Thiophene-2-aryl-2 H -benzotriazole-thiophene oligomers with adjustable electronic properties

Article abstract of DOI:10.1021/ol2005853

Chemical equations presented. The synthesis and the optical and electrochemical properties of 2-aryl-2H-benzotriazole based thiophene oligomers with chemical modifications either on the phenyl side group or on the backbone are presented. All data, support

Full text of DOI:10.1021/ol2005853

ORGANIC
LETTERS
2011
Vol. 13, No. 9
2338–2341
Thiophene-2-aryl-2H-benzotriazole-
thiophene Oligomers with Adjustable
Electronic Properties
†
Felix M. Pasker, Stephan M. Le Blanc, Gregor Schnakenburg, and Sigurd Hoger*
†
‡
,†
€
ꢀ
€
€
Kekule-Institut fur Organische Chemie und Biochemie and Institut fur Anorganische
€
Chemie, Rheinische Friedrich-Wilhelms-Universitat Bonn, Gerhard-Domagk-Strasse 1,
53121 Bonn, Germany
hoeger@uni-bonn.de
Received March 4, 2011
ABSTRACT
The synthesis and the optical and electrochemical properties of 2-aryl-2H-benzotriazole based thiophene oligomers with chemical modifications
either on the phenyl side group or on the backbone are presented. All data, supported by DFT calculations, show that modification on the
backbone has a major impact on the electronic properties while the side groups can fine-tune the electronic properties. In addition, one compound
exhibits a thermotropic mesophase.
Thiophene based linear oligomers and polymers have
been among the most attractive materials in various fields
of organic electronics such as organic field-effect transis-
tors (OFETs), organic light-emitting diodes (OLEDs), or
organic photovoltaics (OPVs).1 With regard to device
fabrication, organic materials are cheap and particularly
simple to deposit (either from vacuum or solution). De-
fined oligomers sometimes have advantages over the cor-
responding polymers because they can, as a matter of prin-
ciple, be obtained in higher purity with respect to defects/
polymerization faults. Moreover, oligomers can act as
model systems allowing estimation of the pure polymer
properties by extrapolation of the oligomer properties
to infinite molecular weight.2 Compounds containing
electron-poor moieties surrounded by electron-rich parts,
so-called donorꢀacceptorꢀdonor (DAD) compounds, offer
the possibility to reduce the HOMOꢀLUMO gap, thereby
shifting the absorption edge toward lower energies. This is
of special importance, as there still is a need for new
materials absorbing the longer wavelength region (>600
nm) of the sunlight spectrum in OPVs3 or giving stable
(3) (a) Zhu, Z.; Waller, D.; Brabec, C. J. New Construction of Low-
Bandgap conducting Polymers. In Organic Photovoltaics; Brabec, C.,
Dyakonov, V., Scherf, U., Eds.; Wiley-VCH: Weinheim, Germany, 2008; p
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129. (b) Thompson, B. C.; Frechet, J. M. J. Angew. Chem. 2008, 120, 62.
(4) (a) Zaumseil, J; Sirringhaus, H. Chem. Rev. 2007, 107, 1296. (b)
Yamashita, Y. Sci. Technol. Adv. Mater. 2009, 10, 024313. (c) Kim, F. S.;
Guo, X. G.; Watson, M. D.; Jenekhe, S. A. Adv. Mater. 2010, 22, 478.
(5) (a) Sonar, P.; Singh, S. P.; Sudhakar, S.; Dodabalapur, A.;
Sellinger, A. Chem. Mater. 2008, 20, 3184. (b) Melucci, M.; Favaretto,
L.; Zanelli, A.; Cavallini, M.; Bongini, A.; Maccagnani, P.; Ostojo, P.;
Derue, G.; Lazzaroni, R.; Barbarella, G. Adv. Funct. Mater. 2010, 20,
†
ꢀ
€
Kekule-Institut fur Organische Chemie und Biochemie.
‡ Institut f€ur Anorganische Chemie.
(1) (a) Fichou, D., Ed. Handbook of Oligo- and Polythiophenes;
€
Wiley-VCH: Weinheim, Germany, 1999. (b) Mishra, A.; Ma, C.-Q.; Bauerle,
P. Chem. Rev. 2009, 109, 1141. (c) Perepichka, I. F.; Perepichka, D. F.;
Meng, H.; Wudl, F. Adv. Mater. 2005, 17, 2281.
€
445. (c) Steinberger, S.; Mishra, A.; Reinold, E.; Muller, C. M.; Uhrich,
€
C.; Pfeiffer, M.; Bauerle, P. Org. Lett. 2010, 13, 90. (d) Velusamy, M.;
€
(2) Mullen, K., Wegner, G., Eds. Electronic Materials: The Oligomer
Thomas, K. R. J.; Lin, J. T.; Hsu, Y. C.; Ho, K. C. Org. Lett. 2005, 7,
1899.
Approach; Wiley-VCH: Weinheim, Germany, 1998.
r
10.1021/ol2005853
Published on Web 04/13/2011
2011 American Chemical Society

ambipolar OFETs.4 For example, in the field of fused
N- and S-heterocycles, benzothiadiazole thiophene hybrids
have already found applications in OFETs and OPVs.5 In
addition, thiophene based DAD oligomers6 and polymers7
with stronger acceptors like [1,2,5]thiadiazolo[3,4-g]qui-
noxaline or diketopyrrolopyrrol exhibit even reduced op-
tical gaps and have entered various fields of organic elec-
tronics. Recently, we introduced 2-aryl-2H-benzotriazoles
as electron-accepting units in conjugated polymers and
could show that copolymers thereof have optical proper-
ties similar to those of their benzothiadiazole analogues.8
of 5 is a result of the restricted solubility of 4 that
precipitated during the course of the reaction. Neverthe-
less, pure 5 could be isolated in sufficient quantities to
continue the multistep synthesis. Reduction with iron
powder in acetic acid and subsequent condensation with
3,30-dimethoxybenzil gave the triazoloquinoxaline 7 in
good yields.
Scheme 2. Synthesis of 9
Scheme 1. Synthesis of 3aꢀd
In the present study we report the synthesis and the
optical and electrochemical characterization of a series of
defined benzotriazole and thiophene hybrids. Specifically
we describe DAD compounds with terminal dithiophene
units and a central electron-poor 2-aryl-2H-benzotriazole
unit that is systematically varied at its aryl- (Scheme 1) and
benzo-position (Scheme 2). The 2-phenyl-2H-benzotria-
zoles 1a and 1b were prepared as described before8 while 1c
and 1d were prepared by nucleophilic aromatic substitu-
tion of the para-fluoride of the 3,4,5-trifluorophenyl group
with the respectivealcoholate at characteristic pointsof the
reaction pathway (Supporting Information). Future inves-
tigations will include both more electron-rich and more
electron-poor substituents on the 2-aryl group. Suzukiꢀ
Miyaura reaction with 2-(50-hexyl-2,20-bithiophen-5-yl)-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2) yielded the
DAD oligomers 3aꢀd (Scheme 1). Scheme 2 shows the
synthesis of the triazolo[4,5-g]quinoxaline oligothiophene
9. 1b was nitrated with sodium nitrate in sulfuric acid to
give a mixture of the mononitro compound 4 and the
dinitro compound 5.9 We assume that the rather low yield
The branched, solubilizing 2-octyldodecyl chain was
introduced by nucleophilic ipso substitution of the fluorine
atom in para position to the benzotriazole group with
sodium-2-octyldodecanolate in THF. A trace amount of
meta substituted byproduct could be removed via recycling
gel permeation chromatography (rec-GPC) after the
SuzukiꢀMiyaura coupling reaction of 8 with 2 and yielded
pure 9. Figure 1 shows the UV/vis absorption spectra of
3aꢀd and 9 and the emission spectra of 3aꢀd (9 does not
show any measurable emission). Both the absorption and
emission spectra follow the same trend.
(6) (a) Peet, J.; Tamayo, A. B.; Dang, X. D.; Seo, J. H.; Nguyen, T. Q.
Appl. Phys. Lett. 2008, 93, 163306. (b) Li, H.; Tam, T. L.; Lam, Y. M.;
Mhaisalkar, S. G.; Grimsdale, A. C. Org. Lett. 2011, 13, 46. (c) Walker,
B.; Tamayo, A. B.; Dang, X.-D.; Zalar, P.; Seo, J. H.; Garcia, A.;
Tantiwiwat, M.; Nguyen, T.-Q. Adv. Funct. Mater. 2009, 19, 3063.
€
(7) (a) Inganas, O.; Zhang, F.; Tvingstedt, K.; Andersson, L. M.;
€
Hellstrom, S.; Andersson, M. R. Adv. Mater. 2010, 22, E100. (b)
Bijleveld, J. C.; Zoombelt, A. P.; Mathijssen, S. G. J.; Wienk, M. M.;
Turbiez, M.; de Leeuw, D. M.; Janssen, R. A. J. J. Am. Chem. Soc. 2009,
131, 16616.
Figure 1. UV/vis absorption spectra (CH2Cl2, 1 ꢁ 10ꢀ5 M, ;)
and emission spectra (CH2Cl2, -Δ-) of 3a (gray), 3b (black), 3c
(red), 3d (blue), and 9 (green).
€
(8) Wettach, H.; Pasker, F.; Hoger, S. Macromolecules 2008, 41,
9513.
(9) 5 could also be crystallized from CH2Cl2/pentane. Details on the
structure of 5 are given in the Supporting Information.
Org. Lett., Vol. 13, No. 9, 2011
2339

Table 1. UV/Vis Absorption (λmaxabs, ε) and Emission Data (λmaxem) As Well As the Optical Band Gapa (Egopt); Redox Potentials of
the Reversible Oxidations and Reductions (E1/2ox1, E1/2ox2, and E1/2red1), the Potential Onset of the Second Reduction (Eonsred2) vs Fc/
CV
Fcþ and the Electrochemical Gap (Eg
)
λmaxabs [nm]/
ε [L molꢀ1 cmꢀ1
]
λmaxem [nm]
Egopt [eV]
E1/2ox1 [V]
E1/2ox2 [V]
E1/2red1 [V]
Eonsred2 [V]
EgCV [eV]
3a
3b
3c
3d
9
474/45500
485/41800
480/40800
482/43100
692/26600
550
588
577
569
ꢀ
2.29
2.22
2.25
2.25
1.52
0.32
0.40
0.37
0.37
0.21
0.64
0.65
0.67
0.66
0.52
ꢀ1.96
ꢀ1.86
ꢀ1.90
ꢀ1.89
ꢀ1.28
ꢀ2.21
ꢀ2.12
ꢀ2.15
ꢀ2.14
ꢀ1.70b
2.28
2.26
2.27
2.26
1.49
a Determined by absorption onset. b The second reduction potential of 9 is reversible; therefore the half wave potential was used.
With increasing electron affinity of the benzotriazole
The absolute energy levels, on the other hand, show the
expected trends as both the first oxidation and the first
reduction potentials shift upon substitution. 3a is easiest to
oxidize and hardest to reduce; 3b on the other hand shows
a higher electron affinity and thereby a less pronounced
tendency of being oxidized. Despite similar HOMOꢀLUMO
gaps, the absolute frontier orbital levels can be adjusted
by the substitution pattern. Interestingly, quinoxaline 9
shows both an increased electron affinity and a slightly
increased tendency of being oxidized. The reversible
second reduction of the highly electron-poor aza
heterocycle 9 is even lower than the first reductions
of 3aꢀd. In order to gain better insight into the elec-
tronic structure of these DAD hybrids, theoretical
calculations based on the density-functional theory
(DFT) were performed.
moiety a slight but measurable bathochromic shift of the
absorption maxima can be observed (Table 1), thus allow-
ing a fine-tuning of the optical properties. Most notably,
introduction of the alkoxy or oligoethyleneoxy chain in 3a
and 3b, respectively, does not affect this behavior. This
shows that the benzotriazole acceptor unit bears the
possibility to alter the solubility and wettability as well as
the morphological behavior of the hybrid compounds
without dramatically influencing their optical properties.
When comparing the absorption characteristics of 9
with 3c a remarkable bathochromic shift is observed (815
vs 550 nm for the absorption onset). Both, the low energy
and the higher energy absorptions of 9 are much broader
than those for 3c and indicate the presence of two or more
overlapping absorption bands.
A detailed theoretical analysis of the optical transitions
will be studied in the near future. Nevertheless, all hybrids
reported here can be viewed as DAD structures with a
reduced optical gap when compared to their constituents.
Cyclic voltammetry (CV) of 3aꢀd in CH2Cl2 shows for all
four compounds two reversible oxidations, one reversible
and one irreversible reduction signal (Figure 2, irreversible
reduction omitted for clarity). The electronic gap was
The ground state geometries, the frontier orbitals for 3c
and 9 (displayed in Figure 3a), and their respective energies
were calculated with the B3LYP functional using a def2-
TZVP basis set (Figure 3b and Supporting Infor-
mation) using the quantum-chemical program package
ORCA.10 The HOMOs are delocalized over the thiophene
and benzo units while there is only limited contribution on
the triazole subunit. In contrast, the LUMOs are deloca-
lized over the whole molecules with larger coefficients at
the electron-poor parts of the molecule. This difference in
the HOMO/LUMO localization is especially pronounced
for 9, in accordance with the assumption of the stronger
electron-accepting behavior of the quinoxaline moiety.
Under the assumption that the long wavelength transitions
are predominantly determined by the HOMOꢀLUMO
transition, the excitation leads to an intramolecular charge-
transfer (CT) from the thiophenes to the benzotriazole or
benzotriazole-quinoxaline units, respectively. These calcu-
lations verify our initial assumption that the electron-rich
thiophene units act as electron donors and the electron-
poor benzotriazole units as electron acceptors in these
intramolecular CT complexes. Notably, there is an almost
linear relationship between the experimentally determined
(CV) and the theoretically calculated energy levels for the
ox1
calculated as EgCV = E1/2 ꢀ E1/2red1 and is only margin-
ally influenced by the substitution pattern (e.g., the values
of 3a and 3b differ by 0.02 eV; Table 1).
Figure 2. Cyclic voltammograms of 3a (gray), 3b (black), 3c (red),
3d (blue), and 9 (green) in CH2Cl2 (c ≈ 2 ꢁ 10ꢀ4 M) at a sweep rate
(10) Neese, F.; Becker, U.; Ganyushin, D.; Kollmar, C.; Kossmann,
S.; Hansen, A.; Liakos, D.; Petrenko, T.; Riplinger, C.; Wennmohs, F.
ORCA - an ab initio, density functional and semiempirical program
package, version 2.8.0; University of Bonn: Bonn, Germany, 2010.
of 100 mV sꢀ1
.
2340
Org. Lett., Vol. 13, No. 9, 2011

Figure 4. DSC signal of heating and cooling 9 with 10 K minꢀ1
and texture of 9 at 150 °C (100 μm ꢁ 74 μm).
original heating curve could be only reproduced after
keeping the sample at room temperature for at least 20 h.
A detailed analysis of the phase behavior of 9 is in progress
and will be reported elsewhere.
In summary, we have synthesized a series of benzo-
triazole-thiophene hybrids with different acceptor units
and studied the impact of these substitutions on the
optical and electronic properties of the compounds. The
UV/vis spectroscopic data support the assumption that
the electron-rich thiophene parts act as donors and the
electron-poor benzotriazole parts as acceptors. This
was further supported by quantum mechanical calcula-
tions. CV data show that the absolute levels of the
frontier orbitals are more sensitive to the substituents
than the optical gap. In addition, a triazoloquinoxaline
has been prepared that has an optical gap of 1.5 eV and
which exhibits a liquid crystalline phase as indicated by
DSC and POM measurements. In addition to tunable
electronic properties, benzotriazoles allow the advanta-
geous introduction of solubilizing side groups.
Figure 3. (a) HOMO (left) and LUMO (right) of 3c (top) and 9
(bottom) calculated with B3LYP/def2-TZVP. (b) Energy level
correlation between cyclic voltammetric and computational data
Y-axis: 3a (gray), 3b (black), 3c (red), and 9 (green).
HOMOs and LUMOs of these benzotriazole thiophene
hybrids (Figure 3b). This relationship shows that, despite
the error in the calculated absolute values, theoretical
calculations can serve as a valuable tool to guide the
synthesis of future DAD materials.
Additionally, 9 exhibits a thermotropic mesophase
which was characterized by differential scanning calorime-
try (DSC) and polarized optical microscopy (POM)
(Figure 4), and that was not observed for the hybrids 3.
The DSC traces show two major transitions during heating
which can be assigned to the melting temperature (Tm =
68.6 °C, ΔH = 17.0 kJ/mol) and the isotropization tem-
perature (Ti = 230.8 °C, ΔH = 3.02 kJ/mol) of the com-
pound. The cooling curve shows that recrystallization of
the sample from the liquid crystalline phase is slow and the
Acknowledgment. We gratefully acknowledge the DFG
(especially through the SPP1355 and SFB813) for financial
support.
Supporting Information Available. Synthesis and char-
acterization of all new compounds. Crystallographic data
for 4 and computational details. This material is available
free of charge via the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 9, 2011
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