Polyaniline: Allowed oxidation states

Synthetic Metals, 41-43 (1991) 715-718

POLYANILINE:

715

A L L O W E D OXTDATIOH STATES

J.G. MASTERS,

Y. SUN, A.G. M A C D I A R M I D

Department of Chemistry,

University of Pennsylvania,

Philadelphia,

PA

19104-6323 (U.S.A.) A.J.

EPSTEIN

Department of Chemistry and Physics,

The Ohio State University,

Columbus,

OH 43210-1106 (U.S.A.)

ABSTRACT The i n t e r c o n v e r s i o n of different oxidation states of polyanilines has been studied.

It is concluded that the polyaniline bases

(NMP solution)

exist only in three discrete oxidation states at the molecular level.

INTRODUCTION The p o l y a n i l i n e s

refer to a class of polymers which can be considered as

being d e r i v e d from a polymer, composition

V ~

"

the base form of which has the generalized

[1,2]:

U

~

"

~

"

~

"

~

.

.

The average oxidation state, {l-y)

can be v a r i e d continuously from zero to give the completely reduced pol~er, pol~er,

~

~

H

H

H

H

.

.

.

.

~

~

. ~

~

N

~

x

~

oxidized p o l ~ e r ,

,

to one to give the completely

m

~

"leucoemeraldine",

, to 0.5 to give the "half-oxidized"

.

~

N

~

.

~

#

~

.

The terms

"emeraldine" and "pernigraniline"

oxidation states of the polymer where

refer to the different

(l-y) - 0, 0.5 and 1.0

[1-3].

No

evidence has b e e n obtained hitherto to indicate whether the polyanilines exist in a c o n t i n u ~

of oxidation states at the molecular level ranging

from (l-y) values of 0 to i.

Early work

[4] suggested that polyaniline

could be i s o l a t e d in five distinct oxidation states having 0.0, 0.25,

0.50,

0.75 and 1.0.

A recent s t u ~

(l-y) values of

[5] has suggested that in

the solid state, phase segregation into different oxidation states may occur.

Elsevier Sequoia/Printed in The Netherlands

716 RESULTS UV/Visible

Spectra of Leucoemeraldine.

Emera]d~ne

and Pernlaranillne

In order to carry out detailed spectroscopic redox interconversion

of polyaniline

bases,

studies

chromophores Fig.

i.

characteristic

Leucoemeraldine

emeraldine

of the three oxidation

base was synthesized

A

Reaction

base base

of

(LB)and

of polyaniline

oxidation

, 3 0 ' nm

@ ~/ I /

Em4ral~ne

Base

Bus

J

6,? n,

~

/

~

~'~"

~

11tl

~

~

states,

(l-y)

(PB + LB)

~

The

values,

$30

of reagents

nm

\(3.79 .v)r__~(~3 eV)

I

400

SO0

calculated

used varied from 0 (pure leucoemeraldine (essentially

" rerniaranl~e Base |

j.~.~'% 327 mm

species.

from the relative quantities

to 0.49

"

1283 nm ' I(4.27 ,v)

260

i.e. mmol PB/n~nol

base)

Laucoemeral~ma

(PB) were mixed in

therefore had the same total molar

average

"

at" --wk,3.7$ ew)

varying proportions in separate reaction vessels yielding solutions all of which

concentration

[7], and

Between

I

NMP solutions

leucoemeraldine

base using

Base and Perniuraniline

Base.

pernigraniline

states are given in

/ I ~\ 34 (3.~1nm e¥)

Self Oxidation/Reductlon

Equimolar

the

from

[3,8].

Leucoemeraldine

spectra of the

solution,

from emeraldine. HCl

base using m-chloroperoxy-

benzoic acid

(NMP)

base was synthesized from emeraldine

N2H 4 [6]. Emeraldine base was synthesized pernigraniline

involving the

the UV/Visible

pure bases were obtained in N-methylpyrrolidinone

Bases

600

700

Wtvelength,

g60

nm

~ g . 1. The el~uonic spectra ~ polyaniline b~es m NMP.

that of emeraldine

base).

After

reaching equilibrium (approximately UV/Visible

0.49

14 hr.) the

spectrum of each

solution was recorded

•

(Fig. 2). As can be seen, two isosbestic present, 260 $SS

400

4SO SO0

5SS

SOS

6SO 700 750

points are

the clearly defined

800 S60

wJ,,,,.stk.., chromophore at 637nm (1.94 Fig.2. The self oxidatio~reduction r~ction ~tween leuco- eV) characteristic of ¢meraldine base ~ d pemigraniline b~e yielding average oxidation states ~tween 0.0 and 0.49. emeraldine base increases

717 with increasing concentration of p e r n i g r a n i l i n e base.

Not unexpectedly,

the a c c o m p a n y i n g changes in the b a n d g a p region are not clearly d e f i n e d because

of the close proximity of the 3 4 3 n m absorption of l e u c o e m e r a l d i n e

base and the 327nm and 283nm a b s o r p t i o n s

of pernigraniline base.

The data

show that regardless of the molar q u a n t i t i e s of the reactants employed, l e u c o e m e r a l d i n e base is oxidized to e m e r a l d i n e base and pernigraniline base is r e d u c e d to emeraldine base in one step without passing through any intermediate, experimental

discrete oxidation states at the molecular level under the conditions employed.

O x i d a t i o n of E m e r a l d i n e Base to P e r n i q r a n i l i n e Base Oxidation of an emeraldine b a s e / N M P solution of m - c h l o r o p e r o x y b e n z o i c

solution with an acetic acid

acid followed by deprotonation with t r i e t h y l a m i n e results in a n a l y t i c a l l y

|.e

,

,

,

,

pure pernigraniline base.

It is found

291sm that the emeraldine base is o x i d i z e d to p e r n i g r a n i l i n e base in one step without

s.s

i $.~

p a s s i n g through any intermediate discrete

0.e

oxidation states at the

molecular

level.

Increasing amounts of

an acetic acid solution of mo.4

chloroperoxybenzoic

acid were a d d e d to a

s o l u t i o n of emeraldine base in NMP, 0.0

~

~

w

~

~

Wavolongth (nm)

e q u i l i b r a t i n g for 15 minutes and then r e c o r d i n g the UV/Visible electronic

spectrum. Sufficient NMP was a d d e d to ~ g . 3. UV-Wis abso~fion spectra of a solution of emeraldine base in N M P to the reaction mixture before r e c o r d i n g which increasing amounts of m ch]oroperbenzoi¢ acid have been added.

each spectrum in order to have identical t o t a l molar concentrations of

polyaniline

species present in each case.

As can be seen from Fig.

3,

three isosbestic points are present on proceeding from pure emeraldine base to where sufficient oxidant has been a d d e d to just convert all the e m e r a l d i n e base

to pernigraniline base.

Deconvolution of the spectra

shows that the m a x i m u m absorbance c h a r a c t e r i s t i c of emeraldine b a s e d e c r e a s e d as the m a x i m u m absorbance for pernigraniline base increased showing that emeraldine base is o x i d i z e d to pernigraniline base in one step without p a s s i n g through any intermediate, molecular varies,

level.

of course,

The average oxidation

discrete oxidation states at the

state of the polyaniline

continuously from that of emeraldine base to

p e r n i g r a n i l i n e base during the a d d i t i o n of oxidant.

species

718 SU~Y~R¥ The c o n t i n u u m of average (in NMP solution)

oxidation states of the polyaniline bases

which varies

from the completely reduced leucoemeraldine

to the c o m p l e t e l y oxidized pernigraniline individual molecules leucoemeraldine,

is actually comprised only of

(or segments within a given molecule)

having the

emeraldine and pernigraniline oxidation states.

The

system is completely described over the whole oxidation state range by three sets of chromophores characteristic of each of the above three oxidation states.

ACKNOWLEDGEMENT This work was supported principally by the Defense A d v a n c e d Research Projects A g e n c y through a grant m o n i t o r e d by the Office of Naval Research.

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