15N NMR of polyaniline

Synthetic Metals, 29 (1989) E243 E249

E243

15N N M R OF P O L Y A N I L I N E

A.F. RICHTER,

A. RAY, K.V. RAMANATHAN,

S.K. MANOHAR,

G.T. FURST,

S.J. O P E L L A AND

A.G. M A C D I A R M I D Department

of Chemistry,

U n i v e r s i t y of Pennsylvania,

Philadelphia,

P A 19104-6323

(USA) A.J. E P S T E I N D e p a r t m e n t of Physics and D e p a r t m e n t of Chemistry, Columbus,

OH 43210-1106

The Ohio State University,

(USA)

ABSTRACT 15N N M R s p e c t r o s c o p y has been u t i l i z e d as a structural p r o b e for the leucoand "emeraldine" base forms of polyaniline.

P r e l i m i n a r y results suggest that the

e m e r a l d i n e base p o l y m e r exists as an a l t e r n a t i n g c o p o l y m e r of o x i d i z e d and r e d u c e d units.

E n d - g r o u p s are not d e t e c t e d at the e x p e r i m e n t a l level of

sensitivity.

INTRODUCTION The t e r m "polyaniline"

is d e s c r i p t i v e of a class of c o n d u c t i n g polymers

d e r i v e d f r o m the base of g e n e r a l c o m p o s i t i o n

N

N

N

N

,0&y~l X

Reduced ( " I A " ) unit The chemical,

electronic,

Oxidized ("2A") unit m a g n e t i c and m e c h a n i c a l p r o p e r t i e s of the p o l y m e r

are s e n s i t i v e to several variables.

These include:

(i) The a v e r a g e o x i d a t i o n state, d e f i n e d b y the t e r m formula

(l-y) in the above general

[i].

(ii) The level of protonation, o x i d a t i o n state a conductor

which has been shown to change the m a t e r i a l of

(l-y)~0.5 from an insulator

(~ ~i S/cm) when d o p e d at pH~0

(~ ~10 -8 S/cm) when d o p e d at pH~7 to [2,3].

(iii) S u b s t i t u t i o n of h y d r o g e n atoms by alkyl groups on the -C6H4 - rings (iv) S u b s t i t u t i o n of h y d r o g e n atoms on -N(H)of alkyl groups to -N = sites

[4].

sites b y alkyl groups or addition

[5].

(v) M o l e c u l a r weight and morphology.

0379-6779/89/$3.50

© Elsevier Sequoia/Printed in The Netherlands

E244

P r o t o n a t i o n of the n i t r o g e n atoms at pH~0 leads to a c o n d u c t i n g form of the polymer having

(i-y)~0.5.

This "protonic acid" d o p i n g process is b e l i e v e d to be

d o m i n a t e d b y the reaction

J H÷AH

H

H

H

X EPR

[6] and m a g n e t i c s u s c e p t i b i l i t y

[7] data suggest that the p r o t o n a t e d polymer

exists m a i n l y as the semiquinone radical cation H

H

H

H

X In this formal structure, the n i t r o g e n atoms.

the u n p a i r e d spins are b e l i e v e d to reside m a i n l y on

It m a y be n o t e d that the n i t r o g e n atoms are also the

reactive sites for "pseudo-protonic"

[5] d o p i n g processes,

where,

for example,

"positive" m e t h y l g r o u p i n s t e a d of a proton adds to the imine sites.

Thus,

a

15N

N M R is e x p e c t e d to be a sensitive and useful probe of the structure and chemistry of the polyanilines.

We report here some e a r l y results in this context as an

indicator of the p o t e n t i a l value of the method.

EXPERIMENTAL 15N aniline

(ICN Biomedicals)

was d i s t i l l e d on a v a c u u m line and u s e d either

d i r e c t l y or at ~50% enrichment by c o - d i s t i l l i n g with 14N-aniline

(Fisher

Scientific). Synthesis of "emeraldine" base C o n v e n t i o n a l Technique(CT) P o l y m e r i z a t i o n was c a r r i e d out in IM HCI u s i n g described elsewhere

(NH4)2S208 as has been

[8~] to y i e l d "emeraldine" hydrochloride,

d e p r o t o n a t e d in air u s i n g 0.1M NH4OH. found to have an o x i d a t i o n state of

(I-y)~0.5.

This was

The base form thus p r e p a r e d is g e n e r a l l y

(l-y)~0.6

[I].

P h a s e - T r a n s f e r Technique(PTT) In this method,

aniline was d i s s o l v e d in CH2CI 2 c o n t a i n i n g a catalytic amount

of t e t r a b u t y l a m m o n i u m perchlorate. A solution of

(NH4)2S208 in aqueous IM HCI was

a d d e d to this, taking care not to m i x the layers

[9].

The p o l y m e r formed at the

interface and was s u b s e q u e n t l y c o l l e c t e d by filtration of the t w o - p h a s e mixture. Subsequent d e p r o t o n a t i o n with 0.1M NH4OH was c a r r i e d out as p r e v i o u s l y d e s c r i b e d [8], except that all operations were p e r f o r m e d in a glove bag under argon.

E245

I n e r t - a t m o s p h e r e syntheses of "emeraldine" base have been f o u n d to y i e l d products having

(l-y) values ~0.4

[i].

S y n t h e s i s of l e u c o e m e r a l d i n e base,

(l-y)~0

~150 mg. of "emeraldine" base(CT), with ~10 m L of h y d r a z i n e hydrate

s y n t h e s i z e d as d e s c r i b e d above, was treated

(Aldrich).

R e d u c t i o n of imine units can be

d e s c r i b e d b y the net reaction

•~ ~ N : : : < ~ N - - +

0.5 N2H4

0.5 N2

The e v o l v e d n i t r o g e n was p e r i o d i c a l l y r e m o v e d by f r e e z e - p u m p - t h a w cycles on the v a c u u m line.

The h e t e r o g e n e o u s m i x t u r e was a l l o w e d to stir for ~72 hours and

u n r e a c t e d h y d r a z i n e was s u b s e q u e n t l y p u m p e d off.

A pale g r e y f r e e - f l o w i n g powder

resulted.

S o l i d State N M R Measurements

on powders were c a r r i e d out on a c u s t o m - b u i l t

f r e q u e n c y = 15.24 MHz)

150 MHz

(15N

instrument u s i n g m a g i c angle s p i n n i n g at 2.5 KHz.

p o l a r i z a t i o n was u s e d to enhance the sensitivity.

Cross-

An optional d e l a y of 50 ~secs

c o u l d be a p p l i e d b e f o r e o b s e r v a t i o n of the 15N FID.

This d e p l e t e d the

p o l a r i z a t i o n of n i t r o g e n species d i r e c t l y b o n d e d to H atoms.

Such cross-

d e p o l a r i z a t i o n d e c r e a s e s the intensity of such -N(H)- species and can thus be u s e d to d i s t i n g u i s h t h e m f r o m imine, u n a f f e c t e d b y the delay.

-N =, nitrogens w h i c h are r e l a t i v e l y

(NH4)2SO 4 was u s e d as an e x t e r n a l reference for

c h e m i c a l shifts. Solution NMR S o l u t i o n e x p e r i m e n t s were p e r f o r m e d on a Bruker WP 2 0 0 S Y 20.28 MHz)

(15N

instrument using a 3:1 m i x t u r e of N - m e t h y l p y r r o l i d i n o n e

dimethylsulfoxide

as the solvent system.

e x t e r n a l reference,

frequency = and d 6-

15N aniline was u s e d as an approximate

t a k e n to be at 29.0 p p m d o w n f i e l d of

p e r m i t c o m p a r i s o n with the s o l i d - s t a t e studies.

(NH4)2SO 4 in order to

An i n v e r s e - g a t e d d e c o u p l i n g

p u l s e s e q u e n c e was u s e d to m i n i m i z e the effect of a p o s s i b l e small n e g a t i v e N u c l e a r O v e r h a u s e r Effect.

R E S U L T S AND D I S C U S S I O N A s o l i d - s t a t e s p e c t r u m of l e u c o e m e r a l d i n e base,

(l-y)~0, is shown in Fig. I.

A single resonance is seen at 54.2 p m d o w n f i e l d from the m o d e l c o m p o u n d p - a m i n o d i p h e n y l a m i n e ,

~

(NH4)2SO 4. The s p e c t r u m of

NH ~ N ~ 2

, is shown in Fig. 2.

C o m p a r i s o n of these spectra shows that l e u c o e m e r a l d i n e base does not contain any detectable end-group population

(expected at ~37 ppm)

on the 15N level of

E246 --54.2 ppm

70 ppm

800 PP 400 U

0

200

37 ppm

I00

-I00

0

ppm

Fig. I. Solid-state 15N NMR spectrum of leucoemeraldine base; Fig. 2. Solid-state 15N NMR spectrum of p-aminodiphenylamine.

sensitivity.

The 54.2 ppm signal arises from -N(H)- groups separated by p-

phenylene moieties. Using this information, base forms of polyaniline,

it is possible to analyze the spectra of "emeraldine" where

spectrum of "emeraldine" base

(I-y)~0.5.

Figs. 3 and 4 compare a solid-state

(CT) with a solution spectrum of "emeraldine" base

(PTT) respectively.

[62.3 ppm

321

ppm 61.0 ppm 55.2

J

~

ppm

~ 333~ppm _I

460

PP?6o U

Fig. 3. Solid-state spinning sidebands;

6

350

300

250

2GO

150

100

So

•

15N NMR spectrum of "emeraldine" base(CT), "x" denotes Fig. 4. Solution 15N NMR spectrum of "emeraldine" base(PTT)

E247

Examination

of the r e l a t i v e

intensities

in the i n f r a r e d

spectra

the CT m a t e r i a l

is m o r e o x i d i z e d

comparison Two

imine-type

resonances

aromatic

CT sample

(Fig.

p p m signal b e i n g The s p e c t r u m least

where

3).

than the PTT.

spectra

in aqueous

(in the range

-N(H)-

region

is s u g g e s t i v e

some

structures

base

imine-imine

acid

studies

[i0]

suggests

that

by a

[i].

a n d o n l y one in the are seen for the

are c o n s i s t e n t

with the

for the -NH- species,

for the -N= species.

62.3

b o n d e d to protons.

This

but at

is c o n s i s t e n t

with a

sequences

are e x p e c t e d

a n d imine

exist

for a p o l y m e r

diads;

respectively.

to be d i f f e r e n t

of the CT " e m e r a l d i n e "

having

(I-y)>0.5.

"type 2" to c o p o l y m e r i c The -NH- signals

f r o m that

base p o l y m e r

and

i"

'a'

for such

for l e u c o e m e r a l d i n e viz.,

"Type

diads;

base,

which

62.3 p p m for the "a2"

a n d 54.2 p p m for the l e u c o e m e r a l d i n e

(al). it can be seen

region

sample, units

ppm)

unique e n v i r o n m e n t

of a

refer to h o m o p o l y m e r i c

However, amine

80% acetic

300-330

only a I diads, as can be seen experimentally,

species

samples

with N atoms which are d i r e c t l y

environments

'i' s t a n d for amine

has

base

c m -I peaks

is also c o n f i r m e d

(expected at 50-70 ppm)

Cross-depolarization

associated

This

a n d ~1600

of the type

resonances and

amine,

two d i f f e r e n t

structure

of the CT and PTT "emeraldine"

of t h e i r U V / v i s i b l e

secondary

of the ~1500

this o b s e r v a t i o n

are i s o l a t e d

present,

(Fig.

a n d one in the imine

4) that the PTT sample region.

is c o n s i s t e n t

has two resonances

U s i n g the same logic

with a

(l-y)

value<0.5,

f r o m e a c h other but some c o n t i g u o u s

in the

as for the CT where

amine-amine

the imine

regions

are

viz.,

By comparison "emeraldine"

w i t h Fig.

I, one can readily

b a s e to the a I species;

the

assign the 55.2 p p m signal 61.0 p p m r e s o n a n c e

a2 species

(seen at 62.3 p p m in the CT case).

identified

at 321 ppm.

Since no i I peaks are o b s e r v e d

material,

to the

An i 2 p e a k can be r e a d i l y

are seen in the u n d e r - o x i d i z e d

in the " o v e r - o x i d i z e d "

of PTT

corresponds

PTT sample

and no a I peaks

it w o u l d a p p e a r that w h e n

(i-y)=0.5

exactly, the s p e c t r u m of the base w o u l d consist of o n l y 2 signals; an i 2 d i a d in the imine special

region

case,

a n d an a 2 d i a d in the amine

the p o l y m e r

region.

It follows

w o u l d t e n d to be c o m p o s e d m a i n l y

of

that

for this

alternating

amine

E248

(IA) and imine(2A)

units,

as has been suggested p r e v i o u s l y

to make a q u a l i t a t i v e p r e d i c t i o n

[ii].

This permits us

(shown in Table i) of the spectral features

a n t i c i p a t e d for "true" e m e r a l d i n e base, where

(l-y) is exactly 0.5.

TABLE 1

~ample

(l-y) value

[eucoemeraldine

I

0

'emeraldine" base <0.5 PTT: under-oxidized) 'emeraldine" base CT: over-oxidized) e m e r a l d i n e base

al

C h e m i c a l Shift {PPm] a2 il i2

54.2

55.2

61.0

>0.5

62.3

0.5

~62

321

333

302 shoulder ~321

~redicted)

The above results are consistent with the CT "emeraldine" base b e i n g overoxidized, y)<0.5.

i.e.

(l-y)>0.5 and with the PTT sample b e i n g u n d e r - o x i d i z e d i.e.

However,

it should be s t r e s s e d that in this discussion,

(i-

a solid-state

s p e c t r u m of the CT p o l y m e r is b e i n g c o m p a r e d with a solution s p e c t r u m of the PTT polymer.

Studies are p r e s e n t l y b e i n g c a r r i e d out in which solid-state spectra of

these two kinds of "emeraldine" base are c o m p a r e d and their respective solution spectra are also compared. It is important to note that p o s s i b l e effects of g e o m e t r i c i s o m e r i s m are not a c c o u n t e d for by this i n t e r p r e t a t i o n of the N M R data.

A l t h o u g h cis-trans

i s o m e r i s m about the rigid q u i n o i d rings of p o l y a n i l i n e has been d e t e c t e d in 13C NMR

[12], the N atoms m a y be too far from the sites of isomeric g e o m e t r y to be

s u b s t a n t i a l l y affected.

However,

the p o s s i b i l i t y cannot be e l i m i n a t e d until

a p p r o p r i a t e m o d e l c o m p o u n d studies have been c a r r i e d out.

We are currently

e x a m i n i n g this issue in our laboratory.

CONCLUSIONS P r e l i m i n a r y 15N NMR studies on "emeraldine" base suggest that the p o l y m e r exists as an a l t e r n a t i n g system of o x i d i z e d and reduced repeat units.

Clustering

of only those units which are present in larger number is o b s e r v e d in samples h a v i n g o x i d a t i o n states species,

(i-y)~0.5.

There is no evidence for end-group,

-NH2,

s u g g e s t i n g that small-chain oligomers are not present in d e t e c t a b l e

quantities.

15N NMR appears to be a p r o m i s i n g technique for p r o b i n g the

structure of polyaniline.

E249 ACKNOWLEDGEMENTS This work was s u p p o r t e d in part b y the Defense A d v a n c e d R e s e a r c h Projects A g e n c y t h r o u g h a contract m o n i t o r e d by the Office of Naval R e s e a r c h S.K.M. and K.V.R.), 85-19059

by the N a t i o n a l Science Foundation,

(A.R.) and by NIH grant no. G M - 2 4 2 6 6

(A.F.R.,

research grant no. DMR-

(K.V.R.).

REFERENCES 1

G.E. Asturias, A.G. MacDiarmid, R.P. McCall and A.J. Epstein, Synth. Met.,

2

J.-C. C h i a n g and A.G. MacDiarmid,

3

R. DeSurville,

29

(1989) E157

Electrocbem. 4

(these Proceedings).

M. Jozefowics,

Acta,

13

(1968)

Svnth. Met..

13 (1986)

193.

L.T. Yu, J. P e r i c h o n and R. Buvet, 1451.

Y. Wei, W.W. Focke, G.E. Wnek, A. Ray and A.G. MacDiarmid,

to be p u b l i s h e d in

J. Phvs. Chem. 5

S.K. Manohar, A.G. MacDiarmid, K.R. Cromack, J.M. Ginder and A.J. Epstein, S~nth. Met., 29

6

A.G. MacDiarmid, Met..18

7

(these Proceedings).

J.-C. Chiang, A.F. Richter and A.J. Epstein,

J.M. Ginder,

A.F. Richter, 63

A.G. MacDiarmid,

(1987)

Dordrecht,

A.G. MacDiarmid,

Holland,

N.L.D.

(ed.), C o n d u c t i n g Polymers,

S.K. M a n o h a r and A.G. MacDiarmid,

10

F. Wudl,

Z.X. Liu and A.J. Heeger, S. Kaplan,

Somasiri,

and A.J.

Reidel Publications,

to be published.

R.O. A n g u s Jr., F.L. Lu, P.M. Allemand,

E.M. Conwell,

J. Am. Chem.

Solid

1987, p. 105 .

9

12

and A.J. Epstein,

97.

J.-C. Chiang, A. F. Richter,

E p s t e i n in Luis A l c a c e r

Ii

Synth.

(1987) 285.

State Commun.. 8

(1989) E349

J. Am. Chem.

Soc..

109

D.J. Vachon, M. Nowak, (1987) 3677.

A.F. R i c h t e r and A.G. MacDiarmid,

to be p u b l i s h e d in

Soc.

M. S a n d b e r g and T. Hjertberg,

Synth. Met., 29 (1989) E257

(these Proceedings).