SYNTHESIS AND CHARACTERIZATION OF HIGHLY CONDUCTING, ENVIRONMENTALLY STABLE, IODINE COMPLEXES OF A SOLUBLE POLY N-METHYL 3,3' CARBAZOLYL

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SYNTHESIS AND CHARACTERIZATION OF HIGHLY CONDUCTING, ENVIRONMENTALLY STABLE, IODINE COMPLEXES OF A SOLUBLE

POLY N-METHYL 3,3’ CARBAZOLYL

S. Wellinghoff, T. Kedrowski, S. Jenekhe, H. Ishida

To cite this version:

S. Wellinghoff, T. Kedrowski, S. Jenekhe, H. Ishida. SYNTHESIS AND CHARACTERIZATION OF

HIGHLY CONDUCTING, ENVIRONMENTALLY STABLE, IODINE COMPLEXES OF A SOLU-

BLE POLY N-METHYL 3,3’ CARBAZOLYL. Journal de Physique Colloques, 1983, 44 (C3), pp.C3-

677-C3-681. �10.1051/jphyscol:19833132�. �jpa-00222644�

JOURNAL DE PHYSIQUE

Colloque C3, supplement au n

6, Tome 44, juin 1983 page C3-677

SYNTHESIS AND CHARACTERIZATION OF HIGHLY CONDUCTING, ENVIRONMENTALLY STABLE, IODINE COMPLEXES OF A SOLUBLE POLY N-METHYL 3 , 3 ' CARBAZOLYL

# * ** . ***

S.T. W e l l i n g h o f f , T. Kedrowski , S. Jenekhe and H. I s h i d a

* University of Minnesota, U.S.A.

**Corporate Technology Center, Honeywell Inc., Minneapolis, Mn, U.S.A.

***Case Western Reserve University, Cleveland, OH, U.S.A.

Résumé - Le poly N-méthyl 3,3' carbazolyl (PC) peut être préparé de la façon suivante : tout d'abord un agent di-Grignard est préparé à partir de 3,3' di- bromo N-méthyl carbazole et de magnésium activé. Cet agent est ajouté à une suspension de tétrahydrofurane contenant une quantité catalytique d'un complexe de nickel. Le PC jaunâtre obtenu par cette réaction, est soluble dans plusieurs solvants "accepteurs" tels que le nitrobenzène. Des films transparents sont obtenus, et le dopage avec Ig et Br2 peut être effectué facilement , rendant les films noirs et résistants. Les semiconducteurs (PC) à dopage complet con- tenant un atome I par carbazole monomère avaient des conductivitës jusqu'à 1 ohnT-'-cnf l avec une énergie d'activation de conduction en courant direct de 0,172 eV. Comme le spectre d'absorption des films dopés avec 12 ne présente pas de raies entre 0,3u et 25y, ces propriétés électriques suggèrent que le trans- port des charges dans ces matériaux amorphes se fait par "hopping". Les radicaux- cations du carbazole sont suffisamment stables dans les complexes pour mainte- nir inchangée cette conductivitë électrique, exposés à l'air à 25°C, durant au moins un mois.

Abstract - Poly N-methyl 3',3 carbazoly (PC) can be prepared by forming the di-Grignard reagent of 3',3 dibromo N-methyl carbazole with highly activated magnesium and subsequently adding this reagent to a tetrahydrofuran suspension containing a catalytic amount of nickle complex. The tan yellow PC isolated from this reaction mixture was soluble in a number of "acceptor" type solvents such as nitrobenzene. Thin, clear yellow films could be cast and were easily doped with l2» Br2 to form tough black films. Completely doped PC semicon- ductors containing one I atom per carbazole monomer unit, exhibited conduc- tivities as high as 1 ohrrr'cm-i with an activation energy for D.C. conduction of 0.172 eV. Together with the featureless absorbance spectrum shown by the I? doped films from 0.3u to 25y. these electrical properties suggest hopping as tne charge transport mechanism in these amorphous materials. The carbazole radical cations were sufficiently stable in the complexes to maintain this electrical conductivity unchanged in room air at 25°C for at least 1 month.

I. Introduction

Organic materials that behave as metals or semiconductors are of interest because 1) solubility in organic solvents or low melting points and glass transi- tions should both minimize the cost of processing and permit composites to be made with thermally sensitive materials such as doped Si or GaAs; 2) the enormous molecular design flexibility of organic chemistry allows precise tayloring of properties that fulfill a wide range of applications as is envisioned, for example, in the amorphous semiconductor field [1]; 3) their high strength and conductivity to weight ratio should facilitate the fabrication of low inertia motors for the computer industry and ultralightweight storage batteries.

A practical goal in the near future is to develop a polymer whereby thin films of some mechanical stability can be cast onto substrates, doped in a controlled fashion, and subsequently exhibit high electrical conductivity for long periods of time in moist air. So far none of the published organic conductors possess all of these properties, all of which are necessary for significant commercial application to bulk devices such as EF1I shielding, wire, battery plates or thin film structures such as microcircuits, large area solar cells, etc.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19833132

C3-678

P r e b e n t l y t h e l a r g e v a r i e t y of o r g a n i ~ metals and semiconductors t h a t are a v a i l a b l e can be s p l i t i n t o two classes: 1) non-polymeric; 2) polymer. Low molecular weight conductors are i n v a r i a b l y h i g h l y c r y s t a l l i n e , b r i t t l e , n o n - f i l m forming m a t e r i a l s . The c r y s t a l s t r u c t u r e of best conductors w i t h i n t h i s group con- s-ists of separate stacks of donor and acceptor molecules t h a t p a r t i a l l y t r a n s f e r charge t o p r o v i d e a one-dimensional c o n d u c t i v i t y p a t h f o r holes and e l e c t r o n s per- p e n d i c u l a r t o t h e plane of t h e f l a t o r - i c molecules [2,3,4].

Polymeric m a t e r i a l s should have considerable higher s t r e n g t h than low molecular weight m a t e r i a l s b o t h i n f i b e r and f i l m form. A l a r g e number o f polymeric conductors have been made [51. Again the most common method of g a i n i n g a h i g h l y conducting p-type m a t e r i a l i s by doping the polymer w i t h a charge t r a n s f e r acceptor such as 12, AsF5 from t h e gas o r w i t h C10-

BFj, o r by electrochemical o x i d a t i o n [6].

A1 k a l i metal doping provides an n-type m a t e r i a l i n many cases. O f the two types o f m a t e r i a l s o n l y the p type show any environmental s t a b i l i t y .

he most u n i f o r m l y doped and environmentally s t a b l e conducting polymers are e l e c t r o c h e n i c a l l y synthesized and simultaneously doped p o l y p y r r o l e f i l m s which a r e as conductive as l o 2 (ohm-cm)-' and are s t a b l e i n d e f i n i t e l y i n a i r [7]. Unfor- t u n a t e l y t h e f i l m s are b r i t t l e , of somewhat v a r i a b l e composition and insoluble_, A t the present time the o n l y two s o l u t i o n p r o c e s s i b l e polymers t h a t can be doped t o h i g h c o n d u c t i v i t i e s are p o l y m-phenylene [8] and p o l y m and p-phenylene s u l f i d e s [9]. Only AsF5 has enough e l e c t r o n a f f i n i t y t o generate r a d i c a l c a t i o n s i n t h e polymers; however, these c a t i o n s are s u f f i c i e n t l y unstable t h a t c r o s s l i n k i n g and r i n g f u s i o n r e a c t i o n s take p l a c e i n a d d i t i o n t o h y d r o l y s i s r e a c t i o n s which severely degrade the t h e c o n d u c t i v i t y .

An a d d i t i o n a l source o f p r o c e s s i b l e environmentally s t a b l e polymers m i g h t be the polymeric phthalocyanines t h a t have been synthesized [ l o ,111. These molecules are q u i t e s i m i l a r t o low molecular weight stacked compounds (TTF-TCNQ) except t h a t t h e stack o f f l a t aromatic r i n g s alongwhich the e l e c t r o n jumps i s coordinated t o a c o v a l e n t l y bound s t r i n g of fSi-OJ, linkages. Counterions such as I - , Ig , form a separate stack surrounding the donor stack. These m a t e r i a l s are s o j u b l e i n s t r o n g acids from which s o l u t i o n f i b e r s can be spun.

Recently we have undertaken a search f o r a new polymer system having t h e f o l - lowing p r o p e r t i e s : 1) p r o c e s s i b i l i t y , 2) h i g h environmental s t a b i 1 i t y of t h e polymer acceptor complex a t h i g h c o n d u c t i v i t y l e v e l s (>0.1 ohm-'cm-I), 3) molecular design f l e x i b i l i t y . The synthesis and c h a r a c t e r i z a t i o n o f t h e polymers and t h e i r i o d i n e complexes having these c h a r a c t e r i s t i c s has been achieved and i s t h e s u b j e c t o f a r e c e n t U.S. p a t e n t a p p l i c a t i o n by some of us [ I 2 1 and a r e c e n t M.S. t h e s i s i n our group [13].

11. Polycarbazoles - Results

Poly M-methyl 3,3' c a r b a z o l y l (PC) was synthesized i n t h e hope t h a t 1 ) a s o l u b l e polymer would be obtained because o f t h e conformational d i s o r d e r induced by t h e 3,3' b i a r y l l i n k a g e , 2) environmentally s t a b l e r a d i c a l c a t i o n s c o u l d be formed by doping w i t h acceptors much lower i n e l e c t r o n a f f i n i t y than AsF5 as f o r example i s found i n N - s u b s t i t u t e d 3,3' b i c a r b a z o l y l r a d i c a l c a t i o n s [14,15] which can be generated through i n t e r a c t i o n s o f the n e u t r a l b i c a r b a z o l y l w i t h e l e c t r o n withdrawing quinones, 3) h i g h e l e c t r i c a l c o n d u c t i v i t i e s s i m i l a r t o t h e s t r u c t u r a l l y analogous p o l y p y r r o l e would be achieved.

A. Synthesis o f t h e Polymer

The synthesis o f PC was s i m i l a r t o t h a t r e c e n t l y r e p o r t e d by workers a t t h e Asahi Glass Co. (Japan)[l6] who used several Kharasch type r e a c t i o n s t o produce an 14-ethylated 3,3' c a r b a z o l y l polymer which they claimed was u s e f u l as a photocon- ductor. However, no attempt was made t o generate r a d i c a l c a t i o n charge c a r r i e r s by doping w i t h charge t r a n s f e r acceptors.

We chose t o use t h e N-methyl s u b s t i t u t e d 3,3' dibromocarbazole as t h e monomer i n s t e a d o f ti-ethyl 1) t o minimize t h e i n t e r m o l e c u l a r d i s t a n c e between polymer chains i n t h e s o l i d , f a c i 1 i t a t i n g i n t e r c h a i n c a r r i e r t r a n s p o r t and 2) t o increase the donor nature o f the carbazole through hyperconjugation.

The f i r s t step was t o prepare t h e d i - G r i g n a r d reagent o f the monomer. T h i s

Was accomplished by using K metal t o p r e c i p i t a t e f i n e l y d i v i d e d a c t i v e Ma, metal from

a suspension o f PlgClp and K I i n r e f l u x i n g THF under N2 [I73 assuring a much more

r a p i d Grignard r e a c t i o n than w i t h Mg t u r n i n g s . A s o l u t i o n of t h e monomer (1 p a r t ) i n THF was subsequently i n j e c t e d i n t o t h e a c t i v a t e d Flg suspension ( 2 p a r t s )

immediately i n i t i a t i n g t h e Grignard r e a c t i o n . A f t e r 1 hour t h e s o u t i o n cleared and anhydrous powdered NiBr2(Pg3) [ I 8 1 was added under N , and then, f i n a l l y , addi- t i o n a l monomer. The r e s u l t a n $ suspension was r e f l u x e B f o r 72 h r . under N2 u n t i l a t a n y e l l o w p r e c i p i t a t e formed. The r e a c t i o n m i x t u r e was then quenched i n t o d i l u t e HC1, washed thoroughly w i t h water and d r i e d t o a y e l l o w powder.

B. Polymer C h a r a c t e r i z a t i o n and P r o p e r t i e s

Although the H t o C r a t i o expected f o r t h e polymer i s obtained i n t h e elemental a n a l y s i s , some unreacted B r end group i s s t i l l present i n t h e polymer. Besides t h e C-H o u t o f plane v i b r a t i o n s a t 800 cm-I and 860 cm-' which a r e c h a r a c t e r i s t i c o f 1,2,4 t r i s u b s t i t u t i o n , weaker C-H o u t of plane v i b r a t i o n s f o r u n s u b s t i t u t e d c a r - bazole end groups formed by h y d r o l y s i s of a c t i v e Grignard end groups were a l s o s l e n i n t h e IR spectrum o f t h e polymer ( F i g u r e 1 ) . The d o u b l e t nature o f t h e 860 cm- peak m i g h t o r i g i n a t e from b o t h 3 ' bromo and 3 ' c a r b a z o l y l s u b s t i t u t i o n .

TGA a n a l y s i s of t h e polymer revealed t h a t thermal degradation s t a r t s a t 300°C w i t h a small l o s s o f absorbed water below t h a t temperature. The t o t a l weight l o s s

i s o n l y 10% a t 700°C i n N2. The IR a n a l y s i s shows t h a t low molecular weight m a t e r i a l w i t h u n s u b s t i t u t e d ends i s l o s t by h e a t i n g i n N2 a t 300°C and t h a t h i g h e r molecular weights are l o s t a t 400°C.

The polymer could be d i s s o l v e d i n acceptor type s o l v e n t s such as nitrobenzene, 2,4 d i n i trochlorobenzene, and 2,4 d i n i troflurobenzene i n h i g h enough concentrations t o c a s t c l e a r y e l l o w f i l n s a t 50-100°C. P r o t i c s o l v e n t s (concentrated s u l f u r i c a c i d and t r i c h l o r o a c e t i c acid-0N02) formed s t a b l e deep green s o l u t i o n s w i t h PC a t room temperature, t y p i c a l of c a r b a z o l y l r a d i c a l c a t i o n . Heating above 160°C caused t h e r a d i c a l c a t i o n t o decompose t o a deep b l u e s o l u t i o n from which b l u e f i l m s c o u l d be cast.

The water i n s o l u b l e polymer i n i t i a l l y p r e c i p i t a t e d could be f u r t h e r subdivided i n t o an acetone s o l u b l e and acetone i n s o l u b l e f r a c t i o n . The GPC o f t h e acetone s o l u b l e f r a c t i o n i s shown i n F i g u r e 2 along w i t h superimposed t r a c e s o f monomer and

1 0 0

Standards

76 -I

g ,.&

B

-.

g

-&.

E

$ 25

!oOO 3600 3200 2800 2400 2 W O 1600 1200 800 400

wemumbmx

m r c w

Figure 1. Infrared spectrum of PC film as a function of doping with iodine.

Figure 2. Gel permeation chranatography r e s u l t s for polycarbazole and a series of standards.

dimer components synthesized f o r c a l i b r a t i o n purposes. 01 igomers up t o t r i m e r were predominant w i t h smaller q u a n t i t i e s o f oligomers up t o DP=10 a t t h e l i m i t of

d e t e c t i o n . S u r p r i s i n g l y t h i s m a t e r i a l c o u l d be r e d i s s o l v e d i n dimethylforamide and c a s t i n t o c l e a r y e l l o w f i l m s w i t h a Tg of 90°C. A t present t h e remaining f r a c t i o n s a l u b l e i n h o t nitrobenzene has n o t y e t been analyzed i n the GPC, b u t i t i s expected t o have considerably h i g h e r average molecular weight.

C. Complex Formation

S o l u t i o n o r m e l t c a s t f i l m s of PC were exposed t o I 2 vapor a t 50°C f o r 1 hour

t o y i e l d mechanically s t a b l e b l a c k f i l m s . Powders o f PC could a l s o be h i g h l y com-

plexed under t h e same c o n d i t i o n s t o produce a complex w i t h one i o d i n e per carbazole

monomer u n i t (elemental a n a l y s i s ) . A t low doping concentrations resonance Raman

spectroscopy revealed t h e presence of b o t h 13, I; b u t no I2 ( F i g u r e 3 ) . I n t e r e s t -

JOURNAL DE PHYSIQUE

i n g l y elemental a n a l y s i s showed t h a t a l l t h e Br end groups disappeared upon doping, a phenomenon seen before a s halogen e l i m i n a t i o n and formation of a bicarbazolyl when t h e r a d i c a l c a t i o n of 3,3,' diiodo N-substituted carbazole was formed [19]. Thus doping could be i n c r e a s i n g t h e molecular weight of t h e PC polymer. The e l e c t r i c a l c o n d u c t i v i t y of t h e complex was thermally a c t i v a t e d with an a c t i v a t i o n energy of only 0.172 eV (Figure 4) showing t h e very small energy d i f f e r e n c e between l o c a l i z e d s t a t e s a t t h e Fermi l e v e l i n agreement with t h e broad f e a t u r e l e s s e l e c t r o n i c absorbtion t h a t was observed between 0 . 3 ~ and 25p (e.g. Figure 1 ) . The DC p r ~ s s e d p e l l e t c o n d u c t i v i t y of t h e complex was always i n t h e v i c i n i t y of 0 . 1 ohm-'cm- a t 25"C, a value which did not change f o r 1 month i n room a i r (Figure 5 ) . The con- d u c t i v i t y of t h e i o d i n e doped f i l m c a s t from acetone i s considerably higher than t h e pressed powder and shows e q u a l l y good s t a b i l i t y .

I ^{Pressed pellet}

Figure 3 . Resonance Raman spectroscopy

of PC doped lightly with iodine Figure 4. Conductivity as a function of a t 50°C f o r 20 min. Laser l i n e temperature for iodine doped

514.5 nm. polycarbazole. Smali thermo-

e l e c t r i c voltage a t 25°C indi- cates p-type carrier.

Both acetone s o l u b l e and i n s o l u b l e f r a c t i o n s can be dissolved i n l i q u i d i o d i n e and c a s t onto s u b s t r a t e s t o y i e l d , a f t e r evaporation of excess i o d i n e , high qua1 i t y black f i l m s of conductivity i n t h e range of 1 ohm-'cm-' o r b e t t e r . D y i n g t h e evaporation of t h e excess i o d i n e an a c i d i c g a s , t h a t was most probably HI orHBr, was evolved. C l e a r l y , a s suspected above, continued polymerization i s occurring during I2 complexing. Obviously t h i s type of processing must produce uniform doping.of t h e polymer. I t should a l s o be mentioned t h a t f i b e r s could be pulled from

iodlne melts a t temperatures j u s t above t h e Tg of t h e mixture. Casting from l i q u i d Br2 a l s o produced f i l m s of high c o n d u c t i v i t y .

Cast film

I I

Me 13-

Radical cation stable on o ,p p o s i t i m s

8 *

I = r

Pressed p e l l e t

Iodine dmed polycarbazole 1

d - spacing (8)

Figure 5. The environmental s t a b i l i t y of. Figure 6. X-ray diffraction patterns f o r cast films and pressed pellets polycarbazole and iodine doped

of iodine doped polycarbazole polycarbazole.

a t 25°C. A

-

-

X-ray s c a t t e r i n g peaks a t about 2.8,,5.0, and 6.OA were v i s i b l e i n t h e i o d i n e

complex of PC powder ( F i g u r e 6 ) . he 5.OA spacing i s seen a l s o i n t h e undoped p a r t i a l l y c r y s t a l l i n e PC. The 2.8 B spacing, which i s n o t as prominant i n the s c a t t e r i n g of the undoped PC, i m p l i e s an a c t u a l spacing i n t h e s o l i d o f about 3.3;

when adjusted by t h e 1.2 f a c t o r t h a t i s used t o c o r r e c t f o r t h e predominantly amorphous s t r u c t u r e . T h i s value corresponds r a t h e r w e l l t o t h e d i s t a n c e t o be expected between o v e r l y i n g aromatic r i n g s on adjacent chains. If t h i s were t r u e s i g n i f i c a n t i n t e r c h a i n o r b i t a l o v e r l a p would occur, e x p l a i n i n g t h e h i g h conduc- t i v i t y i n t h e presence o f a l a r g e amount o f i n t r a c h a i n conformational d i s o r d e r . I n f a c t , i t has r e c e n t l y been observed t h a t amorphous p o l y p y r r o l e complexes have a h i g h e r c o n d u c t i v i t y than c r y s t a l l i n e complexes [20], thus p o i n t i n g o u t the importance o f i n t e r c h a i n t r a n s p o r t [21].

111. Conclusions

C o n d u c t i v i t y E. temperature and thermopower data on the almost amorphous PC-I2 complex a r e c o n s i s t e n t w i t h an i n t e r c h a i n h o l e hopping mechanism a t 25'C.

The h i g h envirsnmental s t a b i l i t y of t h e c a r r i e r s most probably has i t s o r i g i n i n t h e s t e r i c hindrance t o chemical a t t a c k a t t h e p o s i t i o n s o r t h o and para t o t h e n i t r o g e n atom, b o t h l i k e l y l o c a t i o n s f o r t h e c a t i o n i c charge. Since the i n t e r - molecular t r a n s p o r t i s r e 1 a t i v e l y e f f i c i e n t long range i n t r a m o l e c u l a r order i s n o t necessary. 3,3' l i n k a g e s can thus be used t o maximize conformational entropy and increase p r o c e s s i b i l i t y . The toughness o f t h e f i l m s was improved considerably by c a s t i n g o u t of an 12 m e l t where s h o r t e r oligomers were provided t h e o p p o r t u n i t y t o polymerize f u r t h e r .

Acknowledgements

The authors wish t o thank Dr. Donald Long and the Honeywell Corporate Tech- nology Center, Ilinneapolis, Minnesota, USA, f o r t h e i r continued support.

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