J O U R N A L O F
Rational design of an electrochromic polymer with high contrast in
the visible region: dibenzyl substituted poly(3,4-
propylenedioxythiophene){
K. Krishnamoorthy, Ashootosh V. Ambade, Mandakini Kanungo, A. Q. Contractor and
Anil Kumar*
C H E M I S T R Y
Department of Chemistry, Indian Institute of Technology, Bombay, Mumbai, 400076, India.
E-mail: anilkumar@iitb.ac.in
Received 24th September 2001, Accepted 5th October 2001
First published as an Advance Article on the web 12th October 2001
A dibenzyl substituted poly(3,4-propylenedioxythiophene)
was designed and synthesized, and exhibited a contrast
of 89% at 632 nm with switching speeds of 400 ms and
as the substituents, we think, will increase the interchain
separation due to its rigid and bulky nature and hence the
contrast. PProDot-Bz exhibits an extremely high contrast of
2
2
21
coloration efficiency of 575 cm
C
.
8
5
9% at l
(632 nm) with coloration efficiencies of the order of
max
21
75 cm C ; in fact, these are the best reported values to date.
2
1
Conjugated polymers belong to an important class of
polymers due to their widespread use in industrial applications
like sensors, electrochromic materials, light emitting diodes
In this communication, we report its synthesis, electrochemical
and optical properties.
The monomer 1 was synthesized by the standard transether-
ification reaction between 3,4-dimethoxythiophene and 2,2-
dibenzylpropane-1,3-diol as shown in Scheme 1 and was
(
materials where the color exhibited is a function of applied
LEDs), actuators, and batteries etc. Electrochromics (ECs) are
2
–4
8
potential.
Both inorganic and organic materials have been
recrystallised from acetone. 2,2-Dibenzylpropane-1,3-diol
was synthesized starting from diethyl malonate. Monomer 1
used as EC materials but there is still a lot of scope for further
improvement in terms of switching speeds, stability, contrast
and ease of synthesis and processing. Conducting or con-
jugated polymers have been found to be more promising as EC
materials because of their better stability, faster switching
speeds and easy processing compared to the inorganic EC
1
was characterized by H NMR, GC–MS and elemental
analysis, and the crystal structure was solved using X-ray
9
crystallography. The crystal structure of 1 (Fig. 1) showed that
the benzyl substituents are orthogonal to the thiophene ring
confirming our design strategy. Interestingly, the two benzyl
groups are on opposite sides of the thiophene plane and hence
will be very effective for interchain separation.
2
materials. EC materials where the color changes from a highly
opaque colored state to highly transmissive bleached state are
highly desirable as they are potential candidates for applica-
tions in display devices. In these systems, the change in
percentage transmittance (D%T) between the two states is
called the contrast and the higher the contrast, the better the
material is for display applications. Conjugated polymers
based on 3,4-alkylenedioxythiophene have attracted a lot of
attention, both in academic as well as in industrial labs, as
potential candidates for display applications because of their
high contrast, low oxidation potential, better stability to air
exposure at ambient and at elevated temperature conditions,
For optoelectrochemical studies, monomer 1 was electro-
polymerized on indium tin oxide (ITO) coated glass slides from
a 30 mM solution of the monomer in 0.1 M tetrabutylammo-
nium tetrafluoroborate (TBAFB) in acetonitrile (ACN) at a
z
constant potential of 1.2 V vs. Ag/Ag . This quickly results in
the formation of nice and homogeneous films on the ITO
electrode. The films were then washed with monomer-free
electrolyte solution and were vacuum dried before measuring
the thickness as a function of charge density using a
profilometer. A linear relationship was observed for change
5
2
2
and faster switching speeds. Recently, Reynolds and co-
workers have extensively studied the effect of ring size and also
of ring substitution on the electrochromic properties of polymers
in film thickness versus charge density (mQ cm ). The change
in film thickness as a function of charge density was then
calculated from the slope and was found to be 54.2 nm. This
value is almost double that reported for tetradecyl substituted
5
based on 3,4-alkylenedioxythiophene. They observed that the
electrochromic contrast improves on increasing the ring size
and also on increasing the interchain separation by the
6
incorporation of a rigid/bulky side chain. A tetradecyl sub-
stituted polyethylenedioxythiophene derivative was found to
exhibit a D%T of 64%. However, the highest contrast (D%T of
7
8%) reported to date was observed in the case of a dimethyl
7
substituted polypropylenedioxythiophene (PProDOT-Me
Therefore, if one has to design a polymer with even better
contrast than PProDOT-Me one has to use more rigid/bulky
2
).
2
substituents instead of methyl groups. Based on these facts, we
designed and synthesized a dibenzyl propylenedioxythiophene
(
ProDOT-Bz
2
) monomer (1). Incorporation of benzyl groups
{Electronic supplementary information (ESI) available: cyclic voltam-
mograms of polymer films in 0.1 M tetrabutylammonium tetrafluoro-
borate–acetonitrile. See http://www.rsc.org/suppdata/jm/b1/b108654e/
Scheme 1 Synthesis of dibenzyl propylenedioxythiophene (ProDOT-
Bz ) monomer.
2
DOI: 10.1039/b108654e
J. Mater. Chem., 2001, 11, 2909–2911
This journal is # The Royal Society of Chemistry 2001
2909