 nm. Therefore, it might be assumed that macromolecules situated very close to the pore boundaries are in a gauche conformation due to interaction with the pore surface. Polymer chains inside the nanopore are likely to be oriented along the pore.15,16 For this orientation, the conformation of the polymer chains should be more ordered than in thin films; therefore, chains in the center of the pore evidently may be of the trans conformation. However, they may also be in the gauche conformation, depending on temperature and the cooling rate. At low temperature, and particularly after slow cooling, the more ordered trans conformation should be formed. Thus, the confinement of polymers in pores evidently leads to two opposite consequences: polymer chains being in contact with the pore surface exist in less ordered gauche conformation, while those being in the center are better ordered and may arrange into the trans conformation. Some of the macromolecules in the trans conformation situated very close to each other may compose the aggregated states due to the intermolecular interaction. Aggregated states can hardly be composed of the polymer chains in the gauche conformation because of the restraint of their close packing in the presence of the disordered side chains. Analysis of the fluorescence properties and particularly of the fluorescence quenching provides us with useful information about the mutual arrangement of different conformational forms. According to the spectral positions of the PL and PLE bands, the excitation energy should be transferred between different polymer forms and, therefore, the aggregated states should quench the PL of the gauche and trans forms. Chains of the trans form should evidently quench the PL of the gauche form. Due to the short interchain distances the energy transfer between

Dementjev et al. chains confined inside the pore of 10 nm in diameter is expected to proceed on a picosecond time scale; therefore, the PL bands of higher energies should be completely quenched if the polymer chain with conformation characterized by the lower energy PL band is present in the vicinity. The fluorescence may also be quenched because of the energy transfer along the pore even when there is no quencher nearby. The quenching efficiency in this case should depend on the length of the pore region free of the quenching chains. If the pore region containing no quenching chains is longer than the energy-transfer distance, the higher energy fluorescence should be not quenched. Investigation of the energy transfer in MEH-PPV embedded into the nanoporous silica glass channels implies the intrachain energy-transfer distance to be of the order of several nanometers.17 The interchain energy transfer should be more efficient.18,19 The PLE spectra contain all three bands when the aggregate (PL3) band is chosen for the fluorescence detection indicating that the excitation is partly transferred to the aggregate state whatever conformation is excited. Similarly the PLE1 band corresponding to the gauche form is present in the PLE spectrum when the PL2 band corresponding to the trans form is chosen for the fluorescence detection. This clearly indicates that the partial energy transfer between different spectral forms indeed is taking place. However, the presence of several PL bands in the spectra of the PDHS/SBA-15 composite also allows us to conclude that the excitation energy transfer is not always effective. At low temperature the gauche phase fluorescence is completely quenched in slowly cooled samples. We observe the fluorescence of individual and aggregated trans phase chains. It indicates that at least one chain in the pore should be in the trans conformation. The spectrum of the trans form is gradually replaced by the gauche form spectrum by increasing the temperature and the opposite transition takes place by cooling the sample. The spectral changes related to the trans-gauche rearrangement were observed in the 200-290 K temperature range (see Figure 3). PDHS is the thermochromic polymer; therefore, the trans phase should transform into the more disordered gauche phase conformation by increasing the temperature. By cooling the sample the opposite transition should take place. Cooling and heating yield a hysteresis, which demonstrates the pathway of the trans-gauche thermochromic transition (see the insert in Figure 3). The presence of such transition confirms our assignment of the related spectral forms to the gauche and trans conformations of the individual polymer chains. The trans-gauche thermochromic phase transition was observed in solutions and in films of the PDHS polymer. The thermochromic transition for PDHS in solutions takes place at 223 K 1,20 and is basically independent of the solvent.21 In the polymer film this transition appears at 315 K.5 The thermochromic transition for the single polymer chain in the PDHS/ SBA-15 composite is observed at about 265 K, which is essentially higher than the transition temperature in solution. Since solutions always contain aggregates in addition to noninteracting chains, the nature of this thermochromic transition was the subject of discussions.1 By fast cooling the gauche polymer chains evidently have not enough time to rearrange into the trans configuration; therefore, the trans chains are formed only in some fraction of pores or their sections. Other sections, where the trans chains are absent, show the fluorescence corresponding to the gauche form. In the case of the PDHS/SBA-15 composite the second thermochromic transition related to disassembling of the ag-

Different Conformer Forms in Nanosize PDHS

Figure 6. Arrangement of different conformational forms of PDHS in SBA-15 at different temperatures. Ellipses indicate pore regions responsible for different PL bands.

gregated states is also determined. This transition appears at about 320 K, i.e., at significantly higher temperature than the trans-gauche transition of individual polymer chains. The higher aggregate disassembling temperature seems to be contradicting the assignment of aggregate states to the trans conformation. The contradiction may be resolved by assuming that the aggregation stabilizes the trans conformation. The aggregated trans conformation is the lowest energy state; therefore, naturally it should be the most stable. The aggregate states are evidently formed already during the sample preparation and cannot be composed again once they were disassembled at high temperature. It is still difficult to conclude if the reversible changes of the aggregated state concentration take place under temperature variations below the disassembling temperature. The temperature dependence of the aggregate PL intensity presented in Figure 3 may be considered as evidence indicating the changes of the aggregates concentration during the trans-gauche transition. However, it may also be caused by variation of the ratio of the nonagregated trans to gauche concentrations because of different efficiency of the excitation energy transfer from the gauche and trans conformations to the aggregated state. The aggregate concentration is evidently never high, because the aggregate states never quench fluorescence of the individual gauche or trans phase chains completely. Analysis of the PL and PLE spectra and their temperature dependences by taking into account the above presented fluorescence quenching scenario leads to the polymer arrangement presented in Figure 6. The single PL band of the gauche form observed at above 320 K is a clear indication that only the gauche form is present at high temperature. At temperatures between 263 and 320 K the trans form aggregates are present (given the composite was never heated above 320 K); however, there are pore regions longer than about 10 nm free of aggregate states, and, therefore, both PL bands of the aggregate and gauche forms are present. Below 265 K most of the polymer chains at the center of the pore are in the trans conformation, which completely quenches the fluorescence of the gauche form. Conclusions In summary, the photoluminescence and photoluminescence excitation spectra of poly(di-n-hexylsilane) embedded in nanoporous silica material SBA-15 with the pore diameter of 10 nm are sensitive to the temperature changes in a wide temperature range from 5 to 320 K and to the cooling rate of the samples. Depending on the temperature and on the cooling

J. Phys. Chem. C, Vol. 111, No. 12, 2007 4721 rate of the sample several bands in the photoluminescence spectrum are distinguished. Their presence is a result of coexistence of several polymer forms inside a single pore and with the relative intensity modulation by the energy transfer between them. Since some forms may be absent in some pore regions, the energy transfer and concomitant photoluminescence quenching are only a partial effect modulating the relative intensities of the corresponding bands. Three bands observed at 5 K after fast sample cooling are attributed to the disordered gauche and the ordered trans conformations and to the aggregated states, while only bands related to the trans conformation and to the aggregated state are distinguished in the slowly cooled samples below 200 K. Two thermochromic transitions at about 265 and 320 K are distinguished in the slowly heated samples. The first one is related to the trans-gauche conformational changes of the single polymer chain. This transition is completely reversible and appears at temperature, which is different from the temperature of an analogous transition observed in the poly(di-n-hexylsilane) films and solutions. The second transition is related to formation and breakup of the aggregated states. Acknowledgment. This work was supported in part by Ministry of Education and Science of Ukraine (Project No. M/128-2005) and by the Ministry of Education and Science of Lithuania. References and Notes (1) Miller, R. D.; Michl, J. Chem. ReV. 1989, 89, 1359. (2) van Laan, G. P.; de Haas, M.; Hummel, A.; Frey, H.; Moller, M. J. Phys. Chem. 1996, 100, 5470. (3) Suzuki, H.; Meyer, H.; Hoshino, S.; Haarer, D. J. Appl. Phys. 1995, 78, 2684. (4) Wuchse, M.; Tasch, S.; Leising, G.; Lunzer, F.; Marschner, G. Croat. Chim. Acta 2001, 74, 867. (5) Kuzmany, H.; Rabolt, J. F.; Farmer, B. L.; Miller, R. D. J. Chem. Phys. 1986, 85, 7413. (6) Ostapenko, N.; Telbiz, G.; Ilyin, V.; Suto, S.; Watanabe, A. Chem. Phys. Lett. 2004, 383, 456. (7) Ostapenko, N.; Kotova, N.; Telbiz, G.; Suto, S.; Watanabe, A. Fiz. Nizk. Temp. 2004, 30, 658. (8) Ostapenko, N.; Kotova, N.; Lukashenko, V.; Telbiz, G.; Gerda, V.; Suto, S.; Watanabe, A. J. Lumin. 2005, 112, 381. (9) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (10) Cadby, A. J.; Tolbert, S. H. J. Phys. Chem. B 2005, 109, 17879. (11) Okazaki, M.; Torijama, K.; Anandan, S. Chem. Phys. Lett. 2005, 401, 363. (12) Despotopoulou, M. M.; Miller, R. D.; Rabolt, J. F.; Frank, C. W. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 2335. (13) Ostapenko, N.; Kozlova, N.; Suto, S.; Watanabe, A. Fiz. Nizk. Temp. 2006, 32, 1363. (14) Thorne, J. R. C.; Hochstrasser, R. M.; Zeigler, J. M. J. Phys. Chem. 1988, 92, 4275. (15) Israelashvili, J. N.; Kott, S. J. J. Chem. Phys. 1988, 88, 7162. (16) Tolbert, S. H.; Wu, J.; Cross, A. F.; Nguyen, T. Q.; Schwarz, B. J. Microporous Mesoporous Mater. 2004, 44-45, 445. (17) Schwartz, B. J.; Nguyen, T. Q.; Wu, J.; Tolbert, S. H. Synth. Met. 2001, 116, 35. (18) Nguyen, T. Q.; Wu, J. J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H. Science 2000, 288, 652. (19) Beljonne, D.; Pourtois, G.; Silva, C.; Hennebicq, E.; Herz, L. M.; Friend, R. H.; Scholes, G. D.; Setayesh, S.; Mu¨llen, K.; Bre´das, J. L. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 10982. (20) Bukalov, S. S.; Leites, L. A.; West, R. Macromolecules 2001, 34, 6003. (21) Harrah, L. A.; Zeigler, J. M. J. Polym. Sci., Polym. Lett. Ed. 1985, 23, 209.