1n situ intercalative polymerization chemistry of …1n situ intercalative polymerization chemistry...

Solid%tatelonics32.:33 (1989) 594 (~()8 North-llolland, -Mnslurdam

1N S I T U I N T E R C A L A T I V E P O L Y M E R I Z A T I O N C H E M I S T R Y O F FeOCI .

G E N E R A T I O N A N D P R O P E R T I E S O F N O V E L , H I G H L Y C O N D U C T I V E

I N O R G A N I C / O R G A N I C P O L Y M E R M I C R O L A M I N A T E S

Mercou r i G. K A N A T Z I D I S ~, H e n r y O. M A R C Y , Wi l l i am ,l. M c C A R T H Y ,

Car l R. K A N N E W U R F a n d T o b i n J. M A R K S :

l)cl~arfnlcnt o/ ( "henll~gcr and t/it llalcria/~ Re.~eareh ("enler. Departnzc~l q/ l:/~,clric'a/ l:n~,im,crin~, and ( "r~m/mler Xcienc< \~rl/?u est~'rn Uptircrs'il)'. l(~an~lOn. IL 6020& { 'St

Rcccixed 22 June 1988: accepted for publication 23 June 1988

Nc~ structural forms of polyp~rrolc and polythiophene arc prepared by a chemical method in which intercalation and simul- taneous polymerization of p,vrrolc and 2,2'-bithiophene are brought about within the constrained van der Waals gap ofa la}cred inorganic solid. FeO('I. The reaction of FeOCI with p.xrrole and 2,2'-bithiophene yields compounds (('~H~N),, ,4FeO(I, 1 and ((~H:S),, ,sFe()('l. 11, respectixel3. ((TartaN L, ~aFeO('I and (CaH:S)~,=~FeO('I arc brown-black microcrystalline solids with a shiu_~ metallic luster. The interlayer spacing in 1 and II (h-axis) is 13.21 ( 1 ) -k and 13.31 ( 1 ) &, respectivcly, compared to 7.98(2) -\ in pristine Fe()('l. X-ra~ powder diffraction data for I and 11 arc consistent with a space group change from Pmnm (m Fc()('l/ to lmmm or 1222 and doubling oftbe h-axis. The body-centered unit cell results from a lateral shift of the ahcrnate Fc()('l layers m the a direction of the a~ plane so that the chlorine atoms lie directl~ on top of each other. The nalurc of the organic material m [ and II ~as probed b.~ chemical, physical and charge transport techniques, all ofx~hich indicate thc presence of a high molccular ~cight, conductix e polymer. 1 and II exhibit high electrical conductivities compared to other FeO('I intercalation compounds. Fnur-probe electrical conductivit 3 data (in the range 4-300 K ) measured on compressed pcllets of the materials show thermally act~x ated behavior x~ ith room temperature r;v 1 ~2 t cm ~. Thermoelectric power measurements indicate predominant hole conduction ~ith metallic behax ior. In ( ('all :S )~, =~FeOCI, a sudden rise in file Seebeck coefficient below - 35 K is observed, suggest- lug a possible metal-semiconducror transition.

1. Introduction

Resea rch on e lec t r ica l ly c o n d u c t i n g p o l y m e r s has

recen t ly focused heavily ' on the syn thes i s , c h a r a c t e r -

i za t ion and s ludy of po lypy r ro l e s a n d po ly th io -

p h e n e s [ 1-6 ]. These m a t e r i a l s are s o m e o f the m o s t

robus t a n d chemica l ly f lexible m e m b e r s of the cur-

rent ly k n o w n c o n d u c t i n g p o l y m e r s a n d ho ld tech-

nological p r o m i s e for a n u m b e r o f a p p l i c a t i o n s [ 7 -

I 5 ] . P r e p a r e d by the e l e c t r o c h e m i c a l or c h e m i c a l

o x i d a t i o n o f pyr ro les a n d t h i o p h e n e s , the

' , (PP ,v )+ ' ( X ) , i , a n d l ( P t h ) + ' ( X - ) , { , , ( X =

CIO,a , BFa , N O ~ , etc. ) p o l y m e r s are c h a r a c t e r i z e d

by high electr ical c o n d u c t i v i t i e s a n d me ta l - l i ke be-

h a v i o r [ 16 -19 ]. H o w e v e r , these p r o p e r t i e s are o f ten

Present addrcss: Department of( 'hemistr>. Michigan Slate /1nix ersit?. East Lansing, Michigan 48824, [;SA. kmhor m ~hom correspondence should be addressed.

s t rongly d e p e n d e n t on the n a t u r e o f the o x i d a t i o n

c o n d i t i o n s , the c o u n t e r i o n , the poss ib le s u b s t i t u e n t s

on the pyr ro le or t h i o p h e n e rings, a n d the degree of

pa r t i a l o x i d a t i o n o f p o l y m e r b a c k b o n e . P r o p e r t y

m o d i f i c a t i o n s however , have been a c h i e v e d by con-

t ro l l ing f i lm f o r m a t i o n cha rac t e r i s t i c s ( e l e c t r o c h e m -

ical m e t h o d s ) a n d via " a l l o y i n g " wi th o t h e r poly-

mer s [ 2 0 - 2 3 ] . T h e great p r e p o n d e r a n c e o f research

has been ca r r i ed ou t on p o l y m e r s p r e p a r e d by elec-

t r o c h e m i c a l t e c h n i q u e s , p r i m a r i l y because o f the

abi l i ty 1o o b t a i n free s t a n d i n g fi lms. R e p o r t s on

c h e m i c a l p o l y m e r i z a t i o n o f pyr ro les a n d / o r th io-

p h c n c s r e m a i n scarce, i l l - d o c u m e n t e d a n d of ten lead

to i n s u l a t i n g p o l y m e r s [ 2 4 - 3 2 ] . Regard less of thc

s y n t h e t i c m e t h o d , these m a t e r i a l s are a l m o s t inva r -

iably o b t a i n e d in an a m o r p h o u s s ta te a n d thus , de-

spi te an ex t ens ive body of e x p e r i m e n t a l and theo-

ret ical work [ 1 -19 ], t he i r m i c r o s t r u c t u r e /

m o r p h o l o g y cha rac t e r i s t i c s b e y o n d the basic pro-

M. G. Kanatzidis et al. / In situ intercalative polymerization chem isto, o f FeOCI 5 95

posed repeat unit (A) and (B) e.g., overall chain conformation and packing, degree of cross-linking, direction of charge transport (intra- or interchain) etc., remains poorly understood and largely uncontrollable.

H

A B

It would be highly desirable, from a characteri- zation viewpoint if such polymer chains were isolated or confined in well defined environments such as crystal lattices or host structures. Such systems would also represent a new class of novel conductive polymer based hybrid materials that might display new interesting properties or improve existing ones.

We report here on a new type of a redox intercalation reaction with oxidatively polymerizable aro- matic five-membered ring heterocyclic monomers and directs. The products are novel intercalation compounds with conducting polymers inserted in the intralamellar space of the host. Specifically, we dis- cuss the chemical, physical, and charge transport properties of new intercalation compounds of polypyrrole [33] and polythiophene with FeOCI.

2. Experimental section

The reagents, FeC13 (anhydrous), Fe20~ and 2,2'- bithiophene were purchased from commercial sources and used without further purification. Ace- tonitrile and pyrrole were dried over molecular sieves and vacuum distilled. FeOC1 was prepared by a literature procedure [34-38]. When batches with large crystallite size were obtained, the FeOCI was soni- cared for 5-10 rain in toluene to decrease particle size and enhance reaction rates. Elemental analyses were performed by Galbraith Laboratories, Knox- ville, TN. Quantitative and semi-quantitative analyses were performed with a Hitachi scanning electron microscope equipped with a Tracor Northern EDAX system. TGA scans were performed on a Du Pont model 9900 instrument.

In a typical reaction, a mixture of FeOC1, pyrrole or 2,2'-bithiophene in ~ 15 ml CH3CN (neat pyr-

role may also be used) was stirred at ~ 40-50°C for a week under N2 (experiments in dry air did not seem to affect the final products). The black solid was col- lected by a filtration, washed with CH3CN and ether and dried in vacuo.

Chemical polymerizations of pyrrole and 2,2'-bithiophene were performed using CH3CN solutions of Fe(NO3)v6H~O and anhydrous FeCI~, respectively [24-30]. Elemental analyses were consistent with the following stoichiometries: (polypyrrole)o.~4FeOC1, (polythiophene)o2sFeOCl, (polypyrrole ) ( NO3 )o.2> ( polythiophene ) ( FeCI4 )o ,~s3. Often however, the H content was higher than that calculated. This problem of high H content is typical of this class of conducting polymers [24-30].

X-ray powder diffractometry was carried out with a Rigaku Geigerflex instrument using Ni-filtered CuKa radiation. The sampling and calibration techniques have been described previously [39].

The optical reflectivity spectra of polycrystalline compactions were measured with a Digilab FTS60 Fourier Transform infrared spectrometer equipped with a universal infrared microsampling accessory. All measurements were made at room temperature and near normal incidence. The samples were pressed with polished dies at ca. 7-10 tons of force to flat disks of 13 mm diameter and 0.5-1.0 mm thick. Transmission infrared spectra were recorded as KBr pellets using a Mattson Instruments Alfa Centauri or a Digilab FTS60 Fourier Transform spectrometer.

Direct-current electrical conductivity and thermopower measurements were obtained from 4.2 to 320 K using the computer-controlled data acquisi- tion and analysis system described elsewhere [40,41 ]. Samples were examined as pressed pellets with contact techniques and measurement precau- tions noted earlier [40,41 ].

3. Results and discussion

3.1. Synthesis and properties

The synthesis of electrically conducting polypyr- roles and polythiophenes is readily accomplished by the electrochemical oxidation of appropriate monomers in various solvents [16-19,24-26,27- 30,42,43]. Good quality materials are usually ob-

596 M. (;. Kanatzidis el al. / ltl silti inlercalative pol)'mcrtzalion chct*lislo' {~tk'eO('l

rained in CH3CN. Generally, pyrroles are more readily oxidized than thiophenes (i.e. Epa for pyrrole and thiophene are 1.20 and 2.06 V versus SCE, respectively [5 ] ), and therefore can be polymerized by a number of chemical oxidants such as FeCI~ and CuCI,. Recently we have reported that FeOCI oxi- dizes pyrrole and concurrently undergoes ion intercalation to form a unique, electrically conducting layered material, I, in which polypyrrole is sand- wiched between the layers of FeOC1 [33 ]. The crystal structure of unintercalated FeOCl is shown in fig. I. Although the redox potential of FeOCI under the reaction conditions is not known, it is thought to be very similar to that of FeCI~ [34].

Initial attempts to extend this intercalative po/),- merization reaction to thiophene or 3-methylthiophene failed, probably because of the exceedingly high first anodic potentials of these heteroeycles.

Orthorhorabic

Layered Structure of FeOCt

S p ~ c e G r o u p P m n m

' ~ v'J

% %

Ct O Fe o~=3.780 ~ , b = 7 . 9 2 0 .,;, , c = 3 . 3 0 3 ).

Layer S e p a r a t i o n : 7 ,920 J.

Fig. I. The layered structure of FcOC1. The t~-axis is shown normal to the layers. The bridging C1 ions form chains that run parallel to the c-axis. The FeOCI slabs are packed so that the CI chains are staggered.

However, the reaction of 2,2'-bithiophene with FeOC1 in CH3CN solution proceeds cleanly and within ca. 7 days intercalation is complete. This new compound II has the stoichiometry (C4HeS).~sFeOCI. Intercalation of organic mole- cules into FeOCI invariably occurs via oxidation and concomitant partial reduction of the FeOCI network [42,43 ]. If the intercalating molecule is stable after oxidation (e.g., Fe(CsHs)e) then its oxidation product (e.g., F e ( C s H s ) ) ) is inserted between the FeOC[ layers [44]. However, when the oxidation product is unstable (e.g. pyridine), radical cation coupling products are often observed in the van der Waals interlayer space [44,47,48]. In pyridine intercalation compounds such as (py)o>TaSe [47,48] and (PY)o ~FeOCI [ 44 ], significant amounts of 4,4'-bipyridine (an oxidative coupling product ) are found. In the case of 2,2'-bithiophene as in pyrrole or thiophene the only stable oxidative coupling products known are polymers connected largely at the 2,5 positions (A, B ) [ 1 - 19 ].

The facile oxidation of 2,2'-bithiophene by FeOC1, but not thiophene, is consistent with the less positive anodic potential of the former (Epa= 1.32 versus 2.06 V versus SCE respectively [5 ] ) . Indeed the potential of 2,2'-bithiophene is similar to that o f pyrrole (Ep~,= 1.20 V), and obviously less positive than the redox couple of FeOCI. The structural similarity of pyrrole and thiophene in conjunction with the 2,2'- bithiophene results suggest that thermodynamic rather than kinetic factors are important in these intercalative polymerization reactions. Although kinetic factors enter in, when carrying out intercalation reactions, in this particular case the similar size and shape of these heterocyclic compounds suggest that kinetics alone are unlikely to be reaction-lim- iting. From the results reported here, upper and lower bounds can be placed on the redox potential of FeOCI at 1.32<Ea.()(.t> ~ <1.86 V (between the £'),,, of 2,2'-bithiophene and 3-methylthiophene, respectively ~ [ 49 ] ).

It should also be pointed out that for intercalative polymerization to occur in a five-membered ring heterocycle, the 2,5 positions must not be blocked by substituents. For example, use of 2,5-dimethylpyr-

Cyclic voltammet~' studies in these systems show tolally irre- versible anodic waves. See ref. [ l-6 ].

M.G. Kanatzidis et al. ~In situ intercalative polymerization chemisto, q['FeOCl 5 97

role, which has a less positive oxidation potential than pyrrole itself, does not form a conductive polymer nor an intercalation compound. This of course de- rives from the fact that polymerization proceeds through coupling at the 2 and 5 positions. In contrast, N-methylpyrrole which possesses similar Epa values [5], does undergo intercalative polymerization in FeOC1 to yield the analogous conducting polymer.

The intercalates (Ppy)o.34FeOC1 (I) and (Pth)o 28FeOCI, (II) are brown-black microcrystal-

line solids with shiny metallic luster. Scanning electron microscopy (SEM) shows a single phase con- sisting of small crystallites with average dimensions of 15 X 5 X 2 p.m, similar in size to those of the starting material. An example is shown in fig. 2. Unlike the starting material, the crystallites show cracks, chipped corners and flaking perpendicular to the layer stacking direction (010) . The poorer crystalline quality of the intercalated compounds relative to pristine FeOC1 is confirmed by the X-ray powder diffraction results which show broader diffraction

Fig. 2. Scanning electron micrographs. Top three: pristine FeOCl. Bottom three: ( Ppy)o 34FeOCI.

598 M.G. Kanatzidis eta/. ~In situ intercalat;ve polymerization chemi.slrv ql'kPO( 7

e g

S

ea

~ I '

(3x) e . , ' ', e ,

40.0 35.0 30.0 25.0 20.0 15.0 I0.0 5.0

2 a~ (degrees)

Fig. 3. X-ray powder diffractometric traces. Top: FeOCI. Bottom: ( Pth)o 2~FeOCI. A similar powder pattcrn is obtained for ( Ppy ), 34FcOCI.

lines 2. A typical example is shown in fig. 3. The interlayer spacing in ( Pth )028FeOC1 (b-axis)

is 13.31 ( 1 ) ,&, an expansion of 5.33 A from 7.980 (2) ,& in pristine FeOCI [50]. This is comparable to what is observed for (Ppy)o.3aFeOCl [33] , 5.23 A; (Py)o33FeOC1 [36,37] , 5.29 A: (Py)ossTaS2 [45,46], 5.19 A, and (TTF)o. jesFeOCI [51 -55 ] , 5.03 A. It has been demonst ra ted for (py)o55TaS2 (py = pyr id ine) and postulated for other pyr idine ~ [56,57] intercalat ion compounds that the plane of the intercalated molecule lies perpendicular to and the c2 axis parallel to the layers of the host solid [ 58- 60]. A similar structural model can be advanced for I and II. The FeOCI layers are anisotropic with chains of doubly bridging chloride ions extending along the c-axis (fig, 1). These rows are parallel and spaced 3.80 A apart creating channels that also extend par-

2 If the intercalation reaction is carried out at higher tempera- lures ( > 80~C) and for longer times ( > 2 weeks), the SEM photographs show what appears to be amorphous polymeric film covering the cry. stal surfaces. X-ray powder diffraction diagrams of these overreacted samples show broad diffraction lines suggesting considerable decrease in the crystallite size and/ or severe disorder resulting from the disruption of the FeOCI lattice.

' Pyridine intercalated in MnPSe3 [52] and CdPS~ [57] is be- lieved to be oriented with the molecular plane parallel to the host layers.

allel to the c-axis. It is then reasonable to propose that the 3.33 J~ thick planar polypyrrole or thiophene subunits may prefer to line up with such valleys in order to minimize steric repulsion, maximize polymer-lat t ice at traction, and achieve more efficient packing. The fact that there is a shift of the FeOCI layers by a / 2 upon intercalat ion in the a direction from a staggered to an eclipsed configuration, cer- tainly supports such a not ion (see e.g., fig. 4). A similar hypothesis has been advanced for metallocene intercalates of FeOCI [ (CsHs) 2M ] o. ~6FeOCI ) [ 85- 90]. EXAFS and X-ray diffraction studies in (TTF)o~vFeOC1 also suggest that the structurally similar planar TTF molecule aligns itself along the c-axis with its molecular plane perpendicular to thc FeOC1 layers [53].

As in (TTF)o 125FEOC1 [51-55] , [(CsHs)2M]o.I~FeOCI [44] ( M = F e , Co) and (Ppy) o.34FeOCl [ 33 ], the diffraction data for II are consistent with a space group change from Pmnm (in FeOCI) to I m m m or I222 and doubling of the b-

axis. The body-centered unit cell results from a lateral shift of the al ternate FeOC1 layers (fig. 1 ) in the a direct ion of the ac plane so that the chlorine a toms lie directly on top of each other (fig. 4). Approxi- mate cell d imensions for I and II, as calculated from the powder X-ray diffraction data are a = 3.81 ( 1 ) A,

M.G. Kanatzidis el al. ~In situ intercalative polvmeri;atwn chemistry q/FeOCl

INTERCALA TEl) FeOC

/!i? 'il r~ r~5 /Lil - ~ ' ~ ~!:.~ ~ ~ , j

O r t h o r h o r n b i c a = 3 . 8 1 b = 2 8 . 4 3 ,,4 c = 3 . 3 3 A

( b o d y - c e n t e r e d )

599

2G. 4 3

~,j, Q o

Cl 0 Fa

: t r a l i o n t 3 . 2 t 6 r~ i r t t e r c a l a l i o n : 5 . 2 9 8 ,4

o p t . . e ~ . , u . p , D~tTrtm or 1 2 2 2

Fig. 4. Structural model of the intercalated polypyrrole chains within the van der Waals gap of FeOCI. The polymer chain direction is parallel to the c-axis. In this configuration the CI chains are eclipsed thereby creating a body-centered unit cell with h= 26.43.4.

b = 2 6 . 4 1 6 ( 1 2 ) / ~ and c = 3 . 3 3 ( 1 ) A and a = 3 . 8 0 ( 1 ) A, b=26 .62( 1 ) A, and c = 3 .33 ( 1 ) A, respectively.

The nature of the organic material in I and It was probed by a variety of chemical, spectroscopic and charge-transport techniques all of which indicate the presence of intercalated high molecular weight, conductive polymers.

Pyrolysis mass spectra of I and II (up to 320°C) show no volatile species (monomer, dimer, etc.) at least up to 700 ainu. This implies thermally stable high molecular weight materials and is reminiscent of the behavior of other conducting polymeric materials such as { (PPy) C1o.27),, [ 33 ] and {(Pth)(FeCl~)o.o53}n [61 ]. This high thermal sta-

bility is clearly evident in thermogravimetric analysis (TGA) curves recorded under He. Typical TGA curves for I and for (Py)o 33FeOCI are shown in fig. 5. From 20-350°C only a 1-2% weight loss is observed from I and is attributed to residual solvent trapped during the reaction and /o r adsorbed at- mospheric moisture. In the case of (Pth)o 2~FeOCI, rapid weight loss is observed at temperatures above 350°C and can be associated with SO2 loss (by mass spectrometrx ). It appears that at these temperatures, reaction between the FeOCI layers and the polymer intercalate occurs which results in decomposition. We believe that the source of oxygen in SO3 is FeOC1. The weight loss continues up to 840 :C with 58.3%

600 M. G. Kanatzidis et al. ~In situ interca/ative polymerization chemtsto, o f FeOCl

of the original weight remaining. No further weight loss is observed up to 1000°C. In (Ppy)o.34FeOCI, dramatic weight loss only begins at even higher tern-

TGA (10 d e g / m i n ) I n He gas

IOO 4

O 200 400 600 800 IOOO 12OO TEMPERATURE (C °)

Fig. 5. Thermogravimetric analysis data for (Ppy)034FeOCI and (pyridine)() ~3FeOCI under He gas.

L~

03

j _ j -J

4000 3000 2000 1000 400

IFA VENUMBERS (errs- 1)

Fig. 6. NI'-IR spectra (KBr pellets) of (A); (Pth)o.28FeOCl, (B); ( Pth ) (FeC14) o 053 prepared chemically from the reaction of 2,2' - bithiophene with FeCI3 in anhydrous CH3CN, (C); (Pth)Clo jj material isolated from dissolution of the FeOC1 portion of (Pth) o 2sFeOCI with aqueous HCI.

peratures in the vicinity of 500°C, with weight sta- bilization occurring above 750°C (fig. 5 ). The black residue remaining from both I and It appears to be a multiphase microcrystalline, highly reduced, finely divided, magnetic iron oxide which immediately turns red (probably Fe203) upon exposure to air. Almost no chloride was detected in such residues, suggesting that most of the hydrocarbon polymer backbone reacts to form volatile chlorinated products at T > 3 3 0 ° C . It should be noted that the thermal behavior of other intercalation compounds which contain discrete molecular species such as (TTF)~.~2FeOC1 [51-55] , (Py),).33FeOC1 [46], is markedly different (fig. 5). In the latter materials, pyridine, 4,4'-bipyridine, and chloropyridine readily volatize and are detected by mass spectroscopy at temperatures as low as I I 0 ° C [61]. In (TTF)o.125FeOC1, TTF evolution is observed [51- 55].

Further evidence for the presence of an intercalated relatively high molecular weight polymer in 1 and I1 comes from experiments involving dissolution of the FeOC1 lattice in aqueous HCI and iso- lation of the remaining organic product. The latter materials are insoluble in all solvents, X-ray amorphous, and thermally stable. Moreover, they are spectroscopically and electrically (vide infra) very similar to chemically [27-321 or electrochemically [24-26] made polythiophene and polypyrrole. We have chemically synthesized polythiophcne by the oxidation of 2,2'-bithiophene with FeCI3 in CH~CN under strictly anhydrous conditions. A black polymeric product, I11 is obtained which is amorphous by X-ray diffraction and has the stoichiometry {(C4H2S)(FeCla)oos},,. Polymers with similar stoichiometries are known with BF2 [ 62,63 ]. The similarity of the F T - I R spectra of (Pth)oasFeOCl, chemically prepared polythiophene and polymer extracted from (Pth)o.28FeOC1 with HCI (fig. 6) and (Ppy)o.34FeOC1, chemically prepared polypyrrole, and polymer extracted from (Ppy)o.34FeOCI, (fig. 7), argues that I and II contain the corresponding intercalated conducting polymers with basic structural characteristics very, similar to those materials prepared chemically. Furthermore, the IR spectra shown in figs. 6 and 7 are in excellent agreement with those reported for iodine-doped polythiophene and electrochemically prepared polypyrrole films, re-

M. G. Kanatzidis et al. / In situ intercalative polymerization chemistry o f FeOC7 6 01

~3 (3 Z

t--

Z

B)

(c)

I I I I I I i~oo ~2oo 800 400

WAVENUMBERS (cm -1)

Fig. 7. FT-IR spectra ( KBr pellets) in the region 1000-400 cm- J showing (a) (Ppy)o.34FeOCl, (b) (Ppy) (N03)o.27 prepared from an aqueous ferric nitrate solution and (c) polymer obtained by dissolving the FeOC1 matrix of (Ppy)o34FeOCI (from refs. [31,32]).

spectively [ 64,65 ]. The small differences in peak in- tensity in the region 500-800 cm-J are due to the sensitivity of such modes (ring deformations, C-H out of plane bending, etc.) to polymerization conditions, counterion and local microstructure [31 ]. Pristine FeOC1 is transparent in the 4000-500 c m - region but absorbs strongly at ~ 475 cm-~ which is also present in the spectra of I and II [61 ].

The long tailing in the transmission IR spectra in the region of 1500 cm -~ to 4000 cm - t (fig. 6) represents the onset of a very broad electronic transition which peaks at approximately 8000 cm-~ (vide infra) and is attributed to the excitation of electrons

to bipolaron states [65-69]. Similar features have been observed in electrochemically prepared polythiophene and polypyrrole films [29].

It is interesting to comment on the degree of polymerization of 2,2'-bithiophene or polymer chain length in II. In the absence of unambiguous and re- liable experiments to determine such an important parameter we could at least place a lower limit ofn 1> 8

in [H--~--~s~.~ff,-H ] based on the following: (a) pyrolysis, mass-spectral evidence as discussed above, (b) the reactivity of 2,2'-bithiophene radical cations is less than those derived from thiophene [71 ]. This is known to result in polymers with smaller chain lengths than when thiophene is used [71], (c) in typical electrochemical preparations the average degree of polymerization of thiophene derived materials is 35-45, and (d) the minimum necessary chain length required for doped thiophene oligomers to observe substantial conductivity is approximately five thiophene units [65]. In fact, the IR spectra of such oligomers are identical to those of the polymers [65 ]. Therefore it is likely that the major species intercalated in II is at least an octamer. Similar arguments may be advanced for (Ppy)o.34FeOCl and will be reported elsewhere [ 104 ].

The reactions of pyrrole and 2,2'-bithiophene with FeOC1 require the loss of two hydrogen atoms per monomer unit. At present, the fate of these hydrogen atoms is not known although on the basis of mass balance arguments they should remain either on the polymer chain or in the FeOC1 lattice. If they remain on the polymer chain then either protonation of the heteroatom occurs (N, S) or double bond hydrogenation. The other possibility of course is their oc- currence in as yet unidentified, unintercalated by- products. The room temperature 57Fe M6ssbauer results (vide infra) from I and II do not indicate a significantly higher degree of Fe reduction and do not support the notion that significant amounts of hydrogen atoms are found as charge-compensating protons in the FeOC1 lattice. A clue that some of this hydrogen may end up saturating double bonds in the polymer chain, is derived from the charge transport measurements (vide infra). Since two H atoms are eliminated per pyrrole monomer versus one such atom per thiophene (2H atoms per 2,2'-bithiophene) it is reasonable to expect that if double bond hydrogenation occurs it would be more extensive in

602 M.G. Kanatzidis et al. ~In situ intercalative polymerization chemistry o/ FeO('l

1 than in II. Such double bond hydrogenation is ex- pected to be detrimental to charge flow in the polymer chain due to decreased conjugation of the bonds. Therefore I should be less conductive than II. This is in accord with the experimental data (vide infra).

3.2. MOssbauer spectroscopy

The -~7Fe M6ssbauer spectrum of II at room temperature is shown in fig. 8 and features an isomer shift of 0.42 m m / s and a quadrupole splitting of 0.82 m m / s. For (Ppy)o34FeOC1, I.S.=0.41 m m / s and QS=0.73 m m / s [33]. These parameters are in con- cert with those for (py)o.33FeOC1 [58] ( I .S .=0.42 mm/s , I .S.=0.73 mm/s , (TTF)oL2FeOC1 [54] (1.S.=0.46 m m / s I .S.=0.75 m m / s ) and other FeOCI intercalates. The value of the isomer shift for (Pth)o2sFeOCl represents a small, but significant increase of 0.04 m m / s relative to pristine FeOCI and it is due to FeOCI host lattice reduction. This ob- servation is consistent with the fact that the intercalation reaction with 2,2'-bithiophene is a redox process. Electrons are transferred to the host solid and are delocalized over all Fe sites, thus yielding an average formal oxidation state of Fe less than + 3.

This mixed valency gives rise to a time-averaged. single doublet at room temperature. From a variable temperature STFe MSssbauer study on (Py), .~FeOCl and (Py)..~oFeOC[, Herber e ta[ . [58] have deter- mined that the degree of reduction per Fe atom in both compounds is 0.095(5) electrons. A similar study by Averill et al. [55 ] on the (TTF),FeOC1 class of compounds found 0.04 to 0.063 electrons per Fe. The very similar M6ssbauer parameters found for 1 and I1 suggest comparable degree of FeOC[ reduction.

3.3. Charge transport properties

3.3. I. Electrical conductivity

ac and dc electrical conductivity measurements on the (Pth)o._,sFeOC1 compound were performed in the four-probe geometry using compressed polycrystalline samples. Similar data for (PPy)~ ~4FeOC1 have been reported earlier [33]. Significant preferential orientation occurs upon pressing pellets for measurements. The FeOC1 layers are found to be oriented perpendicular to the pressing axis (as evi- denced by X-ray powder diffraction experiments on such pellets). The conductivity measurements were carried out parallel to the FeOC1 layers. In the absence of large single crystals suitable for such mea-

1 . 0 1 0 -

1 . 0 0 0 -

, 990 -

- - . 9 8 0 -

. 9 7 0

. 9 6 0

.95C

( P o l y t h i o p h e n e ) o . 2 8 F e O C l

+ +

l

~.000 - 9 . 0 0 0 - 6 . 0 0 0 - 3 , 0 0 0 . 0 0 0 3 . 0 0 0 6 . 0 0 0 9 , 0 0 0 ] 2 . 0 0 0

VELOCI3~',MM/SEC

Fig. 8.57Fe MiSssbauer spectrum of (Pth)o.2sFeOCI I at 300 K. Isomer shifts are plotted versus Fe~O~ reference.

M . G . Kanatz id i s et al. ~In situ intercalative po lymer iza t ion chemis t ry o f FeOCl 603

surements and based on a large body of empirical data [ 72-78], the conductivity values obtained from such polycrystalline samples will most likely be 10- 2 or 10 -3 less than those from single crystals of the same samples.

At room temperature, the conductivity of (Pth)o2sFeOC1 is 5 ~ ~ cm-~. Although comparable to other conductive polycrystalline organic or polymeric materials, this value is the largest ever recorded for an FeOC1 intercalation compound, exceeding even that of the related (Ppy)o.33FeOC1 [33]. Typically, FeOC1 intercalated materials such as (PY)o33FeOC1 [79] and (TTF)oi25FeOCI [ 51,52 ] exhibit conductivities (polycrystalline sam- pies) in the range between 10 -4 and 1 0 - 3 ~ ' ) - 1 c m - 1 .

It should be noted that CrRv~ 5 ~-1 cm-~ for II is still lower than the best reported value for electrochemically prepared polythiophene (i.e. in (Pth)(BF4)o3 a~100 f2 -~ cm -1 [80,81] 4), but roughly one order of magnitude larger than in the chemically prepared polythiophene (C4H,S) [ FeC14 ] o o53. The disparities in conductivities of these samples probably reflect the fact that in an electrochemically prepared polythiophene sample a contiguous smooth film is obtained with good interparticle/interfibrillar electrical contact, thus fa- cilitating charge flow. This would be less true in powder or polycrystalline samples where the accompanying high interparticle contact resistance will be detrimental to macroscopic charge transport. Furthermore, it should also be noted that I is only 17.6% polythiophene by weight. The remainder is FeOC1 which acts as "dopant" and does not contribute significantly (vide infra) to the conductivity. If an adjustment is made for this "dilution" then ac- tual conductivities should be higher yet.

In other FeOC1 intercalation compounds where the guest species are molecular in nature, the observed room temperature conductivities are much lower than in I or II and are due to the small but finite mo- bility of 3d Fe electrons in the FeOC1 slabs [ 58]. The FeOC1 layers therefore are incapable of conductivities greater than 10 -3 ~-~ cm-~ and hence, the high values obtained in I and II must be due to the ex-

4 In the majority of cases [81] the conductivity of polythiophene ranges between 10 -j and 102 ~ - J cm-L

istence of a conducting polymer within the in FeOC1 matrix.

Due to the significant interparticle contact resistance in pellets of I and II, variable temperature conductivity measurements (fig. 9) down to liquid he- lium temperatures show thermally activated behavior with a decrease in conductivity with falling temperature. However, even at 4.2 K, the materials remain fairly conductive with a conductivity value close to 0.1 ~ - ~ cm- ~. This is to be contrasted with the behavior of chemically prepared polythiophene (i.e., (C4H2S) [FeCI4 ]o.53) which rapidly becomes insu- lating on cooling. Interestingly, the polythiophene isolated from II by extraction/removal of the FeOCI skeleton exhibits lower conductivities than II with aRT ~ 0.1 ~ - ~ cm ~. We tentatively suggest that the structural control over the polymer backbone ex- erted by the inorganic matrix on II facilitates charge transport by maintaining interchain electrical contact.

The effect of annealing (at ~ 150°C) on the electrical properties of (Pth)o.2sFeOC1 and (Ppy)o 34FeOCI was studied in an effort to explore the possibility of obtaining a more conductive material. It was hoped that such a treatment might pro- mote coupling of polythiophene chain-ends to increase chain lengths or induce/increase intralamellar ordering of polymer chains. However, it was found that such a treatment was harmful to the electrical properties of the material with resulting conductivities generally being an order of magnitude lower. The

/ ] 7 (Pth) 0.28F eOC ]

L

. . . . .

j

2 I- i

0 ,/'0 45 bO 8G 200 22G 140 lUG: d8U gG'D

s. 7 <,csoo. , ;

Fig. 9. Four probe variable temperature electrical conductivity data for a polycrystalline pressed pellet of (Pth)o 28FeOCI from 300 to 4.5 K.

604 M.G. Kanatzidis et al. ~In situ intercalative polymer~zatton chemistry o/'FeOCI

X-ray powder diffraction pattern did not change significantly after annealing. Conductivity data for vacuum annealed (Pth)o 2sFeOCl, chemically prepared ( Pth ) (FeCI~)o.o~3, and (Pth) Clo. ~ ~ obtained by HCI extraction of (Pth)o.2~FeOC1 are shown in fig. 10.

The conduction mechanism in either polycrystalline or amorphous macromolecular materials has been the subject of considerable discussion [82 -94] . The issue is complicated by the unavailability of charge transport data on single crystals. Significant interparticle resistance and other effects cause thermally activated electrical conductivity behavior but cr versus 1 / T plots do not typically [82-94 ] follow standard intrinsic semiconductor behavior [ ~ r = ~ r o e x p ( - K / T ) ] . In the case of II, exhaustive numerical attempts were made via computer to ar- rive at an appropriate mathematical fit to the experimental data. Fig. 11 shows that a reasonable fit (straight line) is obtained when a variable-range hopping model [82] (eq. ( 1 )) is considered

a = ~r~ exp( - T o / T °'2"s ) . ( 1 )

Electrochemically prepared polythiophene films generally exhibit variable range hopping behavior as well [95,96]. In agreement with the literature [97- 99 ], conductivity data from (Pth) (FeCl4)o.os~ can also be fit to a variable-range hopping model (fig. 12). Thus, for both I and II the conductivity char-

/ t

i '

i

- \ • ( ~ t k c.: !gbeC~ ]

" ( P t b ' ' b o : ]~ ' , : : , a5 a

,, ( p ~ h ) i u l :

(A)

/ 7 ( l )[ jL; ~ ,

Fig. 10. Four probe variable temperature electrical conductivity data (pressed pellets) on A. (Pth)o.~sFeOC1 vacuum annealed, B. chemically prepared ( Pth ) (FeCI4)o.o53, C. (Pth)Clo ~, material obtained from (Pth)o 2sFeOCl after dissolution of the FeOCI portion in aqueous HCI,

I

J , ~ t ' , o. a s r e J i

. ~ i i i ;

Fig. 1 I. Conduct iv i ty data for a polycrystall ine (P th )~ ~sFeOCI pellet plotted in a log u versus ( 1 / T ) ~'~s format to il lustrate correspondence with the variable-range hopping model (eq. ( I ) ).

~ t

, ' \ ~ \ P t b , ) ( F e E l , t . sh J

J ; \ \

~ i "\\

I

j ~

Fig. 12. Conductivity data for a polycrystalline ( Pth ) ( FeCI4 )~ ~,~

pellet (chemically prepared) plotted in a log o-versus ( l / T ) ~' -'~ format to illustrate correspondence with the variable-range hopping model•

acteristics are compatible with the known transport characteristics of polypyrrole and polythiophene, respectively.

3.3.2. Thermoelectric power In contrast and complementary to electrical con-

ductivity measurements, thermoelectric power is a zero current technique and thus, less sensitive to interparticle resistance effects. It is a more direct probe of the intrinsic charge transport characteristics of the material• The thermoelectric power (or Seebeck

M.G. Kanatzidis et al. ~In situ intercalative polymerization chemistry of FeOCl 605

coefficient ) of polycrystalline pellets of (Pth)o.2sFeOC1 was measured in the range 4 to 300 K and typical data are shown in fig. 13. The thermopower is small ( ~ 5 gV/deg at room temperature) and positive, and decreases linearly with temperature. This is typical of a metal and similar to electrochemically prepared polythiophene samples [95,96]. The positive sign of the thermoelectric power identifies holes as the predominant charge carriers in (Pth)o.28FeOCl. This is consistent with the presence of partially oxidized polythiophene chains and further rules out the importance of the FeOC1 matrix as the charge carrier. At temperatures below ~ 35 K a reversal in the slope of S(T) and a rapid rise to high positive values is observed in II (fig. 13). Although independent evidence would be desirable, this type of behavior is generally indicative of a phase transition to a semiconducting state [ 101 ] 5. In contrast, the corresponding conductivity data do not exhibit any anomaly indicative of such metal to semiconductor transition. However, it should be emphasized that such transitions may be masked in polycrystalline sample measurements [101 ] and thermoelectric power data are more re- liable in this respect. Similar transitions in electrochemically prepared polythiophenes have not been previously observed and it may be that the enforced structural dimensionality in I1 is a major driving force.

5 Similar behavior is observed in Ni(Pc)l.

The thermoelectric power of polycrystalline pellets of (Ppy)o.34FeOC1 and for comparison (Ppy) (N03)o.27 was measured in the range 100 K to 300 K (fig. 14). Although there are significant differences in variable temperature behavior, both materials show a positive Seebeck coefficient indicative of p-type conductivity. The small magnitude and linear decrease with temperature of the thermoelectric power of (Ppy)(NO3)o.27 is in agreement with the literature [ 97-99 ] consistent with metallic character very similar to (Pth)o2sFeOC1 and other polythiophene samples (vide supra). The thermoelectric power behavior of (Ppy)o 34FeOC1 is more complex (fig. 14 ). As the temperature is lowered, a major rise in the Seebeck coefficient is observed indicative of semiconductive behavior followed by a decrease (below 240°C) which is consistent with a metallic regime. Although the origin of this phenomenon is still under study we tentatively attribute this to the lower conductivity of the intercalated polypyrrole chains (relative to the polythiophene analog) and to competing conduction mechanisms through the polypyrrole chains and FeOC1 slabs. It has been dem- onstrated in other FeOCI intercalated materials that the FeOCI slabs are semiconducting [102 ]. There- fore, at higher temperatures thermal activation of electrons within the FeOCI band structure may be significant enough to contribute to the conductivity at these temperatures. At low temperatures ( < 240 K) conductivity through the polypyrrole chains appears to be dominant.

[ . . . .

24r i (P th ) ~. ~e~eOC.'

[Jmoor ' rec ted Cow Au

: p

c~ z uj

(5 [

F . . - - " " ' i

I

o 50 1 oo 150 ~oo , 5 o 300

r ~ P E R A 7d#E <K)

Fig. 13. Variable temperature thermoelectric power data for a compressed (Pth)o 28FeOCl pellet.

3ok

] (PPY) o. a4 FeOC] L- 2O

{ '~ i * P° 1 yPy r ' r ° i e ('~'

/

/ ©© ] 50 200 250 300 350

TEHPERA TURk (K)

Fig. 14. Variable temperature thermoelectric power data for compressed (Ppy)o s4FeOCl and (Ppy) (NO3)o 27 pellets.

606 M. G. Kanatzidis et al. ~In s~tu intercalattve polvmerization chemisto, qf FeOCl

0.50

0 . 4 0

0 . 3 0 r-

0 . 2 0

0 . 1 0

(pth)o.asFeOC1

\

0 . 0 0 . . . . . 1 0 0 0 1 0 0 0 0

co, c m -1

Fig. 15. Optical reflectivity as a function of frequency for a polycrystalline compaction of ( Pth )o ~sFeOCI.

Thermopower studies [ 102 ] on [ Fe (CsH 5) 2 ]o t ~FeOCI in the temperature range 300 to 100 K show qualitatively similar trends to those observed for I, but with much larger S values indicative of a less conductive material. The semiconductive thermopower behavior in [Fe(CsHs)~], ~,FeOCI was interpreted in terms of a hopping mechanism between Fe :+ and Fe ~+ sites in the FeOCI slabs [102].

3.3.3. Optical reflectance Optical reflectance data were obtained on pressed

polycrystalline pellets of (Pth)oe8FeOCl in the region 700 cm -t to 10000 cm t and are shown in fig. 15. The data are in agreement with literature results for heavily doped, electrochemically prepared polythiophene films [103]. A particularly character- istic feature of this reflectivity spectrum is the steady increase in reflectance with decreasing photon en- ergy from the minimum at ca. 8000 cm- L. This edge- like feature is indicative of a metallic band structure in accord with the thermopower data. A full analysis of the optical data will be reported elsewhere [ 104].

4. Summary and conclusions

Oxidatively polymerizable five-membered ring heterocycles such as pyrrole and 2,2'-bithiophene undergo intercalative polymerization in the layered FeOC1 to yield novel, electrically interesting materials. These materials consist of alternating organic

conducting polymer/inorganic host layers and therefore can be characterized as microlaminates. The ex- tent of intercalative polyrnerization strongly depends upon the anodic oxidation potentials of the heterocyclic five-membered ring monomers which must match the cathodic reduction potential of the inorganic host. We believe that the intercalative polymerization reaction is general in nature provided that the above premise regarding redox potentials is ful- filled. The microlaminate materials reported here, (Ppy)o34FeOCI and (Pth)o2~FeOCl are thermally stable up to ~ 400°C and 320°C, respectively. Elec- trical charge transport measurement, show that the intercalated conducting polypyrrole and polythiophene polymers are very similar to other corresponding polymers prepared either chemically or electrochemically. Although (Pth)~ 2~FeOCI is more conductive than (Ppy)o 34FeOCI, both possess metal- like character in the charge transport properties. At temperatures below 35 K, a possible metal to semiconductor transition is observed in (Pth)o >FeOC1. The latter is the most conductive FeOCI intercalation compound known (in the range 35 to 300 K).

Acknowledgement

This research was supported at Northwestern by the Office of Naval Research and the Northwestern Materials Research Center (NSF-MRL Grant DMR 85-20280). We thank Professor K. Poeppelmeier for access to TGA equipment.

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