Discrete photopatternable π-conjugated oligomers for electrochromic devices

Article abstract of DOI:10.1021/ja7112273

Three discrete oligomeric systems including an all-thiophene (T6) system, a thiophene/phenylene (TPTTPT) system, and a thiophene/EDOT/phenylene (TPEEPT) system have been constructed and characterized with emphasis on structural, optical, electrochemical,

Full text of DOI:10.1021/ja7112273

Published on Web 07/02/2008
Discrete Photopatternable π-Conjugated Oligomers for
Electrochromic Devices
Christian B. Nielsen, Alex Angerhofer, Khalil A. Abboud, and John R. Reynolds*
The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry,
Center for Macromolecular Science and Engineering, UniVersity of Florida,
GainesVille, Florida 32611
Received January 4, 2008; E-mail: reynolds@chem.ufl.edu
Abstract: Three discrete oligomeric systems including an all-thiophene (T6) system, a thiophene/phenylene
(TPTTPT) system, and a thiophene/EDOT/phenylene (TPEEPT) system have been constructed and
characterized with emphasis on structural, optical, electrochemical, and spectroelectrochemical properties.
For all three chromophores, the radical cation, the dication, and the π-dimer have been identified and
characterized. EPR spectroscopy reveals that the radical cations of TPTTPT and TPEEPT have g values
of 2.008-2.012 and peak-to-peak widths in the range 4.2-5.3 G. Formation of the radical cation takes
place at a lower potential for TPEEPT than for TPTTPT and T6, whereas subsequent oxidation to the
dication occurs more easily for TPTTPT than for TPEEPT and T6. We ascribe this observation to more
localized charges in the oxidized species of TPEEPT, which is supported by our finding that the radical
cation of TPEEPT is less prone to undergo π-dimerization than the radical cations of TPTTPT and T6. All
the oxidized species are sufficiently stable to allow for optical characterization, and the relative positions
of all absorption bands are found to be in agreement with the electrochemical data. For further solid-state
modifications of these materials, we have effectively modified the synthetic design and grafted terminal
functionalities (e.s. acrylates) onto the discrete oligomers. Of these novel materials, TPEEPT proves to be
the most promising anodically coloring material for electrochromics, and it undergoes reversible switching
between two different colored states (bright yellow and clear blue) and one almost transparent and color
neutral state. Contrast ratios, measured as ∆%T at λmax, are as high as 62.5%, and switching times are in
the range 2-5 s for the coloration process, though significantly longer for the bleaching process. As a
proof of concept, we have successfully constructed a simple photopatterned electrochromic device by
exploiting the terminal acrylate functionalities of the oligomers in a UV-initiated cross-linking process. To
the best of our knowledge, this is the first oligomer-based photopatterned electrochromic device reported
in the literature.
in electroactive devices such as organic field-effect transistors,7–12
organic light-emitting diodes,13–16 and in photovoltaic devices,
Introduction
Ever since the discovery of electrical conductivity in doped
polyacetylene, extensive research has been focused on new
conjugated materials, their design, synthesis, and application
in electronic devices.1 This class of materials primarily includes
a vast number of conjugated polymers, but also more well-
defined systems such as dendrimers and discrete oligomers.2–6
Considering the discrete π-conjugated oligomers, they not only
serve as important model compounds for understanding funda-
mental properties of π-conjugated polymers, but also have been
shown to possess attractive properties themselves for application
either on their own or as part of a donor-acceptor system with
fullerenes.17–20 The major advantage of discrete oligomers
(7) Facchetti, A.; Deng, Y.; Wang, A. C.; Koide, Y.; Sirringhaus, H.;
Marks, T. J.; Friend, R. H. Angew. Chem., Int. Ed. 2000, 39, 4547–
4551.
(8) Yoon, M. H.; Facchetti, A.; Stern, C. E.; Marks, T. J. J. Am. Chem.
Soc. 2006, 128, 5792–5801.
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State Commun. 1989, 72, 381–384.
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(12) Dimitrakopoulos, C. D.; Purushothaman, S.; Kymissis, J.; Callegari,
A.; Shaw, J. M. Science 1999, 283, 822–824.
(13) Geiger, F.; Stoldt, M.; Schweizer, H.; Bauerle, P.; Umbach, E. AdV.
Mater. 1993, 5, 922–925.
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A. J. J. Chem. Soc., Chem. Commun. 1977, 1977, 578–580.
(2) Lo, S. C.; Burn, P. L. Chem. ReV. 2007, 107, 1097–1116.
(3) Mullen, K.; Wegner, G. Electronic Materials: The Oligomer Approach;
Wiley-VCH: Weinheim, Germany, 1998.
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Phys. Lett. 1999, 75, 439–441.
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(16) Hung, L. S.; Chen, C. H. Mater. Sci. Eng., R 2002, 39, 143–222.
(17) Jorgensen, M.; Krebs, F. C. J. Org. Chem. 2004, 69, 6688–6696.
(18) Huisman, C. L.; Huijser, A.; Donker, H.; Schoonman, J.; Goossens,
A. Macromolecules 2004, 37, 5557–5564.
(4) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350–
1377.
(5) Fichou, D. J. Mater. Chem. 2000, 10, 571–588.
(6) Skotheim, T. A.; Reynolds, J. R. Handbook of Conducting Polymers;
Marcel Dekker: New York, 2007.
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2575.
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Discrete Photopatternable π-Conjugated Oligomers
A R T I C L E S
compared to their polymeric counterparts is monodispersity and
hence a more direct and unambiguous correlation between
structure and properties. From a synthetic point of view, the
synthesis of discrete oligomers will often be more tedious and
time-consuming than synthesis of conjugated polymers. How-
ever, structural complexity, such as incorporation of reactive
end groups, can often be more easily achieved for well-defined
oligomers than for polymers as a consequence of the different
synthetic approaches.
Here, we report on a new family of discrete conjugated
oligomers and their electrochromic properties. Whereas conju-
gated polymers (e.g., polyanilines, polypyrroles, and poly-
thiophenes), which have been used widely for electrochromic
applications,21–26 generally have broad electronic absorption
bands, discrete oligomers have more narrow and well-defined
electronic absorption bands and are thus expected to undergo
more clear and distinct color changes during the redox processes
exploited for electrochromic behavior. This has been investigated
nicely by Shirota and co-workers, who have reported on the
electrochromic behavior of several polymeric systems containing
small isolated chromophores.27–29 By attaching conjugated
oligomers of varying length to vinyl and methacrylate polymers
as pendant groups, they were able to create several electro-
chromes, which all underwent clear and reversible color changes
(i.e., from blue to pale yellow, green to yellow, and orange to
purple) upon oxidation. Similar systems have been described
by other researchers including Barbarella and co-workers and
Ferraris and co-workers.30,31 As opposed to attaching the isolated
chromophores as a pendant group, they can also be incorporated
into a polymer backbone using one of several approaches. As
one example, McCulloch and co-workers have illustrated this
well by synthesizing liquid crystalline organic semiconductors
with photopolymerizable end groups;32,33 the isolated chro-
mophores could thus be incorporated into a highly ordered cross-
linked network by UV-curing subsequent to film processing.
Inspired by several of the aforementioned contributions to
recent advances in electrochromism and oligomer design and
Figure 1. Schematic illustration of the idealized concept of processing (i)
of telechelic oligomers (A) to create π-stacked domains (B) that can be
further modified by photoinduced cross-linking (ii) to give cross-linked
networks (C).
processing, we have recently taken interest in new π-conjugated
materials that encompass the desired electroactive properties,
processability, and stability and also provide an opportunity to
incorporate the π-conjugated system covalently into a more
complex macromolecular system. As illustrated in Figure 1A,
we have been focusing on a series of discrete oligomers with
an overall coil-rod-coil structure consisting of two alkyl
spacers and a central π-conjugated oligomer segment. By adding
terminal acrylate functionalities, we aim to exploit the self-
organizing capability of the system induced by π-stacking
between neighboring π-conjugated segments during processing
(Figure 1B) and to introduce a second degree of connectivity
by photoinduced cross-linking at the termini as depicted in a
simplified manner in Figure 1C. Because of the extended
connectivity, the soluble π-conjugated material is converted into
an insoluble cross-linked structure. Moreover, if the cross-linking
takes place without disrupting the intermolecular ordering and
the electroactive properties drastically, then this approach can
be used for photopatterning of optoelectronic devices with
cooperative interactions between chromophores.
(20) Segura, J. L.; Martin, N.; Guldi, D. M. Chem. Soc. ReV. 2005, 34,
31–47.
(21) Argun, A. A.; Aubert, P. H.; Thompson, B. C.; Schwendeman, I.;
Gaupp, C. L.; Hwang, J.; Pinto, N. J.; Tanner, D. B.; MacDiarmid,
A. G.; Reynolds, J. R. Chem. Mater. 2004, 16, 4401–4412.
(22) Garnier, F.; Tourillon, G.; Gazard, M.; Dubois, J. C. J. Electroanal.
Chem. 1983, 148, 299–303.
This strategy has thus far resulted in the design of three new
thiophene- and phenylene-based oligomeric systems, and we
present here the synthesis and characterization of these oligomers
as well as our work toward photopatterning and electrochromic
display fabrication.
(23) Hyodo, K. Electrochim. Acta 1994, 39, 265–272.
(24) (a) Mortimer, R. J.; Dyer, A. L.; Reynolds, J. R. Displays 2006, 27,
2–18. (b) Dyer, A. L.; Reynolds, J. R. In Handbook of Conducting
Polymers, 3rd ed.; Skotheim, T. A., Reynolds, J. R., Eds.; CRC Press:
Boca Raton, FL, 2007.
(25) Sapp, S. A.; Sotzing, G. A.; Reynolds, J. R. Chem. Mater. 1998, 10,
2101–2108.
Oligomer Synthesis and Characterization
(26) Thompson, B. C.; Kim, Y. G.; McCarley, T. D.; Reynolds, J. R. J. Am.
Chem. Soc. 2006, 128, 12714–12725.
Synthesis of Model Oligomers. To establish the fundamental
properties of the π-conjugated segments of the target com-
pounds, we initially synthesized the two model compounds
TPTTPT-Me2 and TPEEPT-Me2 (T for thiophene, P for
phenylene, and E for EDOT), as depicted in Scheme 1.34
1,4-Dibromo-2,5-bis(dodecyloxy)benzene was converted to
the 1-aryl-4-bromo-2,5-bis(dodecyloxy)benzenes 1 and 3 via
Stille couplings with the appropriate 2-stannylated thiophenes
using stoichiometric imbalance to ensure dominant monoreac-
tion. Subsequent Negishi couplings with 2-chlorozinc-5-meth-
ylthiophene afforded the monomethylated bisthienylbenzenes
(27) Ohsedo, Y.; Imae, I.; Shirota, Y. J. Polym. Sci., Part B: Polym. Phys.
2003, 41, 2471–2484.
(28) Shirota, Y. J. Mater. Chem. 2000, 10, 1–25.
(29) Nawa, K.; Miyawaki, K.; Imae, I.; Noma, N.; Shirota, Y. J. Mater.
Chem. 1993, 3, 113–114.
(30) Melucci, M.; Barbarella, G.; Zambianchi, M.; Benzi, M.; Biscarini,
F.; Cavallini, M.; Bongini, A.; Fabbroni, S.; Mazzeo, M.; Anni, M.;
Gigli, G. Macromolecules 2004, 37, 5692–5702.
(31) Meeker, D. L.; Mudigonda, D. S. K.; Osborn, J. M.; Loveday, D. C.;
Ferraris, J. P. Macromolecules 1998, 31, 2943–2946.
(32) McCulloch, I.; Zhang, W. M.; Heeney, M.; Bailey, C.; Giles, M.;
Graham, D.; Shkunov, M.; Sparrowe, D.; Tierney, S. J. Mater. Chem.
2003, 13, 2436–2444.
(33) McCulloch, I.; Bailey, C.; Genevicius, K.; Heeney, M.; Shkunov, M.;
Sparrowe, D.; Tierney, S.; Zhang, W. M.; Baldwin, R.; Kreouzis, T.;
Andreasen, J. W.; Breiby, D. W.; Nielsen, M. M. Philos. Trans. R.
Soc. London, Ser. A 2006, 364, 2779–2787.
(34) Wan, J. H.; Feng, J. C.; Wen, G. A.; Wang, H. Y.; Fan, Q. L.; Wei,
W.; Huang, C. H.; Huang, W. Tetrahedron Lett. 2006, 47, 2829–2833.
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Nielsen et al.
Scheme 1
2 and 4, which were then oxidatively homocoupled to give
TPTTPT-Me2 and TPEEPT-Me2 in good yields.
only 8.0(2)°; S-O distances of 2.899(2) Å between the two
EDOT rings and 2.627(2) Å between the alkoxy group and the
thiophene again demonstrate highly favorable S-O interactions,
which most likely contribute greatly to the coplanar arrangement.
Furthermore, we note that the four dodecyl side chains in PEEP
exhibit an all-trans configuration and are nearly coplanar to the
aromatic core.
Synthesis of Telechelic Oligomers. The synthesis of telechelic
oligomers with identical conjugated backbones as the model
compounds (Scheme 2) was achieved using a similar approach
as described for the model oligomers above. Starting from the
protected 6-(2-thienyl)-1-hexanol (5), which was easily synthe-
sized from the corresponding bromide and 2-lithiothiophene, a
Negishi coupling with either 1 or 3 afforded the monofunc-
tionalized bisthienobenzenes 6 and 8. In this instance, homo-
coupling was carried out by lithiation and subsequent treatment
with ferric acetylacetonate, since the previously applied ferric
chloride conditions were incompatible with the hydroxyl protec-
tion group.37 Deprotection of 7 and 9 were easily achieved under
mildly acidic conditions to give the functional oligomers
TPTTPT-diol and TPEEPT-diol in excellent yields; further
modification of the terminal hydroxy functionality with acryloyl
chloride afforded TPTTPT-diacrylate and TPEEPT-diacry-
late.
Structural Analysis. For X-ray diffraction studies, single
crystals of TPTTPT-Me2 and TPEEPT-Me2 were grown, but
unfortunately, all crystallization experiments afforded crystals
that were not suitable for structural analysis. Instead, the model
compounds PTTP and PEEP depicted in Figure 2 were
designed to resemble the central conjugated moieties of TPT-
TPT-Me2 and TPEEPT-Me2, respectively. These model com-
pounds were synthesized using a similar method (Supporting
Information), and suitable single crystals were successfully
obtained by slow evaporation of a dichloromethane/hexane
mixture (PTTP) and a dichloromethane/acetonitrile mixture
(PEEP).
The crystal structures of PTTP and PEEP are displayed in
Figures 3 and 4. Both molecules are located on inversion centers
and have a completely planar central bithiophene moiety in the
anti configuration. For PTTP, the outer phenyl rings are twisted
out of the plane by 24.4(7)° relative to the inner thiophenes;
favorable S-O interactions are evident from an intramolecular
S-O distance of 2.747(1) Å, which is significantly shorter than
the sum of the van der Waals radii (3.35 Å).35,36 Single crystals
of PEEP adapt an even more coplanar structure with a dihedral
angle between the inner thiophene and the outer phenyl ring of
Additionally, an all-thiophene oligomer was synthesized as
described in Scheme 3. The dialkylated quaterthiophene 10 was
dibrominated (11) and then successfully converted to the
Figure 2. Structure of model compounds PTTP and PEEP.
Figure 3. Molecular structure of a PTTP single crystal.
Figure 4. Molecular structure of a PEEP single crystal.
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Discrete Photopatternable π-Conjugated Oligomers
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Scheme 2
Scheme 3
confirmed by elemental analysis and high-resolution mass
spectrometry (see Supporting Information for experimental
procedures and full characterization of all new compounds).
Solution Properties
Electrochemistry. Cyclic voltammetry (CV) of oligomers and
polymers tends to give broad peaks, which can obstruct an
accurate characterization of the redox-active system investigated.
In differential pulse voltammetry (DPV), on the other hand, the
faradaic current is extracted and the redox processes can
consequently be analyzed more precisely. We have studied the
redox properties of TPTTPT-Me2, TPEEPT-Me2, and T6-diol
in solution by both CV and DPV, and the results are presented
in Figure 5 and Table 1.
The CV for TPTTPT-Me2 displays two well-resolved
consecutive redox processes with half-wave potentials around
0.24 and 0.44 V (vs Fc/Fc+).40 Likewise, the voltammogram
for TPEEPT-Me2 displays two fairly well-resolved consecutive
redox processes with half-wave potentials around 0.15 and 0.50
V. The differential pulse voltammograms for TPTTPT-Me2 and
TPEEPT-Me2 each reveal two well-defined and highly revers-
ible anodic and cathodic processes with a high degree of
symmetry; the half-wave potentials are now observed at slightly
lower potentials than in the CV: 0.07 and 0.32 V for TPTTPT-
Me2 and at -0.01 and 0.40 V for TPEEPT-Me2. As expected,
the first oxidation process occurs at a lower potential for
TPEEPT-Me2 than for TPTTPT-Me2 because of the electron-
donating ethylenedioxy substituents. The second oxidation
process where the radical cation is converted to the dication
occurs at a lower potential for TPTTPT-Me2 than for TPEEPT-
Me2. The same trend was observed for similar oligomeric
systems by Roncali and co-workers and was ascribed to
destabilization due to Coulombic repulsion between adjacent
EDOT moieties in the dicationic state.41 The values for T6-
sexithiophene 12 in high yield by reaction with the THP-
protected 6-(5-tributylstannyl-2-thienyl)-1-hexanol using modi-
fied Stille conditions inspired by the work of Fu and co-workers
and Baldwin and co-workers.38,39 Deprotection to give T6-diol
and subsequent conversion to T6-diacrylate were accomplished
using the same protocol as described for the TPTTPT and
TPEEPT systems in Scheme 2.
All oligomers were characterized by 1H and 13C NMR
spectroscopy and show narrow and well-defined signals as
expected for monodisperse samples; identity and purity were
(35) Turbiez, M.; Frere, P.; Roncali, J. J. Org. Chem. 2003, 68, 5357–
5360.
(36) Turbiez, M.; Frere, P.; Allain, M.; Videlot, C.; Ackermann, J.; Roncali,
J. Chem.-Eur. J. 2005, 11, 3742–3752.
(37) Deprotection of compounds 6 and 8 and subsequent oxidative coupling
with ferric chloride afforded TPTTPT-diol and TPEEPT-diol in
similar overall yields.
(40) In all of the CV experiments, a linear relationship between the oxidation
current peak values and the square root of the scan rate was observed
as expected for a diffusion-limited reaction at the electrode surface
(see Supporting Information for voltammograms illustrating the scan
rate dependence).
(41) Turbiez, M.; Frere, P.; Allain, M.; Videlot, C.; Ackermann, J.; Roncali,
J. Chem.-Eur. J. 2005, 11, 3742–3752.
(38) Littke, A. F.; Schwarz, L.; Fu, G. C. J. Am. Chem. Soc. 2002, 124,
6343–6348.
(39) Mee, S. P. H.; Lee, V.; Baldwin, J. E. Angew. Chem., Int. Ed. 2004,
43, 1132–1136.
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Nielsen et al.
Figure 5. Cyclic voltammograms (A) and differential pulse voltammograms (B) of TPTTPT-Me2 (dashed lines), TPEEPT-Me2 (solid lines), and T6-diol
(dotted lines) in dichloromethane. CV experiments performed with 0.1 mM oligomer and 0.1 M tetrabutylammonium perchlorate (scan rate 25 mV/s) and
DPV with 1 mM oligomer and 0.1 M tetrabutylammonium hexafluorophosphate.
temperature;44–47 lowering of the temperature shifts the equi-
librium from the radical cation (733, 1630 nm) toward the
Table 1. Summarized Electrochemical Properties for the
Oligomers in Solutiona
Epa,1
Epc,1
Epa,2
Epc,2
dimeric species (665, 1187 nm). Upon further oxidation with
nitrosonium hexafluorophosphate, the absorption bands associ-
ated with the radical cation and the π-dimer species disappear
and a new transition from the dication (1188, 1029 nm) is
observed (Figure 6C). As also depicted in Figure 6 (inserted
photographs), the neutral solution of TPTTPT-Me2 is yellow,
the radical cation is blue, and the dication is quite transparent
and color neutral since it mostly absorbs outside the visible
region.
b
c
b
c
TPTTPT-Me2
TPTTPT-Me2
TPEEPT-Me2
TPEEPT-Me2
T6-diolb
0.27
0.10
0.20
0.01
0.52
0.53
0.21
0.04
0.10
-0.03
0.46
0.48
0.35
0.57
0.42
0.64
0.66
0.39
0.30
0.42
0.38
0.58
0.62
T6-diolc
0.49
a All potentials are reported in V vs ferrocene/ferrocenium. b CV data
with 0.1 mM oligomer and 0.1 M tetrabutylammonium perchlorate in
dichloromethane and scan rate 25 mV/s. c DPV data with 1 mM
Solution oxidation of TPEEPT-Me2 (Figure 7 and Table 2)
gives results very similar to what was described for TPTTPT-
Me2 above. Additionally, the solution oxidation of T6-diol was
examined (absorption spectra included in Supporting Informa-
tion), and in agreement with previous optical studies of
sexithiophenes,48,49 absorption bands were assigned to the
neutral state (424 nm), the radical cation (806, 1557 nm), the
π-dimer (702, 1124 nm), and the dication (893, 1003 nm).
Compared to that of 3′,3′′′′-didodecylsexithiophene, which has
unsubstituted R-positions, the absorption bands of T6-diol are
red-shifted 20-50 meV (neutral and π-dimer species) to 60-80
meV (radical cation and dication species).
While the neutral species and the dications of the oligomers
show one HOMO-LUMO transition, the radical cations show
two transitions, one corresponding to a HOMO-SOMO transi-
tion (low-energy transition) and one corresponding to a
SOMO-LUMO transition (high-energy transition), as illustrated
schematically in Figure 8.
oligomer and 0.1
dichloromethane.
M tetrabutylammonium hexafluorophosphate in
diol are in good agreement with earlier electrochemical studies
of alkylated sexithiophenes under similar conditions.42,43 In
comparison with the sexithiophene oligomer, the TPTTPT and
TPEEPT systems have lower half-wave potentials as one would
expect when taking into account their electron-rich 2,5-dialkoxy-
1,4-phenylene moieties.
Optical Spectroscopy. All of the oligomers show a strong
absorption band in the violet-to-blue region of the visible
spectrum due to the π-π* transition. A broad and featureless
absorption band is observed for the T6 (λmax 423-425 nm; εmax
45-49 000 M-1 cm-1) and the TPTTPT oligomers (λmax
430-432 nm; εmax 55 000-67 000 M-1 cm-1), whereas the
TPEEPT system (λmax 444-445 nm; εmax 68 000-79 000 M-1
cm-1) shows vibronic resolution attributed to the fact that the
EDOT units induce a more coplanar structure due to favorable
S-O interactions between neighboring phenylene and EDOT
rings as was illustrated in the crystal structure of PEEP
(Figure 4).
The electron-pairing energy stabilizes both the neutral and
the dication ground states (but not the radical cation ground
state), which explains why the absorption bands of the neutral
species and the dication are observed at shorter wavelengths
Chemical oxidation of TPTTPT-Me2 in dichloromethane
with silver triflate affords a new transition with two peaks at
665 and 733 nm as well as a broad transition in the infrared
(∼1630 nm) while the π-π* transition of the neutral state
gradually disappears (Figure 6A). Investigation of the temper-
ature dependence in this oxidized state (Figure 6B) reveals that
the radical cation coexists with the π-dimer below room
(44) Bauerle, P.; Segelbacher, U.; Maier, A.; Mehring, M. J. Am. Chem.
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Figure 6. Solution oxidation of TPTTPT-Me2 in dichloromethane. (A) Conversion from neutral to radical cation and π-dimer species, (B) temperature
dependence of the monomer/dimer equilibrium, (C) conversion from radical cation to dication, and photographs of the observed color changes.
than the high- and low-energy transitions of the radical cation,
respectively. The two transitions from the π-dimer are blue-
shifted (0.17-0.23 eV for the high-energy transition and
0.19-0.30 eV for the low-energy transition) compared to the
monomeric radical cation; this is indicative of a face-to-face
interaction and is normally referred to as a Davydov blue-shift.44
The absorption bands from the radical cation and the π-dimer
overlap significantly for all three oligomers, which makes it
complicated to determine the dimer formation enthalpy. Quali-
tatively, the dimer formation enthalpy is found to be largest for
the TPTTPT system and smallest for the TPEEPT system. This
is most likely governed by the degree of charge delocalization;
the radical cation of TPEEPT has a more localized charge
distribution than TPTTPT and T6 and will therefore, because
of charge repulsion, be less prone to form a π-dimer. For the
dications, we observe vibronic fine structure in all three cases,
a phenomenon caused by a more quinoidal structure and hence
increased planarity in the oxidized systems. We furthermore
note that these vibronic splittings fall in the range ∆E )
0.15-0.17 eV, which is in good agreement with previous studies
of other oligomeric thiophene systems.44,45,50
of 2.012 with a peak-to-peak width of 5.3 G, while the signal
from radical cation of TPEEPT-Me2 is centered at g ) 2.008
with ∆Hpp ) 4.2 G; these EPR characteristics compare well to
earlier studies of oligothiophenes by Tour and co-workers.50
No hyperfine coupling was observed for either of the two
systems, a finding that could be attributed to the delocalization
of the radical cation wave function, which results in sampling
a large number of different isotropic interactions with neighbor-
ing hydrogens. While hyperfine coupling has been observed for
short oligothiophenes,46,47 longer π-conjugated oligomers gener-
ally give rise to broad and featureless EPR signals.51,52 The
slightly broader line width observed for the TPTTPT system
than that for the TPEEPT system is most likely due to the larger
number of hydrogens attached directly to the aromatic core of
TPTTPT-Me2 (12 vs eight hydrogen atoms). Incomplete
conversion from the radical cation to the dication explains the
small EPR signals observed for the dication preparation of both
TPTTPT-Me2 and TPEEPT-Me2 (Figure 10, dotted lines).
Solid-State Properties
Electrochemistry. To investigate the redox properties of
Electron Paramagnetic Resonance Spectroscopy. Electron
paramagnetic resonance (EPR) spectroscopy of TPTTPT-Me2
and TPEEPT-Me2 in dichloromethane confirmed the presence
of the paramagnetic radical cation upon oxidation (the π-dimer
is EPR silent) and the subsequent conversion to the EPR silent
dication upon further oxidation (Figures 9 and 10). The radical
cation of TPTTPT-Me2 has an EPR signal centered at a g value
the oligomers in the solid state, we focused on thin films of the
(50) Guay, J.; Kasai, P.; Diaz, A.; Wu, R. L.; Tour, J. M.; Dao, L. H.
Chem. Mater. 1992, 4, 1097–1105.
(51) Janssen, R. A. J.; Moses, D.; Sariciftci, N. S. J. Chem. Phys. 1994,
101, 9519–9527.
(52) Kanemoto, K.; Kato, T.; Aso, Y.; Otsubo, T. Phys. ReV. B 2003, 68,
092302.
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A R T I C L E S
Nielsen et al.
Figure 7. Solution oxidation of TPEEPT-Me2 in dichloromethane. (A) Gradual conversion from neutral to radical cation and π-dimer species, (B) temperature
dependence of the monomer/dimer equilibrium, (C) conversion from radical cation to dication, and photographs of the observed color changes.
Table 2. Absorption Maxima (nm) for the Neutral (M), Radical
diacrylate also. Subsequent conversion of the two inner
Cation (M+·), π-Dimer (M22+), and Dication (M2+) of TPTTPT-Me2,
thiophenes into the more electron-rich EDOTs raises the HOMO
TPEEPT-Me2, and T6-Diol in Dichloromethane Solution (10-5 M).
value to approximately 5.2 eV as observed for TPEEPT-
diacrylate.
TPTTPT-Me2
TPEEPT-Me2
T6-diol
M
430
445
424
Numerous studies have illustrated that dioxythiophene-
containing systems have a significantly increased stability of
the oxidized states as compared to unsubstituted thiophene and
alkylthiophene systems.53–57 This observation is also made in
this study when comparing the redox processes of TPTTPT-
diacrylate, TPEEPT-diacrylate, and T6-diacrylate and is
particularly pronounced when investigating the solid-state
electrochemistry in a dichloromethane-supported electrolyte.
While TPTTPT-diacrylate and T6-diacrylate films in dichlo-
romethane show limited current responses upon electrochemical
oxidation and continuously lose electroactivity, incorporation
of EDOT moieties greatly enhances the redox stability. This is
illustrated for TPEEPT-diacrylate in Figure 11; both the CV
and the DPV experiments reveal two well-separated oxidation
peaks followed by two reduction peaks offering a high degree
of symmetry and thus a high degree of chemical reversibility.
The first half-wave potential is observed at 0.05 V in both CV
M•+
733, 1630
665, 1187
1029, 1188
705, 1389
630, 1198
1007, 1167
806, 1557
702, 1124
893, 1003
2+
M2
M2+
diacrylate oligomers; upon UV irradiation of these films, the
diacrylates polymerize to form cross-linked networks, which
renders them insoluble in all common organic solvents and
hence suitable for solid-state electrochemistry.
Redox properties of the cross-linked films in acetonitrile-
supported electrolyte are depicted in Figure 10, and the results
are summarized in Table 3. For TPEEPT-diacrylate, the two
consecutive redox processes are well-resolved, whereas two
poorly resolved peaks (TPTTPT-diacrylate) or one broad peak
with shoulder features (T6-diacrylate) are observed for the other
two systems. Some electrochemical irreversibility is expressed
through the lower current passed during the cathodic sweep as
compared to the current passed during the anodic sweep.
Hysteresis effects are observed for all three systems, and
potential differences as large as 450 mV between the oxidation
and the reduction peaks are seen for T6-diacrylate.
(53) Jonas, F.; Heywang, G. Electrochim. Acta 1994, 39, 1345–1347.
(54) Yamato, H.; Ohwa, M.; Wernet, W. J. Electroanal. Chem. 1995, 397,
163–170.
(55) Pei, Q.; Zuccarello, G.; Ahlskog, M.; Inganas, O. Polymer 1994, 35,
1347–1351.
From the onset of oxidation, it is estimated that T6-diacrylate
has a HOMO value of 5.4 eV; changing the two 3-alkylth-
iophenes into two 2,5-dialkoxy-1,4-phenylenes does not affect
the HOMO value as it is found to be 5.4-5.5 eV for TPTTPT-
(56) Dietrich, M.; Heinze, J.; Heywang, G.; Jonas, F. J. Electroanal. Chem.
1994, 369, 87–92.
(57) Heywang, G.; Jonas, F. AdV. Mater. 1992, 4, 116–118.
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9740 J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008

Discrete Photopatternable π-Conjugated Oligomers
A R T I C L E S
Figure 8. (A) Frontier molecular orbitals and dipole-allowed transitions for a neutral species (M), a radical cation (M•+), and a dication (M2+). (B) Frontier
orbitals and allowed transitions for the π-dimer based on linear combination of molecular orbitals for the radical cation.
Figure 9. EPR spectra of the neutral (solid line), radical cation (dashed line), and dication (dotted line) of (A) TPTTPT-Me2 and (B) TPEEPT-Me2 in
dichloromethane (1 mM).
Figure 10. Cyclic voltammograms (A) and differential pulse voltammograms (B) of cross-linked films of TPTTPT-diacrylate (dashed lines), TPEEPT-
diacrylate (solid lines), and T6-diacrylate (dotted lines) on ITO-coated glass in 0.1 M tetrabutylammonium perchlorate in acetonitrile (CV scan rate 150
mV/s).
and DPV (Table 3), while the second half-wave potential is
identified at 0.64 V (CV) and 0.56 V (DPV).40 These potentials
are significantly lower than the values obtained in acetonitrile-
supported electrolyte but compare well with the solution data
for TPEEPT-Me2 also obtained in dichloromethane. From the
onset of oxidation, the HOMO energy level for TPEEPT-
diacrylate is estimated to be approximately 5.0 eV under these
conditions.
chemically stable materials of the three. Films of TPTTPT-
diacrylate and T6-diacrylate show poor electrochemical
stability in both acetonitrile and dichloromethane. Films of
TPEEPT-diacrylate, on the other hand, switch nicely,
especially in a dichloromethane-supported electrolyte, be-
tween a yellow neutral state, a blue radical cation state, and
a more transparent blue/purple dication state. Thus, compared
to that of TPTTPT and T6, the TPEEPT chromophore
proves to be a far more promising candidate for electrochro-
mic applications, and the diacrylate functionality offers a
convenient means of controlling solubility.
The electrochemical results presented here on the three
oligomer systems in the solid state clearly show that cross-
linked films of TPEEPT-diacrylate are the most electro-
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J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008 9741

A R T I C L E S
Nielsen et al.
Table 3. Summarized Electrochemical Properties for the Diacrylate
photographs show the distinct color changes from a bright
yellow neutral film to a clear blue (radical cation), and
subsequently on to an almost transparent dicationic state with
a residual bluish component from the radical cation and a slight
purple color from the 685-nm band. Comparison with the
solution oxidation data (Figure 7) reveals that the positions of
the various absorption bands are more or less unchanged during
the transition from solution to the solid state. However, the bands
in the solid state are broadened and lack the resolution observed
in solution, most likely due to the rigidification of the films
taking place during cross-linking, which locks the conformation
and hinders any substantial reorganization in the film during
the oxidation processes.
Oligomers in the Solid Statea
Eonset
Epa,1
Epc,1
Epa,2
Epc,2
TPTTPTb
TPTTPTc
TPEEPTb
TPEEPTc
T6b
0.41
0.33
0.20
0.06
0.33
0.35
-0.06
-0.13
0.72
0.68
0.39
0.27
0.70
0.58
0.07
0.10
0.52
0.62
0.37
0.20
0.25
0.53
0.03
0.00
1.05
0.98
0.99
0.73
0.91
0.93
0.94
0.89
0.71
0.46
T6c
TPEEPTd
TPEEPTe
0.66
0.61
0.61
0.50
a All potentials are reported in
V vs ferrocene/ferrocenium. All
experiments were performed with cross-linked films on ITO-coated glass
with 0.1 M tetrabutylammonium perchlorate as electrolyte. b CV data in
acetonitrile and scan rate 150 mV/s. c DPV data in acetonitrile. d CV
data in dichloromethane and scan rate 25 mV/s. e DPV data in
dichloromethane.
Photopatterning and Switching Studies. After having estab-
lished the chemical stability of the TPEEPT system in its
various oxidized states, the high degree of redox reversibility,
and excellent multicolored electrochromic behavior, the next
step was to employ the diacrylate functionality of TPEEPT-
diacrylate for photopatterning purposes. Thin films of TPEEPT-
diacrylate cross-link successfully within a few hours with no
sign of degradation of the TPEEPT chromophores. Figure 14
shows how the absorption increases and the emission decreases
when the films are thermally annealed and subsequently
photopolymerized, a finding that indicates that the degree of
structural order is increasing during the process.58
As illustrated in Figure 15, this strategy (in combination with
simple mask fabrication) can be applied in a straightforward
photopatterning process to create defect-free patterns with at
least submillimeter resolution. Photopatterned films of TPEEPT-
diacrylate show the same optical, electrochemical, and spec-
troelectrochemical characteristics as already discussed for
homogeneous films of TPEEPT-diacrylate; the capability of
manifesting three distinct colors when going from neutral to
radical cation and to dication is also conserved as obvious from
Figure 15B-D.
Optical Spectroscopy. Thin film absorption spectra (Figure
12, Table 4) indicate the expected more coplanar structure in
the solid state compared to solution for all three systems,
although indications of a less ordered packing are observed for
TPEEPT-diacrylate. Figure 12 additionally pictures the changes
in the optical properties when cross-linking the spray-cast films.
The cross-linking process is associated with a small blue-shift
(7-11 nm) for TPTTPT-diacrylate and T6-diacrylate, which
indicates that these two systems rearrange to slightly less
coplanar conformations upon the photopolymerization. TPEEPT-
diacrylate, on the other hand, adapts a more coplanar structure
overall during the cross-linking process as judged from the
increased intensity of the low-energy peaks and decreased
intensity of the high-energy peaks. For all three systems, loss
of order during the UV irradiation is indicated by the loss of
vibronic coupling in the absorption spectra. It is conceivable
that the cross-linking process is associated with molecular
rearrangements that would lead to an overall loss of order in
the film.32 The optical band gaps, as determined from the low-
energy band-edges, fall in the range 2.3-2.4 eV.
To further probe the electrochromic properties of the cross-
linked TPEEPT-diacrylate material, the switching character-
istics for several films were investigated by monitoring the
transmittance at the wavelength of maximum contrast during
repeated redox stepping experiments (Table 5). Figure 16 shows
the chronoabsorptometry at a wavelength of 600 nm for the
electrochromic switching between the neutral state (yellow, λmax
452 nm) and the radical cation (blue, λmax 603 nm) with 30-s
delays between each redox switch. For a relatively thick film
(225 nm, Figure 16A), the percent transmittance is 89.2% in
the bleached state (yellow) and 26.7% in the colored state (blue),
which gives a contrast value of 62.5% (∆%T). For a thinner
film (85 nm, Figure 16B), the percent transmittances are 99.3%
(bleached) and 61.5% (colored), resulting in a contrast ratio of
37.8%; after 400 switches (7 h) of the thinner film, the contrast
ratio decayed to approximately 30%. A good correlation was
observed between film thickness and contrast ratio as obvious
from the data in Table 5 (see Supporting Information for the
complete set of switching data). Response times for achieving
the blue state (600 nm coloration, tc) and the yellow state (600
nm bleaching, tb) were determined as the time it took to reach
95% of the ultimate change in %T.59 As expected, the switching
times were highly dependent on film thickness: for the thinner
films, response times of 2.3-2.4 s for coloration and ap-
Spectroelectrochemistry. To further demonstrate the high
stability of the TPEEPT system in its oxidized states, spectro-
electrochemistry was carried out for a spray-cast and subse-
quently cross-linked film of TPEEPT-diacrylate as depicted
in Figure 13. At -0.24 V (vs Fc/Fc+), the film is neutral and
therefore displays only the π-π* transition at 452 nm. As the
potential is increased stepwise to 0.34 V, the absorption band
from the neutral film disappears gradually, while two broad
bands ascribed to the radical cation (monomeric and/or dimeric)
emerge at 603 and 1180 nm; the latter blue-shifts gradually to
1140 nm as it increases in intensity (Figure 13A). Further
increment of the potential to 0.86 V is accompanied by the
gradual appearance of a transition from the dication (1000 nm)
at the expense of the absorption bands from the radical cation
as illustrated in Figure 13B. Visualization of the optical changes
is aided by observation of the difference spectra obtained by
subtraction of the first spectrum in a series from the following
spectra as illustrated in Figure 13C,D. Especially for the
conversion from radical cation to dication, the regular absorption
spectra are complicated by the overlap of the radical cation and
dication absorption bands, but this is overcome by examination
of the difference spectra instead. This deconvolution reveals
the gradual disappearance of the two bands from the radical
cation and the corresponding appearance of the dication
absorption band. It is furthermore revealed by this approach
that a low-intensity absorption band around 685 nm is associated
with the dication absorption band at 1000 nm. The inserted
(58) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant,
J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C. S.; Ree, M.
Nat. Mater. 2006, 5, 197–203.
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Discrete Photopatternable π-Conjugated Oligomers
A R T I C L E S
Figure 11. Cyclic voltammograms (A) and differential pulse voltammograms (B) of cross-linked films of TPEEPT-diacrylate on ITO-coated glass in 0.1
M tetrabutylammonium perchlorate in dichloromethane (CV scan rate 25 mV/s).
switching time observed for the bleaching process, which is
approximately four times slower than the coloration process.
The cross-linking process is likely to cause a tighter organization
with more closely packed chains. Therefore, the coloration
process, which takes the oligomer from its neutral to its oxidized
form, should take place easily since it is associated with a
planarization of the oligomer backbone as the quinoidal radical
cation forms. On the other hand, the bleaching process is
associated with the conversion from the quinoidal oxidized
species to the slightly less planar neutral form and could
therefore be hindered by the tight locked packing in the cross-
linked network. Further indication of the tight packing is also
seen with the significantly increased switching times with
increased film thickness.
Conclusions
We presented two novel chromophores (TPTTPT and
TPEEPT), which were synthesized both as methyl-terminated
model compounds and telechelic analogues together with two
novel telechelic sexithiophene derivatives (T6-diol and T6-
diacrylate). Structural characterization based on single crystal
structures of PTTP and PEEP indicates that a central biEDOT
unit (due to attractive intramolecular sulfur-oxygen interactions)
causes a higher degree of molecular planarity than a bithiophene
unit. Comparison with the crystal structure of 3,3′′′-didocylqua-
terthiophene62 suggests a close resemblance between the PEEP
motif and the dialkylated quaterthiophene, while PTTP is
somewhat less planar. The overall structural rigidifying effect
of the EDOTs is confirmed by the vibronic resolution observed
in the optical absorption spectra for TPEEPT, but not for
TPTTPT or T6.
Figure 12. Normalized absorption spectra of films of TPTTPT-diacrylate
(top), TPEEPT-diacrylate (middle), and T6-diacrylate (bottom) spray-
cast from toluene/dichloromethane (9:1) before (dashed lines) and after (solid
lines) cross-linking.
Table 4. Summarized Optical Properties for the Oligomers
λmax (nm)a
λmax (nm)b
λmax (nm)c
Eonset (eV)c
TPTTPT-diacrylate
TPEEPT-diacrylate
T6-diacrylate
432
445
425
454, 480
420, 447, 485 452, 482
444 437
443
2.4
2.4
2.3
a In THF solution. b Films spray-cast from toluene/dichloromethane.
c Cross-linked films.
proximately 10 s for bleaching were observed, while the thicker
films showed coloration times of 3.6 s (115 nm film) and 4.6 s
(225 nm film) and bleaching times of 14.5 s (115 nm) and 21.1 s
(225 nm). Apart from the slow bleaching process, these
switching and contrast characteristics are comparable to other
EDOT-containing electrochromic materials described in the
literature,60 but it should be noted that other electrochromic
π-conjugated polymers with much faster switching times
(100-300 ms) and enhanced lifetimes have been reported.25,61
The instability observed during prolonged switching is
primarily caused by an incomplete return to the neutral state
(Figure 16B) and seems to be connected with the relatively long
In dichloromethane, we found the oxidized species of all three
oligomeric systems to be quite stable, which allowed for a
thorough characterization of the redox properties and the
different oxidized species involved. The all-thiophene T6 system
has the highest first oxidation potential; introduction of the more
(60) Kumar, A.; Welsh, D. M.; Morvant, M. C.; Piroux, F.; Abboud, K. A.;
Reynolds, J. R. Chem. Mater. 1998, 10, 896–902.
(61) Lu, W.; Fadeev, A. G.; Qi, B. H.; Smela, E.; Mattes, B. R.; Ding, J.;
Spinks, G. M.; Mazurkiewicz, J.; Zhou, D. Z.; Wallace, G. G.;
MacFarlane, D. R.; Forsyth, S. A.; Forsyth, M. Science 2002, 297,
983–987.
(59) Gaupp, C. L.; Welsh, D. M.; Rauh, R. D.; Reynolds, J. R. Chem.
Mater. 2002, 14, 3964–3970.
(62) Azumi, R.; Gotz, G.; Debaerdemaeker, T.; Bauerle, P. Chem.-Eur.
J. 2000, 6, 735–744.
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A R T I C L E S
Nielsen et al.
Figure 13. Spectroelectrochemistry for a cross-linked film of TPTTPT-diacrylate on ITO-coated glass in 0.1 M tetrabutylammonium perchlorate in
dichloromethane. (A) Gradual conversion from neutral to radical cation species (potential stepped from -0.24 to 0.34 V vs Fc/Fc+), (B) conversion from
radical cation to dication (potential stepped from 0.34 to 0.86 V vs Fc/Fc+), (C) difference absorption spectra showing the optical changes during the
conversion from neutral to radical cation, and (D) difference absorption spectra for the conversion from radical cation to dication.
Figure 14. UV-vis (A) and fluorescence (B) spectra of a thin film of TPEEPT-diacrylate. Film was spray-cast and then annealed (60 °C in vacuum
overnight) before being cross-linked (UV-curing for 10 min) and then rinsed with tetrahydrofuran.
electron-rich dialkoxybenzene unit lowers the oxidation potential
(TPTTPT), and a further decrease is seen when introducing
the EDOT unit. Although TPEEPT thus has the most stable
radical cation, the next one-electron oxidation occurs more easily
for T6 (∆E ) 120-130 mV) and for TPTTPT (∆E ) 200-250
mV) than for TPEEPT (∆E ) 350-410 mV) because of charge
repulsion in the dication of TPEEPT due to a lower degree of
charge delocalization along the backbone. These electrochemical
properties are reflected very well by the optical properties of
the oxidized species. The red-shift associated with the first
oxidation (from neutral to radical cation) is small for TPEEPT
(1.03 eV), somewhat larger for TPTTPT (1.19 eV), and even
larger for T6 (1.38 eV). For the second oxidation (from radical
cation to dication), on the other hand, the trend is opposite with
the red-shift being only 0.30 eV for T6, 0.65 eV for TPTTPT,
and 0.70 eV for TPEEPT. Each of the radical cations is in
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9744 J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008

Discrete Photopatternable π-Conjugated Oligomers
A R T I C L E S
we can take advantage of the very convenient solution proces-
sibility as well as subsequent solid-state processing, which
“locks” the film into an insoluble solid state. In general, we
found the oligomers to behave very similarly in the solid state
to what was observed in solution. The cross-linked film of
TPEEPT-diacrylate has the lowest first oxidation potential, but
opposite to what was observed in solution, T6 (E1/2 ) 0.47 V)
is now more easily oxidized to the radical cation than TPTTPT
(E1/2 ) 0.62 V), which is probably caused by a more coplanar
organization for T6 in the solid state than for TPTTPT as also
indicated in the single crystal structures. Again, we observe the
next one-electron oxidation to the dication to occur more easily
for T6 (∆E ) 210 mV) and for TPTTPT (∆E ) 310-370
mV) than for TPEEPT (∆E ) 460-480 mV).
In particular, we discovered that a cross-linked film of
TPEEPT-diacrylate is quite stable in its different oxidized
states and thus undergoes two consecutive and very reversible
redox processes even under ambient conditions. Further char-
acterization by spectroelectrochemistry reveals two distinct color
changes when going from the bright yellow neutral material to
the clear blue radical cation and subsequently to the dication,
which is transparent with a small residual gray color. The
promising electrochromic properties of this material are further
verified by our initial switching studies showing good contrast
ratios and reasonable coloration and bleaching times during at
least 400 cycles. To utilize the cross-linkable acrylate function-
ality to its fullest, we have (as a proof of concept) shown how
a TPEEPT-diacrylate film successfully can be subjected to a
simple photopatterning process.
Figure 15. Photopatterning process for a spray-cast film of TPEEPT-
diacrylate (A). After (i) applying a mask, (ii) irradiating with UV light,
and (iii) washing with tetrahydrofuran, we found that the cross-linked and
patterned film appears as illustrated in (B). Subsequent electrochemical
oxidation affords the blue radical cation (C) and the pale blue/transparent
dication (D).
Table 5. Summarized Switching Properties for Four Cross-Linked
TPEEPT-Diacrylate Films of Various Thicknesses
film thickness (nm)
∆%T
tc (s)
tb (s)
80
85
115
225
34.4
37.8
45.4
62.5
2.4
2.3
3.6
4.6
10.4
9.6
14.5
21.1
To summarize, we developed a series of telechelic oligomers
with various π-conjugated cores. From our studies, it is obvious
that 3,4-dioxythiophene moieties (here in the form of EDOT
units) offer an enhanced stability of the oxidized states, which
makes these oligomers superior for electrochromic devices as
compared to the thiophene- and phenylene-based systems.
Further synthetic efforts including modifications of the 3,4-
dioxythiophene (incorporation of ProDOT, dialkyl-ProDOT,
etc.), the relative positions of the 3,4-dioxythiophene units in
the oligomer, and variations in the length of the oligomer and
the attractive intramolecular forces determining the molecular
planarity could help identify new electrochromic materials with
altered optical properties.
a All experiments were performed with cross-linked films on
ITO-coated glass with 0.1 tetrabutylammonium perchlorate as
electrolyte. Switching was monitored as the percent transmittance (%T)
at 600 nm with a 30-s delay for each potential (-0.65 and 0.25 V), and
switching times were determined as the time it took to reach 95% of the
ultimate change in %T.
M
equilibrium with its corresponding π-dimer in solution; again,
we see evidence for the fact that the EDOTs tend to favor a
more localized charge distribution since TPEEPT is less prone
to dimerization than TPTTPT and T6.
The herein reported oligomers are all highly soluble in many
common organic solvents and can thus easily be spin- or spray-
cast onto various substrates to form thin homogeneous films.
Furthermore, by the attachment of acrylate functionalities to
the oligomers, we were able to convert the solution-processed
films into insoluble cross-linked networks by UV curing. Thus,
In addition to the presented electrochromic properties of
TPTTPT, TPEEPT, and T6, these oligomeric systems are also
promising candidates for new transistor materials. Therefore,
we are currently investigating charge-carrier mobilities of these
Figure 16. Electrochromic switching of cross-linked TPEEPT-diacrylate films (0.1 M tetrabutylammonium perchlorate in dichloromethane) monitored as
the percent transmittance (%T) at 600 nm with a 30-s delay for each potential (-0.65 and 0.25 V). Ten switching cycles for a 225-nm film (A) and first 10
and last 50 switching cycles for an 85-nm film (B).
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J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008 9745

A R T I C L E S
Nielsen et al.
oligomers in our laboratories, while also pursuing a deeper
understanding of the morphologies of these materials in their
solid state.
voltammograms showing the scan rate dependence for TPT-
TPT-Me2, TPEEPT-Me2, and T6-diolin solution as well as
for TPEEPT-diacrylate in the solid state. UV-vis spectra
displaying the solution oxidation of T6-diol. Chronoabsorp-
tometry showing the electrochromic switching of cross-linked
TPEEPT-diacrylate films. Listings of crystal data and structure
refinement parameters, atomic coordinates, bond lengths and
angles, anisotropic displacement parameters, hydrogen coordi-
nates, and isotropic displacement parameters for PTTP and
PEEP (PDF and CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. We appreciate funding of this work by the
AFOSR (FA9550-06-1-0192) and via a grant to EIC Laboratories
(FA9550-05-C-0147). K.A.A. acknowledges the National Science
Foundation and the University of Florida for funding the purchase
of the X-ray equipment. We thank Geeta Kheter Paul for the supply
of compound 10.
Supporting Information Available: Experimental procedures
including full characterization for all new compounds. Cyclic
JA7112273
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9746 J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008