Conformational locking for band gap control in 3,4-propylenedioxythiophene based electrochromic polymers

Article abstract of DOI:10.1039/b517819c

We report a tethered poly(3,4-propylenedioxythiophene) derivative with a built-in polymer conformation restriction which locks the conjugated chain at a specific dihedral angle, thus providing a handle in which to tune the optical and electronic properties of the material. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b517819c

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Conformational locking for band gap control in 3,4-
propylenedioxythiophene based electrochromic polymers{
Ryan M. Walczak, John S. Cowart, Jr., Khalil A. Abboud and John R. Reynolds*
Received (in Bloomington, IN, USA) 14th December 2005, Accepted 21st February 2006
First published as an Advance Article on the web 16th March 2006
DOI: 10.1039/b517819c
We report a tethered poly(3,4-propylenedioxythiophene) deri-
vative with a built-in polymer conformation restriction which
locks the conjugated chain at a specific dihedral angle, thus
providing a handle in which to tune the optical and electronic
properties of the material.
In recent years, extensive research has been devoted to new
1–5
variable gap conjugated polymers. Often, band gap control is
attained by modifying conformational or aggregate behavior
6–8
through functional group modifications.
Properties such as
color, conductivity, ion selectivity, etc. are tuned by changing a
pendant group. Perhaps the most important principle behind these
types of materials property studies is that the modifications made
to the systems are accomplished without making fundamental
changes to the chemical identity of the conjugated polymer
backbone. This elimination of variability in repeat unit composi-
tion allows for the systematic study of a family of materials whose
properties are governed by a single parameter. One such systematic
modification consists of polymer repeat units connected by a
functional group that serves to restrict the rotational angle between
them. Ground-breaking work by Benincori et al. has shown that
conformation locking of polythiophene derivatives allows for a
variable-length tether to control the electronic and optical
Fig. 1 Front view (left) and side view (right) 3D representations (not to
scale) of top: PProDOT, and bottom: a conformationally locked analogue.
Atom colors: yellow, S; red, O; gray, C; white, H.
with the ditosylate of 1,5-pentanediol in DMF with sodium
hydride to yield monomer 2 in good yield. Macrocycle 3 was
formed in moderate yields by reacting compound 2 with two
equivalents of n-butyllithium, followed by oxidative coupling with
Fe(acac) . Monomer 4 was synthesized from compound 1 and
3
9–14
properties of the polymer.
The groups of Swager and
Roncali have used PEO-tethered polythiophene and PEDOT
15–19
derivatives for applications in sensors.
Here, we use tetheredmacrocyclic dimers of 3,4-propylenedioxy-
thiophenes (ProDOT), as schematically illustrated in Fig. 1
with alkylene bridged PProDOT, to restrict polymer back-
bone rotation and decrease the pi overlap as a means to
elevate the electronic band gap. This new class of materials
is the first known example of using the conformational
restriction motif to form macromolecules with well-defined
repeat unit structures composed of electron-rich subunits.
We demonstrate that the convenient low polymer oxidation
potential similar to the untethered parent system is main-
tained while the band gap of the material is elevated.
Furthermore, these materials are seen to maintain their
electroactivity, displaying a brilliant colored to transmissive
electrochromism upon electrochemical switching.
As outlined in Scheme 1, synthesis began with compound 1,
which was prepared from 3,4-dimethoxythiophene via transether-
20,21
ification with 1,1,1-tris(hydroxymethylethane),
and was reacted
The George and Josephine Butler Polymer Research Laboratory,
Department of Chemistry, Center for Macromolecular Sciences and
Engineering, University of Florida, Gainesville, Florida, 32611, USA
{ Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b517819c
Scheme 1 a) TsO(CH
THF, 278 uC A RT; (2) THF, 2.5 equiv of Fe(acac)
TsO(CH CH , NaH, DMF (63%); (d) (1) 1.2 equiv of n-BuLi, THF,
278 uC A RT; (2) Fe(acac) , reflux (15%); (e) anodic oxidation.
2
)
5
OTs, DMF (71%); (b) (1) 2.1 equiv of n-BuLi,
3
, reflux (15%); (c)
2
)
3
3
3
1
604 | Chem. Commun., 2006, 1604–1606
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Fig. 4 UV/vis/NIR spectra of neutral polymer films of (pink) Poly3;
Fig. 2 Overlaid first scan CVs of monomers (a) 5; (b) 3; (c) 4; (d) 2.
Concentrations were approx. 20 mM monomer in a solution of 0.2 M
TBAP in PC. Current was normalized to facilitate visualization. Inset:
potentiodynamic deposition of Poly3.
(green) PEDOT; (blue) Poly5; (red) Poly4; (dark blue) Poly2; and of
(brown) oxidized Poly3. To aid the eye, the baselines were zeroed, and the
absorbances were normalized to lmax. Photographs: the neutral forms of
PEDOT, Poly5, and the switching of Poly3.
n-butyl tosylate, and dimer 5 was synthesized in moderate yields
from compound 4 via oxidative coupling.
rings, and the macrocyclic bridge adopted syn-connectivity. This
angle is in stark contrast to previously reported dimeric ProDOT
23,24
and EDOT derivatives, which usually have
Shown in Fig. 2, single-ring monomers 4 and 2 exhibited peak
oxidation potentials of 1.08 V and 1.05 V, respectively as is
common for many electropolymerizable dioxythiophenes. Due to
extended conjugation, dimer 5 showed an oxidation peak at 0.54 V
and macrocycle 3 at 0.73 V. Polymer films were electrodeposited
on platinum button electrodes via repeated potential scanning as
shown for Poly3 in the inset to Fig. 2. The electropolymerization
was well-behaved, as evidenced by the low potential redox process
that evolves with multiple scans, and even after 15 scans the
resulting polymer redox peaks remained sharp, and of constant
potential. It was believed that due to its tetrafunctionality,
polymerization of 2 would form a cross-linked network of Poly2
on the electrode surface, thus limiting the effective conjugation
length. Indeed, it was observed that during the electropolymeriza-
tion of Poly2, anodic shifts of both the polymer and monomer
redox processes occurred, passivating the electrode and strongly
suggesting cross-linking. In contrast, the electropolymerization of
control polymers Poly4 and Poly5 exhibited broad oxidation
processes and a lower polymer oxidation potential with respect to
torsion angles close
to 180u. It can be seen that the macrocyclic cavity resembles a
crown ether.
Fig. 4 shows UV/vis/NIR spectra for all four polymers
potentiostatically deposited onto ITO/glass anodes with the
spectrum for PEDOT as a comparison. The spectrum for the
oxidized form of Poly3, which was nearly transparent (with a
bluish hue), is also shown. It is evident that the band gap of Poly3
is elevated in comparison to the polymers formed from the simple
monomeric and untethered dimeric dioxythiophenes. As such,
where PEDOT is blue, and Poly4 and Poly5 are blue-purple, the
conformationally locked Poly3 is translucent orange. The extreme
color shift seen in the tethered Poly3 is unprecedented in a
structurally comparable family of polymers, and can be attributed
to the alkylene tether restricting the extent of p-overlap along the
polymer backbone. This is evident in the spectra where Poly4 and
Poly5 had band gaps (measured as the p, p* absorption onset)
equal to 1.75 eV and PEDOT of 1.61 eV. Fine structure observed
in PEDOT, Poly4 and Poly5, normally attributed to vibronic
+
Poly3. It was also found that the E1/2 values of Poly3, when Li ,
+
+
2+
Na , K , and Ba were used as supporting electrolytes, were
25
coupling, suggests inter-chain p-stacking interactions. In con-
trast, Poly2 and Poly3 have elevated band gaps equal to 2.13 eV
+
approximately 225 ¡ 30 mV. However, when Cs was used, the
(0.38 higher than the control) and no fine structure was observed.
E1/2 of Poly3 was found to be 506 mV. Future studies will address
This lack of fine structure may originate from inefficient p-stacking
of the twisted backbone and may also explain the sharp, well-
defined polymer redox process. While the optical properties of a
thin film of Poly2 are essentially identical to those of Poly3, it is
much less electroactive, precluding its use as an effective
electrochrome, and this demonstrates that the macrocyclic
monomer is critical in producing a material with desirable
electrooptical properties.
this potential ion sensitivity in greater detail.
22
As seen in Fig. 3, the X-ray crystal structure of macrocycle 3
showed an approximately 30.1u torsion angle between thiophene
In conclusion, conformational locking of a tethered PProDOT
derivative has provided an electrochromic polymer with an
elevated band gap and a well-characterized repeat unit structure
due to the monomer’s bifunctionality. The X-ray crystal structure
of the macrocycle indicates that the locked torsion angle reduces
the extent of p-overlap relative to the control polymers. This is the
first known example of a dioxythiophene-based orange neutral
polymer that becomes highly transmissive when oxidized. The
Fig. 3 X-ray crystal structure for 3. Left: Side view. Right: Front view.
Torsion angle between thiophene rings: 30.14(5)u. Thermal ellipsoids are at
the 50% probability level.
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1
2 T. Benincori, E. Brenna, F. Sannicolo, L. Trimarco, G. Moro, D. Pitea,
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8
tethered repeat unit structure shows potential in finely tuning
optical properties by changing the composition of the tether. This
is being explored in further studies.
81.
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1
The AFOSR (F49620-03-1-0091) is gratefully acknowledged for
funding of this project. KAA wishes to acknowledge the National
Science Foundation and the University of Florida for funding of
the purchase of the X-ray equipment.
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2 C23 , M
1
˚
2
H
30
O
6
S
2
r
= 466.59, Monoclinic, P2
1
/n, a = 12.8140(7) A, b =
3
˚
˚
˚
1
1.3396(6) A, c = 15.5221(8) A, b = 92.432(1)u, V = 2253.4(2) A , Z = 4,
23
˚
calc. = 1.375 g cm , Mo Ka (l = 0.71073 A), T = 173 K. 282
D
parameters were refined in the final cycle of refinement using 3856 (out
of 5124 unique reflections and a total of 14603; Rint. = 5.3%) reflections
8
9
1 2
with I > 2s(I) to yield R and wR of 3.47 and 8.7, respectively.
2
Refinement was done using F . CCDC 293188. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b517819c.
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606 | Chem. Commun., 2006, 1604–1606
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