Solution processible poly(1-alkyl-2,5-pyrrolenevinylenes): new low band gap
conductive polymers
In Tae Kim and Ronald L. Elsenbaumer*
Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019, USA
Bis(phenylthiomethylene) derivatives of 1-alkylpyrroles
afford conjugated polymers by base induced elimination of
thiophenyl groups.
i
Me2N
NMe2
N
N
R
R
5
4
Electrochemically prepared polypyrrole is employed as an
electrode in capacitors and batteries owing to its high con-
ductivity ( > 100 S cm21), relative stability to air and moisture
in its oxidized form, ability to form strong coherent films and
ease of synthesis.1 Conductive properties are related to the
nature of the counter anions in polypyrrole films.2 Properties
have also been modified by polymerizing pyrrole derivatives
with 3-methyl and 3,4-dimethyl substituents. Conductivities of
these polymers are 4 and 10 S cm21, respectively.3,4 However,
the room temperature conductivities of poly(1-alkyl-2,5-pyrrol-
ylene) (alkyl = methyl, hexyl, dodecyl) prepared by chemical
ii
I –
+
Me3N
I –
+
NMe3
iii
PhS
SPh
N
N
R
2
R
3
iv
a R = Me
oxidation fall in the range 1023 to 1026 S cm21 5,6
The large
.
b R = (CH2)5Me
c R = (CH2)11Me
N
decrease in conductivity of doped poly(pyrrolylenes) with an
N-alkyl substitutent and an increase in size of the alkyl
substituent has been rationalized on a steric basis.7
R
n
1
Scheme 1 Reagents and conditions: i, aq. CH2O, HClHN(CH3)2, 0–25 °C;
ii, MeI, THF, room temp.; iii, PhSNa, THF, reflux; iv, ButOK, THF,
reflux
Reduction of band gaps in conjugated polymers such as
poly(p-phenylene) and poly(thienylene) has been achieved by
the insertion of vinylene linkages between the aromatic rings in
the polymer chain to give poly(phenylenevinylene) and poly-
(thienylenevinylene),8–10 respectively. The backbone of these
polymers is made of aromatic rings bridged by vinylene
linkages wherein the vinylene linkage not only reduces steric
hindrance between backbone rings attached to them, but also
has a beneficial effect on electronic properties as evidenced by
experimental and theoretical data on both poly(p-phenylene-
vinylene)4 and poly(thienylenevinylene).11
Recently, we reported a new precursor and polymerization
route for the preparation of high molecular mass poly(3,4-
dialkoxy-2,5-thienylenevinylene)s as low band gap conductive
polymers made by base/acid and thermally induced elimination
of sulfoxide groups.12 We now report a convenient synthesis of
1-alkyl-2,5-bis(phenylthiomethylene)pyrroles (2a–c) using the
Mannich reaction.13 These can be readily polymerized using 4
equiv. of ButOK (Scheme 1). The present account is the first
report of the use of phenylthio as a leaving group in forming
poly(1-alkyl-2,5-pyrrolenevinylene)s (1a–c).
temperature and then gradually warmed to reflux. This
monomer polymerized to give poly(1-hexyl-2,5-pyrrolylene-
vinylene) (PHxPyV), 1b. This polymer was purified by four
reprecipitations from THF into MeOH and acetone and had a
deep blue color with a gold luster. On exposure to air, the
polymer became slightly doped reaching a limiting conductivity
of 1026 S cm21. On doping with AuCl3, PHxPyV films (cast on
quartz) exhibited conductivities of 2.5 S cm21 (four-in-line
probe) in air. The undoped conjugated polymer 1b was soluble
in many common organic solvents. Conversion from the bis-
phenylthio monomer 2b afforded PHxPyV in 72% yield, which
is considerably higher than that typically obtained for poly-
(phenylenevinylene) prepared from bis-sulfonium salt pre-
cursors (20–40%).15 As indicated by the UV–VIS–NIR spec-
trum shown in Fig. 1(a) on an undoped film of cast polymer 1b,
1-Alkylpyrroles (5a–b) were prepared as described in the
literature.14 Treatment of 5a–c with aq. HCHO and
Me2NH·HCl at room temperature for 7 d, the Mannich reaction,
gave 70–85% yields of 1-alkyl-2,5-bis[(dimethylamino)-
methyl]pyrroles (4a–c) and these, in turn, were converted into
bisquaternary ammonium salts (3a–c) (yield: 75–92%) after
reaction with MeI in THF at room temperature. We chose
2.0
1.5
1.0
0.5
0.0
(b)
1-alkyl-2,5-bis[(trimethylammonio)methyl]pyrrole
diiodide
(3a–c) as initial target monomers because of their crystallinity
and good stability. However, attempts to polymerize these
monomers (3a–c) with 5–10% aq. NaOH in a temperature range
from 0 °C to reflux have not been successful. Manipulation of
the bisquaternary ammonium salt functional groups in 3a–c into
a phenylthio leaving group 2a–c (yield: 55–70%) provided a
facile approach to a new class of polymerizable monomers
(Scheme 1).†
(a)
5
10
15
20
25
30
2
l / 10 nm
The bis(phenylthiomethylene) derivative 2b of 1-alkylpyr-
Fig. 1 UV–VIS–NIR absorption spectra of (a) undoped and (b) FeCl3 doped
PHxPyV (1b) (cast film on quartz)
role was treated with ButOK (4 mol) in THF at room
Chem. Commun., 1998
327
Kim, In Tae
