Synthesis and characterization of amino-capped oligothiophene-based hole-transport materials

Article abstract of DOI:10.1039/b510826h

A series of seven thiophene-based oligomers were prepared as hole transport materials. The DBATn series is composed of six oligothiophenes (n = 1-6) capped with two dibutylaminostyryl electron donating moieties. The band gaps for these compounds range fro

Full text of DOI:10.1039/b510826h

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and characterization of amino-capped oligothiophene-based
hole-transport materials{
Olivier Clot,a Don Selmarten*a and Michael J. McNevinb
Received 29th July 2005, Accepted 5th September 2005
First published as an Advance Article on the web 11th October 2005
DOI: 10.1039/b510826h
A series of seven thiophene-based oligomers were prepared as hole transport materials. The
DBATn series is composed of six oligothiophenes (n = 1–6) capped with two dibutylaminostyryl
electron donating moieties. The band gaps for these compounds range from 2.47 to 2.19 eV. It was
found that the bis(amino) substituents control the ionization potential, while the electron affinity
and energy gap depend directly on the oligothiophene chain length. The bulk crystalline
properties of these materials depend strongly on whether the number of thiophene rings in the
bridge is even or odd. This results in the compounds that have an odd number of thiophene units
in the bridge to exhibit depressed melting points with respect to the structures with an even
number of rings. X-Ray structure determination was obtained from single crystals of DBAT1 and
-2. Good solubility, thermal stability and film forming properties were found throughout the
series of compounds. A seventh compound, DH2E4T, was also prepared. This molecule is
comparable to a,v-dihexylsexithiophene but with greater solubility and reduced optical band
gap (2.32 eV).
materials in field-effect transistors, light emitting diodes, solar
cells or electrographic materials.4,5 Monodisperse oligomers
Introduction
In both the academic and industrial research community, an
allow for a precise control of the structure and relevant
intense effort into the investigation of organic solar cells has
electronic properties such as ionization potential, electron
recently emerged. Within the field of organic solar energy
affinity and energy gap. In this work, we have designed and
conversion there are several approaches that have been taken
prepared a series of thiophene-based p-conjugated compounds
to achieve usable electricity from solar photons. Amongst
to be used as the solid-state hole transport layer (HTL) in
sensitized solar cells (SSC).2,6 The performance of solid-state
these, we are investigating a variant of the dye sensitized
‘‘Gra¨tzel’’ type cell.1,2 The problem with these existing cells is
SSCs partially depends on a rapid charge separation at the
that the hole conduction phase is a volatile organic electrolyte.
interface between the inorganic nanoporous electrode and
Such a cell would not be practical when exposed to real-world
outdoor conditions. In order for a solar cell that could
function as a marketable product to be realized, a non-volatile
hole transport medium is needed. For this reason we turned
our attention to solid-state hole-conductive, pi-conjugated
systems.
the organic HTL. Therefore, a good energetic alignment
between inorganic and organic semiconductors is of para-
mount importance. Another property that is important for cell
performance is how well the polymer physically covers the
metal oxide and fills the pores. Finally, even assuming perfect
surface coverage and pore filling, said material must exhibit
swift hole transport. Unlike the bulk heterojunction cell, which
is concerned with exciton mobilities, we are interested in hole
mobilities. Although very large hole mobilities have been
reported for oligomers such as a,v-dihexylsexithiophene
(DH6T), the low solubility of that material has restricted
device processing to vacuum deposition.7,8 Not only is this
process costly, such a technique is unlikely to completely fill
the convoluted mesoporous structure of a SSC electrode.
Therefore, along with efficient hole transport capability, we
have pursued two main goals in the design of the oligomers:
solubility and thermal stability. Improving on the solubility
gives us the ability to solution process the HTL. Thermal
stability allows for thermal annealing. This allows us to melt
and or anneal the organic HTL and study the impact this has
on the films properties. We have then chosen to design the
DBATn and DH2E4T oligomers (Chart 1) to mimic existing
highly efficient hole transport compounds such as DH6T or
TPD, but with improved solubility.
Pi-conjugated polymers are known to behave as hole
conductors. For them to function well in a solar cell
application, it would be a great benefit to understand their
fundamental properties (electronic structure, physical order-
ing, etc.).3 Attempting to interrogate the basic science of
polymers can be challenging for several reasons. Therefore,
we chose to focus our efforts on a series of tailored molecular
moieties that are analogs to the conductive polymer poly-
(thiophene). Conjugated oligomers with well-defined chemical
structures have received considerable attention as active
aNational Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO,
80401, USA. E-mail: don_selmarten@nrel.gov
bDepartment of Chemistry and Biochemistry, University of Colorado,
Boulder, CO, USA 80309-0215
{ Electronic supplementary information (ESI) available: table of
crystallographic data and collection parameters and complete tables
of bond lengths and angles for DBAT1 and -2. See DOI: 10.1039/
b510826h
4934 | J. Mater. Chem., 2005, 15, 4934–4942
This journal is ß The Royal Society of Chemistry 2005

View Article Online
Chart 1 Compounds’ structures
confirmed by 1H-NMR spectroscopy that, throughout the
DBATn series, all the compounds are pure E-isomers with a
trans coupling constant of 16.0 Hz for the olefinic protons and
that the compounds possess a center of symmetry as both
olefin bonds are identical. All the compounds of the DBATn
Results and discussion
Synthesis
DBAT1, -2 and -3 were prepared in 80, 74 and 76% yield
respectively via a two-fold Horner–Emmons–Wittig olefina-
tion of the appropriate oligothiophene dialdehyde with dibutyl-
aminobenzylphosphonate (Scheme 1a). Because of the poor
solubility of oligothiophene dialdehydes beyond quaterthio-
phene, a Stille coupling in DMF was chosen as the route to
prepare DBAT4, -5 and -6. DBAT4 was prepared by coupling
2 with dibromobithiophene with a 52% yield, while DBAT5
and -6 were prepared in 52 and 35% yield by coupling 3
with bis(tributylstannyl)thiophene or -bithiophene, respec-
tively (Scheme 1a). The Horner–Emmons–Wittig olefination
chosen to prepare DBAT1, -2 and -3 as well as the building
blocks 2 and 3 favors the formation of E-isomers. It was
series, except for DBAT6, have good solubilities (10 to 20 g L21
)
in common polar solvents such as methylene chloride, chloro-
form or THF. The solubility of DBAT6 in the same solvents
is lower (2–3 g L21) but remains significantly larger than
that of DH6T (1 g L21 in CHCl3).7 Such solubilities allow
processing by spin-coating to form micron-thick films which is
not possible with compounds such as DH6T. Compound
DH2E4T was synthesized via Stille coupling between
bis(stannyl)bis(EDOT) and bromohexylbithiophene and was
obtained as an orange–red powder (Scheme 1b). Its solubility
in chloroform was close to 2.5 g L21, which is 2.5 times that of
the DH6T parent compound.7
Scheme 1 Synthesis of the compounds.
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 4934–4942 | 4935

View Article Online
Table 1 Thermal properties, UV-vis and fluorescence maxima
Compound
Tma/uC
Tca/uC
Td/uC
lmaxc/nm (e/L mol21 cm21
)
Eogptd/eV
lemce/nm
DBAT1
DBAT2
DBAT3
DBAT4
DBAT5
DBAT6
80–81
N/a
321
365
375
383
383
390
446 (4.64 6 104)
467 (4.95 6 104)
473 (4.74 6 104)
479 (5.99 6 104)
480 (5.78 6 104)
489 (4.66 6 104)
2.47
2.33
2.26
2.23
2.21
2.19
514
543
561
570
578
580
153–154
123–124
226–228
206–207
103–105f
274–276g
202–203
135–136
102–103
215–216
N/a
96–97h
DH2E4T
a
170–171
374
470 (6.17 6 104)
2.32
536, 571
Measured by DSC, Tm (melting point at 10 uC min21) and Tc (crystallization point at 2 uC min21). Td (decomposition onset) measured by
b
TGA (5 uC min21) in N2. In chloroform. Calculated by extrapolation of the low-energy edge of the UV-vis absorption. Excitation at lmax
Crystalline to amorphous glass transition. Not present during the 2nd heating cycle. Glass state crystallization.
.
c
d
e
f
g
h
Thermal behavior
13C-NMR studies on various odd- and even-numbered oligo-
thiophenes, which demonstrate the influence of the presence of
a center of symmetry in the crystal packing and conformation
of oligothiophenes.15–17 This supposition is part confirmed by
direct observation of the various compounds of the DBATn
series. While spin-coated films of DBAT2 and -4 are visually
grainy and highly crystalline, DBAT1, -3 and -5 yield, under
similar spin-coating conditions, very smooth and glassy films.
Moreover, the DSC traces of DBAT2, -4 and -6 showed a very
sharp and distinct endothermic transition at the melting point.
In contrast, the transition recorded for DBAT3 was broader
and that of DBAT5 almost indistinct. This indicates that in the
solid-state, the DBAT3 is less crystalline than -2 or -4 while
DBAT5 is almost amorphous. XRD studies on solid-state films
as well as single crystal X-ray spectroscopy are under way in
order to explain this interesting solid-state behavior.
The DSC trace for DBAT6 displayed two endothermic
peaks at 103 and 274 uC. The first peak was attributed to a
solid-state transition to a glass state while the second was
assigned to a solid–liquid transition. Upon cooling, only
one exothermic transition was observed at 96 uC, and was
attributed to the crystallization from an amorphous solid glass.
After the first melt–cool cycle, the second heating only showed
the lower endothermic transition at 104 uC. Even when heated
up to 350 uC, the second melting endotherm was not seen. The
second cooling cycle was identical to the first with only one
transition at 96 uC.
The melting points of the compounds in the DBATn series
shown in Table 1 range between 80 (DBAT1) and 274 uC
(DBAT6) while DH2E4T melts at 202 uC. An interesting trend
was seen with the melting temperatures within the DBATn
series. Instead of monotonously increasing with the oligothio-
phene length, as it is generally the case for conjugated
oligomers,9,10 the melting points (MP) can be placed on two
separate parallel lines (Fig. 1). The higher line contains the
three compounds with an even number of thiophene units.
While the lower line contains the remaining three compounds
with an odd number of thiophene units. The melting tem-
peratures did not depend on the solvent system used to
recrystallize the compounds, although several different combi-
nations were attempted for each structure. The same trend was
observed within the series when considering the crystallization
temperatures (Tc).
Although, with most unsubstituted or a,v-substituted
oligothiophenes, MPs are reported to monotonously increase
with the chain length,7,8,11,12 it also has been reported,
for instance, within amine-13 or a,v-diformyl-substituted14
oligothiophene series that the MP for the odd-numbered
members are depressed. In the case of the DBATn series, the
MP trend is very clearly marked and is likely due to the
symmetry of the structures that directs the crystal packing.
This is supported by X-ray crystal structures and solid-state
Compound DH2E4T displayed a larger recrystallization
point interval of 32 uC, which showed that the compound
remained in a super-cooled liquid phase between 202 and
170 uC.
For all the compounds, Td under nitrogen is significantly
higher than their respective Tm under nitrogen (Table 1).
Therefore, all compounds shown here satisfy the chemical
stability requirements for thermal annealing. They can be
heated into a liquid melt under nitrogen or vacuum in order to
efficiently fill the pores of the inorganic SSC electrode without
decomposition.
Crystal structures
Single crystals of DBAT1 and -2 were obtained from a chloro-
form–methanol mixture.{ The X-ray structure determinations
Fig. 1 Evolution of the melting temperature for the DBATn series as
a function of the odd (squares) and even (triangles) number of
thiophene rings in the bridge. The lines are shown on the graph only to
guide the eye.
{ CCDC reference numbers 279764–279764. For crystallographic data
in CIF format see DOI: 10.1039/b510826h
4936 | J. Mater. Chem., 2005, 15, 4934–4942
This journal is ß The Royal Society of Chemistry 2005

View Article Online
Fig. 2 (a) View of DBAT1 structure, (b) view of DBAT2 structure, (c) packing for DBAT1, the butyl chains were omitted for clarity and (d)
herringbone packing for DBAT2, the butyl chains were omitted for clarity.
(Fig. 2) confirm the E-conformation of the double bonds on
either side of the thienyl moieties. For DBAT1, one of the
terminal phenyl rings shows a large 20u twist while the other is
almost in plane with the thiophene vinyl moiety (twist angle of
4u) (Fig. 2a). Such a twist was apparently not observed in the
structure of the reported 2,5-bis(4-styryl)thiophene.18 DBAT1
shows a layered packing with a slip-stack pattern (Fig. 2c).
Within a layer, the intermolecular p–p distances range from
poly(3-hexylthiophene), which can fold over onto itself in
certain solvents and thus display strong solvochromic pro-
perties due to intramolecular interactions. The band-gaps,
obtained by extrapolation of the low-energy edge of the
UV-vis absorption, decrease from 2.47 (DBAT1) to 2.19 eV
(DBAT6). As expected, Eg decreases as the length of the
thiophene bridge is expanded attaining relatively low values
for the longest members of the series.
˚
3.99 to 4.64 A. For DBAT2, two separate structures coexist
For DH2E4T, the UV-vis spectrum shows a maximum at
470 nm with a well-defined shoulder at 496 nm in chloroform
and show an optical band gap of 2.32 eV. Again, no solvo-
chromism was observed with this molecule. The absorbance
maximum of DH2E4T is strongly red shifted with respect to
that of DH6T (470 nm vs. 432 nm).
within the crystal with twist angles of the two terminal phenyl
rings with respect to the bithienyl vinyl core of 10 and 16u
(Fig. 2b). The bithienyl moiety for DBAT2 is perfectly planar
with no measurable twist. The compound shows a herringbone
packing patten pattern with a 27u inter-plane angle (Fig. 2d).
Electronic and photoluminescence spectra
The UV-vis absorption spectra of the DBATn series have one
band in the visible (Fig. 3). The absorption maxima red-shifts
from 446 nm for DBAT1 to 489 nm for DBAT6 (Table 1).
Considering that the oligothiophene core length increases from
one to six rings, the red-shift of the absorption maximum is
relatively small. In a similar series, from thiophene to sexithio-
phene, the absorption maximum goes from 230 to 430 nm.
This suggests that, in the DBATn series, the amino groups play
a major role in controlling the electronic behavior. When
compared to DH6T, the absorption maximum for all the
members of the DBATn series is red-shifted (DH6T lmax
432 nm), and reduced optical band gaps are observed. No
solvochromism was observed when the polarity of the solvent
was varied from toluene to chloroform, acetone or THF as
can be expected for short, symmetrical oligomers having no
dipole moment. This is unlike polymeric species, such as
Fig. 3 UV-vis spectra for the DBATn series in chloroform.
J. Mater. Chem., 2005, 15, 4934–4942 | 4937
This journal is ß The Royal Society of Chemistry 2005

View Article Online
Fig. 4 Normalized photoluminescence spectra for DBATn series in
chloroform.
Fig. 5 Cyclic voltammograms for DBAT1 to -5 in CH2Cl2 with
0.1 M [n-Bu4N][PF6] as supporting electrolyte and at a scan rate of
All the compounds in the series show strong solution
photoluminescence when excited at their absorption maximum
(Fig. 4). The fluorescence maximum red-shifts with the
lengthening of the bridge as expected for such conjugated
structures. For several compounds within the series, no
variations in their fluorescence spectrum were observed
when the excitation wavelength was varied. This shows that
independent from the excitation wavelength, the emission
came from the same excited state.
100 mV s21
.
solvent (methylene chloride) combined to the simultaneous
oxidation of the two amino groups. However, no splitting
of this wave was seen on the voltammograms, even at
slow scan rates, which indicates that little or no electronic
communication is observed through the conjugated bridge
between the two amino moieties even with the shortest bridge
(i.e. DBAT1).
Electrochemistry
The potential of the third oxidation process however
decreases from 1.40 V for DBAT1 to 0.75 V for DBAT5.
This potential was assigned to the first oxidation of the
oligothiophene bridge. The shift to lower redox potentials with
the lengthening of the bridge is consistent with increasing
conjugation length from one to five thiophene rings and a
similar behavior has frequently been reported in the literature.4
It is to be noted that the DBAT compounds were sturdy and
able to withstand multiple oxidations without decomposition
as attested by the reversibility of the first two oxidation waves.
Unfortunately, as the solubility of DBAT6 in CH2Cl2 was too
low, a CV could not be measured in this solvent. In THF
however, DBAT6 shows a first oxidation wave at 0.53 V vs.
SCE. For the remaining compounds of the series, the first
The compounds’ electrochemical behavior was investigated by
cyclic voltammetry (CV) in solution and the redox potentials
are summarized in Table 2. All compounds in the DBATn
series show a first oxidation wave, which is twice as large as
the second oxidation wave (Fig. 5). This 2-electron oxidation
was assigned to the simultaneous oxidation of the terminal
amino moieties. The potential of this redox wave, close to
0.4 V vs. SCE, remain almost constant throughout the DBATn
series, irrespective to the extent of the conjugated system
(Fig. 5). A similar behavior was reported for a series of
bis(phenyl)amino-capped oligothiophenes.19,20 It was observed
that the 1st oxidation waves are very broad, which is likely
due to a combination of the relatively poor electrochemical
Table 2 Electrochemical properties of the DBATn series
Compound
E1/2 oxidation/V vs. SCEa
E1/2 reduction/V vs. SCEb
EEg chem/eVc
EOg pt/eV
DBAT1
DBAT2
DBAT3
DBAT4
DBAT5
DBAT6
DH2E4T
a
0.40, 1.50
22.27d
2.42
2.41
2.27
2.28
2.21
2.19
2.29
2.47
2.33
2.26
2.23
2.21
2.19
2.32
0.43, 1.12, 1.70d
0.45, 0.91, 1.37, 1.63d
0.43, 0.79, 1.12
0.40, 0.75, 0.96
0.53e
22.19, 22.30d
22.00,d 22.18d
21.83, 21.94
21.80d
21.81, 21.91
21.96
0.33, 0.88d
In CH2Cl2 with 0.1 M [NBu4][PF6] as supporting electrolyte, scan rate 100 mV s21
.
Measured at the onset of the first oxidation and reduction waves in THF (Fig. 5). Irreversible wave. CV
In THF with 0.1 M [NBu4][PF6] as supporting
d e
b
electrolyte, scan rate 100 mV s21
taken in THF due to the poor solubility of DBAT6 in CH2Cl2.
.
c
4938 | J. Mater. Chem., 2005, 15, 4934–4942
This journal is ß The Royal Society of Chemistry 2005

View Article Online
oxidation in THF was also constantly found close to 0.5 V
(Fig. 6). Therefore, it can be concluded that in all the
compounds of the series, the amino moieties oxidize first,
followed by the oligothiophene bridge. At higher potential
(Table 2), one to two additional oxidation waves were
observed. The latter waves were irreversible which indicates
the probable decomposition of the compounds.
was recorded at 21.96 V. The band gap value for DH2E4T
(2.29 V) is comparable to DBAT2 or -3 although its ionization
potential is slightly greater. Both the 1st oxidation process
and band gap value for DH2E4T are comparable with other
EDOT-thiophene-based oligomers of comparable lengths
recently published in the literature.21–24
As no reduction potentials could be observed with
methylene chloride as the solvent, the reduction potentials
were measured in THF. In all cases, it was possible to record
the first oxidation wave within the same scan as the reduction
Conclusion
In conclusion, we have prepared a series of novel conjugated
thiophene-based oligomers that exhibit good thermal stability
as well as interesting electronic and solid-state properties.
Throughout the DBATn series, we found that the energy gap
and electron affinity are dependent on the length of the
conjugated bridge while the ionization potential remains
almost constant, being dominated by the amino capping
groups. We also showed clearly the crystal packing dependence
on the odd or even number of thiophene units contained in the
bridge. This is apparent in the visual aspect of spin-coated
films, the unusual variations in melting points and preliminary
X-ray crystal structure determinations. Further solid-state
studies to elucidate this behavior including thin film XRD and
single crystal studies are being pursued. DH2E4T was found to
compare favorably with DH6T with respect to energy gap,
ionization potential and electron affinity, but with greater
solubility. Charge carrier mobility measurements and use of all
the compounds as the hole transport layer in solar energy
conversion are in progress.
wave, thus allowing for
a direct measurement of the
electrochemical band gap (EEg chem). Voltammograms in THF
for a few representative compounds are shown in Fig. 6. With
the exception of DBAT1, the compounds of the DBATn
series showed two closely spaced reductions (Table 2), whose
potentials increase with increasing oligothiophene chain
length. The reduction waves are not reversible up to DBAT3,
at which point both waves become reversible.
The energy gap, which was measured at the onset of the first
reduction and oxidation waves in THF, decreases throughout
the series with increased oligothiophene bridge length and the
values obtained from the electronic spectra and from the cyclic
voltammograms are in good agreement following the same
decreasing trend. It is interesting that the trend seen for Eg is
mainly due to an increase in electron affinity from DBAT1 to
DBAT6 while the first ionization potential (the oxidation of
the amine capping group) shows little dependence on the
length of the conjugated system.
Experimental section
DH2E4T shows two oxidation waves at 0.33 and 0.88 V with
the second wave being irreversible. In THF, a reduction wave
2,5-Thiophene dicarboxaldehyde,25 5,59-(2,29-bithiophene)
dicarboxaldehyde,25 5,50-(2,29:59,20-terthiophene) dicarbox-
aldehyde,14 5,59-dibromo-2,29-bithiophene,11 5-bromo-59-
formyl-2,29-bithiophene,14 2,5-bis(tributylstannyl)thiophene,14
5,59-bis(tributylstannyl)-2,29-bithiophene,14 and 5,59-bis(tri-
butylstannyl)-2,29-bis(3,4-ethylenedioxythiophene)26
were
prepared following literature procedures. Diethyl-[4-(N,N-
dibutyl)aminobenzyl] phosphonate was prepared by refluxing
commercially available diethyl-(4-aminobenzyl) phosphonate
(Aldrich) with K2CO3 in ten equivalents of bromobutane as
the solvent. THF and diethyl ether were dried over sodium–
benzophenone and distilled fresh before use. All other reagents
were purchased from Sigma-Aldrich or Fisher Chemicals and
used as received without further purification. 1H NMR spectra
were taken on a Varian Unity INOVA AS400 spectrometer at
400 MHz and referenced to residual solvent. Elemental
analyses were performed at Huffman Laboratories (Golden,
CO). DSC analyses were done on a TA Instruments DSC 2910
at 10 uC min21. TGA analyses were done on a TA Instruments
TGA2950 at 5 uC min21. UV-vis spectra were taken on a
Varian Cary 500 spectrophotometer. Emission spectra
were taken on a Fluorolog 1680 spectrophotometer. Cyclic
voltammetry was done on a Pine AFCBP1 bipotentiostat
with a platinum disk working electrode, a platinum coil
counter electrode and a silver wire quasi reference electrode.
Decamethylferrocene (Strem) was added to the electrolyte
solution as an internal reference and the potentials corrected
vs. SCE.27
Fig. 6 Cyclic voltammograms for DBAT1, DBAT3 and BDAT4 in
THF with 0.1 M [n-Bu4N][PF6] as supporting electrolyte and at a scan
rate of 100 mV s21
.
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 4934–4942 | 4939

View Article Online
General procedure for DBAT1, DBAT2 and DBAT3
1.35 (4 H, m, 2 6 NCH2CH2CH2CH3), 0.95 (6 H, t, J 7.2, 2 6
NCH2CH2CH2CH3).
To a stirred THF solution of diethyl-[4-(N,N-dibutyl)amino-
benzyl] phosphonate and the appropriate thiophene
dicarboxaldehyde (cooled at 5 uC under argon) was very
slowly added a solution of potassium tert-butoxide in THF.
After the addition, the mixture was stirred for 1 h at 25 uC. The
reaction mixture was poured in sat. NaHCO3, and the aqueous
layer extracted with diethyl ether. The combined organic
fractions were washed with water and dried over MgSO4.
Removal of the solvent yielded the crude product which was
purified by recrystallization from a hot methylene chloride–
methanol mixture.
2-[(E)-(4-N,N-Dibutylaminobenzylidene)methyl]thiophene (1).
This molecule is a slight variation from a known compound.28
The general procedure for DBAT1 was followed with
2-thiophene carboxaldehyde (1.49 g, 13.3 mmol), diethyl-[4-
(N,N-dibutyl)aminobenzyl] phosphonate (4.30 g, 12.1 mmol)
in 30 mL of THF and tBuOK (13.5 mL, 1 M in THF,
diluted with 15 mL of THF, 13.5 mmol). The crude product
was purified with a silica gel plug eluted with hexanes–ethyl
acetate (9 : 1 v/v). The first yellow band was collected and
the product obtained as a yellow oil (3.82 g, 91%). dH
(400 MHz; CDCl3; Me4Si) 7.30 (d, 2 H, d, J 8.8, NPh), 7.08
(1 H, m, Th-H), 6.98 (1 H, d, J 16.0, Ph–CH2LCH2–), 6.95
(2 H, m, 2 6 Th-H), 6.83 (1 H, d, J 16.0, Ph–CH2LCH2–),
6.59 (d, 2 H, d, J 8.8, NPh), 3.26 (4 H, t, J 7.6, 2 6
NCH2CH2CH2CH3), 1.56 (4 H, m, 2 6 NCH2CH2CH2CH3),
1.34 (4 H, m, 2 6 NCH2CH2CH2CH3) and 0.94 (6 H, t, J
7.2, 2 6 NCH2CH2CH2CH3).
2,5-Bis[(E,E)-(4-N,N-dibutylaminobenzylidene)methyl]thio-
phene (DBAT1). The general procedure was followed with
2,5-thiophene dicarboxaldehyde (0.70 g, 5.00 mmol), diethyl-
[4-(N,N-dibutyl)aminobenzyl]phosphonate (3.91 g, 11.0 mmol)
in 30 mL of THF and t-BuOK (11.5 mL, 1 M in THF diluted
with 30 mL of THF, 11.5 mmol). The product (2.17 g, 80%)
was obtained as an orange powder (Found: C 79.84; H 9.11; N
5.15. C36H50N2S requires C, 79.70; H, 9.23; N, 5.17%); dH
(400 MHz; CDCl3; Me4Si) 7.30 (2 H, d, J 8.2, NPh), 6.94 (1 H,
d, J 16.0, Ph–CH2LCH2–), 6.72–6.82 (2 H, m Ph–CH2LCH2–
and Th-H), 6.60 (2, H, d, J 8.2, NPh), 3.28 (4 H, t, J 7.6, 2 6
NCH2CH2CH2CH3), 1.57 (4 H, m, 2 6 NCH2CH2CH2CH3),
1.35 (4 H, m, 2 6 NCH2CH2CH2CH3), 0.95 (6 H, t, J 7.2, 2 6
NCH2CH2CH2CH3).
(2-Tributylstannyl-5-[(E)-(4-N,N-dibutylaminobenzylidene)-
methyl]thiophene (2). To a solution of 2-[4-(N,N-dibutyl-
amino)styryl]-thiophene (3.10 g, 9.90 mmol) in THF (90 mL)
at 278 uC was added dropwise n-BuLi (4.50 mL, 2.5 M solution
in hexanes, 11.0 mmol). After the addition, the mixture was
stirred for 1 h at 278 uC and then the temperature was raised
to 25 uC. Tributylstannyl chloride (4.22 g, 13.0 mmol) was
added slowly and the reaction stirred for 1 h. The mixture was
diluted with hexanes (250 mL) and washed with sat. NaHCO3,
water and dried over MgSO4. Removal of the solvent yielded a
brown oil which was used without further purification. dH
(400 MHz; CDCl3; Me4Si) 7.30 (d, 2 H, d, J 8.8, NPh), 7.06
(1 H, d, J 3.2, Th-H4), 7.03 (1 H, d, J 16.0, Ph–CH2LCH2–),
5,59-Bis[(E,E)-(4-N,N-dibutylaminobenzylidene)methyl]-
2,29-bithiophene (DBAT2). The general procedure was followed
with 5,59-bithiophene dicarboxaldehyde (1.25 g, 5.62 mmol),
diethyl-[4-(N,N-dibutyl)aminobenzyl] phosphonate (4.22 g,
t
11.9 mmol) in 100 mL of THF and BuOK (12.4 mL, 1 M in
THF, diluted with 30 mL of THF, 12.4 mmol). The product
(2.59 g, 74%) was obtained as a red powder (Found: C 76.53;
H 8.22; N 4.48. C40H52N2S2 requires C, 76.92; H, 8.33; N,
4.49%); dH (400 MHz; CDCl3; Me4Si) 7.30 (2 H, d, J 8.2,
NPh), 7.00 (1 H, d, J 3.8, Th-H4), 6.93 (1 H, d, J 16.0,
Ph–CH2LCH2–), 6.83 (1 H, d, J 3.8, Th-H3), 6.79 (1 H,
m Ph–CH2LCH2–), 6.59 (2 H, d, J 8.2, NPh), 3.27 (4 H, t,
J 7.6, 2 6 NCH2CH2CH2CH3), 1.56 (4 H, m, 2 6
NCH2CH2CH2CH3), 1.35 (4 H, m, 2 6 NCH2CH2CH2CH3)
and 0.95 (6 H, t, J 7.2, 2 6 NCH2CH2CH2CH3).
7.02 (1 H, d,
J 3.2, Th-H3), 6.84 (1 H, d, J 16.0,
Ph–CH2LCH2–), 6.58 (d, 2 H, d, J 8.8, NPh), 3.26 (4 H, m,
2 6 NCH2(CH2)2CH3), 1.54 (10 H, 2 6 NCH2CH2CH2CH3
and
3 6 SnCH2(CH2)2CH3), 1.33 (16 H, m, 2 6
N(CH2)2CH2CH3 and 3 6 SnCH2(CH2)2CH3), 0.94 (15 H,
m, 2 6 N(CH2)3CH3 and 3 6 Sn(CH2)3CH3).
59-Bromo-5-[(E)-(4-N,N-dibutylaminobenzylidene)methyl]-
2,29-bithiophene (3). This molecule is a slight variation from
a known compound.28 The general procedure for DBAT1
was followed with 5-bromo-59-formyl-2,29-bithiophene
(6.91 g, 25.3 mmol), diethyl-[4-(N,N-dibutyl)aminobenzyl]
phosphonate (9.89 g, 27.9 mmol) in 60 mL of THF and
tBuOK (28.0 mL, 1 M in THF, diluted with 30 mL of THF,
28.0 mmol). The crude product was purified with a silica gel
plug eluted with hexanes–ethyl acetate (9 : 1 v/v. The first
yellow band was collected and the product obtained as an
orange powder (9.82 g, 82%). dH (400 MHz; CDCl3; Me4Si)
7.30 d, 2 H, d, J 8.6, NPh), 6.95 (1 H, d, J 4.0, Th-H49), 6.93
(1 H, d, J 4.0, Th-H39), 6.91 (1 H, d, J 16.0, Ph–CH2LCH2–),
6.87 (1 H, d, J 4.0, Th-H4), 6.82 (1 H, d, J 4.0, Th-H3), 6.80
(1 H, d, J 16.0, Ph–CH2LCH2–), 6.59 (d, 2 H, d, J 8.6, NPh),
3.27 (4 H, t, J 7.6, 2 6 NCH2CH2CH2CH3), 1.56 (4 H, m, 2 6
NCH2CH2CH2CH3), 1.34 (4 H, m, 2 6 NCH2CH2CH2CH3)
nad 0.95 (6 H, t, J 7.2, 2 6 NCH2CH2CH2CH3).
5,50-Bis[(E,E)-(4-N,N-dibutylaminobenzylidene)methyl]-
2,29:59,20-terthiophene (DBAT3). The general procedure was
followed with 5,50-terthiophene dicarboxaldehyde (0.72 g,
2.37 mmol), diethyl-[4-(N,N-dibutyl)aminobenzyl] phosphonate
(1.85 g, 5.21 mmol) in 90 mL of THF and tBuOK (5.50 mL, 1 M
in THF, diluted with 10 mL of THF, 5.50 mmol). The
product (1.27 g, 76%) was obtained as a red powder
(Found: C 74.49; H 7.73; N 4.00. C44H54N2S3 requires
C, 74.79; H, 7.65; N, 3.97%); dH (400 MHz; CDCl3;
Me4Si) 7.30 (d, 2 H, d, J 8.2, NPh), 7.00–7.05 (2 H, m,
Th-H4 and Th-H39), 6.93 (1 H, d, J 16.0, Ph–CH2LCH2–),
6.83 (1 H, d, J 3.8, Th-H3), 6.78 (1 H, d, J 16.0, Ph–
CH2LCH2–), 6.59 (2 H, d, J 8.2, NPh), 3.27 (4 H, t, J 7.6, 2 6
NCH2CH2CH2CH3), 1.56 (4 H, m, 2 6 NCH2CH2CH2CH3),
4940 | J. Mater. Chem., 2005, 15, 4934–4942
This journal is ß The Royal Society of Chemistry 2005

View Article Online
General procedure for DBAT4, DBAT5, DBAT6 and DH2E4T
5,59-Bis[(2-hexyl-2,29-bithien-59-yl)]-2,29-bis(3,4-ethylene-
dioxythiophene) (DH2E4T). The general procedure was
followed with 5-bromo-59-hexyl-2,29-bithiophene (2.71 g,
8.25 mmol), 5,59-bis(tributylstannyl)-2,29-bis(3,4-ethylenedioxy-
thiophene) (2.80 g, 3.30 mmol) and Pd(PPh3)4 (50.0 mg,
0.04 mmol) in DMF (60 mL). The crude product was
recystallized from acetone to give DH2E4T (1.08 g, 42%)
as an orange crystalline powder (Found: C 61.35; H 5.48.
C40H42O4S6 requires C, 61.70; H, 5.40%); dH (400 MHz;
THF-d8; Me4Si) 7.10 (1 H, d, J 4.0, Th-H49), 7.03 (1 H, d, J
4.0, Th-H39), 6.99 (1 H, d, J 4.0, Th-H4), 6.70 (1 H, dt, J
4.0 and 0.8, Th-H3), 4.41 (4 H, s, –OCH2CH2O–),
2.80 (2 H, t, J 7.4, Th–CH2(CH2)4CH3), 1.69 (2 H, t, J
7.4, Th–CH2CH2(CH2)3CH3), 1.43–1.34 (6 H, m, Th–
(CH2)2(CH2)3CH3) and 0.91 (3 H, m, Th–(CH2)5CH3).
The appropriate stannylthiophene and bromothienyl starting
materials were dissolved in DMF, argon was bubbled through
the solution for 30 min and Pd(PPh3)4 was then added. The
reaction mixture was heated to 100 uC for 18 h under argon.
After cooling, the mixture was poured into 300 mL of
methanol under vigorous stirring. The resulting red suspension
was stirred for 30 min and the suspended solid filtered.
The crude material was recrystallized from neat acetone or
methylene chloride–methanol.
5,5--Bis[(E,E)-(4-N,N-dibutylaminobenzylidene)methyl]-
2,29:59,20:50,2--quaterthiophene
(DBAT4).
The
general
procedure was followed with
2
(6.00 g, 9.97 mmol),
5,59-dibromo-2,29-bithiophene (1.29 g, 3.99 mmol) and
Pd(PPh3)4 (0.15 g, 0.13 mmol) in DMF (60 mL). DBAT4
(2.16 g, 69%) was obtained as a bright red powder from
acetone (Found: C 73.31; H 7.10; N 3.43. C48H56N2S4 requires
C, 73.10; H, 7.11; N, 3.55%); dH (400 MHz; CDCl3; Me4Si)
7.31 (2 H, d, J 8.8, NPh), 7.01–7.07 (3 H, m, 3 6 Th-H), 6.93
(1 H, d, J 16.0, Ph–CH2LCH2–), 6.85 (1 H, d, J 3.8, Th-H3),
6.81 (1 H, d, J 16.0, Ph–CH2LCH2–), 6.57 (2 H, d, J 8.8, NPh),
3.28 (4 H, t, J 7.6, 2 6 NCH2CH2CH2CH3), 1.56 (4 H, m, 2 6
NCH2CH2CH2CH3), 1.35 (4 H, m, 2 6 NCH2CH2CH2CH3),
0.95 (6 H, t, J 7.2, 2 6 NCH2CH2CH2CH3).
X-Ray crystallography of DBAT1 and -2{. Crystals suitable
for X-ray diffraction studies were grown from chloroform–
methanol solutions at room temperature. A crystal of DBAT1
(orange) or -2 (red) of appropriate size was mounted on a glass
fiber using Paratone-N oil, transferred to a Siemens SMART
diffractometer/CCD area detector, centered in the beam (Mo
˚
Ka; l = 0.71073 A; graphite monochromator), and cooled to
2116 uC by a nitrogen low-temperature apparatus (which had
been previously calibrated by thermocouple) placed at the
same position as the crystal. Preliminary orientation matrix
and cell constants were determined by collection of 60 10-s
frames, followed by spot integration and least-squares refine-
ment. A minimum of a hemisphere of data was collected using
0.3u ? scans at 30 s per frame. The raw data were integrated
and the unit cell parameters refined using SAINT. Data
analysis was performed using XPREP. Absorption correction
was applied using SADABS. The data were corrected for
Lorentz and polarization effects, but no correction for crystal
decay was applied. Structure solutions and refinements were
performed (SHELXTL-Plus V5.0) on F-squared. Refer to the
electronic supplementary information for more data.
5,5-9-Bis[(E,E)-(4-N,N-dibutylaminobenzylidene)methyl]-
2,29:59,20:50,2-:5-,2-9-quinquethiophene (DBAT5). The general
procedure was followed with 3 (4.00 g, 8.44 mmol), 2,5-
bis(tributylstannyl)thiophene (2.54 g, 3.84 mmol) and
Pd(PPh3)4 (0.10 g, 0.09 mmol) in DMF (60 mL). The crude
material was recrystallized from methylene chloride–methanol
to yield DBAT5 (1.73 g, 52%) as a red powder (Found: C
71.80; H 6.51; N 2.99. C50H58N2S5 requires C, 71.72; H, 6.67;
N, 3.22%); dH (400 MHz; CDCl3; Me4Si) 7.31 (2 H, d, J 9.2,
NPh), 7.02–7.08 (4 H, m, 4 6 Th-H), 6.93 (1 H, d, J 16.0,
Ph–CH2LCH2), 6.85 (1 H, d, J 4.0, Th-H3), 6.81 (1 H,
d, J 16.0, Ph–CH2LCH2), 6.59 (2 H, d, J 9.2, NPh), 3.27
(4 H, t, J 7.6, 2 6 NCH2CH2CH2CH3), 1.56 (4 H, m, 2 6
NCH2CH2CH2CH3), 1.35 (4 H, m, 2 6 NCH2CH2CH2CH3)
and 0.95 (6 H, t, J 7.2, 2 6 NCH2CH2CH2CH3).
DBAT1: Preliminary data indicated a monoclinic cell. The
choice of the centric space group P21/n (#14) was confirmed by
the successful solution and refinement of the structure.
Disorder on two of the alkyl chains was modelled. The
occupancy of all the disordered atoms was refined isotropi-
cally. The same x, y, and z parameters and isotropic
displacement parameters were used for the following pairs of
atoms: C15, C15A; C35, C35A. This was done using the
EXYZ and EADP line commands. Constraints were placed on
bond distances within one of the alkyl chains in order to
achieve uniformity. The extinction coefficient refined to
0.0023. All other non-H atoms were refined anisotropically.
All H-atoms were placed in idealized positions and were
included in structure factor calculations but were not refined.
Crystal data for DBAT1: C36H50N2S, M = 542.84, monoclinic,
5,50--Bis[(E,E)-(4-N,N-dibutylaminobenzylidene)methyl]-
2,29:59,20:50,2-:5-,2-:5-,2-0-sexithiophene
(DBAT6).
The
general procedure was followed with 3 (2.80 g, 5.91 mmol),
5,59-bis(tributylstannyl)-2,29-bithiophene (2.00 g, 2.69 mmol)
and Pd(PPh3)4 (70.0 mg, 0.06 mmol) in DMF (50 mL). The
crude material was recrystallized from methylene chloride–
methanol to give DBAT6 (0.73 g, 35%) as a fine dark red
powder (Found: C 70.49; H 6.38; N 3.04. C52H60N2S6
requires C, 70.59; H, 6.30; N, 2.94%); dH (400 MHz; CDCl3;
Me4Si) 7.31 (2 H, d, J 8.8, NPh), 7.02–7.08 (5 H, m, 5 6
Th-H), 7.93 (1 H, d, J 16.0, Ph–CH2LCH2), 6.85 (1 H, d,
J 4.0, Th-H3), 6.80 (1 H, d, J 16.0, Ph–CH2LCH2), 6.59 (2 H,
d, J 8.8, NPh), 3.27 (4 H, t, J 7.6, 2 6 NCH2CH2CH2CH3),
1.56 (4 H, m, 2 6 NCH2CH2CH2CH3), 1.35 (4 H, m,
2 6 NCH2CH2CH2CH3) and 0.95 (6 H, t, J 7.2, 2 6
NCH2CH2CH2CH3)
˚
˚
space group P21/n (#14), a = 12.301(5) A, b = 16.426(6) A, c =
3
˚
˚
16.316(7) A, a = 90u, b = 99.026(9)u, c = 90u, U = 3256(2) A ,
Z = 4, T = 2116 uC, m(Mo Ka) = 1.9 mm21, 18719 reflections
measured from which 4536 (Rint 0.0974) were independent,
wR2 (all data) = 0.2232.
DBAT2: Preliminary data indicated a triclinic cell. The
choice of the centric space group P-1 (#2) was confirmed by
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 4934–4942 | 4941

View Article Online
9 P. Ba¨uerle, in Electronic Materials: The Oligomeric Approach, ed.
K. Mu¨llen, G. Wegner,Wiley-VCh, Weinheim, 1998, pp. 105–197.
10 P. Ba¨uerle, In Handbook of Oligo- and Polythiophenes, ed.
D. Fichou, Wiley-VCH, Weinheim, 1998, pp. 89–184.
11 C. Van Pham, A. Burkhardt, R. Shabana, D. D. Cunningham,
H. B. J. Mark and H. Zimmers, Phosphorus, Sulfur Silicon Relat.
Elem., 1989, 46, 153–168.
12 P. Fre`re, J. M. Raimundo, P. Blanchard, J. Delaunay,
P. Richomme, J. L. Sauvajol, J. Orduna, J. Garin and J. Roncali,
J. Org. Chem., 2003, 68, 7254–7265.
the successful solution and refinement of the structure.
Disorder on two of the alkyl chains was modelled. The
occupancy of all the disordered atoms was refined isotro-
pically. The same x, y, and z parameters and isotropic
displacement parameters were used for the following pairs of
atoms: C33, C33A; C39, C39A. This was done using the
EXYZ and EADP line commands. All other non-H atoms
were refined anisotropically. All H-atoms were placed in
idealized positions and were included in structure factor
calculations but were not refined. Crystal data for DBAT2:
C40H52N2S2, M = 624.96, triclinic, space group P-1 (#2),
13 G. Go¨tz, S. Scheib, R. Klose, J. Heinze and P. Ba¨uerle, Adv. Funct.
Mater., 2002, 12, 723–728.
14 Y. Wei, Y. Yang and J.-M. Yeh, Chem. Mater., 1996, 8,
2659–2666.
15 G. Barbarella, D. Casarini, M. Zambianchi, L. Favaretto and
S. Rossini, Adv. Mater., 1996, 8, 69–73.
16 R. Azumi, M. Goto, K. Honda and M. Matsumoto, Bull. Chem.
Soc. Jpn., 2003, 76, 1561–1567.
˚
˚
a = 12.0888(6) A, b = 12.1517(7) A, c = 12.8142 A,
˚
a = 84.4140(10)u, b = 73.1610(10)u, c = 87.552(2)u, U =
1792.92(16) A , Z = 2, T = 2116 uC, m(Mo Ka) = 1.9 mm
3
21
˚
,
17 M. Melucci, M. Gazzano, G. Barbarella, M. Cavallini, F. Biscarini,
P. Maccagnani and P. Ostoja, J. Am. Chem. Soc., 2003, 125,
10266–10274.
14383 reflections measured from which 3229 (Rint 0.0659) were
independent, wR2 (all data) = 0.2708.
18 V. D. Zobel and G. Ruban, Acta Crystallogr., Sect. B: Struct.
Crystallogr. Cryst. Chem., 1978, 34, 1652–1657.
19 T. Noda, I. Imae, N. Noma and Y. Shirota, Adv. Mater., 1997, 9,
239–241.
20 T. Noda, H. Ogawa, N. Noma and Y. Shirota, Adv. Mater., 1997,
9, 720–722.
21 M. Turbiez, P. Fre`re, M. Allain, C. Videlot, J. Ackermann and
J. Roncali, Chem.–Eur. J., 2005, 11, 3742–3752.
22 M. Turbiez, P. Fre`re and J. Roncali, J. Org. Chem., 2003, 68,
5357–5360.
Acknowledgements
This research was supported by Xcel Energy, contract FIA-
021521. We thank Dr Joe Bozell for the gracious loan of his
synthetic laboratory space.
References
23 J. J. Apperloo, L. Groenendaal, H. Verheyen, M. Jayakannan,
R. A. J. Janssen, A. Dkhissi, D. Beljonne, R. Lazzaroni and
J. L. Bre´das, Chem.–Eur. J., 2002, 8, 2384–2396.
24 R. G. Hicks and M. B. Nodwell, J. Am. Chem. Soc., 2000, 122,
6746–6753.
25 Y. Wei, B. Wang, W. Wang and J. Tian, Tetrahedron Lett., 1995,
36, 665–668.
26 A. K. Mohanakishnan, A. Hucke, M. A. Lyon,
M. V. Lakshmikantham and M. P. Cava, Tetrahedron, 1999, 55,
11745–11754.
1 S. E. Shaheen, D. S. Ginley and G. E. Jabbour, MRS Bull., 2005,
30, 10–19.
2 B. O’Reagan and M. Gratzel, Nature, 1991, 353, 737–740.
3 S. E. Shaheen and D. S. Ginley, in Dekker Encyclopedia
of Nanoscience of Nanotechnology, ed. J. A. Schwarz, C.I.
Contescu and K. Putyera, Marcel Dekker, Inc., New York, 2004,
pp. 2879–2895.
4 K. Mu¨llen and G. Wegner, Electronic Materials: The Oligomer
Approach, Wiley-VCH, Weinheim, 1998.
5 U. Mitschke and P. Ba¨uerle, J. Mater. Chem., 2000, 10, 1471–1507.
6 M. Gra¨tzel, Nature, 2003, 421, 586–587.
7 F. Garnier, A. Yassar, R. Hajlaoui, G. Horowitz, F. Deloffre,
B. Servet, S. Ries and P. Alnot, J. Am. Chem. Soc., 1993, 115,
8716–8721.
27 A. J. Bard and L. R. Faulkner, Electrochemical Methods,
Fundamentals and Applications, John Wiley & Sons, New York,
1980.
28 J.-M. Raimundo, P. Blanchard, N. Gallego-Planas, N. Mercier,
I. Ledoux-Rak, R. Hierle and J. Roncali, J. Org. Chem., 2002, 67,
205–218.
8 F. Garnier, Acc. Chem. Res., 1999, 32, 209–215.
4942 | J. Mater. Chem., 2005, 15, 4934–4942
This journal is ß The Royal Society of Chemistry 2005