Derivatives of phthalocyanine prepared for deposition as thin films by the Langmuir-Blodgett technique

Thin Solid Films, 133 (1985) 197 206

197

PREPARATION AND CHARACTERIZATION

DERIVATIVES O F P H T H A L O C Y A N I N E P R E P A R E D F O R D E P O S I T I O N AS T H I N F I L M S BY THE L A N G M U I R - B L O D G E T T TECHNIQUE* W. R. BARGER, A. W. SNOW, H. W O H L T J E N t AND N. L. JARVIS~

Chemistry Division, Naval Research Laboratory, Washington, DC 20375-5000 (U.S.A.) (Received June 14, 1985; accepted September 19, 1985)

Film pressure v e r s u s area data for a series of organic derivatives of phthalocyanine are presented in detail. Classical monomolecular films were not observed. Compounds studied include tetraphenoxy, dicumylphenoxy and tetracumylphenoxy, tetraoctadecoxy and tetraneopentoxy phthalocyanine. Spread films of metal-substituted tetracumylphenoxy phthalocyanines that contained iron, cobalt, nickel, copper, zinc, palladium, platinum and lead were also examined along with 1 : 1 mole ratio mixed films of these compounds with octadecanol. Mixed films with varying mole ratios of nickel tetracumylphenoxy phthalocyanine and octadecanol are also described. On the basis of these data and considerations of recent studies of t e t r a - t e r t - b u t y l phthalocyanine by other investigators, a stacked phthalocyanine structure is indicated. The stack axis is not parallel to the plane of the film.

1. INTRODUCTION For more than 20 years it has been known that phthalocyanines could interact with gases 1, and in 1978 Sadaoka e t al. 2 described a sensor using a thin film of metalsubstituted phthalocyanine that changed conductivity on exposure to certain gases. The use of the Langmuir-Blodgett (LB) technique for precise control of the deposition of thin films of phthalocyanines for gas sensing has been of interest recently. Baker and coworkers have reported on gas-sensitive LB films of asymmetrically-substituted copper phthalocyanine 3, and in earlier work described the preparation of monolayers of metal-free phthalocyanine and t e t r a - t e r t - b u t y l phthalocyanine 4. Recently, Fryer et al. 5 have described imaging by electron microscopy of t e t r a - t e r t - b u t y l phthalocyanine monolayers, and Kovacs e t al. 6 have described the structure ofmonolayers of this material in detail. In 1984 we reported on the synthesis of a series of tetracumylphenoxy * Paper presented at the Second International Conference on Langrnuir-Blodgett Films, Schenectady, NY, U.S.A.,July 1-4, 1985. t Present address: MicrosensorSystemsInc., P.O. Box 90, Fairfax, VA 22030, U.S.A. ++Present address: Chemical Research and Development Center, SMCCR-RS, Aberdeen Proving Ground, MD 21010, U.S.A. 0040-6090/85/$3.30

© Elsevier Sequoia/Printed in The Netherlands

198

w.R. BARGERe t al.

derivatives of phthalocyanine prepared specifically for use in the formation of monolayer films 7. The ultimate aim of our research in this area is to produce a very small gas sensor that utilizes a gas-sensitive semiconducting coating film on the surface ofmicrofabricated interdigital electrodes. Such coated devices, which change their electrical resistance as a result of chemical interactions with certain vapors, are called chemiresistors. We have been successful at preparing chemiresistors from tetracumylphenoxy phthalocyanine and its iron, cobalt, nickel, palladium, platinum, copper, zinc and lead complexes. Changes in the conductivity of the chemiresistors prepared with 45-layer LB films of the above series of compounds mixed with octadecanol were determined for low concentrations of the vapors of N H 3 (2 ppm), SO2 (20 ppm) and dimethyl methyl phosphonate (2 ppm). Details of that series of experiments involving exposing gases to chemiresistors have been reported elsewhere 8. Descriptions of the test apparatus and electrodes have also been published 9. An interesting observation was that the strongest responses, specially for N H 3, were with metal-substituted tetracumylphenoxy phthalocyanines containing metals of the d s and d 9 electron configurations, i.e. nickel, palladium, platinum and copper. Other observations were that nickel, palladium, platinum and copper were more highly associated in solution than the other metal-substituted tetracumylphenoxy phthalocyanine compounds investigated, according to vapor phase osmometry (VPO) measurements and that conductivity was greatly reduced if the mixed monolayer film temperature was raised above the melting point of the octadecanol, resulting, presumably, in a homogeneous film compared with the ordered LB film that was deposited. In a recent paper on electrically conductive macrocyclic assemblies, Marks 1° pointed out that, when planar conjugated molecules are arranged in a stack, an extended path for electronic charge movement exists, and one method to achieve that condition is by covalently linking phthalocyanines in a cofacial orientation. Deposition in LB layers may be another. A connection between conductivity and the structure of the LB film is indicated, so we are attempting to define the structure of the LB films made with these materials in order to guide the synthesis of improved gas-sensitive coatings. We report here a more thorough study of the film pressure v e r s u s area characteristics of these materials and of their mixtures with octadecanol. 2.

EXPERIMENTAL DETAILS

Syntheses of compounds described here were carried out by the procedures described by Snow and Jarvis 7. Characterization was by electronic, IR, nuclear magnetic resonance and electron spin resonance spectra of the compounds. In addition, molecular weights of the metal-substituted tetracumylphenoxy phthalocyanine compounds were also obtained by fast atom b o m b a r d m e n t mass spectrometry11. Recently, resonance Raman spectra of monolayers on water and deposited multilayers of several of the compounds have been measured 12. Figure 1 illustrates the series of compounds studied. The compounds are mixed isomers with the sidegroups at either the 2 or 3 position of each benzo ring in the phthalocyanine molecule. The side-groups in Fig. 1, identified from top to bottom, are phenoxy, cumylphenoxy, octadecoxy and neopentoxy.

199

LB FILMS OF PHTHALOCYANINE DERIVATIVES

R

00 R

R

CH

-0--~-~

I 3

CH 3

~.

M

-O-(CH~z-CH a

/,N

N..... ~ N ' ~ '

R

~ H3

R

CH 3

Fig. 1. Organic derivatives of phthalocyanine examined in this study. From top to bottom the pendant groups are phenoxy, cumylphenoxy, octadecoxy and neopentoxy.

Film pressure v e r s u s area measurements and multilayer depositions were carried out in a constant temperature room using a thermostated (25 °C) rectangular trough of our own design, based on earlier designs by Zisman and coworkers used in this laboratory for over 30 years. An illustration of this type of trough appears in the paper by Wohltjen e t al. 9 The paraffin-coated trough (14 cm wide; 3 mm deep) had a 7.5 cm deep well near one end. Surface tension was measured by the Wilhelmy plate technique using a platinum foil plate 1.7 cm wide suspended from a Gould UC-2 strain gauge. The Wilhelmy plate, screw-driven bar and dipping device were all interfaced to a microcomputer. With this automated apparatus over 100 layers of the mixed octadecanol-metal-substituted tetracumylphenoxy phthalocyanine films could be routinely deposited onto small electrodes or silica plates. The screw-driven bar reduced the area at a fixed rate of 6.7 x 10- 5 m 2 s- 1. For nearly all the runs reported here, 0.2 ml of 4 x 10 -4 M solutions was spread from a 0.25 ml capacity micrometer-type pipet. Substrates for deposition were subjected to a final cleaning with freshly distilled chloroform in a Soxhlet extractor. They were then submerged in the triply distilled water of the trough prior to film spreading. Deposition was begun by raising the device from below the water surface. The velocity of the dipping device was 4.2 x 10 4 m s 1. Attempts to transfer the pure metal-substituted tetracumylphenoxy phthalocyanine compounds to solid substrates resulted in poor quality multilayer films. Since phthalocyanines are intensely colored, irregular deposition is easily observed. However, metal-substituted tetracumylphenoxy phthalocyanines mixed with octadecanol as a transfer promoter produced high quality multilayer films of the Y type with a good 1: 1 match of the area change on the water surface with the product of the surface area of the solid and the number of times it passed through the interface. Absorption spectra of films deposited onto fused silica microscope slides 25 mm x 50 mm were recorded from 250 to 750 nm with a VarianTechtron UV-visible spectrophotometer. The coated slide was put in the normal location for an optical cell, and an uncoated slide was placed in the reference cell location.

200

3.

W.R.

BARGER et al.

RESULTS A N D DISCUSSION

3.1. Metal-free phthalocyanine derivatives Figure 2 shows the film pressure v e r s u s area data for a series of organic derivatives of metal-free phthalocyanine. The full curves represent materials synthesized and studied in our laboratory. All the curves reported here represent at least duplicate runs that gave identical results. The labels in Fig. 2 correspond to the following derivatives of metal-free phthalocyanine spread from chloroform: curve a, tetraphenoxy phthalocyanine; curve b, tetracumylphenoxy phthalocyanine; curve c, tetraoctadecoxy phthalocyanine; curve d, dicumylphenoxy phthalocyanine; curve h, tetraneopentoxy phthalocyanine. Curve e is for tetraoctadecoxy phthalocyanine spread from toluene and curve f is for tetracumylphenoxy phthalocyanine spread from benzene. Collapse of films a, b and c near 27 m N m - t is indicated by the arrow in Fig. 2. An expansion of the film pressure v e r s u s area curve was observed when benzene and toluene were used as solvents for the compounds that had large side-groups. Whether these two aromatic solvents remained trapped in the spread films or altered the average degree of aggregation of monomers is unknown. To avoid the possibility of incorporation of aromatic solvents in the films, chloroform was used as a spreading solvent. The broken lines in Fig. 2 indicate data for tetrat e r t - b u t y l phthalocyanine reported by Kovacs e t al. 6 (curve g) and by Fryer e t al. 5 (curve i). Toluene or toluene-chloroform-mesitylene was used as a solvent for curve g and xylene for curve i.

3.2. Metal-substituted tetracumylphenoxy phthalocyanines The film pressure v e r s u s

area isotherms for a series of metal-substituted

tetracumylphenoxy phthalocyanine compounds are shown in Fig. 3. Identification 3O 7 E

3O

20

' '. ~'-.-h

El0 E h2

\

~ 20 ~.

~ 10

E 0

0

20 Area

40

60

(A 2 /monomer)

80

100

0 0

20 Area

40 60 80 (A2 /monomer)

100

Fig. 2. Fi•mpressurevs.areais•thermsf•r•rganicderivatives•fmeta••freephtha••cyaninespreadfr•m c h l o r o f o r m : c u r v e a, t e t r a p h e n o x y p h t h a l o c y a n i n e ; c u r v e b, t e t r a c u m y l p h e n o x y p h t h a l o c y a n i n e ; c u r v e c, tetraoctadecoxy p h t h a l o c y a n i u e ; c u r v e d, d i c u m y l p h e n o x y p h t h a l o c y a n i n e ; c u r v e h, tetraneopentoxy p h t h a l o c y a n i n e . C u r v e e represents tetraoctadecoxy phthalocyanine spread from toluene, while c u r v e f shows tetracumylphenoxy phthalocyanine spread from benzene. The broken c u r v e g indicates data of K o v a c s et al. 6 for tetra-tert-butyl phthalocyanine spread from toluene o r t o l u e n e - c h l o r o f o r m mesitylene, and c u r v e i is data from Fryer et al. ~ for tetra-tert-butyl phthalocyanine spread f r o m xylene. Fig. 3. F i l m p r e s s u r e vs. area isotherms for metal-substituted tetracumylpheno×y phthalocyanines spread f r o m c h l o r o f o r m . In s o m e cases the curves coincide. There is more than one curve for s o m e c o m p o u n d s . I d e n t i f i c a t i o n o f the curves b y the central atoms of the ring is as follows: c u r v e a, H2 and nickel ; c u r v e b, c o b a l t ; c u r v e c, lead and nickel ; c u r v e d, i r o n ; c u r v e e, c o b a l t ; c u r v e f, p a l l a d i u m ; c u r v e g, platinum; c u r v e h, p l a t i n u m ; c u r v e i, zinc; c u r v e j, palladium.

201

LB FILMS OF P H T H A L O C Y A N I N E DERIVATIVES

of the curves by the central a t o m of the phthalocyanine is as follows: curve a, H2 and nickel ; curve b, cobalt; curve c, lead and nickel; curve d, iron; curve e, cobalt; curve f, palladium; curve g, platinum; curve h, platinum; curve i, zinc; curve j, palladium. The solvent was chloroform in every case. As in Fig. 2, the arrow indicates the collapse of the films near 27 m N m - 1 . Each reported curve represents at least duplicate identical runs. In the cases of cobalt, nickel, palladium and platinum two different curves are shown in Fig. 3. The repeated entries are for runs with different operators, troughs, solutions or c o m p o u n d s from a different synthesis batch. (The c o m p o u n d s were prepared as mixed isomers.) An influence of the metal on the film area is observed as noted in our previous paper on films spread from benzene 7. Films of palladium, platinum and zinc clearly occupy more area per m o n o m e r than the other compounds. Table I lists the average values and standard deviations of film area at several selected film pressures for the whole series of 13 pure metalsubstituted tetracumylphenoxy phthalocyanine compounds. In general, for the whole class, the area per m o n o m e r is surprisingly small. These data suggest the presence of aggregated molecules in the films. There is evidence from V P O measurements 7 that the metal-substituted tetracumylphenoxy phthalocyanine c o m p o u n d s are already associated in solution prior to spreading as films. In general, the association is increased with increasing concentration. Early film studies were made with more concentrated (1 m g m l 1) chloroform solutions. We have consistently observed that the c o m p o u n d s indicated by V P O measurements as more associated in solution produced film pressure versus area isotherms with larger average areas per monomer. This is the reverse of what would be expected from a simple vertically stacked c o m p o u n d . Tilting of stacks and variations of the tilt angles with different central metals in the phthalocyanines might be a possible explanation, but this remains to be proved. TABLE l AVERAGE VALUES OF FILM AREA AT SELECTED FILM PRESSURES

Film pressure (mN m 1)

A veragefilm area for pure M- Pc ( Cp ) a a compounds (Fig. 3) ( ~ m o n o m e r - ')

Average film area]br 1:1 mixtures with octadecanol b (Fig. 4) (~2 m o n o m e r l)

1 5 10 15 20 25

59.7±12.6 44.8±8.7 41.2±7.9 38.7±7.5 36.4±7.1 34.2±6.7

37.0±5.9 29.5±3.1 28.2±2.8 27.0±2.6 26.2±2.5 25.4±2.5

" M-Pc(Cp)4, m e t a l - s u b s t i t u t e d t e t r a c u m y l p h e n o x y p h t h a l o c y a n i n e . b The area is divided by the n u m b e r of o c t a d e c a n o l plus M-Pc(Cp),~ m o n o m e r s .

3.3. M i x e d f i l ms with octadecanol

Since p o o r quality LB multilayer films were obtained from the singlec o m p o n e n t metal-substituted tetracumylphenoxy phthalocyanine films discussed above, octadecanol was used to prepare mixed films which were easily transferred to solid surfaces. All the film pressure versus area curves fell within the b a n d shown in

202

w . R . BARGER el al.

Fig. 4. T a b l e 1 a l s o lists t h e a v e r a g e a r e a a n d s t a n d a r d d e v i a t i o n a t s e l e c t e d film p r e s s u r e s for t h e series o f m i x e d films s h o w n in Fig. 4. A series of films o f n i c k e l t e t r a c u m y l p h e n o x y p h t h a l o c y a n i n e m i x e d in v a r y i n g proportions with octadecanol showed the typical behavior of the cumylphenoxy d e r i v a t i v e s . T h e c u r v e s for t h e s e films a r e s h o w n in Fig. 5. T h e c o r r e s p o n d i n g n u m e r i c a l d a t a a r e g i v e n i n T a b l e II. T h e c u r v e s in Fig. 5 a r e a s s o c i a t e d w i t h t h e f o l l o w i n g m o l e f r a c t i o n s o f n i c k e l t e t r a c u m y l p h e n o x y p h t h a l o c y a n i n e : c u r v e a, p u r e o c t a d e c a n o l ; c u r v e b, 0.169; c u r v e c, 0 . 3 3 6 ; c u r v e d, 0.504; c u r v e e, 0 . 6 7 0 ; c u r v e f, 0 . 8 0 2 ; c u r v e g, 0.835; c u r v e h, 1.00. D a t a i l l u s t r a t i n g t h e v a r i a t i o n in a v e r a g e a r e a p e r m o n o m e r w i t h m o l e f r a c t i o n a r e s h o w n in Fig. 6 for a series o f d i f f e r e n t film pressures.

1 /

l°I I, 0

20

40

60

80

100

20

0

40

60

Area (A 2 m o n o m e r ) Area (A2 /monomer) Fig. 4. Film pressure vs. area isotherms for 1:1 mole ratio mixtures of metal-substituted tetracumylphenoxy phthalocyanines spread from chloroform. Fig. 5. Film pressure ts. area isotherms for mixtures of nickel tetracumylphenoxy phthalocyanine and octadecanol with the following mole ratios of the nickel compound: curve a, pure octadecanol; curve b, 3.169: curve c, 0.336: curve d, 0.504; curve e, 0.670: curve f, 0.802; curve g, 0.835: curve h, 1.00.

TABLE II MIXED FILMS O1- N I C K E L T E T R A C U M Y L P H E N O X Y P H T H A L O C Y A N I N E AND O C T A D E C A N O L

Moh: fi'action ~?fNiPc(Cp).~ ~

0 0.169 0.336 0.504 0.670 0.802 0.835 1.000

At~erage m i x e d film areasb (~ 2 m o n o m e r

t ).[br the [bllowing f i l m pressures

(mN m ~) l

5

10

15

20

25

21.7 24.5 26.9 32.0 36.2 41.0 44.4 49.9

20.6 23.3 24.4 25.8 26.7 31.4 32.0 38.2

20.4 22.9 23.8 24.5 24.9 28.9 29.2 34.9

20.2 22.6 23.4 23.6 23.9 27.2 27.5 32.6

20.0 22.3 23.0 22.9 23.0 25.7 25.9 30.4

19.8 22.0 22.6 22.3 22.2 24.2 24.5 28.3

"NiPc(Cp)4, nickel tetracumylphenoxy phthalocyanine. b The area is divided by the number of octadecanol plus NiPc(Cp),, monomers. M i x e d films w e r e t r a n s f e r r e d t o s o l i d s u b s t r a t e s a t a p r e s s u r e o f 20 m N m - 1. I n a d d i t i o n t o o b s e r v i n g a 1 :l c o r r e s p o n d e n c e o f t h e a r e a c h a n g e o n t h e t r o u g h w i t h the product of substrate area and number of passes through the interface, the quality o f d e p o s i t i o n c o u l d a l s o b e c h e c k e d s p e c t r o s c o p i c a l l y for films d e p o s i t e d o n t o f u s e d

203

LB FILMS OF PHTHALOCYANINE DERIVATIVES

silica. In Fig. 7 the spectrum of 22 layers of 1:1 nickel tetracumylphenoxy phthalocyanine: octadecanol is shown as a full line and compared with the dotted line spectrum of the mixture in a solution of chloroform. Measurement at 616 nm of the absorbance of deposited films against the number of layers resulted in a straight line, demonstrating uniformity of deposition.

.6

.2 .4 .6 .8 1.0 Mole Fraction NiPc(Cp)4

250

350

450 550 650 Wavelength (nm)

750

Fig. 6. Variation in the average area per m o n o m e r with the mole fraction of nickel tetracumylphenoxy phthatocyanine in mixed films with octadecanol. Fig. 7. Spectra of i:1 nickel tetracumylphenoxy phthalocyanine:octadecanol in chloroform solution ( . . . . . ) and deposited onto silica ( ). The silica had 11 layers of mixed film on each side. The linear increase in absorbance with the n u m b e r of deposited layers is shown in the inset.

3.4. M o l e c u l a r o r i e n t a t i o n

Each of these different compounds represents a complete synthetic preparation; the side-groups were not attached to a pre-existing metal phthalocyanine. In spite of the variation in the area per m o n o m e r with different metals, the average values of the film areas are unambiguously small compared with the known size of phthalocyanine molecules, and generalizations can be made about the structure of films of this whole class of metal-substituted tetracumylphenoxy phthalocyanine compounds. On the basis of the film pressure versus area data presented here and in our previous paper v in addition to the recent observations of others studying tetrat e r t - b u t y l phthalocyanine, it seems to be clear that we are not working with monomeric phthalocyanine species oriented with the plane of the ring parallel to the water surface in our films even though the film pressure versus area isotherms are quite reproducible. The sizes, shapes and stacking arrangements in crystals of the underivatized metal phthalocyanines have been well known for many years t t. The macrocy.clic ring alone would occupy more than 100/~2 lying flat on a surface. X-ray analysis by Kovacs et al. 6 for t e t r a - t e r t - b u t y l phthalocyanine showed three d spacings of 3.34 ~, 5.47/~ and 17.15/~ which were interpreted as the perpendicular interplanar spacing, the distance between tilted rings along the stack axis and the in-plane length of the molecule respectively. The electron diffraction data of Fryer et al. s indicated an intermolecular separation of 3.3/~ with an intercolumnar separation of 19 ~. We have previously reported an X-ray powder diffraction spectrum for tetracumylphenoxy phthalocyanine, indicating a d spacing" of 3.4/~. Taking 17.15/~ as one side of a square molecule results in a planar area for t e t r a - t e r t - b u t y l phthalocyanine

204

w.R. BARGER et al.

of 306/~2. The metal-substituted tetracumylphenoxy phthalocyanine monomers are certainly larger than this. A space-filling molecular model showed that, when arranged in a tight planar spiral configuration, the tetracumylphenoxy phthalocyanine molecule would fill a square 20/~ on a side and occupy a minimum area of about 400 ~2 in a plane. Figure 8 illustrates some possible stacking arrangements. Figures 8(a) and 8(b) represent phthalocyanine monomers with small attached groups, and Fig. 8(c) represents a phthalocyanine monomer with large groups attached. Two general types of stacking arrangements are shown in Figs. 8(d) and 8(e). Figure 8(e) shows a slipped stack with its axis 45 ° to the edge of the page. Figure 8(f) illustrates that there is ample room in such a geometry for very large side-groups. The teams studying tetra-tert-butyl phthalocyanine have interpreted their data to indicate that a slipped stack oftetra-tert-butyl phthalocyanine molecules lies with the stack axis parallel to the water surface. If our tetracumylphenoxy phthalocyanine has roughly the same stacking (because of the observed interplanar spacing), can the stack axis be horizontal? Figure 9 shows why we think the answer is no. Given a 20 ~ edge and a 3.4 ,~ interplanar spacing, the absolute minimum area that could be occupied per m o n o m e r (Fig. 9(a)) would be 68/~2. If we assume a slipped stack and the 5.7 ,~ axial distance this increases to 114/~2 per monomer (Fig. 9(b)). Every one of our measured film pressure versus area isotherms is incompatible with these geometries. However, a stack arranged as shown in Fig. 9(c) could account for the data reported here and remain compatible with the model proposed by workers studying tetra-tert-butyl phthalocyanine. Orientation of the stack as shown in Fig. 9(c) would also be favored by hydrogen bonding of the ether linkage oxygens at the water surface. Resonance Raman data obtained recently for mixed films containing octadecanol indicate that at least in the case of cobalt tetracumylphenoxy phthalocyanine deposited onto gold, for some of the stacks, the first macrocyclic ring in the stack is parallel to the plane of the surface I i.

d

e

f

Fig. 8. Some simple stacking arrangements ofderivatized phthalocyanines. Phthalocyanines containing very large groups can exist in a slipped stack arrangement that maintains a close cofacial packing of the macrocyclic rings. Fig. 9. Projections of the occupied unit area on the surface for several stack orientations. For a vertical or slipped stack (c) oriented away from the plane of the surface, the average area per monomer can be smaller than the minimum area per monomer in a horizontal stack (a), if there are many monomers in the stack.

LB FILMS OF P H T H A L O C Y A N I N E DERIVATIVES

205

3.5. N u m b e r o f molecules in a stack On the assumption that the interpretations above are correct, an estimate of the number of molecules in the stack can be made. For example, let us consider the data for nickel tetracumylphenoxy phthalocyanine in Table II. The observation of only 49.9 ~2 per monomer at a film pressure of 1 mN m - t implies a stack of at least eight molecules if each metal-substituted tetracumylphenoxy phthalocyanine occupies an area of 400 ~2. Film collapse was observed at approximately 27 mN m 1 for many of the films, so the area at 25 mN m - 1from Table II should correspond closely to the minimum area occupied by the stack if tilted by compression such as shown in Fig. 10(b). If the plane of the 20 ~ x 20/~ ring were tilted at a 45 ° angle to the surface, the occupied surface area would be reduced to 283 ~2. Division of this area by 28.3 (the area at 25 mN m-~) indicates a stack of ten molecules. These values are also consistent with the mixed film results, whereas values due to true monolayers of metal-substituted tetracumylphenoxy phthalocyanines are not. For example, in the 1:1 mole ratio mixture of nickel tetracumylphenoxy phthalocyanine and octadecanol the average area per monomer at 1 mN m -~ would be (400+21.7)/2 or 211 ]~2 if single molecules of the phthalocyanine were flat on the surface. If a phthalocyanine stack ten molecules high was surrounded by ten molecules of octadecanol at 1 mN m t the area would be (400+217)/20 or 31 ~2 per monomer. Likewise, at 25 mN m-1 a phthalocyanine stack of ten with ten octadecanol molecules would yield (283 + 198)/20 or 24/~2 per monomer. We observe corresponding values of 32.0 and 22.3 ~2 (Table II). A stack often phthalocyanine derivatives with the base parallel to the plane of the surface would only be 34/~ high, on the assumption that the interplanar spacing mentioned earlier is maintained. Proof of this postulated structure awaits further experiments, but it seems to be clear at this point that the films we are working with are definitely not classical monolayers of monomers.

Fig. 10. A m i x e d film c o n t a i n i n g stacKeo m o l e c u l e s as o n e c o m p o n e n t .

4. CONCLUSIONS Film pressure versus area data have been presented for a series of organic derivatives of phthalocyanine in which the side-group was attached to the macrocyclic ring by an ether linkage. Tetracumylphenoxy phthalocyanine compounds containing different metals were studied extensively as pure films and as mixed films with octadecanol. Consideration of recently published data from research teams studying tetra-tert-butyl phthalocyanine together with the results reported here lead to the conclusion that, in the case of the metal-substituted tetracumylphenoxy phthalocyanine series of compounds, stacks of phthalocyanine

206

w.R. BARGERe t al.

molecules with the axis of the stack not parallel to the plane of the surface are contained in both the single-component films and the mixed films. In the case of nickel tetracumylphenoxy phthalocyanine, an average of eight to ten phthalocyanine rings per stack would account for the observed film pressure v e r s u s area data. At this time we have only a small quantity of data for other classes of phthalocyanine derivatives, so the model presented here is based almost entirely on the behavior of the metal-substituted tetracumylphenoxy phthalocyanine compounds. However, considering the small areas per monomer indicated by the pressure v e r s u s area curves for tetraphenoxy phthalocyanine and tetraoctadecoxy phthalocyanine, the model may be appropriate for these compounds. However, the tetraneopentoxy phthalocyanine seems to behave more like the t e t r a - t e r t - b u t y l phthalocyanine that has been studied by other research groups. ACKNOWLEDGMENTS

This work was supported by the Office of Naval Research. The authors also want to thank Mr. Mark Klusty for carrying out a number of the film pressure v e r s u s area measurements. REFERENCES 1 A . B . P . Lever, in H. Emeleus and A. G. Sharpe (eds.), Advances in Inorganic Chemistry and Radiochemistry, Vol. 7, Academic Press, New York, 1965, pp. 27-114. 2 Y. Sadaoka, N. Yamazoe and T. Seiyama, Denki Kagaku Oj'obi Kogyo Butsuri Kakagu, 46 (1978) 597. 3 S. Baker, G. G. Roberts and M. C. Petty, Proe. Inst. Electr. Eng., 130 (1983) 260. 4 S. Baker, M. C. Petty, G. G. Roberts and M. V. Twigg, Thin Solid Films, 99 (1983) 53. 5 J.R. Fryer~ R. A. Hann and B. L. Eyres, Nature (London J, 313 (1985) 382. 6 G.J. Kovacs, P. S. Vincett and J. H. Sharp, Can. J. Phys., 63 (1985) 346. 7 A.W. Snow and N. L. Jarvis, J. Am. Chem. Soc., 106 (1984) 4706. 8 W.R. Barger, H. Wohltjen and A. W. Snow, Transducers '85, Int. Con[] on Solid-State Sensors and Actuators, 1985, Dig. Tech. Pap., IEEE, New York, 1985, pp. 410 413. 9 H. Wohltjen, W. Barger, A. Snow and N. L. Jarvis, 1EEE Trans. Electron Devices, 32 (7) (1985) 1170. 10 T.J. Marks, Science, 227(1985) 881. 11 R.B. Freas and J. B. Campana, lnorg. Chem., 23 (1984) 4654. 12 D.P. DiLella, W. R. Barger, A. W. Snow and R. R. Smardzewski, Thin Solid Films~ 133 (1985) 207.