Supramolecular Architecture - American Chemical Society

Chapter 15

Crystalline Inorganic Hosts as Media for the Synthesis of Conductive Polymers 1

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M . G. Kanatzidis , C.-G. Wu , H. O. Marcy , D. C. DeGroot , J. L. Schindler , C. R. Kannewurf , M. Benz , and E. LeGoff 2

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Downloaded by UNIV OF ARIZONA on August 6, 2012 | http://pubs.acs.org Publication Date: July 14, 1992 | doi: 10.1021/bk-1992-0499.ch015

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Department of Chemistry and the Center of Fundamental Materials Research, Michigan State University, East Lansing, MI 48824 Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL 60208 2

In this paper we present work on the insertion of selected conductive polymers into several crystalline inorganic layered hosts. Specifically, we describe the insertion of polyaniline and polyfuran in FeOCl. We also describe the reaction of the Hofmann-type inclusion compounds Ni(CN) NH (pyrrole) and Ni(CN) NH (aniline) with various oxidants which yield polypyrrole/Ni(CN) NH and polyaniline/Ni(CN) NH composites. Magnetic, infra-red spectroscopic, scanning and transmission electron microscopic (TEM) and X-ray diffraction (XRD) data on the state of the polymers inside FeOCl and Ni(CN) NH are presented. Variable temperature electrical conductivity and thermoelectric power data are reported. Based on XRD andTEMdata a structural model for the orientation of polyaniline in FeOCl is proposed. 2

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The idea of having well ordered organic macromolecules inside inorganic host structures is intriguing and during the last five years notable progress towards producing such systems has been made. For example polypyrrole, polythiophene, polyaniline and polyacetylene, the four archetype conducting polymers, have been reported by several groups*^ to be synthesized into layered and three-dimensional hosts. Intercalated polymers in structurally defined hosts are of interest for several reasons, including (a) the possibility of direct structural characterization by crystallographic methods, (b) the opportunity for detailed spectroscopic studies on isolated polymer chains without interchain interactions (provided no interference is presented by the host) and (c) the prospect of obtaining conductive polymers with fewer defects and with greater orientation than otherwise possible. They also represent a new class of materials with anisotropic properties derived from the different structural and electronic nature of their individual components.^ A demonstrated method to insert polymer chains into host structures is by in-situ intercalative polymerization of a monomer using the host itself as the oxidant. FeOCl is one of the most convenient redox-intercalation hosts for a great variety of molecules-^ including conducting polymers.4>5 The in-situ intercalative polymerization of pyrrole, 2,2-bithiophene and aniline in the interlayer space of this material has been reported.^ The resulting intercalation compounds are composed 0097-6156/92/0499-0194$07.50/0 © 1992 American Chemical Society

In Supramolecular Architecture; Bein, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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of alternating monolayers of positively charged conductive polymer chains and negatively charged FeOCl layers. The mechanism of this intercalative process is not well understood. Apart from the in situ intercalative polymerization into strongly oxidizing hosts, another synthetic method to obtain intercalated polymers is to oxidatively polymerize an appropriate monomer already intercalated into an inorganic host using an outside oxidant. This was applied in the case of anilinium ion which was included in the framework of a zeolite and then polymerized to polyaniline by reaction with (NH4)2S208.1b In this case an external oxidant acts as the electron acceptor instead of the host material. In yet another way, the host material is intercalated by an oxidant and then it is exposed to the appropriate monomer vapor. For example, the inclusion polymerization of polypyrrole, polythiophene and polyaniline in an non-oxidizing zeolite host was accomplished by first introducing an oxidizing ion (e.g. F e , C u ) in the zeolite followed by exposure to monomer (pyrrole, thiophene, aniline) vapor. Here we report our recent progress in using crystalline hosts to insert conductive polymers by some of the methods mentioned above. Specifically, we describe the insertion of polyaniline and polyfuran in FeOCl. We also describe the reaction of the Hofmann-type inclusion compounds Ni(CN)2NH (C4HsN) and Ni(CN)2NH3(C6H5NH2) with various oxidants which yield electrically conductive polypyrrole/Ni(CN)2NH3 and polyaniline/Ni(CN)2NH3 type composites.


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EXPERIMENTAL SECTION Reagents. FeOCl is prepared by heating at 380 ° C in a sealed pyrex tube a mixture of FeCl3 (anhydrous) and Fe2Û3 in 1.2:1 ratio. The purple FeOCl crystals are isolated by washing away the excess FeCl3 with acetone and ether. The synthesis of 2,2':5 ,2 -terfuran was reported earlier. Pyrrole and aniline were distilled under reduced pressure. Ni(CN)2NH3 (pyrrole) and Ni(CN)2NH3 (aniline) were prepared according to the Uterature. ,

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Insertion of Polyaniline in Single Crystal F e O C l . 0.34 g (3.1 mmol) of single crystals were suspended in a solution of 2.00 g aniline in 50 ml acetonitrile. The mixture remained undisturbed for 60 days. The black crystals were collected by filtration and washed with acetone and ether. Elemental analysis gave (polyaniline) FeOCl (I). The χ can vary from 0.23-0.28. x

Reaction of terfuran with F e O C l . 0.3g (2.8 mmole) of FeOCl and 0.2g (1 mmole) of terfuran were mixed with 1.5 ml MeOH, frozen in the bottom of a pyrex tube which was subsequently flame-sealed under reduced pressure. The tube was heated atlOO °C for 6 days. The black shiny solid was isolated by filtration washed with acetone and dried in vacuum. Elemental analysis gave (C4H20) (MeOH) FeOCl (Π) with χ ranging between 0.63-0.81. Due to the presence of methanol we do not know the exact ammount of polyfuran present. For simplicity we will refer to this material as polyfuran/FeOCl. x

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Reaction of N i ( C N ) N H ( p y r r o l e ) (III), with Fe*+. 0.20g (1 mmole) of Ni(CN)2NH3(pyrrole) were suspended in an aqueous solution of 0.8g (5 mmole) of FeCl3. The mixture was stirred at room temperature for 1.5 hs and the black solid was isolated by filtration, washed with H2O and acetone and dried in vacuum. Elemental analysis suggests the formula [Ni(CN)2NH3]{(C4H2NH)Clo.22Jo.9 (V). The same 2

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reaction can also be run in C H C I 3 . The corresponding reactions with single crystals were carried out in a similar manner as described above except that 3 days were required. 3 +

Reaction of Ni(CN) NH (aniline) (IV), with F e . The procedure was similar to the one employed for the pyrrole analog. Elemental analysis suggests the formula [Ni(CN)2NH3]{(C H4NH)Cl .52}o.l5.(VI) The corresponding reactions with single crystals were carried out in a similar manner as described above except that 1 week was required. 2

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Removal of the Ni(CN)2NH3 host. In a typical experiment [Ni(CN)2NH3]{polymerjx is suspended in 50ml 1M aqueous HC1 and stirred for 2 days until no C N vibration peak (2161 c m ' l ) is detected in the Fourier transform infrared (FTIR) spectrum and no N i by SEM-EDS. The black powder was washed with ethanol and ether and dried in vacuo. Ethylene diamine also can be used to dissolve the Ni(CN)2NH3 matrix if non-aqueous conditions are desired. Physical Measurements. X-ray diffraction experiments were carried out with a Phillips XRG-3000 instrument using C u K a radiation. Infrared spectroscopy was performed with a Nicolet 740 FTIR spectrometer. A Nicolet P3F four-circle diffractometer was used for single crystal studies. Direct current electrical conductivity and thermopower measurements were obtained from 5 to 320 Κ using a data acquisition and analysis system described elsewhere. Samples were measured as pressed pellets using conditions and protocols described earlier. Scanning electron microscopic (SEM) and transmission electron microscopic (TEM) studies were performed on JEOL-JSM35CF and JEOL-100CX-(II) instruments respectively. 83

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RESULTS A N D DISCUSSION F e O C l as an Intercalation Host: FeOCl possesses a two-dimensional polymeric structure in which FeOCl layers are separated by van der Waals gaps between chlorine atoms. The coordination number of iron in this compound is six and the geometry is distorted octahedral. There are two axial trans chloride ligands and four equatorial oxygen ligands. The oxygen atoms are shared by four iron atoms while the chlorine atoms are shared by two iron atoms. The crystallographic b-axis is 7.92 Â and runs perpendicular to the layers. It is this axis which expands upon intercalation, while the a- and c- axes at 3.30 Â and 3.78 Â respectively do not change significantly. The structure of FeOCl and its unit cell (a- and c- axes) are shown in Figure 1. The average Fe-0 and Fe-Cl distances are 2.032 Â and 2.368 Â respectively. The Fe-Fe distance is 3.30 Â. The ability of FeOCl to intercalate guest ions and molecules derives by its redox properties and specifically its ability to be reduced ( F e —> F e ) to a certain extent, without undergoing significant structural transformations. When intercalated with species such as pyridinium, ferrocenium, K , R b , C s etc, the FeOCl layers become semiconducting due to the thermally activated carrier hopping between F e and F e sites. Over-reduction of the framework (>~30%) can result in destruction of its crystallinity. 9

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Insertion of Polyaniline i n F e O C l Single Crystals. The intercalation of single crystals of many materials at room temperature is known to be very slow, it is seldom complete, and usually it destroys the crystals. Nevertheless we still attempted the reaction of FeOCl with aniline to see if similar difficulties existed in this system as



15. KANATZIDIS ET AL.

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Ο—έί—Ο—α—Ο—α—Ο—α—ο Ο—u—Ο—ο—Ο—Η—Ο—Η—ο—»—ο φ—65—Ο—η—Ο—η—Ο—65—π Ο — — Π — 6 5 — Ο — 6 5 — Ο — 6 5—Ο—η—Ο Ο

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Π 65 Q 65 Π 65 Λ Ο 1 Τ 1 Τ 1 Τ 1 Τ Π (5 Π 6 5 Π 6 5 Ο Η Ο ^ Ο 6 5 Ô 0kl h0l>hkl>hk0. Axial X-ray photographs from single crystals are shown in Figure 3. This is to be expected considering that the FeOCl crystals have undergone a topotactic intercalation reaction. t

Interestingly, the diffraction pattern of the hkO zone reveals a set of strong reflections associated with the parent 3.30 X 3.78 Â cell, and a set of several weak reflections half way between the strong ones, suggesting that the periodicity along the a- and caxes have doubled relative to the original FeOCl unit cell. The new unit cell is orthorhombic with a=6.60 Â, b=28.86 Â, c=7.56 Â and V=1459 Â . The origin of this superlattice is due to substantial long-range order of polyaniline in FeOCl. Long range order of polyaniline in the intralamellar space of FeOCl could be achieved by orientation of the polymer chains along certain crystallographic directions, such that a doubling of the periodicity of a- and c- axes is caused (vide infra). 3

Although the structure of polyaniline is not yet known in detail, it has been proposed, based on X-ray diffraction studies on powders, that it is similar overall to that of polyphenyleneoxide. » This is shown in scheme (A). 11

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2-fold screw

10.05 A

5.02 A-

repeating unit

Scheme (A) 12

Based on single crystal crystallographic studies of oligoanilines, by Baughman et al , the repeating unit of polyaniline is estimated to be 10.05 Â (ignoring the details in individual phenyl groups). The highest symmetry possible for the polymer chain corresponds to a 2-fold screw axis in the chain direction. The step of the 2-fold screw axis is 10.05/2=5.02 Â. If we now attempt to orient such a chain along either the a or the c-axes of the FeOCl layers in Figure 4 we see that the repeating unit of the polymer





Figure 2. Scanning electron micrographs of large single crystals of a-(I)


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Figure 3. Oscillation photographs of •δ Ο Ο Ο Φ -Q ω CO

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TEMPERATURE (Κ)

Figure 13. (A): Variable temperature electrical conductivity data of (V) as pressed polycrystalline pellet and in "single" crystal form. (B): Variable temperature thermolectric power data (Seebeck coefficient) of (V) as pressed polycrystalline pellet and in "single" crystal form.



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by dissolution in ethylene diamine (or aq. HC1) the black polymer which is isolated is a good electrical conductor with room temperature conductivity of 1 S/cm. Location of the polymer chains in (V) and (VI). The inside vs. outside question. We note here that we could not obtain unequivocal evidence that the conductive polymers in [Ni(CN)2NH3]{(polymer)(A) } are indeed completely intercalated between the Ni(CN)2NH3 sheets. The X-ray powder diffraction (XRD) patterns of (V) and (VI) are very similar. The original 002 peak present in [Ni(CN)2NH ]{monomer} is absent in the corresponding patterns of (V) and (VI) and no reflection can be identified which can be assigned to the interlayer spacing in [Ni(CN)2NH ]{(polymer)(A) } . The X R D patterns show primarily hkO type reflections. There are several explanations for these observations. First, the absence of any order along the stacking c-direction may be due to random stacking of the N i ( C N ) 2 N H sheets. This could happen if there was random staging (not every interlayer gap was occupied by polymer). A second possibility is that the polymers are not intercalated at all but intimately mixed with a crystalline phase such as Ni(CN)2NH (H20)o.25- This however is in contrast with the EPR data discussed above. The X R D patterns cannot rule out such a phase and cannot confirm that the oxidation reaction is really topotactic. It is likely that in the case of (V) both of the above possibilities occur simultaneously. This is suggested by the following. When single crystals of (V) are cleaved and examined under the scanning electron microscope a separate polypyrrole phase (small spheres of amorphous material -1-2 x

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μπι in diameter) is visible at the edges and surface of the crystals. The spheres however are well separated from each other and do not seem to make a continuous path from one end of the sample to the other. Nevertheless, the fact that "single" crystals of (V) are more conductive than powder samples, argues that at least a significant portion of polypyrrole is aligned with, and intercalated in the Ni(CN)2NH3 framework. The above data suggest that the oxidative polymerization reaction is not truly topotactic. Auger and X-ray photoelectron spectroscopic (XPS) studies are needed to further probe this point. In contrast to (V), we see no evidence, under SEM conditions, of separate polyaniline phase in crystals of (VI). This, in combination with our EPR spectroscopic data (vide supra) suggests that polyaniline is intercalated between the Ni(CN)2NH3 layers. Acknowledgement. Financial support from the National Science Foundation (grant DMR-8917805 to M G K ) is gratefully acknowledged. At Northwestern University this work made use of Central Facilities supported by NSF through the Materials Research Center. This work made use of the S E M facilities of the Center for Electron Optics at Michigan State University. REFERENCES 1)

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