Oligomers Containing Mixed Thiophene and Pyrrole Units
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x
S
n
N / S: C?-
n n
1 X=S
2 X=NH
3 R = H, Me,C 12H 25 n = 1,2,3... In our hands, the oxidation of 3 (R - Me, n =1) with NOPF6 yielded a black powder,which after being washed free of oxidant, showed 0< 10- 5 S/cm [5], a value considerably smaller in magnitude than that reported by Ferraris. Our results were also consistent with a detailed esr study carried out by Bakker, which indicated that very few radical ions were present. (Table 1) [6].The apparent contradiction between our results and those of Ferraris can be explained in one of two ways.We had washed the NOPF6 treated material with methanol, whereas Ferraris reported that their sample was washed only with methylene chloride.The methanol could well have partially compensated some cationic centers in the doped polymer. Alternatively, the methanol may simply have washed out excess NOPF6. In this case, the poor conductivity may 179 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
be attributed to sructural defects caused by branching, resulting from appreciable radical character at the 03positions of the pyrrole ring (Scheme 1).
S
I
S
Sc
S
-
\/ N+ R
i3"
P3J
R
Sc
a
'
a
Scheme 1 Table 1. Results of Conductivity and EPR Study on NOPF 6 Oxidized 3 (R = Me, n = 1). Conductivity Polymer Rings/charge Rings/polaron S/cm
S
f\
\ /~
S
,
2.7 x 10"2
~*PF6
[4]
2.1
63 5 < 10"o
[6]
960
36
A series of variously substituted analogs(4-8) became our targets.The new compounds 5-8 were synthesized as shown in Scheme 2.Dilithiation of 3 (R = Me, n = 1) followed by quenching of the dianion with bromine furnished the dibromo compound 5.Treatment of the dianion with methyl iodide furnished 6.Direct bromination of 3 (R = Me, n = 1) led to the tetrabromide 7.The anion derived from 2propionylthiophene was oxidatively coupled, followed by reaction with methylamine to give 8 [7].
T\
N
FS
\
Is,
NH3 CH3
\
QH3 N
Br
Br 5
4
Br- ' \S
H3
Br
\
SN
6
/ S\ Br
QH3 N CH 3
Br 7
CH 3 8
180
/33 QCH
i. 2nBuLi/TMEDA
\/
ii.2Br2
CH 3 N
Br-
\\i.
OH 3
\
/
3
S \H
\'N
CH
3
N '
5
n=H
~0
3j TBr 2 '
Br
2nBuLi/TMEDA ii.2CH31
S•
OH3
6
\Br
Br 7 Br H3CO
i. LIDA
CH3 /MeNH
S0i.Cu(Tf)2
2
f)\_H 3 OH3 OH3
8
Scheme 2
The steps involved in the electrochemical oxidation of a dithienyl pyrrole (3, n = 1), assuming a regular linear coupling, are shown in Scheme 3. The first step is the removal of an electron to form a radical cation.This species rapidly dimerizes (ki) to a dication, which loses two protons in a slow step (k2) to give the neutral dimer, and the process then presumably repeats itself. Cyclic Voltammetry data for 4-8 is presented in Table 2.
S
/
S
-e
e
N-
R
\S
R k ,(fast) -
HS
R
S
-
S
-
k2(slow)
-2 H÷
S
/\
\ \ SI
N
N
S
\S
R
R Scheme 3
In collaboration with Carlin , fast scan cyclic voltammetry was carried out and the kinetic rate constants for the radical ion coupling step kl ,as well as that of the half life
181
(tN/2) of the radical ion were determined ; the results are sh
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