N-Acyldithieno[3,2- b:2′,3′-d]pyrroles: Second generation dithieno[3,2-b:2′,3′-d]pyrrole building blocks with stabilized energy levels

Article abstract of DOI:10.1021/ol101647f

A new class of dithieno[3,2-b:2′,3′-d]pyrroles (DTPs) incorporating N-acyl groups have been prepared from 3-bromothiophene via copper-catalyzed amidation. The utilization of various electron-withdrawing acyl groups has allowed stabilization of the HOMO an

Full text of DOI:10.1021/ol101647f

ORGANIC
LETTERS
2010
Vol. 12, No. 18
4054-4057
N-Acyldithieno[3,2-b:2′,3′-d]pyrroles:
Second Generation
Dithieno[3,2-b:2′,3′-d]pyrrole Building
Blocks with Stabilized Energy Levels
Sean J. Evenson and Seth C. Rasmussen*
Department of Chemistry and Biochemistry, North Dakota State UniVersity, NDSU
Department 2735, P.O. Box 6050, Fargo, North Dakota 58108-6050
seth.rasmussen@ndsu.edu
Received July 15, 2010
ABSTRACT
A new class of dithieno[3,2-b:2′,3′-d]pyrroles (DTPs) incorporating N-acyl groups have been prepared from 3-bromothiophene via copper-
catalyzed amidation. The utilization of various electron-withdrawing acyl groups has allowed stabilization of the HOMO and LUMO energy
levels of these popular building blocks for conjugated materials. The synthesis and characterization of this new class of compounds is
described, including electrochemical and photophysical data for all compounds and X-ray structural data for the octanoyl, benzoyl, and
cyclohexanoyl functionalized compounds. Initial polymers generated via electropolymerization are also reported.
Conjugated organic materials have received considerable
fundamental and technological interest due to their combina-
tion of the electronic and optical properties of inorganic
semiconductors with many of the desirable properties of
organic plastics, including mechanical flexibility and low
production costs.1,2 In addition, one of the key advantages
of these materials is the ability to tune the electronic and
optical properties at the molecular level via synthetic
modification. One approach to such synthetic modification
is the introduction of fused aromatic units into the conjugated
backbone, resulting in materials exhibiting enhanced carrier
mobilities and lowered band gaps. Such fused-ring building
blocks that have found recent popularity are the N-alkyl- and
N-aryl-dithieno[3,2-b:2′,3′-d]pyrroles (DTPs).3 Since their
introduction, these building blocks have been incorporated
into various polymeric,4-14 oligomeric,15-17 and molecular
materials18-20 to give high carrier mobilities, enhanced
(3) Ogawa, K.; Rasmussen, S. C. J. Org. Chem. 2003, 68, 2921.
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10.1021/ol101647f  2010 American Chemical Society
Published on Web 08/20/2010

solution and solid-state fluorescence, and materials with
reduced and low band gaps. As a result, these materials have
been applied to organic light-emitting diodes (OLEDs),9
organic photovoltaic cells (OPVs),10-14 electrochromics,13
and field effect transistors (FETs).7,8,11,13 One limitation of
the current DTP building blocks, however, is the high energy
of the HOMO, which limits stability and the effective
application of DTP-based materials to various devices. As a
solution to this limitation, we report herein the successful
synthesis of a new class of DTPs that incorporate N-acyl
groups to significantly stabilize the HOMO and LUMO
energy levels.
Scheme 1. Synthesis of N-Acyldithieno[3,2-b:2′,3′-d]pyrroles
utility of 1, new synthetic methods for its production were
desirable. By utilizing LDA, rather than BuLi, selective
deprotonation of the more cost-effective 3-bromothiophene
can be used to generate the lithiated intermediate. Trans-
metalation to the copper species is then facilitated by first
reacting with ZnCl2, followed by CuCl2.24 Oxidative coupling
of the copper intermediate is assisted with the addition of
dry O2 to produce 1 in high yield (85-90%) via this one-
pot method. It should be acknowledged that during the
preparation of this manuscript, a related synthesis of 1 from
3-bromothiophene and LDA was reported.9 However, with-
out the additional modifications to the oxidative coupling
steps, yields were still limited to 73%.
The new family of N-acylDTPs (2a-e) was prepared via
copper-catalyzed amidation21 of 3,3′-dibromo-2,2′-bithiophene
(1) as shown in Scheme 1. After our initial introduction of
the preparation of DTPs via Pd-catalyzed amination of
3-bromothiophene,3 other groups reported variant routes
utilizing 1 as an intermediate.22 As 1 is commonly produced
in moderate yield (60-75%) via the lithiation and oxidative
Cu-coupling of 2,3-dibromothiophene,23 its use increases the
material cost of the resulting DTPs. Thus, to improve the
(7) Liu, J.; Zhang, R.; Sauve, G.; Kowalewski, T.; McCullough, R. D.
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Hashimoto, K. J. Mater. Chem. 2010, 20, 2362.
With good access to 1, the production of N-acylDTPs was
then investigated utilizing Cu-catalyzed tandem C-N bond
formation methods developed by Buchwald.21b These meth-
ods had previously been successful in the production of the
isomeric dithieno[2,3-b:3′,2′-d]pyrrole and thus looked prom-
ising for the production of DTPs. The initial application of
the previously reported conditions successfully produced the
desired DTP 2b but only in 19% yield. While a variety of
reaction conditions were investigated (Supporting Informa-
tion, Table S1), the only condition that enhanced product
yield was the increase of catalyst loading to 10 mol %,
resulting in isolated yields of ca. 40%. Further increases in
the amount of catalyst did not improve product yields, and
at this time, it is not clear what is limiting product formation.
No significant side products are observed, and all unreacted
1 can be completely recovered. However, further production
of 2a stops at ca. 40%, even with the addition of further
fresh catalyst, leading us to suspect that the product itself is
inhibiting the catalyst. In fact, the addition of N-acylDTP
product to the initial reactant/catalyst mixture results in little
to no product under the standard conditions discussed above.
Similar inhibition has been previously observed in the Cu-
catalyzed amination of bromothiophenes.25 Attempts to apply
alternate Pd-catalyzed methods have not been successful.
The structures of three N-acylDTPs were examined via
X-ray crystallography and selected bond lengths are given
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X.; Liu, Y.; Li, Y.; Yang, Y. J. Polym. Sci., Part A: Polym. Chem. 2009,
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Org. Lett., Vol. 12, No. 18, 2010
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Table 1. Selected Bond Lengths for Various DTPs
Table 2. Electronic and Optical Data of N-AcylDTPsa
Epa
(V)
EHOMO
(eV)
λmax
ELUMO
(eV)
molecule
(nm) ε (M-1 cm-1
)
N-alkylDTPb 0.56
5.61
5.70
5.85
5.83
5.82
5.87
5.85
310
298
310
300
305
289
305
289
305
289
320
290
305
289
26100
29300
42400
46700
14800
26300
15400
27600
13100
23500
11000
27000
14500
27200
1.61
1.70
2.13
2.10
2.10
2.25
2.13
N-arylDTPc
0.65
0.80
0.78
0.77
0.82
0.80
2a
2b
2c
2d
2e
a Reference 3.
in Table 1. Data for the previously reported N-octylDTP are
included for comparison.3 As can be seen, the overall
structures of the N-acylDTPs agree well with N-octylDTP
and the only significant differences are a shortening of the
N-C bond between the pyrrole and the acyl group and an
elongation of the two internal N-C bonds of the pyrrole.
These deviations are indicative of additional conjugation
between the acyl CdO and the fused-ring DTP unit and is
consistent with differences previously observed for N-
arylDTPs,3 although to a greater extent. For 2b and 2d, the
CdO unit is nearly coplanar with the DTP backbone, with
a dihedral angle of 7.8 and 2.3°, respectively. In contrast,
the bulkier N-cyclohexanoyl group of 2e resulted in a larger
angle of 18.4°.
a In CH3CN. Potentials vs Ag/Ag+ in 0.1 M TBAPF6. EHOMO values
26
were determined in reference to ferrocene (5.1 eV vs vacuum).
E
)
LUMO
EHOMO - λonset
.
b R ) C8H17.3 c R ) Ph-C6H13.3
to pyrrole.3 However, the addition of the N-acyl group still
has a significant effect and shifts the peak potentials of the
N-acylDTP oxidations to ∼0.78 V (vs Ag/Ag+), approximately
220 mV more positive than the previous N-alkylDTPs. Calcu-
lations show that the addition of the N-acyl group does not
change the nature of the DTP HOMO and the functional
group still resides at a node (Supporting Information, Figure
S14). As a result, the observed stabilization of the HOMO
energy level is the result of an inductive effect, rather than
a direct contribution to the HOMO.
A representative cyclic voltammogram (CV) of 2a is
shown in Figure 1, and the combined electrochemical data
A representative UV-visible spectrum of 2a is shown in
Figure 2, and the combined spectroscopic data for the series
Figure 1. Cyclic voltammograms of N-alkyl- and N-acylDTPs.
for the series are given in Table 2. As with most oligo-
thiophenes, N-acylDTPs exhibit a well-defined, irreversible
oxidation, corresponding to a removal of an electron from
the monomer π-system and the formation of the radical
cation. However, unlike previous DTPs, the N-acylDTPs
exhibit a well-defined reduction coupled to the initial
oxidation. This would indicate some partial rereduction of
the radical cation formed and suggests the acyl group helps
stabilize the radical cation intermediate.
Figure 2. UV-visible spectra of N-alkyl- and N-acylDTPs.
are given in Table 2. All of the N-acylDTPs exhibit two
primary transitions, a sharp transition at 289 and a broader
transition at 305 nm. A prominent shoulder is also observed
for all species at ∼280 nm.
As previously reported, the larger π system of the DTP
attenuates the effect of N-functional groups in comparison
While all of the transitions of the previous N-alkylDTPs
were assigned as various vibrational components of the same
electronic transition, it seems that the N-acylDTPs exhibit
two separate electronic transitions, with the high energy
(26) Thompson, B. C.; Kim, Y.-G.; McCarley, T. D.; Reynolds, J. R.
J. Am. Chem. Soc. 2006, 128, 12714.
4056
Org. Lett., Vol. 12, No. 18, 2010

transition consisting of a simple π-π* transition and the
lower energy transition exhibiting some charge-transfer (CT)
character. The CT assignment is consistent with the broad
nature and lower extinction coefficients (∼11-15 × 103 M-1
cm-1) of this transition. In addition, calculations indicate that
while the acyl group does not contribute to the DTP HOMO,
it does contribute significantly to the LUMO and LUMO+1.
Thus, HOMO-LUMO excitation would result in charge-
transfer from the localized DTP unit out onto the acyl group.
Calculations indicate that the lowest energy transition arises
from excitation between the HOMO and a combination of
the LUMO and LUMO+1, supporting the CT nature of this
transition. The contribution of the acyl group to the DTP
LUMO also results in greater stabilization of the LUMO in
comparison to the HOMO, producing a red-shift in the onset
of absorption of the N-acylDTPs in comparison to the
previous N-alkylDTPs as shown in Figure 2.
Figure 4. Solid-state absorption spectra of electropolymerized films
of N-acyl- and N-alkylDTP homopolymers.
poly(2b) is similar to that of the N-octylDTP polymer, the
N-acylDTP polymer exhibits a low energy shoulder consis-
tent with the previously discussed monomeric N-acylDTP
absorption spectra. As a result, the absorption onset of
poly(2b) is red-shifted by ∼55 nm, giving a polymeric
band gap of 1.60 eV. This results in an ∼0.1 eV reduction
in band gap in comparison to the previously reported
poly(N-alkylDTPs).5
To illustrate the effect of the stabilized N-acylDTPs on
their respective conjugated materials, the homopolymer of
DTP 2b was generated via electropolymerization.27 The
resulted CV of the poly(2b) film is shown in Figure 3 and
In conclusion, a new class of dithieno[3,2-b:2′,3′-d]-
pyrroles incorporating N-acyl groups has been prepared via
copper-catalyzed amidation. Electrochemical and optical
characterization of the resulting N-acylDTPs shows that both
the HOMO and LUMO energy levels are significantly
stabilized in comparison to previous N-alkylDTPs, thus
demonstrating how the electronic properties of DTPs can
be tuned through N-functionalization. In addition, initial
polymeric materials prepared by electropolymerization of the
monomeric N-acylDTPs confirm that their application to
conjugated materials results in both deeper HOMO levels
and reduced band gaps.
Figure 3. Cyclic voltammograms of electropolymerized films of
N-acyl- and N-alkylDTP homopolymers.
Acknowledgment. We thank the ND EPSCoR Flexible
Electronics program (EPS-0814442) for support of this
research. We would also like to thank Dr. Angel Ugrinov
(NDSU) for the collection of the X-ray crystal data and Dr.
Christopher L. Heth (NDSU) for assistance with the calcula-
tions.
compared to the analogous poly(N-octylDTP). As expected,
the polymer of 2b exhibits an oxidative onset significantly
more positive (∼400 mV) than the previous N-alkylDTP
polymers. In addition, the N-acylDTP polymers exhibit two
well-defined redox waves in comparison to the single broad
response observed for the previous N-alkylDTP polymers.5
To further characterize the overall effect of the new
N-acylDTPs, polymer films were grown on ITO to measure
the resulting solid-state absorption spectra, and illustrative
visible spectra are given in Figure 4. While the λmax of
Supporting Information Available: Experimental and
synthetic details for compounds 1 and 2a-e. Full UV-vis
data for 2a-e, X-ray data for 2b, 2d, and 2e, and calculations
for N-acetylDTP. This material is available free of charge
via the Internet at http://pubs.acs.org.
(27) Heth, C. L.; Tallman, D. E.; Rasmussen, S. C. J. Phys. Chem. B
2010, 114, 5275.
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