J. Phys. Chem. 1980, 84, 3223-3230 Shelef, M. J. Catal. 1969, 75, 289. Kasai, P. H. J. Phys. Chem. 1977, 87, 1527. Jacobs, P. A.; Buyer, H. K. J. Phys. Chem. 1979, 83, 1174. Durrant, P. J.; Durrant, B. “Introduction to Advanced Inorganlc Chemistry”, 2nd ed.; Wiley: New York, 1970; p 1025. Cotton, F. A.; Wilkinson, G. “Advanced Inorganlc Chemistry”, 3rd ed.; Interscience: New York, 1972; p 843. Kortum, 1G. “Reflectance Spectroscopy”; Springer-Verlag: Berlln, 1969. Lunsford, J. H.; Jayne, J. P. J. Chem. fhys. 1966, 44, 1487. Enemark, J. H.; Fleltham, R. D. Coord. Chem. Rev. 1974, 73,370. Fenske, R. F.; HnII, M. B. Inorg. Chem. 1972, 1 7 , 768. Clementi, E. IBM J. Res. Dev. 1965, 9 , 2. Radtke, [I. D. Ph.lD. Dissertation, University of Wisconsin, Madison, WI, 1966. Richardson, J. W.; Neiuwpoort, W. C.; Powell, R. R.; Edgell, W. F. J. Chem. fhys. 1962, 36, 1057. Carrlngton, A,; Mdachlan, A. D. “Introductionto Magnetic Resonance with Appllcation to Chemistry and Chemical Physics”; Harper and Row: Netw York, 1967; p 167. Anderson, J. H. J. Catal. 1973, 2 8 , 76. Poole, C. P.; MacJver, D. S.Adv. Catal. 1967, 77, 262. Zecchina,,A,; Garrone, E.; Ghiitti, G.; Mortena, L.; Borelio, E. J. phys. Chem. 1975, 79, 978. Laane, J.; Ohisenl, J. Adv. Inorg. Chem., In press. Windhorst, K. A,; Lunsford, J. H. J. Am. Chem. Soc. 1975, 97, 407. Niwa, M.; Minami, T.; Kodama, H.;Hattcri, T.; Murakaml, Y. J. Cafal. 1970, 53, 198.
3223
(34) Tevault, D. C.; Andrews, L. Spectrochim. Acta. f a r t A 1974, 30, 969. (35) Chao, C. C.; Lunsford, J. H. J. Am. Chem. Soc. 1971, 93, 71. (36) Nakamoto, K. “Infrared Spectra of Inorganic and Coordination Compounds”, 2nd ed.; Interscience: New York, 1963; p 160. (37) Cotton, F. A.; Wilklnson, 0. “Advanced Inorganic Chemistry”, 3rd ed.; Intersclence: 1972; p 696. (38) Beck, D.; Lunsford, J. H. J . Catal. I n press. (39) Bernai, I. Chem. Commun. 1965, 62, 571. (40) Vogt, F.; Bremer, H.; Rublnstejn, A. M.; Dasevskij, M. I.; Slinkln, A. A.; Klajcko, A. L. Z. Anorg. A@. Chem. 1976, 423, 155. (41) Sulelmanov, A. S.; Namazova, F. A,; Guseln-Zade, S. M. Issled. Ob/. Neorg. Fk. KMm. from Ref. Zh.,Khlm. 1971, Abstract No. 218577. (42) Kellerman, R.; Kller, K. “Molecular Sieves 11”; American Chemical Society: Washington, D.C., 1977; p 120. (43) Delgass, W. 1%; Garten, R. L.; Boudart, M. J. fhys. Chem. 1969, 73, 2970. (44) Huang, Y. Y.; Anderson, .I.R. J. Catal. 1975, 40, 143. (45) Nesterov, V. IK.; Mikheikin, I.D.; Khodakov, Y. S.; Kazanskii, V. B.; Minachev, K. M. Kinet. Katal. 1973, 74, 1348. (46) Gallezot, P.; Alarcon-Diaz, A,; Delmon, J. A.; Renouprez, A. J.; Imlii, B. J. Catal. 1975, 39, 334. (47) Jacobs, P. A.; Teilen, M.; Linart, J. P.; Uytterhoeven, J. B. J. Chem. Soc., Faraday Trans. 7 1976, 72, 2793. (48) Simpson, H. [I.; Stelnfink, H. J. Am. Chem. Soc. 1966, 97, 6225. (49) ”Handbook of Chemistry and Physics”, 54th ed.; CRC Press: Cleveland, OH, 1974; p D61.
Solvent Effects on Crystal Growth and Transformation of Zinc Phthalocyanine Fumio Iwatsu,+ Takashi Kobayashi, and Natsu Uyeda” Institute for Chemlcal Research, Kyoto University, vi,Kyoto-Fu 61 1, Japan (Received: December 26, 1979)
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The process of a P crystal transformation of zinc phthalocyanine in vapor of the various alcohols was studied by means of X-ray diffractometry and electron microscopy. The existence of the intermediate crystal form through which the CY form transformed into the P form was confirmed, and the crystallographic behavior of this form was investigated. The difference in the transformation process was interpreted in terms of the properties of alcohols, such as the ionization potential and the steric hindrance of alcohol as well as the solubility of zinc phthalocyanine in alcohol.
Introduction The dimorphic properties of various phthalocyanine compounds have been well known. Hamm and van Norman1 examinled the microcryst& of copper phthalocyanine as well as indanthrene by electron microscopy and diffraction. More detailed results on similar transformation of copper phthalocyanine taking place in an electron microscope were reported by Suito and Uyeda.2 In both cases they identified two crystal forms; the a form was produced by spontaneous recr:ystallization in the electron microscope when the original material of the 0 form was exposed to heavy electron bombardment. Some powdered crystals, which show the polymorphic transformatica on thermal treatment, also undergo similar phase changes when they are suspended in organic solvents. For instance, the powdered copper phthalocyanine of the metastable a form undergoes crystal transformation into the stable P form in various organic solvents used as suspension media.s ‘Theprocess of the transformation was investigated by X-ray powder diffraction and electron microscopy. It was found that the transformation was not a simple process, but the metastable powder grew to a considerable extent without changing its crystal structure before the actual transformation took place. The same ‘Department of Coordinated Science, Nagoya Institute of Technology, Gokiso-cho, Syowaku Nagoya 466, Japan.
0022-3654/80/2084-3223$0 1.OO/O
effect was observed more clearly in the case of vacuumcondensed small particles which grew to the metastable slender crystallites similar to those of the stable form. Hartman4demonstrated that the strong interaction in the structure has a remarkable influence on the crystal habit. In the case of phthalocyanine compounds, the crystal habits are decided by the x electronic interaction, which binds most strongly two neighboring planar molecules in the crystal. The molecular stacking5 of both slender crystallites are similar as shown in Figure 1, and slight differences in the manner of superposition of stacked planar molecules exist between both crystal forms. The appearance of such polymorphs is considered to be due to a slight difference in the x electronic interaction among stacked planar molecules. Besides the a P transformation, zinc, iron, manganese, and cobalt phthalocyanines are known to form many complexes with n-donor molecule^.^ New polymorphs were observed by Kobayashi et aL6in the intermediate stage of the a p transformation of zinc phthalocyanine, depending on whether the solvents in which the powder was dispersed are electron donors or electron acceptors. The results are listed in Table I. Up to now only very few results of basic investigations have been reported with respect to the mechanism of the transformation and the effect of solvents on the transformation process. The purpose of this paper is to elu-
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0 1980 American Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No. 24, 1980
Iwatsu et al.
TABLE I : Classification of Suspension Media and Their Effect in the Transformation and Complex Formation of Zinc Phthalocyanine benzene, toluene, and xylene 01 form is directly transformed to the p form n donors ethyl alcohol, acetone, ethyl ether, 01 form is transformed into the p form, weak n donors and carbon disulfide with intermediate stage of the metastable x form nitromethane, nitrobenzene, carbon tetrachloride, 01 form is transformed into the p form, acceptors and thiophene with intermediate stage of the metastable e form N-methylaniline and N, N-dimethylaniline strong n donors, unstable solvated complex is formed during the transformation steric hindrance pyridine, picoline, piperidine, methylamine, 01 form and the p form both give solvated strong n donors dimethylamine, dimethyl sulfoxide, and 1,4-dioxane complex with solvent molecule n
a-form .rl VI
8-form
O... zinc 0 . '
Nitrogen
Flgure 1. Schematic diagrams of lattice orientations and comparison of molecular stacking of zinc phthalocyanine in a and 0modifications: for these molecular stackings are based on the figure of Suito et copper phthalocyaninemodifing the spacings for zinc phthalocyanine.
cidate the overall process of the transformation and the effect of the interaction between phthalocyanine and solvents on the transformation mechanism. In the present work, the system of vacuum-condensed film of zinc phthalocyanine exposed to the vapor of various alcohols was investigated, and it was revealed that the ionization potential and the steric hindrance of alcohol molecules as well as the solubility of zinc phthalocyanine in alcohol have considerable effects on the formation of intermediate crystal form. Experimental Section Materials. Zinc phthalocyanine was prepared according to the method of Barrett et al.7 A mixture of o-phthalodinitrile and pure zinc dust was heated at 260-270 "C for 20 min. The product was ground after cooling and washed with hot acetone for 6 h and then washed with hot ethyl alcohol for 6 h in a Soxhlet extractor. The powder was purified by sublimation in a reduced nitrogen gas flow at -550 "C. The resulting needlelike cyrstals were found to be the 0 form of zinc phthalocyanine by X-ray diffraction analysis. The a-form powdered crystal was obtained by the following procedure. The needlelike crystals were completely dissolved in concentrated sulfuric acid, resulting in a viscous, dark-green paste. This paste was poured slowly into
a large amount of cold water and vigorously stirred, yielding fresh precipitates of zinc phthalocyanine. After filtration followed by repeated washing with water, the precipitates were again dispersed into pure water and boiled for 1 h. This treatment was repeated several times until the residue of sulfuric acid in the filtrate was not able to be detected by the addition of a drop of an aqueous solution of Ba(OH)? The precipitates were ground to a fine powder after drying. The powder was identified as the a form of zinc phthalocyanine by X-ray diffraction analysis. The organic solvents were commercial special grade and used without further purification. Procedure of Treatment. Zinc phthalocyanine was used in a form of powder in order to identify the crystal form by X-ray diffraction analysis. For the handling in electron microscopy and X-ray diffractometry, however, the main process of the crystal transformation of zinc phthalocyanine was investigated with thin films which were obtained by a vacuum-condensation method. A few milligrams of the pure P-zinc phthalocyanine crystals were sublimed from a small crucible of fused silica heated with a furnace of coiled tantalum wire and were condensed onto glass plates or quartz plates kept at room temperature. The pressure in the evaporator was -1 X lo* torr, and the film thickness was controlled in a range of 3000-6000 A for X-ray diffractometry and -1000 A for optical absorption spectra. These films on glass plates were placed in the vapors of alcohols, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, and tert-butyl alcohol, which were held in an air bath at constant temperatures, 45 and 55 O C , and regulated to an accuracy of ltO.1 "C. In order to trace the transformation of zinc phthalocyanine, the films on glass plates were successively taken out from the ambient vapor of individual alcohols and directly examined with an X-ray diffractometer at regular intervals of time. The operating conditions for the chart recording of the diffraction pattern were as follows: radiation, Ni-filtered Cu Ka! (A = 1.54 A); accelerating potential and tube current, 35 kV and 15mA, respectively; divergent angle of incident X-ray beam, 3.5 X rad (2'); receiving angle of reflected X-ray beam, 8.7 X low3rad (0.5O);scanning speed, 0.5 or loper minute; time constant, 4 or 2 s; record chart speed, 10 mm min-l. The kinetic treatment of crystal growth and the transformation process was not attempted with powdered materials, because the process was found to be affected by many uncertain factors. In order to observe the external appearance of individual crystals under transformation, we also prepared similar vacuum-condensed films on thin carbon films attached to copper grids for electron microscopy. The film thickness of the phthalocyanine was controlled at -500 A for this purpose. The specimens were treated similarly in the vapor of each alcohol and were examined by electron microscopy and selected area diffraction.
The Journal of Physlcal Chemistty, Vol. 84, No. 24, 1980 3225
Solvent Effects and Ziric Phthalocyanine
0
5
6
7 8 9 1 2 8 0)
0
5 6 7 8 9 1 0 28("1
5
6
7 8 9 2ev)
10
Flgure 3. Changes in the line profile of X-ray diffraction patterns of zinc phthalocyanine film undergoing transformation and growth at 55 OC: (A) in MeOH; (U)In i-BuOH; (C) in EtOH.
iris
TABLE I1 : Type of Transformation of Zinc Phthalocyanine Vacuum-Condensed Film on Glass Treated with Various Alcohol Vapor
TP, IP, temp, C eV K,,,a nm " C type MeOH 64.7 -97.8 10.85 0.206 (665.4) 45, 55 1 EtOH 78.4 -114.1 10.55 0.773 (666.5) 45 2 55 3 n-PrOH 97.2 -127.0 10.46 2.195 (667.1) 45 2 55 3 i-PrOH 82.5 -88.5 10.15 0.588 (666.3) 45 1 55 4 n-BuOH 117.5 --90.0 10.30 2.725 (668.9) 45, 55 3 i-BuOH 108.0 -108.0 10.17 0.689 (668.3) 45, 55 4 t-BuOH 82.9 24.7 9.92 0.500 (666.4) 45, 55 5 Type of transformation : 1 , a form -+ developed a form 2, a form + x form t form -+ developed x form t P form -+ p form 3, a form -+ x form t 0 form + p form 4, a form -+ x form 5,01form -+ a form t x' form -+ p form a Values in parentheses: extinction coefficient of maximum band of' absorption spectra of zinc phthalocyanine saturated solution of each alcohol at 25 " C and position of maximum band. solvent
5
10
15
20
28
25
30
35
("1
Flgure 2. X-ray diffraction patterns on three different forms of powdered crystal. The a form is obtained by the acid-paste method; the fl form is obtained by grinding crystals glven by sublimation: the x form is obtained by treatment of the CY form in i-PrOH at 55 OC for 24 h.
In an attempt to obtain the information on the changes of the electronic structure of phthalocyanine molecules in the crystal due to the interaction with alcohol, electronic absorption spectra and infrared absorption spectra were also measured. The specimen films on quartz plates were treated with the vapor of each alcohol for a few minutes and were kept in air for 24 h at room temperature in order to remove the vapor molecules condensed on the films, and then their absorption spectra were examined in a range of 350-750 nm. The absorption spectra of zinc phthalocyanine alcohol solutions saturated at 25 "C were also examined in the same way. For infrared spectra measurement, the zinc phthalocyanine was also sublimed on thin polyethylene film. The specimen thickness was controlled to be -5000 A. Polyethylene substrate was chosen to minimize the interaction between zinc phthalocyanine and the substrate. The specimens were treated in the vapor of each alcohol and examined with an infrared spectrometer at definite time intervals.
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Results Successive Trace of the a Transformation. Figure 2 shows X-ray diffraction patterns of the metastable a form, the stable P form, and the intermediate x form of individual powder crystals of zinc phthal~cyanine.~ The x form was obtained by dispersing the a-form powder into isopropyl alcolhol for 24 h at 55 "C. A common characteristic of these three diffraction patterns is that very strong reflection appears at lower angular positions corresponding to lattice spacings of 9-13 A. They are indicated in Figure 2 by letters m for the metastable a-form crystal, r and s for the stable P-form crystal, and x for the intermediate x-form crystal. Therefore the process of the transformation which is normally accompanied by crystal growth was followed by X-ray diffraction patterns in terms of lower angular position of the series of vacuum-condensed films treated with alcohols.
FP, C
The change in X-ray diffraction patterns obtained in sequence has been reproduced in Figure 3A-C for methyl, isobutyl, and ethyl alcohols, respectively. Three types of transformation and/or growth mechanisms of crystals were found. In the first type, the original metastable form does not change into the intermediate or the stable form but shows only the increase in crystal size as shown in Figure 3A (a a type). In the second type, the original film changes into the iintermediate form immediately after the contact of the specimen film with the vapor of alcohols, although the intermediate form does not change any further into the most stable p form as shown in Figure 3B (a x type). In the third type, the original film immediately changes into the intermediate form which finally changes 0 into the stable form as shown in Figure 3C (a x type). In the later stage of the last type, the intermediate form grows in some cases without changing its crystal form. These results are summarized in Table 11. Intensity changes of some X-ray diffraction peaks indicated as m, r, s, and x in Figure 2 were plotted as a function of the treatment time as shown in Figure 4. These peak intensities were also plotted on a logarithmic scale as shown in Figure 5. Since the crystal undergoing the growth transformation on the glass plate has a strong tendency to take lamellar shapes in preferred orientation,
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The Journal of Physical Chemlstry, Vol. 84, No. 24, 1980
Iwatsu et ai.
TABLE 111: Gradient of the Diffraction Intensity of x Form of the Plot in Figure 5
"Imax
solvent EtOH n-ROH n-BuOH
0
30
90 (min)
60
Flgure 4. Intensity variation of reflected X-ray from vacuum-condensed film of zinc phthalocyanine treated in n-BuOH at 55 O C as a function of time. Intenslty was estimated by the product of the peak height and the half-value width of the peak: (m) 12.96, a form: (r) 12.54 and (s) 9.485, @ form: (x) 11.4, form.
x
0
10
20
30
40
5 0 (min)
-0.4
-0.5
log(I/Imax)
Figure 5. Variation of the logarithm of the peak intensity of reflected X-ray from a vacuum-condensed fllm treated in n-BuOH at 55 O C as a function of time.
it is very difficult to elucidate quantitatively the amount of crystals of individual forms from the diffraction intensities for those samples. Nevertheless, as is apparent from Figure 5, the plots of the decrease of the x form and the increase of the p form are almost linear on a logarithmic scale in consecutive processes during the transformation, and this result indicates that the transformation of the x form into the p form is a first-order phenomenon with respect to the xform phthalocyanine. The rate constant of the transformation of the x form into the p form can be estimated from the gradient of the plot of the diffraction intensities of the x form in its decreasing stage. The values are listed in Table I11 together with the ionization potential of each alcohol and the optical absorbance of zinc phthalocyanine saturated solution of each alcohol from which the solubility of zinc phthalocyanine can be estimated. The rate constants show a tendency to increase with the decrease in ionization potential and the increase in solubility. The results suggest that the transformation is related somehow to the charge-transfer mechanism, although no definite formation of crystalline charge-transfer complex was detected by infrared and optical spectra, as will be discussed later. Changes in Crystal Shape. The changes in crystal shape as well as particle size can be clearly observed with a series of electron micrographs, displayed in Figure 6, which shows as an example the change of crystal habit in the vacuum-
IP, eV 10.55 10.46 10.30
Kmax
0.773 2.195 2.725
grad, h-'
-
- 0.269 - 0.436
- 0.614
condensed film treated in n-propyl alcohol at 55 O C . The total process in other alcohols follows the same pattern. The original particles of the a form changed into the crystals of the intermediate form which appeared in various shapes according to the species of the solvent. The intermediate crystals transformed finally into the large slender crystal lamellae of the form when exposed to the vapor of alcohols such as ethyl, n-propyl, and n-butyl alcohol (a x p type). In the vapor of other alcohols like isopropyl, isobutyl, and tert-butyl alcohols, the original crystals changed into the crystals of the intermediate form, and they remained unchanged even after a treatment of 10 days in these alcohol vapors at 55 "C (a x type). In both cases the growth of the a-form crystals was not observed in contrast with the case of the a a type of zinc phthalocyanine and the transformation of copper phthalocyanine.8 The X-ray diffraction pattern only showed the common peak of the x form at 11.4 A reported by Kobayashi et al.6 However, the selected area electron diffraction pattern revealed several different crystal forms at the intermediate stage of the transformation in these alcohol vapors, as this method is more sensitive for the detection of minor constituents. Results of Infrared Spectroscopy. The zinc phthalocyanine vacuum-condensed film on polyethylene was treated in alcohol vapor for the appropriate interval of time at room temperature and was examined with infrared spectroscopy and X-ray diffractometry. It was found that x transformation of zinc phthalothe rate of the a cyanine on polyethylene was extremely reduced as compared with that on glass plate, This reduction in the rate of the transformation makes it possible to investigate in more detail the process of the a x transformation which takes place too rapidly on glass plate to trace by a present IR method. The example of the change in infrared spectra was shown in Figure 7 with reference to X-ray diffraction patterns. In the infrared spectra (Figure 7B), an absorption at 1132 cm-l was observed for the specimen in the stage where no transformation into the x form took place despite the contact with alcohol vapor for 1 h. This absorption band, which could be assigned to C-0 stretching, ordinarily appeared at 1050 cm-l for primary alcohols existing as liquid and remained even when the specimen was dried in air. However, it disappeared not only when the speciment changed its crystal form into the x or p form but also when the specimen, treated in alcohol vapor, was placed in a vacuum (-1 X lo4 torr) for 10 h, although no change in X-ray diffraction pattern from the original was observed. The result indicates that alcohols play an important role in the process of the transformation of the a form into the x form. It seems that the original crystals of the a form are very strongly solvated with the condensed alcohol when they are placed in the vapor, and subsequently the solvents are excluded as the transformation into the x form or the 0 form proceeds. Results of Electronic Absorption Spectroscopy. The maximum extinction coefficients to zinc phthalocyanine alcohol solutions saturated at 25 "C are listed in Table 11, with which the solubility of zinc phthalocyanine has been estimated. In Figure 8, A and B, the absorption spectra
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-
-
-
Flaun 0. SBauOntiBi W o n micrommhs of zinc DMhabcvanhe Rm lndergohg mansfarmtionh nAOH at 55 OC: (A) 0 min: (B) 1 mh; (C) 5 i i n ; (0) 10 min; (E) 6 h: (F) 70-h.'
of ethyl alcohol solution and vacuum-deposited film treated ethyl alcohol vapor are reproduced, as an example, and the wavenumbers of the absorbances numbered 1.2, and 3 for the solution and 4 and 5 for the film are plotted against the ionization potential of the corresponding alcohols. In both graphs the wavenumber of each absorbance increased with the ionization potential as regards primary alcohols, which, except methyl alcohol, cause the a x B type transformation. And as regards other alcohols, the wavenumber and the ionization potential were against such a relation. This tendency was larger in vacuum-deposited film than in solution; it seemed to be due to the effect of the steric hindrance and the bulkiness of alcohols. The resemblance of both graphs suggests that although the solvents condensed on film were removed by placing the film in open air for 24 h a t room temperature, the phthalocyanine molecules in the a-form film are in an enviroment like that of the solution, presumably being somewhat solvated with alcohols.
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Discussion In the process of the transformation of zinc phthalocyanine in alcohol vapors, two steps are found to exist. One is the step from the original a form into the x form, and the other is from the x form into the stable B form. These steps will be discussed separately. In each step alcohols play an important role, and their effects are interpreted in t e r n of several factors, such as the ionization potential, the steric hindrance, and the solubility of zinc phthalocyanine in alcohols. In addition, some other factors which are less important are considered to affect the transformation and the crystal growth as well. Process of Transformation of the a Form into the x Form. The transformation is induced by a weak charge
transfer to zinc phthalocyanine from strongly adsorbed alcohol around the small a-form particles. A small change in the *-electron energy level of zinc phthalocyanine may occur through the charge transfer. The resulting r-electron distribution in the molecule requires a new packing mode in a crystal of the x form. Such a mechanism of the transformation has been suggested by the fact that the adsorbed alcohols surrounding the fine a-form particles cannot be removed easily even in air, whereas the alcohol molecule evaporates as soon as the a-form crystallites transform into the x form. The existence of a chargetransfer mechanism is confirmed by a strong band shift in the infrared spectra appearing a t 1132 cm-'. The absorption band of the C-0 stretching is observed usually at 1030,1070,1065,1060,and 1040 cm-', for methyl. ehtyl, n-propyl, n-butyl, and isobutyl alcohols, respectively, when they remain as liquid without charge-transfer effect? The rate of the transformation is very rapid as revealed by X-ray diffractometry. The crystals of the a-form phthalocyanine made by vacuum deposition are ordinarily very small, as shown in Figure 6A. The original a-form crystalline particles retain a large specific area which promotes the possibility for phthalocyanine to interact with alcohol molecules. Although the thickness of the specimen films for X-ray studies are -3oo(t6000 A, each crystallite in the film is also very small as estimated from the half-value width of the X-ray diffraction pattern by using the equationg DMl = 0.9h/B1,2 coe 8. The crystallite size is found to be 147 A for Dm as shown in Figure 9, rad, 8 is 6.9/2", and A is 1.54 A where BL,2 is 9.38 X (Cu Ka). As a unit cell length of the a axis is 25.9 A for the a form, the mean thickness of each crystallite in this direction is less than six unit cells length. These small a-form crystals have a large specific area extending up to
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The Journal of Physical Chemistty, Vol. 84, No. 24, 1980
Iwatsu et al.
As shown in Table 11, the a form is transformed into the
P form through the intermediate x-form crystals in the vapor of straight-chain primary alcohols, such as ethyl, n-propyl, and n-butyl alcohols. Table I11 shows that the rate constant of transformation of the x form into the form increases with the decrease in the ionization potential or the increase in the electron-donating ability of alcohol as well as with the increase in solubility of zinc phthalocyanine in alcohol. Uyeda et a1.I0 have revealed that, in the case of copper phthalocyanine, aromatic solvents, such as xylene, toluene, and benzene, are active on the a! transformation with this sequence which corresponds to the increase of the a-electron-donating ability. The aelectronic interaction between stacking molecules is most strong along the b axis in the crystal of phthalocyanine, as shown in Figure 1. The lattice period along the b axis of the crystal is 3.78 A for the a! form and 4.85 A for the P form. Ordinarily the a-electron system is considered to be the origin of interaction between the planar molecules forming a column in a crystal. The change in the a-electron density causes the change in the intermolecular distance as well as the change in the a-electron overlapping, which are primary factors of transformation. In the present system the observed relationship between the electron-donating ability and the rate of transformation gives active support to the inference that the lone-pair electrons of the oxygen atom in an alcohol molecule affect the a-electronic state of the phthalocyanine through the central metal ion, as was observed in the case of strong electron-donating molecules such as amine.ll On the other hand, though the solubility of phthalocyanine in alcohol is extremely small, it may be reasonable to consider that the rearrangements of molecules take place at some corners of individual crystallites. Here the solvent acts to relax the crystal lattice and triggers the formation of nuclei of the stable f h m . If individual molecules are given enough freedom, they finally settle themselves in the new stable position in the crystal. This inference is also supported by the fact that the higher solubility gives the higher rate of transformation. In some other alcohols, such as isopropyl, isobutyl, and tert-butyl alcohols, the x form did not transform into the P form in spite of the considerably low ionization potential and the high solubility. In the case of isopropyl and tert-butyl alcohols, which are secondary and tertiary alcohols, respectively, these results may be considered to be due to the steric hindrance around the oxygen atom, preventing the lone-pair electrons on the oxygen atom from interacting with the central metal of phthalocyanine. However, in the case of isobutyl alcohol (primary alcohol) where the steric hindrance around the oxygen atom does not exist, this idea cannot explain the result. Isobutyl alcohol has a branched chain and may not break into the crevice among phthalocyanine molecules in a crystal because of its bulkiness. The most reasonable explanation is that the lone-pair electrons on the oxygen atom of alcohol interact with phthalocyanine molecules through the central melt not only at the surface but also at the inner part of crystals to some extent to change the a-electronic state of phthalocyanine and further to change the potential between phthalocyanine molecules in the crystal from which the transformation of the metastable phthalocyanine crystals into the stable ones is triggered. Temperature Effect. The phthalocyanine crystallites on the glass plates are considered to be surrounded by saturated alcohol solution as the result of interparticle condensation of alcohol by a capillary effect. In this sense, the system studied here is a solid-liquid system rather than
-
5
6
7
8
ze(")
9
1200 1100 1000
Wavenumber (cm-l)
Figure 7. Sequential infrared spectra and X-ray diffraction patterns of films on polyethylene: (A) original vacuum-condensed film on polyethylene, a form; (B)specimen obtained from the treatment of specimen A in n-BuOH at 55 O C for 1 h; (C) specimen obtained from keeping specimen B under vacuum (lo-' torr) for 10 h; (D) specimen obtained from the treatment of specimen C in n-BuOH at 55 OC for 24 h, x form: (E) specimen obtained from the treatment of specimen D in n-BuOH at 55 ' C for 48 h, /3 form.
250 m2/g. These large specific areas of the original a! form will explain the rapid rate of the transformation, in the case of the X-form crystals which are not additive complexes of zinc phthalocyanine and alcohol. The alcohol molecules are not adsorbed so strongly and cannot remain on the crystal when the specimen film is placed in open air. This fact also suggests that the charge transfer takes place only on the crystal surfaces or at a depth of a few molecular layers. When the X-form crystals grow into large crystals through Ostwald's ripening mechanism, the phthalocyanine molecules in the inner part of the crystal suffer no effect of charge-transfer interaction, and hence a new stable nuclei of the /3-form crystal has a chance to form. The total free energy of the system decreases at this stage because the specific area of the crystals has decreased, and their growth occurs gradually at this stage, as will be discussed in the next section. Process of Transformation of the x Form into the P Form. This process is greatly affected by the properties of alcohol in contrast with the preceding a x process. -+
The Journal of Physical Chemistty, Vol. 84, No. 24, 1980 3229
Solvent Effects and Zinic Phthalocyanine
I
(B) vacuum-condensed f i l m
~~
400
500
600
I
700 (nm)
400
WAVE LENGTH t-BuOH
i-PrOH n-BuOH I-BuOH
n-PrOH EtOH
MeOH
t-BuOH
I
I
I
600 700(nm) WAVE LENGTH i-PrOH n-BuOH n-PrOH I-BuOH EtOH 500
I
1
1
MeOH I
16.30b
0
z er;
14*70
0
5 14*60 -
'
14.50
3
0
14.40 14.30
9.92
10.15 10.30 10.46 10.17 10.55
-
-
9.92
10.85 (ev) I . P .
10.15
10.30
LO. 17
10.46 10.55
10.85 (ev) I . P .
Figure 8. Opticis1 absorption spectra and wavenumber of ab?--ption as a function of the ionlziitlon potential of alcohol: (A) saturated solution of zinc phthalocyanine in alcohol at 25 "C; (B) film treated for a few minutes and kept in air for 24 h; the ordinate represents the wavenumber of the absorption peak and the abscissa represents the ionization potential of alcohol (kllokayser = 1000 cm-').
Figure 9. Half-value width of X-ray diffraction attern of the a form of zinc Phthalocyanine film. /3,* is 9.38 X 10- rad, 0 is 6.9/2", and A is 1.54 A.
P
a solid-vapor system. Therefore, the change of the vapor pressure with the teinperature will not affect the rate of transformation. The temperature effect on the transformation and growth process appears through the solubility, the diffusion rate, and the lattice vibration of the phthalocyanine molecules. In fact, the a x transformation with isopropyli alcohol did not occur at 45 "C, while this transformation proceeded at 55 "C. More detailed results will be obtained by controlling the temperature at a wider range. Other Factors. It was reported that the metastable powder crystal grew to a considerable extent before the actual transformation without changing its crystal structure in the process of the transformation of copper
-
phthalocyanine in organic suspension media: and that the a-form particle in evaporated thin films grew also in the initial stage of the formation of a charge-transfer complex of zinc phthalocyanine with pyridine.12 In the present case, the a-form particles treated with methyl alcohol do not change into the x or p form but grow remarkably without changing their crystal structure, as shown in Figure 3A. The particle growth of the x form preceeding the transformation into the ,f?form was also observed when the a-form fine particle was treated with ethyl and n-propyl alcohols at 45 "C. These results are reasonably explained by the concept known as Ostwald's ripening based on the balance between the surface energy and the internal energy of the fine crystal. The crystal structure of the intermediate form also may be decided by the balance between them.
References and Notes (1) (2) (3) (4)
F. Hamm and E. van Norman, J . Appl. Phys., 19, 1097 (1968).
E. Suit0 and N. Uyeda, Proc. Jpn. Acad., 33, 398 (1957). E. Sulto and N. Uyeda, Kolloid Z. Z . Po/ym., 193, 7 (1963). P. Hartman and W. G. Perdok, Acta Crystallogr.,8,49 and 521 (1955); P. Hartman, "Crystal Growth: An Introductlon", North-Holland Publishing Co., Amsterdam, 1973, p 358. (5) J. M. Assour, J. Am. Chem. Soc., 87, 4701 (1965); C. Ercolani, J . Chem. SOC.A , 2123 (1968); D. V. Stynes and B. R. James, J. Am. Chem. Soc., 96, 2733 (1974); J. F. Myers, G. W. R. Canham, and A. B. P. Lever, Inorg. Chem., 14, 461 (1975); F. Carlatl, D. Galkoll, F. Morauonl, and C. Busetto, J. Chem. Soc.,Dakon, Trans., 556 (1975); F. Cariati, F. Morauonl, and C. Busetto, lbld., 496 (1976); D. A. Sweigart, ibM., 1476 (1976); F. Carlatl, F. Morauonl, and M. Zocchl, ibid., 1018 (1978); R. 0. Loutfy and J. H. Sharp, J. Phys. Chem., 82, 2787 (1978). (6) T. Kobayashl, N. Uyeda, and E. Sulto, J. Phys. Chem., 72, 2446
(1968).
J. Phys. Chem. 1980, 84, 3230-3232
3230
(7) P. A. Barrett, C. E. Dent, and R. P. Linstead, J. Chem. Soc., 1719 (1936). (8) H. A. Szymanski, "Interpreted Infrared Spectra", Vol. 2,Plenum Press, New York, 1964. (9) W. L. Bragg, "The Crystalline State", Vol. 1, 1933,p 189;C. C. Murdock, Phys. Rev., 31, 304 (1928);B. E. Warren, 2.Krisfallogr.,
99, 488 (1938). (10) Unpublished results. (11) T. Kobayashi, T. Ashida, N. Uyeda, E. Suito, and M. Kakudo, Boll. Chem. SOC. Jpn. 44, 2095 (1971). (12) Y. Saito, T. Kobayashi, N. Uyeda, and E. Suito, Bull. Insf. Chem. Res. Kyoto Unlv., 49, 256 (1971).
Chemisorption of Methanol on Magnesium Oxide. Observations by Carbon-I 3 NMR Ian D. Gay Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A lS8,Canada (Received: December 27, 1979; In Final Form: June 18, 1980)
The adsorption of methanol on magnesium oxide is studied by high-resolution solid-state 13CNMR techniques. It is found that adsorption to coverages of less than about half a monolayer produces a rigidly bound methoxide-like species. Higher coverages produce in addition an isotropically rotating methanol species.
Carbon-13 NMR has become a nearly routine method for studying weakly adsorbed species on solid s~rfaces.l-~ In order to obtain useful results by conventional techniques, one must use adsorbed molecules that undergo isotropic rotation on a timescale of less than lo4 s, in order that the C-H dipolar interactions be averaged to small values. Such techniques are therefore not applicable to strongly chemisorbed species. Recent advances in highresolution solid-state NMR techniques4 have shown that this dipolar coupling may be suppressed by high-power irradiation of the protons, and high-resolution 13Cspectra obtained. The decoupling may readily be combined with cross polarizationH to produce a valuable increase in signal to noise. Surprisingly, these newer techniques have been little used in surface studies. There exist to date only two studies7i8in which results have been achieved at natural abundance, an equal number7v9with enriched compounds, and a few studies1&12of tightly bound aprotic molecules. One reason for this is that 13C shielding anisotropies can be rather large. This aggravates the signal-to-noise problem, and precludes surface studies on complicated molecules, where the necessity for polycrystalline samples would lead to intractably overlapping powder patterns. An obvious solution to this problem is the use of selectively enriched compounds. The utility of this approach has recently been demonstrated by SefcikSg In the present work we present the results of studies of methanol adsorbed on MgO, at natural isotopic abundance. The magnesium oxide was prepared from reagent grade magnesium hydroxide (Matheson Coleman and Bell). The hydroxide was decomposed under vacuum at 300 "C in a thin bed. This gave a product having a surface area of 250 m2/g. The initial oxide was then loaded into 12-mm NMR tubes and slowly heated to 500 "C in vacuo. The oxide samples had areas of 200-210 m2/g after this treatment. Measured amounts of methanol vapor were then allowed to adsorb from the gas phase, with the oxide at room temperature. The samples were then sealed off and allowed to stand for several days at room temperature before NMR measurements. 0022-3654/80/2084-3230$0 1.OO/O
TABLE I molecule 6 la 8 1IU HOCH, 73 10 KOCH, 78 18 Mg(OCH3 I* 74 10 Ca(OCH3l2 73 20 In parts per million downfield of liquid tetramethylsilane.
Samples of potassium, magnesium, and calcium methoxides were used for comparison. These were prepared by reaction of the appropriate metal with dried methanol, followed by heating in vacuo at a sufficient temperature to decompose s0lvates.~~9~~ These samples were kept sealed under vacuum. Spectra were measured at 15.08 MHz on a modified TT-14 spectrometer. Single-contact Hartmann-Hahn cross polarization4 was used, with rf field strengths of 70 kHz (yH1/27r) at both frequencies, and a contact time of 2 ms. The proton TIvalues of the adsorbed species lay in the range of 50-200 ms, permitting a relatively high data rate. Sample temperature during the measurements was 35 f 5 "C. The spectra were not changed on repetition after some months storage of the samples at room temperature. The solid methoxides were all found to give an axially symmetric shielding pattern, which could be well fitted by the theoretical powder pattern convolved with a Lorentzian line shape. The results of these computer fits are given in Table I, together with our measurement of solid methanol. The uncertainties in these data are estimated as f 2 ppm, based on the results of least-squares fitting and on reproducibility of different measurements on the same sample. In the case of magnesium methoxide, reproducibility was also checked on samples from different preparations, and agreement within the above limits obtained. Our result for solid methanol agrees with the values of 74 and 11 f 6 ppm found by Pines et al.16 Figure 1 shows the spectra obtained for the various adsorbed samples. The vertical lines in this figure represent the shielding components of magnesium methoxide, as given in Table I. At low coverage, it can be seen that 0 1980 American Chemical Society
