Novel electrochromic films via anodic oxidation of carbazolyl substituted polysiloxanes

Synthetic Metals, 41--43 (1991) 2935-2938

29 35

NOVEL ELECTROCHROMIC FILMS VIA ANODIC OXIDATION OF CARBAZOLYL SUBSTITUTED

POLYSILOXANES

A.R. HEPBURN=, J.M. MARSHALLa and J.M. MAUDm Departments of Chemistryb and Materials Engineeringa, University College of Swansea, Singleton Park, Swansea, SA2 8PP, U.K.

ABSTRACT Polysiloxanes containing pendant carbazole groups attached to the polymer backbone

via

long

spacer groups undergo anodic

oxidation

to

give

electrochromic thin fi|ms which are characterised by two well defined

redox

couples associated with dicarbazolyl groups.

INTRODUCTION We [ I ]

and others

[2] have described

the

anodic

cross-linking of

carbazole substituted polysiloxanes (I-2) to give electrochremic thin containing dicarbazolyl groups. the

films

Potential uses include display systems, and

observation of two d i s t i n c t redox processes suggests application

tristate

logic

devices.

However films

containing short spacer groups,

derived

from

in

polymers ( I - 2 ) ,

exhibit poorly resolved cyclic v o l t a m e t r i c

(CV) waves. We now report that polymers (3-5),

incorporating long

spacer

groups, afford thin films in which the CV waves are clearly resolved.

RESULTS and DISCUSSION Synthesis Polymers

(3-5)

were obtained from platinum

reactions

between

carbazole

terminated

polymethylhydrosiloxane alk-l-enes

(6-8).

catalysed

(PNHS,

n~35,

Syntheses

of

hydrosilylation Petrarch) polymer

(3)

and and

precursor (6) have been described previously [3]. 0379-6779/91/$3.50

© Elsevier Sequoia/Printed in The Netherlands

2936

'

I

!

Ar-Z-CH2CH2-?i-CH3

(1) (2) (3) & (6) (4) & (7) (5) & (8)

(1-5) O rl

Ar-Z-CH=CH2

Ar

-Z-

Ar 1 Ar2 Ar2 Ar2 Ar3

-CH2-CH2-(CH2)9-(CH2CH20)3CH2-

-(CH2)4OCH2-

(6-8)

Br

I Intermediate hydride, ether ether,

I Et

I (7) was obtained in three steps :

(i)

alkylation

(sodium

dimethylformamide) of carbazole with the tetrahydropyranyl

o f t r i e t h y ] e n e glycol monoch]orohydrin, and

(iJi)

alkylation

(ally1

(ii)

c h l or id e ,

(THP)

acJdJolysis of the sodium

hydroxide,

THP phase

t r a n s f e r conditions) of the r e s u l t i n g alcohol. Precursor (8) was obtained in four steps :

(J) Friedel Crafts

acylation

(ClCO(CH2)zCOONe-SnC14) of commercially a v a i l a b l e N-ethylcarbazole give keto-ester (10), give acid (11),

(9),

to

( i J ) Wolff-Kishner reduction (HzNNH2-KOH) of (10)

to

(JiJ) reduction (LiA1H4) o f (11) to give alcohol (12),

and

( i v ) a l k y l a t i o n of (12) with a l l y 1 chloride.

/X

I Et

(9) (10) (11) (12) (8)

X= X= X= X= X =

H COCH2CH2COOMe (CH2)3COOH (CH2)4OH (CH2)4OCH2CH=CH2

Electrochemistry Cyclic

voltammetry

(0 - 1.5 - 0 V,

100 mV s - l )

at

a

gold

wire

working electrode (Ag pseudo reference and A1 counter) in a CHzClz s o l u t i o n of polymer (5) (2.5 x 10-3 M in polymer repeat u n i t ) containing NBu4PFe (0.1 M) e x h i b i t e d peaks at ca.

1.15 and 1.3 V (Figure 1) which were

by the growth o f a green polymer f i l m ,

accompanied

during the anodic scan, and peaks at

2937

ca.

0.7 and 1.2 V during the cathodic sweep when the f i l m was bleached. The

observations can be interpreted in terms of a reaction sequence similar those proposed for the polymers ( I ) [ I ] and (2) [2]. groups

at

1.15 V is followed by dimerisation of

to

Oxidation of carbazole the

resulting

radical

Is j .l

t o o c

<

0 - ~ .o o A~

o

+

~

t o

~5 0 c

<

O-

L

.o o c-

(b)

O

+ 0.5

1.0

115

F_JV vs. "Ag"

F i g . 1. C y c l i c voltammograms (a) taken a t a g o l d w i r e w o r k i n g e l e c t r o d e i n a CHzC12 s o l u t i o n o f polymer ( 5 ) , and (b) o f t h e r e s u l t i n g e l e c t r o c h r o m i c f i l m ; scan r a t e = 100 mV s - i ; S = I O 0 ~ A om-z.

cations,

proton

dications

near 1.3 V.

loss

and

oxidation of the

In t h e c a t h o d i c scan,

carbazolyl

dimrs

to

their

r e d u c t i o n t o t h e dimer r a d i c a l

c a t i o n occurs near 1.2 V and t o t h e n e u t r a l dimer near 0.7 V. F o l l o w i n g removal from t h e film

revealed

polymer s o l u t i o n ,

cyclic

voltammetry of

be

a s s o c i a t e d [ 5 ] w i t h c o n s e c u t i v e one e l e c t r o n o x i d a t i o n s (and r e d u c t i o n s )

of

groups. The waves were w e l l s e p a r a t e d w i t h a peak w i d t h a t h a l f

height for the first for

which

the

can

dicarbazolyl

two r e v e r s i b l e waves (E1/z 0.73 and 1.25 V),

a n o d i c peak c l o s e t o 90 mY,

a one e l e c t r o n t r a n s f e r

a t room t e m p e r a t u r e .

the theoretical

value

[4]

Corresponding v a l u e s

for

t h e f i l m s from polyTners (1) and (2) were some t h r e e t i m e s l a r g e r .

2938

The role o f the Ion9 spacer group voltammogram o f dissimilar

in (5) in

determining the form o f the

the cross-linked f i l m was demonstrated

polymers

(3)

and

(4)

when s t r u c t u r a l l y

underwent anodic

oxidation

to

give electrochromic films with s i m i l a r features.

Electronic Characterisation We are c u r r e n t l y performing an exhaustive e l e c t r o n i c characterisation of

carbazole substituted polysiloxanes (1-5) and other analogues. We are

examining forms,

the

virgin

polymers

and t h e i r

electrochemically cross-linked

both when reduced and at various states o f oxidation. Preliminary

experiments

have

b e e n performed

with

polymer

(2)

photoconductivity measurements, with f i e l d s o f I0S-I0 6 V cm- i , evidence f o r f i e l d enhanced conduction and y i e l d o f the order 5 x I0 -s cm2 V- i

Transient reveal no

c a r r i e r m o b i l i t i e s values

s- i . These values are two orders of magnitude

greater than those reported previously f o r the same material [6]

ACKNOWLEDGEMENT We thank the SERC (UK) f o r f i n a n c i a l support.

REFERENCES 1

T . W . Booth, S. Evans and J.N. Naud, J. Chem. Soc., Chem. Cow,nun., (1989)

2

B. Tieke and N.O. Chard, Polymer, 30 (1989) 1150-4.

196-197. 3

P. S t r o h r i e g l , Nakromol. Chem., Rapid Conmun., 7 (1986) 771-5.

4

A.J. Bard and L.R. Faulkner, Electrochemical Nethods, Wiley, New York, 1980, Ch. 12, p. 522.

5

J.F. Ambrose and R.F. Nelson, J. Electrochem. Soc., 115 (1968) 1159-64.

6

H. Schnorer, H. Domes and D. Haarer, P h i l . Nag. L e t t . , 58 (1988) 101-5.