Novel base-dopable poly(2,7-fluorenylene) derivatives
Maxime Ranger and Mario Leclerc*
De´partement de Chimie, Universite´ de Montre´al, Montre´al, Que´bec, Canada H3C 3J7
A novel acidic poly(fluorenylene) derivative has been
synthesized which, upon base-doping, shows electrical
conductivities of 1026–1025 S cm21; this new doping method
for conjugated polymers opens the way to the preparation of
air-stable n-type conducting polymers.
iv, v
H17C8 C8H17
vi
i
Conjugated polymers, such as poly(acetylene)s, poly(pyrrole)s,
poly(thiophene)s, etc., can undergo redox processes (partial
oxidation or reduction) which create mobile charge carriers
Br
Br
Br
Br
Br
(radical cations, dications or their negative analogs) and lead to
ii, iii
H17C8 C8H17
vii, viii
high electrical conductivities.1 These reversible redox reactions
(also called doping reactions) modify the electronic structure of
the conjugated polymers and are also accompanied by colour
Br
changes (electrochromism). Another major development in the
field of conducting polymers appeared with the possibility of
creating positive charge carriers through the protonation of
basic conjugated polymers. For instance, poly(aniline)s can
exhibit an insulating-to-conducting transition through a simple
protonation of the imine moieties (without any external redox
O
O
EtO
H
B
B
O
O
O
H17C8 C8H17
Scheme 1 Reagents and conditions: i, CuBr2, Al2O3, CCl4, reflux, 5 h, 97%;
ii, LDA (2.1 equiv.), THF, 278 °C; iii, ClCO2Et (1 equiv.), 92% over two
steps; iv, BunLi (2 equiv.), THF, 278 °C; v, C8H17Br (3 equiv.), 99% over
two steps; vi, Br2 (2 equiv.), FeCl3 (2 mol%), CHCl3, O?25 °C, 24 h, 99%;
vii, BunLi (2.1 equiv.), THF, 278 °C; viii, 2-isopropoxy-4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane (2.5 equiv.), 65% over two steps
process).2 Once again, this doping reaction induces colour
changes which are related to the formation of the charge
carriers. Conversely, acidic conjugated polymers could be base-
dopable and this approach should lead to negative charge
carriers which could be particularly interesting for the develop-
ment of electron-injecting electrodes in light-emitting or
electrochemical devices. Along these lines, interesting work on
poly(2-hydroxy-1,4-phenylene) was recently published by Pei
et al.,3 where delocalization of negative charges on the side
chains onto the conjugated main chain led to conductivities of 5
3 1029 S cm21. However, it could be more interesting to create
charge carriers directly on the backbone and for this purpose,
9-monosubstituted poly(2,7-fluorenylene)s are certainly prom-
ising candidates. Indeed, fluorenes bearing a cyano or an ester
group at the 9-position exhibit relatively low pKa (8–10) values
in organic solvents, which can be rationalized by the fact that
the deprotonation reaction induces the aromatization of the
inner ring.4 Therefore, we report here the first synthesis of a
well-defined acidic poly(2,7-fluorenylene) derivative and the
characterization of its electrical and optical properties upon
base-doping.
use of organolithium compounds in the preparation of the boron
derivatives. All monomers were characterized by NMR spec-
troscopy and mass spectrometry and complete details of their
synthesis will be published in a forthcoming publication. As
shown in Scheme 2, this method yields a well-defined
alternating copolymer where 50% of the repeat units can be
base-doped. Nevertheless, this alternating structure can indeed
be useful since it has been shown that fully protonated
polyanilines are less conducting than 50% acid-doped poly-
anilines.8
The synthesis of this alternated poly(2,7-fluorenylene) deriv-
ative gives a pale yellow polymer, completely soluble in THF
O
O
B
B
As shown in Scheme 1, ethyl 2,7-dibromofluorene-9-car-
boxylate was easily synthesized in two steps from fluorene.
Recently, Pei and Yang5 reported the nickel-catalysed reduction
with metallic zinc of 2,7-dibromo 9,9-disubstituted fluorenes to
yield well-defined 9,9-disubstituted poly(2,7-fluorenylene)s,
but first attempts with this method were found to be inefficient
in the case of acidic 2,7-dibromofluorene-9-carboxylates. The
oxidative synthesis of fluorene-9-carboxylates, following the
method reported by Yoshino and co-workers,6 gave only highly
cross-linked polymers. Finally, as reported for the synthesis of
well-defined processable poly(p-phenylene)s,7 it has been
found that palladium-catalysed Suzuki couplings between
2,7-dibromofluorene-9-carboxylates and fluorene derivatives
bearing diboronic moieties lead to well-defined poly(2,7-
fluorenylene) derivatives. For this purpose, as also described in
Scheme 1, 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-9,9-dioctylfluorene was synthesized, in good yield, in
three steps from fluorene. This synthetic procedure could not be
applied to 2,7-dibromofluorene-9-carboxylate because of the
O
O
H17C8 C8H17
+
Br
Br
EtO
H
O
i
H17C8 C8H17
n
EtO
O
H
Scheme 2 Reagents and conditions: i, [(PPh3)4]Pd (2 mol%), PhMe–
Na2CO3 (2 m aq.), Ar, reflux, 48 h, 65%
Chem. Commun., 1997
1597
Ranger, Maxime
