Synthesis and electronic, photophysical, and electrochemical properties of a series of thienylcarbazoles

Article abstract of DOI:10.1021/jo202625p

A series of thienylcarbazoles were synthesized by Suzuki-Miyaura and Ullmann coupling reactions. In these compounds, the 2-thienyl or 2,2′-bithiophen-5-yl group is connected at the N-, 1,8-, 3,6-, 2,7-, 2,7,N-, or 1,8,N-positions of the carbazole ring. The effects of structural variations on their electronic, photophysical, and electrochemical properties were explored by UV-vis and fluorescence spectroscopies, cyclic voltammetry (CV), and DFT calculations in evaluation of their potential as material components. The thienyl substituents at the 2,7-positions in 4, 5, and 10 are responsible for a high degree of π-conjugation and strong emission with fluorescence quantum yields up to 0.61. The CV on a series of thienylcarbazoles revealed a good electron-donating ability of 3,6-substituted carbazoles 3 and 9. The number of thiophene units was found to affect the extent of π-conjugation, the resulting HOMO-LUMO gaps, and fluorescence efficiency. The crystal structures of 5 and 9 were also disclosed.

Full text of DOI:10.1021/jo202625p

Article
pubs.acs.org/joc
Synthesis and Electronic, Photophysical, and Electrochemical
Properties of a Series of Thienylcarbazoles
Shin-ichiro Kato, Satoru Shimizu, Hiroaki Taguchi, Atsushi Kobayashi, Seiji Tobita,
and Yosuke Nakamura*
Department of Chemistry and Chemical Biology, Graduate School of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan
S
* Supporting Information
ABSTRACT: A series of thienylcarbazoles were synthesized by
Suzuki−Miyaura and Ullmann coupling reactions. In these
compounds, the 2-thienyl or 2,2′-bithiophen-5-yl group is connected
at the N-, 1,8-, 3,6-, 2,7-, 2,7,N-, or 1,8,N-positions of the carbazole
ring. The effects of structural variations on their electronic,
photophysical, and electrochemical properties were explored by
UV−vis and fluorescence spectroscopies, cyclic voltammetry (CV),
and DFT calculations in evaluation of their potential as material
components. The thienyl substituents at the 2,7-positions in 4, 5, and
10 are responsible for a high degree of π-conjugation and strong emission with fluorescence quantum yields up to 0.61. The CV
on a series of thienylcarbazoles revealed a good electron-donating ability of 3,6-substituted carbazoles 3 and 9. The number of
thiophene units was found to affect the extent of π-conjugation, the resulting HOMO−LUMO gaps, and fluorescence efficiency.
The crystal structures of 5 and 9 were also disclosed.
extensively investigated by Leclerc and co-workers.7b−d In any
of these cases, the nitrogen atom of the carbazole moiety can be
INTRODUCTION
The development of new π-conjugated organic materials is
■
also functionalized by alkylation or arylation reaction to
attracting growing attention due to their potential applications
for various electronic and opto-electronic devices,1 such as
enhance the solubility and other properties of the resulting
organic light-emitting diodes (OLEDs),2 organic field-effect
carbazole derivatives. Thus, carbazole would be recognized as a
transistors (OFETs),3 organic photovoltaics (OPVs),4 and
useful scaffold to allow the large structural diversity for fine-
sensors.5 Much effort has been devoted to establish structure−
property relationships in order to develop new materials with
tuning of optical and electrochemical properties.
Thiophene derivatives, in particular oligothiophens, are now
excellent candidates for a variety of advanced materials
desired properties in an intelligent way. Such systematic studies
applications.15 Therefore, mixed π-conjugated polymers and
on conjugated oligomers also provide valuable insights into
relevant fundamental properties of π-systems.6
oligomers made of carbazoles and thiophenes are a new class of
functional compounds through the use of their highly electron-
The choice of building blocks has to be carefully examined in
donating ability. For example, Jen and co-workers reported the
the molecular design of functional materials because they tend
high performance of poly(3,6-carbazole)s incorporating dieth-
to dominate the properties in many cases. Among many
ynylthiophene units in OLED.16 Zotti, Leclerc, and co-workers
heteroaromatic systems, carbazoles, their derivatives, polymers,
synthesized poly(2,7-carbazole)s incorporating thiophene units,
and oligomers are a highly interesting family of functional
and their conductive properties were investigated.17 More
organic molecules because carbazole has fine optical properties,
recently, Melucci and co-workers synthesized 3,6-bis(2,2′-
bithiophen-5-yl)-9-methyl-9H-carbazole derivatives with suit-
a low redox potential, and high chemical stability. Oligo-/
polycarbazoles7 have been representative benchmark materials
in OLEDs,8,9 OFETs,10 and OPVs11 to date. Carbazole can be
able alkyl chains as liquid-crystalline semiconducting materi-
als.18 Some oligomers consisting of carbazole and thiophene
easily functionalized by electrophilic aromatic substitution at its
3,6-positions (para position from the nitrogen atom) with high
units were also synthesized;19,20 however, no systematic study
on structurally well-defined carbazole−thiophene systems
focusing on the effect of the conjugation connectivity between
the carbazole and thiophene units on their electronic, optical,
and electrochemical properties has been reported so far, to the
best of our knowledge. In this context, we became interested in
thienyl-substituted carbazole derivatives, namely, thienylcarba-
electron density, and hence a large number of 3,6-
functionalized carbazole derivatives have been studied. The
1,8-functionalized carbazoles can be obtained when the 3,6-
positions of carbazole are first protected, because its 1,8-
positions (ortho position from the nitrogen atom) are also
activated.12−14 Although the synthesis of 2,7-functionalized
carbazoles is not straightforward, the efficient synthetic
pathways to them have been established in the past decade,
and their applications to various functional devices have been
Received: December 23, 2011
Published: February 28, 2012
© 2012 American Chemical Society
3222
dx.doi.org/10.1021/jo202625p | J. Org. Chem. 2012, 77, 3222−3232

The Journal of Organic Chemistry
Article
Chart 1
zoles 1−10, in which a thienyl group is incorporated into a
carbazole moiety at the N-, 1,8-, 3,6-, 2,7-, 2,7,N-, and 1,8,N-
positions (Chart 1). A large series of thienylcarbazoles 1−10
varying the conjugation connectivity and the number of
thiophene units allow us to study the structure−property
relationships of carbazole−thiophene-based materials. Here, we
report the synthesis and fundamental properties, such as the
structural features and the electronic, photophysical, and redox
properties, of 1−10 on the basis of X-ray crystallographic study,
UV−vis and fluorescence spectroscopies, cyclic voltammetry,
and theoretical calculations.21
Scheme 1. Synthesis of 2
performed on 16 and 12 to give 5 in 92% yield. The Ullmann
coupling reaction of 3,6-di-tert-butylcarbazole (17) with
bromide 18 afforded 6 in 20% yield (Scheme 3).
Thienylcarbazole 7 was synthesized following a two-step
transformation from diiodocarbazole derivative 19. The
ethylation of 19 with iodoethane was carried out in the
presence of NaOH as a base in DMF, and then the cross-
coupling reaction of 20 with 14 afforded the desired 7. The
synthesis of 8 was achieved by sequential coupling reactions.
Thus, 8 was prepared from 19 by the Suzuki−Miyaura coupling
with 14 followed by the Ullmann coupling with 18. The
Suzuki−Miyaura coupling reaction of dibromocarbazole
derivative 22 with 14 gave 9 in 85% yield (Scheme 4). All
compounds were fully characterized by various spectroscopic
methods, such as 1H NMR, 13C NMR, and mass spectroscopy.
Structural Properties. The single crystals of 5 and 9
suitable for X-ray crystallographic analyses were successfully
obtained by slow diffusion of hexane into CHCl3 at 0 °C
(Figure 1a,b).27 The structure determined for 5 exhibited the
statistical disorder of the two thiophene rings at the 2,7-
positions (Supplementary Figure S1 in the Supporting
Information). In 5, the 2,7-di(2-thienyl) and carbazole moieties
are quasi-coplanar with dihedral angles for C9−C10−C24−
RESULTS AND DISCUSSION
■
Synthesis. In our molecular design, the 2-thienyl or 2,2′-
bithiophen-5-yl group was incorporated into the carobazole
moiety. To functionalize the 1,8-positions of carbazole, tert-
butyl groups were first introduced to the 3,6-positions in 2, 7,
and 8 by Friedel−Crafts reactions with 2-chloro-2-methyl-
propane.
Compounds 1a,22 1b,23 and 324 are known in the literature.
Thienylcarobazoles 2 and 4−10 were synthesized via Suzuki−
Miyaura cross-coupling25 and/or Ullmann coupling reactions26
as key steps (Schemes 1−4). For Suzuki−Miyaura cross-
coupling, Pd(PPh3)4 as a catalyst was used and 1,8-
disubstituted carbazole 2 was obtained from 1,8-dibromocarba-
zole derivative 11 and 2-thiopheneboronic acid pinacol ester
(12) in 32% yield (Scheme 1). Similar cross-coupling reactions
were performed on 2,7-dibromocarbazole derivative 13 with
either 12 or 2,2′-bithiophene-5-boronic acid pinacol ester (14)
to give thienylcarbazoles 4 and 10 in 68% and 81%, respectively
(Scheme 2). Compound 5 was obtained from 2,7-dibromo-
carbazole (15) in two steps. The Ullmann coupling reaction of
15 with 2-bromothiophene in the presence of copper powder
and K2CO3 in nitrobenzene afforded dibromide 16 in 75%
yield. The Suzuki−Miyaura cross-coupling reaction was then
3223
dx.doi.org/10.1021/jo202625p | J. Org. Chem. 2012, 77, 3222−3232

The Journal of Organic Chemistry
Article
Scheme 2. Synthesis of 4, 5, and 10
Scheme 3. Synthesis of 6−8
C23 and C2−C3−C17−C20 of 170.1° and 14.9°, respectively
(Figure 1a). The N-(2-thienyl) and carbazole moieties are
nearly orthogonal with a dihedral angle for C14−C13−N1−C1
of −107.9°. This is apparently ascribed to steric repulsions
between the 1,8-hydrogen atoms in the carbazole moiety and
the attached N-(2-thienyl) moiety. Thienylcarbazole 9 also
exhibited the disorder of the peripheral thiophene rings in the
solid state. There are three crystallographically independent
3224
dx.doi.org/10.1021/jo202625p | J. Org. Chem. 2012, 77, 3222−3232

The Journal of Organic Chemistry
Article
molecules for 9 in the unit cell. The largest difference among
them is the conformation between the thiophene rings and the
carbazole π-system. The two molecules adopt the two-s-cis
conformations, while the s-cis and s-trans conformations coexist
in the one molecule (Figure 1b and Supplementary Figure S2).
The torsional angles between the carbazole π-system and the
attached 3,6-bis(2,2′-bithiophen-5-yl) moiety vary from 0.6° to
33.9°. In the density functional theory (DFT)-optimized
structure of 9 at the B3LYP/6-31G* level of theory without
any symmetry constrains by the Gaussian 03 suit of program,28
the torsional angle is estimated to be approximately 25°.
Therefore, it is likely that the observed deviation of the
torsional angles from the calculated one is derived from the
crystal packing force. The bithiophene systems in the solid-state
structure of 9 are nearly planar with a torsional angle of
approximately 16° as a mean value. Pyramidalization of the
nitrogen atoms is insignificant in both 5 and 9. This is
expressed by the sums of the three bond angles at these
nitrogen atoms of 354.8° and 359.6° in 5 and 9, respectively.
The selected geometrical parameters of 5 and 9 obtained by the
DFT calculations are given in Supplementary Tables S1 and S2,
respectively, and compared with the crystallographic bond
lengths and torsion angles. In general, there is good agreement
between the experiment and theory. The differences in
carbon−carbon bond lengths in the carbazole moieties are
within 0.02 Å. The large deviation occurs for the above-
mentioned torsional angles between the carbazole π-system and
the thienyl units and the carbon−sulfur bond lengths in the
thiophene moieties. Thus, the DFT results provide slightly
longer bond lengths by 0.023−0.039 Å compared with the X-
ray results.
Scheme 4. Synthesis of 9
Electronic Absorption Spectroscopy. The UV−vis
absorption spectral data of 1−10 in CH2Cl2 are summarized
in Table 1. Notably, the extent of π-conjugation is highly
dependent on the substituent pattern of the carbazole moiety
and the number of thiophene units.
The absorption spectra of 1a and 2−5 having the 2-thienyl
functionality are shown in Figure 2a. Thienylcarbazole 1a
features the longest absorption maximum (λmax) of 333 nm
with a molar extinction coefficient (ε) of 4000 M−1 cm−1, while
1b displays the bathochromically shifted longest λmax of 340 nm
with ε of 3900 M−1 cm−1 (Figure 2b). The absorption curves of
1a and 1b are similar to those of N-ethylcarbazole (23) and 3,6-
Figure 1. ORTEP plots of (a) 5 and (b) 9 with displacement ellipsoids
at 50% probability level at 153 K. Arbitrary numbering; hydrogen
atoms are omitted for clarity. One of the three different molecules in
the unit cell is shown for 9.
Table 1. Optical Data and Calculated Lowest Excitation Energies of 1−10
a
b
a
c
d
d
λmax [nm]
λonset [nm]
λem [nm]
Φem
calcd λmax [nm] (f)
composition of band
e
1a
1b
2
285, 291, 322, 333
345
355
380
385
395
390
390
415
390
430
455
370
0.014
f
307 (0.015)
313 (0.038)
335 (0.074)
329 (0.330)
352 (1.128)
352 (1.179)
375 (0.216)
374 (0.274)
372 (0.013)
395 (1.146)
421 (1.964)
H → L, 33%; H → L + 1, 60%
H → L, 23%; H → L + 1, 69%
H−1 → L + 3, 3%; H → L, 88%
H−1 → L + 1, 9%; H → L + 1, 81%
H → L, 84%
e
285, 295, 329, 340
368
e
e
e
290, 305, 345, 360
395
0.064
0.047
0.610
0.210
0.129
0.106
0.092
0.168
0.482
e
3
256, 267, 314, 345
392, 411
391, 412
387, 407
417
e
4
269, 290, 353, 370
5
261, 292, 352
H → L, 84%
e
e
e
6
260, 285, 296, 325, 340
H−2 → L, 4%; H → L, 89%
H−2 → L, 9%; H → L, 89%
H → L, 76%; H → L + 1, 14%; H → L + 2, 3%
H → L, 87%
e
e
7
310, 325, 360
440
e
8
305, 340
434
e
e
9
353, 380
430, 450
447, 472
10
256, 313, 399
H → L, 87%
a
b
c
In CH2Cl2. The longest energy absorption wavelength with a molar absorptivity ε = 500 M−1 cm−1. Absolute quantum yields determined by a
d
calibrated integrating sphere system in cyclohexane. TD-DFT (TD/B3LYP/6-31G*) calculations were carried out with the use of optimized
structures at B3LYP/6-31G* level of theory, and only energies with f > 0.01 are shown; f = oscillator strength; H = HOMO, L = LUMO. Peak as
shoulder. Not measured.
e
f
3225
dx.doi.org/10.1021/jo202625p | J. Org. Chem. 2012, 77, 3222−3232

The Journal of Organic Chemistry
Article
Figure 2. Electronic absorption spectra of (a) 1a and 2−5 and (b) 1b and 6−10 and normalized fluorescence spectra of (c) 1a and 2−5 and (d) 6−
10 in CH2Cl2.
di-tert-butyl-N-ethylcarbazole (24), respectively. The longest
λmax values of 1a and 1b are hypochromically shifted relative to
those of 23 and 24, respectively: 23, 346 nm, 24, 353 nm
(Supplementary Figure S4). These results show almost no π-
conjugation between the carbazole and thiophene moieties,
which should reflect steric hindrance between the N-(2-thienyl)
moiety and the 1,8-hydrogen atoms in the carbazole moiety as
expected from the crystal structure of 5. The longest λmax of 1,8-
disubstituted carbazole derivative 2 is bathochromically shifted
relative to that of 24 by only approximately 5 nm, indicative of
less effective conjugation between the carbazole and thiophene
moieties in 2 probably due to the twist around the C−C bonds
connecting them. Compound 3 displays a different absorption
curve from those of 1a,b and 2 along with the significantly
increased absorptivity in the whole region. When compared to
3, 4 features the significantly bathochromic shift of the longest
λmax of 353 nm with higher ε of 46400 M−1 cm−1. Interestingly,
5 displays almost the same absorption curve as that of 4 in a
region of 320−400 nm, featuring the absorption maximum of
352 nm with ε of 46900. Thus, the electronic property of 5 in
the ground state is mainly dominated by the 2,7-di(2-thienyl)
substitution, which is further confirmed by theoretical
calculations described below.
The UV−vis spectra of 6−10 having the 2,2′-bithiophen-5-yl
functionality are shown in Figure 2b. The 2,2′-bithiophen-5-yl
substitutions in 6, 7, 9, and 10 intrinsically resulted in the
extension of π-conjugation, that is, the reduction of HOMO−
LUMO gaps relative to those of the corresponding 1b, 2, 3, and
4. For example, the extension of the chromophore in nearly
planar 9 readily gives rise to the spectral change as compared to
3. Thus, the spectrum of 9 is highly broadened, and its
absorption onset (λonset) as well as the longest λmax are
bathochromically shifted, clearly reflecting the effective π-
conjugation as expected by its X-ray crystal structure. Similarly,
the longest λmax of 10 is bathochromically shifted relative to
that of 4 by approximately 30 nm, and hence 10 has the
smallest optical HOMO−LUMO gap in the series of 1−10
(Table 1). The similar effect of the number of thiophene units
on the extension of conjugation is also seen in nonplanar 6−8.
Compound 6 features the longest λmax of 340 nm with ε of
18900 M−1 cm−1, and its λonset (390 nm) is bathochromically
shifted by approximately 35 nm as compared with that of 1b
(355 nm). The spectrum of 6 is apparently different from the
superposition of the spectra of 24 and 2,2′-bithiophene (25),
which shows the effective π-conjugation between the 2,2′-
bithiophen-5-yl group and the carbazole backbone in 6
3226
dx.doi.org/10.1021/jo202625p | J. Org. Chem. 2012, 77, 3222−3232

The Journal of Organic Chemistry
Article
(Supplementary Figure S5). The DFT calculations at the
B3LYP/6-31G* level of theory predict that there is no
significant difference of the dihedral angles between the
carbazole and thiophene moieties in 1b and 6: 69° in 1b and
66° in 6 (Figure 3).29 The longest λmax for 7 appears at 360 nm
as a shoulder, which is almost consistent with that of 2;
however, the λonset of 7 (415 nm) is red-shifted relative to that
extent of π-conjugation is found in our recent bicarbazole
systems30 as well as in others.7b,c,8g
Fluorescence Spectroscopy. All compounds in this study
are fluorescent. The fluorescence spectra for 1a−5 and 6−10 in
CH2Cl2 are shown in Figure 2c and d, respectively. The data
are also summarized in Table 1. The fluorescence spectra were
recorded in the dilute regime (10−5−10−6 M). No characteristic
excimer fluorescence was observed.31 The color of the
fluorescence of 1−5 and 6−10 ranges from violet to light
blue and from blue to light green, respectively. Similar to the
change of the longest λmax values of 1−10 as described above,
the emission maxima (λem) of 6−10 are bathochromically
shifted compared to those of the corresponding 1−5,
depending on the number of thiophene units. The substitution
pattern of the carbazole moiety also has a substantial effect on
the fluorescence wavelength. For instance, 3,6-functionalized
carbazole derivative 9 exhibits emission with λem values of 430
and 450 nm, while 2,7-functionalized carbazole 10 features λem
values of 447 and 472 nm, which is the longest among the
compounds in this study. The two well-defined vibronic
maxima were observed in 3−5, 9, and 10, while a single
maximum appeared in 1a,b, 2, and 6−8. This finding indicates
a somewhat large structural change and/or electronic
distribution change between the ground and excited states for
1a,b, 2, and 6−8. The electronic absorption spectra of 4, 5, and
10 do not seem to be mirror images of fluorescence spectra.
The longest absorption bands of the 2,7-functionalized
carbazoles 4, 5, and 10 are probably ascribed to two different
transitions, which may reflect the two well-resolved peaks in
their fluorescence spectra. We note a shoulder around 370 nm
in the absorption spectra of 4 in addition to the strong
absorption peak at 353 nm.
Figure 3. Optimized structures of (a) 1b, (b) 2, (c) 6, (d) 7, and (e) 8
at the B3LYP/6-31G* level of theory.
The fluorescence quantum yields (Φem) of 1−10, which were
determined by a calibrated integrating system, vary from 0.014
to 0.610 (Table 1). In a series of 1−5, the Φem values were
dramatically affected by the substitution pattern. The lowest
Φem value was observed in 1a to be 0.014, which is almost 1/
30th of that (ca. 0.4) of 23.32 Both 2 and 3 also display smaller
Φem values than 23 by about 1 order of magnitude. In sharp
contrast to 2 and 3, compound 4 displays the significantly high
Φem of 0.610, which is the highest value in a series of 1−10 in
the present study and also higher than that of 23. The Φem
value (0.210) of 5 is almost one-third that of 4, which should be
attributed to the N-(2-thienyl) group in 5. The 2,2′-bithiophen-
5-yl substitution essentially brings about higher Φem values than
the 2-thienyl substitution as found in 6−9. The Φem values of 7
and 9 are almost 2−2.5 times as high as those of 2 and 3,
respectively, while 10 displays the slightly low Φem value
(0.482) relative to that of 4. Thienylcarbazoles 4 and 10 are
remarkably emissive, which clearly confirms that the thienyl
substitution at the 2- and/or 7-positions of the carbazole
moiety is crucial to ensure high Φem values,33,34 although the
reason for this finding is unclear at present. Lee and co-workers
reported that 4,4′-di(2-thienyl)biphenyl shows a remarkably
high fluorescence quantum yield of 0.79.35 Compounds 4 and
10 can be regarded as the nitrogen-bridged analogues. The
aforementioned high Φem values of 4 and 10 may be attributed
to the structural similarity between the 2,7-functionalized
carbazoles and 4,4′-di(2-thienyl)biphenyl.
of 2 (380 nm). On the basis of the calculations, the twist
around the C−C single bonds connecting the carbazole moiety
to the thienyl moieties in 7 is expected to be smaller than that
in 2: 61° and 65° in 2 and 48° and 60° in 7. This may be one of
the reasons for the extension of π-conjugation of 7 relative to 2.
However, we do not have a suitable explanation for the π-
conjugative effect in 6 as compared to 1b at present. Further
introduction of the 2,2′-bithiophen-5-yl group on the nitrogen
atom in 8 leads to a hypochromic shift of the longest λmax as
well as the absorption onset as compared to 7, although the ε
value around 300 nm of 8 is slightly increased. The sterically
crowded 2,2′-bithiophen-5-yl groups in 8 induce the significant
twist around the C−C and/or N−C single bonds connecting
the carbazole π-system to the 2,2′-bithiophen-5-yl group, which
should result in the less effective π-conjugation in 8 as
compared to 7. Indeed, the DFT calculations predict that the
dihedral angles between the carbazole and the 2,2′-bithiophen-
5-yl moieties increase from 48° and 60° in 7 to 58° and 69° in
8, and the N-(2,2′-bithiophen-5-yl) moiety in 8 is twisted from
the central carbazole moiety by 71° (Figure 3).
Importantly, the effect of the thienyl functionalization at the
different positions of the carbazole moiety on the extent of π-
conjugation is clearly observable in the UV−vis spectral data of
6−10, because they differ much in both the longest λmax values
and the absorption onset. Thus, thienylcarbazole systems follow
the established trend of the substitution pattern for the extent
of π-conjugation: N-, 1,8- < 3,6- < 2,7-substitution. Similar
effect of the substitution pattern of the carbazole moiety on the
Electrochemistry. To investigate the effect of the thienyl
functionalization of the carbazole moiety on the electron-
donating ability, that is, the HOMO level, we carried out the
cyclic voltammetry (CV) for 1−10 as well as 23 and 24 in
3227
dx.doi.org/10.1021/jo202625p | J. Org. Chem. 2012, 77, 3222−3232

The Journal of Organic Chemistry
Article
CH2Cl2 with n-Bu4NPF6 (0.1 M) as the supporting electrolyte.
The oxidation potentials (Epa) versus Fc+/Fc (ferrocenium/
ferrocene couple) are listed in Table 2. Thienylcarbazoles 1−10
displayed a carbazole-type 1 e− oxidation step, which was
by the N-(2-thienyl) group in 5, which is nearly orthogonal to
the carbazole backbone as confirmed by the X-ray analysis,
lowers the HOMO level relative to that of 4.
Upon increasing the number of thiophene units from 1b, 2,
3, and 4 to 6, 7, 9, and 10, respectively, the oxidations occur at
potentials cathodically shifted by 90−150 mV.36 Compound 6
exhibits a substantial cathodic shift of the Epa value by 130 mV
as compared to the corresponding 1b with the N-(2-thienyl)
group. Similarly, 7 is oxidized more cathodically than 2 by 110
mV. The efficient π-conjugation in 6 and 7 relative to 1b and 2,
respectively, as observed in the electronic absorption spectra
should make their oxidation potentials cathodic. Interestingly,
the Epa value (+0.68 V) of 8 is less negative as compared to
those of 6 and 7, despite the presence of the three 2,2′-
bithiophene units. It is clear that the somewhat high Epa value
of 8 is a consequence of the less effective π-conjugation
between the 2,2′-bithiophen-5-yl and carbazole moieties by the
steric effects. From 3 to 9, the cathodic shift is observed to be
70 mV, and thus the oxidation of 9 occurs at +0.47 V, which is
the lowest in a series of compounds in this study. The oxidation
potential of 10 shifts cathodically by 150 mV relative to that of
4 upon 2,7-bis(2,2′-bithiophen-5-yl) substitution. Thus, the
oxidation of 10 occurs at +0.50 V, which is higher than that of 9
by only 30 mV.
Table 2. Oxidation Potential (Epa) by Cyclic Voltammetry in
a
CH2Cl2 (0.1 M n-Bu4NPF6), Theoretically Calculated
b
HOMO and LUMO Levels, and Optical HOMO−LUMO
c
Gaps (ΔEopt
)
Epa
d
HOMO
[eV]
LUMO
[eV]
ΔEHOMO−LUMO
ΔEopt
[V]
[eV]
[eV]
1a
1b
2
+0.88
+0.73
+0.75
+0.54
+0.65
+0.74
+0.60
+0.64
+0.68
+0.47
+0.50
+0.89
+0.71
−5.74
−5.54
−5.47
−5.31
−5.44
−5.47
−5.47
−5.36
−5.41
−5.14
−5.21
−5.55
−5.37
−1.05
−0.97
−1.26
−1.21
−1.66
−1.70
−1.68
−1.67
−1.60
−1.67
−1.98
−0.96
−0.89
4.69
4.57
4.21
4.09
3.78
3.77
3.79
3.69
3.81
3.47
3.22
4.58
4.48
3.59
3.49
3.26
3.22
3.13
3.17
3.17
2.98
3.17
2.88
2.72
3.49
3.39
3
4
5
6
7
8
9
10
23
24
Theoretical Calculations. To obtain further insights into
the electronic properties, we performed the time-dependent
(TD) DFT calculations for 1−10. The results for absorption
energies of 1−10 at the TD-B3LYP/6-31G*//B3LYP/6-31G*
level of theory with the ground-state geometries are
summarized in Table 1. The absorption maxima in the low-
energy region of 4, 5, 9, and 10 are related to the HOMO−
LUMO transitions, whereas those of 1a,b, 2, 3, and 6−8 are
attributed to the transitions from the HOMO, HOMO-1, and
HOMO-2 into the LUMO, LUMO+1, LUMO+2, and LUMO
+3 levels. The calculated λmax values are blue-shifted with
respect to the experimental values by 14−32 nm except for 5
and 6, which probably reflects that the calculations were
performed under the gas-phase conditions. Compounds 1−10
a
All potentials are given versus the Fc+/Fc couple used as external
b
standard; scan rate = 0.1 V s−1. B3LYP/6-311G**//B3LYP/6-31G*.
c
d
The values are obtained from λonset. No reversible peak was observed
except for 2 and 24.
irreversible except for 2, between +0.47 and +0.88 V. The
oxidation peak amplitude in the CV was quite large after the
first oxidation, which should be attributed to the accumulation
of the compounds at the electrode surface by electrochemical
polymerization.
follow the experimentally observed trend for the longest λmax
,
where the introduction of the thienyl groups at the 2,7-
positions than the 1,8- and/or N- and 3,6-positions resulted in
the more red-shifted absorption bands (vide supra). The red
shifts of the longest λmax values observed in the electronic
absorption spectra upon increasing the number of thiophene
units from 3 (345 nm) to 9 (380 nm) and from 4 (370 nm) to
10 (399 nm) are nicely reproduced in the calculations.
The electronic effects of the thienyl groups in the carbazole
moiety were studied by molecular orbital calculations. The
frontier molecular orbital (FMO) plots and the HOMO and
LUMO levels of 1−10 were obtained by the single-point
calculations at the B3LYP/6-311G**//B3LYP/6-31G* level of
theory. The results are summarized in Table 2, and the
HOMOs and LUMOs of 1−5 and 6−10 are shown in Figures
4 and 5, respectively. The calculated HOMO−LUMO gaps are
higher than those obtained from the UV−vis absorption spectra
by ca. 1 eV, which probably reflects that the optical gaps are
estimated under the solution conditions, whereas the
theoretical gaps are estimated under the gas-phase conditions
as described above. However, it is noted that the calculations
qualitatively reproduce the findings that the 2,2′-bithiophen-5-yl
substitution generally resulted in the decrease of the HOMO−
LUMO gaps and the increase of the donor ability as observed
in the absorption spectra and the CV, respectively. The
Thienylcarbazoles 1a and 1b showed oxidation peaks at
+0.88 and +0.73 V, respectively, which are almost comparable
to those of 23 and 24, respectively. This clearly demonstrates
that there is almost no effective π-conjugation between the
carbazole and thiophene moieties in 1a,b by the steric effects as
discussed above, and thereby, their N-(2-thienyl) groups do not
contribute to the donor potency. Again, the 1,8-di(2-thienyl)
groups in 2 have almost no influence on the electron-donating
ability, and thus 2 is oxidized at +0.75 V. On the contrary, 3 and
4 are oxidized cathodically relative to 23 by 350 and 240 mV,
respectively. The value of +0.54 V in 3 is the lowest in a series
of 1−5. Compound 4 exhibits an anodic shift of the Epa value
by 110 mV as compared to 3, which is readily explained by the
substitution pattern of the carbazole moiety. The 3,6-di(2-
thienyl) substituents of 3 are located at the para positions
relative to the nitrogen atom of the carbazole moiety, while the
2,7-di(2-thienyl) substituents of 4 are at the meta positions.
Hence, the electron-donating ability of the carbazole moiety in
3 is enhanced by the 3,6-di(2-thienyl) groups, while the
enhancement of that in 4 is insufficient due to the cross-
conjugation. The presence of the additional N-(2-thienyl)
group in 5 brings about an anodic shift of its oxidation potential
relative to 4 by 90 mV. This implies that the σ-inductive effect
3228
dx.doi.org/10.1021/jo202625p | J. Org. Chem. 2012, 77, 3222−3232

The Journal of Organic Chemistry
Article
3 and 4 is again clearly rationalized by the calculations, where
the HOMO density of 3 has a large contribution from the
electron-rich nitrogen atom, whereas that of 4 is not found on
the nitrogen atom. Both the HOMO and LUMO densities of 5
have almost no contribution from the N-(2-thienyl) group.
This justifies the nearly identical absorption profiles of 4 and 5
as shown in Figure 2a. Compared to 1a, 1b, and 2, the HOMO
densities of 6−8 are more delocalized over the carbazole−
bithiophene systems by the π-conjugation. This probably shifts
their Epa values cathodically compared to 1a, 1b, and 2. The
majority of LUMO densities of 6−8 resides on the 2,2′-
bithiophen-5-yl unit. Therefore, the longest wavelength
absorption bands of 6−8 may have a somewhat charge-transfer
character, which causes the broad emission with large Stokes
shifts.38 The HOMOs and LUMOs of 9 and 10 have an
extension of π-conjugation over the whole conjugated back-
bone.
Figure 4. Molecular orbital plots (B3LYP/6-311G**//B3LYP/6-
31G*) of (a) 1a, (b) 1b, (c) 2, (d) 3, (e) 4, and (f) 5. The lower plots
represent the HOMOs, and the upper plots represent the LUMOs.
CONCLUSION
■
In conclusion, we have synthesized a series of thienylcarbazoles
1−10 by Suzuki−Miyaura and Ullmann coupling reactions.
The electronic structure was approached by UV−vis and
fluorescence spectral measurements, CV, and DFT calculations.
We have demonstrated that the substitution pattern of a
carbazole moiety and the number of thiophene units play an
important role in the electronic structure. The thienyl
substituents at the 2,7-positions in 4, 5, and 10 cause a high
degree of π-conjugation and high fluorescence quantum yields,
while those at the 3,6-positions increase the donor ability as
demonstrated by the CV for 3 and 9. Compound 10 also
possesses the high donor ability presumably due to the 2,2′-
bithiophen-5-yl group. The introduction of the 2,2′-bithiophen-
5-yl group in 9 and 10 was found to ensure the extension of π-
conjugation compared to 3 and 4, respectively. It has to be
mentioned that the number of thiophene units sufficiently
affects the extent of π-conjugation in nonplanar N- and/or 1,8-
substituted carbazole derivatives: there is almost no π-
conjugation between the carbazole and thiophene units in 1a,
1b, and 2, whereas the carbazole and thiophene units in 6−8
are substantially π-conjugated. The experimental observations
as above are in qualitative agreement with the results obtained
by the DFT calculations. Further studies addressing the
synthesis and the characteristics of the π-conjugation, emission,
and donor ability of new carbazole−thiophene systems are
currently underway in our laboratory. We believe that the
present study provides valuable information for the synthesis of
new carbazole-based π-systems.
Figure 5. Molecular orbital plots (B3LYP/6-311G**//B3LYP/6-
31G*) of (a) 6, (b) 7, (c) 8, (d) 9, and (e) 10. The lower plots
represent the HOMOs, and the upper plots represent the LUMOs.
calculated HOMO levels of 6, 7, 9, and 10 are higher than
those of 1b, 2, 3, and 4, respectively. Again, the LUMO levels of
6, 7, 9, and 10 are lower than those of 1b, 2, 3, and 4,
respectively. The LUMO levels are more affected than the
HOMO levels by the 2,2′-bithiophen-5-yl substitution.
Consequently, the calculated gaps of 6, 7, 9, and 10 are
smaller than those of 1b, 2, 3, and 4, respectively. Both the
calculated HOMO and LUMO levels of 5 are lower than those
of 4, and no significant difference in their ΔEHOMO−LUMO values
is observed. This also agrees well with the results in the
absorption spectra and the CV.
All HOMO densities are found in the electron-donating
carbazole moiety as expected. Almost no π-conjugation
between the carbazole and thienyl moieties in 1a,b is
observable in the FMOs as well as the UV−vis spectra and
CV (vide supra), which may explain the fact that the electron-
donating ability of 1a and 1b is not enhanced as compared to
23 and 24, respectively.37 The HOMO of 2 is also localized in
the carbazole moiety, while the HOMOs of 3 and 4 are
delocalized over the whole π-skeleton, which results in the
destabilization of the HOMOs compared to that of 23 as
confirmed by the CV. The difference of the Epa values between
EXPERIMENTAL SECTION
■
General Suzuki−Miyaura Cross-Coupling Procedure for
Preparation of Thienylcarbazoles. A mixture of haloarene (1
equiv) and Na2CO3 (2.6 equiv) in DME/water (7:1, ∼0.5 M) was
bubbled with argon with stirring for 15 min. Pd(PPh3)4 (0.02 equiv)
and boronic acid pinacol ester (1.2 equiv) were added to the mixture,
and the resulting mixture was refluxed for 4−20 h under argon
atmosphere. After addition of 5% aqueous NH4Cl to the mixture, the
organic phase was separated, and the aqueous phase was extracted with
CHCl3. The combined organic phase was washed with brine, dried
over anhydrous MgSO4, and evaporated in vacuo. The residue was
purified by column chromatography. An analytically pure material was
obtained by recycling gel permeation chromatography.
3,6-Di-tert-butyl-1,8-di(2-thienyl)-9-ethyl-9H-carbazole (2).
Compound 11 (0.20 g, 0.43 mmol) was allowed to react with 2-
thiopheneboronic acid pinacol ester 12 (0.23 g, 1.07 mmol) for 12 h
3229
dx.doi.org/10.1021/jo202625p | J. Org. Chem. 2012, 77, 3222−3232

The Journal of Organic Chemistry
Article
according to the general Suzuki−Miyaura cross-coupling procedure.
The crude material was purified by column chromatography (SiO2;
hexane/toluene 9:1) to give 2 (64 mg, 32%) as a white solid. Mp 261−
263 °C; 1H NMR (300 MHz, CDCl3) δ 0.47 (t, J = 6.9 Hz, 3H), 1.46
(s, 18H), 3.66 (q, J = 6.9 Hz, 2H), 7.10 (dd, J = 3.6, 5.1 Hz, 2H), 7.17
(dd, J = 1.2, 3.6 Hz, 2H), 7.36 (dd, J = 1.2, 5.1 Hz, 2H), 7.40 (d, J =
2.1 Hz, 2H), 8.08 (d, J = 2.1 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ
14.31, 32.06, 34.70, 40.19, 116.37, 118.81, 125.45, 126.17, 127.04,
127.06, 127.87, 139.15, 142.09, 142.73; UV−vis (CH2Cl2) λmax
(relative intensity) 290 (sh, 0.91), 303 (1.00), 345 (sh, 0.22), 359
127.99, 135.62, 137.89, 140.06, 144.54; UV−vis (CH2Cl2) λmax
(relative intensity) 353 (1.00), 380 (0.97) nm; HR-FAB-MS (NBA,
+
positive) m/z calcd for C30H21NS4 523.0557, found 523.0593 (M+).
2,7-Bis(2,2′-bithiophen-5-yl)-9-ethyl-9H-carbazole (10).
Compound 13 (80 mg, 0.22 mmol) was allowed to react with 2,2′-
bithiophene-5-boronic acid pinacol ester 14 (0.14 g, 0.68 mmol) for 17
h according to the general Suzuki−Miyaura cross-coupling procedure.
The crude material was purified by column chromatography (SiO2;
hexane/toluene 4:1 to toluene) to give 10 (97 mg, 81%) as a yellow
solid. Mp 255−256 °C; 1H NMR (300 MHz, CDCl3) δ 1.53 (t, J = 7.3
Hz, 3H), 4.45 (q, J = 7.3 Hz, 2H), 7.05 (dd, J = 3.6, 4.4 Hz, 2H), 7.20
(d, J = 3.6 Hz, 2H), 7.23 (d, J = 3.6 Hz, 2H), 7.23 (d, J = 4.4 Hz, 2H),
7.34 (d, J = 3.6 Hz, 2H), 7.51 (dd, J = 1.4, 8.1 Hz, 2H), 7.58 (d, J = 1.4
Hz, 2H), 8.05 (d, J = 8.1 Hz, 2H); 13C NMR (125 MHz, CDCl3): not
available due to the low solubility; UV−vis (CH2Cl2) λmax (relative
intensity) 256 (0.56), 313 (0.20), 399 (1.00) nm; HR-FAB-MS (NBA,
+
(0.23) nm; HR-FAB-MS (NBA, positive) m/z calcd for C30H33NS2
471.2054, found 471.2059 (M+).
9-Ethyl-2,7-di(2-thienyl)-9H-carbazole (4). Compound 13
(0.30 g, 0.85 mmol) was allowed to react with 2-thiopheneboronic
acid pinacol ester 12 (0.45 g, 2.12 mmol) for 20 h according to the
general Suzuki−Miyaura cross-coupling procedure. The crude material
was purified by column chromatography (SiO2; hexane/toluene 4:1)
+
positive) m/z calcd for C30H21NS4 523.0557, found 523.0565 (M+).
1
to give 4 (0.21 g, 68%) as a white solid. Mp 196−198 °C; H NMR
1,8-Bis(2,2′-bithiophen-5-yl)-3,6-di-tert-butyl-9H-carbazole
(21). Compound 19 (0.23 g, 0.43 mmol) was allowed to react with
2,2′-bithiophene-5-boronic acid pinacol ester 14 (0.38 g, 1.30 mmol)
for 6 h according to the general Suzuki−Miyaura cross-coupling
procedure. The crude material was purified by column chromatog-
raphy (SiO2; hexane/toluene 4:1) to give 21 (0.12 g, 60%) as a yellow
solid. Mp 220−223 °C; 1H NMR (300 MHz, CDCl3) δ 1.50 (s, 18H),
7.05 (dd, J = 3.9, 5.4 Hz, 2H), 7.24−7.26 (m, 4H), 7.28 (d, J = 3.6 Hz,
2H), 7.36 (d, J = 3.6 Hz, 2H), 7.65 (d, J = 2.1 Hz, 2H), 8.08 (d, J = 2.1
Hz, 2H), 8.90 (s, 1H); 13C NMR (125 MHz, CDCl3) δ 32.16, 34.96,
116.57, 117.26, 123.22, 123.98, 124.54, 124.71, 124.76, 125.19, 128.06,
135.32, 136.87, 137.31, 140.57, 143.45; UV−vis (CH2Cl2) λmax
(relative intensity) 325 (1.00), 379 (0.69) nm; HR-FAB-MS (NBA,
(300 MHz, CDCl3) δ 1.50 (t, J = 7.2 Hz, 3H), 4.43 (q, J = 7.2 Hz,
2H), 7.13 (dd, J = 3.6, 5.1 Hz, 2H), 7.31 (dd, J = 1.2, 5.1 Hz, 2H), 7.42
(dd, J = 1.2, 3.6 Hz, 2H), 7.51 (dd, J = 1.5, 8.1 Hz, 2H), 7.59 (d, J =
1.5 Hz, 2H), 8.04 (d, J = 8.1 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ
14.04, 37.70, 105.88, 118.00, 120.87, 122.39, 123.24, 124.77, 128.23,
132.31, 141.04, 145.68; UV−vis (CH2Cl2) λmax (relative intensity) 269
(0.92), 290 (sh, 0.36), 353 (1.00), 370 (sh, 0.81) nm; HR-FAB-MS
(NBA, positive) m/z calcd for C22H17NS2+ 359.0802, found 359.0795
(M+).
2,7,9-Tri(2-thienyl)-9H-carbazole (5). Compound 16 (0.15 g,
0.36 mmol) was allowed to react with 2-thiopheneboronic acid pinacol
ester 12 (0.19 g, 0.92 mmol) for 4 h according to the general Suzuki−
Miyaura cross-coupling procedure. The crude material was purified by
column chromatography (SiO2; hexane/CH2Cl2 10:1) to give 5 (140
+
positive) m/z calcd for C36H33NS4 607.1496, found 607.1479 (M+).
General Ullmann Coupling Procedure for Preparation of
Thienylcarbazoles. A mixture of carbazole derivative (1 equiv),
bromoarene (1.2 equiv), K2CO3 (3 equiv), and copper powder (3
equiv) in nitrobenzene (∼0.5 M) was stirred at 180 °C for 24 h. After
the precipitate was removed by filtration, the filtrate was concentrated
in vacuo. The residue was purified by column chromatography. An
analytically pure sample was obtained by recycling gel permeation
chromatography.
1
mg, 92%) as a pale yellow solid. Mp 203−204 °C; H NMR (300
MHz, CDCl3) δ 7.08 (dd, J = 3.6, 5.1 Hz, 2H), 7.22 (dd, J = 1.6, 3.8
Hz, 1H). 7.24−7.26 (m, 1H), 7.27 (dd, J = 1.2, 5.1 Hz, 2H), 7.35 (dd,
J = 1.2, 3.6 Hz, 2H), 7.44 (dd, J = 1.6, 5.4 Hz, 1H), 7.57 (dd, J = 1.2,
8.1 Hz, 2H), 7.62 (d, J = 1.2 Hz, 2H), 8.04 (d, J = 8.1 Hz, 2H); 13C
NMR (125 MHz, CDCl3) δ 107.48, 119.61, 120.69, 122.78, 123.45,
124.82, 124.95, 125.32, 126.56, 128.18, 132.95, 138.22, 143.14, 145.15;
UV−vis (CH2Cl2) λmax (relative intensity) 261 (0.76), 292 (0.33), 352
9-(2,2′-Bithiophen-5-yl)-3,6-di-tert-butyl-9H-carbazole (6).
Compound 17 (0.28 g, 1.00 mmol) was allowed to react with 18
(0.29 g, 1.20 mmol) according to the general Ullmann coupling
procedure. The crude material was purified by column chromatog-
raphy (SiO2; hexane/toluene 4:1) to give 6 (89 mg, 20%) as a yellow
solid. Mp 189−191 °C; 1H NMR (300 MHz, CDCl3) δ 1.46 (s, 18H),
7.07 (dd, J = 3.6, 5.1 Hz, 1H), 7.08 (d, J = 3.6 Hz, 1H), 7.20 (d, J = 3.6
Hz, 1H), 7.21 (dd, J = 3.6, 5.1 Hz, 1H), 7.26 (dd, J = 3.6, 5.1 Hz, 1H),
7.45 (d, J = 8.7 Hz, 2H), 7.50 (dd, J = 1.5, 8.7 Hz, 2H), 8.09 (d, J = 1.5
Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 32.12, 34.91, 109.78,
116.37, 122.43, 123.65, 123.99, 124.07, 124.83, 124.91, 128.05, 135.31,
137.32, 138.00, 140.25, 143.81; UV−vis (CH2Cl2) λmax (relative
intensity) 260 (sh, 0.76), 285 (sh, 0.75), 296 (1.00), 325 (sh, 0.66),
340 (0.74) nm; HR-FAB-MS (NBA, positive) m/z calcd for
+
(1.00) nm; HR-FAB-MS (NBA, positive) m/z calcd for C24H15NS3
413.0367, found 413.0356 (M+).
1,8-Bis(2,2′-bithiophen-5-yl)-3,6-di-tert-butyl-9-ethyl-9H-
ethylcarbazole (7). Compound 20 (64 mg, 0.11 mmol) was allowed
to react with 2,2′-bithiophene-5-boronic acid pinacol ester 14 (100 mg,
0.34 mmol) for 4 h according to the general Suzuki−Miyaura cross-
coupling procedure. The crude material was purified by column
chromatography (SiO2; hexane/toluene 10:1) to give 7 (46 mg, 63%)
as a pale yellow solid. Mp 220−222 °C; 1H NMR (300 MHz, CDCl3)
δ 0.53 (t, J = 6.9 Hz, 3H), 1.47 (s, 18H), 3.94 (q, J = 6.9 Hz, 2H), 7.03
(dd, J = 3.6, 5.1 Hz, 2H), 7.10 (d, J = 3.6 Hz, 2H), 7.19 (d, J = 3.6 Hz,
2H), 7.20−7.24 (m, 4H), 7.44 (d, J = 1.8 Hz, 2H), 8.08 (d, J = 1.8 Hz,
2H); 13C NMR (125 MHz, CDCl3) δ 14.20, 32.05, 32.15, 34.74,
116.56, 118.69, 123.73, 124.43, 125.19, 126.54, 127.60, 127.75, 127.98,
137.36, 137.54, 139.32, 140.96, 143.11; UV−vis (CH2Cl2) λmax
(relative intensity) 310 (1.00), 325 (sh, 0.92), 370 (0.52) nm; HR-
+
C28H29NS2 443.1741, found 443.1743 (M+).
3,6-Di-tert-butyl-1,8,9-tris(2,2′-bithiophen-5-yl)-9H-carba-
zole (8). Compound 21 (80 mg, 0.13 mmol) was allowed to react
with 18 (39 mg, 0.15 mmol) according to the general Ullmann
coupling procedure. The crude material was purified by column
chromatography (SiO2; hexane/toluene 3:1) to give 8 (65 mg, 65%)
as a yellow solid. Mp 213-214 °C; 1H NMR (300 MHz, CDCl3) δ 1.49
(s, 18H), 6.35 (d, J = 3.9 Hz, 1H), 6.40 (d, J = 3.9 Hz, 1H), 6.48 (d, J
= 3.6 Hz, 2H), 6.74 (d, J = 3.6 Hz, 2H), 6.79 (dd, J = 0.9, 3.3 Hz, 1H),
6.86 (dd, J = 3.6, 5.1 Hz, 1H), 6.95 (dd, J = 3.6, 5.1 Hz, 2H), 7.00 (dd,
J = 0.9, 3.6 Hz, 2H), 7.11 (dd, J = 0.9, 3.6 Hz, 1H), 7.15 (dd, J = 0.9,
5.1 Hz, 2H), 7.45 (d, J = 2.1 Hz, 2H), 8.15 (d, J = 2.1 Hz, 2H); 13C
NMR (125 MHz, CDCl3) δ 32.03, 34.81, 116.65, 118.67, 121.36,
123.42, 123.53, 123.94, 124.19, 124.60, 124.89, 127.66, 127.78, 128.65,
128.77, 128.94, 136.19, 137.00, 137.31, 137.59, 139.28, 139.52, 143.63
(1 peak was missing); UV−vis (CH2Cl2) λmax (relative intensity) 305
+
FAB-MS (NBA, positive) m/z calcd for C38H37NS4 635.1809, found
635.1805 (M+).
3,6-Bis(2,2′-bithiophen-5-yl)-9-ethyl-9H-carbazole (9). Com-
pound 22 (0.08 g, 0.23 mmol) was allowed to react with 2,2′-
bithiophene-5-boronic acid pinacol ester 14 (0.20 g, 0.68 mmol) for 17
h according to the general Suzuki−Miyaura cross-coupling procedure.
The crude material was purified by column chromatography (SiO2;
hexane/CH2Cl2 10:1) to give 9 (102 mg, 85%) as a yellow solid. Mp
171−174 °C; 1H NMR (300 MHz, CDCl3) δ 1.47 (t, J = 6.9 Hz, 3H),
4.39 (q, J = 6.9 Hz, 2H), 7.05 (dd, J = 3.6, 5.0 Hz, 2H), 7.19 (d, J = 3.6
Hz, 2H), 7.21 (d, J = 5.0 Hz, 2H), 7.22 (d, J = 3.6 Hz, 2H), 7.28 (d, J
= 3.6 Hz, 2H), 7.41 (d, J = 8.4 Hz, 2H), 7.75 (dd, J = 1.8, 8.4 Hz, 2H),
8.35 (d, J = 1.8 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 14.05, 37.94,
109.17, 117.85, 122.77, 123.38, 123.42, 124.15, 124.38, 124.81, 125.73,
3230
dx.doi.org/10.1021/jo202625p | J. Org. Chem. 2012, 77, 3222−3232

The Journal of Organic Chemistry
Article
(1.00), 335 (sh, 0.70) nm; HR-FAB-MS (NBA, positive) m/z calcd for
4436−4451. (c) Murphy, A. R.; Frechet, J. M. Chem. Rev. 2007, 107,
́
+
C44H37NS6 771.1250, found 771.1315 (M+).
1066−1096. (d) Anthony, J. E. Angew. Chem., Int. Ed. 2008, 47, 452−
483. (e) Mas-Torrent, M.; Rovira, C. Chem. Soc. Rev. 2008, 37, 827−
838. (f) Yamashita, Y. Sci. Technol. Adv. Mater. 2009, 10, 024313.
(4) (a) Organic Photovoltaics; Brabec, C., Cyakonov, V., Scherf, U.,
2,7-Dibromo-9-(2-thienyl)-9H-carbazole (16). Compound 15
(400 mg, 1.24 mmol) was allowed to react with 2-bromothiophene
(1.2 mL, 12.5 mmol) according to the general Ullmann coupling
procedure. The crude material was purified by column chromatog-
raphy (SiO2; hexane/toluene 4:1) give 16 (374 mg, 75%) as a yellow
solid. Mp 233−235 °C; 1H NMR (300 MHz, CDCl3) δ 7.19−7.22 (m,
2H), 7.41 (dd, J = 1.8, 8.7 Hz, 2H), 7.44 (dd, J = 2.4, 4.8 Hz, 1H), 7.53
(d, J = 1.8 Hz, 2H), 7.90 (d, J = 8.7 Hz, 2H); 13C NMR (125 MHz,
CDCl3) δ 113.63, 120.39, 121.52, 121.90, 124.38, 125.42, 125.83,
126.67, 137.14, 143.07; UV−vis (CH2Cl2) λmax (relative intensity) 294
(0.71), 303 (1.00), 323 (0.22), 336 (0.14) nm; HR-FAB-MS (NBA,
positive) m/z calcd for C16H9Br2NS+ 404.8822, found 404.8836 (M+).
3,6-Di-tert-butyl-1,8-diiodo-9-ethyl-9H-carbazole (20). A mix-
ture of 19 (50 mg, 0.094 mmol), NaOH (15 mg, 0.37 mmol), and
iodoethane (15 μL, 0.19 mmol) in DMF was stirred at 45 °C for 3 h.
The mixture was poured into water, and the resulting solution was
extracted with toluene/ethyl acetate. The organic layer was dried over
anhydrous MgSO4 and evaporated in vacuo. The residue was purified
by column chromatography (SiO2; hexane) to give 20 (50 mg, 95%)
as a white solid. Mp 171−172 °C; 1H NMR (300 MHz, CDCl3) δ 1.22
(t, J = 7.2 Hz, 3H), 1.41 (s, 18H), 5.22 (q, J = 7.2 Hz, 2H), 7.97 (d, J =
1.8 Hz, 2H), 7.99 (d, J = 1.8 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ
16.61, 31.89, 34.52, 38.02, 73.55, 116.05, 126.00, 137.87, 139.09,
144.89; UV−vis (CH2Cl2) λmax (relative intensity) 254 (1.00), 265 (sh,
0.73), 275 (sh, 0.55), 290 (sh, 0.40), 300 (0.49), 335 (sh, 0.09), 348
(0.13), 362 (0.12) nm; HR-FAB-MS (NBA, positive) m/z calcd for
C22H27I2N+ 559.0233, found 559.0146 (M+).
́
Eds.; Wiley-VCH: Weinheim, 2008. (b) Thompson, B. C.; Frechet, J.
M. Angew. Chem., Int. Ed. 2008, 47, 58−77. (c) Cheng, Y.-J.; Yang, S.-
H.; Hsu, C.-S. Chem. Rev. 2009, 109, 5868−5923.
(5) (a) Chen, J.; Reed, M. A.; Dirk, S. M.; Price, D. W.; Rawlett, A.
M.; Tour, J. M.; Grubisha, D. S.; Bennet, D. W. In Molecular
Electronics: Bio-Sensors and Bio-Computers; NATO Science Series II:
Mathematics, Physics, Chemistry; Plenum: New York, 2003; Vol. 96,
pp 59−195. (b) Thomas, S. W. III; Joly, G. D.; Swager, T. M. Chem.
Rev. 2007, 107, 1339−1386.
(6) (a) Electronic Materials: The Oligomer Approach; Mullen, K.,
̈
Wegner, G., Eds., Wiley-VCH; Weinheim, 1998. (b) Martin, R. E.;
Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350−1377. (c) Segura,
J. L.; Martín, N. J. Mater. Chem. 2000, 10, 2403−2435.
(7) For recent reviews, see: (a) Grazulevicius, J. V.; Strohriegl, P.;
Pielichowski, J.; Pielichowski, K. Prog. Polym. Sci. 2003, 28, 1297−
1353. (b) Morin, J.-F.; Leclerc, M.; Ades, D.; Siove, A. Macromol.
̀
Rapid Commun. 2005, 26, 761−778. (c) Blouin, N.; Leclerc, M. Acc.
Chem. Res. 2008, 41, 1110−1119. (d) Boudreault, P.-L. T.; Morin, J.-
F.; Leclerc, M. In Design and Synthesis of Conjugated Polymers; Leclerc,
M., Morin, J.-F., Eds.; Wiley-VCH: Weinheim, 2010; pp 205−226.
(8) For selected examples of light-emitting materials, see: (a) Morin,
J.-F.; Bourdreault, P.; Leclerc, M. Macromol. Rapid Commun. 2002, 23,
1032−1036. (b) Liu, Y.; Nishiura, M.; Wang, Y.; Hou, Z. J. Am. Chem.
Soc. 2006, 128, 5592−5593. (c) Adhikari, R. M.; Mondal, R.; Shah, B.
K.; Neckers, D. C. J. Org. Chem. 2007, 72, 4727−4732. (d) Zhao, Z.;
Zhao, Y.; Lu, P.; Tian, W. J. Phys. Chem. C 2007, 111, 6883−6888.
(e) Zhao, Z.; Xu, X.; Wang, H.; Lu, P.; Yu, G.; Liu, Y. J. Org. Chem.
2008, 73, 594−602. (f) Zhao, Z.; Li, J.-H.; Chen, X.; Wang, X.; Lu, P.;
Yang, Y. J. Org. Chem. 2009, 74, 383−395. (g) Wang, H.-Y.; Liu, F.;
Xie, L.-H.; Tang, C.; Peng, B.; Huang, W.; Wei, W. J. Phys. Chem. C
2011, 115, 6961−6967.
ASSOCIATED CONTENT
■
S
* Supporting Information
General experimental methods, X-ray data, UV−vis data,
1
Cartesian coordinates of all calculated molecules, and H and
13C NMR spectra of all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
(9) For selected examples of host materials for emitters, see:
(a) Brunner, K.; van Dijken, A.; Borner, H.; Bastiaansen, J. J. A. M.;
̈
AUTHOR INFORMATION
Corresponding Author
*E-mail: nakamura@gunma-u.ac.jp. Tel: +81 277 30 1310. Fax:
+81 277 30 1314.
■
Kiggen, N. M. M.; Langeveld, B. M. W. J. Am. Chem. Soc. 2004, 126,
6035−6042. (b) van Dijken, A.; Bastiaansen, J. J. A. M.; Kiggen, N. M.
M.; Langeveld, B. M. W.; Rothe, C.; Monkman, A.; Bach, I.; Stossel,
̈
P.; Brunner, K. J. Am. Chem. Soc. 2004, 126, 7718−7727. (c) Tsai, M.-
H.; Lin, H.-W.; Su, H.-C.; Ke, T.-H.; Wu, C.-c.; Fang, F.-C.; Liao, Y.-
L.; Wong, K.-T.; Wu, C.-I. Adv. Mater. 2006, 18, 1216−1220. (d) Tao,
Y.; Wang, Q.; Yang, C.; Wang, Q.; Zhang, Z.; Zou, T.; Qin, J.; Ma, D.
Angew. Chem., Int. Ed. 2008, 47, 8104−8107. (e) Kim, S. H.; Cho, I.;
Sim, M. K.; Park, S.; Park, S. Y. J. Mater. Chem. 2011, 21, 9139−9148.
(f) Chen, Y.-M.; Hung, W.-Y.; You, H.-W.; Chaskar, A.; Ting, H.-C.;
Chen, H.-F.; Wong, K.-T.; Liu, Y.-H. J. Mater. Chem. 2011, 21, 14971−
14978.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Dr. Soichiro Kyushin (Gunma University) for
generous permission to use an X-ray diffractometer and Prof.
Dr. Teruo Shinmyozu (Kyushu University) for mass spectrom-
etry measurements. This work was performed under the
Cooperative Research Program of “Network Joint Research
Center for Materials and Devices” (Kyushu University).
(10) (a) Drolet, N.; Morin, J.-F.; Leclerc, N.; Wakim, S.; Tao, Y.;
Leclerc, M. Adv. Funct. Mater. 2005, 15, 1671−1682. (b) Sonntag, M.;
Kreger, K.; Hanft, D.; Strohriegl, P. Chem. Mater. 2005, 17, 3031−
3039. (c) Wakim, S.; Blounin, N.; Gingras, E.; Tao, Y.; Leclerc, M.
Macromol. Rapid Commun. 2007, 28, 1798−1803. (d) Song, Y.; Di, C.-
a.; Wei, Z.; Zhao, T.; Xu, W.; Liu, Y.; Zhang, D.; Zhu, D. Chem.Eur.
J. 2008, 14, 4731−4740. (e) Li, Z.; Liu, Y.; Yu, G.; Wen, Y.; Guo, Y.; Ji,
L.; Qin, J.; Li, Z. Adv. Funct. Mater. 2009, 19, 2677−2683.
REFERENCES
■
(1) (a) Special issue on organic electronics: J. Mater. Res. 2004, 19,
1887−2203; Faupel, F., Dimitrakopoulos, C., Kahn, A., Woll, C., Eds.
(b) Organic Electronics: Materials, Manufacturing and Applications;
Klauk, H., Ed.; Wiley-VCH: Weinheim, 2006. (c) Special issue on
organic electronics and opto-electronics: Chem. Rev. 2007, 107, 923−
1386; Forrest, S. R., Thompson, M. E., Eds.
̈
(11) For a review on carbazole derivatives for photovoltaic devices,
see: (a) Beaupre,
2010, 22, E6−E27. See also: (b) Blounin, N.; Michaud, D.; Gendron,
D.; Wakim, S.; Blair, E.; Neagu-Plesu, R.; Belletete, M.; Durocher, G.;
́
S.; Boudreault, P.-L. T.; Leclerc, M. Adv. Mater.
̂
Tao, Y.; Leclerc, M. J. Am. Chem. Soc. 2008, 130, 732−742.
(c) Blounin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19,
2295−2300. (d) Peng, B.; Najari, A.; Liu, B.; Berrouard, P.; Gendron,
D.; He, Y.; Zhou, K.; Leclerc, M.; Zou, Y. Macromol. Chem. Phys. 2010,
211, 2026−2033.
(2) (a) Organic Light Emitting Devices: Synthesis, Properties and
Applications; Mullen, K., Scherf, U., Eds.; Wiley-VCH: Weinheim,
̈
2006. (b) Organic Electroluminescence; Kakafi, Z. H., Ed.; CRC Press:
New York, 2005.
(3) (a) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater. 2002,
14, 99−117. (b) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.;
Bredas, J.-L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16,
́
(12) (a) Moser, M.; Wucher, B.; Kunz, D.; Rominger, F.
Organometallics 2007, 26, 1024−1030. (b) Mudadu, M. S.; Singh, A.
3231
dx.doi.org/10.1021/jo202625p | J. Org. Chem. 2012, 77, 3222−3232

The Journal of Organic Chemistry
Article
N.; Thummel, R. P. J. Org. Chem. 2008, 73, 6513−6520. (c) Barbe, J.-
M.; Habermeyer, B.; Khoury, T.; Gros, C. P.; Richard, P.; Chen, P.;
Kadish, K. M. Inorg. Chem. 2010, 49, 8929−8940.
(29) We thank referees for checking the optimized structures of 1b
and 2.
(30) Kato, S.-i.; Noguchi, H.; Kobayashi, A.; Tobita, S.; Nakamura, Y.
Unpublished results. The results will be reported elsewhere.
(31) The fluorescence excitation spectra correspond to the
absorption spectra.
(32) Ohkita, H.; Ito, S.; Yamamoto, Y.; Tohda, Y.; Tani, K. J. Phys.
Chem. A 2002, 106, 2140−2145.
(33) Poly(2,7-carbazole)s display high fluorescence quantum yields.
See ref 7c and 33.
(34) Fu, Y.; Bo, Z. Macromol. Rapid Commun. 2005, 26, 1704−1710.
(35) Lee, S. A.; Hottta, S.; Nakanishi, F. J. Phys. Chem. A 2000, 104,
1827−1833.
(36) We assume that the first oxidation potentials of 6−10 are
ascribed to the carbazole-centered oxidations on the basis of
comparison with the oxidation potential of 2,2′-bithiophene (+0.96 V).
(37) It seems that the N-(2-thienyl) substituents of 1a and 1b slightly
contribute to their HOMOs by the σ-π conjugation rather than the π-
conjugation.
(13) (a) Hassan, L. A.; Norouzi-Arasi, H.; Wagner, M.; Enkelmann,
V.; Mullen, K. Chem. Commun. 2011, 47, 970−972. (b) Maeda, C.;
̈
Yoneda, T.; Aratani, N.; Yoon, M.-C.; Lim, J. M.; Kim, D.; Yoshioka,
N.; Osuka, A. Angew. Chem., Int. Ed. 2011, 50, 5691−5694.
(14) (a) Michinobu, T.; Osako, H.; Shigehara, K. Macromol. Rapid
Commun. 2008, 29, 111−116. (b) Michinobu, T.; Osako, H.;
Shigehara, K. Macromolecules 2009, 42, 8172−8180. (c) Michinobu,
T.; Osako, H.; Murata, K.; Shigehara, K. Chem. Lett. 2010, 39, 168−
169.
(15) (a) Mishra, A.; Ma, C.-Q.; Bauerle, P. Chem. Rev. 2009, 109,
̈
1141−1276. (b) Handbook of Thiophene-based Materials; Perepichka, I.
F., Perepichka, D. F., Eds.; Wiley-VCH: Weinheim, 2009.
(16) Zhan, X.; Liu, Y.; Zhu, D.; Jiang, X.; Jen, A. K.-Y. Macromol.
Chem. Phys. 2001, 202, 2341−2345.
(17) Zotti, G.; Schiavon, G.; Zecchin, S.; Morin, J.-F.; Leclerc, M.
Macromolecules 2002, 35, 2122−2128.
(38) With decreasing the solvent polarity from CH2Cl2 to
cyclohexane, the λem values of 6−8 are hypochromically shifted by
4−15 nm.
(18) Melucci, M.; Favaretto, L.; Bettini, C.; Gazzano, M.; Camaioni,
N.; Maccagnani, P.; Ostoja, P.; Monari, M.; Barbarella, G. Chem.Eur.
J. 2007, 13, 10046−10054.
(19) (a) Morin, J.-F.; Drolet, N.; Tao, Y.; Leclerc, M. Chem. Mater.
̂
2004, 16, 4619−4626. (b) Belletete, M.; Bouchard, J.; Leclerc, M.;
Durocher, G. Macromolecules 2005, 38, 880−887. (c) Lu, J.; Xia, P. F.;
Lo, P. K.; Tao, Y.; Wong, M. S. Chem. Mater. 2006, 18, 6194−6203.
(d) Promark, V.; Pankvuang, A.; Ruchirawat, S. Tetrahedron Lett. 2007,
48, 1151−1154. (e) Promarak, V.; Ruchirawat, S. Tetrahedron 2007,
63, 1602−1609. (f) Promarak, V.; Punkvuang, A.; Sudyoadsuk, T.;
Jungsuttiwong, S.; Saengsuwan, S.; Keawin, T.; Sirithip, K. Tetrahedron
2007, 63, 8881−8890.
̂ ́
(20) (a) Belltete, M.; Bedard, M.; Leclerc, M.; Durocher, G. Synth.
Met. 2004, 146, 99−108. (b) Yang, Li.; Feng, J.-K.; Ren, A.-M.; Sun, J.-
Z. Polymer 2006, 47, 1397−1404. (c) Doskocz, J.; Doskocz, M.;
Roszak, S.; Soloducho, J.; Leszczynski, J. J. Phys. Chem. A 2006, 110,
13989−13994.
(21) For a preliminary communication of this work, see: Shimizu,
H.; Kobayashi, A.; Itoi, S.; Yoshihara, T.; Tobita, S.; Nakamura, Y.;
Nishimura, J. Heterocycles 2009, 78, 1265−1269.
(22) Wu, I.-Y.; Lin, J. T.; Tao, Y.-T.; Balasubramaniam, E.; Su, Y. Z.;
Ko, C.-W. Chem. Mater. 2001, 13, 2626−2631.
(23) Hameurlaine, A.; Dehaen, W. Tetrahedron Lett. 2003, 44, 957−
959.
(24) Park, M.; Buck, J. R.; Rizzo, C. J. Tetrahedron 1998, 54, 12707−
12714.
(25) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457−2483.
(26) Hassan, J.; Sevignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M.
Chem. Rev. 2002, 102, 1359−1469.
(27) For the crystal packing structures of 5 and 9, see the Supporting
Information.
(28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J.
B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J.
J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision
C.02; Gaussian, Inc.: Wallingford, CT, 2004.
3232
dx.doi.org/10.1021/jo202625p | J. Org. Chem. 2012, 77, 3222−3232