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Solid-State Polymerization Behaviors of Crystalline Diacetylene Monolayers on Hydrophilic Surfaces Keisuke Kuriyama, Hirotsugu Kikuchi, and Tisato Kajiyama* Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812, Japan Received September 29, 1995. In Final Form: January 19, 1996X Solid-state polymerization behaviors of crystalline lithium 10,12-heptacosadiynoate monolayers on the hydrophilic SiO substrate and on the water surface were investigated on the basis of the ultraviolet (UV) light irradiation time dependence of the UV-visible absorption spectrum. In the case of the crystalline monolayer on the SiO substrate, the polydiacetylene (PDA) monolayer in a blue form was formed upon UV photoirradiation. On the other hand, in the case of the crystalline monolayer on the water surface, the PDA monolayer in a red form was formed by the polymerization reaction. The PDA blue and red forms exhibit absorption peaks at 640 and 540 nm, respectively. The two-dimensional (2D) molecular packings of the PDA monolayers in both forms were clarified by the electron diffraction (ED) analysis for the first time. Each form had the distinct 2D molecular packing relating to its electronic structure. The difference in the solid-state polymerization behaviors for the crystalline lithium 10,12-heptacosadiynoate monolayers on the different substrate surfaces was discussed in terms of the thermal mobility of molecules in the monolayer.
Introduction Polydiacetylenes (PDAs) are promising candidates for third-order nonlinear optical materials, because of their potentially large third-order nonlinear optical susceptibility, χ(3).1 For practical applications such as nonlinear guided wave devices, however, the effective value of χ(3) for PDAs is not sufficient yet at the present time. It is apparent from theoretical calculations that the magnitude of χ(3) for PDAs strongly depends on the effective delocalization length of the π-electron along a conjugated polymer main chain.2 Then, in order to increase the magnitude of χ(3), it is required to form PDA with both a longer delocalization length of the π-electron and a sharp distribution of the delocalization length of the π-electron. PDA can be obtained only by the solid-state (topochemical) photopolymerization of diacetylene monomeric single crystals.3 This also indicates that the regular arrangement of diacetylene molecules is essentially required for the longer delocalization length of the π-electron, as shown in Figure 1. It is, therefore, of great importance to investigate the topochemical photopolymerization behavior of DAs for preparation of PDAs with a longer delocalization length of the π-electron and a sharp distribution of the delocalization length of the π-electron. The polymerization behaviors of diacetylene derivatives in the bulk (three-dimensional) crystalline solid have been wellestablished by Enkelman et al.4 However, the polymerization behavior of diacetylene derivatives in the monolayer state has not been clarified yet, especially with respect to the molecular packing in the unit cell or the molecular aggregation state in the crystal phase. Diacetylene derivatives in the monolayer state are of potential interest for the ability to form a large twodimensional single crystal with crystallographically superior quality in various crystal systems, which cannot be * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) For a review, see: Chemla, D. S.; Zyss, J. Nonlinear Optical Properties of Organic Molecules and Crystals; Academic: Orlando, FL, 1987. (2) Agrawal, G. P.; Cojan, C.; Flytzanis, C. Phys. Rev. 1978, B17, 776. (3) Wegner, G. Z. Naturforsch 1969, 24b, 824. (4) Enkelman, V. In Polydiacetylenes; Cantow, H.-J., Ed.; Springer-Verlag: New York, 1984.
S0743-7463(95)00810-9 CCC: $12.00
prepared in the bulk state.5-7 Thus, diacetylene derivatives in the monolayer form might be suitable for preparation of the PDA with both a longer delocalization length of the π-electron and a sharp distribution of the delocalization length of the π-electron. We have developed an effective technique to investigate the aggregation structure of the fatty acid monolayer on the water surface on the basis of the electron diffraction (ED) and the areal modulus studies.8 Using this technique, we have investigated the two-dimensional (2D) solid-state polymerization behavior of the monolayer composed of the diacetylene derivative diynoic acid.9,10 The polymerization behavior of the monolayer was strongly dependent on the molecular aggregation state in the monomeric monolayer. Especially, PDA with a relatively longer delocalization length of the π-electron could be obtained only if the monomeric monolayer is in a highly ordered crystalline state. The monolayer in a crystalline state, that is the crystalline monolayer, can be prepared under appropriate conditions on the basis of the structural and thermomechanical investigations.5 The solid-state polymerization of the crystalline monolayer can be carried out on the water surface or on a hydrophilic solid substrate such as the SiO substrate by ultraviolet (UV) light irradiation. However, the difference in the solid-state polymerization behaviors for the crystalline monolayers on the water surface and on the hydrophilic solid surface has not been clarified yet. The molecules on the water surface were supposed to have a larger mobility than those on the solid substrate. It was reported that a transformation of the crystal lattice due to reorientation of the monomeric molecules was induced during the polymerization of diacetylenes.11 Thus, the mobility of the diacetylene molecules in the crystal lattice (5) Kuriyama, K.; Kajiyama, T. Bull. Chem. Soc. Jpn. 1993, 66, 2522. (6) Fukuda, K.; Shibasaki, Y.; Nakahara, H.; Endo, H. Thin Solid Films 1989, 179, 103. (7) Fuchs, H.; Ohst, H.; Prass, W. Adv. Mater. 1991, 3, 10. (8) Kajiyama, T.; Oishi, Y.; Uchida, M.; Tanimoto, Y.; Kozuru, H. Langmuir 1992, 8, 1563. Kajiyama, T.; Oishi, Y.; Uchida, M.; Morotomi, N.; Ishikawa, J.; Tanimoto, Y. Bull. Chem. Soc. Jpn. 1992, 65, 864. (9) Kuriyama, K.; Kikuchi, H.; Kajiyama, T. Trans. Mater. Res. Soc. Jpn. 1994, 15A, 571. (10) Kuriyama, K.; Kikuchi, H.; Oishi, Y.; Kajiyama, T. Langmuir 1995, 11, 3536. (11) Liao, J.; Martin, D. C. Science 1993, 260, 1489.
© 1996 American Chemical Society
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Figure 1. Scheme for solid-state polymerization of diacetylene units.
might be an important factor with respect to the photopolymerization reactivity and the extension of the conjugated main chain. In this paper, a crystalline lithium 10,12-heptacosadiynoate monolayer was used as a monomer of PDA. The solid-state polymerization behaviors of the crystalline lithium 10,12-heptacosadiynoate monolayers on the different hydrophilic surfaces, i.e. the water surface and the surface of the SiO solid substrate, were investigated by UV-visible absorption spectra, infrared absorption spectra measurements, and ED analysis. The SiO solid substrate has a hydrophilic surface similar to the water surface with respect to the magnitude of interfacial free energy between the hydrophilic group of the diynoic acid molecule and the surface. Furthermore, in order to examine the difference in the degree of mobility of monomeric diacetylene molecules in the monolayer, the melting temperatures in both cases of the monolayers on the water surface and on the hydrophilic SiO substrate were also evaluated by the ED analysis. Experimental Section Monolayer Preparation on the Water Surface. 10,12Heptacosadiynoic acid with chromatographic reference quality was used without further purification. Benzene with spectroscopic quality was used as a spreading solvent. A benzene solution of 10,12-heptacosadiynoic acid was prepared with a concentration of 2.0 × 10-3 mol‚L-1.12 The subphase water was purified by a Milli-QII system (Millipore Co., Ltd.). The monolayer was prepared by spreading the benzene solution of 10,12-heptacosadiynoic acid on the water subphase containing 2.0 × 10-3 mol‚L-1 of LiOH.13,14 Polymerization of the Monolayer on the SiO Substrate. A commercially available Langmuir trough (USI System Co., Ltd.) was used for preparation of the monolayer. The dimensions of the trough were 404 mm in length, 150 mm in width, and 5 mm in depth. The crystalline monolayer of lithium 10,12heptacosadiynoate was prepared at the subphase temperature (Tsp) of 293 K.5 The crystalline monomeric monolayer was transferred onto a hydrophilic SiO substrate by the upward drawing method at the surface pressure of 20 mN‚m-1. At this surface pressure, the lithium 10,12-heptacosadiynoate monolayer was confirmed to be morphologically homogeneous on the basis of the bright field electron microscopic observation.5 The hydrophilic SiO substrate was prepared by vapor deposition of SiO onto a Formvar-covered electron microscope grid (200-mesh) for ED study. The static water contact angle was less than 30°. The hydrophilic SiO substrate is suitable for the electron microscopic structural investigation of the lithium 10,12-heptacosadiynoate monolayer on the water surface, since the crystal system of the monolayer on the water surface is stably maintained on the hydrophilic SiO substrate upon transferring the monolayer by the upward drawing method.8 Also, the surface of the hydrophilic SiO substrate was confirmed to be smooth and amorphous, on the basis of morphological and ED studies, respectively. Then, the hydrophilic SiO substrate was used to transfer the monolayer by the upward drawing method. Polymerization of the crystalline monolayer on the SiO substrate was carried out by UV light irradiation (UL1-5EB-6A, (12) Tieke, B.; Weiss, K. J. Colloid Interface Sci. 1984, 101, 129. (13) Miyano, K.; Mori, A. Thin Solid Films 1989, 168, 141. Miyano, K.; Mori, A. Jpn. J. Appl. Phys. 1989, 28, 252. (14) Day, D.; Lando, J. B. Macromolecules 1980, 13, 1478, 1483. Day, D.; Ringsdorf, H. J. Polym. Sci., Polym. Lett. Ed. 1978, 16, 205.
50 W, Ushio Co., Ltd.) at a distance of 28 cm in a N2 atmosphere. The temperature during UV light irradiation was adjusted to Tsp. The UV-visible absorption spectrum of the monolayer was recorded using a Shimadzu MPS-2000. Polymerization of the Monolayer on the Water Surface. The crystalline monolayer of lithium 10,12-heptacosadiynoate was prepared at Tsp ) 293 K. A small trough was designed so that the trough can be enclosed in the glovebag and purged with N2 gas before and during polymerization of the spread monolayer. The dimensions of the trough were 152 mm in length, 62 mm in width, and 2 mm in depth. Polymerization of the crystalline monolayer on the water surface was carried out by UV light irradiation at a distance of 28 cm. The monolayer was maintained at the constant surface pressure of 20 mN‚m-1 during polymerization. The polymerized monolayer on the water surface was easily visible to the naked eye. The color was red. For the electron diffraction study, the polymerized monolayer was transferred onto a Formvar-covered glass substrate with an electron microscope grid (200-mesh) by the horizontal lifting method. Since the observed color of the polymerized monolayer on the water surface did not change after the process of transferring it to the substrate surface and since the polymerized monolayer has a rigid structure, the crystal system of the monolayer on the water surface is thought to be stably maintained on the substrate. The monolayer on that substrate was also used for obtaining the UV-visible absorption spectrum. Infrared Absorption Spectrum Measurements of the Polymerized Monolayer. Fourier transform infrared external reflection spectra (FT-IR/ERS) of the polymerized monolayers were recorded on a Nicolet Model 510 FT-IR spectrophotometer equipped with a liquid-nitrogen-cooled HgCdTe MCT detector. The ERS technique enables us to measure the infrared spectrum of the polymerized monolayer on the substrate prepared for obtaining the UV-visible absorption spectrum.15,16 The FT-IR/ ERS were measured at the angle of incidence of about 60° using a Specac 19650 monolayer/grazing angle accessory attached to the spectrophotometer. Three-thousand interferograms were coadded, apodized with the Happ-Genzel function, and Fouriertransformed with one level of zero filling to yield spectra with a resolution of 2 cm-1. Electron Diffraction Analysis of the Monolayer. ED patterns of the monolayer before and after polymerization were taken with a Hitachi H-7000 electron microscope, which was operated at an acceleration voltage of 75 kV and a beam current of 0.5 µA. The electron beam was 10 µm in diameter. Evaluation of the Crystallographic Quality of the Polymerized Monolayer. The crystallographic quality of the polymerized monolayer was evaluated in terms of the magnitude of the crystallographic lattice distortion (Dlat) in the 020 direction, being parallel to the monolayer surface. Dlat corresponds to the root mean square value of the differential rate between the positions of the ideal crystal lattice and the real crystal lattice, in other words, to the degree of deviation from the intrinsic atom or molecular position in a defect-free crystal. This value was quantitatively evaluated by a modified single-line method based on the Fourier analysis of ED peak profiles.17,18 Evaluation of the Melting Temperatures of the Monomeric Monolayers on the Water Surface and on the SiO Substrate. The melting temperature of the monomeric monolayer on the water surface was evaluated on the basis of the Tsp dependence of the ED pattern.5,8 Tsp was varied in the tem(15) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78, 946. (16) Sakai, H.; Umemura, J. Chem. Lett. 1993, 2167. (17) Kajiyama, T.; Umemura, K.; Uchida, M.; Oishi, Y.; Takei, R. Chem. Lett. 1989, 1515. (18) Hofmann, D.; Walenta, E. Polymer 1987, 28, 1271.
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Figure 2. UV light irradiation time dependences of the UV-visible absorption spectrum of the monolayers (a) on the SiO substrate and (b) on the water surface. perature range 283-308 K. The monomeric monolayer was transferred onto the hydrophilic SiO substrate by the upward drawing method at various Tsp’s. The ED patterns were taken at Tsp using a cryotransfer system (Gatan Co., Ltd.). Also, the melting temperature of the monomeric monolayer on the SiO substrate was evaluated from the substrate temperature (Tss) dependence of the ED pattern for the crystalline monomeric monolayer transferred onto the SiO substrate. The Tss of the transferred monolayer was varied in the temperature range 293-318 K by changing the temperature in the sample chamber of the transmission electron microscope.
Results and Discussion Photopolymerization Behaviors of the Monolayers on the SiO Substrate and on the Water Surface. Parts a and b of Figure 2 show the UV light irradiation time dependences of the UV-visible absorption spectrum of the monolayer on the SiO substrate and on the water surface, respectively. Both monomeric monolayers were prepared under the surface pressure of 20 mN‚m-1 at 293 K and confirmed to be in a crystalline state on the basis of the ED pattern of the monolayer.5 The appearance of the spectrum indicates the extension of conjugated chains and the progress of polymerization in the monolayer, since there is no absorption peak of the monomeric monolayer in an observed wavelength range of 400-800 nm an absorption peak at a wavelength of less than 300 nm. In the case of the monolayer irradiated with UV light on the SiO substrate as shown in Figure 2a, the main absorption peak at 640 nm and the weak peak or the broad shoulder at ca. 580 nm were observed. These characteristic absorptions were assigned to the π-π* transition (excitonic absorption) and the phonon sideband of PDA, respectively.19 The absorption intensity increased with the UV irradiation time and almost saturated at the UV irradiation time of around 60 min. The PDA with the absorption peak at 640 nm was designated the PDA blue form.20 Then it is reasonable to conclude that the PDA monolayer in the blue form can be formed by photopolymerization of the crystalline monomeric monolayer on the SiO substrate. On the other hand, in the case of the monolayer irradiated with UV light on the water surface as shown in Figure 2b, the absorption peaks at 540 and 500 nm (19) Tokura, Y.; Oowaki, Y.; Kaneko, Y.; Koda, T.; Mitani, T. J. Phys. Soc. Jpn. 1984, 53, 4054. (20) Chance, R. R.; Banghman, R. H.; Muller, H.; Echhardt, J. G. J. Chem. Phys. 1977, 67, 3616.
were observed. The absorption intensity increased remarkably with the UV irradiation time and saturated at UV irradiation times of around 10 min. The PDA with the absorption peak at 540 nm was designated the red form.20 Thus, it is reasonable to conclude that the PDA monolayer in the red form can be formed by photopolymerization of the crystalline monomeric monolayer on the water surface. These results indicate that the PDAs in the different forms can be prepared by changing the substrate surface on which the crystalline monomeric monolayer undergoes photoirradiation. This finding is noteworthy for elucidation of the 2D polymerization mechanisms of diacetylene derivatives. In order to clarify the photopolymerization process of the monolayers on the different substrate surfaces, characterizations of the resultant PDAs in the blue form and the red one were performed as follows. From the absorption spectra shown in Figure 2, it is apparent that the electronic structure of the PDA red form is different from that of the PDA blue form. The difference in the electronic structure between the red and the blue forms may arise from that of the effective delocalization length of the π-electron. That is, the PDA red form is thought to have a shorter delocalization length of the π-electron than the PDA blue form, as suggested from the wavelengths of the main absorption peaks, 540 and 640 nm, respectively. Parts a and b of Figure 3 show the FT-IR/ERS of the PDA monolayers in the blue form and the red one, respectively. Infrared absorption peaks at 2950, 2930, 2920, 2870, and 2850 cm-1 were assigned to CH3 antisymmetric, CH3 symmetric, CH2 antisymmetric, CH3 symmetric, and CH2 symmetric stretching vibrations of alkyl side groups, respectively.21 Since the spectral features for the red form were considerably broadened in comparison with those of the blue form, the two CH3 bands at 2930 and 2870 cm-1 became hardly discrenible as separate structures. The difference in the spectral features between the forms is similar to that reported by Tokura et al.22,23 They reported that the vibrational bands of PDA became broader with the blue-to-red phase (21) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85. (22) Tokura, Y.; Nishikawa, S.; Koda, T. Solid State Commun. 1986, 59, 393. (23) Tomioka, Y.; Tanaka, N.; Imazeki, S. J. Chem. Phys. 1989, 91, 5694.
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Figure 3. FT-IR/ERS of the PDA monolayer in the blue form and the red form.
transition. The broadening of these vibrational bands implies the conformational disordering of the hydrocarbon side chain due to an increase of the gauche conformation in the all-trans regular conformation.24,25 It is, therefore, reasonable to conclude from Figure 3 that the red form has a relatively disordered structure in the hydrocarbon side chain and that this disordered structure of the side chain is inherent in the red form. On the basis of the valence effective Hamiltonian (VEH) and modified neglect of diatomic overlap (MNDO) calculations, Eckhardt et al. have shown that the conformational disorder of the side chain might induce the distortion along a conjugated main chain of the PDA.26 That is, the distortion induces less planarity with respect to the zigzag PDA main chain, and then, the lower planarity of the main chain disturbs the conjugation of the π-orbitals along the series of alternate single, double, single, and triple bonds in an acetylenic main chain. In other words, the distortion along a main chain might reduce the effective delocalization length of the π-electron. Then it is apparent from Figures 2 and 3 that the PDA red form has a shorter delocalization length of the π-electron than the PDA blue form owing to the distortion along a conjugated main chain. Molecular Packing in Monolayers. Figure 4 shows the ED patterns of the monomeric monolayer and the PDA monolayers in the blue and red forms which were polymerized on the SiO substrate and the water surface, respectively. Since all the ED patterns exhibited crystalline spots, it seems reasonable to consider that all the monolayers are composed of fairly large crystalline domains in comparison with an electron beam diameter of 10 µm.5 Then Figure 4 indicates that the 2D PDA crystals are successfully formed by the topochemical photopolymerization in both cases of the monolayers on the SiO substrate and on the water surface. Figure 5 shows the molecular packings for the monomeric monolayer and the PDA monolayers in the blue and red forms. The side chains of the polymerized monolayers are not drawn in the schematic for simplification. The 2D lattice parameters, the lengths along the a and the b axes, and the areas of the unit cells are also shown in Figure 5. These possible molecular packings were decided on the basis of the ED patterns shown in Figure 4. The direction of the main PDA chain was determined to be along the same direction as that of a axis, because the a axis length of 0.49 nm agrees well with the repeating unit distance along the acetylenic main chain, which was calculated by the ab initio method.27 Since the unit cells for both the monomeric and the PDA blue form monolayers are 2D oblique crystal lattices, (24) Saperstein, D. D. J. Phys. Chem. 1986, 90, 1408. (25) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82, 2136. (26) Eckhardt, H.; Boudreaux, D. S.; Chance, R. R. J. Chem. Phys. 1986, 85, 4116. (27) Karpfen, A. J. Phys. 1980, C13, 5673.
Figure 4. ED patterns of the monomeric monolayer and the PDA monolayers in the blue and red forms.
the crystallographic symmetry was maintained through the conversion from the monomeric monolayer to the PDA blue form one. On the other hand, the unit cell for the PDA red form monolayer exhibits a 2D rectangular crystal lattice, indicating the modification in crystallographic symmetry through the polymerization. The unit cell for the PDA red form monolayer is similar to that for the poly(10,12-nonacosadiynoic acid) bilayer in the red form, reported by Day et al.14 The areas of the unit cells for both polymerized monolayers were smaller than that of the monomeric monolayer. This indicates that the polymerization process induces the contraction of the crystal lattice owing to the covalent bond formation between
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Figure 6. Tsp dependence of the ED pattern of the monomeric monolayer.
Figure 5. Schematic molecular packings of the monomeric monolayer and the PDA monolayers in the blue and red forms. Table 1. Dlat for the Monomeric Monolayer and the PDA Monolayers in the Blue Form and the Red Form crystallographic distortion, Dlat (%) monomer polydiacetylene blue form polydiacetylene red form
6.7 2.2 4.6
adjacent molecules. Also, it is apparent that molecules in the PDA blue form are more closely packed than those in the PDA red form. Table 1 shows the values of the crystallographic lattice distortion, Dlat, for the monomeric monolayer and the PDA monolayers in the blue form and the red one. The magnitudes of Dlat were evaluated from the ED peak profile of each monolayer, as explained in the Experimental Section. Since the ED peak profiles from the (020) ED spots were used, the magnitudes of Dlat correspond to the degree of spatial fluctuation for the polymeric chain packings in the direction perpendicular to the main chain. The value of Dlat for the PDA red form was almost twice as large as that for the PDA blue form. This result clearly indicates that the PDA red form has a larger spatial fluctuation with respect to the polymeric chain packings than the PDA blue form. This corresponds to the result that a PDA main chain in the red form must occupy a larger area than that in the blue form. This larger fluctuation of the red form might arise from its conformational disorder of the side chain not only induces the distortion along a PDA chain but also disturbs the closed
packing of the PDA chains. Also, it is reasonable that the conformational disorder of the side chain should vary the crystal system in the monolayer during the polymerization process of the red form, as shown in Figure 5. It is clear from these results mentioned above that each form of PDA has a distinct 2D molecular packing determining its electronic structure. Evaluation of the Melting Temperatures of the Monomeric Monolayers on the Water Surface and on the SiO Substrate. The difference in the 2D molecular packing for the formed PDAs should be attributed to the difference in the polymerization behavior of the monomer. For example, in the case of polymerization in the monolayer on the water surface corresponding to the formation process of the PDA red form, the distortion along a conjugated PDA main chain may be caused by the procession of the polymerization reaction. This is because the diacetylene molecules on the water surface are supposed to be more disordered even in the crystalline state owing to remarkable thermal mobility compared to those on the SiO substrate. The active thermal mobility of molecules on the water surface may induce the molecular disorder in the monolayer and then reduce the cohesive forces among molecules. It is reasonable to consider that the decrease in the cohesive forces makes the melting temperature of the monolayer depressed (lowered). Thus, in order to investigate the difference in the degree of thermal mobility of diacetylene monomeric molecules in the monolayer, the melting temperatures in both cases of the monomeric monolayers on the water surface and on the SiO substrate were measured. Figure 6 shows the Tsp dependence of the ED pattern for the monomeric monolayer transferred onto the hydrophilic SiO substrate. The ED patterns were taken at Tsp using a cryotransfer system. The ED patterns at 293, 298, and 303 K exhibited crystalline oblique spots, an almost crystalline Debye ring, and an amorphous halo, respectively. Then Figure 6 indicates that the monolayer on the water surface melts in a temperature range between 298 and 303 K. Figure 7 shows the substrate temperature, Tss, dependence of the ED patterns for the crystalline monomeric monolayer transferred onto the SiO substrate at Tsp ) 293 K. The ED pattern at Tss ) 293 K was the same as that at Tsp ) 293 K, as shown in Figure 6. The ED patterns for the crystalline monomeric monolayer on the SiO substrate exhibited crystalline spots at 298, 303, 308, and 313 K and an amorphous halo at 318 K, respectively. The symmetrical extinction of spots shown in Figure 7 indicates that molecules are tilted on the SiO substrate at higher temperatures.28 The melting temperature of the crystalline monomeric monolayer on the SiO substrate was evaluated to be around 315 K (compare Figure 7). Since the monomeric monolayer on the SiO substrate exhibited a higher melting temperature than that on the water surface, it is reasonable to conclude from Figures 6 and (28) Seitz, R.; Mitchell, E. E.; Peterson, I. R. Thin Solid Films 1991, 205, 124.
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Figure 7. Tss dependence of the ED pattern of the crystalline monomeric monolayer.
7 that diacetylene molecules on the water surface have an increased thermal mobility compared to those on the SiO substrate. As shown in Figure 2, the polymerization reaction is completed in a much shorter irradiation time in the case of the monolayer on the water surface, compared with the case of the monolayer on the SiO substrate, although the polymerizability evaluated by the absorbance is almost the same. This apparently indicates the relatively larger reaction rate of the molecules in the monolayer on the water surface. In the polymerization process of diacetylenes accompanying a reorientation of the molecules in the crystal lattice, the reaction rate would be strongly dependent on the mobility of molecules rather than the aggregation structure of the monomer. As previously reported, the difference in the aggregation structure of the monomeric monolayer, whose influence cannot be eliminated completely in this study, does not so much affect the reaction rate, while it causes the difference in the resultant polymer.10,29 Thus, Figure 2 suggests that the mobility of diacetylene molecules in the crystal lattice is presumably a dominant factor in determining the polymerization behavior in this study. In other words, the difference in the polymerization behaviors for the crystalline monolayers on the different substrate surfaces is attributable to the difference in the thermal mobility of molecules. Possible Polymerization Process of the Monolayers on the SiO Substrate and on the Water Surface. On the basis of the structural studies mentioned above, a possible polymerization process of the monolayers on the different surfaces could be described as follows. Figure 8 shows the schematic representation for the possible polymerization processes in the lithium 10,12heptacosadiynoate monolayers on the SiO substrate (a) and on the water surface (b). In the case of the monomeric monolayer on the SiO substrate (Figure 8a), diacetylene molecules in the crystalline monolayer are closely and regularly packed, and the mobility of molecules is relatively depressed. Then the conversion from monomer to polymer through a 1-4 addition of the conjugated triple bonds (1,3-butadiyne-1,4-diyl groups) between adjacent molecules preferentially occurs without the generation of a large distortion along a conjugated main chain. Because of that, PDA chains have relatively longer delocalization lengths of the π-electron in a regularly packed state. On the other hand, in the case of the monomeric monolayer (29) Tieke, B.; Lieser, G.; Weiss, K. Thin Solid Films 1983, 99, 95. Tamura, H.; Mino, N.; Ogawa, K. Thin Solid Films 1989, 179, 33.
Figure 8. Schematic representation for the polymerization process of the monolayers on the SiO substrate and on the water surface.
on the water surface (Figure 8b), the diacetylene molecules are more mobile or disordered even in a crystalline state owing to a remarkable thermal mobility of the molecules on the water surface, as schematically shown in Figure 8b. Thus, the polymerization reaction proceeds rapidly and causes a distortion along the conjugated main chain being attributed to the conformational disorder of the side chain. The distortion along a conjugated main chain disturbs the conjugation of the π-orbitals along an acetylenic main chain. Therefore, the formed polymer chains have a relatively shorter delocalization length of the π-electron. Furthermore, the conformational disorder of the side chain prevents the close and regular packing of the PDA chains. From the results mentioned above, the role of the interfacial interaction between the diacetylene molecules in the monolayer and the surface of the substrate has been clarified on the basis of the regularity of molecular packing induced during the two-dimensional polymerization of the diacetylene derivatives. Conclusion The polymerization behavior of the lithium 10,12heptacosadiynoate monolayer was strongly affected by the interfacial interaction between the molecules in the monolayer and the surface of the substrate. The PDA blue form could be prepared by the photopolymerization of the crystalline monomeric monolayer on the hydrophilic SiO substrate, while the PDA red form was formed by the photopolymerization of the monomeric monolayer on the water surface. These PDA monolayers have different crystal structures attributed to the difference in the polymerization behavior of the monomer. This difference in the polymerization behavior, which also influences the electronic structure of the formed polymer, can be explained in terms of the mobility of molecules in the crystalline monolayer. Acknowledgment. This research was supported in part by a grant from the Ministry of Education, Science and Culture, Japan. Keisuke Kuriyama is a Fellow of the Japan Society for the Promotion of the Science. The authors thank Dr. Y. Oishi for helpful discussions and a critical reading of the manuscript. LA9508102
