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J. Phys. Chem. 1994,98, 2726-2728
Thermochromism of Specific Crystal Form Oxotitanium Phthalocyanines Studied by Electroabsorption and X-ray Diffraction Measurements Toshiro Saito,' Yasushi Iwakabe, Toshiyuki Kobayashi, Shigeo Suzuki, and Takao Iwayanagi Hitachi Research Laboratory, Hitachi, Ltd.. Hitachi, Ibaraki, 31 9- 12, Japan Received: October 25, 1993; In Final Form: January 3, 1994"
Thermochromism was found in phase I1 and Y-form oxotitanium phthalocyanines (TiOPcs) from the observed temperature dependence of the absorption and X-ray diffraction spectra of various crystal forms of TiOPcs. Thermochromism has not been observed in amorphous and phase I TiOPcs. The temperature dependence of the electroabsorption spectra of phase I1 was also different from that of phase I. This distinctive behavior was discussed in terms of the crystalline packing structure of each TiOPc.
Introduction Phthalocyanine(Pc) compounds are commonly used functional organic pigments, and their applications to various optical and electric devices are well reported.' For example, Pcs have been used as the charge generation materials in organic electrophotographic receptors because of their high photoconductivity,high stability, and nontoxicity.2 It is well-known that many Pcs show several polymorphs with different crystalline packing structures, and their optical and electrical properties depend on their crystal forms. The absorption spectra, photoconductivity, and electroabsorption spectra are distinctive among different crystal forms of oxotitaniumPcs (TiOPcs).3 This fact implies that theelectronic structures depend on the crystal structures, namely, on the orientation of the Pc molecules. In order to elucidate more details about the crystal form dependence of the properties of the electronic states, we studied temperature effects for the absorption spectra of various TiOPcs. We found that thermochromism took place only in the specific crystal form TiOPcs. This suggests that the electronic states of these TiOPcs vary with temperature. Here, we report on the temperature dependence of absorption, X-ray diffraction, and electroabsorption spectra of various crystal forms of TiOPcs. Experimental Section TiOPc was obtained from Dainichiseika Color C Chemicals Mfg. Co., Ltd., and it was purified by vapor sublimation under flowing nitrogen gas. Thin film samplesof TiOPc were prepared as follows. TiOPc was evaporated in a vacuum of 10-5-10-6 Torr using a conventional bell jar system. Then TiOPc films (90-270 nm thick) were depositedat room temperature onto I T 0 substrates which had been coated with an evaporated SiO, thin layer ( 140 nm). Someof the depositedTiOPc thin film sampleswere exposed to tetrahydrofuran (THF) vapor or chlorobenzene-water (1O:l) mixed vapor for 13-1 5 h. Other samples were left in xylene for 13 h. These treatments were done at room temperature. As described previously,3 we could assign the crystal form of the treated TiOPc samples as follows. The THF-treated, xylenetreated, and chlorobenzene-water mixture-treated samples were assigned to phase 11, phase I, and Y-form, respectively. The samples for electroabsorptionmeasurement had semitransparent aluminum electrodes with a transmittance of about 10-201 formed on their Pc layers by vacuum deposition. Absorption spectra were measured by a conventional spectrophotometer (Hitachi 330). TiOPc thin films were placed in a cryostat for measurement of their X-ray diffraction spectra by an X-ray diffractometer (Rigaku RU-200with thin film attachN
* Abstract
published in Advance ACS Abstracts, March 1 , 1994.
0022-365419412098-2726$04.50/0
ment) using Cu Ka!monochromatic radiation. The experimental setup used to measure the electroabsorption spectra has been described in detail elsewhere.' When the absorption and electroabsorption measurements were carried out at 77 K, the thin film samples were placed in a quartz cell filled with liquid nitrogen.
Results and Discussion Figure 1 shows the absorption spectra of various TiOPcs measured at room temperature (298 K)and 77 K. No difference was observed in spectra of the amorphous sample for the two temperatures. On theother hand, in phase I1 and Y-formsamples the peaks of the longest wavelength absorption bands observed at 298 K were shifted to even longer wavelengths at 77 K. The observed red shifts for phase I1 and Y-form samples were about 20 and 15 nm, respectively. The peak of the absorption band around 830 nm moved smoothly to a longer wavelength with decreasing temperature and returned to its original, room temperature position by raising the temperature, indicating thermochromic behavior. While the absorption bands around 680 and 770 nm appeared clearly in phase I sample when the temperature was lowered to 77 K, no red shift was observed. The temperature effects on the absorption spectra were dependent on the crystal form as clearly shown in Figure 1. Although in metalfree and copper Pcs the red shift of the absorption spectra was not observed, in phase I1 oxovanadium Pc it was.4 These findings indicate that the red shift may be correlated with stacking structures of PCS. X-ray diffraction spectra of various TiOPcs measured at 298, 77, and 20 K are depicted in Figure 2. No change in diffraction pattern with decreasing temperature was observed in the amorphous sample. On the other hand, in crystal samples, major peaks shifted to the higher diffraction angle (20) side, indicating that the crystal lattice of each TiOPc shrunk with decreasing temperature. The lattice shrinkage should result in a shortened distance between neighboring molecules and hence a change in intermolecular interactions. We can discuss how the crystal lattice shrinks with decreasing temperature on the basis of the crystal structure information obtained from the X-ray diffraction data. The absolute crystal structures of phase I and phase I1 TiOPcs have been determined by Hiller et al.5 By using their reported lattice constants and the coordinates, we could assign the main diffraction peaks as shown in Figure 2. In phase I1 TiOPc the interplanar distance of (212) was about 3.1 A, which seemed to correspond to the distance between neighboring Pc planes. With decreasing temperature, the diffraction peak assigned to (212) shifted to the higher angle side. This indicated that the distance between adjacent molecular planes decreased when the temperature was lowered. The shift Q 1994 American Chemical Society
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2 8 (degree) Qure 2, X-ray diffraction spectra of TiOPc vapor-deposited thin films measured at 298,77, and 20 K. Black circles indicate diffraction pcaks due to the IT0 mating on the glass substrate.
of the absorption spectra induced by lowering the temperature seemed to correlate with shrinkage of the crystal lattice. The absolute crystal structures of phase I and phase I1 TiOPcs determined by Hiller's group are displayed in Figure 3. In phase I1 (Figure 3b), molecules across the ac lattice plane can be considered to form a column, where T i 4 bonds are arranged in the same direction and the angle between the stacking axis and the Pc planes is small. This packing form seems to be similar to that observed in J-aggregates of some dyes which show a red shift of their abeorption spectra compared with the monomer spectra and a larger red shift for smaller stacking angle.6 With decreasing temperature, the decrease in the intermolecular distance and in the angle between the stacking axis and the Pc planes would result in a shift of the absorption band to longer wavelengths. The fact that phase I1 TiOPc has absorption bands in quite a longer
wavelength region than for the absorption spectrum observed in solution is also consistent with the view of a similarity to the J-aggregate form. In contrast to the phase I1 crystal, no thermochromic behavior was observed for phase I (Figure lb), although the shift to the higher angle side of the X-ray diffraction peak assigned to the (004) plane with decreasing temperature (Figure 2b) implied to a decrease in the distance between the molecular planes. A clear explanation for this dichotomy is not possible yet. In phase I (Figure 3a) one lattice includes four layers, and the TiOPc molecules are aligned together with equally directed T i 4 bonds in each layer. While the top and bottom layers and the two intermediate layers are parallel to each other, the two pairs are not parallel. The fact that the molecular planes between the two kinds of layers are not parallel may lead to a weaker dipole-
2128 The Journal of Physical Chemistry, Vol. 98, No. 11, 1994
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dipole interaction compared with phase 11, causing a difference in both the position of the longest absorption band and its thermochromic behavior from the phase I1 crystals. Recently, we reported that the electroabsorption spectra of TiOPcs depend on the crystal form. In electroabsorption spectra of phase I1 and Y-form samples, the second-derivativepatterns, which imply rich CT character in the excited states, wereclearly ob~erved.~ These samples showed higher photoconductivitythan other TiOPcs. Therefore, extensiveCT interactions in the excited state seemed to cause the high photoconductivity. Figure 4 indicates the electroabsorption spectra of phase I and phase I1 TiOPcs measured at 298 and 77 K. In phase I1 samples at the low temperature, the electroabsorption intensity decreased and the spectrum had a slight red shift compared with that observed at 298 K. Furthermore, the clear second-derivative pattern at 298 K was distorted at 77 K. The decrease in the electroabsorption intensity and the shape change of the spectrum suggested that the CT interactions in the excited state were weakened with decreasing temperature. If the lattice shrunk in a way such that the angle j3 (the angle between the a and c axes) became larger at the low temperature, the overlap between the adjacent molecules would probably decrease so that the CT interaction would be weakened. Again in contrast to phase 11, the phase I sample showed marginal changes except in the fine peaks appearing around 770 nmat 77 K. Thissuggested that in phaseIsample thecontribution
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800 900 1000 Wavelength (nm) Figure 4. Electroabsorptionspectra of phase I and phase I1 TiOPc thin films measured at 298 and 77 K. The applied fields were 9 X 104 (phase I) and 6 X 101 V/cm (phase 11). of CT interactions in the excited electronic states hardly changed with decreasing temperature. This conclusion was in agreement with the temperature effects of the absorption spectra. In all TiOPc samples except the amorphous one the maximum absorbance increased when the temperature was lowered to 77 K as shown in Figure 1. At present, the detailed reason was not clear. It is possible that the change of the mutual orientation between neighboring TiOPc molecules induced by loweting temperature will change the electronic transition probability. More detailed studies on temperature effectsof absorptionand electroabsorption spectra are now in progress.
References and Notes (1) Moser, F. H.;Thomas, A. L. The Phthalocyanines; CRC Press: Boca Raton, FL, 1983; Vol. I, pp 185-187. (2) Kakuta, A.; Mori, Takano, S.;Sawada, M.; Shibuya, I. J . Imaging Technol. 1985, 11, 7. ( 3 ) Saito, T.; Sisk, W.; Kobayashi, T.; Suzuki, S.;Iwayanagi, T. J. Phys. Chem. 1993, 97, 8026. (4) Saito, T. Unpublished raults. (5) Hiller, W.; Striihle, J.; Kobel, W.; Hanack, M. Z. Kristallogr. 1982, 159, 173. ( 6 ) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371.
