Solid State Chemistry: A Contemporary Overview - American

10 Conducting Polymers: A Comparison of the Properties of Polythiazyl (SN) and Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1970-0186.ch010

x

Polyacetylene (CH)x and Their Derivatives G. B.

STREET

and T .

C.

CLARKE

IBM Research Laboratory, San Jose, CA 95193

The

properties

of the

to exhibit metallic discussed.

Although

tine (CH) gens.

pristine

a low-band

x

becomes

only two

intriguingly

The

gap

l

(SN) (SN)

polymers

semiconductor,

effects of halogens

electronic,

and

their

after treatment and

are

x

is a semimetal

x

known

and doped (CH) ,

x

analogous

cussed from a structural, of

synthetic

conductivity,

pris-

behavior with

other dopants

halo-

are

dis-

and spectroscopic

point

view.

h e interest i n s y n t h e t i c m e t a l s , w h i c h h a d its o r i g i n s i n

Little's

t

-•-theoretical w o r k on room temperature superconductors

has

(1,2)

m a i n t a i n e d its i m p e t u s , e n c o u r a g e d b y systematic advances i n m a t e r i a l s r e s u l t i n g f r o m the s t r o n g l y i n t e r a c t i v e efforts of chemists a n d p h y s i c i s t s . T h e earliest m a t e r i a l s of interest w e r e the K r o g m a n n t y p e of salts

(3)

and

(4).

t h e o r g a n i c c h a r g e transfer salts e x e m p l i f i e d b y T T F - T C N Q

F o l l o w i n g these m a t e r i a l s c a m e (SN)

(5-7)

X

(8-11),

a n d its h a l o g e n d e r i v a t i v e s

t h e first p o l y m e r i c metals a n d s t i l l t h e o n l y examples of s u p e r

conducting polymers. exhibit a range

of

Most recently polyacetylene conductivity from

has b e e n s h o w n to

semiconductor

c h e m i c a l l y t r e a t e d w i t h a v a r i e t y of dopants

(12,13).

to

metal

when

A s t h i s class of

s y n t h e t i c metals has e x p a n d e d , so has the p o s s i b i l i t y of a c h i e v i n g m e t a l l i c a n d s e m i c o n d u c t i n g p r o p e r t i e s i n p o l y m e r s w h i l e r e t a i n i n g the t e c h n o l o g i c a l l y d e s i r a b l e characteristics of

plastics.

I n this p a p e r

we

will

e x a m i n e t h e p r o p e r t i e s of ( S N ) * a n d its d e r i v a t i v e s together w i t h those of t h e m e t a l l i c d e r i v a t i v e s of

(CH)^.

T h e c h e m i s t r y (6)

a n d physics

0-8412^0472r-l/80/33-186-177$05.00/1 © 1980 American Chemical Society Holt et al.; Solid State Chemistry: A Contemporary Overview Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

178

SOLID S T A T E

(5,7)

of

CHEMISTRY:

A

CONTEMPORARY OVERVIEW

( S N ) a . h a v e b e e n r e v i e w e d p r e v i o u s l y a n d its p r o p e r t i e s

have

b e e n c o m p a r e d w i t h those of t h e o r g a n i c c h a r g e t r a n s f e r salts (14).

The

p o l y t h i a z y l h a l i d e s h a v e b e e n the subject of a m o r e r e c e n t r e v i e w

(15).

T h e early w o r k on linear and substituted polyacetylenes cussed b y M e i e r

has b e e n d i s

(16).

History

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B o t h ( S N ) a . a n d ( C H ) ^ were w e l l k n o w n i n the c h e m i c a l literature f o r m a n y years p r i o r to t h e i r r e l a t i v e l y recent e x p l o i t a t i o n b y t h e s o l i d state c o m m u n i t y . most

I t is p e r t i n e n t to n o t e t h a t a n a p p r e c i a t i o n of t h e i r

significant properties

h a d to a w a i t extensive

materials w o r k

p r o d u c e h i g h - q u a l i t y crystals of ( S N ) a . a n d films of ( C H ) ^ .

to

S i n c e 1953

( S N ) a . has b e e n k n o w n to e x h i b i t i n t e r e s t i n g e l e c t r i c a l p r o p e r t i e s b u t i t w a s n o t u n t i l 1973 (18)

(17),

t h a t crystals of sufficient q u a l i t y t o d e m o n

strate its i n t r i n s i c m e t a l l i c p r o p e r t i e s w e r e o b t a i n e d . T h e difficulties w i t h ( C H ) a . , w h i c h is p r e p a r e d b y p o l y m e r i z i n g a c e t y l e n e i n p r e s e n c e of a Z i e g l e r - N a t t a catalyst, w e r e p e r h a p s h a v e n o t yet b e e n p r e p a r e d .

more

severe.

B e r e t s a n d S m i t h (19)

C r y s t a l s of

l e m s of p r o d u c i n g p o l y a c e t y l e n e free f r o m c a t a l y s t a n d o x y g e n . i t w a s t h e extensive w o r k of S h i r i k a w a a n d h i s c o - w o r k e r s finally

(CH)^

discussed the p r o b However, that

(20,21)

l e d to t h e o p t i m i z a t i o n of this process a n d the first h i g h l y p u r e

p o l y a c e t y l e n e , i n t h e f o r m of t h i n

films.

Nevertheless, Berets a n d S m i t h

w e r e a b l e to s h o w t h a t the c o n d u c t i v i t y of c o m p r e s s e d pellets of powder

c o u l d b e v a r i e d o v e r 7 orders of m a g n i t u d e .

(CH)a.

The conductivity

w a s i n c r e a s e d b y a factor of 1 0 b y t r e a t m e n t w i t h acceptors s u c h as B F , 3

3

w h e r e a s t r e a t m e n t w i t h a m m o n i a r e d u c e d the c o n d u c t i v i t y b y a f a c t o r of 10 . 4

C h i a n g et a l . (13)

i n conjunction w i t h Shirakawa, using donors

as w e l l as acceptors, h a v e v a r i e d t h e c o n d u c t i v i t y of p o l y a c e t y l e n e o v e r 12 o r d e r s of m a g n i t u d e .

We

have

films

shown that certain transition

m e t a l salt solutions, for e x a m p l e , A g C 1 0 / t o l u e n e , are also effective 4

c h a n g i n g the c o n d u c t i v i t y of

( C H ) ^ films (22).

E x a m i n a t i o n of

in

these

s i l v e r d e r i v a t i v e s of ( C H ) ^ has b e e n h e l p f u l i n e l u c i d a t i n g t h e m e c h a n i s m b y w h i c h other acceptors i n f l u e n c e the c o n d u c t i v i t y of ( C H ) ^ , a n d t h e y w i l l b e d i s c u s s e d i n some d e t a i l .

Structures

and Bonding

of

(SN)

X

and

(CH)

X

T h o u g h this r e v i e w w i l l b e c o n c e r n e d l a r g e l y w i t h t h e p r o p e r t i e s o f d e r i v a t i v e s of (SN)-,. a n d ( C H ) ^ r a t h e r t h a n the p r i s t i n e m a t e r i a l s , t h e similarities

between

the

structures of

(SN)*

and

(CH)*

themselves

deserve some a t t e n t i o n a n d are also significant to t h e d i s c u s s i o n of t h e i r

Holt et al.; Solid State Chemistry: A Contemporary Overview Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

10.

STREET

Conducting

AND CLARKE

physical properties.

179

Polymers

T h o u g h ( S N ) a . is f o r m e d v i a t h e s o l i d state p o l y

m e r i z a t i o n of S N , t h e r e s u l t i n g crystals a r e n o t h i g h l y p e r f e c t 2

2

(6).

T h e y a r e fibrous a n d u s u a l l y h i g h l y t w i n n e d besides c o n t a i n i n g a l a r g e n u m b e r of defects (23).

B y contrast, e v e n t h e best films of ( C H ) * a r e

p o l y c r y s t a l l i n e a n d n o t v e r y o r d e r e d (20), w e a k reflections i n X - r a y (24)

s h o w i n g o n l y a b o u t 10 b r o a d

a n d e l e c t r o n d i f f r a c t i o n . T h e films a r e

a c t u a l l y a m a t of r a n d o m l y a l i g n e d

fibers

p a c k e d w i t h o n l y 3 0 % of

t h e o r e t i c a l d e n s i t y ( 2 5 ) . E l e c t r o n d i f f r a c t i o n studies of i n d i v i d u a l

fibers


s h o w p r e f e r r e d o r i e n t a t i o n e v e n t h o u g h t h e i n d i v i d u a l fibers are p o o r l y c r y s t a l l i n e (26).

B y c o m b i n i n g the information from X - r a y diffraction

studies w i t h t h e p r e s u m e d g e o m e t r y of t h e ( C H ) a . c h a i n a n d m o l e c u l a r p a c k i n g c o n s i d e r a t i o n s , B a u g h m a n et a l . (24) for ds-polyacetylene.

have proposed a structure

T h e structures of b o t h (SN)-,. (27)

and

cis-(CH)

a}

are g i v e n i n F i g u r e 1. ( S N ) a . a n d ( C H ) ^ consist of essentially flat c h a i n s , t h e l e n g t h of w h i c h is u n k n o w n . I n t h e case of ( S N ) * , t h e c h a i n s p a c k i n p a r a l l e l p l a n e s , w h e r e a s t h e c h a i n s i n successive p l a n e s of (CH)

X

r o t a t e d s u c h t h a t t h e i r ?r c l o u d s

are

are almost orthogonal, r e d u c i n g the

nonbonded electron interaction. O b v i o u s l y , i t w o u l d b e d e s i r a b l e t o p r e p a r e ( C H ) a , i n t h e f o r m of s i n g l e crystals o r e v e n as o r i e n t e d films i n o r d e r to o b t a i n a m o r e

com

p l e t e u n d e r s t a n d i n g of m a n y of t h e p h e n o m e n a t h a t a r e of interest i n t h i s m a t e r i a l . I t o et a l . (20)

w e r e a b l e to p a r t i a l l y a l i g n films b y s t r e t c h i n g .

T h i s t e c h n i q u e has b e e n s i g n i f i c a n t l y i m p r o v e d r e c e n t l y b y S h i r a k a w a and Ikeda

(28)

to g i v e films t h a t e x h i b i t a n i s o t r o p i c p r o p e r t i e s .

The

s i m i l a r i t y i n t h e c h a i n axis r e p e a t i n g u n i t of ( S N ) a . a n d ( C H ) ^ , as s h o w n i n F i g u r e 1, e n c o u r a g e d

us to a t t e m p t to g r o w ( C H ) ^ e p i t a x i a l l y o n

c h a i n - a l i g n e d films of ( S N ) ^ . H o w e v e r , t h e n a t u r e of t h e p o l y m e r i z a t i o n process is s u c h t h a t i t o c c u r s at t h e a c e t y l e n e - c a t a l y s t s o l u t i o n i n t e r f a c e , a n d no epitaxy took place.

A t t e m p t s t o g r o w o r i e n t e d ( C H ) ^ films o n

s t r e t c h e d M y l a r f a i l e d f o r s i m i l a r reasons.

Intercalation

of

(SN)

X

and

(CH)

X

T h e comparatively weak interchain bonding i n ( S N ) ^ a n d ( C H ) a , i n d i c a t e s t h a t b o t h m a t e r i a l s m i g h t b e subject to i n t e r c a l a t i o n b y a p p r o p r i a t e guest m o l e c u l e s . T h e r e a d y f o r m a t i o n of a d d u c t s (29,30) S N 4

4

or S N 2

2

between

a n d L e w i s a c i d s , t o g e t h e r w i t h t h e o b s e r v a t i o n t h a t these

t y p e s o f m o l e c u l e s i n t e r a c t w i t h ( C H ) ^ as w e h a v e d e s c r i b e d , suggests t h a t t h e y c o u l d i n t e r c a l a t e i n b o t h c h a i n structures.

Exposing

(SN)a-

to m a n y s u c h m o l e c u l e s g a v e n o e x p a n s i o n c h a r a c t e r i s t i c of i n t e r c a l a t i o n , e x c e p t i n t h e case of b r o m i n e (31—40).

H o w e v e r , X - r a y studies of ( C H ) ^

e x p o s e d to a v a r i e t y o f m o l e c u l e s , i n c l u d i n g b r o m i n e , s h o w e d t h a t i n t e r c a l a t i o n takes p l a c e r e a d i l y i n this system.


180

CHEMISTRY: A



SOLID S T A T E

Figure I .

Comparison

of the structures of (SN) the chain configurations

X

and cis-(CH)

x

showing


10.

STREET AND

Conducting

CLARKE

B e r n a r d et a l . (31)

first

181

Polymers

reported that b r o m i n e c o u l d be intercalated

i n t o ( S N ) a - . T h o u g h t h e o r i g i n a l shape of t h e ( S N ) a . crystals is r e t a i n e d , at r o o m t e m p e r a t u r e b r o m i n e v a p o r causes t h e m to e x p a n d a p p r o x i m a t e l y 50%

i n v o l u m e p e r p e n d i c u l a r to the c h a i n axis, to g i v e a c o m p o s i t i o n

(SNBr .4)*. 0

D e s p i t e this e x p a n s i o n i n the v o l u m e of t h e c r y s t a l s , the

( S N ) a . u n i t c e l l v o l u m e contracts b y 6 % t h e c h a i n axis (32).

i n d i r e c t i o n s p e r p e n d i c u l a r to

N e i t h e r the crystals n o r the u n i t c e l l s h o w a n y

c h a n g e i n d i m e n s i o n s a l o n g t h e c h a i n axis. T h e fact t h a t t h e s e p a r a t i o n


b e t w e e n 102 p l a n e s decreases o n b r o m i n a t i o n s t r o n g l y suggests t h a t t h e b r o m i n e is i n t e r c a l a t i n g b e t w e e n t h e c h a i n s i n t h e 102 p l a n e a n d n o t b e t w e e n t h e 102 p l a n e s themselves. I n t e r c a l a t i o n of ( C H ) ^ w a s first d e m o n s t r a t e d b y B a u g h m a n et a l . They showed

(24).

f r o m X - r a y d a t a that i n t h e i o d i n e - d o p e d

r e p o r t e d b y C h i a n g et a l . (11-13) (100) films

planes.

We

films

the i o d i n e intercalates b e t w e e n

h a v e also p r e p a r e d

(CHIo^)^ by

exposing

the

(CH)^

to the r o o m t e m p e r a t u r e v a p o r pressure of i o d i n e o v e r n i g h t .

In

a g r e e m e n t w i t h B a u g h m a n , o u r X - r a y d a t a ( T a b l e I ) s h o w t h a t the i o d i n e enters the ( C H ) ^ l a t t i c e s u c h t h a t it causes a n e x p a n s i o n of t h e plane spacing.

The

(100)

(100)

p e a k , n o r m a l l y of z e r o i n t e n s i t y i n p r i s t i n e

( C H ) ^ , b e c o m e s q u i t e s t r o n g a n d shifts i n v a l u e , o w i n g to t h e i o d i n e o c c u p y i n g t h e ( C H ) ^ c h a i n sites i n t h e ( 1 0 0 )

a n d (200) planes i n an or

d e r e d f a s h i o n . S i m i l a r studies of ( C H B r o ^ y ) * (22) i n d i c a t e t h a t these I n the case of d(002)

and [ C H ( AsF ) .i5]# 5

species i n t e r c a l a t e i n a f a s h i o n s i m i l a r to

0

iodine.

( C H I 0 . 2 ) * w e w e r e also a b l e to m e a s u r e t h e s e p a r a t i o n

a n d s h o w t h a t the v o l u m e of the u n i t c e l l h a d e x p a n d e d

about 9 . 5 % .

by

F r o m the d a t a i n T a b l e I i t is p o s s i b l e to c a l c u l a t e the

v a n d e r W a a l s d i a m e t e r , D , of t h e i n t e r c a l a n t b y u s i n g t h e f o r m u l a D(A) =

2 d (200) 1

w h e r e d ? ( 2 0 0 ) is the v a l u e of d ( 2 0 0 )

3.80

after i n t e r c a l a t i o n . T h e

values

o b t a i n e d for i o d i n e a n d b r o m i n e , 4.06 A a n d 3.46 A r e s p e c t i v e l y , are i n g o o d a g r e e m e n t w i t h the l i t e r a t u r e (41).

T h e v a l u e o b t a i n e d for A s F

5

is 4.98 A , c o m p a r e d to 4.75 A d e t e r m i n e d f o r a n A s F - g r a p J i i t e i n t e r 5

calation compound

(42).

It is i n t e r e s t i n g to note t h a t b r o m i n e causes a

c o n t r a c t i o n i n u n i t c e l l p a r a m e t e r s for b o t h ( S N ) * a n d ( C H ) ^ .

Table I.

X - r a y D a t a for Pristine and Intercalated d(WO)

(A)

( C H ) .

(CHBro.47)*

(CHIo.2)^

[CH(ASF )O.IG]. b

7.31 7.96 8.83

&(200) (A) 3.80 3.63 3.93 4.39

(CH)j, &(002)

(A)

2.19 2.30


182

SOLID S T A T E

Structure

of the

CHEMISTRY: A


Intercalants

F r o m X - r a y d i f f r a c t i o n , e l e c t r o n d i f f r a c t i o n (9,34), a n d R a m a n s p e c t r o s c o p y (32-36),

(37),

infrared

(IR)

magnetic susceptibility

(38),

a n d e x t e n d e d X - r a y a b s o r p t i o n fine structure ( E X A F S )

data (39),

it

appears t h a t the b r o m i n e i n ( S N B r o . ) * is present as B r ~ ions as w e l l as 4

neutral B r molecules (15). 2

3

Besides b e i n g i n t e r c a l a t e d i n t h e 102 p l a n e s ,

the b r o m i n e species are also d i s t r i b u t e d i n t h e i n t e r f i b r i l l a r

regions.

B e c a u s e of the s m a l l d i a m e t e r (30 A ) of the fibers of ( S N ) * after b r o m i Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1970-0186.ch010

n a t i o n (9,34), total.

t h e i n t e r f i b r i l l a r b r o m i n e m a y b e as m u c h as 9 0 % of t h e

E l e c t r o n d i f f r a c t i o n (9)

show that the bromine

a n d diffuse X - r a y - s c a t t e r i n g d a t a

gives rise to a o n e - d i m e n s i o n a l

(40)

(1-D)

com

m e n s u r a t e s u p e r l a t t i c e o r i e n t e d a l o n g the c h a i n axis w i t h a p e r i o d

of

2b, t w i c e t h a t of the ( S N ) ^ c h a i n . F r o m g e o m e t r i c a l considerations, i t seems p l a u s i b l e t h a t the 2b s u p e r l a t t i c e is d u e to B r " o r i e n t e d p a r a l l e l 3

to t h e ( S N ) * . c h a i n s . I n o r d e r to g i v e rise t o t h e 1-D s u p e r l a t t i c e , t h e B r ~ m u s t be a t t a c h e d to t h e

( S N ) ^ chain i n a regular manner.

3

As

halogens are u s u a l l y a t t a c h e d to s u l f u r atoms i n p o l y t h i a z y l h a l i d e s , i t seems l i k e l y t h a t the B r ~ ions o r d e r w i t h t h e i r axes p a r a l l e l to the ( S N ) * 3

c h a i n s , i n a m a n n e r t h a t m a x i m i z e s s u l f u r - b r o m i n e i n t e r a c t i o n s , as s h o w n i n F i g u r e 2. Br

2

S i m i l a r p r i n c i p l e s w o u l d d e t e r m i n e the a r r a n g e m e n t of t h e

molecules. R a m a n s p e c t r o s c o p y shows that t h e b r o m i n e i n t e r c a l a t e d i n t o ( C H )

also exists as B r " ions (22,43).

x

F r o m X - r a y studies the axes of t h e B r "

3

3

ions w o u l d also a p p e a r to b e p a r a l l e l to the c h a i n (22).

However, in

this, case there is no e v i d e n c e of a n y o r d e r i n g of the B r " ions a l o n g t h e 3

c h a i n d i r e c t i o n , n o r is there a n y significant a m o u n t of n e u t r a l B r . A s 2

the ( C H ) a . fibers are a b o u t 200 A i n d i a m e t e r , almost a n o r d e r of m a g n i t u d e l a r g e r t h a n ( S N ) ^ , t h e a m o u n t of b r o m i n e o n the o u t s i d e of t h e fibers

is s m a l l c o m p a r e d

w i t h that intercalated between

the c h a i n s .

D e s p i t e these differences, t h e m o d e l s of b r o m i n a t e d ( S N ) ^ a n d

(CH)^

are r e m a r k a b l y s i m i l a r . I n b o t h cases the b r o m i n e oxidizes the c h a i n to give an extended polymeric

c a t i o n , a n d the l i n e a r B r " anions a l i g n 3

p a r a l l e l to the axis of the c h a i n s . IC1, I B r , a n d I

2

a l l react w i t h ( S N ) * (8,10,45),

b u t the p r o d u c t s

are not s i m p l e i n t e r c a l a t i o n c o m p o u n d s a n d t h e y h a v e n o t b e e n exten s i v e l y s t u d i e d . T h e s e differences m a y relate to the l a r g e r size of i o d i n e r e l a t i v e to b r o m i n e , b u t the l a c k of affinity of s u l f u r for i o d i n e m a y m o r e significant. I n the case of I C 1 b o t h e l e c t r o n d i f f r a c t i o n (44) R a m a n studies ( 3 3 )

be and

i n d i c a t e that the I C 1 is a b s o r b e d p r i m a r i l y o n t h e

surface as the n e u t r a l m o l e c u l e

a n d t h a t no

o x i d a t i o n of

the

chain

takes p l a c e . IC1, I B r , a n d I

2

a l l react w i t h ( C H ) ^ also, a n d i n t h e case of i o d i n e

the r e a c t i o n is o x i d a t i v e i n t e r c a l a t i o n , as w e h a v e a l r e a d y d i s c u s s e d f o r



10.

STREET

AND

Conducting

CLARKE

Polymers

183

Figure 2. Model for brominated (SN) showing possible orientation of Br ~ relative to the (SN) chain. The van der Waals dimensions of the Br ~ ion are shown to scale with the (SN) chain. X

s

X

3

X

b r o m i n e . T h e i o d i n e is present as I " a n d also as a s e c o n d species, either 3

I " or I 5

(25).

2

Though A s F

5

does not react s i m p l y w i t h ( S N ) * , i t does

give a n i n t e r c a l a t i o n c o m p o u n d

with

(CH)^

k n o w n a b o u t the final state of the A s F

5

H o w e v e r , m u c h less is

after i n t e r c a l a t i o n . I f the i n t e r

c a l a t i o n i n v o l v e s o x i d a t i o n of the c h a i n , as the e l e c t r i c a l p r o p e r t i e s suggest a n d as is t h e case for b r o m i n e a n d i o d i n e , the e x p e c t e d r e a c t i o n w o u l d b e 3AsF (where

e =

electron).

5

+ 2e -> 2 A s F " + 6

AsF

3

T h e experimental evidence

m e c h a n i s m is s u m m a r i z e d elsewhere

(46).

i n s u p p o r t of t h i s

E X A F S measurements

sug-

gst t h a t this r e a c t i o n also occurs i n t h e i n t e r c a l a t i o n of g r a p h i t e b y A s F

5

(47). Spectroscopy T h e I R (48,49)

a n d R a m a n spectra of

( S N ) * ( 4 9 , 5 0 ) have

been

i n v e s t i g a t e d s e v e r a l t i m e s ; h o w e v e r , m u c h of the e a r l y I R d a t a (51) c o m p l i c a t e d b y the presence of i m p u r i t i e s . R e c e n t studies b y et a l . (37)

is

Macklin

s h o w that o n b r o m i n a t i o n the I R a n d R a m a n s p e c t r a c h a n g e

i n a s i m i l a r m a n n e r : the peaks b r o a d e n a n d shift s o m e w h a t i n f r e q u e n c y . I n a d d i t i o n , s t r o n g I R peaks a p p e a r i n the R a m a n spectra a n d v i c e v e r s a because of the loss of the i n v e r s i o n c e n t e r r e l a t i n g t h e t w o c h a i n s i n t h e u n i t c e l l of p r i s t i n e ( S N ) * .

A l l these changes c a n b e e x p l a i n e d b y t h e

increase i n d i s o r d e r that takes p l a c e o n b r o m i n a t i o n . A f t e r b r o m i n a t i o n , the

difference

between

corresponding

IR

a n d R a m a n frequencies

is

r e d u c e d , r e f l e c t i n g the d e s t r u c t i o n of t h e v i b r a t i o n a l association b e t w e e n the ( S N )

x

chains. T h e 2 0 - 6 0 - c m "

1

shift i n the I R peaks r e l a t i v e to p r i s t i n e

(SN)-,. is a measure of the s t r o n g i n t e r a c t i o n b e t w e e n t h e ( S N ) * l a t t i c e


184

SOLID S T A T E

CHEMISTRY: A


a n d b r o m i n e . T h e o b s e r v e d increase i n f r e q u e n c y of t h e S N s t r e t c h i n g m o d e s o n b r o m i n a t i o n is consistent w i t h t h e o x i d a t i o n of t h e c h a i n b y t h e b r o m i n e , w h i c h w o u l d r e m o v e a n t i b o n d i n g ir*

(SN)*

electrons.

A l t h o u g h no n e w b a n d s a p p e a r t h a t c a n b e r e l a t e d to S - B r o r N - B r b o n d s , the p r e s e n c e of b r o m i n e as B r " a n d B r c a n b e d e t e r m i n e d f r o m 3

2

b a n d s i n t h e r e g i o n b e l o w 400 cm"" . I n the R a m a n (33,36)

the presence

1

of B r " is i d e n t i f i e d b y its s y m m e t r i c a n d a s y m m e t r i c s t r e t c h i n g f r e q u e n 3

cies at 150 a n d 190 c m "


i n t e r p r e t e d as B r

2

respectively.

(37),

1

A p e a k at 230 c m "

is

1

T h e l a r g e shift, f r o m the v a l u e of 325 c m '

(33,37).

1

o b s e r v e d f o r gaseous b r o m i n e , is a m e a s u r e of the s t r e n g t h of its associa t i o n w i t h the ( S N ) * l a t t i c e . U n l i k e ( S N ) * , t h e ( C H ) * c h a i n exists i n b o t h cis a n d trans f o r m s . A s p r e p a r e d at — 78 ° C , the stable f o r m has the cis g e o m e t r y , b u t o n h e a t i n g to 2 0 0 ° C , i t is c o n v e r t e d to the t h e r m o d y n a m i c a l l y m o r e stable trans f o r m (20,21).

T h e s e isomers are r e a d i l y d i s t i n g u i s h a b l e b y t h e i r c h a r a c

teristic I R a n d R a m a n s p e c t r a , as s h o w n i n T a b l e I I . T h e s e d a t a w e r e r e p o r t e d b y I t o et al. (21).

T h e y r e p o r t e d a C - C s t r e t c h at 1100 c m '

1

f o r t h e trans, b u t w e h a v e o b s e r v e d that this b a n d is a c t u a l l y a d o u b l e t at 1080 c m " a n d 1120 c m " 1

S y m m e t r y analysis (52)

(22).

1

a n d I R m o d e s leads to assignment of

of t h e R a m a n

cis-transoid a n d trans-transoid

geometries f o r the cis a n d trans forms of p o l y a c e t y l e n e , as s h o w n i n F i g u r e 3. O n a p p r o p r i a t e d o p i n g ( C H ) * b e c o m e s m e t a l l i c a n d absorbs s t r o n g l y i n t h e I R (12);

thus R a m a n t e c h n i q u e s h a v e p r o v i d e d m o s t i n f o r m a t i o n

a b o u t these m e t a l l i c d e r i v a t i v e s .

H o w e v e r , i n our experience

as

the

a m o u n t of a c c e p t o r ( B r , I , A s F , A g C 1 0 ) increases, t h e R a m a n b a n d s 2

2

5

4

associated w i t h p r i s t i n e ( C H ) * b e c o m e s p r o g r e s s i v e l y w e a k e r , a n d n o n e w b a n d s c h a r a c t e r i s t i c of the e l e c t r o n - d e p l e t e d

Table II.

( C H ) * chain appear

Infrared and Raman Spectra of and fr*MM-Polyacetylene

cis-

Observed Frequency Assignment C — H stretch C — H i n plane deformation C — H o u t of p l a n e d e f o r m a t i o n C — C — C bond deformation C = C stretch C — C stretch

1

trans

CIS

3057 3044 1329 1249 740 446 1552* 1262* 920*

(cm' )

(vw) (w) (s) (w) (vs) (vs)

3013 (m) 1292 ( w w ) 1015 (vs) 1474* 1080*, 1120* 1016*

° Raman bands.


10.

STREET

AND

H H \ / C=C /

Conducting

CLARKE

H \

\

/

H / C=C

c=c / \ H H

\

/ H

185

Polymers

/

c=c \ H

(a) H

H


I

H

I

I

I

I

H

I

H

H

(

t

h

Figure 3. The cis-transoid (a) and trans-transoid (b) forms of (CH)

e

X

T h i s m a y result f r o m a s c r e e n i n g effect associated w i t h the free

(22).

e l e c t r o n a b s o r p t i o n of the m e t a l l i c p o l y m e r .

D o p i n g mixed cis-trans

films leads to i n t e r e s t i n g effects o n the r e l a t i v e rates of d i s a p p e a r a n c e of cis a n d trans b a n d s . D o p i n g w i t h halogens or A s F

5

causes a decrease i n

t h e cis b a n d s r e l a t i v e to trans, w h i l e the reverse effect is o b s e r v e d treatment w i t h silver perchlorate.

on

T h i s does not result f r o m a c i s - t r a n s

i s o m e r i z a t i o n c a t a l y z e d b y the d o p a n t , because the absolute i n t e n s i t y of the trans b a n d s does not a c t u a l l y increase o n d o p i n g . I n s t e a d , the r e l a t i v e intensities of the cis a n d trans b a n d s c h a n g e ; b o t h g r a d u a l l y decrease w i t h increasing dopant concentration.

It appears m o r e l i k e l y t h a t this

is a k i n e t i c p h e n o m e n o n , t h e c i s - ( C H ) * r e a c t i n g faster t h a n trans w i t h AsF

5

a n d the halogens, w h e r e a s the reverse is t r u e for A g C 1 0 . 4

This

i n t e r p r e t a t i o n is r e i n f o r c e d b y the o b s e r v a t i o n t h a t o n t r e a t m e n t w i t h a m m o n i a the cis a n d trans R a m a n b a n d s of b o t h A s F - a n d A g C 1 0 5

4

t r e a t e d ( C H ) * films r e a p p e a r w i t h a p p r o x i m a t e l y the same r e l a t i v e a n d a b s o l u t e intensities as i n t h e u n d o p e d s t a r t i n g m a t e r i a l . T h e t w o C - C stretch b a n d s of trans-(CH)* also b e h a v e

differently w i t h different dopants.

p e r c h l o r a t e causes the 1120-cm"

1

at 1080 a n d 1120 c m "

1

Treatment w i t h silver

b a n d to decrease m o r e r a p i d l y t h a n t h e

1080-cm" b a n d . S u b s e q u e n t t r e a t m e n t w i t h a m m o n i a restores b o t h b a n d s 1

to t h e i r o r i g i n a l intensities. B r o m i n e s e l e c t i v i t y reduces t h e b a n d r e l a t i v e t o t h e b a n d at 1120 c m " . 1

1080-cm"

T h e s e results w o u l d seem

1

to

suggest the presence of t w o different types of trans e n v i r o n m e n t s , a l t h o u g h t h e n a t u r e of this difference is not clear. Bonding

in

(SN)

X

and

(CH)

X

B e f o r e d i s c u s s i n g t h e e l e c t r i c a l p r o p e r t i e s of these p o l y m e r s , i t is i n s t r u c t i v e to discuss t h e i r b o n d i n g .

F r o m simple linear combination

of a t o m i c o r b i t a l s a n d m o l e c u l a r o r b i t a l ( L C A O - M O ) t h e o r y a n d f r o m


186

SOLID S T A T E

CHEMISTRY: A


e l e m e n t a r y b a n d t h e o r y , it is possible to r a t i o n a l i z e t h e i r o b s e r v e d trical properties. treatment (SN),

elec

A m o r e r i g o r o u s , b u t to m a n y chemists less s a t i s f y i n g ,

is o b t a i n e d

from

detailed

band

structure c a l c u l a t i o n s

for

(7) and ( C H ) * (53-55).

I n the s i m p l e s t m o d e l for a n i s o l a t e d S N u n i t , b o t h s u l f u r a n d n i t r o g e n c a n b e a s s u m e d to be

sp

2

h y b r i d i z e d , w i t h the r e m a i n i n g p o r b i t a l

p e r p e n d i c u l a r to the p l a n e of the c h a i n . E i g h t electrons c a n b e m o d a t e d i n t h e t w o b o n d i n g a n d one n o n b o n d i n g s p


atom.

2

accom

orbitals o n

each

T w o electrons w o u l d t h e n go i n t o a ir o r b i t a l f o r m e d b y the p

orbitals o n the s u l f u r a n d n i t r o g e n a n d one e l e c t r o n w o u l d reside i n a n unfilled 7T*

antibonding orbital.

I n a c h a i n of s u c h S N u n i t s t h e

orbitals f o r m a d e l o c a l i z e d ir n e t w o r k a l o n g the c h a i n , r o u g h l y

p

corre

s p o n d i n g to t h e 7T a n d T T * o r b i t a l s of t h e i n i t i a l S N u n i t . T h e ir b a n d w i l l be

filled

b y t w o electrons p e r S N u n i t a n d t h e ir* l e v e l w i l l r e m a i n

h a l f - f i l l e d , w i t h one

electron per

(SN)

unit.

I t is this h a l f - f i l l e d ir*

o r b i t a l that, i n this s i m p l e m o d e l , is responsible f o r the m e t a l l i c c o n ductivity i n ( S N ) * .

B a n d structure c a l c u l a t i o n s r e v e a l t h a t ( S N ) * is

actually a semimetal (7). I n the case of ( C H ) * e a c h c a r b o n of t h e basic C H u n i t m a y a g a i n b e c o n s i d e r e d to b e s p h y b r i d i z e d , w i t h one e l e c t r o n f o r m a l l y i n v o l v e d 2

i n e a c h of the o- b o n d s to n e i g h b o r i n g atoms a n d t h e f o u r t h

valence

e l e c t r o n i n the p o r b i t a l p e r p e n d i c u l a r to the p l a n e of the c h a i n . W i t h i n e a c h basic C H 2

and

2

u n i t the p orbitals w i l l t h e n c o m b i n e to f o r m a filled w

a n e m p t y T T * o r b i t a l . I n a n e x t e n d e d p o l y o l e f i n these

molecular

orbitals t h e n g i v e rise to a filled ir b a n d a n d a n u n f i l l e d ir* b a n d . I n the l i m i t of c o m p l e t e e l e c t r o n d e r e a l i z a t i o n a n d u n i f o r m c a r b o n carbon bond

lengths, L C A O - M O

t h e o r y p r e d i c t s t h a t the

separation

b e t w e e n the ir a n d ir* b a n d s w o u l d a p p r o a c h zero, c a u s i n g ( C H ) * itself to

exhibit metallic behavior.

B o t h spectroscopic

evidence

(56)

and

t h e o r e t i c a l p r e d i c t i o n s ( 5 3 - 5 5 ) suggest, h o w e v e r , t h a t b o n d a l t e r n a t i o n and hence incomplete d e r e a l i z a t i o n w i l l be observed even i n very l o n g chain polyenes.

T h i s i n t r o d u c e s a finite g a p b e t w e e n t h e ir a n d ir* b a n d s ,

c a u s i n g ( C H ) * to b e h a v e

as a s e m i c o n d u c t o r ,

as is i n fact

observed

experimentally. R e c e n t b a n d s t r u c t u r e c a l c u l a t i o n s of G r a n t a n d B a t r a ( 5 5 ) that b o n d - a l t e r n a t e d ( C H ) * is a s e m i c o n d u c t o r .

confirm

I n case of trans-(CH)

^

the c a l c u l a t e d b a n d gap of 0.8 e V is c a u s e d b y b o n d a l t e r n a t i o n . I n t h e a b s e n c e of s u c h a l t e r n a t i o n t h e b a n d s t r u c t u r e of trans- ( C H ) * i n d i c a t e s that it w o u l d be a metal, i n agreement

w i t h our simpler arguments.

H o w e v e r , c i s - ( C H ) * , a c c o r d i n g to the b a n d s t r u c t u r e c a l c u l a t i o n s , w o u l d h a v e a b a n d gap of a b o u t 1.0 e V , r e s u l t i n g f r o m t h e c r y s t a l s y m m e t r y , e v e n f o r the case of u n i f o r m b o n d l e n g t h s . C h e m i c a l o x i d a t i o n of ( C H ) * films to g i v e a p o l y o l e f i n i c c a t i o n leads to r e m o v a l of electrons f r o m the filled ir v a l e n c e b a n d a n d the o b s e r v e d


10.

STREET AND

Conducting

CLARKE

increase i n c o n d u c t i v i t y .

187

Polymers

S i m i l a r l y , c h e m i c a l r e d u c t i o n to f o r m a p o l y -

olefinic a n i o n w o u l d l e a d to t h e i n t r o d u c t i o n of electrons i n t o t h e e m p t y 7r* c o n d u c t i o n b a n d , a g a i n g i v i n g rise to c o n d u c t i v i t y .

T o the

that d e r e a l i z a t i o n is i n c r e a s e d i n the i o n i c d e r i v a t i v e s of

extent

( C H ) * , the

w i d t h a n d s e p a r a t i o n of the IT a n d w* b a n d s m i g h t also b e e x p e c t e d t o change on doping.


Electrical

Properties of

(SN)

and

X

(CH)

X

T h e fact that ( C H ) * is a s m a l l - b a n d gap s e m i c o n d u c t o r ,

whereas

( S N ) * is a s e m i m e t a l , makes i t m u c h m o r e sensitive to d o p a n t s . the c o n d u c t i v i t y of ( C H ) * c a n b e v a r i e d o v e r 12 orders of

Indeed,

magnitude,

w h i l e t h a t of ( S N ) * c a n be v a r i e d o n l y b y one o r d e r of m a g n i t u d e .

In

a d d i t i o n , t h e reasons for t h e c o n d u c t i v i t y changes are not t h e same i n b o t h systems.

I n the case of

( C H ) * the observed

changes can

be

e x p l a i n e d i n terms of changes i n t h e n u m b e r of carriers, w h i l e i n ( S N ) * changes i n the l i f e t i m e or t h e m o b i l i t y of the carriers d o m i n a t e

the

conductivity. T h e increase i n t h e c o n d u c t i v i t y of ( S N ) * o n p a r t i a l o x i d a t i o n w i t h b r o m i n e is s h o w n i n F i g u r e 4 ( a )

as a f u n c t i o n of t e m p e r a t u r e

(9).

S i m i l a r changes i n the c o n d u c t i v i t y , p e r p e n d i c u l a r a n d p a r a l l e l to ( S N ) * c h a i n s , h a v e b e e n r e p o r t e d b y C h i a n g et a l . (11).

the

E v e n if a l l the

b r o m i n e w e r e present as B r ~ , the n u m b e r of carriers g e n e r a t e d is n o t 3

sufficient to e x p l a i n the increase i n c o n d u c t i v i t y .

Grant (57)

has c a l c u

l a t e d t h e shift of t h e F e r m i l e v e l as a f u n c t i o n of c h a r g e transfer, u s i n g a r i g i d b a n d m o d e l for ( S N ) * . A c h a r g e transfer, consistent w i t h c u r r e n t estimates of the a m o u n t of B r ~ i n ( S N B r o . O * , appears to r e m o v e t h e 3

e l e c t r o n p o c k e t i n the b a n d structure of electron-hole

scattering.

predominant

mechanism

pristine ( S N ) * .

( S N ) * a n d h e n c e eliminates

T h i s t y p e of s c a t t e r i n g is b e l i e v e d to b e responsible

for

limiting the

S u p p r e s s i o n of e l e c t r o n - h o l e

be m a n i f e s t e d b y a c h a n g e f r o m a T

2

to a l i n e a r t e m p e r a t u r e ( T )

of

s c a t t e r i n g process s h o u l d

d e p e n d e n c e of t h e r e s i s t i v i t y

dependence.

the

conductivity

A l t h o u g h the observed

(58) tem

p e r a t u r e d e p e n d e n c e is not l i n e a r , the t r e n d t o w a r d s m a l l e r t e m p e r a t u r e exponents

is a p p a r e n t for the r e s i s t i v i t y of

F u r t h e r m o r e , the t h e r m o p o w e r ,

brominated

shown i n Figure 4 ( b ) ,

(SN)*

(11).

becomes p-type

o n b r o m i n a t i o n , a g a i n consistent w i t h the r e m o v a l of t h e e l e c t r o n p o c k e t . T h u s the o r d e r - o f - m a g n i t u d e

increase i n c o n d u c t i v i t y o n b r o m i n a t i o n is

p r e d o m i n a n t l y the r e s u l t of a n increase i n the c a r r i e r l i f e t i m e r a t h e r t h a n a n increase i n the n u m b e r of carriers ( 9 ) .

C h e m i c a l l y , one c a n v i e w the

c h a r g e transfer as l e a d i n g to t h e f o r m a t i o n of a n e x t e n d e d p o l y t h i a z y l c a t i o n , w i t h u n i t p o s i t i v e charge for every B r " . 3

G r e e n e et a l . ( 5 9 )

h a v e s h o w n that t h e s u p e r c o n d u c t i n g t r a n s i t i o n

t e m p e r a t u r e of ( S N ) * increases b y a b o u t 1 0 % to 0.316 K o n b r o m i n a t i o n ,


188

SOLID

STATE CHEMISTRY:

OVERVIEW

Electrical Conductivity vs. Temperature .for (SN) & (SNBr ) X

10

A CONTEMPORARY

(

Q4

i

x

\

a


-

_

-

•—-—•

_

[ ( C H ) / ( C 1 0 - ) J + yAg° +

4

W e b e l i e v e t h a t other acceptors

behave

(1)

4

similarly, creating simple

p o l y o l e f i n i c cations. T h u s there is n o n e e d to i n v o k e b r o m o n i u m ions or s i m i l a r ir complexes.

Conclusions T h e s i m i l a r i t i e s a n d differences b e t w e e n ( S N ) * a n d ( C H ) * a n d t h e i r derivatives have been

examined.

p o l y m e r s are s i m i l a r , b u t semiconductor.

T h e b a s i c c h a i n structures of

( S N ) * is a s e m i m e t a l w h e r e a s

both

( C H ) * is a

A f t e r treatment w i t h b r o m i n e , h o w e v e r , b o t h m a t e r i a l s

e x h i b i t m e t a l l i c c o n d u c t i v i t y . A l t h o u g h the p h y s i c s r e s p o n s i b l e f o r these increases i n c o n d u c t i v i t y is not the same i n b o t h systems, the c h e m i s t r y t a k i n g p l a c e u p o n b r o m i n a t i o n is q u i t e s i m i l a r . T h e chains are o x i d i z e d to p o l y m e r i c cations a n d t h e b r o m i n e is r e d u c e d to B r " . T h e s e l i n e a r 3

B r " ions are i n t e r c a l a t e d b e t w e e n the p o l y m e r c h a i n s , w i t h t h e i r axes 3

p a r a l l e l . I n the case of ( S N ) * this o x i d a t i o n is l i m i t e d to b r o m i n e , b u t ( C H ) * reacts w i t h a v a r i e t y of o t h e r acceptors as w e l l as some donors. T h e p h y s i c s a n d c h e m i s t r y of these processes are b e l i e v e d to be s i m i l a r to b r o m i n a t i o n , w i t h the ( C H ) * f o r m i n g a p o l y o l e f i n i c c a t i o n i n the case of acceptors a n d a p o l y o l e f i n i c a n i o n i n the case of donors. of s i m i l a r d o n o r complexes

T h e absence

f o r ( S N ) * m a y reflect t h e f a c t t h a t

(SN)*

a l r e a d y has one a n t i b o n d i n g e l e c t r o n p e r S N u n i t i n t h e h i g h e s t o c c u p i e d orbital. T h e final s t o i c h i o m e t r y of these i n t e r c a l a t i o n c o m p o u n d s a n d ( C H ) * p r o b a b l y is d e t e r m i n e d b y s e v e r a l factors. bromine, B r

2

of

(SN)*

I n t h e case of

enters the m a t e r i a l a n d o x i d i z e s the c h a i n s to a degree

d e t e r m i n e d b y the respective

oxidation a n d reduction potentials


and

192

SOLID S T A T E

CHEMISTRY: A


M a d e l u n g energy, f o r m i n g the p o l y m e r i c c a t i o n a n d B r ~ c o u n t e r ions. 3

I f m o r e b r o m i n e c a n fit i n t o the l a t t i c e t h a n c a n b e r e d u c e d b y

the

p o l y m e r , the excess w i l l r e m a i n as n e u t r a l b r o m i n e . S u c h appears to b e t h e case f o r ( S N ) * , w h e r e b o t h B r " a n d B r 3

(SNBro.-i)*.

2

have been identified i n

I n t h e case of ( C H ) * , a l l t h e b r o m i n e is r e d u c e d to B r " . 3

A f u r t h e r f a c t o r that m a y also influence the s t o i c h i o m e t r y as w e l l as the electrical properties b r o m i n e complexes.

of

tions tends to f o r m S N 4


these

polymers

is the m e t a s t a b i l i t y of

these

( S N ) * t r e a t e d w i t h b r o m i n e u n d e r less m i l d c o n d i 3

+

B r " , a n d i n t h e case of ( C H ) * b r o m i n e has a 3

t e n d e n c y to a d d across the e t h y l e n i c d o u b l e b o n d . T h e e l e c t r i c a l p r o p e r t i e s of these c o n d u c t i n g p o l y m e r s offer

many

e x c i t i n g p o s s i b i l i t i e s . Progress t o w a r d a c h i e v e m e n t of t h e i r f u l l p o t e n t i a l s h o u l d b e e n c o u r a g e d b y d e v e l o p m e n t s l e a d i n g to m o r e c h e m i c a l l y a n d t h e r m a l l y stable m a t e r i a l s .

Glossary of Symbols D =- v a n der W a a l s diameter d(hkl)

= s e p a r a t i o n of the hkl planes

ef(hfcZ) == s e p a r a t i o n of t h e hkl planes after i n t e r c a l a t i o n EXAFS =

e x t e n d e d X - r a y a b s o r p t i o n fine structure

m K == m i l l i d e g r e e s K e l v i n R =

resistance

S =

thermopower

1-D — one-dimensional = CI =

microvolts ohms

T T F —»tetrathiasulvalene TCNQ =

tetracyanoquinodimethane

I R = infrared b = l a t t i c e constant i n t h e c h a i n d i r e c t i o n f o r b o t h ( S N )* a n d (CH)* e =

electron

N M R = n u c l e a r m a g n e t i c resonance L C A O = l i n e a r c o m b i n a t i o n of a t o m i c o r b i t a l s M O =— m o l e c u l a r o r b i t a l T =

temperature

Acknowledgments W e thank . W . D . G i l l , P. M . Grant, R. L . Greene, and J . B . Torrance f o r c r i t i c a l l y r e a d i n g this m a n u s c r i p t a n d t h e Office of N a v a l R e s e a r c h for partial support.


10.

STREET

A N D

CLARKE

Conducting

Polymers

193

Literature Cited 1. 2. 3. 4. 5. 6. 7.


8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

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