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Langmuir 1998, 14, 1130-1138
Chromatic Phase of Polydiacetylene Langmuir-Blodgett Film 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 July 24, 1997. In Final Form: December 3, 1997 The thermochromic phase-transition behavior of polydiacetylene (PDA) Langmuir-Blodgett (LB) films in the blue form and the bluish-green one was investigated from the viewpoint of the molecular lateral packings evaluated by the electron diffraction (ED) analysis. The PDA blue and bluish-green forms exhibit absorption peaks at the wavelengths 640 and 704 nm in the visible absorption spectrum, respectively. At an elevated temperature, the PDA LB films in both forms were converted into the red form with the absorption peak at the wavelength 540 nm. The chromatic phase transition of the PDA LB films was accompanied by the characteristic crystal phase transition in both cases. Further, the detailed structural analyses based on the ED measurement, the Fourier transform infrared external reflection spectrum, small-angle X-ray scattering, and X-ray photoelectron spectrum measurements revealed that the difference in the chromatic phase of the PDA LB film could be explained by the difference in the exciton energy level being strongly dependent on both the crystal symmetry and the resonance backbone structure.
Introduction Polydiacetylenes (PDAs) are pseudo-one-dimensional conjugated polymers and are regarded as promising candidates for third-order nonlinear optical materials. PDAs can be produced in the form of anisotropic chainextended crystal by the topochemical photopolymerization of the monomer crystal.1 PDAs have also been studied for fundamental understandings of the various photophysical processes in the highly delocalized π-conjugated system.2,3 Even for a linear optical process such as light absorption, however, the mechanism has not been clarified yet. It has been reported that PDAs have two spectroscopically distinct phases designated the blue form and the red one according to their colors.4 The blue and the red forms show the exciton absorption peaks around the wavelengths of 640 and 540 nm in the visible absorption spectrum, respectively. In the early stage of the study, the difference between these forms was ascribed to the distinct resonance backbone structures, that is, the butatrienic type and the acetylenic one, as shown in Figure 1.5 Recent ultraviolet photoelectron spectroscopic studies, however, revealed that both forms have the acetylenic type main chains.6,7 At the present time, the widely accepted model is based on the difference in the effective delocalization length of the π-electron.8 That is, the red form is thought to have a shorter delocalization length of the π-electron than the blue form, as expected from the wavelengths of the absorption peaks, 540 and 640 nm. The model explains that the effective delocalization length * To whom correspondence should be addressed. (1) Wegner, G. Z. Naturforsch. 1969, 24b, 824. (2) Guo, D.; Mazumdar, S.; Dixit, S. N.; Kajzar, F.; Jarka, F.; Kawabe, Y.; Peyghambarian, N. Phys. Rev. 1993, 48, 1433. (3) Kobayashi, T. J. Lumin. 1992, 53, 159. (4) Kanetake, T.; Tokura, Y.; Koda, T. Solid State Commun. 1985, 56, 803. (5) Chance, R. R.; Banghman, R. H.; Muller, H.; Echhardt, J. G. J. Chem. Phys. 1977, 67, 3616. (6) Nakahara, H.; Fukuda, K.; Seki, K.; Asada, S.; Inokuchi, H. Chem. Phys. 1987, 118, 123. (7) Seki, K.; Morisada, I.; Tanaka, H.; Edamatsu, K.; Yoshiki, M.; Takata, Y.; Yokoyama, T.; Ohta, T. Thin Solid Films 1989, 179, 15. (8) Eckhardt, H.; Boudreaux, D. S.; Chance, R. R. J. Chem. Phys. 1986, 85, 4116.
Figure 1. Two resonance backbone structures of PDA.
of the π-electron is attributed to the degree of lattice strain on a conjugated PDA main chain, since the lattice strain on a main chain can disturb the conjugation of π-orbitals along the series of alternate single, double, single, and triple bonds constituting an acetylenic main chain. Further, the lattice strain on a conjugated main chain is supposed to be induced by the conformational disorder of the side chain.9,10 On the other hand, it was reported that the absorption peak at the wavelength 704 nm was detected upon photopolymerization of the annealed Langmuir-Blodgett (LB) film of cadmium 10,12-tricosadiynoate.11,12 The new spectroscopic phase of PDA was designated the bluish(9) Tomioka, Y.; Tanaka, N.; Imazeki, S. J. Chem. Phys. 1989, 91, 5694. (10) Tokura, Y.; Nishikawa, S.; Koda, T. Solid State Commun. 1986, 59, 393. (11) Fukuda, K.; Shibasaki, Y.; Nakahara. H. Thin Solid Films 1988, 160, 43. (12) Shibata, M.; Kaneko, F.; Aketagawa, M.; Kobayashi, S. Thin Solid Films 1989, 179, 433.
S0743-7463(97)00831-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/13/1998
Chromatic Phase of Polydiacetylene LB Film
green form. The bluish-green form was expected to have the longest delocalization length of the π-electron with less conformational disorder of the side chain, compared with the blue and the red forms. However, our previous electron diffraction (ED) studies on the polymerization mechanism of the PDA LB film indicated that the bluishgreen form was composed of the butatrienic type main chains rather than the acetylenic ones corresponding to the resonance backbone structure of the blue and the red forms.13 Thus, though the PDA has three distinct chromatic phases as mentioned above, three forms have not been investigated systematically. In order not only to understand the photophysical properties of the PDAs but also to obtain the PDA with superior nonlinear optical characteristics, it is of great importance to clarify the electronic natures specifying these chromatic phases of PDA. It has been well-known that the blue form can be altered to the red one upon temperature elevation.14-16 However, the detailed mechanism of the phase transition still remains unrevealed as yet. Further, in the case of the bluish-green form, even the thermochromic behavior has not been clarified. These thermochromic phase-transition behaviors of PDAs should be an important phenomenon for elucidation of the electronic nature of these contrasting forms. In this paper, the thermochromic phase-transition behavior of the blue form and the bluish-green one for the polymerized cadmium 10,12-tricosadiynoate LB films was investigated by the temperature dependence of visible absorption spectra and the ED pattern. The aggregation structures of the LB films in the blue, the red, and the bluish-green forms were also investigated on the basis of the Fourier transform infrared external reflection spectroscopy (FT-IR/ERS), small-angle X-ray scattering (SAXS), and X-ray photoelectron spectroscopic (XPS) measurements. Finally, an electronic nature specifying the chromatic phase of the PDA LB film was proposed. Experimental Section Preparation of the Monomeric LB Film. 10,12-Tricosadiynoic acid with chromatographic reference quality was used without further purification. Benzene with spectroscopic quality was used as a spreading solvent. A benzene solution of 10,12tricosadiynoic acid was prepared with a concentration of 2.0 × 10-3 mol‚L-1. The subphase water was purified by a Milli-QII system (Millipore Co., Ltd.). The dimensions of the trough were 404 mm in length, 150 mm in width, and 5 mm in depth. The monolayer was prepared by spreading the benzene solution of 10,12-tricosadiynoic acid on the water subphase containing 4.0 × 10-4 mol‚L-1 of CdCl2 and 5.0 × 10-5 mol‚L-1 of KHCO3 at the subphase temperature of 286 K.11 The monolayer formed on the water surface was transferred onto the SiO substrate by the upward drawing method at the surface pressure of 35 mN‚m-1. The hydrophilic SiO substrate was prepared by vapor-deposition of SiO onto a Formvar-covered electron microscope grid (200mesh) attached to a glass substrate for ED analysis.17,18 The surface of the hydrophilic SiO substrate was confirmed to be smooth and amorphous, on the basis of morphological and ED studies, respectively. The SiO substrate with the grids was used for all measurements in this paper. The number of transferred (13) Kuriyama, K.; Kikuchi, H.; Kajiyama, T. Langmuir 1996, 12, 6468. (14) Kanetake, T.; Tokura, Y.; Koga, T. Solid State Commun. 1985, 56, 803. (15) Lieser, G.; Tieke, B.; Wegner, G. Thin Solid Films 1980, 68, 77. (16) Tamura, H.; Mino, N.; Ogawa, K. Thin Solid Films 1989, 33, 179. (17) Kajiyama, T.; Oishi, Y.; Uchida, M.; Tanimoto, Y.; Kozuru, H. Langmuir 1992, 8, 1563. (18) Kajiyama, T.; Oishi, Y.; Uchida, M.; Morotomi, N.; Ishikawa, J.; Tanimoto, Y. Bull. Chem. Soc. Jpn. 1992, 65, 864.
Langmuir, Vol. 14, No. 5, 1998 1131 monolayers for the LB film was 19. Transfer ratios were almost unity for all cases. Photopolymerization Procedure of the LB Film. Polymerization of the LB film on the SiO substrate was carried out by UV light irradiation (UL1-5EB-6A, 500 W, Ushio Co., Ltd.) at a distance of 28 cm from the substrate in a N2 gas atmosphere at 293 K. The PDA blue form was prepared by photopolymerization of the monomeric LB film. For the preparation of the bluish-green form, the monomeric LB film was preannealed at 323 K for 1 h before photopolymerization of the LB film.11 The formation of each chromatic phase was confirmed by the Visible absorption spectrum. The visible absorption spectra of the LB films were obtained using a Shimadzu MPS-2000. Chromatic Phase Change of the Polymerized LB Films. The chromatic phase change of the polymerized LB films was induced by annealing of the LB films at various temperatures ranging from 303 to 363 K for 1 h. The annealed LB films were subsequently quenched at the temperature 293 K, and all measurements were carried out at 293 K. It was confirmed from the visible absorption spectrum that the LB films retained their chromatic phases during all measurements. Infrared Absorption Spectrum Measurements of the Polymerized LB Film. The FT-IR/ERS spectra of the polymerized LB films were recorded on a Nicolet Model 510 FT-IR spectrophotometer equipped with a liquid-nitrogen-cooled MCT detector. The ERS technique enables us to measure the infrared spectrum of the polymerized LB film on the substrate prepared for the measurement of the visible absorption spectrum.19,20 The FT-IR/ERS spectra were measured at the angle of incidence 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 Fourier-transformed with one level of zero filling to yield spectra with a resolution of 2 cm-1. Electron Diffraction Analysis of the Polymerized LB Film. The ED patterns of the LB films 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 2 µm in diameter. The incident electron beam was irradiated perpendicular to the film surface. Small-Angle X-ray Scattering Measurements of the LB Film. The small-angle X-ray scattering measurements of the LB films were carried out by using a Kratky U-slit camera attached to a Rigaku X-ray generator RU-300.21 An X-ray beam was generated with Cu KR radiation filtered by nickel foil. The incident X-ray beam was irradiated parallel to the film surface. X-ray Photoelectron Spectroscopic Analysis of the LB Film. The X-ray photoelectron spectra of the LB films were acquired with a Shimadzu ESCA 850 spectrometer with a Mg KR anode. The emission angle of the photoelectrons was 90°. The LB films prepared on the SiO substrate were attached to the probe of the spectrometer.
Results and Discussion Thermochromic Phase Transition Behavior of the Blue Form. Figure 2 shows the annealing temperature dependence of the visible absorption spectrum for the PDA LB film in the blue form. The PDA blue form obtained by the photopolymerization of the as-prepared cadmium 10,12-tricosadiynoate showed the typical main absorption peak at 640 nm with the phonon side band at approximately 580 nm. Since the spectral feature of the blue form did not change up to 303 K, Figure 2 shows the spectra above this temperature. The intensity of the absorption peak at 640 nm decreased with increasing annealing temperature, whereas the other absorption peak at 540 nm appeared and the intensity of it increased inversely with an increase of the annealing temperature. At the annealing temperature 353 K, only the main (19) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78, 946. (20) Sakai, H.; Umemura, J. Chem. Lett. 1993, 2167. (21) Kajiyama, T.; Hanada, I.; Shuto, K. Oishi, Y. Chem. Lett. 1989, 193.
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Figure 2. Annealing temperature dependence of the visible absorption spectrum for the PDA LB film in the blue form.
absorption peak at 540 nm with the phonon side band at 500 nm was observed. Then, it is clear from Figure 2 that the chromatic phase change from the blue form to the red one occurred at an elevated temperature. Since, in the case of the blue-to-red phase transition, the isosbestic point was observed, it is apparent that there exists a complementarity between both forms, implying that only the two chromatic phases with no other intermediate are concerned in this phase change. Therefore, this indicates that the blue and the red forms exist in the distinct stable states, that is, the chromatic phases of the PDA LB film. The spectroscopic difference between two forms might correspond to the difference in the effective delocalization length of the π-electron along a conjugated main chain, which is principally dependent on the degree of conformational disorder of the side chain. That is, it is considered that the red form has a shorter delocalization length of the π-electron than the blue form, owing to its disordered side chain. The degree of conformational disorder could be evaluated by the vibrational spectral feature of the alkyl side chain. The vibrational bands would become broader with an increase of conformational disordering of the hydrocarbon side chain due to formation of the gauche conformation from the all-trans regular conformation.22,23 It is reported that the spectral features for the red form were considerably broadened in comparison with those of the blue form. Parts a and b of Figure 3 show the FT-IR/ERS spectra for the PDA LB films in the blue form at 303 K and the red one at 353 K, respectively. The infrared absorption peaks at 2950, 2930, 2920, and 2850 cm-1 are assigned to the CH3 antisymmetric, the CH3 symmetric, the CH2 antisymmetric, and the CH2 symmetric stretching vibrations of alkyl side groups, respectively.24 As shown in Figure 3, the spectral feature for the red form is almost the same as that for the blue form and the broadening of vibrational bands in the red form was not discernible in this study. It is, therefore, reasonable to consider from Figure 3 that both forms have a relatively similar ordered structure in the hydrocarbon side chain and that the degree of conformational disorder is not apparently influenced by the annealing treatment. This result indicates that the lattice strain on the main chain may not be a main factor determining the chromatic phase of PDA. (22) Saperstein, D. D. J. Phys. Chem. 1986, 90, 1408. (23) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82, 2136. (24) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85.
Figure 3. FT-IR/ERS spectra of the PDA LB films in the blue form (a) and the red one (b).
Figure 4 shows the annealing temperature dependence of the schematic ED pattern for the PDA LB film prepared originally in the blue form. The ED patterns were described as the contour plot of the diffraction intensity for clarification. The ED pattern of the PDA LB film in the blue form at 303 K showed six crystalline spots in the reciprocal lattice plane. The diffraction pattern showed three different lattice spacings, indicating that the crystal system was classified as the two-dimensional (2D) oblique. As the annealing temperature increased, the diffraction spots became broader along the azimuthal direction and the crystal system changed into the 2D rectangular one with two distinct lattice spacings. This change in the ED patterns indicates that the crystallographic axes of the PDA crystallites in the LB film would fluctuate to be disordered and that the crystal phase change would be induced at higher temperature. Parts a and b of Figure 5 show the schematic molecular packings in the PDA LB films in the blue and the red forms prepared at the annealing temperatures 303 and 343 K, respectively. The alkyl side chains were not shown in the schematic drawing for simplification. The 2D lattice parameters, the lengths along the a and the b axes, the
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Figure 4. Annealing temperature dependence of the ED pattern of the PDA LB film in the blue form.
angle between the a- and the b-axes, and the areas of the unit cells were shown in Figure 5. These possible molecular packings were determined from the observed ED patterns in consideration that the repeating unit distance of the main chain along the a-axis agrees with 0.49 nm for the acetylenic backbone structure calculated by the ab initio method.13,25 It has been reported that the repeating unit distance of the main chain for PDA is approximarely 0.49 nm, irrespective of the side chain substituents.26 As mentioned above, the molecular packings of the PDA blue and red forms took 2D oblique and rectangular crystal systems, respectively. This means that the conjugated main chains are inclined-stacked with respect to the translational lateral-chain axis, that is, the b-axis in the case of the blue form, while the conjugated main chains in the red form are stacked perpendicular to the b-axis. The relationship between the molecular packings and the visible absorption properties of PDA as shown in Figures 2 and 5 is similar to that of a dye molecule with a large electric dipole transition moment along its long axis. A number of dye molecules (25) Karpfen, A. J. Phys. 1980, C13, 5673. (26) Day, D.; Lando, J. B. Macromolecules 1980, 13, 1483.
have two characteristic exciton energy levels corresponding to distinct packings of the molecules, the so-called j-aggregate and h-aggregate.27 Parts a and b of Figure 6 illustrate the schematic molecular packings of the jaggregate and the h-aggregate for the dye molecules, respectively. In the case of the j-aggregate, the dye molecules are inclined-stacked with respect to the translational lateral axis. On the other hand, the dye molecules in the h-aggregate are perpendicularly stacked to the translational lateral axis, as shown in Figure 6b. The h-aggregate has a higher exciton energy level than the j-aggregate. The difference in the exciton energy level between the j- and h-aggregates of the dye molecules can be explained in terms of the electrostatic interaction of transition dipole moments among adjacent molecules. Therefore, the difference in the spectroscopic properties (exciton energy levels) between blue and red forms of PDA might be attributed to their molecular packings; that is, the stacking manner or the orientation of the main chains, (27) MacRae, E. D.; Kasha, M. J. Chem. Phys. 1958, 28, 721. Kasha, M. In Spectroscopy of the Excited-State; Bartolo, B. D., Ed.; Plenum: New York, 1976.
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Figure 7. Annealing temperature dependence of the visible absorption spectrum for the PDA LB film in the bluish-green form.
Figure 5. Schematic molecular packings for the PDA LB films in the blue (a) and the red form (b).
Figure 6. Schematic molecular packings of the j-aggregate and the h-aggregate for the dye molecules.
since the PDA chain also has a large electric transition dipole moment along the conjugated main chain. Campbell et al. reported that the red form of PDA could be classified into both the disordered and ordered phases with respect to the conformational state of the alkyl side chain on the basis of the resonant Raman spectrum measurement.28 Also, our previous IR measurements showed that the spectral features for the alkyl side chain of a red form prepared from the diynoic acid monolayer on the water surface were considerably broadened compared to the case of Figure 3.29 This result suggested that the side chain of the red form was in a disordered gauchecontained conformation. These previous studies as well as the IR results in Figure 3 indicate that the conformational regularity of the side chain is not so important in determining the chromatic phase of PDA. Thermochromic Phase-Transition Behavior of the Bluish-Green Form. Figure 7 shows the annealing temperature dependence of the visible absorption spectrum for the PDA LB film in the bluish-green form. The (28) Campbell, A. J.; Davis, C. K. L. Polymer 1995, 36, 675. (29) Kuriyama, K.; Kikuchi, H.; Oishi, Y.; Kajiyama, T. Langmuir 1995, 11, 3536.
PDA bluish-green form obtained by the photopolymerization of the annealed cadmium 10,12-tricosadiynoate LB film showed the main absorption peak at 704 nm with the phonon side band at approximately 640 nm. The wavelength of the absorption peak of the bluish-green form decreased with increasing annealing temperature up to around 343 K. In the case of the higher annealing temperature above 353 K, the absorption peak at 540 nm appeared with the phonon side band at 500 nm. It is apparent from Figure 7 that the chromatic phase change of the bluish-green form into the red one was induced by an annealing treatment. In the case of the bluish-greento-red phase transition, the isosbestic point could not be detected. This indicates that the chromic phase transition is not straightforward between two stable forms. Figure 8 shows the annealing temperature dependence of the schematic ED pattern of the PDA LB film asprepared in the bluish-green form. The ED pattern of the PDA LB film in the bluish-green form at 303 K showed 16 crystalline spots near the direct beam in the reciprocal lattice plane. The diffraction pattern clearly showed that the crystal was classified as a 2D square system with one characteristic lattice spacing. As the annealing temperature increased, the number of diffraction spots decreased to six. The ED pattern with six spots at higher temperature showed the crystal system 2D rectangular. This change in the ED pattern in Figure 8 indicate that the crystal phase change might be responsible for the chromic change of the bluish-green form, similarly to the case of the blue form. Parts a and b of Figure 9 show the schematic molecular packings for the PDA LB films in the bluish-green and the red forms prepared at the annealing temperatures 303 and 363 K, respectively. In the case of the bluishgreen form shown in Figure 9a, the direction of the main chain was fixed to the same direction as that of the a-axis, since the 2D crystal lattice exhibited the square system in which the a- and the b-axes could be assigned arbitrarily. Though the conjugated main chains in the bluish-green form were perpendicularly stacked to the b-axis (the translational lateral axis), the stacking manner of individual chains was different from that in the red form in Figure 5b. Also, the repeating unit distance of the main chain was 0.47 nm, whose magnitude was apparently different from the 0.49 nm of the blue and the red forms. This indicates that the resonance backbone structure of the bluish-green form might be the butatrienic type rather than the acetylenic type, as shown in Figure 10, since the
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Langmuir, Vol. 14, No. 5, 1998 1135
Figure 8. Annealing temperature dependence of the ED pattern of the PDA LB film in the bluish-green form.
repeating unit distance for the butatrienic type is predicted to be less than 0.483 nm on the basis of the ab initio and the molecular mechanics calculations.13,25 On the other hand, the molecular packing of the red form in Figure 9b was the same as that of the red form transformed from the blue form. The red form had the acetylenic type backbone structure with the repeating unit distance 0.49 nm. Therefore, in the case of the bluishgreen-to-red chromic phase transition, not only the deformation of the molecular packings but also the change in the resonance backbone structure would occur. The
crystal phase transition varies both the energy level of the unit molecule defined by the resonance backbone structure and the degree of the electrostatic interaction among the transition dipole moments on the individual chain. It seems reasonable that the complicated chromic phase change in Figure 7, which lacks the isosbestic point, is based on these two phenomena. Aggregation Structures of the LB Films in the Blue, the Red, and the Bluish-Green Forms. To characterize the layer configuration and the aggregation structure on the polar group-counterion moiety, SAXS
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Figure 9. Schematic molecular packings for the PDA LB films in the bluish-green (a) and the red form (b).
Figure 11. SAXS spectra of the PDA LB films. Table 1. Longitudinal Lattice Parameters of the LB Films in the Blue, the Red, and the Bluish-Green Form d001/nm bluish-green form blue form red form
Figure 10. Atomic positions of PDA backbone structures based on the ab initio calculation. The average value from the molecular mechanics calculation is also shown in parentheses.
and XPS measurements of the polymerized LB films were also carried out. Figure 11 shows the SAXS spectra of the PDA LB films. Table 1 shows the longitudinal lattice parameters of the LB films in the blue, the red, and the bluish-green form, evaluated from the SAXS peaks. All the calculated lattice parameters were more than 5 nm. This apparently suggests the typical Y type configurations of LB films for all forms, since the thickness of the monolayer with the all-trans conformation of the side chain is estimated to be approximately 2.6 nm, on the basis of the molecular mechanics calculation of the molecule.30 The Y type configurations of LB films lead to the fact that the distance
5.16 5.17 5.41
between adjacent two conjugated chains in the longitudinal direction is more than 1.93 nm. Thus, the electrostatic interaction of the transition dipole moment between molecules in this direction is negligible compared to that of the case in the b-axis direction, where the distance between adjacent chains is less than 0.47 nm. In general, the electrostatic interaction of the transition dipole moment is known to be inversely proportional to its cubed distance. The results in Table 1, therefore, support the previous explanation for the difference in the exciton energy level between the blue and the red forms of PDA, on the basis of their in-plane (2D) molecular packings. Figure 12 shows the survey scan XPS spectrum of the bluish-green form. The spectrum showed the C1s, the Cd3d5/2, the Cd3d3/2, and the O1s peaks at approximately 285, 406, 413, and 540 eV, respectively.31 Also, both the blue form and the red one showed the similar spectrum. Table 2 shows the atomic ratios of [Cd]/[C] in the bluishgreen, the blue, and the red forms calculated from the XPS spectra. The peak areas of the C1s and the Cd3d5/2 (30) Walsh, S. P.; Lando, J. B. Mol. Cryst. Liq. Cryst. 1994, 240, 201. (31) Ohnishi, T.; Ishitani, A.; Ishida, H.; Yamamoto, N.; Tsubomura, H. J. Phys. Chem. 1978, 18, 1989.
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Figure 14. Two limiting arrangements of the cadmium cation in the LB film.
Figure 12. Survey scan XPS spectrum of the bluish-green form. Table 2. Composition Ratio of [Cd]/[C] for the Bluish-Green Form, the Blue Form and, the Red Form [Cd3d5/2]/[C1s] bluish-green form blue form red form
0.0221 0.0225 0.0202
Figure 13. Cd3d spectra of the LB films in the bluish-green, the blue, and the red form.
its surroundings. The cadmium ion is known to possess covalent character in some cases. The cadmium ion in the bluish-green form has more ionic character in comparison with the ones in the blue and the red forms. Figure 14 shows the two limiting arrangements of cadmium cation in the LB film. Figure 14a shows that a cadmium cation links two carbanion molecules in the separate layers with two specific bonds. On the other hand, Figure 14b shows that a cadmium cation is placed in the center of four adjacent carbanion molecules. This cadmium cation-carbanion molecule aggregation structure is formed only by the lateral shift of the cadmium cation along the layer plane. In this case, a divalent cadmium cation links not only two adjacent layers of molecules but also another two adjacent molecules in the same layer without two specific bonds. If the bivalent Cd cation has covalent character, it forms the two specific bonds to two adjacent carbanion molecules. On the other hand, if the bivalent Cd cation has ionic character, the ions in the film are held together simply by attraction of the opposite charges without two specific bonds. Therefore, the cadmium cations with two specific bonds as shown in Figure 14a are considered to have more covalent character than those shown in Figure 14b. It is reasonable to conclude from XPS results that the blue and the red forms have the cadmium ion-counterion moiety, as shown in Figure 14a rather than that in Figure 14b, while the bluish-green form forms the aggregation structure shown in Figure 14b rather than that in Figure 14a. The cadmium cations in the bluish-green form have the tricosadiynoic carbanion molecules in the same layer being packed more closely. The abnormally short repeating unit distance of 0.47 nm for the bluish-green form may be stabilized by this bridging effect of cadmium cation in the LB film, because the butatrienic structure is less stable than the acetylenic structure with respect to the lattice energy.13,33
peaks were adopted for the calculation. The peak intensities were corrected for the photoionization cross section of each element. The photoelectron mean free paths for the C1s and the Cd3d5/2 peaks were assumed to be the same.32 Table 2 reveals that all forms have almost the same [Cd]/[C] value, approximately 0.02. This value agrees well with the ideal value 0.0217, which is derived from the composition ratio of [tricosadiynoic carbanion molecule] [Cd2+] ) 2:1. Each form, therefore, has one cadmium cation for two anion molecules in the LB film. Also, Figure 13 shows the Cd3d spectra of the LB films in the bluish-green, the blue, and the red forms. The two peaks of Cd3d5/2 and Cd3d3/2 for the blue and the red forms were shifted to a lower binding energy range in comparison with the corresponding peaks for the bluish-green form. This implies that the cadmium ion in the blue and the red forms has more covalent character than one in the bluishgreen form, since the Coulombic attraction between the atomic nuclei and the electron in the cation decreases as the cation is stabilized by the donation of an electron from
On the basis of the structural studies mentioned above, the possible chromatic phase-transition processes of the blue and the bluish-green forms of PDA might be discussed as follows. Figure 15 shows the schematic representation for the blue-to-red (a) and the bluish-green-to-red (b) phasetransition processes of the polymerized cadmium 10,12tricosadiynoate LB film. In the case of the blue-to-red phase transition shown in Figure 15a, the as-prepared blue form 2D oblique crystal is composed of the acetylenic conjugated main chains. When the blue form crystal is
(32) Kajiyama, T.; Zhang, L.; Uchida, M.; Oishi, Y.; Takahara, A. Langmuir 1993, 9, 760.
(33) Sixl, H. In Polydiacetylenes, Proceedings of a NATO Advanced Research Workshop, Dordrecht, Nijdorf, 1984.
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lattice is composed of the butatrienic conjugated main chains. When the bluish-green form crystal is heated to the phase-transition temperature above 313 K (see Figure 7), the rearrangement of the main chains would occur accompanied by the change in the resonance backbone structure from butatrienic to acetylenic. Crystallographically, this phenomenon was observed as the crystal phase transition which is from 2D square to 2D rectangular, as shown in Figure 9. In the bluish-green-to-red phase transition, therefore, not only the stacking manner of the main chains but also the conjugation of the π-electrons on the main chain would change. This is the reason the isosbestic point could not be detected in the temperature dependence of the visible absorption spectrum, as shown in Figure 7. Since the acetylenic type conjugation has a larger energy gap between HOMO and LUMO than the butatrienic type conjugation, the wavelength of the absorption maxima would be shortened with progress of the bluish-green-to-red phase transition. Also, the butatrienic-to-acetylenic transition may be ascribed to the deformation of the linkage of the cadmium ions in the LB film.
Figure 15. Schematic representation for the blue-to-red (a) and the bluish-green- to-red (b) phase-transition processes of the polymerized cadmium 10,12- tricosadiynoate LB film.
heated to the phase-transition temperature, which was estimated to be above 313 K from the result in Figure 2, the rearrangement of the main chains would occur. Crystallgraphically, this phenomenon was observed as the crystal phase transition which is from 2D oblique to 2D rectangular, as shown in Figure 5. However, the backbone structure maintains the acetylenic type conjugation of π-electron. Since the change in the stacking manner of the conjugated main chains made the exciton energy level of the system raised, the wavelength of the absorption maxima for the red form might be shortened. On the other hand, in the case of the as-prepared bluishgreen form shown in Figure 15b, the 2D square crystal
Conclusion The difference in the spectroscopic properties of PDA in the three different forms could be elucidated in terms of the PDA molecular packings. The PDA blue and red forms have the acetylenic type main chains with different stacking manner, causing the difference in the exciton energy level between the two forms. On the other hand, the bluish-green form has the butatrienic type main chain. The electronic natures of the PDAs are strongly dependent on their resonance backbone structures and, also, the stacking manners of the main chains. Acknowledgment. This research was supported in part by a grant from the Ministry of Education, Science and Culture, Japan. KK. is a Fellow of the Japan Society for the Promotion of the Science. The authors thank Dr. Y. Oishi for helpful discussions. One of us (K.K.) is also grateful to Dr. K. Tanaka for his help with the XPS measurements. LA970831R
