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Magnetic orientation of phthalocyaninato- polysiloxanes
T/tin SolidFi/ms, 201 (1991) 187 195
187
LANGMUIR--BLODGETT AND RELATED FILMS
M A G N E T I C O R I E N T A T I O N OF P H T H A L O C Y A N I N A T O POLYSILOXANES Z. ALI-ADIB, K. DAVIDSON, H. NOOSHIN AND R. H. TREDGOLD
School of Physics and Materials, Lancaster University, Lancaster LA l 4 YB (U.K.) (Received September 17, 1990; revised N ovem ber 12, 1990; accepted November 26, 1990)
The synthesis of phthalocyaninato-polysiloxanes bearing all butoxy or all decyloxy side-chains is described. The behaviour of both the intermediate monomers and also the polymers at the air-water interface is discussed. Langmuir-Blodgett films of the polymers have been investigated and are described. Films of the polymers cast from solution in a magnetic field of 5 T have been formed and are shown to give a dichroic ratio of up to 7.3 at 555 nm.
1. INTRODUCTION
During the last few years there has been an intensive study of ordered thin organic films. The majority of papers published in this field have been devoted to the Langmuir Blodgett (LB) technique ~'2 although other methods of imposing order on such films have been discussed. One possibility which has heretofore received little attention is to employ a material which forms either a lyotropic or a thermotropic liquid crystal and to cast a thin film of it by either evaporating the solvent or cooling the mesophase depending on which of the two initial states is employed. In order to impose order in the mesophase over macroscopic distances it is necessary to impose either an electric or a magnetic field and to retain it during the transition from liquid crystal to solid. The use of an electric field introduces problems associated with electrical conduction in the case of many materials. We have thus examined the possibility of using magnetic fields in this context. If a molecule includes a moiety involving a conjugated ring structure it will show diamagnetic anisotropy, the lowest energy state corresponding to the magnetic field being tangential to the plane of the rings. The energies involved are minute as compared with k T at room temperatures. However, in the case of a liquid crystal, the cooperative interaction enhances the influence of the diamagnetic anistropy and substantial effects can be observed. If the material involved is a rigid rod polymer in which conjugated ring structures lie with their planes oriented in a definite way with respect to the rod axis an even larger effect can be observed. In our initial work 3 we studied poly(~,-benzyl-L-glutamate) (PBLG) which forms rigid rods based on the s-helix structure such that the benzene rings are all nearly tangential to 0040-6090/91/$3.50
~C',Elsevier Sequoia/Printed in The Netherlands
lea
z. ALI-ADIB e t al.
the rod's axis. The difficulty about this material is that it is chiral in structure and thus, over a sufficiently long distance, there is a tendency for the rod direction to twist round so that it is difficult to obtain a thin film of reasonable area in which all the material is oriented so that the R-helix axes all lie along the applied magnetic field. In the case of P B L G we overcame this difficulty by using a solvent which produced a mesophase and which could be cross-linked by irradiation with UV light while a large magnetic field was applied. Clearly a rigid rod polymer which is capable of forming a mesophase phase, which contains suitably oriented conjugated rings and which is not chiral would be a more suitable candidate for study. After an examination of various polyamides and polyesters we decided that the ideal materials to examine would be phthalocyaninato-polysiloxanes. These materials can be rendered soluble by suitable chemical modification and consist, to a large extent, of large conjugated planar structures arranged so that the rod axis corresponds to the normal to the ring planes. Thus the polymer rods will tend to lie with the axis perpendicular to the magnetic field. If the film is cast on a horizontal substrate to which the magnetic field is tangential the combined effect of the field and the substrate should produce a highly ordered film. Wegner and his collaborators 4 lO have shown how it is possible to synthesize phthalocyaninato-polysiloxanes bearing alkane side-chains attached to the phthalocyanine ring by an ether bond. Such materials are soluble in chloroform and various other c o m m o n organic solvents. The materials which they have used have four methyl groups and (usually) four octy[ groups attached to each phthalocyanine ring in a random manner. They have carried out extensive studies of these materials and show that it is possible to form LB films from them. The rod axis tends to become aligned in the dipping direction so that an anisotropic structure is formed. These researchers s have thus obtained dichroic ratios as high as 2.3 for light propagated normally to the LB plane and at a wavelength of 555 nm. In our studies which we describe below, we have used synthetic techniques based on those of these workers and have also made LB studies of our materials although the main thrust of our work involved magnetic alignment. We have also studied the physical properties of the m o n o m e r intermediates used in forming the polymers. Our materials differ from those of Wegner in that we attach alkyl chains of uniform length to the periphery of each phthalocyanine ring. 2. EXPERIMENTAL METHODS
2.1. S y n t h e s i s o]'the materials
The phthalocyanine monomers were synthesized using, in general, the method of Sauer and Wegner 1° which is outlined in reaction Scheme 1. However, the bromination of catechol was carried out using a method developed by K o h n 11 which was slightly different from the method described by Sauer and Wegner TM. Polymerization was carried out according to the method described by Caseri et al. 6 However, as the phthalocyanine monomers contain eight similar pendant groups which restrict the polycondensation reaction, particularly where the pendant groups are long, two different methods of polycondensation were used successively to ensure m a x i m u m conversion. Initially dichloro monomers were polymerized
189
MAGNETIC ORDERING OF POLYPHTHALOCYANINES
OH
Br" ~
" OH
B r f ~
OR
I Cu CN
OR SiCI4 I
Where
C1
C Ng"",,,.~~
OR
R = CaHgor CloH21
Ion exchange
RO RO
RO RO
Scheme 1. Synthesis of soluble octa-substituted silicondichloro- and silicondihydroxo-phthalocyanines.
using two equivalents of thallium(I) trifluoromethanesulphonate as a catalyst at 100°C for 30h in a nitrogen atmosphere. The end groups of the polymers so produced by this initial process were then converted to a hydroxy form by ion exchange using Amberlyst A26 employing the method of Sauer and Wegner 1°. Further polycondensation was then carried out in toluene using 4 ~ (by weight) FeC13 as a catalyst. The solution was refluxed and stirred for 120h. Figure 1
02
i 0L~
J
I
300
t~O0
500
wavetengFh,
I
I
I
600 nm
700
800
Fig. 1. Optical absorbance ~;s. wavelength for chloroform solutions of the butoxy-substituted materials: curve a, monomer; curve b, polymer.
190
z. ALI-ADIBet al.
illustrates the change in absorption spectra with polymerization. It is difficult to use any of the standard techniques to measure the degree of polymerization obtained in the case of a rigid rod polymer of this kind. However, the high degree of dichroism in the cast films of the butoxy-substituted material indicates that the polymers must be behaving as long thin rods and are thus likely to contain at least 30 monomeric units in each chain. The structure of the polymer is shown in Fig. 2. RO
RO
RO
R0
, -OR
N
~
OR
N
oRRO~
N
OR
" N
OR
OR
R=Cn H2n+l Fig. 2. Schematic structure of the polymer.
2.2. Langmuir-Blodgett studies The study of the isotherms of the monomers and polymers was carried out using a semiautomated constant perimeter trough constructed in the physics department at Lancaster. This trough and also a fully automated trough constructed "in house" were used for depositing LB films of the polymers. Spreading solutions were made up in chloroform at a concentration of 0.2 mg m l - 1 and were filtered through a 0.5 pm Teflon filter immediately before use. The subphase employed was doubly distilled Millipore filtered water. N o n e of the materials studied was sensitive to small variations in p H and the p H was thus allowed to take up a value of about 5.6 in equilibrium with the atmosphere. Isotherms were studied and deposition made at approximately 23 °C. Isotherms were recorded using a barrier speed of 22 m m m i n t LB films of the polymers were deposited on Chance select microscope slides. These were rendered hydrophilic by etching in a solution consisting of 190 ml H 2 0 , 10 ml H 2 0 2 3 0 % and 4 g N a O H and subsequently washing in pure water. The first layer was deposited on the up stroke and good Y-deposition was obtained at a dipping rate of 8 m m rain ~. The polymer bearing butoxy side-groups were dipped at 20 m N m ~ and the polymer bearing decyloxy side-groups were dipped at 13 m N m 1. For thick specimens the fully automated trough was used. For X-ray studies 400 layers of the butoxy-substituted polymer and 620 layers of the decyloxysubstituted polymer were deposited. X-ray diffraction patterns were obtained using Cu Kcz radiation with a Raymax RX 3D diffractometer. Optical dichroism was studied using a Cecil Instruments CE 505 double-beam
MAGNETIC ORDERING OF POLYPHTHALOCYANINES
191
spectrophotometer fitted with polarizing attachments and, qualitatively, with a Nikon polarizing microscope. 2.3. Cast.films In order to obtain a lyotropic nematic mesophase we found that the best solvent for the polymers was 1,1,1,3,3,3-hexafluoro-2-propanol which was obtained at a purity of better than 99% from Aldrich. A solution was allowed to spread over a microscope slide which was held in a horizontal plane in a magnetic field of 5 T provided by a superconducting solenoid. If the solvent was allowed to evaporate in air insufficient time elapsed before solidification for the magnetic field to align the mesophase. Accordingly the slide was placed in a small Teflon box from which the rate of evaporation could be controlled. Hydrophilic slides were obtained as for the LB technique described above. Hydrophobic slides were obtained by exposing dry hydrophilic slides to the vapour of hexamethyldisilizane for 12 h. Measurements of dichroism were carried out as for the LB films. The thickness of the cast films was obtained using a Talystep stylus device having previously protected the film surface by a thin film ofaluminium of known thickness deposited in vacuo.
3.
EXPERIMENTAL RESULTS AND DISCUSSION
3.1. Isotherms Figure 3 shows the isotherms obtained from the phthalocyanine monomers with an Si(OH)2 group at the centre of the ring system. The isotherm for the material having butoxy side-groups exhibits a sharp discontinuity corresponding to an area per molecule of about 2.6 nm 2. Measurements made on an M C P space-filling model of the molecule show that, allowing for a small degree of interdigitation, this area 60 55 50
\ \ \
A
45
\ \
\
E4o z E ~5
\
~ 30 ~ 25
\\\?\\
~_ 20 ~15 O ~ 101
-
5 0-5
1-0
\ 1-5
2.0
2.5
3.0
Surfoce Areo Per Repeot Unit (nm2) Fig. 3. Isotherms for monolayers of the monomers: curve a, decyloxy-substituted material; curve b, butoxy-substituted material.
192
z. ALI-ADIBet al.
corresponds to the area occupied by the molecule when in a prone position at the air-water interface. Thus we believe that, at low densities, the molecules lie prone on the water but, as the system is compressed, they stand up and ultimately tend towards an arrangment in which the planes of the phthalocyanines are nearly vertical. The monomer bearing decyloxy groups do not show a corresponding behaviour and we incline to the belief that, with compression, the molecules tend to slide over one another in this case. Figure 4 shows isotherms for the polymers. It will be seen that the pressure rises rapidly over a fairly small region of area and that, as is usual with LB materials, this indicates that it should be possible to form good LB multilayers. 3.2. Langmuir--Blodgett multilayers
Both polymers could be deposited in the Y-mode but the butoxy material behaved much better than the decyloxy material. It is difficult to measure the molecular weight of rigid rod polymers but we believe that the superior behaviour of the butoxy material is associated with its relatively high molecular weight. Unlike the materials studied by Wegner and his collaborators 4-9 which have four methyl groups per monomer, our materials have all eight positions bearing butoxy or decyloxy chains. This probably renders the region involved in polymerization rather inaccessible. Thus one might expect the butoxy-substituted phthalocyanines to polyrnerize more readily than the decyloxy phthalocyanines and this would explain the superior behaviour of the former material. Figure 5 shows the low angle X-ray diffraction patterns obtained with the two polymers. The layer spacings obtained from these correspond well to the spacings predicted from the molecular structure. These results are similar to those obtained by Sauer et al. 8 who explain why one
6O 55 50
"rE~5 z ,t.O
~35 ~3o
ffl
~2s ~2o
,\x
5 13 0
0.s
1.0
1.s
2.0
2-5
3.0
Surface Area Per Repeat Unit (nm21
Fig. 4. Isotherms for the polymers: curve a, decyloxy-substituted material; curve b, butoxy-substituted material.
MAGNETIC ORDERING OF POLYPHTHALOCYANINES
193
m
0
1
2
3
l+
5 6 7 8 9 '70 11 12 13 14 15 Bragg Angle 2e, Degrees
Fig. 5. X-ray diffraction patterns obtained from multilayers of the polymers: curve a, decyloxy; curve b, butoxy.
would only expect to observe a single peak in each pattern. At 555 nm we observe an optical dichroism of 1.73 as compared with the value of 2.3 reported by Sauer et al. 8 It should be pointed out that the degree of alignment obtained depends in a critical manner on the mean molecular weight of the polymer. Thus one should not read too much into this discrepancy. On the contrary, as the same batch of polymer was used for both experiments, comparison of the value 1.73 obtained from an LB film and 7.3 obtained from the cast film (see below) is significant. 3.3. Cast.films
As we have pointed out above the main objective of our study was to examine the effect of a magnetic field applied during the casting of films of polyphthalocyanines. The polymers were dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol at an initial concentration of 7.3 mg ml ~ and the solvent was allowed to evaporate slowly in a field of 5 T as described above. It was found that the nature of the substrate was critical to the nature of the film obtained. Figure 6 shows a cast film viewed through a polarizing microscope and deposited on a hydrophilic substrate. All the films which we cast on hydrophilic substrates behaved in a similar way and contained a regular array of cracks which were oriented in the same direction as the polymer rod axis. As the outer surface of a polymer rod was covered with hydrophobic groups it is not surprising that the polymer adheres to itself preferentially to the substrate and hence produces this structure. On the contrary, films cast on a hydrophobic substrate were clear and uniform. Figure 7 gives the visible and near-UV spectra of a film cast having a thickness of 0.5 I.tm__+20~o on a hydrophobic substrate in a field of 5 T with the direction of polarization being in one case collinear with the original magnetic field direction and in the other case at right angles to it. At 555 nm a dichroic ratio of 7.3 is obtained which is substantially larger than results reported 8 for LB films of similar materials. X-ray diffraction studies of these films give results very similar to those obtained from the films deposited by the LB technique. The results reported here show that it is possible to synthesize phthalocyaninato-polysiloxanes in which all the pendant
z. ALI-AD[B et al.
194
side chains are of equal length. The results on cast films show that the effect of a magnetic field applied when the casting takes place leads to more complete orientation and a higher dichroic ratio than is obtainable by the LB technique.
Fig. 6. Microphotograph of a film of the butoxy-substituted polymer cast from 1,1,1,3,3,3- hexafluoro-2propanol on a hydrophilic substrate in a field of 5 T viewed through crossed polarizers, (Magnification, 100 x .)
o8
m0e,,c I' A
/5
flew [ ~Si~O JS,~-0-
\
0.2 ~
E
~.00
500
600
700
waveI.engi'h, nm
Fig. 7. Absorbance v s . wavelength obtained from a film of the butoxy-substituted polymer (0.5+_0.1 p_m thick) cast in a field of 5 T showing the high degree of dichroism observed.
MAGNETIC ORDERING OF POLYPHTHALOCYANINES
195
ACKNOWLEDGMENTS W e wish to t h a n k P r o f e s s o r G. W e g n e r a n d Dr. T. S a u e r for v a l u a b l e a d v i c e in c o n n e c t i o n w i t h the synthesis. W e also w i s h to t h a n k the S c i e n c e a n d E n g i n e e r i n g R e s e a r c h C o u n c i l for f i n a n c i a l s u p p o r t a n d for the p r o v i s i o n of t h e s u p e r c o n d u c t i n g solenoid. REFERENCES 1 2 3 4 5 6 7
K. B, Blodgett, J. Am. Chem. Soc., 57 (1935) 1007. R.H. Tredgold, Rep. Prog. Phys., 50 (1987) 1609. R.H. Tredgold and Z. Ali-Adib, J. Phys. D, 21 (1988) 1467. E. Orthmann and G. Wegner, Makromol. Chem., Rapid Commun., 7 (1986) 243. E. Orthmann and G. Wegner, Angew. Chem., Int. Edn. Engl., 25 (1986) 1105. W. Caseri, T. Sauer and G. Wegner, Makromol. Chem., Rapid Commun., 9 (1988) 65 I. C. Bubeck, D. Nehor, A. Kaltbeitzel, G. Duda, T. Arndt, T. Sauer and G. Wegner, Nonlinear Optical Effects in Organic Polymers, Kluwer, Dordrecht, 1989, pp. 185-193. 8 T. Sauer, T. Arndt, D. N. Batchelder, A. A. Kalachev and G. Wegner, Thin Solid Films, 187 (1990) 357. 9 A.A. Kalachev, T. Sauer, V. Vogel, N. A. Plate and G. Wegner, Thin Solid Films, 18~ (1990) 341. 10 T. Sauer and G. Wegner, Mol. Cryst., Liq. Crvst. B, 162 (1988) 97. II M. Kohn, J. AmChem. Soc.,73(1951)480.
187
LANGMUIR--BLODGETT AND RELATED FILMS
M A G N E T I C O R I E N T A T I O N OF P H T H A L O C Y A N I N A T O POLYSILOXANES Z. ALI-ADIB, K. DAVIDSON, H. NOOSHIN AND R. H. TREDGOLD
School of Physics and Materials, Lancaster University, Lancaster LA l 4 YB (U.K.) (Received September 17, 1990; revised N ovem ber 12, 1990; accepted November 26, 1990)
The synthesis of phthalocyaninato-polysiloxanes bearing all butoxy or all decyloxy side-chains is described. The behaviour of both the intermediate monomers and also the polymers at the air-water interface is discussed. Langmuir-Blodgett films of the polymers have been investigated and are described. Films of the polymers cast from solution in a magnetic field of 5 T have been formed and are shown to give a dichroic ratio of up to 7.3 at 555 nm.
1. INTRODUCTION
During the last few years there has been an intensive study of ordered thin organic films. The majority of papers published in this field have been devoted to the Langmuir Blodgett (LB) technique ~'2 although other methods of imposing order on such films have been discussed. One possibility which has heretofore received little attention is to employ a material which forms either a lyotropic or a thermotropic liquid crystal and to cast a thin film of it by either evaporating the solvent or cooling the mesophase depending on which of the two initial states is employed. In order to impose order in the mesophase over macroscopic distances it is necessary to impose either an electric or a magnetic field and to retain it during the transition from liquid crystal to solid. The use of an electric field introduces problems associated with electrical conduction in the case of many materials. We have thus examined the possibility of using magnetic fields in this context. If a molecule includes a moiety involving a conjugated ring structure it will show diamagnetic anisotropy, the lowest energy state corresponding to the magnetic field being tangential to the plane of the rings. The energies involved are minute as compared with k T at room temperatures. However, in the case of a liquid crystal, the cooperative interaction enhances the influence of the diamagnetic anistropy and substantial effects can be observed. If the material involved is a rigid rod polymer in which conjugated ring structures lie with their planes oriented in a definite way with respect to the rod axis an even larger effect can be observed. In our initial work 3 we studied poly(~,-benzyl-L-glutamate) (PBLG) which forms rigid rods based on the s-helix structure such that the benzene rings are all nearly tangential to 0040-6090/91/$3.50
~C',Elsevier Sequoia/Printed in The Netherlands
lea
z. ALI-ADIB e t al.
the rod's axis. The difficulty about this material is that it is chiral in structure and thus, over a sufficiently long distance, there is a tendency for the rod direction to twist round so that it is difficult to obtain a thin film of reasonable area in which all the material is oriented so that the R-helix axes all lie along the applied magnetic field. In the case of P B L G we overcame this difficulty by using a solvent which produced a mesophase and which could be cross-linked by irradiation with UV light while a large magnetic field was applied. Clearly a rigid rod polymer which is capable of forming a mesophase phase, which contains suitably oriented conjugated rings and which is not chiral would be a more suitable candidate for study. After an examination of various polyamides and polyesters we decided that the ideal materials to examine would be phthalocyaninato-polysiloxanes. These materials can be rendered soluble by suitable chemical modification and consist, to a large extent, of large conjugated planar structures arranged so that the rod axis corresponds to the normal to the ring planes. Thus the polymer rods will tend to lie with the axis perpendicular to the magnetic field. If the film is cast on a horizontal substrate to which the magnetic field is tangential the combined effect of the field and the substrate should produce a highly ordered film. Wegner and his collaborators 4 lO have shown how it is possible to synthesize phthalocyaninato-polysiloxanes bearing alkane side-chains attached to the phthalocyanine ring by an ether bond. Such materials are soluble in chloroform and various other c o m m o n organic solvents. The materials which they have used have four methyl groups and (usually) four octy[ groups attached to each phthalocyanine ring in a random manner. They have carried out extensive studies of these materials and show that it is possible to form LB films from them. The rod axis tends to become aligned in the dipping direction so that an anisotropic structure is formed. These researchers s have thus obtained dichroic ratios as high as 2.3 for light propagated normally to the LB plane and at a wavelength of 555 nm. In our studies which we describe below, we have used synthetic techniques based on those of these workers and have also made LB studies of our materials although the main thrust of our work involved magnetic alignment. We have also studied the physical properties of the m o n o m e r intermediates used in forming the polymers. Our materials differ from those of Wegner in that we attach alkyl chains of uniform length to the periphery of each phthalocyanine ring. 2. EXPERIMENTAL METHODS
2.1. S y n t h e s i s o]'the materials
The phthalocyanine monomers were synthesized using, in general, the method of Sauer and Wegner 1° which is outlined in reaction Scheme 1. However, the bromination of catechol was carried out using a method developed by K o h n 11 which was slightly different from the method described by Sauer and Wegner TM. Polymerization was carried out according to the method described by Caseri et al. 6 However, as the phthalocyanine monomers contain eight similar pendant groups which restrict the polycondensation reaction, particularly where the pendant groups are long, two different methods of polycondensation were used successively to ensure m a x i m u m conversion. Initially dichloro monomers were polymerized
189
MAGNETIC ORDERING OF POLYPHTHALOCYANINES
OH
Br" ~
" OH
B r f ~
OR
I Cu CN
OR SiCI4 I
Where
C1
C Ng"",,,.~~
OR
R = CaHgor CloH21
Ion exchange
RO RO
RO RO
Scheme 1. Synthesis of soluble octa-substituted silicondichloro- and silicondihydroxo-phthalocyanines.
using two equivalents of thallium(I) trifluoromethanesulphonate as a catalyst at 100°C for 30h in a nitrogen atmosphere. The end groups of the polymers so produced by this initial process were then converted to a hydroxy form by ion exchange using Amberlyst A26 employing the method of Sauer and Wegner 1°. Further polycondensation was then carried out in toluene using 4 ~ (by weight) FeC13 as a catalyst. The solution was refluxed and stirred for 120h. Figure 1
02
i 0L~
J
I
300
t~O0
500
wavetengFh,
I
I
I
600 nm
700
800
Fig. 1. Optical absorbance ~;s. wavelength for chloroform solutions of the butoxy-substituted materials: curve a, monomer; curve b, polymer.
190
z. ALI-ADIBet al.
illustrates the change in absorption spectra with polymerization. It is difficult to use any of the standard techniques to measure the degree of polymerization obtained in the case of a rigid rod polymer of this kind. However, the high degree of dichroism in the cast films of the butoxy-substituted material indicates that the polymers must be behaving as long thin rods and are thus likely to contain at least 30 monomeric units in each chain. The structure of the polymer is shown in Fig. 2. RO
RO
RO
R0
, -OR
N
~
OR
N
oRRO~
N
OR
" N
OR
OR
R=Cn H2n+l Fig. 2. Schematic structure of the polymer.
2.2. Langmuir-Blodgett studies The study of the isotherms of the monomers and polymers was carried out using a semiautomated constant perimeter trough constructed in the physics department at Lancaster. This trough and also a fully automated trough constructed "in house" were used for depositing LB films of the polymers. Spreading solutions were made up in chloroform at a concentration of 0.2 mg m l - 1 and were filtered through a 0.5 pm Teflon filter immediately before use. The subphase employed was doubly distilled Millipore filtered water. N o n e of the materials studied was sensitive to small variations in p H and the p H was thus allowed to take up a value of about 5.6 in equilibrium with the atmosphere. Isotherms were studied and deposition made at approximately 23 °C. Isotherms were recorded using a barrier speed of 22 m m m i n t LB films of the polymers were deposited on Chance select microscope slides. These were rendered hydrophilic by etching in a solution consisting of 190 ml H 2 0 , 10 ml H 2 0 2 3 0 % and 4 g N a O H and subsequently washing in pure water. The first layer was deposited on the up stroke and good Y-deposition was obtained at a dipping rate of 8 m m rain ~. The polymer bearing butoxy side-groups were dipped at 20 m N m ~ and the polymer bearing decyloxy side-groups were dipped at 13 m N m 1. For thick specimens the fully automated trough was used. For X-ray studies 400 layers of the butoxy-substituted polymer and 620 layers of the decyloxysubstituted polymer were deposited. X-ray diffraction patterns were obtained using Cu Kcz radiation with a Raymax RX 3D diffractometer. Optical dichroism was studied using a Cecil Instruments CE 505 double-beam
MAGNETIC ORDERING OF POLYPHTHALOCYANINES
191
spectrophotometer fitted with polarizing attachments and, qualitatively, with a Nikon polarizing microscope. 2.3. Cast.films In order to obtain a lyotropic nematic mesophase we found that the best solvent for the polymers was 1,1,1,3,3,3-hexafluoro-2-propanol which was obtained at a purity of better than 99% from Aldrich. A solution was allowed to spread over a microscope slide which was held in a horizontal plane in a magnetic field of 5 T provided by a superconducting solenoid. If the solvent was allowed to evaporate in air insufficient time elapsed before solidification for the magnetic field to align the mesophase. Accordingly the slide was placed in a small Teflon box from which the rate of evaporation could be controlled. Hydrophilic slides were obtained as for the LB technique described above. Hydrophobic slides were obtained by exposing dry hydrophilic slides to the vapour of hexamethyldisilizane for 12 h. Measurements of dichroism were carried out as for the LB films. The thickness of the cast films was obtained using a Talystep stylus device having previously protected the film surface by a thin film ofaluminium of known thickness deposited in vacuo.
3.
EXPERIMENTAL RESULTS AND DISCUSSION
3.1. Isotherms Figure 3 shows the isotherms obtained from the phthalocyanine monomers with an Si(OH)2 group at the centre of the ring system. The isotherm for the material having butoxy side-groups exhibits a sharp discontinuity corresponding to an area per molecule of about 2.6 nm 2. Measurements made on an M C P space-filling model of the molecule show that, allowing for a small degree of interdigitation, this area 60 55 50
\ \ \
A
45
\ \
\
E4o z E ~5
\
~ 30 ~ 25
\\\?\\
~_ 20 ~15 O ~ 101
-
5 0-5
1-0
\ 1-5
2.0
2.5
3.0
Surfoce Areo Per Repeot Unit (nm2) Fig. 3. Isotherms for monolayers of the monomers: curve a, decyloxy-substituted material; curve b, butoxy-substituted material.
192
z. ALI-ADIBet al.
corresponds to the area occupied by the molecule when in a prone position at the air-water interface. Thus we believe that, at low densities, the molecules lie prone on the water but, as the system is compressed, they stand up and ultimately tend towards an arrangment in which the planes of the phthalocyanines are nearly vertical. The monomer bearing decyloxy groups do not show a corresponding behaviour and we incline to the belief that, with compression, the molecules tend to slide over one another in this case. Figure 4 shows isotherms for the polymers. It will be seen that the pressure rises rapidly over a fairly small region of area and that, as is usual with LB materials, this indicates that it should be possible to form good LB multilayers. 3.2. Langmuir--Blodgett multilayers
Both polymers could be deposited in the Y-mode but the butoxy material behaved much better than the decyloxy material. It is difficult to measure the molecular weight of rigid rod polymers but we believe that the superior behaviour of the butoxy material is associated with its relatively high molecular weight. Unlike the materials studied by Wegner and his collaborators 4-9 which have four methyl groups per monomer, our materials have all eight positions bearing butoxy or decyloxy chains. This probably renders the region involved in polymerization rather inaccessible. Thus one might expect the butoxy-substituted phthalocyanines to polyrnerize more readily than the decyloxy phthalocyanines and this would explain the superior behaviour of the former material. Figure 5 shows the low angle X-ray diffraction patterns obtained with the two polymers. The layer spacings obtained from these correspond well to the spacings predicted from the molecular structure. These results are similar to those obtained by Sauer et al. 8 who explain why one
6O 55 50
"rE~5 z ,t.O
~35 ~3o
ffl
~2s ~2o
,\x
5 13 0
0.s
1.0
1.s
2.0
2-5
3.0
Surface Area Per Repeat Unit (nm21
Fig. 4. Isotherms for the polymers: curve a, decyloxy-substituted material; curve b, butoxy-substituted material.
MAGNETIC ORDERING OF POLYPHTHALOCYANINES
193
m
0
1
2
3
l+
5 6 7 8 9 '70 11 12 13 14 15 Bragg Angle 2e, Degrees
Fig. 5. X-ray diffraction patterns obtained from multilayers of the polymers: curve a, decyloxy; curve b, butoxy.
would only expect to observe a single peak in each pattern. At 555 nm we observe an optical dichroism of 1.73 as compared with the value of 2.3 reported by Sauer et al. 8 It should be pointed out that the degree of alignment obtained depends in a critical manner on the mean molecular weight of the polymer. Thus one should not read too much into this discrepancy. On the contrary, as the same batch of polymer was used for both experiments, comparison of the value 1.73 obtained from an LB film and 7.3 obtained from the cast film (see below) is significant. 3.3. Cast.films
As we have pointed out above the main objective of our study was to examine the effect of a magnetic field applied during the casting of films of polyphthalocyanines. The polymers were dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol at an initial concentration of 7.3 mg ml ~ and the solvent was allowed to evaporate slowly in a field of 5 T as described above. It was found that the nature of the substrate was critical to the nature of the film obtained. Figure 6 shows a cast film viewed through a polarizing microscope and deposited on a hydrophilic substrate. All the films which we cast on hydrophilic substrates behaved in a similar way and contained a regular array of cracks which were oriented in the same direction as the polymer rod axis. As the outer surface of a polymer rod was covered with hydrophobic groups it is not surprising that the polymer adheres to itself preferentially to the substrate and hence produces this structure. On the contrary, films cast on a hydrophobic substrate were clear and uniform. Figure 7 gives the visible and near-UV spectra of a film cast having a thickness of 0.5 I.tm__+20~o on a hydrophobic substrate in a field of 5 T with the direction of polarization being in one case collinear with the original magnetic field direction and in the other case at right angles to it. At 555 nm a dichroic ratio of 7.3 is obtained which is substantially larger than results reported 8 for LB films of similar materials. X-ray diffraction studies of these films give results very similar to those obtained from the films deposited by the LB technique. The results reported here show that it is possible to synthesize phthalocyaninato-polysiloxanes in which all the pendant
z. ALI-AD[B et al.
194
side chains are of equal length. The results on cast films show that the effect of a magnetic field applied when the casting takes place leads to more complete orientation and a higher dichroic ratio than is obtainable by the LB technique.
Fig. 6. Microphotograph of a film of the butoxy-substituted polymer cast from 1,1,1,3,3,3- hexafluoro-2propanol on a hydrophilic substrate in a field of 5 T viewed through crossed polarizers, (Magnification, 100 x .)
o8
m0e,,c I' A
/5
flew [ ~Si~O JS,~-0-
\
0.2 ~
E
~.00
500
600
700
waveI.engi'h, nm
Fig. 7. Absorbance v s . wavelength obtained from a film of the butoxy-substituted polymer (0.5+_0.1 p_m thick) cast in a field of 5 T showing the high degree of dichroism observed.
MAGNETIC ORDERING OF POLYPHTHALOCYANINES
195
ACKNOWLEDGMENTS W e wish to t h a n k P r o f e s s o r G. W e g n e r a n d Dr. T. S a u e r for v a l u a b l e a d v i c e in c o n n e c t i o n w i t h the synthesis. W e also w i s h to t h a n k the S c i e n c e a n d E n g i n e e r i n g R e s e a r c h C o u n c i l for f i n a n c i a l s u p p o r t a n d for the p r o v i s i o n of t h e s u p e r c o n d u c t i n g solenoid. REFERENCES 1 2 3 4 5 6 7
K. B, Blodgett, J. Am. Chem. Soc., 57 (1935) 1007. R.H. Tredgold, Rep. Prog. Phys., 50 (1987) 1609. R.H. Tredgold and Z. Ali-Adib, J. Phys. D, 21 (1988) 1467. E. Orthmann and G. Wegner, Makromol. Chem., Rapid Commun., 7 (1986) 243. E. Orthmann and G. Wegner, Angew. Chem., Int. Edn. Engl., 25 (1986) 1105. W. Caseri, T. Sauer and G. Wegner, Makromol. Chem., Rapid Commun., 9 (1988) 65 I. C. Bubeck, D. Nehor, A. Kaltbeitzel, G. Duda, T. Arndt, T. Sauer and G. Wegner, Nonlinear Optical Effects in Organic Polymers, Kluwer, Dordrecht, 1989, pp. 185-193. 8 T. Sauer, T. Arndt, D. N. Batchelder, A. A. Kalachev and G. Wegner, Thin Solid Films, 187 (1990) 357. 9 A.A. Kalachev, T. Sauer, V. Vogel, N. A. Plate and G. Wegner, Thin Solid Films, 18~ (1990) 341. 10 T. Sauer and G. Wegner, Mol. Cryst., Liq. Crvst. B, 162 (1988) 97. II M. Kohn, J. AmChem. Soc.,73(1951)480.